LTC5100
1
sn5100 5100fs
FEATURES
APPLICATIO S
U
DESCRIPTIO
U
TYPICAL APPLICATIO
U
■
Gigabit Ethernet and Fibre Channel Transceivers
■
SFF and SFP Transceiver Modules
■
Proprietary Fiber Optic Links
, LTC and LT are registered trademarks of Linear Technology Corporation.
■
155Mbps to 3.2Gbps Laser Diode Driver for VCSELs*
■
60ps Rise and Fall Times, 10ps Deterministic Jitter
■
Eye Diagram is Stable and Consistent Across
Modulation Range and Temperature
■
1mA to 12mA Modulation Current
■
Easy Board Layout, Laser can be Remotely Located
if Desired
■
No Input Matching or AC Coupling Components
Needed
■
On-Chip ADC for Monitoring Critical Parameters
■
Digital Setup and Control with I
2
C
TM
Serial Interface
■
Emulation and Set-Up Software Available**
■
Operates Standalone or with a Microprocessor
■
On-Chip DACs Eliminate External Potentiometers
■
Constant Current or Automatic Power Control
■
First and Second Order Temperature Compensation
■
On-Chip Temperature Sensor
■
Extensive Eye Safety Features
■
Single 3.3V Supply
■
4mm × 4mm QFN Package
3.3V, 3.2Gbps VCSEL Driver
I
2
C is a trademark of Philips Electronics N.V.
*Vertical Cavity Surface Emitting Laser
**Downloadable from www.linear.com
Figure 1. VCSEL Transmitter with Automatic Power Control
The LTC
®
5100 is a 3.2Gbps VCSEL driver offering an
unprecedented level of integration and high speed perfor-
mance. The part incorporates a full range of features to
ensure consistently outstanding eye diagrams. The data
inputs are AC coupled, eliminating the need for external
capacitors. The LTC5100 has a precisely controlled 50Ω
output that is DC coupled to the laser, allowing arbitrary
placement of the IC. No coupling capacitors, ferrite beads
or external transistors are needed, simplifying layout,
reducing board area and the risk of signal corruption. The
unique output stage of the LTC5100 confines the modula-
tion current to the ground system, isolating the high speed
signal from the power supply to minimize RFI.
The LTC5100 supports fully automated production with its
extensive monitoring and control features. Integrated 10-bit
DACs eliminate the need for external potentiometers. An on-
board 10-bit ADC provides the laser current and voltage,
as well as monitor diode current and temperature. Status
information is available from the I
2
C serial interface for feed-
back and statistical process control.
An internal digital controller compensates laser tempera-
ture drift and provides extensive laser safety features.
3.2Gbps Electrical Eye Diagram
1mA/DIV
50ps/DIV
5100 TA01
–
+
ADC
3.3V
SCL
EN
SDA
V
DD
V
SS
24LC00 EEPROM
IN SOT-23 PACKAGE
MD
MODA
SRC
MODB
DAC
DAC
10nF
3.2Gbps
MODULATOR
FAULT
WARNING: POTENTIAL EYE HAZARD.
SEE “EYE SAFETY INFORMATION”
100Ω
IN
+
IN
–
50Ω
5100 F01
DIGITAL
CONTROLLER
ARBITRARY
DISTANCE
SERIALIZER
LTC5100
2
sn5100 5100fs
V
DD
, V
DD(HS)
............................................................. 4V
IN
+
, IN
–
(Cml_en = 1) (Note 6)
Peak Voltage........... V
DD(HS)
– 1.2V to V
DD(HS)
+ 0.3V
Average Voltage...... V
DD(HS)
– 0.6V to V
DD(HS)
+ 0.3V
IN
+
, IN
–
(Cml_en = 0) (Note 4).. –0.3V to V
DD(HS)
+ 0.3V
Cml_en = 0 (Note 4)
Peak Difference Between IN
+
and IN
–
.............. ±2.5V
Average Difference Between IN
+
and IN
–
....... ±1.25V
MODA, MODB (Transmitter Disabled) ....–0.3V to 2.75V
MODA, MODB
(Transmitter Enabled) ............ V
DD(HS)
– 2.75V to 2.75V
EN, SDA, SCL, FAULT .....................–0.3V to V
DD
+ 0.3V
MD, SRC................................................... –0.3V to V
DD
Ambient Operating Temperature Range.. –40°C to 85°C
Storage Temperature Range ................ –65°C to 125°CConsult LTC Marketing for parts specified with wider operating temperature ranges.
ABSOLUTE AXI U RATI GS
W
WW
U
PACKAGE/ORDER I FOR ATIO
UUW
(Note 1)
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1%
resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to
VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5.
ORDER PART
NUMBER
UF PART MARKING
5100
LTC5100EUF
T
JMAX
= 125°C, θ
JA
= 37°C/W
EXPOSED PAD IS V
SS
(PIN 17)
MUST BE SOLDERED TO PCB GROUND PLANE
16 15 14 13
5 6 7 8
TOP VIEW
17
UF PACKAGE
16-LEAD (4mm × 4mm) PLASTIC QFN
9
10
11
12
4
3
2
1VSS
IN+
IN–
VSS
VSS
MODA
MODB
VSS
VDD
EN
SRC
MD
FAULT
SDA
SCL
VDD(HS)
1/16 of Full-Scale I
S
Current
PARAMETER CONDITIONS MIN TYP MAX UNITS
Power Supply
V
DD
, V
DD(HS)
Operating Voltage ●3.135 3.3 3.465 V
V
DD
+ V
DD(HS)
Quiescent Current, V
DD
= 3.465V
Excluding the SRC Pin Current (Note 2)
Transmitter Disabled, Power_down_en = 1 4.5 mA
Transmitter Enabled, Is_rng = Im_rng = 3 54 mA
Impp = 24mA
High Speed Data Inputs (IN
+
and IN
–
Pins) (Test Circuit, Figure 5)
Input Signal Amplitude Peak-to-Peak Differential Voltage (The Single- 500 to 2400 mV
P-P
Ended Peak-to-Peak Voltage is One Half the
Differential Voltage)
Common Mode Input Signal Range (Note 3) Cml_en = 0 (Note 4) 0 V
DD(HS)
V
Differential Input Resistance 80 to 120 Ω
Common Mode Input Resistance Cml_en = 0 (Note 5) 50 kΩ
Open-Circuit Voltage Cml_en = 0 (Note 5) 1.65 V
SRC Pin Current, I
S
Full-Scale I
S
Current Is_rng = 0 6 9 mA
Is_rng = 1 12 18 mA
Is_rng = 2 18 27 mA
Is_rng = 3 24 36 mA
Minimum Operating Current (Note 7)
Resolution 10 Bits
SRC Pin Voltage Range 1.2 V
DD
–V
200mV
LTC5100
3
sn5100 5100fs
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1%
resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to
VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5.
1/8 of Full-Scale Peak-to-Peak
Modulation Current
PARAMETER CONDITIONS MIN TYP MAX UNITS
Laser Bias Current, I
B
Full-Scale Current (Note 8) Is_rng = 0 6 – I
M
9 – I
M
mA
Is_rng = 1 12 – I
M
18 – I
M
mA
Is_rng = 2 18 – I
M
27 – I
M
mA
Is_rng = 3 24 – I
M
36 – I
M
mA
Absolute Accuracy SRC Pin and MODA, MODB Pin Currents Within ±25 %
Specified Voltage Ranges
Resolution 10 Bits
Linear Tempco Resolution 122 ppm/°C
Linear Tempco Range ±15625 ppm/°C
Second Order Tempco Resolution 3.81 ppm/°C
2
Second Order Tempco Range ±488 ppm/°C
2
Temperature Stability Ib_tc1 = 0, Ib_tc2 = 0 ±500 ppm/°C
Off-State Leakage Transmitter Disabled, V
SRC
= 1.2V 50 µA
MODA, MODB Pin Current, I
M
Full Scale, Peak-to-Peak Modulation Current (Note 9) Im_rng = 0 6 9 mA
Im_rng = 1 12 18 mA
Im_rng = 2 18 27 mA
Im_rng = 3 24 36 mA
Minimum Operating Current (Note 10)
Resolution (Note 11) 9 Bits
Current Stability Im_tc1 = 0, Im_tc2 = 0 ±500 ppm/°C
Voltage Range Peak Transient Voltage on MODA and MODB 1.2 2.7 V
Absolute Accuracy of the Modulation Current ±25 %
Linear Tempco Resolution 122 ppm/°C
Linear Tempco Range ±15625 ppm/°C
Second Order Tempco Resolution 3.81 ppm/°C
2
Second Order Tempco Range ±484 ppm/°C
2
Maximum Bit Rate 3.2 Gbps
Modulation Current Rise and Fall Times 20% to 80% Measured with K28.5 Pattern at 60 ps
2.5Gbps
Deterministic Jitter, Peak-to-Peak (Note 12) Measured with K28.5 Pattern at 3.2Gbps 10 ps
Random Jitter, RMS (Note 13) 1ps
RMS
Pulse Width Distortion 10 ps
Automatic Power Control (Note 14)
Minimum Operating Current for the Monitor Diode 20% of Full Scale
(Note 15) Monitor Diode Current
Temperature Stability Imd_tc1 = 0, Imd_tc2 = 0 ±500 ppm/°C
Monitor Diode Bias Voltage (Note 16) I
MD
≤ 1600µA 1.45 V
LTC5100
4
sn5100 5100fs
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1%
resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to
VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Automatic Power Control (Note 14)
Temperature Compensation (Note 17)
Linear Tempco Resolution 254 • Imd_nom/1024 ppm/°C
Linear Tempco Range ±32300 • Imd_nom/1024 ppm/°C
ADC
Resolution 10 Bits
Source Current Measurement, I
S
(SRC Pin Current)
Full Scale Is_rng = 0 9 mA
Is_rng = 1 18 mA
Is_rng = 2 27 mA
Is_rng = 3 36 mA
Accuracy ±3% of Full Scale
±25% of Reading
Average Modulation Current Measurement, I
M
(Note 18)
Full Scale Im_rng = 0 9 mA
Im_rng = 1 18 mA
Im_rng = 2 27 mA
Im_rng = 3 36 mA
Accuracy ±3% of Full Scale
±25% of Reading
Laser Diode Voltage Measurement
Full Scale 3.5 V
Accuracy ±150mV ±10% of Reading
Monitor Diode Current Measurement (Note 19)
Full Scale Imd_rng = 0 34 µA
Imd_rng = 1 136 µA
Imd_rng = 2 544 µA
Imd_rng = 3 2176 µA
Zero Scale ADC Code = 0 1/8 of Full Scale
Resolution Relative to Reading 0.2 %
Accuracy ±25% of Reading
Temperature Measurement
Full Scale Celsius 239 °C
Sensitivity 0.500 °C/LSB
Termination Resistor Voltage Measurement
Full Scale Is_rng = 0 400 mV
Is_rng = 1 800 mV
Is_rng = 2 1200 mV
Is_rng = 3 1600 mV
Accuracy ±30mV ±10% of Reading
Safety Shutdown, Undervoltage Lockout (UVLO)
Undervoltage Detection V
DD
Decreasing 2.8 V
Undervoltage Detection Hysteresis 150 mV
LTC5100
5
sn5100 5100fs
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1%
resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to
VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Bias Current Limit, I
B(LIMIT)
Set Point Resolution 7 Bits
Set Point Range Is_rng = 0 9 mA
Is_rng = 1 18 mA
Is_rng = 2 27 mA
Is_rng = 3 36 mA
Optical Power Limit Automatic Power Control Mode Only, Apc_en = 1
Overpower Limit Expressed in % Over the Imd Set Point 50 %
Underpower Limit Expressed in % Under the Imd Set Point –50 %
Safety Shutdown Response Time Time from the Fault Occurance to Reduction of 100 µs
the Laser Bias Current to 10% of Nominal
FAULT Output, Open-Drain Mode, Flt_drv_mode = 0
Output Low Voltage I
OL
= 3.3mA 0.4 V
Output High Leakage Current V
FAULT
= 2.4V 10 µA
FAULT Output, Open-Drain Mode with 330µA Internal Pull Up, Flt_drv_mode = 1
Output Low Voltage I
OL
= 3.3mA 0.4 V
Output High Current V
FAULT
= 2.4V –280 µA
FAULT Output, Open-Drain Mode with 500µA Internal Pull Up, Flt_drv_mode = 2
Output Low Voltage I
OL
= 3.3mA 0.4 V
Output High Current V
FAULT
= 2.4V –425 µA
FAULT Output, Complementary Drive Mode, Flt_drv_mode = 3
Output High Voltage I
OH
= –3.3mA 2.4 V
Output Low Voltage I
OL
= 3.3mA 0.4 V
EN Input, Ib_gain or (Apc_gain in APC Mode) = 16, Im_gain = 4, Is_rng = 0, Im_rng = 0
Input Low Voltage 0.8 V
Input High Voltage 2V
Input Low Current En_polarity = 0 (EN Active Low), V
EN
= 0V –10 µA
Input High Current En_polarity = 0 (EN Active Low), V
EN
= V
DD
–10 to 10 µA
Input Low Current En_polarity = 1 (EN Active High), V
EN
= 0V –10 to 10 µA
Input High Current En_polarity = 1 (EN Active High), V
EN
= V
DD
10 µA
Transmit Enable Time Time from Active Transition on EN to 95% of 100 ms
Nominal Laser Power and 95% of Full Modulation.
First Time Transmission is Enabled After Power
On or with Rapid_restart_en = 0
Transmit Re-Enable Time Time from Active Transition on EN to 95% of 1 ms
Nominal Laser Power and 95% of Full Modulation.
When Transmission is Re-Enabled After the First
Time and with Rapid_restart_en = 1
Transmit Disable Time Time from Inactive Transition on EN to 5% of 10 µs
Nominal Laser Power
Minimum Pulse Width Required to Clear 10 µs
a Latched Fault
LTC5100
6
sn5100 5100fs
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1%
resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to
VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: The quiescent V
DD
and V
DD(HS)
currents refer to the current with
zero SRC pin current (i.e., the laser is operating with zero bias current and
zero modulation current). The total power supply current is the quiescent
current plus the SRC pin current, I
S
, plus any current sinked from IN
+
and
IN
–
.
Note 3: The peak transient voltage at the IN
+
and IN
–
pins must not
exceed the range of –300mV to V
DD(HS)
+ 300mV.
Note 4: When Cml_en = 0 (not in CML mode), the termination is 100Ω
differential with 50k common mode to V
DD(HS)
/2.
Note 5: The common mode input resistance is measured relative to
V
DD(HS)
/2 with the inputs tied together.
Note 6: When Cml_en = 1 (CML mode), the termination is nominally 50Ω
to V
DD(HS)
on each of the IN
+
and IN
–
pins.
Note 7: The SRC pin current can be programmed to near zero in each
range, but the recommended minimum operating level is 1/16 of full scale.
Note 8: The laser bias current is the average current delivered to the laser.
It is equal to the SRC pin current minus the average modulation current at
the MODA and MODB pins, or I
B
= I
S
– I
M
. Full scale for the bias current
therefore depends on Is_rng and the actual modulation current.
Note 9: The MODA and MODB pins are connected on-chip. The modulation
current refers to the sum of the currents on these pins. I
M
refers to the
total average current at the MODA and MODB pins. I
MPP
refers to the total
peak-to-peak modulation current at the MODA and MODB pins. I
MPP
differs
from the laser modulation current, I
MOD
. I
MPP
splits between the laser and
the termination resistor according to I
MOD
= I
MPP
• R
T
/(R
T
+ R
LD
), where
R
T
is the value of the termination resistor and R
LD
is the dynamic
resistance of the laser diode.
PARAMETER CONDITIONS MIN TYP MAX UNITS
SCL, SDA
SCL, SDA Input Low Voltage, V
IL
–0.5 0.3 • V
V
DD
SCL, SDA Input High Voltage, V
IH
0.7 • V
DD
+V
V
DD
0.5
SCL, SDA Input Low Current (Note 21) V
SDA
, V
SCL
= 0.1 • V
DD
–100 µA
SCL, SDA Input High Current (Note 21) V
SDA
, V
SCL
= 0.9 • V
DD
–100 µA
SCL, SDA Output Low Voltage I
OL
= 3mA 0 0.4 V
Hysteresis 280 mV
Serial Interface Timing (Note 20)
SCL Clock Frequency 100 kHz
Hold Time (Repeated) START Condition. After This 4 µs
Period the First Clock Pulse is Generated
Low Period of the SCL Clock 4.7 µs
High Period of the SCL Clock 4µs
Set-Up Time for a Repeated START Condition 4.7 µs
Data Hold Time 0 3.45 µs
Data Set-Up Time 250 ns
Input Rise Time of Both SDA and SCL Signals 1000 ns
Output Fall Time of SCL and SDA from V
IH(MIN)
to 300 ns
V
IL(MAX)
with a Bus Capacitance from 10pF to 400pF
Set-Up Time for STOP Condition 4µs
Bus Free Time Between a STOP and START Condition 4.7 µs
Capacitive Load for Each Bus Line 400 pF
Noise Margin at the LOW Level for Each Connected 0.1 • V
Device (Including Hysteresis) V
DD
Noise Margin at the HIGH Level for Each Connected 0.2 • V
Device (Including Hysteresis) V
DD
LTC5100
7
sn5100 5100fs
ELECTRICAL CHARACTERISTICS
Note 10: The modulation current can be programmed to near zero in each
range, but the high speed performance is not guaranteed for currents less
than the specified minimum.
