General Description
The MAX3740A is a high-speed VCSEL driver for small-
form-factor (SFF) and small-form-factor pluggable (SFP)
fiber optic LAN transmitters. It contains a bias genera-
tor, a laser modulator, and comprehensive safety fea-
tures. The automatic power control (APC) adjusts the
laser bias current to maintain average optical power
over changes in temperature and laser properties. The
driver accommodates common cathode and differential
configurations.
The MAX3740A operates up to 3.2Gbps. It can switch
up to 15mA of laser modulation current and source up
to 15mA of bias current. Adjustable temperature com-
pensation is provided to keep the optical extinction
ratio within specifications over the operating tempera-
ture range. The MAX3740A interfaces with the Dallas
DS1858 to meet SFF-8472 timing and diagnostic
requirements. The MAX3740A accommodates various
VCSEL packages, including low-cost TO-46 headers.
The MAX3740A safety circuit detects faults that could
cause hazardous light levels and disables the VCSEL
output. The safety circuits are compliant with SFF and
SFP multisource agreements (MSA).
The MAX3740A is available in a compact 4mm ✕4mm,
24-pin thin QFN package and operates over the -40°C
to +85°C temperature range.
Applications
Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules
Gigabit Ethernet Optical Transmitters
Fibre Channel Optical Transmitters
Infiniband Optical Transmitters
Features
♦Supports all SFF-8472 Digital Diagnostics
♦2mA to 15mA Modulation Current
♦1mA to 15mA Bias Current
♦Optional Peaking Current to Improve VCSEL Edge
Speed
♦Supports Common Cathode and Differential
Configuration
♦Automatic Power Control
♦Safety Circuits Compliant with SFF and SFP
MSAs
♦4mm ✕4mm 24-Pin Thin QFN Package
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
VCC
REF
COMP
MD
BIAS
OUT+
OUT-
MODSET
BIASSET
RBIASSET
PWRMON
TC1
TC2
IN+
IN-
TX_DISABLE
FAULT
SQUELCH
GND
0.01µF
0.01µF50Ω
L1*
0.047µF
+3.3V
0.1µF
0.1µF
RTC†
†OPTIONAL COMPONENT
*FERRITE BEAD
PEAKSET
RPEAKSET†
CF†
RF†
BIASMON
RBIASMON
4.7kن
RPWRSET
RMODSET
MAX3740A
19-3118; Rev 0; 12/03
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART
TEMP RANGE
PIN-PACKAGE
MAX3740AETG
-40°C to +85°C
24 Thin QFN (4mm x 4mm)
Typical Application Circuit
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage (VCC)..............................................-0.5V to 6.0V
Voltage at TX_DISABLE, IN+, IN-, FAULT,
SQUELCH TC1, TC2, MODSET, PEAKSET, BIASSET,
BIAS, BIASMON, COMP, MD, REF,
PWRMON ...............................................-0.5V to (VCC + 0.5V)
Voltage at OUT+, OUT-.........................(VCC - 2V) to (VCC + 2V)
Current into FAULT ............................................ -1mA to +25mA
Current into OUT+, OUT-....................................................60mA
Continuous Power Dissipation (TA= +85°C)
24-Lead Thin QFN
(derate 20.8mW/°C above +85°C).................................1354mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-55°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA=
+25°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IMOD = 2mAP-P 32
SQUELCH set low,
TX_DISABLE set low,
peaking is not used
(Note 1) IMOD = 15mAP-P 55 68
Additional current when peaking is used
(Note 2) 15 20
ICC
Additional current when SQUELCH is high 5 10
Supply Current
ICC-SHDN Total current when TX_DISABLE is high 3.9 5
mA
FAULT OUTPUT
Output High Voltage VOH RLOAD = 10kΩ to 2.97V 2.4 V
