General Description
The MAX3737 is a 3.3V laser driver designed for
multirate transceiver modules with data rates from
155Mbps to 2.7Gbps. Lasers can be DC-coupled to the
MAX3737 for reduced component count and ease of
multirate operation.
Laser extinction ratio control (ERC) combines the features
of automatic power control (APC), modulation compensa-
tion, and built-in thermal compensation. The APC loop
maintains constant average optical power. Modulation
compensation increases the modulation current in pro-
portion to the bias current. These control loops combined
with thermal compensation maintain a constant optical
extinction ratio over temperature and lifetime.
The MAX3737 accepts differential data input signals.
The wide 5mA to 60mA (up to 85mA AC-coupled) mod-
ulation current range and up to 100mA bias current
range makes the MAX3737 ideal for driving FP/DFB
lasers in fiber-optic modules. External resistors set the
required laser current levels. The MAX3737 provides
transmit disable control (TX_DISABLE), single-point
fault tolerance, bias-current monitoring, modulation-cur-
rent monitoring, and photocurrent monitoring. The
device also offers a latched failure output (TX_FAULT)
to indicate faults, such as when the APC loop is no
longer able to maintain the average optical power at the
required level. The MAX3737 is compliant with the SFF-
8472 transmitter diagnostic and SFP MSA timing
requirements.
The MAX3737 is offered in a 5mm x 5mm 32-pin thin QFN
and QFN package and operates over the -40°C to +85°C
extended temperature range.
Applications
Multirate OC-3 to OC-48 FEC Transceivers
Gigabit Ethernet SFF/SFP and GBIC
Transceivers
1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC
Transceivers
Features
♦Single 3.3V Power Supply
♦47mA Power-Supply Current
♦85mA Modulation Current
♦100mA Bias Current
♦Automatic Power Control (APC)
♦Modulation Compensation
♦On-Chip Temperature Compensation
♦Self-Biased Inputs for AC-Coupling
♦Ground-Referenced Current Monitors
♦Laser Safety, Shutdown, and Alarm Outputs
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
________________________________________________________________
Maxim Integrated Products
1
32
31
30
29
28
27
26
MODTCOMP
TH_TEMP
MODBCOMP
MODSET
APCSET
APCFILT2
APCFILT1
25 VMD
9
10
11
12
13
14
15
MC_MON
GND
VCC
TX_FAULT
SHUTDOWN
VBS
GND
16GND
17
18
19
20
21
22
23
BIAS
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND
TO ACHIEVE SPECIFIED PERFORMANCE.
VCC
OUT-
OUT-
OUT+
OUT+
VCC
8
7
6
5
4
3
2
BC_MON *EP
PC_MON
VCC
IN-
IN+
VCC
TX_DISABLE
MAX3737EGJ
1GND 24 MD
TOP VIEW
5mm x 5mm
QFN
Pin Configurations
Ordering Information
19-2818; Rev 3; 6/11
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART TEMP RANGE PIN-PACKAGE
MAX3737ETJ -40°C to +85°C 32 Thin QFN-EP*
MAX3737ETJ+ -40°C to +85°C 32 Thin QFN-EP*
MAX3737EGJ -40°C to +85°C 32 QFN-EP*
Functional Diagram and Typical Application Circuit appear
at end of data sheet.
Pin Configurations continued at end of data sheet.