Note 11: The effective resolution of the modulation current is 9 bits
because the modulation servo system uses only one-half of the 10-bit
ADC range.
Note 12: As defined in ANSI x3.230, Annex A, deterministic jitter is the
peak-to-peak deviation of the 50% crossings of the modulation signal
when compared to the ideal time crossings. The specification for the
LTC5100 pertains to the electrical modulation signal. The K28.5 pattern is
the repeating sequence 00111110101100000101.
Note 13: Random jitter is the standard deviation of the 50% crossings of
the electrical modulation signal as measured by an oscilloscope. It is
measured with a 1GHz square wave after quadrature subtraction of the
random jitter of the pulse generator and oscilloscope. Peak-to-peak
random jitter is defined as 14 times the RMS random jitter.
Note 14: The LTC5100 digitizes and servo controls the logarithm of the
monitor diode current. Many of the characteristics of the APC system,
such as range and resolution, are determined by the ADC.
Note 15: The minimum practical operating current for the monitor diode is
determined by servo settling time considerations.
Note 16: I
MD
must be less than 25µA, 100µA, 400µA and 1600µA
corresponding to Imd_rng = 0, 1, 2, 3.
Note 17: The temperature coefficients of the monitor diode current depend
on the I
MD
setting because of the logarithmic relationship between the set
point and the monitor diode current. Imd_nom is the digital code setting
for the nominal monitor diode current. Imd_nom lies between 0 and 1023.
Note 18: The ADC digitizes the average modulation current, which is 50%
of the peak-to-peak current for a 50% duty cycle signal.
Note 19: The LTC5100 ADC digitizes the logarithm of the monitor diode
current. This implies that the ADC resolution is a constant percentage of
reading and that the monitor diode current is non-zero when the ADC
reads zero. See the Design Notes for further information.
Note 20: Serial interface timing is guaranteed by design from
–40°C to 85°C.
Note 21: The LTC5100 has 100µA nominal pull-up current sources on the
SCL and SDA pins to eliminate the need for external pull-up resistors when
connected to a single EEPROM device. The LTC5100 meets the maximum
rise time specification of 1000ns with external I
2
C bus capacitances up to
25pF. Example: 10pF EEPROM + 150mm trace ~ 25pF.
Note 22: V
DD
and V
DD(HS)
must be tied together on the PC board.
LTC5100
8
sn5100 5100fs
TYPICAL PERFOR A CE CHARACTERISTICS
UW
VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit shown in Figure 5.
Optical Eye Diagram at 3.2Gbps
with 850nm VCSEL
100µW/DIV
50ps/DIV 5100 G01
EMCORE MODE LC-TOSA VCSEL
2
15
PRBS, 7dB EXTINCTION RATIO, 300µW AVG
PWR, 2.4GHz 4TH ORDER BESSEL-THOMPSON
LOWPASS FILTER
Optical Eye Diagram at 2.5Gbps
with 850nm VCSEL
125µW/DIV
60ps/DIV 5100 G02
EMCORE MODE LC-TOSA VCSEL
2
15
PRBS, 10dB EXTINCTION RATIO, 300µW AVG
PWR, 1.87GHz 4TH ORDER BESSEL-THOMPSON
LOWPASS FILTER
Effect of Peaking Control on the
Electrical Eye Diagram
3mA/DIV
50ps/DIV 5100 G03
3.2Gbps, 2
23
PRBS, Im_rng = 2, I
MPP
= 12mA,
PEAKING = 4, 8, 16, 30
Electrical Eye Diagram at 25°C
3.2Gbps, 223 PRBS, IMPP = 3mA
0.5mA/DIV
49ps RISING 50ps/DIV 5100 G04
54ps FALLING
Im_rng = 0
PEAKING = 16
Electrical Eye Diagram at 25°C
3.2Gbps, 223 PRBS, IMPP = 12mA
2mA/DIV
51ps RISING 50ps/DIV 5100 G05
59ps FALLING
Im_rng = 2
PEAKING = 16
Electrical Eye Diagram at 25°C
3.2Gbps, 223 PRBS, IMPP = 24mA
4mA/DIV
50ps RISING 50ps/DIV 5100 G06
62ps FALLING
Im_rng = 3
PEAKING = 16
Electrical Eye Diagram at –40°C,
3.2Gbps, 223 PRBS, IMPP = 12mA
2mA/DIV
47ps RISING 50ps/DIV 5100 G07
56ps FALLING
Im_rng = 2
PEAKING = 16
Electrical Eye Diagram at 85°C,
3.2Gbps, 223 PRBS, IMPP = 12mA
2mA/DIV
57ps RISING 50ps/DIV 5100 G09
67ps FALLING
Im_rng = 2
PEAKING = 16
2mA/DIV
LTC5100
9
sn5100 5100fs
TYPICAL PERFOR A CE CHARACTERISTICS
UW
VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit shown in Figure 5.
Supply Current vs Modulation
Level (Excluding Laser Current) Supply Current vs Temperature Modulator Output Resistance
vs Modulation Level
Impp (mA)
0
0
IDD + IDD(HS) (mA)
10
20
30
40 Im_rng
0
60
481216
5100 G16
20 24
50 Im_rng
1
Im_rng
2Im_rng
3
TRANSMIT DISABLED
AND Power_down_en = 0
TRANSMIT DISABLED
AND Power_down_en = 1
TRANSMIT
ENABLED
TEMPERATURE (°C)
–40
48.1
48.0
47.9
47.8
47.7
47.6
47.5
47.4 20 60
5100 G17
–20 0 40 80
I
DD
+ I
DD(HS)
(mA)
Im_rng = 3
Impp = 12mA
Ib = 0.0mA
(INCLUDES SRC PIN
CURRENT REQUIRED
TO SUPPLY THE AVERAGE
MODULATION CURRENT
Impp (mA)
1
100
OUTPUT RESISTANCE (Ω)
1000
10000
10 100
5100 G18
Laser Bias Current
vs Temperature in CCC Mode Monitor Diode Current
vs Temperature in APC Mode
TEMPERATURE (°C)
–40
0.97
I
B
(NORMALIZED)
0.98
0.99
1.00
1.01
–20 0 20 40
5100 G20
60 80
NORMALIZED TO UNITY AT 25°C
Ib_tc1 = Ib_tc2 = 0
TEMPERATURE (°C)
–40
0.97
IMD (NORMALIZED)
0.98
0.99
1.00
1.01
–20 0 20 40
5100 G21
60 80
NORMALIZED TO UNITY AT 25°C
Imd_tc1 = Imd_tc2 = 0
Laser Modulation Current
vs Temperature, Im_rng = 1
TEMPERATURE (°C)
–40
0.97
I
MPP
(NORMALIZED)
0.98
0.99
1.00
1.01
–20 0 20 40
5100 G22
60 80
NORMALIZED TO UNITY AT 25°C
Im_tc1 = Im_tc2 = 0
Rise and Fall Times vs IM at the
Midpoint of Each Im_rng Deterministic Jitter vs IMPP for
Im_rng = 3
I
MPP
(mA)
0
50
20% TO 80% RISE AND FALL TIMES (ps)
52
54
56
58
60
62
36912
5100 G13
15
Im_rng
0
1
1
2
2
3
3
t
FALL
t
RISE
Im_rng
0
I
MPP
(mA)
0
40
20% TO 80% RISE AND FALL TIMES (ps)
45
55
60
65
612 15 27
5100 G14
50
39 18 21 24
t
FALL
t
RISE
I
MPP
(mA)
0
DETERMINISTIC JITTER (ps)
5
6
7
24
5100 G15
4
3
1
0612 18
327
915 21
2
8
Rise and Fall Times
vs IMPP for Im_rng = 3
LTC5100
10
sn5100 5100fs
TYPICAL PERFOR A CE CHARACTERISTICS
UW
VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted.
Transmitter Enable
5µs/DIV
5100 G26
V
DD
FAULT
LASER OUTPUT
EN
Transmitter Disable
5µs/DIV
5100 G27
V
DD
FAULT
LASER OUTPUT
EN
Transmitter Enable, Rapid Restart
10µs/DIV
5100 G28
V
DD
FAULT
LASER OUTPUT
EN
Response to Fault Fault Recovery Time
10µs/DIV
5100 G29
FAULT
FAULT
LASER OUTPUT
V
MD
10ms/DIV
5100 G30
FAULT
LASER OUTPUT
EN
5µs WIDE
PULSE ON EN
Hot Plug with EN Active in
CCC Mode
10ms/DIV
5100 G23
V
DD
SCL
FAULT
LASER OUTPUT
Hot Plug with EN Active in
APC Mode
10ms/DIV
5100 G24
V
DD
SCL
FAULT
LASER OUTPUT
Start-Up with Slow Ramping
Supply in APC Mode
10ms/DIV
5100 G25
V
DD
FAULT
LASER OUTPUT
EEPROM READ
SCL
LTC5100
11
sn5100 5100fs
PI FU CTIO S
UUU
VSS (Pins 1, 4, 9, 12, 17): Ground for Digital, Analog and
High Speed Circuitry. These pins are internally connected.
Connect Pins 1, 4, 9 and 12 to the ground plane with
minimal trace lengths. Place a minimum of four vias
(preferably nine vias) to the ground plane in the Exposed
Pad area. Most of the high speed modulation current is
returned through the Exposed Pad (Pin 17).
IN
+
, IN
–
(Pins 2, 3): High Speed Laser Modulation Inputs.
The inputs are differential with internal termination resis-
tors. The input amplifier is internally AC coupled. With
current mode logic (CML) enabled, the inputs are indepen-
dently terminated to V
DD(HS)
with 50Ω resistors. With
CML disabled, the inputs provide 100Ω differential termi-
nation and permit rail-to-rail common mode range. The
input pins can be AC coupled with external capacitors.
When externally AC coupled, the input pins self-bias to
V
DD(HS)
/2. The Cml_en bit selects the termination mode.
FAULT (Pin 5): Signals One of Five Safety Fault Con-
ditions: laser overcurrent, overpower, underpower, power
supply undervoltage and memory load error. The pin can
be programmed active high or active low with the
Flt_pin_polarity bit. The FAULT pin can be programmed to
four different drive modes with the Flt_drv_mode bits.
SDA, SCL (Pins 6, 7): Serial Interface Data and Clock
Signals. The pins are open drain with a 100µA internal pull-
up current. An external pull-up resistor can be added to
drive larger capacitive loads.
V
DD(HS)
(Pin 8): Power Input for the High Speed Laser
Modulation Circuitry. Filter this pin with a ferrite bead and
bypass the pin directly to the ground plane with a 10nF
ceramic capacitor.
MODA, MODB (Pins 11, 10): High Speed Laser Modula-
tion Outputs. MODA and MODB are connected on-chip
and driven by an open-drain output transistor. One of
these pins should be connected to the laser. The other
should be connected to a termination resistor. See the
Applications Information section for details.
MD (Pin 13): Monitor Diode Input for Automatic Power
Control of the Laser Bias Current. The MD pin allows
connection to the cathode or anode of the monitor diode.
The Md_polarity bit selects the polarity of the monitor
diode.
SRC (Pin 14): Current Source for Biasing the Laser. See
the Applications Information section for details.
EN (Pin 15): Transmitter Enable and Disable Input. This
input is TTL compatible and can be programmed for active
high or active low operation with the En_polarity bit. An
internal 10µA current source disables the transmitter if the
EN pin becomes disconnected. This safety feature oper-
ates whether the EN pin is active high or active low.
V
DD
(Pin 16): Power Input for Digital and Low Speed
Analog Circuitry. Connect this pin to V
DD(HS)
(Pin 8) with
a short trace. No bypassing is needed at the V
DD
pin if the
trace length to the V
DD(HS)
bypass capacitor is less than
10mm long.
LTC5100
12
sn5100 5100fs
BLOCK DIAGRA
W
–
+
–
+
I
M(MON)
I
M
V
TERM(MON)
V
LD
MODA
MODB
Transmit_en
Transmit_en
Im_rng
Is_rng
Is_rng
PEAKING DAC
5 BITS
I
M
DAC
10 BITS
I
S
DAC
10 BITS
I
S
V
DD
Im_rng Is_rng
I
S(MON)
I
MD(MON)
Over_current
Over_pwr
Under_pwr
En_polarity
Over_pwr
Under_pwr
I
M(MON)
11
10
V
SS
5100 F02
9
V
SS
12
SRC
14
I
S(MON)
Over_current
–
–
+
–
+
Imd_rngMd_polarity
LOGARITHMIC
AMPLIFIER
Flt_pin_polarity
Transmit_en
Flt_drv_mode
FAULT PIN
DRIVER
Ib LIMIT DAC
7 BITS
10-BIT ADC
I
S(MON)
I
M(MON)
V
LD
I
MD(MON)
V
TERM(MON)
TEMP
SENSOR
sel
LASER POWER
CONTROLLER
REGISTER
SET
SERIAL DIGITAL
INTERFACE
UNDERVOLTAGE
DETECTION
Under_voltage
POWER LIMIT
COMPARATORS
TRANSMIT AND
FAULT
CONTROLLER
CURRENT
ATTENUATOR
6
7
SDA
EN
16
V
DD
SCL
2
IN+
3
IN–
V
SS
100µADATA BUS
100µA
MD
13
FAULT
5
4
17
V
SS
1
V
DD(HS)
8
50Ω50Ω
CML_en
V
DD(HS)
20pF
15
V
SS
(EXPOSED PAD)
Mem_load_errorr
Fault
Figure 2. Block Diagram
LTC5100
13
sn5100 5100fs
Figure 3. Functional Diagram—Automatic Power Control Mode
FU CTIO AL DIAGRA S
UU
W
–
+
DAC
ADC
+
+
Im set
Imd set
Im error
Peaking
Im nom
Im tc2
Im tc1
Imd tc2
Imd tc1
Ext temp en
T nom
T int adc
T ext
Im gain
Im dac
Im adc
IN
+
IN
–
V
SS
100Ω
V
SS
8
V
DD(HS)
2
3
4
1
SDA
EN
16
V
DD
6
SCL 7
15
11
10
9
12
∑DAC
+–
Imd error
Imd nom
Apc gain
Is dac
Imd adc
∑DAC
ADC
ADC
V
DD
ADC
ADC V
SS
5100 F03
V
SS
V
LD
V
TERM
I
S
I
S
MODB
MODA
I
M
USER_ADC. Data
USER_ADC. Data
USER_ADC. Data
ADC
+
–
+
–
+
–
14 SRC
13 MD
5FAULT
Im rng
Is rng
Imd rng Md polarity
TEMP
COMP
TEMP
COMP
MINIMUM
GAIN CTRL
CURRENT
ATTENUATOR
TEMP
SENSOR
I
MD
LOG AMP
+
–
–
–
17
V
SS
(EXPOSED PAD)
LTC5100
14
sn5100 5100fs
FU CTIO AL DIAGRA S
UU
W
Figure 4. Functional Diagram—Constant Current Control Mode
USER_ADC. Data
USER_ADC. Data
–
+
DAC
+
–
Ib_set
Im nom
Im tc2
Im tc1
Ib tc2
Ib tc1
Ext temp en
T nom
T int adc
T ext
Im gain
Im dac
Im adc
IN
+
IN
–
V
SS
100Ω
V
SS
8
V
DD(HS)
2
3
4
1
SDA
EN
16
V
DD
6
SCL 7
15
11
10
9
12
∑DAC
+–
Ib_error
Im_error
Ib nom
Ib gain
Is rng
Im rng
Is dac
Is adc
∑DAC
ADC
V
DD
ADC
ADC V
SS
5100 F04
V
SS
V
LD
V
TERM
I
S
I
S
MODB
MODA
I
M
ADC
+
–
+
–
+
–
14 SRC
13 MD
5FAULT
Im rng
Is rng
TEMP
COMP
TEMP
COMP
TEMP
SENSOR
(BIAS CURRENT)
+
–
Peaking
+
–
17
V
SS
(EXPOSED PAD)
ADC
+
–
LTC5100
15
sn5100 5100fs
TEST CIRCUIT
Figure 5. Test Circuit
EQUIVALE T I PUT A D OUTPUT CIRCUITS
UUU
15 EN
V
DD
En_polarity
0 EN ACTIVE LOW
10µA
V
DD
V
SS
10µA
5100 F06
1 EN ACTIVE HIGH
LTC5100
100µA
5100 F07
SDA, SCL
6, 7
VDD
LTC5100
VDD
Figure 6. Equivalent Circuit for the EN Pin Figure 7. Equivalent Circuit for the SDA and SCL Pins
6
3.3mA
DRIVER
COMPLEMENTARY
DRIVE FAULT
5100 F08
250µA
PULL-UP
250µA
3.3mA
DRIVER
400µA
PULL-UP
400µA
V
DD
LTC5100
V
DD
13
V
DD
M2 Md_polarity
MD
5100 F09
Imd
Imd
M1
1
0
LTC5100
V
DD
Figure 8. Equivalent Circuit for the FAULT Pin.
