General Description
The MAX3735/MAX3735A are +3.3V laser drivers for
SFP/SFF applications from 155Mbps up to 2.7Gbps.
The devices accept differential input data and provide
bias and modulation currents for driving a laser. DC-
coupling to the laser allows for multirate applications
and reduces the number of external components. The
MAX3735/MAX3735A are fully compliant with the SFP
MSA timing and the SFF-8472 transmit diagnostic
requirements.
An automatic power-control (APC) feedback loop is incor-
porated to maintain a constant average optical power
over temperature and lifetime. The wide modulation cur-
rent range of 10mA to 60mA (up to 85mA AC-coupled)
and bias current of 1mA to 100mA make this product
ideal for driving FP/DFB laser diodes in fiber-optic mod-
ules. The resistor range for the laser current settings is
optimized to interface with the DS1858 SFP controller IC.
The MAX3735/MAX3735A provide transmit-disable con-
trol, a single-point latched transmit-failure monitor out-
put, photocurrent monitoring, and bias-current
monitoring to indicate when the APC loop is unable to
maintain the average optical power. The MAX3735A
also features improved multirate operation.
The MAX3735/MAX3735A come in package and die
form, and operate over the extended temperature
range of -40°C to +85°C. Applications
Gigabit Ethernet SFP/SFF Transceiver Modules
1G/2G Fibre Channel SFP/SFF Transceiver
Modules
Multirate OC3 to OC48-FEC SFP/SFF Transceiver
Modules
Features
♦SFP Reference Design Available
♦Fully Compliant with SFP and SFF-8472 MSAs
♦Programmable Modulation Current from 10mA to
60mA (DC-Coupled)
♦Programmable Modulation Current from 10mA to
85mA (AC-Coupled)
♦Programmable Bias Current from 1mA to 100mA
♦Edge Transition Times <51ps
♦27mA (typ) Power-Supply Current
♦Multirate 155Mbps to 2.7Gbps Operation
♦Automatic Average Power Control
♦On-Chip Pullup Resistor for TX_DISABLE
♦24-Pin 4mm ×4mm QFN package
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
IN+
0.1µF
GND
MODSET
APCSET
APCFILT2
APCFILT1
SHUTDOWN
TX_FAULT
TX_DISABLE
BC_MON
PC_MON
MD
BIAS
OUT+
OUT+
OUT-
OPTIONAL SHUTDOWN
CIRCUITRY
VCC
IN-
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE
SERDES MAX3735
MAX3735A
0.1µF
+3.3V
+3.3V
CMD
FERRITE BEAD
+3.3V
10Ω
RBC_MON
RPC_MON
15Ω0.01µF
DS1858/DS1859
CONTROLLER
IC CAPC
M0N2
M0N3
MON1
H1
H0
+3.3V
Typical Application Circuit
19-2529; Rev 2; 7/04
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.
*Dice are designed to operate from -40°C to +85°C, but are
tested and guaranteed only at TA= +25°C.
**EP = Exposed pad.
+Denotes lead-free package.
PART TEMP RANGE PIN-PACKAGE
MAX3735E/D -40°C to +85°C Dice*
MAX3735ETG -40°C to +85°C 24 Thin QFN-EP**
MAX3735EGG -40°C to +85°C 24 QFN-EP**
MAX3735AETG -40°C to +85°C 24 Thin QFN-EP**
MAX3735AETG+ -40°C to +85°C 24 Thin QFN-EP**
Pin Configuration appears at end of data sheet.
EVALUATION KIT
AVAILABLE
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless
otherwise noted.) (Note 1)
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
Current into BIAS, OUT+, OUT-......................-20mA to +150mA
Current into MD.....................................................-5mA to +5mA
Voltage at IN+, IN-, TX_DISABLE, TX_FAULT,
SHUTDOWN...........................................-0.5V to (VCC + 0.5V)
Voltage at BIAS, PC_MON, BC_MON,
MODSET, APCSET .................................-0.5V to (VCC + 0.5V)
Voltage at OUT+, OUT-.............................+0.5V to (VCC + 1.5V)
Voltage at APCFILT1, APCFILT2..............................-0.5V to +3V
Continuous Power Dissipation (TA = +85°C )
24-Lead Thin QFN (derate 20.8mW/°C
above +85°C).............................................................1354mW
24-Lead QFN (derate 20.8mW/°C
above +85°C).............................................................1354mW
Operating Ambient Temperature Range (TA)......-40°C to +85°C