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = 2.97V to 3.63V, TA = -40°C to +85°C. Typical values are at VCC = 3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25°C, unless other-
wise noted.) (Notes 1, 2)
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
IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN,
MC_MON, BC_MON, PC_MON, VBS, VMD,
APCFILT1, APCFILT2, MD, TH_TEMP,
MODTCOMP, MODBCOMP, MODSET, and
APCSET Voltage .......................................-0.5V to VCC + 0.5V
OUT+, OUT-, BIAS Current.............................-20mA to +150mA
Continuous Power Dissipation (TA= +85°C)
QFN/TQFN (derate 21.2mW/°C above +85°C) ................1.3W
Operating Junction Temperature Range...........-55°C to +150°C
Storage Temperature Range .............................-55°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow)
Lead(Pb)-free...............................................................+260°C
Containing lead(Pb) .....................................................+240°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
POWER SUPPLY
Supply Current ICC (Note 3) 47 60 mA
Power-Supply Noise Rejection PSNR f 1MHz, 100mVP-P (Notes 4, 6) 33 dB
I/O SPECIFICATIONS
Differential Input Swing VID DC-coupled, Figure 1 0.2 2.4 VP-P
Common-Mode Input VCM 1.7 VCC -
VID/4 V
LASER BIAS
Bias-Current Setting Range 1 100 mA
Bias Off Current TX_DISABLE = high 0.1 mA
Bias-Current Monitor Ratio IBIAS/IBC_MON 62 76 90 mA/mA
LASER MODULATION
Modulation-Current Setting
Range IMOD (Note 5) 5 85 mA
5mA IMOD 10mA 71 80
Output Edge Speed 20% to 80%
(Notes 6, 7) 10mA < IMOD 85mA 52 80 ps
Output Overshoot/Undershoot (Note 7) ±6 %
Random Jitter (Notes 6, 7) 0.65 1.3 ps
5mA IMOD 10mA 25.6 40
2.7Gbps 10mA < IMOD 85mA 16 40
5mA IMOD 10mA 32 41
1.25Gbps 10mA < IMOD 85mA 15 41
5mA IMOD 10mA 39 46
622Mbps 10mA < IMOD 85mA 21 46
5mA IMOD 10mA 65 100
Deterministic Jitter (Notes 6, 8)
155Mbps 10mA < IMOD 85mA 46 70
psP-P
Modulation-Current Temperature
Stability (Note 6) ±150 ±480 ppm/°C
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 2.97V to 3.63V, TA = -40°C to +85°C. Typical values are at VCC = 3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25°C, unless other-
wise noted.) (Notes 1, 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
5mA IMOD 10mA ±20
Modulation-Current Setting Error 15 load,
TA = +25°C 10mA < IMOD 85mA ±15 %
Modulation Off Current TX_DISABLE = high 0.1 mA
Modulation-Current Monitor Ratio IMOD/IMC_MON 223 262 302 mA/mA
EXTINCTION RATIO CONTROLS
Monitor-Diode Input Current Range IMD Average current into the MD pin 18 1500 μA
MD Pin Voltage 1.4 V
MD-Current Monitor Ratio IMD/IPC_MON 0.85 1.0 1.15 mA/mA
APC Loop Time Constant CAPC_FILT = 0.01μF, IMD/IBIAS = 1/70 3.3 μs
APC Setting Stability ±100 ±480 ppm/°C
APC Setting Accuracy TA = +25°C ±15 %
IMOD Compensation Setting
Range by Bias K K = IMOD/IBIAS 0 1.5 mA/mA
IMOD Compensation Setting
Range by Temperature TC TC = IMOD/ (Note 6) 0 1.0 mA/°C
Threshold Setting Range for
Temperature Compensation TTH (Note 6) 10 60 °C
LASER SAFETY AND CONTROL
Bias and Modulation Turn-Off
Delay
CAPC_FILT = 0.01μF, IMD/IBIAS = 1/80
(Note 6) 5 μs
Bias and Modulation Turn-On
Delay
CAPC_FILT = 0.01μF, IMD/IBIAS = 1/80
(Note 6) 600 μs
Threshold Voltage at Monitor Pins VREF Figure 5 1.14 1.3 1.39 V
INTERFACE SIGNALS
TX_DISABLE Input High VHI 2.0 V
TX_DISABLE Input Low VLO R
PULL = 7.5k 0.8 V
VHI = VCC 15
TX_DISABLE Input Current VLO = GND -450 -800 μA
TX_FAULT Output Low Sinking 1mA, open collector 0.4 V
Shutdown Output High Sourcing 100μA VCC -
0.4 V
Shutdown Output Low Sinking 100μA 0.4 V
Note 1: AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent test pattern.