All Switches are Open in Open-Drain Mode Figure 9. Equivalent Circuit for the MD Pin
FAULT SDA SCL VDD(HS)
VDD
VDD
VSS
IN+
IN–
VSS
VSS
MODA
MODB
VSS
EN SRC
1.8V POWER SOURCE
LTC5100
MD
13
12
10nF
ZO = 50Ω
MICROWAVE
BLOCKING
CAPACITOR
TO
SCOPE
50Ω
RESISTORS: 0402 SURFACE MOUNT
CAPACITORS: 0402 SURFACE MOUNT, X7R DIELECTRIC
11
10
9
1
2
3
4
141516
8
10nF
5100 F05
765
10nF
ZO = 50Ω
ZO = 50Ω
FROM
BERT
LTC5100
16
sn5100 5100fs
EQUIVALE T I PUT A D OUTPUT CIRCUITS
UUU
2IN
+
3IN
–
50Ω
50k
V
DD(HS)
TO INPUT
AMPLIFIER
50k
5100 F10
25k
20pF
R
ON
≅ 3ΩCml_en
50Ω
LTC5100
V
DD(HS)
V
DD(HS)
14
V
DD
M2
25k
SRC
11
MODA
10
MODB
5100 F11
1M
M1
LTC5100 V
DD(HS)
V
DD(HS)
Figure 10. Equivalent Circuit for the IN+ and IN– Pins Figure 11. Equivalent Circuit for the SRC, MODA and MODB Pins
OPERATIO
U
OVERVIEW
(Refer to Figure 1 and the Block Diagram in Figure 2)
The LTC5100 is optimized to drive common cathode
VCSELs in high speed fiber optic transceivers. The chip
incorporates several features that make it very compact
and easy-to-use while delivering exceptional high speed
performance. Only a capacitor, a resistor and a small
EEPROM (excluding laser diode and power supply filter-
ing) are needed to build a complete fiber optic transmitter.
Digital control over the I
2
C serial interface allows fully
automated laser setup to improve manufacturing effi-
ciency. The LTC5100’s extensive set of eye safety features
meet GBIC and SFF requirements but go beyond the
standards with open-pin protection, redundant transmit-
ter enable controls and other interlocks.
10-bit integrated DACs set laser bias and modulation
levels, eliminating the cost and space of digital potentiom-
eters. A multiplexed ADC allows monitoring of tempera-
ture and laser operating conditions in production or field
operation. Laser bias and modulation currents are digi-
tally temperature compensated to second order for tight
control of average power and extinction ratio. The LTC5100
provides both constant current and automatic power
control of the laser bias current. In automatic power
control mode, special circuitry maintains constant set-
tling time in spite of variations in the laser slope efficiency
and monitor diode response characteristics.
The high speed inputs of the LTC5100 are internally
terminated in 50Ω and internally AC coupled, eliminating
all external components at the inputs. The modulation
output is DC coupled to the laser and presents a high
quality resistive drive impedance to deliver very fast and
clean eye diagrams in spite of laser impedance variations.
The modulation output is capable of driving significant
lengths of transmission line, allowing the LTC5100 to be
placed at an arbitrary distance from the laser. This feature
allows for packaging flexibility within the module.
The LTC5100 minimizes electromagnetic interference (EMI)
with several architectural features. The unique design of
the driver output forces the high speed modulation current
to circulate only in the laser and ground system. The high
speed amplifier chain and the digital circuitry are internally
filtered and decoupled to further reduce power supply
noise generation.
LTC5100
17
sn5100 5100fs
LASER BIAS AND MODULATION
Modulator Architecture
The LTC5100 drives common cathode lasers using a
method called “shunt switching”. As shown in Figure 12,
shunt switching involves sourcing DC current into the
laser diode and shunting part of that current with a high
speed current switch to produce the required modulation.
The SRC pin provides the DC current and the MODA,
MODB pins (which are connected on chip) provide the
high speed modulation current. This technique results in
a very fast, single-ended driver that confines the high
speed modulation current to the laser and ground system.
The LTC5100 actually uses a modified shunt switching
scheme in which the source current is delivered through
a “termination” resistor, R
T
, that is bypassed to ground
with a large capacitor. The resistor brings three advan-
tages to the modulation stage. First, it gives the modulator
a precise resistive output impedance to damp ringing and
absorb reflections from the laser. Second, the resistor
isolates the capacitance of the SRC pin from the high
speed signal path, further improving modulation speed.
Third, the resistor and capacitor heavily filter the high
speed output signal so that it does not modulate the power
supply and cause radiation or interference. On-chip
decoupling of the high speed amplifiers further reduces
power supply noise generation.
OPERATIO
U
Terminology and Basic Calculations
Figure 12 through Figure 16 define terminology that is
used throughout this data sheet. The current delivered by
the SRC pin is called I
S
. The
average
modulation current
delivered by the chip at the MODA, MODB pins is called I
M
.
The laser bias current, I
B
, is defined as the average current
in the laser. I
B
is the difference between the source current
and average modulation current.
The
peak-to-peak
modulation current delivered by the
chip is called IMPP. IMPP is twice the value of IM because
the high speed data is assumed to have a 50% duty cycle.
The peak-to-peak modulation current is divided between
the termination resistor and the laser. The peak-to-peak
modulation amplitude
in the laser
is called IMOD. The
relationship between IMPP and IMOD depends on the
relative values of the termination resistor and the laser
dynamic resistance.
Figure 12. Simplified Laser Bias and Modulation Circuit
V
SS
I
M
I
S
= I
B
+ I
M
I
B
I
MOD
I
S
V
DD
MODA, MODB
11, 10
SRC
LTC5100
R
T
50Ω
TYP
C
T
10nF
I
MPP
= 2 • I
M
3.2Gbps
MODULATOR
14
Figure 13. Components of the LTC5100 Source and
Modulation Currents (The Laser Bias Current is Also Shown)
0
IM
ISIM
IB
IMPP
5100 F13
The relationships between the source, bias, and modula-
tion currents are as follows.
I
B
= I
S
– I
M
(1)
I
MPP
= 2 • I
M
(2)
IR
RR I
MOD T
TLD
MPP
=+
()
•
(3)
where
R
T
is the termination resistor value.
R
LD
is the dynamic resistance of the laser, defined in
Figure 15.
The expression for I
B
in Equation 1 shows that the maxi-
mum achievable laser bias current is a function of the
maximum source current, I
S
, and the average modulation
LTC5100
18
sn5100 5100fs
current, I
M
. The maximum value of I
S
is given in the
Electrical Characteristics and the value of I
M
depends on
the laser characteristics and the termination resistor value.
The logic “1” and “0” current levels in the laser are given
by:
III
BMOD
12
=+
(4)
III
BMOD
02
=–
(5)
OPERATIO
U
Figure 14. Components of the Laser Bias
and Modulation Currents
0
I
B
I
TH
I
MOD
5100 F14
The power levels corresponding to I1 and I0 are P1 and P0,
as shown in Figure 16.
P1 = η(I1 – I
TH
) (6)
P0 = η(I0 – I
TH
) (7)
where η is the slope efficiency and Ith is the laser threshold
current, defined in Figure 16.
The average optical power and extinction ratio are given
by:
PPP
AVG
=+10
2
(8)
ER P
P
=1
0
(9)
The average voltage on the laser diode relative to ground
is V
LD
(see Figure 12 and Figure 15). The voltage on the
SRC pin is:
V
S
= V
LD
+ I
S
• R
T
(10)
= V
LD
+ (I
B
+ I
M
) • R
T
The value V
S
is important because V
S
must not exceed the
limits given in the Electrical Characteristics.
V
LD
V
I0 I
B
I
LD
5100 F15
I1
R
LD
Figure 15. Approximate VI Curve for a Laser Diode
Figure 16. Approximate LI Curve for a Laser Diode
P1
P
AVG
P0
L
I0I
TH
I
B
I
LD
I
MOD
5100 F15
I1
η
The voltage across the termination resistor is:
V
TERM
= V
SRC
– V
MODA
(11)
= Is • R
T
The LTC5100 can digitize the voltage across the termina-
tion resistor using the on-chip ADC, which can give a more
accurate measurement of Is than that given by digitizing
the current internally. See the Electrical Characteristics for
details.
Temperature Compensation
The LTC5100 digitally compensates the temperature drift
of the laser bias current, laser modulation current and
LTC5100
19
sn5100 5100fs
monitor photodiode current. In each case the fundamental
calculation is the same. The LTC5100’s digital controller
multiplies the nominal value of the quantity (I
B
, I
M
or I
MD
)
by a quadratic function of temperature. Temperature mea-
surements are supplied either by an on-chip temperature
sensor or by an external microprocessor, according to the
setting of Ext_temp_en. The general temperature com-
pensation formula is:
I = I_nom • (TC2 • 2
–18
• ∆T
2
+ TC1 • 2
–13
• ∆T + 1) (12)
where I is the digital representation of the laser bias
current, modulation current or monitor diode current (I
B
,
I
M
or I
MD
).
When using the internal temperature sensor (Ext_temp_en
= 0), the temperature measurements are taken by the on-
chip ADC, and ∆T is the change in the LTC5100 die tem-
perature relative to a user defined nominal temperature:
∆T = T_int_adc – T_nom (13)
When using an external temperature source (Ext_temp_en
= 1), the temperature measurements are provided in
digital form by a microprocessor or host computer and ∆T
is the change in temperature relative to a user defined
nominal temperature:
∆T = T_ext –T_nom (14)
T_int_adc, T_ext, and T_nom are 10-bit, unsigned numbers
scaled at 0.5K/LSB. The maximum temperature that can be
represented is therefore 2
10
• 0.5°K = 512°K or 239°C.
TC1 and TC2 are the first and second order temperature
coefficients. They correspond to the registers Im_tc1 and
Im_tc2 for modulation current, Ib_tc1 and Ib_tc2 for bias
current and Imd_tc1 and Imd_tc2 for monitor diode
current. In each case TC1 and TC2 are 8-bit signed
numbers in two’s complement format. The range of the
temperature coefficients is therefore –128 to +127. When
TC1 is multiplied by its weighting coefficient of 2
–13
in
Equation 12, the effective value of the first order tempera-
ture coefficient is 122ppm/°C per LSB. The full-scale
range is approximately ±15500 ppm/°C. When TC2
is
multiplied by its weighting coefficient of 2
–18
in Equation
12, the effective value of the second order temperature
coefficient is 3.81ppm/°C
2
per LSB. The full-scale range is
approximately ±484 ppm/°C
2
.
Note that Equation 12 is applied to the digital representa-
tion of the currents, not the physical current themselves.
This is a particularly important point where monitor diode
current is concerned, because the digital representation of
the monitor diode current is the logarithm of the current.
Thus the temperature compensation is of the logarithm of
the monitor diode current and not the current itself.
Notation Used for Registers and Bit Fields
The LTC5100 has a large set of registers, many of which
are subdivided into fields of bits. Register names are given
in all capitals (SYS_CONFIG) and bit fields are given in
mixed case (Apc_en). For example, the bit that enables
Automatic Power Control mode is contained in the System
Configuration register. This bit is denoted by:
SYS_CONFIG.Apc_en
In many cases this bit field will simply be referred to as
“Apc_en.”
The functional diagrams of Figure 3 and Figure 4 show
registers and bit fields within registers between horizontal
bars. For example, the “Data” field in the ADC register is
shown as:
USER_ADC.Data
A write operation to this register is shown as:
USER_ADC.Data
A register read operation is shown as:
Peaking
Range Selection for the Source
and Modulation Currents
The source and modulation currents each have four ranges
of operation to optimize ADC and DAC resolution as well
as high frequency performance. The source current range
is controlled by two bits called Is_rng. Similarly, the modu-
lation current range is controlled by two bits called Im_rng.
The maximum current that can be delivered is proportional
to the range, so the current output is 1, 2, 3 or 4 times the
typical base value of 9mA for the source current and
4.5mA for the average modulation current or 9mA peak-
to-peak.
OPERATIO
U
LTC5100
20
sn5100 5100fs
Figure 17 depicts the current ranges for the source cur-
rent. The guaranteed full scale is 6mA per range. The
minimum operating level should be limited to 1/16 of full
scale to avoid the coarse relative quantization seen in any
ADC or DAC when operated at low levels. The source
range, Is_rng, should be selected as low as possible such
that the source current, I
S
, stays within the guaranteed
current limits over temperature, considering the laser
temperature characteristics. From Equation 1 we can see
that the source current is the sum of the laser bias and the
average modulation currents:
I
S
= I
B
+ I
M
(15)
Is_rng should be chosen to support the total current
required for laser bias and modulation, taking temperature
changes in I
B
and I
M
into account.
Figure 18 depicts the current ranges for the average
modulation current. This is the average modulation cur-
rent at the MODA and MODB pins of the chip (recall that the
MODA and MODB pins are connected on-chip). The peak-
to-peak modulation at the pins of the chip is twice the
average. Guaranteed full scale is 3mA average or 6mA pp
per range. The minimum operating level should be limited
to 1/8 of full scale to preserve the quality of the eye
diagram. Operating below 1/8 full scale causes increased
overshoot and undershoot. The modulation range, Im_rng,
should be selected as low as possible such that the
modulation current, I
M
, stays within the guaranteed cur-
rent limits over temperature. The modulation current
varies over temperature to compensate the loss in slope
efficiency typical of most VCSELs. Therefore, the choice of
Im_rng should take temperature changes into account.
High Speed Aspects of the Modulation Output
The LTC5100 modulation output presents a resistive drive
impedance with very low reflection coefficient. This output
design suppresses ringing and reflections to maintain the
quality of the eye diagrams in spite of laser impedance
variations. The reflection coefficient is sufficiently low that
the LTC5100 can drive the laser over an arbitrary length of
transmission line, as shown in Figure 19. A well designed
transmission line stretching the entire length of a typical
transceiver module goes virtually unnoticed in this sys-
tem. The only practical limitation on interconnect length to
the laser is high frequency line loss.
OPERATIO
U
Figure 17. Ranges for the Source Current
Figure 18. Ranges for the Modulation Current Figure 19. High Speed Details of the Modulation Output
0
9
Is_rng = 0
5100 F17
4.5
18
27
36
Is_rng = 1 RECOMMENDED
MINIMUM IS 1/16
OF FULL SCALE
Is_rng = 2
Is_rng = 3
I
S
(mA)
0
4.5
Im_rng = 0
5100 F18
9
13.5
18
Im_rng = 1 RECOMMENDED
MINIMUM IS 1/8
OF FULL SCALE
Im_rng = 2
Im_rng = 3
I
M
(mA)
V
SS
I
M
L
BWB
M1
I
S
= I
B
+ I
M
Z
O
= R
T
I
B
I
S
V
DD
SRC
MODA
MODB
LTC5100
R
T
50Ω
TYP
C
T
10nF
3.2Gbps
MODULATOR TRANSMISSION
LINE
14
11
10
C1 L
BWA
LTC5100
21
sn5100 5100fs
Figure 19 shows how the LTC5100 achieves a low reflec-
tion coefficient. The unavoidable capacitance of the high
speed driver transistor, bond pads and ESD protection
circuitry (C1) is compensated by the inductance of the
bond wires (L
BWA
and L
BWB
).
The high speed behavior of the circuit in Figure 19 can be
understood in greater detail by examining the simplified
circuit in Figure 20. In Figure 20 the switched current
source (M1 in Figure 19) launches a current step (1)
toward the termination resistor (2A) and toward the trans-
mission line (2B) connected to the laser. The laser is
typically mismatched to the line impedance and reflects a
portion of the incident wave (3) back toward the MODB
pin. There it encounters an L-C-L structure composed of
the bond wires and driver capacitance. This structure is
carefully designed as a lumped element approximation to
the transmission line impedance. It therefore transmits
wave (3) through the IC package without reflecting energy
back toward the laser. The traveling wave passes through
the chip largely unimpeded (4) and is absorbed by the
matched termination resistor, R
T
.