Storage Ambient Temperature Range...............-55°C to +150°C
Die Attach Temperature...................................................+400°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER
SYM B O L
CONDITIONS
MIN TYP MAX
UNITS
POWER SUPPLY
Power-Supply Current ICC Excludes the laser bias and modulation
currents (Note 2) 27 50 mA
I/O SPECIFICATIONS
Differential Input Voltage VID VID = (VIN+) - (VIN-), Figure 1
200 2400
mVP-P
Common-Mode Input Voltage 0.6 × VCC V
Differential Input Resistance 85
100
115 Ω
TX_DISABLE Input Pullup
Resistance RPULL 4.7 7.4
10.0
kΩ
VHIGH = VCC 15
TX_DISABLE Input Current VLOW = GND, VCC = 3.3V, RPULL = 7.4kΩ
-450
µA
TX_DISABLE Input High Voltage
VIH 2V
TX_DISABLE Input Low Voltage VIL 0.8 V
TX_FAULT Output High Voltage VOH IOH = 100µA sourcing (Note 3) 2.4 V
TX_FAULT Output Low Voltage VOL IOL = 1mA sinking (Note 3) 0.4 V
SHUTDOWN Output High Voltage
VOH IOH = 100µA sourcing VCC - 0.4 V
SHUTDOWN Output Low Voltage
VOL IOL = 100µA sinking 0.4 V
BIAS GENERATOR
Bias On-Current Range IBIAS Current into BIAS pin 1 100 mA
Bias Off-Current
IBIASOFF
Current into BIAS pin during TX_FAULT or
TX_DISABLE 100 µA
Bias Overshoot During SFP module hot plugging
(Notes 4, 5, 11) 10 %
External resistor to GND defines the voltage
gain, IBIAS = 1mA, RBC_MON = 69.28kΩ
10.0
12
13.5
Bias-Current Monitor Gain
IBC_MON
IBIAS = 100mA, RBC_MON = 693.25Ω
11.5
13
13.5
mA/A
MAX3735 -8 +8
Bias-Current Monitor Gain
Stability
1mA ≤ IBIAS ≤ 100mA
(Notes 4, 6) MAX3735A -6 +6 %
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless
otherwise noted.) (Note 1)
PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS
AUTOMATIC POWER-CONTROL LOOP
MD Reverse Bias Voltage 18µA ≤ IMD ≤ 1500µA 1.6 V
MD Average Current Range IMD Average current into MD pin 18 1500 µA
IBIAS = 1mA
(MAX3735) -880 +880
IBIA S = 1m A
( M AX 3735A) -110 +110
Average Power-Setting Stability APC closed loop
(Notes 4, 7)
IBIA S = 100m A-650 +650
ppm/°C
Average Power Setting Accuracy APC Closed Loop
1mA ≤ IBIAS ≤ 100mA (Note 8) -16 +16 %
MAX3735 0.8 1 1.23
External resistor to GND
defines the voltage gain;
IMD = 18µA, RPC_MON =
50kΩMAX3735A 0.9 1.1
MD-Current Monitor Gain IPC_MON
IMD = 1.5mA, RPC_MON = 600Ω0.95 1 1.05
A/A
MAX3735 -10 +10
MD-Current Monitor Gain Stability 18µA ≤ IMD ≤ 1500µA
(Notes 4, 6) MAX3735A -4 +4 %
LASER MODULATOR
Current into OUT+ pin; RL ≤ 15Ω, VOUT+,
VOUT- ≥ 0.6V (DC-coupled) 10 60
Modulation On-Current Range IMOD Current into OUT+ pin; RL ≤ 15Ω_, V OUT+,
VOUT- ≥ 2.0V (AC-coupled) 10 85
mA
Modulation Off-Current IMODOFF Current into OUT+ pin during TX_FAULT or
TX_DISABLE 100 µA
IMOD = 10mA -480 +480
Modulation-Current Stability
(Note 4) IMOD = 60mA -255 +255 ppm/°C
Modulation-Current Absolute
Accuracy 10mA ≤ IMOD ≤ 60mA (Note 8) -15 +15 %
Modulation-Current Rise Time tR20% to 80%, 10mA ≤ IMOD ≤ 60mA (Note 4) 42 65 ps
Modulation-Current Fall Time tF20% to 80%, 10mA ≤ IMOD ≤ 60mA (Note 4) 50 80 ps
10mA ≤ IMOD ≤ 60mA at 2.67Gbps
(Notes 4, 9, 10) 18 38
At 1.25Gbps (K28.5 pattern) 11.5
At 622Mbps (Note 9) 18
Deterministic Jitter
At 155Mbps (Note 9) 40
ps
Random Jitter RJ 10mA ≤ IMOD ≤ 60mA (Note 4) 0.7 1.0 psRMS
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
4_______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless
otherwise noted.) (Note 1)
Note 1: Specifications at -40°C are guaranteed by design and characterization. Dice are tested at TA= +25°C only.
Note 2: Maximum value is specified at IMOD = 60mA, IBIAS = 100mA.
Note 3: TX_FAULT is an open-collector output and must be pulled up with a 4.7kΩto 10kΩresistor.
Note 4: Guaranteed by design and characterization.
Note 5: VCC turn-on time must be ≤0.8s, DC-coupled interface.
Note 6: Gain stability is defined by the digital diagnostic document (SFF-8472, rev. 9.0) over temperature and supply variation.
Note 7: Assuming that the laser diode to photodiode transfer function does not change with temperature.