Output Low Voltage VOL RLOAD = 4.7kΩ to 3.63V 0.4 V
TX_DISABLE INPUT
Input Impedance 4.7 10.0 kΩ
Input High Voltage VIH 2.0 V
Input Low Voltage VIL 0.8 V
Power-Down Time The time for ICC to reach ICC-SHDN when
TX_DISABLE transitions high 50 µs
SQUELCH
Squelch Threshold 25 85 mVP-P
Squelch Hysteresis 10 mVP-P
Time to Squelch Data (Note 3) 0.02 5.00 µs
Time to Resume from Squelch (Note 3) 0.02 5.00 µs
BIAS GENERATOR (Note 4)
Minimum 1
Bias Current IBIAS Maximum 15 mA
5mA ≤ IBIAS ≤ 15mA -8 +8
Accuracy of Programmed Bias
Current ∆BIAS 1mA ≤ IBIAS ≤ 5mA -12 +12 %
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA=
+25°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Bias Current During Fault IBIAS_OFF Current out of the BIAS pin 1.5 10 µA
1mA < IBIAS < 3mA 0.0875 0.105 0.1375
BIASMON Gain 3mA ≤ IBIAS ≤ 15mA 0.085 0.105 0.125 mA/mA
BIASMON Stability (Notes 5,6) -10 +10 %
AUTOMATIC POWER CONTROL (APC)
MD Nominal Voltage VMD APC loop is closed 1 VREF -
0.2 2V
Voltage at REF VREF 1.2 1.8 2.2 V
MD Voltage During Fault 0V
MD Input Current Normal operation (FAULT = low) -2 0.7 +2 µA
APC Time Constant CCOMP = 0.047µF (Note 6) 5 20 µs
PWRMON Nominal Gain VPWRMON / (VREF - VMD)1.85 2.15 2.45 V/V
LASER MODULATOR (Note 7)
Minimum 250
Data Input Voltage Swing VID Maximum 2200 mVP-P
Single-ended resistance at OUT+ 80 105
Output Resistance Single-ended resistance at OUT- 72 100 Ω
Minimum 2
Modulation Current IMOD Maximum 15 mAP-P
Minimum Peaking Current Range 0.2 mA
Maximum Peaking Current Range 2mA
Peaking Current Duration 80 ps
Tolerance of Programmed
Modulation Current TC1 is shorted to TC2 -10 +10 %
Minimum Programmable
Temperature Coefficient 0ppm/°C
Maximum Programmable
Temperature Coefficient Temperature range 0°C to +70°C+5000 ppm/°C
Modulation Transition Time tR, tF5mA ≤ IMOD ≤ 15mA, 20% to 80% (Note 6) 65 95 ps
Deterministic Jitter DJ 5mA ≤ IMOD ≤ 15mA, 3.2Gbps (Notes 6, 8) 12 20 psP-P
Random Jitter RJ (Note 6) 1.3 4 psRMS
Laser Modulation During Fault or
while Squelch is Active IMOD_OFF 15 50 µAP-P
Input Resistance Differential resistance 85 100 115 Ω
Input Bias Voltage VIN VCC -
0.3 V
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
4_______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA=
+25°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
SAFETY FEATURES (see the Typical Operating Characteristics section)
High-Current Fault Threshold VBMTH VBIASMON > VBMTH causes a fault 0.7 0.8 0.9 V
VBIAS Fault Threshold VBTH VBIAS referenced to VCC -0.250 -0.2 -0.150 V
Power-Monitor Fault Threshold VPMTH VPWRMON > VPMTH causes a fault 0.7 0.8 0.9 V
TX Disable Time t_OFF
Time from rising edge of TX_DISABLE to
IBIAS = IBIAS_OFF and IMOD = IMOD_OFF
(Note 6)
1.8 5 µs
TX Disable Negate Time t_ON
Time from rising edge of TX_DISABLE to
IBIAS and IMOD at 99% of steady state
(Note 6)
55 500 µs
Fault Reset Time t_INIT1Time to set VFAULT = low after power-on or
after rising edge of TX_DISABLE (Note 6) 60 200 ms
Power-On Time t_INIT2Time after power-on to transmitter-on with
TX_DISABLE low (Note 6) 60 200 ms
Fault Assert Time t_FAULT
Time from fault occurrence to VFAULT =
high; CFAULT < 20pF, RFAULT = 4.7kΩ
(Note 6)
1.4 50 µs
Fault Delay Time t_FLTDLY Time from fault to IBIAS = IBIAS_OFF and
IMOD = IMOD_OFF (Note 6) 15µs
TX_DISABLE Reset t_RESET Time TX_DISABLE must be held high to
reset FAULT (Note 6) 1µs
Note 1: Supply current measurements exclude IBIAS from the total current.
Note 2: Tested with RPEAK = 1.18kΩ.