The matched termination is provided by the termination
resistor, R
T
, decoupled by capacitor C
T
. C
T
forms an AC
short across the entire frequency range contained in the
modulation data.
The termination resistor, R
T
, need not be 50Ω. 50Ω is best
for electrical testing because it matches the impedance of
most high frequency instruments. R
T
can be made smaller,
35Ω, for example, to more closely match a laser with low
dynamic impedance or to allow more voltage headroom at
the SRC pin. This may be necessary for lasers that run at
high voltages or high bias currents. R
T
can be made larger,
70Ω for example, to more closely match a laser with high
dynamic impedance or if a narrow, high impedance PC
board trace is needed to connect to the laser.
Figure 21 shows that the high speed modulation current is
confined to the ground system, laser and back termination
network. No high speed current circulates in the power
supply where it could cause radiation and interference
problems.
HIGH SPEED DATA INPUTS
The high speed data inputs, IN+ and IN–, are internally
terminated in 50Ω and internally AC coupled, eliminating
the need for external termination resistors and AC cou-
pling capacitors. Figure 10 shows the equivalent circuit
for the high speed data pins. By default, the high speed
data inputs are terminated differentially with 100Ω for
compatibility with LVDS, PECL and similar differential
signaling standards (Cml_en = 0). Alternately, the inputs
can be programmed for 50Ω single-ended termination to
the power supply for biasing a current mode logic (CML)
driver. To select CML compatibility, program Cml_en to 1.
Although internally AC coupled, the inputs are biased with
high valued resistors (50k equivalent) to VDD(HS)/2, so the
LTC5100 remains compatible with external AC coupling
capacitors. When externally AC coupled, the inputs self-
bias to approximately VDD(HS)/2.
Internal AC coupling gives the LTC5100 rail-to-rail input
common mode capability. The inputs can be driven as
much as 300mV beyond the rail during peak excursions.
The AC coupling circuit is a distributed highpass filter with
OPERATIO
U
Figure 21. High Speed Current Flow in the Modulation Output
11
MODA
10
MODB TRANSMISSION
LINE
Z
O
= R
T
5100 F20
C1
R
T
50Ω
TYP
L
BWA
L
BWB
1
2B2A
4 3
V
SS
M1
EXPOSED
PAD
V
DD
SRC
MODA
MODB
5100 F21
LTC5100
10nF
NO HIGH
SPEED
CURRENT
50Ω
3.2Gbps
MODULATOR
14
11
12
10
9
Figure 20. Wave Propagation in the Laser Interconnect
LTC5100
22
sn5100 5100fs
approximately second order characteristics. The design
maximizes the flatness of the step response over extended
periods, giving optimal performance during long strings
of ones or zeros in the data.
MODULATION CURRENT CONTROL
IN APC AND CCC MODES
The LTC5100 controls the modulation current with a
digital servo control loop using feedback from the on-chip
ADC. Figure 3 and Figure 4 are Functional Diagrams of the
LTC5100 operating in Automatic Power Control (APC)
mode and Constant Current Control (CCC) modes, respec-
tively. These diagrams show the organization and opera-
tion of the servo control loops for laser bias and laser
modulation. Either diagram can be used to understand the
modulation current control loop.
Servo Control
The average modulation current is controlled by a digital
servo loop (shown in the lower half of Figure 3). The
nominal modulation current, Im_nom, is multiplied by a
temperature compensation factor, producing a 10-bit
digital set point value, Im_set. Im_set is the target value
for average modulation current. The ADC digitizes the
average modulation current, producing a 10-bit value
Im_adc. The difference between the target value and the
actual value produces the servo loop error signal, Im_error.
Im_error is multiplied by a constant, Im_gain, to set the
loop gain. The result is integrated in a digital accumulator
and applied to a 10-bit DAC, increasing or decreasing the
modulation amplitude as required to drive the loop error to
zero. The servo loop adjusts the modulation amplitude
every four milliseconds, producing 250 servo iterations
per second.
The modulation servo loop operates on the average modu-
lation current, which is one-half of the peak-to-peak value
for a 50% duty cycle signal. The analog electronics in the
high speed modulator ensure that controlling the average
modulation current is equivalent to controlling the peak-
to-peak current.
The ADC input for average modulation current is scaled
such that code 512 is the nominal full-scale value, corre-
sponding to 4.5mA per range. Thus, if Im_rng = 0 and
Im = 4.5mA, the ADC digitizes code 512. The control
system for the modulation current effectively has 9-bit
resolution, because at most one-half of the 10-bit ADC
range is utilized. This provision maximizes the compliance
voltage range of the modulation output.
The difference equation for the modulation servo loop is:
Im_ Im_ Im_ •Im_ ( )
Im_ Im_ • Im_ – Im_
–
adc adc gain error
adc gain set adc
nn
nn
=+
=+
()
−−
1
11
816
8
Im_gain is a 3-bit digital value, so the scaling factor,
Im_gain/8, takes on the discrete values 0, 1/8, 2/8, …, 7/8.
If Im_gain = 4, then Im_gain/8 = 0.5 and the error in the
control loop is cut in half with each servo iteration. In this
case the step response of the loop is given by:
Im_ Im_ • – – Im_
adc set gain
n
n
=
11 8
(17)
The step response has the familiar exponential settling
characteristic of a first order system. The step response is
shown in Figure 22 for Im_gain = 4. The remaining error
is reduced by one-half with each servo iteration. In seven
iterations, or about 28ms, the modulation current settles
to under 1% in this example. The measured step response,
including the modulation envelope, is shown in the Typical
Performance Characteristics.
OPERATIO
U
Figure 22. Step Response of the Average Modulation Current
for Im_gain = 4
Im_adc
0 4 8 12 16 20 24 28 32
5100 F22
21 345678
TIME (ms)
SERVO
ITERATIONS
Im_set
LTC5100
23
sn5100 5100fs
Reducing Im_gain slows the settling time and increasing
Im_gain speeds the settling time. For example, with Im_gain
= 1, the residual loop error is cut by 1/8 with each servo
iteration. In this case it would take 35 servo iterations
(about 140ms) to settle to 1%. With Im_gain = 7, the
residual servo loop error is cut by 7/8 with each servo
iteration. In this case it would take only three servo
iterations (about 12ms) to settle to 1%, but the servo loop
will tend to “hunt” or oscillate at a low level with such a
high loop gain.
Temperature Compensation
The set point value for the modulation current, Im_set in
Figure 3 and Figure 4, changes with temperature to com-
pensate the temperature dependence of the laser diode’s
slope efficiency. Temperature measurements are supplied
either by an on-chip temperature sensor or by an external
microprocessor, according to the setting of Ext_temp_en.
The temperature compensated expression for Im_set is
given by:
Im_ Im_ • Im_ • •
Im_ • •
–
–
set nom tc T
tc T
=∆
+∆+
22
12 1
18 2
13
(18)
Im_tc1 and Im_tc2 are the first and second order tempera-
ture coefficients for the modulation current.
LASER BIAS CURRENT CONTROL IN APC MODE
Figure 3 is a functional diagram of the LTC5100 operating
in automatic power control (APC) mode. In APC mode, the
LTC5100 servo controls the average optical power with
feedback from a monitor photodiode. Setting Apc_en = 1
selects this mode. In APC mode the monitor diode current
can be temperature compensated with first and second
order temperature coefficients.
Figure 9 shows an equivalent circuit for the MD pin and
Figure 23 shows details of the monitor diode circuit. The
Md_polarity bit selects whether the monitor diode sources
or sinks current from the MD pin. A programmable attenu-
ator and logarithmic amplifier permit a very wide range of
monitor diode currents spanning 4.25µA to 2176µA (typi-
cal) with constant 0.2% set point resolution. The attenu-
ator divides the monitor diode current by 1, 4, 16 or 64
depending on the value of Imd_rng. Two bits called Imd_rng
control the attenuator setting, selecting a full scale current
range of 34, 136, 544 or 2176µA typical. A 5kHz lowpass
filter provides antialiasing and limits noise. The logarithmic
amplifier compresses the dynamic range of the monitor
diode current and plays a role in maintaining constant and
predictable settling times regardless of the photodiode
characteristics.
Range Selection
F
igure 24 depicts the current ranges for the monitor diode
current. The full-scale range of the monitor diode current
is 34µA • 4Imd_rng typical where Imd_rng = 0, 1, 2 or 3. The
minimum operating level should be limited to 20% of full
scale to ensure adequate settling time of the optical power
output of the laser. The range should be selected so that
the monitor diode current stays within the guaranteed
current limits over temperature.
OPERATIO
U
Figure 23. Detail of the Monitor Photodiode Circuit
13
LOG AMP 10-BIT ADC
LTC5100
MD
V
DD
5kHz
LOWPASS
FILTER
ATTENUATOR
÷1, ÷4, ÷16, ÷64 POLARITY
CONTROL
Md_polarity
Md_polarity = 1
Md_polarity = 0
5100 F32
Imd_rng
Imd_adc
LTC5100
24
sn5100 5100fs
The SRC pin current range, Is_rng, should be chosen so
that the SRC pin can supply the required bias current over
temperature. See the section titled Range Selection for the
Source and Modulation Currents.
Servo Control
The average optical power is controlled by a digital servo
loop shown in the upper half of Figure 3. The loop sets and
controls the
logarithm
of the monitor diode current. The
logarithm of the nominal monitor diode current, Imd_nom,
is multiplied by a temperature compensation factor, pro-
ducing a 10-bit digital set point value, Imd_set. Imd_set is
therefore the temperature compensated
logarithm
of the
target value for monitor diode current. The ADC digitizes
the logarithm of the monitor diode current, producing a
10-bit value called Imd_adc. The difference between the
target value and the actual value produces the servo loop
error signal, Imd_error. Imd_error is multiplied by a
constant, Apc_gain, to set the loop gain. Imd_error is also
multiplied by the set point value of the modulation current
to further stabilize the servo dynamics, as explained
below. The result is integrated in a digital accumulator and
applied to a 10-bit DAC, increasing or decreasing the SRC
pin current (and consequently the laser bias current) as
required to drive the loop error to zero. The servo loop
adjusts the laser bias current every four milliseconds,
producing 250 servo iterations per second.
The open-loop gain of the APC loop is proportional to the
laser slope efficiency, η (Watts/Amp), and monitor diode
response, γ (Amps/Watt). These parameters vary widely
from laser to laser. If nothing is done to compensate the
variations in η and γ, the settling time of the optical power
output will vary over an unacceptably wide range. For
example, a 4:1 variation in slope efficiency and a 5:1
variation in monitor diode response could create a 20:1
variation in settling time.
The LTC5100 uses two techniques to fully compensate for
variations in the laser and monitor diode characteristics,
achieving constant settling times under all conditions.
First, taking the logarithm of the monitor diode current
precisely compensates variations in the monitor diode
response. Second, multiplying the error signal by the
modulation current precisely compensates for variations
in laser slope efficiency.
The difference equation for the APC loop is:
Im_ Im _ •Im _ ( )
Im _ • Im _ –Im _
adc d adc A d error
d adc A d set d adc
nn
nn
=+
=+
()
−
−−
1
11
19
where A is the small-signal loop gain, given by:
AApc_gain
32
=+
+
++
•_
Im_ •ln( ) ()
•–•
1
1
1
820
1
1
Is rng
rng
ER
ER
RR
R
TLD
T
where:
ln(8) = 2.079 is the natural logarithm of 8
ER is the extinction ratio
R
T
is the termination resistance
R
LD
is the dynamic resistance of the laser diode
Apc_gain is a 5-bit digital value, so the scaling factor,
Apc_gain/32, takes on the discrete values 0, 1/32, 2/32,
…, 31/32.
In practice, the extinction ratio is usually high (ER >> 1),
and R
T
~ R
LD
, so Equation 20 simplifies to:
AApc gain Is rng
rng
≈+
+
_•_
Im_32
1
1
(21)
OPERATIO
U
Figure 24. Operating Ranges for the Monitor Diode Current
0
34
Imd_rng = 0
5100 F24
7
136
544
2176
Imd_rng = 1
RECOMMENDED
MINIMUM IS 20%
OF FULL SCALE
I
MD
(µA) (LOG SCALE)
Imd_rng = 2
Imd_rng = 3
LTC5100
25
sn5100 5100fs
Equation 20 shows that the loop gain is completely inde-
pendent of the slope efficiency and monitor diode re-
sponse. Consequently the servo dynamics and settling
time are independent of these highly varying quantities.
The Apc_gain quantity can be set to compensate for the
selected values of Is_rng and Im_rng as well as the
extinction ratio, termination resistance and laser dynamic
resistance.
The step response of the APC loop is:
Imd_adc
n
= Imd_set • [1 – (1 – A)
n
] (22)
The step response given in Equation 22 has the familiar
exponential settling characteristic of a first order system.
The step response is shown in Figure 25 for A = 0.5. The
remaining error is reduced by one-half with each servo
iteration. In seven iterations, or about 28ms, the modula-
tion current settles to under 1% in this example. The
measured step response, including the modulation enve-
lope, is shown in the Typical Performance Characteristics.
Choosing A = 0.5 is nearly optimal because it results in
smooth, exponential settling. A = 1 will settle in about two
servo iterations or 8ms, but “hunting” or low level oscil-
lation will be seen in the laser bias current. A > 1 results
in overshoot and A > 2 results in sustained high level
oscillation.
microprocessor, according to the setting of Ext_temp_en.
The temperature compensated expression for Imd_set is
given by:
Im _
Im _ • Im _ • •
Im _ • •
–
–
d set
d nom dtc T
dtc T
=
∆
+∆+
22
12 1
18 2
13
(23)
Imd_tc1 and Imd_tc2 are the first and second order
temperature coefficients for the monitor diode current.
Equation 23 applies to the digital representation of the
monitor diode current. Recall that Imd_set is the digital set
point for the
logarithm
of the monitor diode current. This
fact has two important implications. First, the first order
temperature coefficient in Equation 23 (Imd_tc1) results
in an exponential change in the physical monitor diode
current with temperature. However, the monitor diode
temperature drift is usually very small, and the exponential
is well approximated as linear. Second, if Imd_tc2 = 0, the
relative temperature sensitivity of the physical current is
given by:
dI
MD
dT I dtc d nom
MD
• ln( )• •Im _ •Im _
–
182 1 1024
13
=
(24)
where I
MD
is the physical monitor diode current in Amps.
Equation 24 shows that the temperature coefficient of the
physical current depends on the nominal monitor diode
current. For example, if Imd_nom = 512 and Imd_tc1 = 4,
the physical temperature compensation would be:
dI
MD
dT I ppm C
MD
• ln( )• • • /
–
182 4512
1024 508
13
==°
(25)
The effect of Imd_tc2 on the physical monitor diode
current has no simple physical interpretation. In most
cases it will be sufficient to set Imd_tc2 to zero and use the
first order temperature coefficient, Imd_tc1 to correct
monitor diode drift.
LASER BIAS CURRENT CONTROL IN CCC MODE
Figure 4 is a functional diagram of the LTC5100 operating
in constant current control (CCC) mode. In CCC mode, the
LTC5100 sets the laser bias current directly. Setting
Apc_en = 0 selects this mode. In CCC mode the laser bias
OPERATIO
U
Figure 25. Step Response of the Monitor
Diode Current for a Total Loop Gain of 0.5
Im_adc
0 4 8 12 16 20 24 28 32
5100 F25
21 345678
TIME (ms)
SERVO
ITERATIONS
Imd_set
Temperature Compensation
The set point value for the monitor diode current, Imd_set
in Figure 3, can be changed with temperature to compen-
sate the temperature dependence of the monitor diode
response. Temperature measurements are supplied either
by an on-chip temperature sensor or by an external
LTC5100
26
sn5100 5100fs
current can be temperature compensated with first and
second order temperature coefficients.
Servo Control
The laser bias current is controlled by a digital servo loop
(shown in the upper half of Figure 4) and can be under-
stood as follows. The nominal bias current, Ib_nom, is
multiplied by a temperature compensation factor, produc-
ing a 10-bit digital set point value, Ib_set. Ib_set is the
target value for the laser bias current. The ADC digitizes the
SRC pin current and the average modulation current,
producing 10-bit values Is_adc and Im_adc. The laser bias
current is the difference between the SRC pin current and
the average modulation current (Equation 1). The system
generates a digital representation of the laser bias current
by calculating:
Ib_adc = Is_rng • Is_adc – Im_rng • Im_adc (26)
where Ib_adc is the result of a calculation. (The ADC never
digitizes the laser bias current directly.)
The difference between the target value and the actual
value is the servo loop error signal, Ib_error. Ib_error is
multiplied by a constant, Ib_gain, to set the loop gain. The
result is integrated in a digital accumulator and applied to
a 10-bit DAC, increasing or decreasing the SRC pin current
as required to drive the loop error to zero. The servo loop
adjusts the SRC pin current every four milliseconds,
producing 250 servo iterations per second.