Note 8: Accuracy refers to part-to-part variation.
Note 9: Deterministic jitter is measured using a 223 - 1 PRBS or equivalent pattern.
Note 10: Broadband noise is filtered through the network as shown in Figure 3. One capacitor,
C < 0.47µF, and one 0603 ferrite bead or inductor can be added (optional). This supply voltage filtering reduces the hot-
plugging inrush current. The supply noise must be < 100mVP-P up to 2MHz.
Note 11: CAPC values chosen as shown in Table 4 (MAX3735A).
PARAMETER SYM B O L CONDITIONS MIN TYP MAX UNITS
SAFETY FEATURES
Excessive Bias-Current
Comparator Threshold Range
TX_FAULT always occurs for VBC_MON ≥
1.38V, TX_FAULT never occurs for
VBC_MON ≤ 1.22V
1.22 1.30 1.39 V
Excessive MD-Current
Comparator Threshold Range
TX_FAULT always occurs for VPC_MON ≥
1.38V, TX_FAULT never occurs for
VPC_MON ≤ 1.22V
1.22 1.30 1.39 V
SFP TIMING REQUIREMENTS
TX_DISABLE Assert Time t_off
Time from rising edge of TX_DISABLE to
IBIAS = IBIASOFF and IMOD = IMODOFF
(Note 4)
0.14 5 µs
CAPC = 2.7nF,
MAX3735 (Note 4) 1ms
TX_DISABLE Negate Time t_on
Time from falling edge
of TX_DISABLE to
IBIAS and IMOD at 95%
of steady state when
TX_FAULT = 0 before
reset
MAX3735A
(Note 11) 600 µs
TX_DISABLE Negate Time
During FAULT Recovery t_onFAULT
Time from falling edge of TX_DISABLE to
IBIAS and IMOD at 95% of steady state when
TX_FAULT = 1 before reset (Note 4)
60 200 ms
TX_FAULT Reset Time or Power-
On Time t_init From power-on or negation of TX_FAULT
using TX_DISABLE (Note 4) 60 200 ms
TX_FAULT Assert Time t_fault Time from fault to TX_FAULT on, CFAULT ≤
20pF, RFAULT = 4.7kΩ (Note 4) 3.3 50 µs
TX_DISABLE to Reset Time TX_DISABLE must be held high to
reset TX_FAULT (Note 4) 5µs
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
_______________________________________________________________________________________ 5
VIN+
(VIN+) - (VIN-)
VIN-
(200mVP-P min,
2400mVP-P max)
(100mV min,
1200mV max)
IMOD
VOLTAGE
CURRENT
TIME
IOUT+
Figure 2. Output Termination for Characterization
50Ω75Ω
30Ω
30Ω
0.5pF
30Ω
OSCILLOSCOPE
OUT+
IOUT+
OUT-
OUT+
MAX3735
MAX3735A
VCC VCC
Figure 1. Required Input Signal and Output Polarity
SOURCE
NOISE
TO LASER
DRIVER VCC
OPTIONAL
OPTIONAL
MODULE
HOST BOARD
C1
0.1µF
C2
10µF
C3
0.1µF
L1
1µH
VOLTAGE
SUPPLY
FILTER DEFINED BY SFP MSA
Figure 3. Supply Filter
Typical Operating Characteristics
(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA= +25°C, unless otherwise noted.)
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
6_______________________________________________________________________________________
OPTICAL EYE
MAX3735 toc01
54ps/div
ER = 8.2dB, 2.7Gbps, 2.3GHz FILTER
27 - 1 PRBS, 1310nm FP LASER
OPTICAL EYE
MAX3735 toc02
115ps/div
ER = 8.2dB, 1.25Gbps, 900MHz FILTER
K28.5 PATTERN, 1310nm FP LASER
OPTICAL EYE
MAX3735 toc03
919ps/div
ER = 12dB, 155Mbps, 117MHz FILTER
27 - 1 PRBS, 1310nm FP LASER
ELECTRICAL EYE
MAX3735 toc04
58ps/div
2.7Gbps, 27 - 1 PRBS,
30mA MODULATION
85mV/div
SUPPLY CURRENT vs. TEMPERATURE
MAX3735 toc05
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
603510-15
25
40
55
70
10
-40 85
EXCLUDES IBIAS
AND IMOD
BIAS-CURRENT MONITOR GAIN
vs. TEMPERATURE
MAX3735 toc06
TEMPERATURE (°C)
GAIN (mA/A)
603510-15
12
14
16
18
20
10
-40 85
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA= +25°C, unless otherwise noted.)