Note 2: Specifications at -40°C are guaranteed by design and characterization.
Note 3: Excluding IBIAS and IMOD. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
Note 4: Power-supply noise rejection (PSNR) = 20log10(Vnoise (on VCC)/ΔVOUT). VOUT is the voltage across the 15Ωload when IN+ is high.
Note 5: The minimum required voltage at the OUT+ and OUT- pins is +0.75V.
Note 6: Guaranteed by design and characterization.
Note 7: Tested with 00001111 pattern at 2.7Gbps.
Note 8: DJ includes pulse-width distortion (PWD).
_______________________________________________________________________________________
3
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.)
OPTICAL EYE DIAGRAM
(2.7Gbps, 27 - 1PRBS, 2.3GHz FILTER)
MAX3737 toc01
54ps/div
1310nm FP LASER
Er = 8.2dB
OPTICAL EYE DIAGRAM
(1.25Gbps, 27 - 1PRBS, 940MHz FILTER)
MAX3737 toc02
116ps/div
1310nm FP LASER
Er = 8.2dB
OPTICAL EYE DIAGRAM
(155Mbps, 27 - 1PRBS, 117MHz FILTER, CAPC = 0.1μF)
MAX3737 toc03
920ps/div
ELECTRICAL EYE DIAGRAM
(IMOD = 30mA, 2.7Gbps, 27 - 1PRBS)
MAX3737 toc04
52ps/div
75mV/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE
(EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3737 toc05
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
60
40
20
0-20
40
45
50
55
60
65
35
-40 80
VCC = 3.63V
VCC = 2.97V
VCC = 3.3V
IMOD = 60mA
IBIAS = 60mA
BIAS-CURRENT MONITOR GAIN
vs. TEMPERATURE
MAX3737 toc06
TEMPERATURE (°C)
IBIAS/IBC_MON (mA/mA)
603510-15
72
74
76
78
80
82
84
86
88
90
70
-40 85
PHOTO-CURRENT MONITOR GAIN
vs. TEMPERATURE
MAX3737 toc07
TEMPERATURE (°C)
IMD/IPC_MON (mA/mA)
6035-15 10
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
0.80
-40 85
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
200
230
220
210
240
250
260
270
280
290
300
-40 10-15 35 60 85
MODULATION-CURRENT MONITOR GAIN
vs. TEMPERATURE
MAX3737 toc08
TEMPERATURE (°C)
IMOD/IMC_MON (mA/mA)
90
0
1 10 100
MODULATION CURRENT
vs. RMODSET
20
10
MAX3737 toc09
RMODSET (kΩ)
IMOD (mA)
40
30
60
70
50
80
0
15
10
5
20
25
30
35
40
45
50
02010 30 5040 70 8060 90
DETERMINISTIC JITTER
vs. MODULATION CURRENT
MAX3737 toc11
IMOD (mA)
DJ (psP-P)
0
0.5
1.0
1.5
2.0
0 20406080100
RANDOM JITTER
vs. MODULATION CURRENT
MAX3737 toc12
IMOD (mA)
RJ (psRMS)
10
0.01
0 0.1 100
COMPENSATION (K)
vs. RMODBCOMP
0.1
1
MAX3737 toc13
RMODBCOMP (kΩ)
K (mA/mA)
110
30
50
40
60
70
80
90
100
-20 200 40 60 80 100
TEMPERATURE COMPENSATION
vs. RTH_TEMP (RMODTCOMP = 500Ω)
MAX3737 toc14
TEMPERATURE (°C)
IMOD (mA)
RTH_TEMP = 12kΩ
RTH_TEMP = 7kΩ
RTH_TEMP = 4kΩ
RTH_TEMP = 2kΩ
Typical Operating Characteristics (continued)
(VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.)