Note 3: Measured by applying a pattern that contains 20µs of K28.5, followed by 5µs of zeros, then 20µs of K28.5, followed by 5µs
of ones. Data rate is equal to 2.5Gbps, with inputs filtered using 1.8GHz Bessel filters.
Note 4: VBIAS < VCC - 0.7V.
Note 5: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA.
Note 6: Guaranteed by design and characterization.
Note 7: Measured electrically with a 50Ωload AC-coupled to OUT+.
Note 8: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern at 3.2Gbps
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
_______________________________________________________________________________________ 5
ELECTRICAL EYE
MAX3740A toc01
50ps/div
73mV/div
3.2Gbps, K28.5, 10mA MODULATION,
PEAKING OFF
ELECTRICAL EYE WITH PEAKING
MAX3740A toc02
50ps/div
73mV/div
3.2Gbps, K28.5, 10mA MODULATION,
RPEAKSET = 2.4kΩ
ELECTRICAL EYE WITH MAX PEAKING
MAX3740A toc03
50ps/div
73mV/div
3.2Gbps, K28.5, 10mA MODULATION,
RPEAKSET = 500Ω
OPTICAL EYE
MAX3740A toc04
68ps/div
ER = 8.2dB, 2.125Gbps, K28.5,
850nm VCSEL, WITH 2.3GHz
O-TO-E CONVERTER
EMCORE SC-TOSA-8585-3420 VCSEL
OPTICAL EYE
MAX3740A toc05
58ps/div
ER = 8.2dB, 2.5Gbps, K28.5,
850nm VCSEL SONET MASK
WITH +20% MARGIN
EMCORE SC-TOSA-8585-3420 VCSEL
IBIASMON vs. BIAS CURRENT
MAX3740A toc06
BIAS CURRENT (mA)
IBIASMON (mA)
1284
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
016
DETERMINISTIC JITTER
vs. MODULATION CURRENT
MAX3740A toc07
IMOD (mAP-P)
DETERMINISTIC JITTER (psP-P)
105
5
10
15
20
25
30
35
40
0
015
RANDOM JITTER
vs. MODULATION CURRENT
MAX3740A toc08
IMOD (mAP-P)
RANDOM JITTER (psRMS)
105
1
2
3
4
5
6
7
0
015
TRANSITION TIME
vs. MODULATION CURRENT
MAX3740A toc09
IMOD (mAP-P)
TRANSITION TIME (ps)
141210864
50
60
70
80
90
100
40
216
RISE
FALL
Typical Operating Characteristics
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ωload AC-coupled to OUT+, TA= +25°C, unless otherwise
noted.)
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
6_______________________________________________________________________________________
BIAS CURRENT vs. RBIASSET
MAX3740A toc10
RBIASSET (Ω)
BIAS CURRENT (A)
10k
1m
10m
100m
100µ
1k 100k
MODULATION CURRENT vs. RMODSET
MAX3740A toc11
RMODSET (Ω)
MODULATION CURRENT (AP-P)
1k
10m
100m
1m
10k100
MEASURED WITH A
50Ω ELECTRICAL LOAD
MONITOR DIODE CURRENT
vs. RPWRSET
MAX3740A toc12
RPWRSET (Ω)
MONITOR DIODE CURRENT (A)
1k
10µ
100µ
1m
10m
1µ
100 10k
MONITOR DIODE CURRENT
vs. TEMPERATURE
MAX3740A toc18
TEMPERATURE (°C)
MONITOR DIODE CURRENT (µA)
6035-15 10
125
150
175
200
225
250
275
300
100
-40 85
SUPPLY CURRENT vs. TEMPERATURE
MAX3740A toc13
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
603510-15
20
30
40
50
60
70
80
10
-40 85
IMOD = 2mA
IMOD = 15mA
INPUT RETURN LOSS
MAX3740A toc14
FREQUENCY (Hz)
S11 (dB)
1G
-35
-30
-25
-20
-15
-10
-5
0
-40
100M 10G
DIFFERENTIAL
MEASUREMENT
OUTPUT RETURN LOSS
MAX3740A toc15
FREQUENCY (Hz)
S22 (dB)
1G
-16
-14
-12
-10
-8
-6
-4
-2
0
-18
100M 10G
SINGLE-ENDED
MEASUREMENT
MODULATION CURRENT
vs. TEMPERATURE
MAX3740A toc16
TEMPERATURE (°C)
MODULATION CURRENT (mAP-P)
8070605040302010
5
6
7
8
9
10
11
4
090
RMOD = 1.35kΩ
RTC = 1kΩ
RTC = 500kΩ
RTC = 100kΩ
RTC = 60kΩ
RTC = 10kΩ
RTC = 5kΩ
RTC = 100Ω
MODULATION CURRENT TEMPCO
vs. RTC
MAX3740A toc17
RTC (Ω)
TEMPCO (ppm/°C)
100k10k1k
500
1500
2500
3500
4500
5500
-500
100 1M
REFERENCED TO +25°C
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ωload AC-coupled to OUT+, TA= +25°C, unless otherwise
noted.)