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
_______________________________________________________________________________________ 7
PHOTOCURRENT MONITOR GAIN
vs. TEMPERATURE
MAX3735 toc07
TEMPERATURE (°C)
GAIN (A/A)
603510-15
0.5
1.0
1.5
2.0
2.5
3.0
0
-40 85
MODULATION CURRENT vs. RMODSET
MAX3735 toc08
RMODSET (kΩ)
IMOD (mA)
10
10
20
30
40
50
60
70
80
90
100
0
1 100
MONITOR DIODE CURRENT vs. RAPCSET
MAX3735 toc09
RAPCSET (kΩ)
IMD (mA)
101
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
0 100
EDGE TRANSITION TIME
vs. MODULATION CURRENT
MAX3735 toc10
IMOD (mA)
EDGE TRANSITION TIME (ps)
50403020
30
40
50
60
70
80
20
10 60
RISE TIME
FALL TIME
DETERMINISTIC JITTER
vs. MODULATION CURRENT
IMOD (mA)
DJ (psP-P)
50403020
10
20
30
50
40
60
0
10 60
DJ (INCLUDING PWD)
PWD
MAX3735 toc11
RANDOM JITTER
vs. MODULATION CURRENT
MAX3735 toc12
IMOD (mA)
RANDOM JITTER (psRMS)
50403020
0.5
1.0
1.5
2.0
2.5
3.0
0
10 60
HOT PLUG WITH TX_DISABLE LOW
MAX3735 toc13
20ms/div
FAULT
VCC
LASER
OUTPUT
TX_DISABLE
t_init = 60ms
LOW
LOW
0V
3.3V
STARTUP WITH SLOW RAMPING SUPPLY
MAX3735 toc14
20ms/div
FAULT
VCC
LASER
OUTPUT
TX_DISABLE
LOW
0V
LOW
3.3V
TRANSMITTER ENABLE
MAX3735 toc15
12µs/div
FAULT
VCC
LASER
OUTPUT
TX_DISABLE t_on = 44µs
LOW
HIGH
3.3V
LOW
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA= +25°C, unless otherwise noted.)
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
8_______________________________________________________________________________________
TRANSMITTER DISABLE
MAX3735 toc16
40ns/div
FAULT
VCC
LASER
OUTPUT
TX_DISABLE
t_off = 134ns
LOW
LOW
3.3V
HIGH
RESPONSE TO FAULT
MAX3735 toc17
1µs/div
FAULT
VPC_MON
LASER
OUTPUT
TX_DISABLE
t_fault = 0.9µs
LOW
LOW
EXTERNALLY
FORCED FAULT
HIGH
FAULT RECOVERY TIME
MAX3735 toc18
100ms/div
FAULT
VPC_MON
LASER
OUTPUT
TX_DISABLE
t_init = 60ms
HIGH
LOW
EXTERNAL
FAULT REMOVED
HIGH
LOW
LOW
FREQUENT ASSERTION OF TX_DISABLE
MAX3735 toc19
4µs/div
FAULT
VPC_MON
LASER
OUTPUT
TX_DISABLE
EXTERNALLY
FORCED FAULT
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
_______________________________________________________________________________________ 9
Detailed Description
The MAX3735/MAX3735A laser drivers consist of three
parts: a high-speed modulation driver, a laser-biasing
block with automatic power control (APC), and safety
circuitry (Figure 4). The circuit design is optimized for
high-speed and low-voltage (+3.3V) operation.
High-Speed Modulation Driver
The output stage are composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3735/MAX3735A are optimized for dri-
ving a 15Ωload; the minimum instantaneous voltage
required at OUT+ is 0.6V. Modulation current swings up
to 60mA are possible when the laser diode is DC-cou-
pled to the driver and up to 85mA when the laser diode
is AC-coupled to the driver.
To interface with the laser diode, a damping resistor
(RD) is required for impedance matching. The com-
bined resistance of the series damping resistor and the
equivalent series resistance of the laser diode should
equal 15Ω. To reduce optical output aberrations and
duty-cycle distortion caused by laser diode parasitic
inductance, an RC shunt network might be necessary.
Refer to Maxim Application Note HFAN 02.0: Interfacing
Maxim’s Laser Drivers to Laser Diodes for more informa-
tion.
At data rates of 2.7Gbps, any capacitive load at the
cathode of a laser diode degrades optical output perfor-
mance. Because the BIAS output is directly connected
to the laser cathode, minimize the parasitic capacitance
associated with the pin by using an inductor to isolate
the BIAS pin parasitics from the laser cathode.
Pin Description
PIN NAME FUNCTION
1, 4, 8, 14, 18 VCC +3.3V Supply Voltage
2IN+ Noninverted Data Input
3IN- Inverted Data Input
5PC_MON Photodiode Current Monitor Output. Current out of this pin develops a ground-referenced voltage
across an external resistor that is proportional to the monitor diode current.
6BC_MON Bias Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an
external resistor that is proportional to the bias current.
7, 12, 22 GND Ground
9SHUTDOWN Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown
circuitry.
10 TX_FAULT Open-Collector Transmit Fault Indicator (Table 1).
11 MODSET A resistor connected from this pad to ground sets the desired modulation current.
13 BIAS Laser Bias Current Output
15, 16 OUT+ Noninverted Modulation Current Output. Connect pins 15 and 16 externally to minimize parasitic
inductance of the package. IMOD flows into this pin when input data is high.
