EVALUATION KIT Multirate Laser Driver with Extinction Ratio Control

General DescriptionThe MAX3737 is a 3.3V laser driver designed for multirate transceiver modules with data rates from155Mbps to 2.7Gbps. Lasers can be DC-coupled to theMAX3737 for reduced component count and ease ofmultirate operation.

Laser extinction ratio control (ERC) combines the featuresof automatic power control (APC), modulation compensa-tion, and built-in thermal compensation. The APC loopmaintains constant average optical power. Modulationcompensation increases the modulation current in pro-portion to the bias current. These control loops combinedwith thermal compensation maintain a constant opticalextinction 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 currentrange makes the MAX3737 ideal for driving FP/DFBlasers in fiber-optic modules. External resistors set therequired laser current levels. The MAX3737 providestransmit disable control (TX_DISABLE), single-pointfault tolerance, bias-current monitoring, modulation-cur-rent monitoring, and photocurrent monitoring. Thedevice also offers a latched failure output (TX_FAULT)to indicate faults, such as when the APC loop is nolonger able to maintain the average optical power at therequired level. The MAX3737 is compliant with the SFF-8472 transmitter diagnostic and SFP MSA timingrequirements.

The MAX3737 is offered in a 5mm x 5mm 32-pin thin QFNand QFN package and operates over the -40°C to +85°Cextended temperature range.

ApplicationsMultirate OC-3 to OC-48 FEC Transceivers

Gigabit Ethernet SFF/SFP and GBICTransceivers

1Gbps/2Gbps Fibre Channel SFF/SFP and GBICTransceivers

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

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Multirate Laser Driver with Extinction Ratio Control

________________________________________________________________ Maxim Integrated Products 1

32 31 30 29 28 27 26

MOD

TCOM

P

TH_T

EMP

MOD

BCOM

P

MOD

SET

APCS

ET

APCF

ILT2

APCF

ILT1

25VM

D

9 10 11 12 13 14 15

MC_

MON GN

D

V CC

TX_F

AULT

SHUT

DOW

N

VBS

GND

16GN

D

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 5mmQFN

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 appearat end of data sheet.

Pin Configurations continued at end of data sheet.

+Denotes a lead(Pb)-free/RoHS-compliant package.*EP = Exposed pad.

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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 functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.

Supply Voltage VCC...............................................-0.5V to +6.0VIN+, 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°CStorage Temperature Range .............................-55°C to +150°CLead Temperature (soldering, 10s) .................................+300°CSoldering Temperature (reflow)

Lead(Pb)-free...............................................................+260°CContaining 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 80Output 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 25.6 402.7Gbps

10mA < IMOD 85mA 16 40

5mA IMOD 10mA 32 411.25Gbps

10mA < IMOD 85mA 15 41

5mA IMOD 10mA 39 46622Mbps

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

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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 ±20Modulation-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 RPULL = 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 15Ω load 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).

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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)

MAX

3737

toc0

1

54ps/div

1310nm FP LASEREr = 8.2dB

OPTICAL EYE DIAGRAM(1.25Gbps, 27 - 1PRBS, 940MHz FILTER)

MAX

3737

toc0

2

116ps/div

1310nm FP LASEREr = 8.2dB

OPTICAL EYE DIAGRAM(155Mbps, 27 - 1PRBS, 117MHz FILTER, CAPC = 0.1μF)

MAX

3737

toc0

3

920ps/div

ELECTRICAL EYE DIAGRAM(IMOD = 30mA, 2.7Gbps, 27 - 1PRBS)

MAX

3737

toc0

4

52ps/div

75mV/div

SUPPLY CURRENT (ICC) vs. TEMPERATURE(EXCLUDES BIAS AND MODULATION CURRENTS)

MAX

3737

toc0

5

TEMPERATURE (°C)

SUPP

LY C

URRE

NT (m

A)