1.6
0
0.1 10 100
PHOTODIODE CURRENT
vs. RAPCSET
0.4
0.2
0.6
0.8
1.0
1.2
1.4
MAX3737 toc10
RAPCSET (kΩ)
IMD (mA)
1
_______________________________________________________________________________________
5
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.)
30
32
34
36
38
40
42
44
-20 200 406080100
TEMPERATURE COMPENSATION
vs. RTH_TEMP (RMODTCOMP = 10kΩ)
MAX3737 toc15
TEMPERATURE (°C)
IMOD (mA)
RTH_TEMP = 12kΩ
RTH_TEMP = 7kΩ
RTH_TEMP = 4kΩ
RTH_TEMP = 2kΩ
20ms/div
HOT PLUG WITH TX_DISABLE LOW
VCC
FAULT
MAX3737 toc16
TX_DISABLE
LASER
OUTPUT
t_init = 60ms
3.3V
0V
LOW
LOW
20μs/div
TRANSMITTER ENABLE
VCC
FAULT
MAX3737 toc17
TX_DISABLE
LASER
OUTPUT
t_on = 75μs
3.3V
LOW
HIGH
LOW
40ns/div
TRANSMITTER DISABLE
VCC
FAULT
MAX3737 toc18
TX_DISABLE
LASER
OUTPUT
3.3V
LOW
LOW t_off = 134ns HIGH
1μs/div
RESPONSE TO FAULT
VPC_MON
FAULT
MAX3737 toc19
TX_DISABLE
LASER
OUTPUT
t_fault = 0.9μs
HIGH
EXTERNALLY
FORCED FAULT
LOW
LOW
100ms/div
FAULT RECOVERY TIME
VPC_MON
FAULT
MAX3737 toc20
TX_DISABLE
LASER
OUTPUT
t_init = 68ms
LOW
EXTERNAL FAULT
REMOVED
LOW
HIGH
HIGH
LOW
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
Pin Description
PIN NAME FUNCTION
1, 10, 15, 16 GND Ground
2 TX_DISABLE
Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is ass erted high or left
unconnected. The laser output is enabled when this pin is asserted low.
3, 6, 11, 18, 23 VCC 3.3V Supply Voltage
4 IN+ Noninverted Data Input
5 IN- Inverted Data Input
7 PC_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.
8 BC_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.
9 MC_MON
Modulation-Current Monitor Output. Current out of this pin develops a ground-referenced voltage
across an external resistor that is proportional to the modulation current amplitude.
12 TX_FAULT Open-Collector Transmit Fault Indicator (Table 1)
13 SHUTDOWN
Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown
circuitry.
14 VBS
Bias Voltage Sense. Isolated tap (3k ±15%) on the bias output reduces component count when
a precision bias sense resistor is used.
17 BIAS Laser Bias-Current Output
19, 20 OUT- Inverted Modulation-Current Output (Connect Pins 19 and 20 Together). IMOD flows into this pin
when input data is low.
21, 22 OUT+ Noninverted Modulation-Current Output (Connect Pins 21 and 22 Together). IMOD flows into this
pin when input data is high.
24 MD
Monitor Photodiode 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.
25 VMD
Monitor Photodiode Voltage Sense. Isolated tap (3k ±15%) on the MD input reduces
component count when a precision photodiode current-sense resistor is used.
26 APCFILT1
Connect a capacitor (CAPC) between pin 26 (APCFILT1) and pin 27 (APCFILT2) to set the
dominant pole of the APC feedback loop.
27 APCFILT2 (See Pin 26.) The maximum capacitance allowed on this pin is 10pF.
28 APCSET
A resistor connected from this pin to ground sets the desired average optical power. The
maximum capacitance allowed on this pin is 10pF.
29 MODSET
A resistor connected from this pin to ground sets the desired constant portion of the modulation
current.
30 MODBCOMP
Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.
Mirrors IBIAS through an external resistor. Leave open for zero coupling.