17 OUT- Inverted Modulation Current Output. IMOD flows into this pin when input data is low.
19 MD Monitor Diode Input. Connect this pin to the anode of a monitor photodiode. A capacitor to ground is
required to filter the high-speed AC monitor photocurrent.
20 APCFILT1 Connect a capacitor (CAPC) between pin 20 (APCFILT1) and pin 21 (APCFILT2) to set the dominant
pole of the APC feedback loop.
21 APCFILT2 See APCFILT1
23 APCSET A resistor connected from this pin to ground sets the desired average optical power.
24 TX_DISABLE Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left
unconnected. The laser output is enabled when this pin is asserted low.
EP Exposed
Pad
Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance
(see the Exposed Pad Package section).
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
10 ______________________________________________________________________________________
Laser-Biasing and APC
To maintain constant average optical power, the
MAX3735/MAX3735A incorporate an APC loop to com-
pensate for the changes in laser threshold current over
temperature and lifetime. A back-facet photodiode
mounted in the laser package is used to convert the
optical power into a photocurrent. The APC loop
adjusts the laser bias current so that the monitor cur-
rent is matched to a reference current set by RAPCSET.
The time constant of the APC loop is determined by an
external capacitor (CAPC). For possible CAPC values,
see the Applications Information section.
Safety Circuitry
The safety circuitry contains an input disable
(TX_DISABLE), a latched fault output (TX_FAULT), and
fault detectors (Figure 5). This circuitry monitors the
operation of the laser driver and forces a shutdown if a
fault is detected (Table 1). A single-point fault can be a
short to VCC or GND. See Table 2 to view the circuit
response to various single-point failures. The transmit
fault condition is latched until reset by a toggle of
TX_DISABLE or VCC. The laser driver offers redundant
laser diode shutdown through the optional shutdown
circuitry (see the Typical Applications Circuit). The
TX_FAULT pin should be pulled high with a 4.7kΩto
10kΩresistor to VCC as required by the SFP MSA.
Safety Circuitry Current Monitors
The MAX3735/MAX3735A feature monitors (BC_MON,
PC_MON) for bias current (IBIAS) and photo current
(IMD). The monitors are realized by mirroring a fraction
of the currents and developing voltages across external
resistors connected to ground. Voltages greater than
1.38V at PC_MON or BC_MON result in a fault state.
For example, connecting a 100Ωresistor to ground on
each monitor output gives the following relationships:
VBC_MON = (IBIAS / 76) x 100Ω
VPC_MON = IMD x 100Ω
VCC
VCC
IAPCSET
CAPC
IBIAS
IMOD
RD
IMD
RAPCSET
x38
VCC
RPC_MON
PC_MON
100Ω
IN-
IN+
IMOD
ENABLE
IBIAS
ENABLE
INPUT BUFFER
SHUTDOWN
DATA
PATH
X270
SAFETY LOGIC
AND
POWER
DETECTOR
TX_FAULT
CMD
TX_DISABLE
APCFILT1 APCFILT2
SHUTDOWN
MODSET
BC_MON x1
76
IBIAS
1
IMD
APCSET
MD
MAX3735
MAX3735A
VCC
VBG
(4.7kΩ
TO 10kΩ)
RBC_MON
VCC
VBG
RMODSET
VCC
OUT-
OUT+
BIAS
VCC
15Ω
Figure 4. Functional Diagram
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
______________________________________________________________________________________ 11
Design Procedure
When designing a laser transmitter, the optical output
usually is expressed in terms of average power and
extinction ratio. Table 3 shows relationships helpful in
converting between the optical average power and the
modulation current. These relationships are valid if the
mark density and duty cycle of the optical waveform
are 50%.
Programming the Modulation Current
For a given laser power (PAVG), slope efficiency (η), and
extinction ratio (re), the modulation current can be calcu-
lated using Table 3. See the Modulation Current vs.
RMODSET graph in the Typical Operating Characteristics
section, and select the value of RMODSET that corre-
sponds to the required current at +25°C.
Programming the APC Loop
Program the average optical power by adjusting -RAPC-
SET. To select the resistance, determine the desired moni-
tor current to be maintained over temperature and
lifetime. See the Monitor Diode Current vs. RAPCSET
graph in the Typical Operating Characteristics section,
and select the value of RAPCSET that corresponds to the
required current.
Interfacing with Laser Diodes
To minimize optical output aberrations caused by sig-
nal reflections at the electrical interface to the laser
diode, a series-damping resistor (RD) is required
(Figure 4). Additionally, the MAX3735/MAX3735A out-
puts are optimized for a 15Ωload. Therefore, the series
combination of RDand RL, where RLrepresents the
laser-diode resistance, should equal 15Ω. Typical val-
ues for RDare 8Ωto 13Ω. For best performance, place
a bypass capacitor (0.01µF typ) as close as possible to
the anode of the laser diode. An RC shunt network
between the laser cathode and ground minimizes opti-
cal output aberrations. Starting values for most coaxial
lasers are RCOMP = 50Ωin series with CCOMP = 8pF.