HOT PLUG WITH TX_DISABLE LOW
MAX3740A toc19
20ms/div
TX_DISABLE
VCC
LASER
OUTPUT
FAULT
LOW t_INIT = 60ms
3.3V
LOW
OV
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
_______________________________________________________________________________________ 7
STARTUP WITH SLOW RAMPING SUPPLY
MAX3740A toc20
20ms/div
TX_DISABLE
VCC
LASER
OUTPUT
FAULT
LOW
t_INIT = 62ms
3.3V
LOW
OV
TX_DISABLE NEGATE TIME
MAX3740A toc21
20µs/div
TX_DISABLE
VCC
LASER
OUTPUT
FAULT
HIGH t_ON = 54µs
3.3V
LOW
LOW
TRANSMITTER DISABLE
MAX3740A toc22
1µs/div
TX_DISABLE
VCC
LASER
OUTPUT
FAULT
LOW
t_OFF = 1.86µs
3.3V
LOW
HIGH
RESPONSE TO FAULT
MAX3740A toc23
200ns/div
TX_DISABLE
VPWRMON
LASER
OUTPUT
FAULT
LOW
t_FAULT = 245ns
LOW HIGH
EXTERNALLY
FORCED
FAULT
FAULT RECOVERY TIME
MAX3740A toc24
40µs/div
TX_DISABLE
VPWRMON
LASER
OUTPUT
FAULT
LOW
t_INIT = 54µs
HIGH
EXTERNAL
FAULT
REMOVED
LOW
LOW
HIGH
FREQUENT ASSERTION OF TX_DISABLE
MAX3740A toc25
200µs/div
TX_DISABLE
VPWRMON
LASER
OUTPUT
FAULT
EXTERNALLY
FORCED FAULT
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ωload AC-coupled to OUT+, TA= +25°C, unless otherwise
noted.)
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
8_______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1, 10, 13 GND Ground
2TX_DISABLE Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The
driver output is enabled when the pin is asserted low.
3IN+ Noninverted Data Input
4IN- Inverted Data Input
5FAULT Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a
fault condition.
6SQUELCH Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is
set low or left open.
7, 16, 20 VCC +3.3V Supply Voltage
8TC1 Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the modulation current.
9TC2 Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the modulation current.
11 MODSET Modulation Set. A resistor connected from MODSET to ground (RMODSET) sets the desired
modulation current amplitude.
12 PEAKSET Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the
peaking current amplitude. To disable peaking, leave PEAKSET open.
14 OUT- Inverted Modulation-Current Output
15 OUT+ Noninverted Modulation-Current Output
17 BIASSET
Bias Current Set. When a closed-loop configuration is used, connect a 1.7kΩ resistor between
ground and BIASSET to set the maximum bias current. When an open configuration is used,
connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current.
18 BIAS Bias Current Output
19 BIASMON
Bias Current Monitor. The output of BIASMON is a sourced current proportional to the bias current.
A resistor connected between BIASMON and ground (RBIASMON) can be used to form a ground-
referenced bias monitor.
21 COMP
Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of
0.047µF is recommended. For open-loop configuration, short the COMP pin to GND to deactivate
the APC.
22 MD Monitor Diode Connection
23 REF Refer ence P i n. Refer ence m oni tor used for AP C . A r esi stor b etw een RE F and M D ( RP WRS E T) sets the
p hoto m oni tor cur r ent w hen the AP C l oop i s cl osed .
24 PWRMON Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop
configuration, connect PWRMON to GND.
EP Exposed Pad Ground. Must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
Detailed Description
The MAX3740A contains a bias generator with automat-
ic power control (APC), safety circuit, and a laser mod-
ulator with optional peaking compensation.