31 TH_TEMP
Threshold for Temperature Compensation. A resistor at this pin programs the temperature, above
which compensation is added to the modulation current.
32 MODTCOMP
Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature
coefficient of the modulation current when above the threshold temperature. Leave open for zero
temperature compensation.
— EP
Exposed Pad. Solder the exposed pad to the circuit board ground for specified thermal and
electrical performance.
_______________________________________________________________________________________
7
MAX3737
Detailed Description
The MAX3737 laser driver consists of three main parts: a
high-speed modulation driver, biasing block with ERC,
and safety circuitry. The circuit design is optimized for
high-speed, low-voltage (3.3V) operation (Figure 4).
High-Speed Modulation Driver
The output stage is composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3737 is optimized for driving a 15Ω
load. The minimum instantaneous voltage required at
OUT+ is 0.7V for modulation current up to 60mA and
0.75V for currents from 60mA to 85mA. Operation
above 60mA can be accomplished by AC-coupling or
with sufficient voltage at the laser to meet the driver
output voltage requirement.
To interface with the laser diode, a damping resistor (RD)
is required. The combined resistance due to the series
damping resistor and the equivalent series resistance
(ESR) of the laser diode should equal 15Ω. To further
damp aberrations caused by laser diode parasitic induc-
tance, an RC shunt network may be necessary. Refer to
Application Note 274:
HFAN-02.0: Interfacing Maxim
Laser Drivers with Laser Diodes
for more information.
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.
Extinction Ratio Control
The extinction ratio (re) is the laser on-state power
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
re = (2PAVG + PP-P) / (2PAVG - PP-P)
Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compen-
sating the modulation current for reduced slope effi-
ciency (η) of the laser over time and temperature:
PP-P = ηx IMOD
PI
AVG MD
MON
=ρ
Multirate Laser Driver with Extinction
Ratio Control
8 _______________________________________________________________________________________
100mV (MIN)
1200mV (MAX)
200mVP-P (MIN),
2400mV (MAX)
IMOD
TIME
SINGLE ENDED
DIFFERENTIAL
VOLTAGE
CURRENT
VIN+
VIN-
IOUT+
(VIN+) - (VIN-)
Figure 1. Required Input Signal and Output Polarity
Z0 = 30Ω
30Ω
0.5pF
30Ω
30Ω
OSCILLOSCOPE
OUT-
OUT-
VCC
IOUT+
VCC
MAX3737
Z0 = 30Ω
75Ω
Z0 = 50Ω
50Ω
OUT+
OUT+
Figure 2. Test Circuit for Characterization
VOLTAGE
SUPPLY
C1
0.1μF
C3
0.1μF
C2
10μF
L1
1μHOPTIONAL
TO LASER
DRIVER VCC
OPTIONAL
SOURCE
NOISE
HOST BOARD
FILTER DEFINED BY SFP MSA
MODULE
Figure 3. Supply Filter
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) need-
ed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note 1119:
HFAN-02.2.1: Maximizing the
Extinction Ratio of Optical Transmitters Using K-Factor
Control
for details:
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power. Slope
efficiency decreases more rapidly as temperature
increases. The MAX3737 provides additional tempera-
ture compensation as temperature increases past a
user-defined threshold (TTH).
Safety Circuitry
The safety circuitry contains a disable, input (TX_DIS-
ABLE), a latched fault output (TX_FAULT), and fault
detectors (Figure 5). This circuitry monitors the opera-
tion of the laser driver and forces a shutdown if a fault
is detected (Table 1). The TX_FAULT pin should be
pulled high with a 4.7kΩto 10kΩresistor to VCC as
required by the SFP MSA. 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 as shown in the
Typical Operating Circuit
. This
shutdown transistor prevents a single-point fault at the
laser from creating an unsafe condition.