Adjust these values experimentally until the optical out-
put waveform is optimized. Refer to Maxim Application
Note: HFAN 02.0: Interfacing Maxim’s Laser Drivers to
Laser Diodes for more information.
POR AND COUNTER
FOR t_init
COUNTER FOR
t_onfault
MODSET
SHORT-
CIRCUIT
DETECTOR
100ns DELAY
MAX3735
MAX3735A
VCC
VCC
VCC
VBG
COMP
VBG
COMP
76
IBIAS
1
IMD
RS
LATCH
RQ
S
PC_MON
BC_MON
IMOD
ENABLE
IBIAS
ENABLE
TX_FAULT
CMOS
TTL
OPEN COLLECTOR
SHUTDOWN
RPC_MON
RBC_MON
TX_DISABLE
Figure 5. Safety Circuitry
Table 1. Typical Fault Conditions
1.
If any of the I/O pins is shorted to GND or VCC (single-
point failure, see Table 2), and the bias current or the
photocurrent exceed the programmed threshold.
2.
End-of-life (EOL) condition of the laser diode. The bias
current and/or the photocurrent exceed the
programmed threshold.
3. Laser cathode is grounded and the photocurrent
exceeds the programmed thresholds.
4.
N o feed b ack for the AP C l oop ( b r oken i nter connecti on,
d efecti ve m oni tor p hotod i od e) , and the b i as cur r ent
exceed s the p r og r am m ed thr eshol d .
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
12 ______________________________________________________________________________________
Pattern-Dependent Jitter
To minimize the pattern-dependent jitter associated
with the APC loop time constant, and to guarantee loop
stability, connect a capacitor between APCFILT1 and
APCFILT2 (see the Applications Information section for
more information about choosing CAPC values). A
capacitor attached to the photodiode anode (CMD) is
also recommended to filter transient currents that origi-
nate from the photodiode. To maintain stability and
proper phase margin associated with the two poles
created by CAPC and CMD, CAPC should be 20x
greater than CMD for the MAX3735. CAPC should be 4x
to 20x greater than CMD for the MAX3735A.
Table 2. Circuit Responses to Various Single-Point Faults
PIN NAME CIRCUIT RESPONSE TO OVERVOLTAGE
OR SHORT TO V CC
CIRCUIT RESPONSE TO UNDERVOLTAGE
OR SHORT TO GROUND
TX_FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE Modulation and bias currents are disabled. Normal condition for circuit operation.
IN+
The optical average power increases and a fault occurs
if VPC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
The optical average power decreases and the APC
loop responds by increasing the bias current. A fault
state occurs if VBC_MON exceeds the threshold voltage.
IN-
The optical average power decreases and the APC
loop responds by increasing the bias current. A fault
state occurs if VBC_MON exceeds the threshold voltage.
The optical average power increases and a fault occurs
if VPC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
MD Disables bias current. A fault state occurs. The APC circuit responds by increasing bias current
until a fault is detected, then a fault state* occurs.
SHUTDOWN Does not affect laser power. If the shutdown circuitry is
used, laser current is disabled and a fault state* occurs. Does not affect laser power.
BIAS In this condition, laser forward voltage is 0V and no light
is emitted.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
OUT+ The APC circuit responds by increasing the bias current
until a fault is detected, then a fault state* occurs.
Fault state* occurs. If the shutdown circuitry is used,
laser current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
PC_MON Fault state* occurs. Does not affect laser power.
BC_MON Fault state* occurs. Does not affect laser power.
APCFILT1 IBIAS increases until VBC_MON exceeds the threshold
voltage.
IBIAS increases until VBC_MON exceeds the threshold
voltage.
APCFILT2 IBIAS increases until VBC_MON exceeds the threshold
voltage.
IBIAS increases until VBC_MON exceeds the threshold
voltage.
MODSET Does not affect laser power. Fault state* occurs.
APCSET Does not affect laser power. Fault state* occurs.
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Table 3. Optical Power Definitions
PARAMETER SYMBOL RELATION
Average Power PAVG PAVG = (P0 + P1) / 2
Extinction Ratio rere = P1 / P0
Optical Power High P1P1 = 2PAVG x re / (re + 1)
Optical Power Low P0P0 = 2PAVG / (re + 1)
Optical Amplitude PP-P PP-P = P1 - P0
Laser Slope
Efficiency ηη = PP-P / IMOD
Modulation Current IMOD IMOD = PP-P / η
Input Termination Requirements
The MAX3735/MAX3735A data inputs are SFP MSA
compliant. On-chip 100Ωdifferential input impedance is
provided for optimal termination (Figure 6). Because of
the on-chip biasing network, the MAX3735/MAX3735A
inputs self-bias to the proper operating point to accom-
modate AC-coupling.
Optional Shutdown Output Circuitry
The SHUTDOWN control output features extended eye
safety when the laser cathode is grounded. An external
transistor is required to implement this circuit (Figure 4).