Bias Generator
Figure 1 shows the bias generator circuitry that contains
a power-control amplifier and smooth-start circuitry. An
internal PNP transistor provides DC laser current to bias
the laser in a light-emitting state. The APC circuitry
adjusts the laser-bias current to maintain average power
over temperature and changing laser properties. The
smooth-start circuitry prevents current spikes to the laser
during power-up or enable, ensuring compliance with
safety requirements and extending the life of the laser.
The MD input is connected to the cathode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the anode of the laser through an
inductor or ferrite bead. The power-control amplifier dri-
ves a current amplifier to control the laser’s bias current.
During a fault condition, the bias current is disabled.
The PWRMON output provides a voltage proportional to
average laser power given by:
VPWRMON = 2 ✕IPD ✕RPWRSET
The BIASMON output provides a current proportional to
the laser bias current given by:
IBIASMON = IBIAS / 9
When APC is not used (no monitor diode, open-loop
configuration) connect the COMP and PWRMON pins
to GND. In this mode, the bias current is set by the
resistor RBIASSET. When a closed-loop configuration is
used, connect a 1.7kΩresistor between ground and
BIASSET to set the maximum bias current.
Safety Circuit
The safety circuit contains an input disable
(TX_DISABLE), a latched fault output (FAULT), and fault
detectors (Figure 2). This circuit monitors the operation
of the laser driver and forces a shutdown (disables
laser) if a fault is detected (Table 1). Table 2 contains
the circuit’s response to various single-point failures.
The transmit fault condition is latched until reset by a
toggle of TX_DISABLE or VCC. The FAULT pin should
be pulled high with a 4.7kΩto 10kΩresistor.
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
_______________________________________________________________________________________ 9
CCOMP
COMP BIASSET
RPWRSET
MD
1.8V
REF CURRENT
AMPLIFIER
POWER-
CONTROL
AMPLIFIER
ENABLE
200Ω
RBIASSET
PWRMON
BIAS GENERATOR
SMOOTH-
START
RBIASMON
BIASMON
FERRITE
BEAD
BIAS
1.6V
(2VBE)
2X
IBIAS
34
IPD
IBIAS
9
MAX3740A
1.2V
Figure 1. Bias Generator
PIN FAULT CONDITION
BIAS VBIAS > VCC - 0.2V
BIASMON VBIASMON > 0.8V
PWRMON VPWRMON > 0.8V
Table 1. Fault Conditions
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
10 ______________________________________________________________________________________
PIN NAME CIRCUIT RESPONSE TO VCC SHORT CIRCUIT RESPONSE TO GND SHORT
FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE Modulation and bias current are disabled. Normal condition for circuit operation.
IN+ Does not affect laser power. Does not affect laser power.
IN- Does not affect laser power. Does not affect laser power.
SQUELCH Does not affect laser power. Does not affect laser power.
TC1 Does not affect laser power. Does not affect laser power.
TC2 The laser modulation is increased, but average power
is not affected. Modulation current is disabled.
MODSET Modulation current is disabled. The laser modulation is increased, but average power
is not affected.
PEAKSET Does not affect laser power. Does not affect laser power.
OUT+ Modulation current is disabled. Modulation current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
BIASSET Laser bias is disabled. Fault state* occurs.
BIAS Fault state* occurs. Note that VCSEL emissions may
continue; care must be taken to prevent this condition. Disables VCSEL.
BIASMON Fault state* occurs. Does not affect laser power.
COMP The bias current is reduced, and the average power of
the laser output is reduced.
IBIAS increases to the value determined by RBIASSET; if
the bias monitor fault threshold is exceeded, a fault is
signaled.
MD
IBIAS increases to the value determined by RBIASSET; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
REF
IBIAS increases to the value determined by RBIASSET; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
PWRMON Fault state* occurs. Does not affect laser power.
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Modulation Circuit
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
3). The modulator drives up to 15mA of modulation into
a 50ΩVCSEL load.