The simplified difference equation for the bias current
servo loop is, assuming Im_nom = 0:
Ib adc
Ib adc Ib gain Is rng Ib error
Ib adc Ib gain Is rng
Ib set Ib adc
n
n
n
n
_()
__•( _ )• _
__•( _ )
•_–_
=
++
=+ +
()
−
−
−
27
32 1
32 1
1
1
1
Ib_gain is a 5-bit digital value, so the scaling factor,
Ib_gain/32, takes on the discrete values 0, 1/32, 2/32, …,
31/32. If Ib_gain • (Is_rng + 1) = 16, then Ib_gain • (Is_rng
+ 1)/32 = 0.5 and the error in the control loop is cut in half
with each servo iteration. In this case the step response of
the loop is given by, assuming Im_nom = 0 :
Ib adc
Ib set Ib gain Is rng
n
n
_
_•––
_•(_ )
=
+
11 1
32
(28)
The step response has the familiar exponential settling
characteristic of a first order system. The step response
is shown in Figure 26 for Ib_gain
• (Is_rng + 1)
= 16. The
remaining error is reduced by one-half with each servo
iteration. In seven iterations, or about 28ms, the laser bias
current settles to under 1% in this example. The mea-
sured step response is shown in the Typical Performance
Characteristics.
OPERATIO
U
Ib_adc
0 4 8 12 16 20 24 28 32
5100 F26
21 345678
TIME (ms)
SERVO
ITERATIONS
Ib_set
Figure 26. Step Response of the Laser Bias
Current for (Ib_gain) • (Is_rngtl ) = 16
Reducing Ib_gain slows the settling time and increasing
Ib_gain speeds the settling time. For example, with Ib_gain
• (Is_rng + 1) = 1, the residual loop error is cut by 1/32 with
each servo iteration. In this case it would take 145 servo
iterations (about 580ms) to settle to 1%. With Ib_gain •
(Is_rng + 1) = 31, the residual servo loop error is cut by 31/
32 with each servo iteration. In this case it would take only
two servo iterations (about 8ms) to settle to 1%.
Setting Im_nom ≠ 0 slows the settling time of the laser
bias current somewhat. This effect can easily be compen-
sated by increasing Ib_gain.
Temperature Compensation
The set point value for the laser bias current, Ib_set in
Figure 4, can change with temperature to compensate the
temperature dependence of the laser diode’s threshold
current. Temperature measurements are supplied either
LTC5100
27
sn5100 5100fs
by an on-chip temperature sensor or by an external
microprocessor, according to the setting of Ext_temp_en.
The temperature compensated expression for Ib_set is
given by:
Ib set Ib nom Ib tc T
Ib tc T
__•
_• •
_• •
–
–
=∆
+∆+
22
12 1
18 2
13
(29)
Ib_tc1 and Ib_tc2 are the first and second order tempera-
ture coefficients for the laser bias current.
TRANSMIT ENABLE, FAULT DETECTION
AND EYE SAFETY
The LTC5100 is compatible with the Gigabit Interface
Converter (GBIC) specification, but includes additional
features and safety interlocks. Figure 27 shows the state
diagram for enabling the transmitter and detecting faults.
The EN pin and Soft_en control bit enable and disable the
transmitter. The EN pin may be programmed for active
high or active low operation with the En_polarity bit.
OPERATIO
U
READY
(DISABLED)
2
SETTLING
(ENABLED)
4
READ
EEPROM
1
WAIT
64ms
3
SETTLED
(ENABLED)
5
READY
(SETTLED
AND
DISABLED)
6
FAULTED
(DISABLED)
8
POWER ON RESET OR
Operating_mode = 0
SUCCEEDED OR
Operating_mode = 1
100ms
TIMEOUT
ENABLE AND
Rapid_restart_en = 0
ENABLEENABLE
Transmitter_enabled = 0
FAULT DETECTION DISABLED
Transmitter_enabled = 1
FAULT PIN ASSERTED
Transmitter_enabled = 0
Transmit_ready = 0
Faulted = 1
SETTLING
(ENABLED)
10 FAULT DETECTION DISABLED
Transmitter_enabled = 1
SETTLING
(ENABLED)
11
FAULTED
TWICE
(DISABLED)
12
FAULT DETECTION ENABLED
Transmit_ready = 1
POWER ON RESET
5100 F27
Faulted = 1
Faulted_once = 1
Faulted_twice = 1
Faulted = 0
Faulted_once = 1
25ms
TIMEOUT
100ms
TIMEOUT
Transmitter_enabled = 0
Transmit_ready = 0
FAULT
FAULT DETECTION ENABLED
Transmit_ready = 1
FAULT DETECTION DISABLED
Transmit_ready = 1
ENABLE = EN AND Soft_en
FAULT = Over_current OR
Over_power OR
Under_power
FAILED
TIMEOUT
Mem_load_error = 1
Operating_mode = 1
READY
(DISABLED)
9
ENABLEENABLE
FAULT
SETTLING
(ENABLED)
7
ENABLE
Faulted_once = 0
ENABLE
40ms
TIMEOUT
ENABLE AND
Rapid_restart_en = 1
ENABLE AND
Rep_flt_inhibit = 0
ENABLE AND
Rep_flt_inhibit = 1
Figure 27. State Diagram for Transmitter Enable and Fault Detection
LTC5100
28
sn5100 5100fs
The EN pin and the Soft_en bit must both be active to
enable the transmitter, providing an extra degree of safety
and allowing full software control of the transmitter enable
function. As shown in Figure 6, the EN pin has a weak 10µA
current source that pulls it to the inactive state in case of
an accidental open on the pin. The EN and Soft_en bits are
inhibited until the LTC5100 has successfully loaded its
registers from an EEPROM or the Operating_mode bit has
been set, signaling that a microprocessor has assumed
control of the chip.
The first time the transmitter is enabled after initial power
up, the servo loops find the correct DAC settings for bias
and modulation current through a feedback process.
Initial settling is typically within 300ms. If the transmitter
is disabled and subsequently re-enabled, the previously
determined DAC settlings are restored. In this case set-
tling occurs typically within 1ms. This feature is called
“Rapid Restart” and can be overridden by setting the
Rapid
–
restart_en bit to zero.
The LTC5100 has sophisticated eye safety and fault han-
dling features. Five types of faults are detected: low supply
voltage, excessive laser bias current, overpower,
underpower and EEPROM memory load failure. Table 1
summarizes these five faults and how they are handled in
the LTC5100.
Faults are latched in compliance with GBIC requirements.
Faults can be independently enabled (except for low sup-
ply voltage and memory load failure) and are recorded in
an internal register for readout over the serial bus. If two
faults occur simultaneously, the fault with the highest
priority (see Table 1) is recorded in the FLT_STATUS
register. This register indicates the cause of the fault and
is cleared only when read (not when the fault itself is
cleared.) Low supply voltage and memory load failure are
considered hard faults and cannot be masked or overrid-
den. They prevent the transmitter from begin enabled until
they are cleared.
Normally, a fault automatically disables the transmitter
and shuts down the laser. In some systems it may be
desirable to allow data transmission to continue after a
OPERATIO
U
Table 1. Fault Detection and Handling
FAULT TYPE
LASER LASER LASER EEPROM MEMORY POWER SUPPLY SOFTWARE
OVERCURRENT OVERPOWER UNDERPOWER LOAD FAULT UNDERVOLTAGE FORCED FAULT
Fault Occurs When Laser Bias Current Monitor Diode Monitor Diode EEPROM Load V
DD
Drops The Flt_pin_override
Exceeds the Value Current is 50% Current is 50% Starts But Fails Below 2.8V and Force_flt Bits
in the IB_LIMIT Greater Than the Less than the to Complete are Set
Register Set Point Set Point
Priority 5 4 3 2 1 NA
Cleared by Yes Yes Yes Yes No Yes
Power-On Reset
Latched in the Yes Yes Yes Yes Yes No (Not Part of the
FLT_STATUS FLT_STATUS Register)
Register
Cleared from the Yes Yes Yes Yes Yes No (Not Part of the
FLT_STATUS FLT_STATUS Register)
Register on Read
Latched at the Yes Yes Yes Yes No (The FAULT Pin Yes (Actually Latched
FAULT Pin Signals a Fault as in the FLT_CONFIG
Long as the Supply Register)
Voltage Remains
Too Low)
Enabled by Over_current_en Over_pwr_en Under_pwr_en Always Enabled Always Enabled Flt_pin_override
and Apc_en and Apc_en
Glitch Rejection 4µs4µs4µs NA 200mV Typical NA
Hysteresis
LTC5100
29
sn5100 5100fs
fault has occurred. For example, the software in the host
system may need to evaluate the cause of the fault before
shutting down the laser. If Auto_shutdn_en = 1, the
LTC5100 automatically disables the transmitter after a
fault. If Auto_shutdn_en = 0, data transmission continues
after a fault. The transmitter is not disabled until the host
system drives the EN pin inactive or clears the Soft_en bit.
Low power supply voltage and memory load errors are
considered hard faults and always disable the transmitter,
regardless of the setting of Auto_shutdn_en.
The LTC5100 implements the GBIC protocol for prevent-
ing software from repeatedly re-enabling a faulted trans-
mitter. When a first fault is detected, it can be cleared by
disabling the transmitter. If the transmitter is re-enabled
and a second fault occurs within 25ms after fault detection
is enabled, the transmitter is permanently disabled. Only
cycling power to the LTC5100 can clear this condition.
This feature is called “Repeated Fault Inhibit” and can be
overridden by setting the Repeated_flt_inhibit bit to zero.
The FAULT pin can be configured active high or active low
with the Flt_pin_polarity bit. The FAULT pin can be pro-
grammed for open drain, 330µA internal pull-up, 500µA
internal pull-up or complementary (push-pull) drive with
the two Flt_drv_mode bits. Refer to Figure 8 for an
equivalent circuit of the FAULT pin.
The FAULT pin can be overridden in software for testing
purposes or to allow a microprocessor in the transceiver
module to fully control the module’s fault output. If the
Flt_pin_override bit is set, then the Force_flt bit fully
controls the state of the FAULT pin.
The state of the LTC5100 can be monitored by reading the
FLT_STATUS register. See Table 21 for a description of the
status bits.
EYE SAFETY INFORMATION
Communications lasers can emit levels of optical power
that pose an eye safety risk. While the LTC5100 provides
certain fault detection features, these features alone do
not ensure that a laser transmitter using the LTC5100 is
compliant with IEC 825 or the regulations of any particu-
lar agency. The user must analyze the safety require-
ments of their transceiver module or system, activate the
appropriate laser safety features of the LTC5100, and take
any
additional
precautions needed to ensure compliance
of the end product with the requirements of the relevant
regulatory agencies. In particular, the LTC5100 produces
laser currents in response to digitally programmed com-
mands. The user must ensure software written to control
the LTC5100 does not cause excessive levels of radiation
to be emitted by the laser.
POWER CONSUMPTION AND POWER MANAGEMENT
The power consumption of the LTC5100 is dependent on
several variables, including the modulation current range
(set by Im_rng), the laser bias and modulation levels, and
the state of the transmitter (whether enabled or disabled.)
If Power_down_en = 1, the LTC5100 turns off its high
speed amplifiers when the transmitter is disabled, reduc-
ing supply current to less than 5mA (typical). See the
Typical Performance Chacteristics for further information.
HIGH SPEED PEAKING CONTROL
The LTC5100 has the ability to selectively peak the falling
edge of the modulation waveform to accelerate the turn-off
of the laser diode. The 5-bit PEAKING register controls this
function. See the Typical Performance Chacteristics for
further information. Lower values in the PEAKING register
increase the falling edge peaking.
ANALOG-TO-DIGITAL CONVERSION
Overview
The ADC in the LTC5100 is a 10-bit, dual slope integrating
converter with excellent linearity and noise rejection. A
multiplexer allows digitizing six quantities:
• SRC pin current, I
S
• Average modulation current, I
M
• Laser diode voltage, V
LD
• Monitor diode current, I
MD
• Termination resistor voltage, V
TERM
• Die temperature, T
All of these measurements are available to the user via the
I
2
C serial bus.
OPERATIO
U
LTC5100
30
sn5100 5100fs
Conversion Sequence
The ADC has a 1ms conversion time and operates in a four-
cycle sequence. Three of these cycles are dedicated to the
needs of the servo controllers for laser bias and modula-
tion current. One cycle is available to the user to convert
any desired quantity. Table 2 shows how the four conver-
sion time slots are allocated. The temperature compensa-
tion and servo loop calculations are done during the User
cycle. The source and modulation DACs are also updated
during this cycle.
Table 2. ADC Conversion Sequence
RESULT STORED IN
CYCLE APC MODE CCC MODE REGISTER
1 T T T_INT_ADC
2I
M
I
M
IM_ ADC
3I
MD
I
S
IMD_ADC/IS_ADC
4 User User USER_ADC
User Access to the ADC
The results of each conversion cycle in Table 2 are stored
in user accessible registers. The last die temperature
measurement can be read over the I
2
C bus at any time by
reading the T_INT_ADC register. Note that the quantity
converted during the third cycle depends on whether the
chip is in APC or CCC mode. The result of the third
conversion cycle is stored in a register that is called
IMD_ADC in APC mode and IS_ADC in CCC mode. There
is only one register, but it is given two names to indicate
the quantity it actually holds.
The fourth cycle, called the user cycle, is available to
digitize any of the six multiplexed signals. The result can
be read out over the I2C serial bus. The signal to be
digitized during the user cycle is selected by setting the
three-bit field USER_ADC.Adc_src_sel (see Table 23).
For example, setting Adc_ src_sel = 2 programs the
multiplexer to select the laser diode voltage, VLD. During
the next user conversion cycle, VLD is converted and
stored to the USER_ADC. Data field. When the conversion
is complete, USER_ADC.Valid is set and
USER_ADC.Adc_src indicates the signal source whose
converted value is stored in USER_ADC.Data. Reading or
OPERATIO
U
writing the USER_ADC register clears the Valid bit. The
Valid bit remains cleared until the next user conversion is
complete. USER_ADC.Adc_src always corresponds to
the signal source whose data is stored in USER_ADC.Data,
not the source that was most recently selected by writing
USER_ADC.Adc_src_sel. The Valid bit and ADC_src field
are useful for monitoring when the ADC has updated the
USER_ADC.Data field. Table 3 gives an extended example
of accessing the USER_ADC register.
Note that the content of the USER_ADC register is different
for writing and for reading, even though the I
2
C command
used to access this register is the same in both cases. See
Table 23 and Table 24 for a detailed definition of the bit
fields in the USER_ADC register. Table 23 also shows how
to convert ADC digital codes to real-world quantities.
DIRECT MICROPROCESSOR CONTROL OF THE LASER
BIAS AND MODULATION CURRENT
Setting Lpc_en to zero turns off the LTC5100’s digital
Laser Power Controller (see Figure 2). The source and
modulation DACs (Is_dac and Im_dac) can then be written
from the I
2
C serial bus, allowing an external microproces-
sor or test computer to directly control the source and
modulation currents.
DIGITAL CONTROL AND THE I
2
C SERIAL INTERFACE
The LTC5100 has extensive digital control and monitoring
features. These features can be used during final assembly
of a transceiver module to set up the laser and verify
performance. In normal operation, the LTC5100 can oper-
ate standalone or under microprocessor supervision. Op-
erating standalone, the LTC5100 automatically loads its
configuration and laser operating parameters (bias cur-
rent, modulation current, monitor diode current) from a
small external EEPROM at power up. Operating under
microprocessor supervision, the microprocessor is in
total control of setting up the LTC5100.