In the event of a fault, SHUTDOWN asserts high, plac-
ing the optional shutdown transistor in cutoff mode and
thereby shutting off the laser current.
Applications Information
An example of how to set up the MAX3735/MAX3735A
follows:
Select a communications-grade laser for 2.488Gbps.
Assume that the laser output average power is PAVG =
0dBm, the operating temperature is -40°C to +85°C,
minimum extinction ratio is 6.6 (8.2dB), and the laser
diode has the following characteristics:
Wavelength: λ= 1.3µm
Threshold current: ITH = 22mA at +25°C
Threshold temperature coefficient: βTH = 1.3% / °C
Laser-to-monitor transfer: ρMON = 0.2A/W
Laser slope efficiency: η= 0.05mW/mA at +25°C
Determine RAPCSET
The desired monitor diode current is estimated by IMD
= PAVG x ρMON = 200µA. The Monitor Diode vs. RAPC-
SET graph in the Typical Operating Characteristics sec-
tion shows that RAPCSET should be 3kΩ. The value can
also be estimated using the equation below:
IMD = 1.23 / (2 ×RAPCSET)
Determine RMODSET
To achieve a minimum extinction ratio (re) of 6.6 over
temperature and lifetime, calculate the required extinc-
tion ratio at +25°C. Assuming the results of the calcula-
tion are re= 20 at +25°C, the peak-to-peak optical
power PP-P = 1.81mW, according to Table 3. The
required modulation current is 1.81mW / (0.05mW/mA)
= 36.2mA. The Modulation Current vs. RMODSET graph
in the Typical Operating Characteristics section shows
that RMODSET should be 9.5kΩ. The value can also be
estimated using the equation below:
IMOD = 1.23 / (0.0037 ×RMODSET)
Determine CAPC
In order to meet SFP timing requirements and minimize
pattern-dependent jitter, the CAPC capacitor value is
determined by the laser-to-monitor transfer and other
variables. The following equations and table can be
used to choose the CAPC values for the MAX3735 and
MAX3735A, respectively. The equations and table
assume a DC-coupled laser. Refer to Maxim
Application Note HFDN 23.0: Choosing the APC Loop
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
______________________________________________________________________________________ 13
24kΩ
16kΩ
50Ω
50Ω
PACKAGE
0.97nH
0.97nH
0.11pF
0.11pF
VCC
VCC
MAX3735
MAX3735A
VCC
IN+
IN-
Figure 6. Simplified Input Structure
VCC
OUT-
OUT+
PACKAGE
0.99nH
MAX3735
MAX3735A 0.81nH
OUT+
0.99nH
0.11pF
K = 0.3
0.11pF
0.11pF
Figure 7. Simplified Output Structure
MAX3735/MAX3735A
Capacitors Used with MAX3735/MAX3735A SFP Module
Designs for more information on choosing CAPC for DC-
and AC-coupled laser interfaces.
MAX3735
Use the following equation to find the CAPC value when
using the MAX3735:
CAPC = 4.04 ×10-9 ×t_on ×η×ρMON (29.3 + 20.6 ITH -
0.22 ITH2) ×(1947 + 833 IMOD - 7.78 IMOD2+ 0.103
IMOD3)
where units are:
CAPC in µF, ITH, and IMOD in mA and tON in µs. CMD can
then be chosen as approximately 20x smaller than CAPC
for the MAX3735.
MAX3735A
Use Table 4 to choose CAPC when using the MAX3735A.
CAPC should be chosen according to the highest gain of
the lasers (generally at cold temperature). CAPC selec-
tion assumes a 34% reduction in the gain of the lasers at
+85°C from the cold (-40°C) values.
Table 4. MAX3735A CAPC Selection
where Gain = IMD/(IBIAS - ITH + 0.5 x IMOD) for DC-cou-
pled lasers. CMD can then be chosen as approximately
4x to 20x smaller than CAPC for the MAX3735A
Using the MAX3735/MAX3735A
with Digital Potentiometers
For more information on using the MAX3735/MAX3735A
with the Dallas DS1858/DS1859 SFP controller, refer to
Maxim Application Note HFAN 2.3.3: Optimizing the
Resolution of Laser Driver Setting Using Linear Digital
Potentiometers for more information.
Modulation Currents Exceeding 60mA
For applications requiring a modulation current greater
than 60mA, headroom is insufficient for proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3735/MAX3735A’s modula-
tion output can be AC-coupled to the cathode of a laser
diode. An external pullup inductor is necessary to DC-
bias the modulation output at VCC. Such a configuration
isolates laser forward voltage from the output circuitry
and allows the output at OUT+ to swing above and
below the supply voltage (VCC). When AC-coupled, the
MAX3735/MAX3735A modulation current can be pro-
grammed from 10mA to 85mA. Refer to Maxim
Application Note HFAN 02.0: Interfacing Maxim’s Laser
Drivers to Laser Diodes for more information on AC-
coupling laser drivers to laser diodes.