The amplitude of the modulation current is set with
resistors at MODSET and temperature coefficient (TC1,
TC2) pins. The resistor at MODSET (RMODSET) pro-
grams the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (RTC)
programs the temperature coefficient of the modulation
current. For appropriate RTC and RMODSET values, see
the Typical Operating Characteristics section.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
260ps or better for 1.25Gbps, or 130ps or better for
2.5Gbps applications. Use a high-efficiency laser that
requires low modulation current and generates a low-
voltage swing. Trim the leads to reduce laser package
inductance. The typical package leads have induc-
tance of 25nH per inch (1nH/mm). This inductance
causes a large voltage swing across the laser. A com-
pensation filter network can also be used to reduce
ringing, edge speed, and voltage swing (see the
Designing the Compensation Filter Network section).
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
______________________________________________________________________________________ 11
S
RQ
R-S LATCH
HIGH-POWER FAULT
HIGH-CURRENT FAULT
VBIAS FAULT
BIAS
BIASMON
PWRMON
POR
TX_DISABLE
TX_DISABLE
0.8V
0.8V
VCC - 0.2V FAULT
ENABLE
SAFETY CIRCUIT
FAULT
OUTPUT
MAX3740A
Figure 2. Safety Circuit
VCC
ROUT+
100Ω
CURRENT AMPLIFIER 30x
ENABLE
IN+
IN-
OUT+
OUT-
TC1 MODSET
RTC
RMODSET
MODULATION
CURRENT GENERATOR
INPUT BUFFER
CURRENT SWITCH
TEMPERATURE
COMPENSATION
TC2
SIGNAL
DETECT
SQUELCH
PEAKING
CONTROL
PEAKSET
200Ω
ROUT-
1V
RPEAKSET
MAX3740A
Figure 3. Modulation Circuit
Programming Modulation Current
The modulation current output of the MAX3740A is con-
trolled by a resistor (RMODSET) placed between
MODSET and ground. The RMODSET resistor controls
the amount of current being sourced to the VCSEL. The
modulation current is given by the following:
It is important to note that the modulation current being
sourced by the MAX3740A is affected by the load
impedance of the VCSEL. The Modulation Current vs.
RMODSET graph in the Typical Operating Characteristics
shows the current into a 50Ωelectrical load.
Programming Bias Current
The bias current output of the MAX3740A is controlled
by a resistor (RBIASSET) placed between BIASSET and
ground. In open-loop operation the RBIASSET controls
the bias current level of the VCSEL. In closed-loop
operation the RBIASSET controls the maximum bias cur-
rent provided by the APC. The bias current is given by
the following:
The Bias Current vs. RBIASSET graph is also shown in
the Typical Operating Characteristics.
Photodiode Selection
To ensure stable operation of the APC circuit, the time
constant of the MD node should be shorter than the
APC time constant. (tAPC = 5µs if CAPC = 0.047µF).
For typical IPD = 400µA, RPWRSET = 500Ω, select a
photodiode with capacitance less than 500pF.
Programming Modulation-Current Tempco
Compute the required modulation tempco from the
slope efficiency of the laser at TA= +25°C and at a
higher temperature. Then select the value of RTC from
the Typical Operating Characteristics. For example,
suppose a laser has a slope efficiency (SE) of
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
From the Typical Operating Characteristics, the value
of RTC, which offsets the tempco of the laser, is 9kΩ. If
modulation temperature compensation is not desired,
short TC1 and TC2.
Programming the APC Loop
Program the average optical power by adjusting
RPWRSET. To select the resistance, determine the
desired monitor current to be maintained over tempera-
ture and lifetime. See the Monitor Diode Current vs.
RPWRSET graph in the Typical Operating Characteristics
section, and select the value of RPWRSET that corre-
sponds to the required current.
Input Termination Requirements
The MAX3740A data inputs are SFP MSA compatible.
On-chip 100Ωdifferential input impedance is provided
for optimal termination (Figure 4). Because of the on-chip
biasing network, the MAX3740A inputs self-bias to the
proper operating point to accommodate AC-coupling.
Laser tempco SE SE
SE E
ppm C
=−
×−×
=−°
()
()
/
85 25
25 85 25 16
2380
tMD tAPC MD MD
RC sns≤×≤µ=
20
5
20 250,
II
IR
BIAS BIASSET
BIAS BIASSET
=
()
×
=+
×
34
12
200 34
.