KI
I
MOD
BIAS
=Δ
Δ
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
MAX3737
DATA
PATH
X1/2
X1
RPULL = 7.5kΩ
PC_MON
IN-
SHUTDOWN
TX_FAULT
TX_DISABLE
IN+
IMD
1
VCC
VCC
INPUT BUFFER
IMOD ENABLE
IMOD
OUT-
SHUTDOWN
OUT+
BIAS
RD
IBIAS ENABLE
SAFETY LOGIC
AND POWER
DETECTOR
BC_MON
IBIAS
82
MC_MON
IMOD
268
IBIAS
IMD CMD
IBIAS
IAPCSET
APCSET
MD
RAPCSET
VCC
VBG
RTH_TEMP
TH_TEMP MODTCOMP MODBCOMP APCFILT2MODSET APCFILT1
T > TH
VBG
xTC
RMODTCOMP RMODSET
RMDMON
RBC_MON
RMC_MON
RMODBCOMP
x268 xK
CAPC
T
Figure 4. Functional Diagram
_______________________________________________________________________________________
9
MAX3737
Safety Circuitry Current Monitors
The MAX3737 features monitors (MC_MON, BC_MON,
PC_MON) for modulation current (IMOD), bias current
(IBIAS), and photocurrent (IMD). The monitors are realized
by mirroring a fraction of the currents and developing volt-
ages across external resistors connected to ground.
Voltages greater than VREF at MC_MON, PC_MON, or
BC_MON result in a fault state. For example, connecting a
Multirate Laser Driver with Extinction
Ratio Control
10 ______________________________________________________________________________________
1If 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 photocurrent exceeds the programming threshold.
4No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
Table 1. Typical Fault Conditions
PIN CIRCUIT RESPONSE TO OVERVOLTAGE
OR SHORT TO VCC
CIRCUIT RESPONSE TO UNDERVOLTAGE
OR SHORT TO GROUND
TX_FAULT Does not affect laser power. Does not effect 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 This 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. 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,
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.
MC_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.
Table 2. Circuit Responses to Various Single-Point Faults
*
A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
100Ωresistor to ground at each monitor output gives the
following relationships:
VMC_MON = (IMOD / 268) ✕100Ω
VBC_MON = (IBIAS / 82) ✕100Ω
VPC_MON = IMD ✕100Ω
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
isolation resistors are included to reduce the number of
components needed to implement this function.
Design Procedure
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 3 gives relationships that are
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 opti-
cal waveform are 50%.
For a desired laser average optical power (PAVG) and
optical extinction ratio (re), the required bias and modula-
tion currents can be calculated using the equations in
Table 3. Proper setting of these currents requires knowl-
edge of the laser to monitor transfer (ρMON) and slope
efficiency (η).
Programming the Monitor Diode Current
Set Point
The MAX3737 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
The APCSET pin controls the set point for the monitor-
diode current. An internal current regulator establishes
the APCSET current in the same manner as the MODSET
pin. See the IMD vs. RAPCSET graph in the
Typical
Operating Characteristics
and select the value of RAPC-
SET that corresponds to the required current at +25°C:
The laser driver automatically adjusts the bias to maintain
the constant average power. For DC-coupled laser diodes:
Programming the Modulation Current with
Compensation
Determine the modulation current from the laser slope
efficiency:
The modulation current of the MAX3737 consists of a
static modulation current (IMODS), a current proportional
to IBIAS, and a current proportional to temperature. The
portion of IMOD set by MODSET is established by an
internal current regulator, which maintains the reference
voltage of VREF across the external programming resis-
tor. See to the IMOD vs. RMODSET graph in the
Typical
Operating Characteristics
and select the value of RMOD-
SET that corresponds to the required current at +25°C:
IPr
r
MOD AVG e
e
=× × +
21
1
η
-
III
AVG BIAS MOD
=+
2
IV
R
MD
REF
APCSET
=×
1
2
PI
AVG MD
MON
=ρ
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
______________________________________________________________________________________
11
PARAMETER SYMBOL RELATION
Average power PAVG PAVG = (P0 + P1) / 2
Extinction ratio rere = P1 / P0
Optical power of a 1 P1P1 = 2PAVG re / (re + 1)
Optical power of a zero 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
Threshold current ITH P0 at I ≥ ITH
Bias current (AC-coupled) IBIAS IBIAS ≥ ITH + IMOD / 2
Laser to monitor transfer ρMON IMD / PAVG
Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark density.