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3735/MAX3735A laser driver. If dice
are used, replace package parasitic elements with
bondwire parasitic elements.
Wire Bonding Die
The MAX3735 uses gold metalization with a thickness of
5µm (typ). Maxim characterized this circuit with gold wire
ball bonding (1-mil diameter wire). Die-pad size is 94 mil
(2388µm) square, and die thickness is 15 mil (381µm).
Refer to Maxim Application Note HFAN-08.0.1:
Understanding Bonding Coordinates and Physical Die
Size for additional information.
Layout Considerations
To minimize inductance, keep the connections between
the MAX3735 output pins and laser diode as close as
possible. Optimize the laser diode performance by
placing a bypass capacitor as close as possible to the
laser anode. Use good high-frequency layout tech-
niques and multiple-layer boards with uninterrupted
ground planes to minimize EMI and crosstalk.
Exposed-Pad Package
The exposed pad on the 24-pin QFN provides a very
low thermal resistance path for heat removal from the IC.
The pad is also electrical ground on the MAX3735/
MAX3735A 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
Using the MAX3735/MAX3735A laser driver alone does
not ensure that a transmitter design is compliant with IEC
825. The entire transmitter circuit and component selec-
tions must be considered. Each user must determine the
level of fault tolerance required by the application, recog-
nizing that Maxim products are neither designed nor
authorized for use as components in systems intended
for surgical implant into the body, for applications intend-
ed to support or sustain life, or for any other application
in which the failure of a Maxim product could create a
situation where personal injury or death may occur.
2.7Gbps, Low-Power SFP Laser Drivers
14 ______________________________________________________________________________________
LASER GAIN (A/A) CAPC (µF)
0.005 0.039
0.007 0.047
0.010 0.068
0.020 0.100
0.030 0.120
0.040 0.120
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
______________________________________________________________________________________ 15
0.079"
(2.007mm)
VCC
15
OUT-16
OUT-17
OUT+18
OUT+19
VCC
20
BIAS21
6
0.060"
(1.524mm)
VCC
5IN+
4IN-
3VCC
2PC_MON
1BC_MON
INDEX
PAD
GND
14
MD
13
APCFILT1
12
APCFILT2
11
GND
10
APCSET
9
TX_DISABLE
8
GND
7
GND
22
GND
23
MODSET
24
TX_FAULT
25
SHUTDOWN
26
VCC
27
GND
28
GND
29
MAX3735 Chip Topography
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
16 ______________________________________________________________________________________
Bonding Coordinates
*Index pad. Orient the die with this pad in the lower-left corner.
**Bond out both pairs of OUT- and OUT+ to minimize series
inductance.
Chip Information
TRANSISTOR COUNT: 327
SUBSTRATE CONNECTED TO GND
DIE SIZE: 60 mils x 79 mils
PROCESS: SiGe Bipolar
Table 5. MAX3735 Bondpad Locations
COORDINATES
PAD NAME XY
1* BC_MON 47 47
2PC_MON 47 229
3V
CC 47 514
4IN- 47 696
5IN+ 47 878
6V
CC 47 1063
7GND 242 1149
8TX_DISABLE 452 1149
9APCSET 636 1149
10 GND 819 1149
11 APCFILT2 1008 1149
12 APCFILT1 1193 1149
13 MD 1383 1149
14 GND 1567 1149
15 VCC 1758 1032
16** OUT- 1758 888
17** OUT- 1758 742
18** OUT+ 1758 579
19** OUT+ 1758 433
20 VCC 1758 289
21 BIAS 1758 93
22 GND 1578 -64
23 GND 1401 -64
24 MODSET 1205 -64
25 TX_FAULT 1016 -64
26 SHUTDOWN 818 -64
27 VCC 623 -64
28 GND 435 -64
29 GND 245 -64
22
MAX3735
MAX3735A
2324 21 20 19
789
10 11 12
1
2
3
4
5
6
18
17
16
15
14
13
VCC
Thin QFN*
(4mm x 4mm)
TOP VIEW
IN+
IN-
VCC
PC_MON
BC_MON
VCC
OUT-
OUT+
OUT+
VCC
BIAS
GND
VCC
SHUTDOWN
TX_FAULT
MODSET
GND
TX_DISABLE
APCSET
GND
APCFILT2
APCFILT1
MD
*THE EXPOSED PAD MUST BE CONNECTED TO CIRCUIT BOARD GROUND FOR PROPER
THERMAL AND ELECTRICAL PERFORMANCE.
Pin Configuration
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
______________________________________________________________________________________ 17
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
12,16,20, 24L QFN.EPS
E
1
2
21-0106
PACKAGE OUTLINE
12,16,20,24L QFN, 4x4x0.90 MM
E
2
2
21-0106
PACKAGE OUTLINE
12,16,20,24L QFN, 4x4x0.90 MM
MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
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.
18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
C
1
2
21-0139
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
C
2
2
21-0139
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