I
2
C Serial Interface Protocol
The digital interface for the LTC5100 is I
2
C, a 2-wire serial
bus standard that is fully documented in “I
2
C-Bus and How
LTC5100
31
sn5100 5100fs
OPERATIO
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Table 3. Example of User ADC Cycle Access
READ FROM
ADC WRITE TO ADC_USER
CYCLE SIGNAL SOURCE Adc_src_sel REGISTER Adc_src Valid Data COMMENT
1T V
TERM
0V
TERM
(1) Selected Signal Source is V
TERM
2I
M
V
TERM
0V
TERM
(1)
3I
S
V
TERM
0V
TERM
(1)
4 User (V
TERM
)V
TERM
0V
TERM
(1)
1T V
TERM
1V
TERM
(2) ADC Updates Data with New Data,
Setting Valid
2I
M
V
LD
V
TERM
0V
TERM
(2) User Selects New Signal Source,
V
LD
, Clearing Valid
3I
S
V
TERM
0V
TERM
(2)
4 User (V
LD
)V
TERM
0V
TERM
(2)
1T V
LD
1V
LD
(1) ADC Updates Data with New Data,
Setting Vaild and Changing Adc_src
to Reflect the Source of the New Data
2I
M
V
LD
1V
LD
(1)
3I
S
V
LD
V
LD
0V
LD
(1) User Reads the ADC_USER Register,
Clearing Valid
4 User (V
LD
)V
LD
0V
LD
(1)
1T V
LD
1V
LD
(2) ADC Updates Data with New Data,
Setting Valid
2I
M
V
LD
1V
LD
(2)
3I
S
V
LD
1V
LD
(2)
4 User (V
LD
)V
LD
1V
LD
(2)
to Use It, V1.0” by Philips Semiconductor. The 7-bit I
2
C
bus address for the LTC5100 is 0x0A (hex). When the
Read/Write bit that follows is a “1”, the resulting 8-bit word
becomes 0x15. When the Read/Write bit is a “0”, the 8-bit
word becomes 0x14. To communicate with the LTC5100,
the bus master transmits the LTC5100 address followed
by a command byte and data as defined by the I
2
C bus
specification and shown in Figure 28 and Table 4. Note that
16 bits of data are always transmitted, low byte first, high
byte last. Within each transmitted byte, the bit order is
Figure 28. I2C Serial Read/Write Sequences (LTC5100 Responses are Shown in Bold Italics)
Table 4. Legend for the I2C Protocol
SYMBOL MEANING
S Start
W Write
R Read
A Acknowledge
NA No Acknowledge
P Stop
SWRITE
READ
W
A
LTC5100
ADDRESS
(7 BITS) 0x0A
COMMAND
BYTE LOW BYTE
A A
HIGH BYTE
A
P
LOW BYTE
A
HIGH BYTE
N
AP
5100 F28
SW
A
LTC5100
ADDRESS
(7 BITS) 0x0A
LTC5100
ADDRESS
0x0A
COMMAND
BYTE S
A A
R
MSB .. LSB. The register set and I
2
C command set for the
LTC5100 are documented in Table 7 through Table 30.
LTC5100
32
sn5100 5100fs
Standalone Operation
On power-up the LTC5100 becomes an I
2
C bus master
and attempts to load its configuration data from an exter-
nal EEPROM. If an EEPROM responds, the LTC5100 reads
16-bytes of data and transfers this data to the internal
register set. When a 16-byte transfer is completed without
error, the LTC5100 becomes ready to enable the transmit-
ter and begin driving the laser. If a bus error occurs during
this transfer, the load sequence is aborted and a
Mem_load_error is generated, preventing the transmitter
from being enabled until a successful memory load at-
tempt is completed or until an external agent sets the
Operating_mode bit. Every 64ms another attempt is made
to load the EEPROM until the memory is read or until
Operating_mode = 1. Table 5 shows the memory map for
the EEPROM.
The LTC5100 generates I
2
C address 0xAE (1010_1110
binary) when accessing the EEPROM, making it compat-
ible with a wide range of EEPROM sizes. Table 6 details
how the LTC5100 interacts with EEPROMs from 128 bits
to 16k bits and from where it gets its data.
The LTC5100 supports hot plugging in standalone mode.
If the Soft_en bit is set in the EEPROM and the EN pin is
active, the LTC5100 loads its configuration data from the
EEPROM and immediately enables the transmitter. The
transmitter is typically enabled and settled within the
300ms t_init period required by the GBIC specification.
OPERATIO
U
Table 5. EEPROM Memory Map
BIT
BYTE 7 6 5 4 3 2 1 0
15 Reserved Peaking (4:0)
14 Ib_gain(4:0)/Apc_gain(4:0) Im_gain(2:0)
13 Reserved Imd_rng(1:0) T_nom(9:8)
12 T_nom(7:0)
11 Im_tc2(7:0)
10 Im_tc1(7:0)
9 Reserved Im_rng(1:0) Im_nom(9:8)
8 Im_nom(7:0)
7 Ib_tc2(7:0)/Imd_tc2(7:0)
6 Ib_tc1(7:0)/Imd_tc2(7:0)
5 Reserved Is_rng(1:0) Ib_nom(9:8)/Imd_nom (9:8)
4 Ib_nom(7:0)/Imd_nom(7:0)
3 Reserved Rep_flt_inhibit Rapid_restart_en Flt_drv_mode
2 Lpc_en Auto_shutdn_en Flt_pin_polarity Flt_pin_override Force_flt Over_pwr_en Under_pwr_en Over_current_en
1 Reserved Ib_limit
0 Cml_en Md_polarity Ext_temp_en Power_down_en Apc_en En_polarity Soft_en Operating_mode
LTC5100
33
sn5100 5100fs
OPERATIO
U
Table 6. Effective Base Addresses for Various Sized EEPROMs
GENERIC PART NUMBER 24LC00 24LC01B 24LC02B 24LC04B 24LC16B
Bits 128 1k 2k 4k 16k
Bytes 16 128 256 512 2048
Device Address (Binary) 1010xxx. 1010xxx. 1010xxx. 1010.xxa 1010cba.
Word Address Space (Binary) xxxx_nnnn xnnn_nnnn nnnn_nnnn nnnn_nnnn nnnn_nnnn
LTC5100 Generates 1010_111. 1010_111. 1010_111. 1010_111. 1010_111.
Device Address = 0xAE = 0xAE = 0xAE = 0xAE = 0xAE
LTC5100 Generates 0110_0000 0110_0000 0110_0000 0110_0000 0110_0000
Word Address = 0x60 = 0x60 = 0x60 = 0x60 = 0x60
Effective Base Address 0000_0000 0110_0000 0110_0000 0001_0110_0000 0111_0110_0000
= 0x00 = 0x60 = 0x60 = 0x160 = 0x760
Comments Minimum Size EEPROM Not Big Standard GBIC LTC5100 Loads LTC5100 Loads
EEPROM. Enough for GBIC EEPROM. from an Area from an Area
Loads Every ID. LTC5100 Smallest EEPROM Outside the GBIC Outside the GBIC
Byte in the Loads from 0x60 That is Big Enough ID Data Area ID Data Area
EEPROM. to 0x6F to Hold the LTC5100
Data and the GBIC ID.
LTC5100 Loads from
0x60 to 0x6F, the First
16 Bytes of the
Vendor Area
Microprocessor Controlled Operation
An external microprocessor or a test computer can take
full control of the LTC5100 by setting the Operating_mode
bit. When this bit is set, the LTC5100 stops searching for
an external EEPROM and takes commands from the mi-
croprocessor. It is even possible to combine standalone
and microprocessor controlled modes. If an EEPROM is
present, the LTC5100 will load its configuration registers
from the EEPROM at power-up. A microprocessor or test
computer can then read and write the LTC5100 registers
at will.
The primary purpose of the Operating_mode bit is to stop
the LTC5100’s EEPROM load attempts. Once the LTC5100
has loaded itself from an EEPROM (if present), it is not
technically necessary to set the Operating_mode bit to
communicate with the LTC5100.
The LTC5100 attempts to read the EEPROM every 64ms
until it successfully loads its registers or until the
Operating_mode bit is set. There is a finite chance that the
microprocessor and the LTC5100 will generate an I
2
C bus
collision if an EEPROM load attempt coincides with the
microprocessor’s attempt to access the LTC5100. In this
case, the microprocessor will receive a NACK (not-ac-
knowledged) response to its transmissions. The micro-
processor needs only to cease transmission in accordance
with the I
2
C protocol and try again. If the microprocessor
makes this second attempt within 64ms (typical), it is
guaranteed not to collide with the LTC5100.
LTC5100
34
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 7. Register Set Overview
REGISTER NAME
CONSTANT CURRENT AUTOMATIC POWER I
2
C COMMAND READ/WRITE REFERENCE
REGISTER GROUP CONTROL MODE CONTROL MODE CODE (HEX) ACCESS INFORMATION
System Operating SYS_CONFIG “ 0x10 R/W Table 8
Configuration LOOP_GAIN “ 0x1E R/W Table 9
PEAKING “ 0x1F R/W Table 10
Reserved “ 0x08 R/W Table 11
Laser Setup Coefficients IB IMD 0x15 R/W Table 12
IB_TC1 IMD_TC1 0x16 R/W Table 13
IB_TC2 IMD_TC2 0x17 R/W Table 14
IM “ 0x19 R/W Table 15
IM_TC1 “ 0x1A R/W Table 16
IM_TC2 “ 0x1B R/W Table 17
Temperature T_EXT “ 0x0D R/W Table 18
T_NOM “ 0x1D R/W Table 19
Fault Monitoring FLT_CONFIG “ 0x13 R/W Table 20
and Eye Safety FLT_STATUS “ 0x12 R Table 21
IB_LIMIT “ 0x11 R/W Table 22
ADC USER_ADC “ 0x18 R/W Tables 23, 24
T_INT_ADC “ 0x05 R/W Table 25
IM_ADC “ 0x06 R/W Table 26
IS_ADC IMD_ADC 0x07 R/W Table 27
DAC IS_DAC “ 0x01 R/W Table 28
IM_DAC “ 0x02 R/W Table 29
PWR_ LIMIT_DAC “ 0x03 R Table 30
LTC5100
35
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 8. Register: SYS_CONFIG—System Configuration (I2C Command Code 0x10)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Cml_en 7 0 Current Mode Logic Enable
0: Floating Differential Input Termination: 100Ω Across IN
+
and IN
–
1: CML Compatible Input Termination: 50Ω from IN
+
to V
DD(HS)
and from IN
–
to V
DD(HS)
.Md_polarity 6 0 Monitor Diode Polarity
0: Cathode Connected to the MD Pin, Sinking Current from the Pin
1: Anode Connected to the MD Pin, Sourcing Current Into the Pin
.Ext_temp_en 5 0 External Temperature Enable
Selects the Source of Temperature Measurements for Temperature Compensation.
0: Internal Temperature Sensor
1: Externally Supplied Through the Serial Interface
.Power_down_en 4 1 Power Down Enable
Allow Power Reduction When the Transmitter is Disabled.
0: No Power Reduction When the Transmitter is Disabled.
1: Reduce Power Consumption When Transmitter is Disabled by Turning Off the High Speed Amplifiers.
.Apc_en 3 0 Automatic Power Control Enable
Select the Means of Controlling the Laser Bias Current.
0: Constant Current Control
1: Automatic Power Control Using Feedback from the Monitor Diode
.En_polarity 2 0 EN Pin Polarity
Set the Input Polarity of the EN Pin.
0: Active Low: A Logic Low Input Level Enables the Transmitter.
1: Active High: A Logic High Input Level Enables the Transmitter.
Note: In order to Enable the Transmitter, Both the EN Pin and Soft_en Bit Must be Asserted.
.Soft_en 1 0 Soft Transmitter Enable
Enables Transmitter Through the Serial Interface.
0: Disable the Transmitter
1: Enable the Transmitter (if the EN Pin is Active)
Note: In order to Enable the Transmitter, Both the EN Pin and Soft_en Bit Must be Asserted.
.Operating_mode 0 0 Digital Operating Control Mode
Select Whether the LTC5100 Operates Autonomously or Under External Control.
0: Standalone Operation: Configuration Parameters are Loaded from an External EEPROM at Power Up.
1: Externally Controlled Operation: Configuration Parameters are Set by an External Microprocessor or
Test Computer.
LTC5100
36
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 9. Register: LOOP_GAIN—Control Loop Gain (I2C Command Code 0x1E)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Ib_gain 7 0 Bias Current or APC Loop Gain
(.Apc_gain in APC 6 0 This Bit Field Modifies the Open-Loop Gain of the Bias Current Servo Control Loop. The Effect of This
Mode) 51
Bit Field Differs in Constant Current Control (CCC) Mode and in Automatic Power Control (APC) Mode.
40
In CCC Mode, This Bit Field is Called lb_gain. In APC Mode, This Bit Field is Called Apc_gain.
30
Constant Current Control (CCC) Mode (Apc_en = 0): The Loop Gain and Settling Time are Independent
of Is_rng. The Default Value of Ib_gain Yields Stable but Slow Settling of the Laser Bias Current for
Any Value of Is_rng.
Automatic Power Control (APC) Mode (Apc_en = 1): The Open-Loop Gain of the Bias Current Servo
Loop Depends on the Value of Is_rng. The Default Value of Apc_gain Yields Stable but Potentially Slow
Settling of the Laser Bias Current for any Value of Is_rng.
Im_gain 2 0 Modulation Current Loop Gain
1 0 This Bit Field Modifies the Open-Loop Gain of the Modulation Current Servo Loop. The Open-Loop.
01
Gain is Approximately Im_gain/32. The Loop Gain and Settling Time are Independent of Im_rng. The
Default Value of Im_gain Yields Stable but Slow Settling of the Laser Modulation Current.
Table 10. Register: PEAKING—High Speed Modulation Peaking (I2C Command Code 0x1F)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:5
.Peaking 4 1 Peaking Control for the Modulation Output
3 0 This Bit Field Controls the High Speed Peaking of the Modulation Output. Decreasing the Value of
20
Peaking Increases the Undershoot on the Falling Edge of the Modulation Signal. The Peaking Control
10
can be Used to Compensate for Slow Laser Turn-Off Characteristics.
00
Table 11. Register: Reserved—Reserved for Internal Use. This Register is for Test Puposes Only. Do Not Write to this Register
(I2C Command Code 0x08)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:7
.Reserved 6 1 Reserved for Internal Use, Do Not Write.
50
40
30
.Reserved 2 1 Reserved for Internal Use, Do Not Write.
10
00
LTC5100
37
sn5100 5100fs
Table 12. Register: IB (IMD)—Laser Bias Current Register (Monitor Diode Current in APC Mode) (I2C Command Code 0x15)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:12
.Is_rng 11 0 Source Current Range
10 0 Is_rng Sets the Full-Scale Range of the SRC Pin Current. The Table Below Shows the Available Ranges.
Values for Is_rng
Nominal Full-
Binary Decimal Scale SRC Pin
Value Value Current (mA)
00 0 9
01 1 18
10 2 27
11 3 36
See the Electrical Specifications for Guaranteed Limits in Each Range.
.Ib_nom 9 0 Bias Current or Monitor Diode Current Setting at the Nominal Temperature
(.Imd_nom in 8 0 This Bit Field has Different Functions Depending on Apc_en.
APC Mode) 7 0 This Bit Field is an Unsigned 10-Bit Integer.
60Constant Current Control (CCC) Mode (Apc_en = 0): Ib_nom Sets the Laser Bias Current at
50
Temperature T = T_nom. The physical Bias Current at T_nom is Given by:
4 0 (typical)
30
20Automatic Power Control (APC) Mode (Apc_en = 1): Imd_nom Sets the Monitor Diode Current at the
10
Temperature T = T_nom. The Physical Monitor Diode Current at T_nom is Given by:
00
REGISTER DEFI ITIO S
UU
I=
Ib_nom
1024
B
•( _ )•Is rng mA+19
IA d nom
MD drng
=µ
425 4 8 1024
. • • exp ln( )• Im _
Im _
Table 13. Register: IB_TC1 (IMD_TC1)—Laser Bias/Monitor Diode Current First Order Temperature Coefficient
(I2C Command Code 0x16)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Ib_tc1 (.Imd_tc1 7 0 First Order Temperature Coefficient for Bias Current or Monitor Diode Current
in APC Mode) 6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127.
5 0 This Bit Field has Different Functions Depending on Apc_en.
40Constant Current Control (CCC) Mode (Apc_en = 0): Ib_tc1 Sets the First Order Temperature
30
Coefficient for the Laser Bias Current. The Nominal Scaling is 2
–13
/°C or 122ppm/°C per LSB.
20Automatic Power Control (APC) Mode (Apc_en = 1): Imd_tc1 Sets the First Order Temperature
10
Coefficient for the Monitor Diode Current. See Laser Bias Current Control in APC Mode
00
in the Operation Section for Details.
LTC5100
38
sn5100 5100fs
Table 14. Register: IB_TC2 (IMD_TC2)—Laser Bias/Monitor Diode Current Second Order Temperature Coefficient
(I2C Command Code 0x17)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Ib_tc2 (.Imd_tc2 7 0 Second Order Temperature Coefficient for Bias Current or Monitor Diode Current
in APC Mode) 6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127.
5 0 This Bit Field has Different Functions Depending on Apc_en.
40Constant Current Control (CCC) Mode (Apc_en = 0): Ib_tc2 Sets the Second Order Temperature
30
Coefficient for the Laser Bias Current. The Nominal Scaling is 2
–18
/°C
2
or 3.81ppm/°C
2
per LSB.
20Automatic Power Control (APC) Mode (Apc_en = 1): Imd_tc2 Sets the Second Order Temperature
10
Coefficient for the Monitor Diode Current. See Laser Bias Current Control in APC Mode
00
in the Operation Section for Details.