MAX3737
An external resistor at the MODBCOMP pin sets current
proportional to IBIAS. Open circuiting the MODBCOMP
pin can turn off the interaction between IBIAS and IMOD:
If IMOD must be increased from IMOD1 to IMOD2 to
maintain the extinction ratio at elevated temperature,
the required compensation factor is:
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
The temperature coefficient of thermal compensation
above TTH is set by RMODTCOMP. Leaving the MODT-
COMP pin open disables additional thermal compensation:
Current Compliance (IMOD
≤≤
60mA),
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
For:
VDIODE—Diode bias point voltage (1.2V typ)
RL—Diode bias point resistance (5Ωtyp)
RD—Series matching resistor (20Ωtyp)
For compliance:
Current Compliance (IMOD > 60mA),
AC-Coupled
For applications requiring 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 MAX3737’s modulation 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
MAX3737 modulation current can be programmed up
to 85mA. Refer to Application Note 274:
HFAN-02.0:
Interfacing Maxim Laser Drivers with Laser Diodes
for
more information on AC-coupling laser drivers to laser
diodes.
For compliance:
Determine CAPC
The APC loop filter capacitor CAPC must be selected to
balance the requirements for fast turn-on and minimal
interaction with low frequencies in the data pattern. The
low-frequency cutoff is:
High-frequency noise can be filtered with an additional
cap CMD from the MD pin to ground:
The MAX3737 is designed so that turn-on time is faster
than 1ms for most laser gain values (η ✕ ρMON).
Choosing a smaller value of CAPC reduces turn-on
time. Careful balance between turn-on time and low-fre-
quency cutoff may be needed at low data rates for
some values of laser gain.
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3737 laser driver. If dice are used,
replace package parasitic elements with bondwire par-
asitic elements.
CC
MD APC
≈4
CF
f kHz
APC DB MON
() ()
( )
.
μηρ≈××
68
3
11
VV
IRR V
OUT C
MOD
DL+=×+
()
≥
C
-
2075 .
VVV I RRI R V
OUT CC DIODE MOD D L BIAS L+=×+
()
×≥
- - - .07
TC Rk
mA
C
MODTCOMP
=+°
±
1
05 10
. () %
Ω
TC M
kR C
TH TH TEMP
=°+ +°± .
. %
_
-70 145
92 10
Ω
Ω
KII
II
MOD MOD
BIAS BIAS
=21
21
-
-
KRMODBCOMP
=+±
1700
1000 10%
ITCTTTT
ITT
MODT TH TH
MODT TH
=×
()
>
=≤
|
|
-
0
IV
R
MODS
REF
MODSET
=×268
II KII
MOD MODS BIAS MODT
=+×+
Multirate Laser Driver with Extinction
Ratio Control
12 ______________________________________________________________________________________
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3737 output and the laser diode as
short as possible. Use good high-frequency layout
techniques and multilayer boards with uninterrupted
ground plane to minimize EMI and crosstalk. Circuit
boards should be made using low-loss dielectrics. Use
controlled-impedance lines for data inputs, as well as
the module output.
Laser Safety and IEC 825
Using the MAX3737 laser driver alone does not ensure
that a transmitter design is IEC 825 compliant. The entire
transmitter circuit and component selections must be
considered. Each customer must determine the level of
fault tolerance required by their application, recognizing
that Maxim products are not designed or authorized for
use as components in systems intended for surgical
implant into the body, for applications intended to support
or sustain life, or for any other application where the fail-
ure of a Maxim product could create a situation where
personal injury or death may occur.