II R
RR
IR
R
RR
MOD MODSET OUT
OUT LOAD
MOD MODSET
OUT
OUT LOAD
=
()
×
[]
×+
=+
×
×+
+
+
+
+
30
1
200 30
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
12 ______________________________________________________________________________________
MAX3740A
IN+
IN- 1nH
1nH
0.5pF
0.5pF
VCC
50Ω
50Ω
16kΩ
24kΩ
VCC
VCC
PACKAGE
Figure 4. Simplified Input Structure
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
______________________________________________________________________________________ 13
Applications Information
Interface Models
Figures 4 and 5 show simplified input and output circuits
for the MAX3740A laser driver. Figure 6 shows the fault
circuit interface.
Layout Considerations
To minimize inductance, keep the connections between
the MAX3740A output pins and laser diode as short as
possible. Use good high-frequency layout techniques
and multilayer boards with uninterrupted ground planes
to minimize EMI and crosstalk.
Designing the Compensation Filter
Network
Laser package inductance causes the laser impedance
to increase at high frequencies, leading to ringing, over-
shoot, and degradation of the laser output. A laser com-
pensation filter network can be used to reduce the laser
impedance at high frequencies, thereby reducing output
ringing and overshoot.
The compensation components (RFand CF) are most
easily determined by experimentation. Begin with RF=
50Ωand CF= 1pF. Increase CFuntil the desired trans-
mitter response is obtained (Figure 7). Refer to
Application Note HFAN-2-0: Interfacing Maxim Laser
Drives with Laser Diodes for more information.
Exposed-Pad (EP) Package
The exposed pad on the 24-pin thin QFN provides a very
low thermal resistance path for heat removal from the IC.
The pad is also electrical ground on the MAX3740A and
must be soldered to the circuit board ground for proper
thermal and electrical performance. Refer to Maxim
Application Note HFAN-08.1: Thermal Considerations for
QFN and Other Exposed-Pad Packages for additional
information.
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions from
fiber optic transmitters. IEC 825 defines the maximum
light output for various hazard levels. The MAX3740A
provides features that facilitate compliance with IEC 825.
A common safety precaution is single-point fault toler-
ance, whereby one unplanned short, open, or resistive
connection does not cause excess light output. Using
this laser driver alone does not ensure that a transmitter
design is compliant with IEC 825. The entire transmitter
circuit and component selections must be considered.
Customers must determine the level of fault tolerance
required by their applications, recognizing that Maxim
products are not designed or authorized for use as com-
ponents in systems intended for surgical implant into the
body, for applications intended to support or sustain life,
or for any other application where the failure of a Maxim
product could create a situation where personal injury or
death may occur.
VCC
MAX3740A
PACKAGE
1nH
1nH
0.5pF
0.5pF
OUT-
OUT+
ROUT+
ROUT-
Figure 5. Simplified Output Structure
TIME
POWER
UNCOMPENSATED
CORRECTLY COMPENSATED
OVERCOMPENSATED
Figure 7. Laser Compensation
MAX3740A
VCC
FAULT
Figure 6. Fault Circuit Interface
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
PART PACKAGE TYPE PACKAGE CODE
MAX3740AETG 24 Thin QFN
(4mm x 4mm x 0.8mm) T2444-1
24
23
22
21
20
19
PWRMON
REF
MD
COMP
VCC
BIASMON
7
8
9
10
11
12
VCC
TC1
TC2
GND
MODSET
PEAKSET
13
14
15
16
17
18
*EXPOSED PAD IS CONNECTED TO GND
GND
OUT-
OUT+
VCC
BIASSET
BIAS
6
5
4
3
2
1
SQUELCH
FAULT
IN-
IN+
TX_DISABLE
GND
MAX3740A
24 THIN QFN (4mm x 4mm)
TOP VIEW
Pin Configuration
Chip Information
TRANSISTOR COUNT: 3806
PROCESS: SiGe BIPOLAR
SAFETY
CIRCUITRY
TX_DISABLE
FAULT
VCC
IN+
IN-
OUT-
OUT+
MODULATION CURRENT
GENERATOR
BIAS
GENERATOR
WITH APC
ENABLE
ENABLE
100Ω
LASER
MODULATOR
PEAKING
CONTROL
COMP MD REF PWRMON
BIAS
BIASSET
BIASMON
PEAKSET
TC1 TC2 MODSET
SQUELCH
SIGNAL
DETECT
MAX3740A
Functional Diagram
Package Information
For the latest package outline information, go to
www.maxim-ic.com/packages.