Table 15. Register: IM—Laser Modulation Current (I2C Command Code 0x19)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:12
.Im_rng 11 0 Modulation Current Range
10 0 Im_rng Sets the Full-Scale Range of the Modulation Current
Binary Decimal Nominal Full-Scale MODA and MODB Pin Current
Value Value Peak-to-Peak (mA) Average (mA)
00 0 9 4.5
01 1 18 9
10 2 27 13.5
11 3 36 18
See the Electrical Specifications for Guaranteed Limits in Each Range.
These Currents Represent the Peak-to-Peak Current at the MODA and MODB Pins. (The MODA and
MODB Pins are Tied Together On Chip).
.Im_nom 9 0 Modulation Current Setting at the Nominal Temperature
8 0 This Bit Field is an Unsigned 10-Bit Integer. Im_nom Sets the Average Modulation Current Delivered at
70
the MODA and MODB Pins. (The MODA and MODB Pins are Tied Together On Chip).
60
The Peak-to-Peak Current is Twice the Average Current for a Data Stream with 50% Duty Cycle.
50
The Modulation Current Reaching the Laser Depends on its Dynamic Resistance Relative to the
40
Termination Resistor.
30
20
10
00
REGISTER DEFI ITIO S
UU
LTC5100
39
sn5100 5100fs
Table 16. Register: IM_TC1—Laser Modulation Current First Order Temperature Coefficient (I2C Command Code 0x1A)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Im_tc1 7 0 First Order Temperature Coefficient for Modulation Current
6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127.
50
Im_tc1 Sets the First Order Temperature Coefficient for the Modulation Current. The Nominal Scaling
40
is 2
–13
/°C or 122ppm/°C per LSB.
30
20
10
00
REGISTER DEFI ITIO S
UU
Table 17. Register: IM_TC2—Laser Modulation Current Second Order Temperature Coefficient (I2C Command Code 0x1B)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:8
.Im_tc2 7 0 Second Order Temperature Coefficient for Modulation Current
6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127.
50
Im_tc2 Sets the Second Order Temperature Coefficient for the Laser Bias Current. The Nominal
40
Scaling is 2
–18
/°C
2
or 3.81ppm/°C
2
per LSB.
30
20
10
00
Table 18. Register: T_EXT—External Temperature (I2C Command Code 0x0D)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.T_ext 9 0 Externally Supplied Temperature for Temperature Compensation Calculations (Unsigned 10-Bit
80
Integer)
7 0 By Convention the Scaling of T_ext is 512K or 239°C Full Scale, Corresponding to 0.5°C/LSB.
60
However, Any Scaling is Permissible as Long as the Temperature Compensation Coefficients are Also
50
Appropriately Scaled.
4 0 T_ext = (T + 273°C)/0.5°C, Where T is the External Temperature in Degrees Celsius.
30
20
10
00
LTC5100
40
sn5100 5100fs
Table 19. Register: T_NOM—Nominal Temperature (Includes Imd_rng) (I2C Command Code 0x1D)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:12
.Imd_rng 11 0 Monitor Diode Current Range
10 0 Imd_rng Sets the Full-Scale Range of the Monitor Diode Current.
Binary Decimal MD Pin Current Range (µA)
Value Value Nom Min Nom Max
00 0 4.25 34
01 1 17 136
10 2 68 544
11 3 272 2176
.T_nom 9 0 Nominal Temperature
8 0 T_nom is the Temperature with Respect to Which All Temperature Compensation Calculations are
70
Made. T_nom is Usually the Temperature at Which the LTC5100 and Laser Diode were Set Up In
60
Production.
5 0 The Scaling is 512K or 239°C Full Scale, Corresponding to 0.5°C/LSB
4 0 T_nom = (T + 273°C)/0.5°C, Where T is the Nominal Temperature in Degrees Celsius.
30
20
10
00
REGISTER DEFI ITIO S
UU
LTC5100
41
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 20. Register: FLT_CONFIG—Fault Configuration (Refer also to Table 1) (I2C Command Code 0x13)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:12
Rep_flt_inhibit 11 0 Repeated Fault Inhibit
0: Allow Repeated Attempts to Clear a Fault and Re-enable the Transmitter.
1: Inhibit Repeated Attempts to Clear a Fault. Only One Attempt to Clear a Fault is Allowed. If the Fault
Recurs Within 25ms of Re-enabling the Transmitter, the Transmitter is Disabled Until Power is
Cycled.
Rapid_restart_en 10 1 Rapid_restart_en
0: Rapid Restart Disabled: The Servo Controller Settings for the Laser Bias and Modulation Currents
are Reset to Zero when the Transmitter is Disabled. When Re-enabled, the Laser Currents Start from
Zero and Settle Typically Within the 300ms Standard Initialization Time, t_int, from the GBIC
Specification.
1: Rapid Restart Enabled: The Servo Controller Settings for the Laser Bias and Modulation Currents are
Retained when the Transmitter is Disabled. When Re-enabled, the Retained Servo Values are Loaded
into the SRC_DAC and MOD_DAC, Allowing Settling Typically Within the 1ms Standard Turn-On
Time, t_on, from the GBIC Specification.
Flt_drv_mode 9 0 FAULT Pin Drive Mode
8 0 00: Open Drain (3.3mA Sink Capability)
01: Open Drain, 280µA Internal Pull Up
10: Open Drain, 425µA Internal Pull Up
11: Push-Pull (3.3mA Source and Sink Capability)
Lpc_en 7 1 Laser Power Controller (LPC) Enable
0: LPC Disabled: Allows External Control of the SRC_DAC and MOD_DAC Registers from the Serial
Interface. This Setting Gives an External Microprocessor or Test Computer Full Control of the
SRC_DAC and MOD_DAC Registers.
1: LPC Enabled: The LPC Continuously Updates the SRC_DAC and MOD_DAC Registers to Servo
Control the Laser. (Any Values Written to These Registers Over the Serial Interface Will be
Overwritten by the LPC.)
Auto_shutdn_en 6 1 Automatic Transmitter Shutdown Enable
0: Disabled: When a Fault Occurs the LTC5100 Continues to Drive the Laser. This Mode Allows a
Microprocessor or Test Computer to Mediate the Decision to Shut Down the Transmitter. The
Microprocessor can Turn Off the Transmitter by Driving the EN Pin Inactive or by Clearing the
Soft_en Bit in the SYS_CONFIG Register.
1: Enabled: When a Fault Occurs, the Transmitter is Automatically Disabled.
Flt_pin_polarity 5 1 FAULT Pin Polarity
0: Active Low: The FAULT Pin is Driven Low to Signal a Fault.
1: Active High: The FAULT Pin is Driven High to Signal a Fault.
Flt_pin_override 4 0 FAULT Pin Override
0: The FAULT Pin is Driven Active when a Fault Occurs.
1: Internal Control of the FAULT Pin is Overridden. When a Fault Occurs, the Fault is Detected and
Latched Internally, but the FAULT Pin Remains Inactive. This Mode Allows a Microprocessor or Test
Computer to Mediate Fault Handling. The Microprocessor can Drive the FAULT Pin Active by Setting
the Force_flt Bit.
Force_flt 3 0 Force the FAULT Pin Output.
Force_flt Gives a Microprocessor or Test Computer Full Control of the FAULT Pin, Allowing External
Mediation of Fault Handling.
0: Force the FAULT Pin Inactive.
1: Force the FAULT Pin Active.
This Bit Has No Effect Unless Flt_pin_override = 1.
Over_pwr_en 2 1 Enables Detection of a Laser Overpower Fault.
0: Disabled, 1: Enabled
Under_pwr_en 1 1 Enables Detection of a Laser Underpower Fault.
0: Disabled, 1: Enabled
Over_current_en 0 1 Enables Detection of a Laser Overcurrent Fault.
0: Disabled, 1 Enabled
LTC5100
42
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 21. Register: FLT_STATUS—Fault Status (I2C Command Code 0x12)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:11
.Transmit_ready 10 0 Transmit Ready
Indicates that the Laser Bias and Modulation Currents Have Settled to Within Specification and the
LTC5100 is Ready to Transmit Data. A Fault Clears This Bit.
0: Not Ready, 1: Ready
.Transmitter_ 9 0 Transmitter Enabled
enabled Indicates That the Transmitter is Enabled and the Laser Bias and Modulation Currents Are on (Though
Not Necessarily Settled.) The Transmitter is Enabled When the EN pin and Soft_en Bits are Active and
No Faults Have Occurred. A Fault Clears This Bit.
0: Transmitter is Disabled, 1: Transmitter is Enabled.
.En_pin_state 8 Varies EN Pin State
Indicates the Logic Level on the EN Pin. The En_polarity Bit Has No Effect on En_pin_state.
The Power-On Reset Value Reflects the State of the EN Pin.
0: EN Pin is Low.
1: EN Pin is High.
.Faulted_twice 7 0 Faulted Twice (Only Active When Rep_flt_inhibit is Set)
0: Either No Faults or Only One Fault Has Been Detected.
1: A Second Fault Has Been Detected Within 25ms of Attempting to Clear a First Fault. The Transmitter
is Disabled and Can Only be Re-enabled by Cycling the Power.
.Faulted_once 6 1 Faulted Once (Only Active When Rep_flt_inhibit is Set)
Indicates That a First Fault Has Been Detected. After a Fault Occurs, Faulted_once Will be Set at the
Moment the Transmitter is Disabled (by Setting the EN pin of Soft_en Bit Inactive). If the Transmitter
is Subsequently Re-enabled and a Second Fault Occurs Within 25ms, the Faulted_twice Bit is Set. If
No Fault Occurs Within 25ms, the Faulted_once Bit is Cleared.
0: A First Fault Has Not Been Detected or Has Been Cleared.
1: A First Fault Has Been Detected.
.Faulted 5 1 Faulted
0: The LTC5100 is Not in the Faulted State.
1: A Fault Has Occurred and the LTC5100 Has Entered the Faulted State (the Transmitter is Not
Disabled Unless Auto_shutdn_en is Set).
.Under_votlage 4 1 Undervoltage Fault Indicator (Always Enabled)
Cleared-on-read Indicates That a Power Supply Undervoltage Event Occurred.
0: No Fault, 1: Undervoltage Fault Detected.
The Undervoltage Bit is Always Set at Power Up. Read the FLT_STATUS Register Immediately After
Power-Up to Clear This Bit.
.Mem_load_error 3 0 Memory (EEPROM) Load Error Indicator (Always Enabled)
Cleared-on-read Indicates That an Attempt to Load the Registers from EEPROM Was Started But Did Not Complete
Successfully.
0: No Fault, 1: EEPROM Load Failed.
.Over_power 2 0 Laser Overpower Fault Indicator (Enabled by Over_pwr_en)
Cleared-on-read Indicates That a Laser Overpower Fault Occurred. Overpower Occurs When the Monitor Diode Current
Exceeds its Set Point. An Overpower Fault Can Occur Only in APC Mode.
0: No Fault, 1: Overpower Fault Detected.
.Under_power 1 0 Laser Underpower Fault Indicator (Enabled by Under_pwr_en)
Cleared-on-read Indicates That a Laser Underpower Fault Occurred. Underpower Occurs When the Monitor Diode
Current Falls Below its Set Point. An Underpower Fault Can Occur Only in APC Mode.
0: No Fault, 1: Underpower Fault Detected.
.Over_current 0 0 Laser Overcurrent Fault Indicator (Enabled by Over_current_en)
Cleared-on-read Indicates That the Laser Bias Current Exceeded the Value Set in the IB_LIMIT Register.
0: No Fault, 1: Overcurrent Fault Detected.
LTC5100
43
sn5100 5100fs
Table 22. Register: IB_LIMIT—Laser Bias Current Limit (I2C Command Code 0x11)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:7
.Ib_limit 6 0 Laser Bias Current Limit
5 0 This Bit Field is an Unsigned 7-Bit Integer
4 0 Sets the Detection Level for an Over_current Fault. When the Laser Bias Current Exceeds This Level an
30
Over_current Fault is Generated (Provided Over_current_en is Set).
20
The Physical Bias Current Level is Given By:
10
00
I=
Ib_limit
128
B(LIMIT)
•( _ )• ( )Is rng mA typical+19
REGISTER DEFI ITIO S
UU
Table 23. Register: USER_ADC—Writing (I2C Command Code 0x18)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:3
.Adc_src_sel 2 0 ADC Source Select
1 0 Selects the Signal to be Converted by the ADC During the User ADC Cycle
00
User ADC Signal Sources
Signal
Select Signal
(Binary) Name Description Scaling
000 I
S
Source Current (SRC Pin I
S
= ADC_code/1024 • (Is_rng + 1) • 9mA
Current)
001 I
M
Average Modulation I
M
= ADC_code/1024 • (Im_rng + 1) • 9mA
Current (MODA +MODB
Pin Current)
010 V
LD
Laser Diode Voltage V
LD
= ADC_code/1024 • 3.5V
011 I
MD
Monitor Diode Current I
MD
= 4.25µA • 4
lmd_rng
• exp[In(8) • ADC_code/
1024]
100 T Temperature T(°C) = ADC_code • 0.5°C – 273°C
101 V
TERM
Termination Resistor V
TERM
= ADC_code/1024 • (Is_rng + 1) • 400mV
Voltage
110 Reserved Reserved
111 Reserved Reserved
LTC5100
44
sn5100 5100fs
REGISTER DEFI ITIO S
UU
Table 24. Register: USER_ADC—Reading (I2C Command Code 0x18)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15
.Adc_src 14 0 ADC Signal Source
13 0 Specifies the Signal Source of the Last User ADC Conversion. See Table 23 for the Definition of These
12 0 Signal Sources. Adc_src Reflects the Last Signal Source Converted. It Does Not Necessarily Hold the
Last Value Written to the ADC_src_sel Bit Field.
.Reserved 11
.Valid 10 0 ADC Data Valid
Indicates That the Result in the Data Bit Field (Defined Below) Contains Newly Converted Data Since
the Last Time Adc_src_sel Was Written or This Register Was Read. Immediately After Power Up
Valid is False. Valid Becomes True as Soon as the First User ADC Conversion is Completed.
0: The ADC Result is Not a Valid Conversion of the Most Recently Selected ADC Source.
1: The ADC Has Finished Conversion and the Result is Valid.
.Data 9 0 ADC Data (10-Bit Unsigned Integer)
8 0 Contains the Result of the Last User ADC Conversion. See Table 23 for the Definition of the Available
70
Signal Sources.
60
50
40
30
20
10
00
Table 25. Register: T_INT_ADC—Internal Temperature ADC (I2C Command Code 0x05)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.T_int_adc 9 0 ADC Reading of the Internal (Die) Temperature (10-Bit Unsigned Integer)
8 0 This Bit Field Contains the Result of the Last Conversion of the LTC5100’s Internal Die Temperature.
70
6 0 The Scaling is 512°K or 239°C Full Scale, Corresponding to 0.5°C/LSB.
50
4 0 T = T_int_adc • 0.5°C – 273°C, Where T is the Internal Temperature in Degrees Celsius.
30
20
10
00
LTC5100
45
sn5100 5100fs
Table 26. Register: IM_ADC—Modulation Current ADC (I2C Command Code 0x06)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.Im_adc 9 0 ADC Reading of the Modulation Current (10-Bit Unsigned Integer)
8 0 Im_adc Contains the Last ADC Conversion of the Average Modulation Current Delivered at the MODA
70
and MODB Pins. (The MODA and MODB Pins are Tied Together On-Chip.) The Peak-to-Peak Current
60
s Twice the Average Current for a Data Stream with 50% Duty Cycle. The Modulation Current
50
Reaching the Laser Depends on its Resistance Relative to the Termination Resistor.
4 0 The Average Physical Current at the MODA and MODB Pins is Given By:
30
20
10
00
REGISTER DEFI ITIO S
UU
Table 27. Register: IS_ADC (IMD_ADC)—Source Current/Monitor Diode Current ADC (I2C Command Code 0x07)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.Is_adc 9 0 ADC Reading of the SRC Pin Current or Monitor Diode Current
(.Imd_adc in 8 0 This Bit Field Has Different Functions Depending on Apc_en.
APC Mode) 70Constant Current Control (CCC) Mode (Apc_en = 0): Is_adc Contains the Last ADC Conversion of
60
the SRC Pin Current. The Physical SRC Pin Current is Given By:
50
40
30Automatic Power Control (APC) Mode (Apc_en = 1): Imd_adc Contains the Last ADC Conversion of
20
the Monitor Diode Current. The Physical Monitor Diode Current is Given By:
10
00
I=
ADC_code
1024
S•( _ )• ( )Is rng mA typical+19
I= A•4
MD Imd_rng
425 8 1024
. •exp ( )• _
µ
In ADC code
Table 28. Register: IS_DAC—Souce Current DAC (I2C Command Code 0x01)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.Is_dac 9 0 DAC Setting for the Source Current (the SRC Pin Current)
8 0 Read Access to This DAC is Always Available. Write Access is Only Valid if LPC_en = 0.