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
______________________________________________________________________________________
13
PC_MON
BC_MON
MC_MON
RPC_MON
RBC_MON
RMC_MON
VCC
VCC MAX3737
COMP
RQ
S
RS
LATCH CMOS
SHUTDOWN
TX_FAULT
TX_DISABLE
TTL
OPEN
COLLECTOR
COMP
COMP
EXCESSIVE
MODULATION
CURRENT
VCC
IMD
1
VCC
VREF
VREF
VREF
IBIAS
82
IMOD
268
IMOD
ENABLE
IBIAS
ENABLE
COUNTER
60ms DELAY
100ns
DELAY
POR AND COUNTER
60ms DELAY
Figure 5. Simplified Safety Circuit
MAX3737
5kΩ
16kΩ
5kΩ
24kΩ
PACKAGE VCC
VCC
0.83nH
0.83nH
0.11pF
0.11pF
VCC
Figure 6. Simplified Input Structure
MAX3737
Exposed-Pad (EP) Package
The exposed-pad on the 32-pin QFN provides a very low
thermal resistance path for heat removal from the IC. The
pad is also electrical ground on the MAX3737 and should
be soldered to the circuit board ground for proper ther-
mal and electrical performance. Refer to Application Note
862: HFAN-08.1:
Thermal Considerations of QFN and
Other Exposed-Paddle Packages at
www.maxim-ic.com
for additional information.
Multirate Laser Driver with Extinction
Ratio Control
14 ______________________________________________________________________________________
MAX3737
VCC
PACKAGE
0.82nH
0.82nH
OUT-
OUT+
0.11pF
0.11pF
Figure 7. Simplified Output Structure
32
31
30
29
28
27
26
MODTCOMP
TH_TEMP
MODBCOMP
MODSET
APCSET
APCFILT2
APCFILT1
25 VMD
9
10
11
12
13
14
15
MC_MON
GND
VCC
TX_FAULT
SHUTDOWN
VBS
GND
16GND
17
18
19
20
21
22
23
BIAS
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND
TO ACHIEVE SPECIFIED PERFORMANCE.
VCC
OUT-
OUT-
OUT+
OUT+
VCC
8
7
6
5
4
3
2
BC_MON
PC_MON
VCC
IN-
IN+
VCC
TX_DISABLE
MAX3737ETJ
1GND 24 MD
TOP VIEW
5mm x 5mm
THIN QFN
*EP
Pin Configurations (continued)
Chip Information
PROCESS: SiGe/BIPOLAR
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
______________________________________________________________________________________
15
IN+
IN-
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE
V
CC
SHUTDOWN
3.3V
OPTIONAL SHUTDOWN
CIRCUITRY
+3.3V
15Ω
10Ω
OUT-
OUT+
BIAS
MD
BC_MON
MC_MON
APCFILT1
APCFILT2
GND
APCSET
MODSET
TX_DISABLE
TX_FAULT
3.3V
CMD
CDR
CAPC
0.01μF
0.1μF
0.1μF
FERRITE BEAD
PC_MON
R
MODSET
R
APCSET
R
MC_MON
R
BC_MON
R
PC_MON
MODBCOMP
MODTCOMP
TH_TEMP
RMODBCOMP
RMODTCOMP
RTH_TEMP
MAX3737
Typical Operating Circuit
Package Information
For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains
to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.
32 TQFN-EP T3255-3 21-0140 90-0001
32 QFN-EP G3255-1 21-0091 90-0279
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
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.
16
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
3 6/11
Added lead and soldering temperature information to the Absolute Maximum Ratings;
changed the Bias-Current Monitor Ratio parameter specs from 68mA/mA (min),
82mA/mA (typ), 95mA/mA (max) to 62mA/mA (min), 76mA/mA (typ), 90mA/mA (max)
in the Electrical Characteristics table; updated the APCFILT2 and APCSET pin
functions in the Pin Description table; added the Package Information table
2, 7, 15