70
60
50
40
30
20
10
00
I=
Is_dac
1024
S
•( _ )• ( )Is rng mA typical+19
I=
ADC_code
1024
M
•(Im_ )• ( )rng mA typical+19
LTC5100
46
sn5100 5100fs
Table 29. Register: IM_DAC—Modulation Current DAC (I2C Command Code 0x02)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:10
.Im_dac 9 0 DAC Setting for the Peak-to-Peak Modulation Current (the Combined MODA and MODB Pin Currents)
8 0 Read Access to This DAC is Always Available. Write Access is Only Valid if LPC_en = 0.
70
60
50
40
30
20
10
00
REGISTER DEFI ITIO S
UU
Table 30. Register: PWR_LIMIT_DAC—Optical Power Limit DAC—Read Only (I2C Command Code 0x03)
REGISTER RESET VALUE
.BITFIELD BIT (BIN) FUNCTION AND VALUES
.Reserved 15:7
.pwr_limit_dac 6 0 DAC Setting for the Over and Underpower Fault Detection Comparator (Read Only)
Read Only 5 0 This Bit Field Has Different Functions Depending on Apc_en.
40Constant Current Control (CCC) Mode (Apc_en = 0): Pwr_limit_dac Has No Function in This Mode.
30
Its Contents are Undefined.
20Automatic Power Control (APC) Mode (Apc_en = 1): Pwr_limit_dac Tracks the Value of the Monitor
10
Diode Current. The Laser Power Controller Continuously Updates the PWR_LIMIT_DAC with the
00
Most Recent ADC Reading of Imd. Reading the DAC Will Return the Value of Imd_adc Shifted Right
by Three Bits.
I=
Im_dac
1024
M
•(Im_ )• ( )rng mA typical+19
LTC5100
47
sn5100 5100fs
APPLICATIO S I FOR ATIO
WUUU
HIGH SPEED DESIGN AND LAYOUT
Figure 29 and Figure 30 show the schematic and layout of
a minimum component count circuit for standalone op-
eration. The exposed pad of the package is soldered to a
copper pad on top of the board, and nine vias couple this
pad to the ground plane. The four V
SS
pins (Pins 1, 4, 9,
and 12) have webs of copper connecting them to the cen-
tral pad to reduce ground inductance. The laser modula-
tion current returns to the ground plane primarily through
the exposed pad. Any measures that reduce the induc-
tance from the pad to the ground plane improve the modu-
lation waveforms and reduce RFI.
FAULT SDA SCL V
DD(HS)
V
DD
V
SS
IN
+
IN
–
V
SS
V
SS
MODA
MODB
V
SS
EN SRC
LTC5100
MD
13
12
R1
50Ω
C1
10nF
Z
O
= 50Ω
11
10
9
1
L1
FERRITE
BEAD
ENABLE
+TX_DATA
–TX_DATA
FAULT
V
DD
+ 3.3V
V
SS
2
3
4
141516
8FIBER
C3
10nF
24LC00
EEPROM
SOT23 PACKAGE PROGRAMMING
PADS
5100 F29
765
Z
O
= 50Ω
V
CC
NC
SCL
V
SS
SDA
IN
+
V
SS
IN
–
EEPROM
C1
R1
L1
C3
2
3
V
SS
V
SS
5100 F30
MODA
MODB
116 15 14 13
V
SS
FAULT
SDA
SCL
V
DD(HS)
V
DD
EN
SRC
MD
4
11
10
12
9
5678
SCL V
CC
V
SS
SDA NC
Figure 29. Schematic of a Minimum Component Count Circuit
Figure 30. Layout of the Minimum Component Count Circuit Using 0402 Passive Components
LTC5100
48
sn5100 5100fs
APPLICATIO S I FOR ATIO
WUUU
The termination resistor, R1, and its decoupling capacitor,
C1, are placed as close as possible to the LTC5100 to
reduce inductance. Inductance in these two components
causes high frequency peaking and overshoot in the
current delivered to the laser. R1 and C1 are folded against
each other so that their mutual inductance and counter-
flowing current partially cancel their self-inductance. C1
has two vias to the ground plane and a trace directly to Pin
12. The layout shows the EEPROM placed next to the
LTC5100. However, placement of the EEPROM is not
critical. It can be placed several centimeters from the
LTC5100 or on the back of the PC board if desired.
The transmission line connecting the MODB pin to the
laser has a short length of minimum width trace. The net
inductance of this section of trace helps compensate on-
chip capacitance to further reduce reflections from the
chip.
FAULT SDA SCL V
DD(HS)
V
DD
V
SS
IN
+
IN
–
V
SS
V
SS
MODA
MODB
V
SS
EN SRC
LTC5100
MD
13
12
R1
50Ω
C1
10nF
Z
O
= 50Ω
11
10
9
1
L1
FERRITE
BEAD
ENABLE
+TX_DATA
–TX_DATA
FAULT
V
DD
+ 3.3V
V
SS
2
3
4
141516
8FIBER
C3
10nF
24LC00
EEPROM
SOT23 PACKAGE PROGRAMMING
PADS
5100 F29
765
Z
O
= 50Ω
C2
10nF
V
CC
NC
SCL
V
SS
SDA
C3
IN
+
V
SS
IN
–
EEPROM
C1
R1
L1
2
3
V
SS
V
SS
5100 F32
MODA
MODB
116 15 14 13
V
SS
FAULT
SDA
SCL
V
DD(HS)
V
DD
EN
SRC
MD
4
11
10
12
9
5678
C2
SCL V
CC
V
SS
SDA NC
Figure 31. Schematic of a Minimum Output Reflection Coefficient Circuit
Figure 32. Layout of the Minimum Reflection Coeffieicnt Circuit Using 0402 Passive Components
LTC5100
49
sn5100 5100fs
APPLICATIO S I FOR ATIO
WUUU
Figure 31 and Figure 32 show the schematic and layout of
a minimum reflection coefficient, minimum peaking solu-
tion. Two capacitors, C1 and C2 are used to further reduce
the inductance in the termination network. C2 has two vias
to the ground plane.
TEMPERATURE COMPENSATION
The LTC5100 has first and second order digital tempera-
ture compensation for the laser bias current, laser modu-
lation current, and monitor diode current. Recall that in
constant current control mode, the LTC5100 provides
direct temperature compensation of the laser bias current
and the laser modulation current. In automatic power
control mode, the laser bias current is under closed-loop
control and the LTC5100 provides temperature compen-
sation for the monitor diode current and the laser modu-
lation current. The simplest procedure for determining the
temperature coefficients (TC1 and TC2 in Equation 12,
Equation 18, Equation 23, and Equation 29) is as follows:
• Select a nominal or representative laser diode and
assemble it into a transceiver module with the LTC5100.
• Set all temperature coefficients to zero.
• Place the transceiver module in a temperature chamber
and find the values of Ib_nom, Im_nom, and Imd_nom
that give constant average optical power and extinction
ratio at several temperature points.
• Record the LTC5100’s temperature reading, T_int, at
each temperature point.
• Select a convenient value for T_nom, the nominal tem-
perature. (It is customary, but not mandatory, to use
25°C for the nominal temperature.)
• Find the best values of TC1 and TC2 by fitting the
quadratic temperature compensation formula (Equa-
tion 12) to the experimental values of Ib_nom, Im_nom,
Imd_nom, and T_int.
To configure the LTC5100 for normal operation, set the
nominal current to the value found at the nominal tem-
perature. Set TC1 and TC2 to the values determined by the
best fit of the data. For standalone operation, store these
values in the EEPROM. For microprocessor operation,
store the values in the microprocessor’s internal non-
volatile memory or in another source of nonvolatile memory
and load them into the LTC5100 after power-up.
The above procedure not only corrects for the laser
temperature drift, but also corrects the small temperature
drift found in the LTC5100’s internal references.
DEMONSTRATION BOARD
Figure 33 shows the schematic of the DC499 demonstra-
tion board. Details of the use of this demo board and
accompanying software can be found in the DC499 demo
board manual. Figure 34 shows the layout of the demo
board and Table 31 gives the bill of materials for the demo
board.
The core applications circuit for the LTC5100 VCSEL
driver appears inside the box in Figure 33. This is the
complete circuit for an optical transceiver module, includ-
ing power supply filtering. It consists of the LTC5100 with
EEPROM for storing setup parameters, L1 and C3 for
power supply filtering, and R1, C1, and C2 for terminating
the 50Ω modulation output. The circuitry outside the box
in Figure 33 is for support of the demonstration. 5V power
enters through 2-pin connector P2 and is regulated by U3
to 3.3V to power the LTC5100. Connector P1 sends 5V
power and serial control signals to another board, allow-
ing a personal computer to control the LTC5100. U4
produces 1.8VDC to bias the modulation output for elec-
trical eye measurements.
High speed data enters the LTC5100 through SMA con-
nectors J1 and J2. The LTC5100 high speed inputs are
internally AC coupled with rail-to-rail common mode input
voltage range. The input signal swing can go as much as
300mV above V
DD
or below V
SS
without degrading perfor-
mance or causing excessive current flow. The high speed
inputs may be AC coupled, in which case the common
mode voltage floats to mid-supply or 1.65V nominally.
A common cathode VCSEL can be attached to the demo
board via SMA connector J3. R1 establishes a precision,
low reflection coefficient 50Ω modulation drive. By main-
taining a wide band microwave quality 50Ω path, the
length of the connection to the laser can be arbitrarily long.
The LTC5100 generates 20% to 80% transition times of
60ps (80ps 10% to 90%), corresponding to an instanta-
neous transition filtered by a 4.4GHz Gaussian lowpass
filter. At these speeds the primary limitation on line length
is high frequency loss. For high grade, low loss laboratory
cabling with silver coated center conductor and foamed
PTFE dielectric, a practical limit is about 30cm.
LTC5100
50
sn5100 5100fs
The laser’s monitor diode (if needed) can be attached to
either pin of 2-pin header H2 (labeled MD) or to the test
turret labeled MD. H2 is a 2mm, 2-pin header with 0.5mm
square pins.
The demo board includes an EEPROM that provides non-
volatile storage for the LTC5100’s configuration settings
and parameters. For example, the EEPROM stores param-
eters for the laser bias and modulation levels as well as
temperature coefficients and fault detection modes. The
LTC5100 transfers the data in the EEPROM to its internal
registers at power up. The LTC5100 is designed for hot
plugging and can be configured to load the EEPROM and
enable the transmitter as soon as power is applied. Be
APPLICATIO S I FOR ATIO
WUUU
careful with this mode of operation! It is possible to
leave the EEPROM in a state that automatically turns the
laser on at power up.
The LTC5100’s FAULT output is available at the test turret
labeled “FAULT.” The FAULT pin can be software config-
ured with several output pull-up options, including open
drain.
The demo board has three jumpers for enabling the
transmitter, observing the electrical eye diagram, and
measuring the LTC5100’s power supply current. Details of
the use of these jumpers are given in the DC499 demo
manual.
9
10
11
12
4
50Ω
FAULT
J1
SMA
J2
SMA
IN
–
IN
+
3
2
1
13141516
8765
U1
LTC5100
U2
24LC00 EEPROM
5-LEAD SOT23 PACKAGE
128 BITS
V
CC
SCL
V
SS
SDANC
50Ω
R1
49.9Ω
C2
10nF
(OPTIONAL)
1
1
JP2
ELEC
EYE
REMOVE JUMPER
BEFORE ATTACHING
A LASER DIODE!
TERMINATION
RESISTOR
50Ω
V
DD
C3
10nF
C1
10nF
J3
SMA
SCLGNDSDA
SCLGNDSDA
H3
GND
MDSRCEN
5V
P1
SDA
SCL
GND1
GND2
5V
GND
P2
JP1
ENABLE
TRANS
12
H2
MD
12
–
+
R5
22.1k
C7
0.1µF
2
4
35U4
LT1812
1
1.8V
V
+
V
OUT
V
–
R4
10Ω
R3
26.7k
C5
10µFD3
R2
22.1k
JP3
3A SCHOTTKY
I
DD
12
C4
10µF
NC
U3
LT1762EMS8-3.3
V
CC
5V ±5%
150mA MAX
OUT
SENSE
BYP
GND
1
2
3
4
8
7
6
5
IN
NC
NC
SHDN
V
DD2
V
DD1
D2
3A SCHOTTKY
C6
10µF
LTC5100 CORE
APPLICATIONS CIRCUIT
5100 F33
L1
BLM15AG121PN1D
3.3V
5V
V
SS
IN
+
IN
–
V
SS
V
SS
MODA
MODB
V
SS
V
DD
EN
SRC
MD
FAULT
SDA
SCL
V
DD(HS)
GND
+
+
+
Figure 33. Schematic Diagram of the DC499 Demo Board
LTC5100
51
sn5100 5100fs
APPLICATIO S I FOR ATIO
WUUU
Figure 34 Layout of the DC499 Demo Board (Silkscreen and Top Layer Copper)
Table 31. Bill of Materials for the DC499 Demo Board
REFERENCE QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE
5V, V
DD1
, V
DD2
, SDA, 12 2501-2 1-Pin Terminal Turret Test Point Mill-Max (516) 922-6000
SCL, FAULT, EN,
SRC, MD, GND(3)
C1, C2, C3 3 GRP155R71E103JA01 0.01µF 25V 5% X7R 0402 Capacitor Murata (770) 436-1300
C4, C5, C6 3 12066D106MAT 10µF 6.3V 20% X5R 1206 Capacitor AVX (843) 946-0362
C7 1 0603YC104KAT 0.1µF 16V 10% X7R 0603 Capacitor AVX (843) 946-0362
D2,D3 2 B320A 3A Schottky Rectifier Diode Diodes, Inc. (805) 446-4800
D4 0 Option (No Load) N/A (No Load) N/A
H2, JP1, JP2, JP3 4 2802S-02G2 2mm 2-Pin Header Comm Con (626) 301-4200
H3 1 2802S-03G2 2mm 3-Pin Header Comm Con (626) 301-4200
J1, J2, J3 3 142-0701-851 50Ω SMA Edge-Lanch Connector Johnson Components (800) 247-8256
L1 1 BLM15AG121PN1D 0402 Ferrite Bead Murata (770) 436-1300
P1 1 70553-0004 5-Pin Right Angle Header Molex (630) 969-4550
P2 1 70553-0001 2-Pin Right Angle Header Molex (630) 969-4550
R1 1 CR05-49R9FM 49.9Ω 1% 1/16W 0402 Resistor AAC (800) 508-1521
R2, R5 2 CR16-2212FM 22.1k 1% 1/16W 0603 Resistor AAC (800) 508-1521
R3 1 CR16-2672FM 26.7k 1% 1/16W 0603 Resistor AAC (800) 508-1521
R4 1 CR16-10R0FM 10Ω 1% 1/16W 0603 Resistor AAC (800) 508-1521
U1 1 LTC5100 QFN 4mm × 4mm IC LTC (408) 432-1900
U2 1 24LC00 128-Bit IC Bus Serial EEPROM 5-Pin SOT-23 Microchip
U3 1 LT1762EMS8-3.3 Low Noise LDO Micropower Regulator IC LTC (408) 432-1900
U4 1 LT1812CS5 Op Amp with Shutdown IC LTC (408) 432-1900
H3 1 CCIJ2mm-138G 2-Pin 2mm Shunt Comm Con (626) 301-4200
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LTC5100
52
sn5100 5100fs
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LTC1773 Current Mode Synchronous Buck Regulator Design Note 295 “High Efficiency Adaptable Power Supply for
XENPAK 10Gbps Ethernet Transceivers”
LTC1923 High Efficiency Thermoelectric Cooler Controller
LT®1930A 2.2MHz Step-Up DC/DC Converter in 5-Lead SOT-23 Design Note 273 “Fiber Optic Communication Systems Benefit
from Tiny, Low Noise Avalanche Photodiode Bias Supply”
LINEAR TE CHNOLOGY CORPORATION 2003
LT/TP 0903 1K • PRINTED IN USA
U
PACKAGE DESCRIPTIO
UF Package
16-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1692)
4.00 ± 0.10
(4 SIDES)
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC)
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
4. EXPOSED PAD SHALL BE SOLDER PLATED
PIN 1
TOP MARK
0.55 ± 0.20
1615
1
2
BOTTOM VIEW—EXPOSED PAD
2.15 ± 0.10
(4-SIDES)
0.75 ± 0.05 R = 0.115
TYP
0.30 ± 0.05
0.65 BSC
0.200 REF
0.00 – 0.05
(UF) QFN 0802
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.72 ±0.05
0.30 ±0.05
0.65 BCS
2.15 ± 0.05
(4 SIDES)
2.90 ± 0.05
4.35 ± 0.05
PACKAGE
OUTLINE