6040200-20

40

45

50

55

60

65

35-40 80

VCC = 3.63V

VCC = 2.97V

VCC = 3.3V

IMOD = 60mAIBIAS = 60mA

BIAS-CURRENT MONITOR GAINvs. TEMPERATURE

MAX

3737

toc0

6

TEMPERATURE (°C)

I BIA

S/I B

C_M

ON (m

A/m

A)

603510-15

72

74

76

78

80

82

84

86

88

90

70-40 85

PHOTO-CURRENT MONITOR GAINvs. TEMPERATURE

MAX

3737

toc0

7

TEMPERATURE (°C)

I MD/

I PC_

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

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200

230

220

210

240

250

260

270

280

290

300

-40 10-15 35 60 85

MODULATION-CURRENT MONITOR GAINvs. TEMPERATURE

MAX

3737

toc0

8

TEMPERATURE (°C)

I MOD

/I MC_

MON

(mA/

mA)

90

01 10 100

MODULATION CURRENTvs. RMODSET

20

10

MAX

3737

toc0

9

RMODSET (kΩ)

I MOD

(mA)

40

30

60

70

50

80

0

15

10

5

20

25

30

35

40

45

50

0 2010 30 5040 70 8060 90

DETERMINISTIC JITTERvs. MODULATION CURRENT

MAX

3737

toc1

1

IMOD (mA)

DJ (p

s P-P

)

0

0.5

1.0

1.5

2.0

0 20 40 60 80 100

RANDOM JITTERvs. MODULATION CURRENT

MAX

3737

toc1

2

IMOD (mA)

RJ (p

s RM

S)

10

0.010 0.1 100

COMPENSATION (K)vs. RMODBCOMP

0.1

1

MAX

3737

toc1

3

RMODBCOMP (kΩ)

K (m

A/m

A)

1 1030

50

40

60

70

80

90

100

-20 200 40 60 80 100

TEMPERATURE COMPENSATIONvs. RTH_TEMP (RMODTCOMP = 500Ω)

MAX

3737

toc1

4

TEMPERATURE (°C)

I MOD

(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

00.1 10 100

PHOTODIODE CURRENTvs. RAPCSET

0.4

0.2

0.6

0.8

1.0

1.2

1.4

MAX

3737

toc1

0

RAPCSET (kΩ)

I MD

(mA)

1

_______________________________________________________________________________________ 5

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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 40 60 80 100

TEMPERATURE COMPENSATIONvs. RTH_TEMP (RMODTCOMP = 10kΩ)

MAX

3737

toc1

5

TEMPERATURE (°C)

I MOD

(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

LASEROUTPUT

t_init = 60ms

3.3V

0V

LOW

LOW

20μs/div

TRANSMITTER ENABLE

VCC

FAULT

MAX3737 toc17

TX_DISABLE

LASEROUTPUT

t_on = 75μs

3.3V

LOW

HIGHLOW

40ns/div

TRANSMITTER DISABLE

VCC

FAULT

MAX3737 toc18

TX_DISABLE

LASEROUTPUT

3.3V

LOW

LOW t_off = 134ns HIGH

1μs/div

RESPONSE TO FAULT

VPC_MON

FAULT

MAX3737 toc19

TX_DISABLE

LASEROUTPUT

t_fault = 0.9μs

HIGH

EXTERNALLYFORCED FAULT

LOW

LOW

100ms/div

FAULT RECOVERY TIME

VPC_MON

FAULT

MAX3737 toc20

TX_DISABLE

LASEROUTPUT

t_init = 68ms

LOW

EXTERNAL FAULTREMOVED

LOW

HIGH

HIGH

LOW

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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 asserted 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.

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Detailed DescriptionThe MAX3737 laser driver consists of three main parts: ahigh-speed modulation driver, biasing block with ERC,and safety circuitry. The circuit design is optimized forhigh-speed, low-voltage (3.3V) operation (Figure 4).

High-Speed Modulation DriverThe output stage is composed of a high-speed differ-ential pair and a programmable modulation currentsource. The MAX3737 is optimized for driving a 15Ωload. The minimum instantaneous voltage required atOUT+ is 0.7V for modulation current up to 60mA and0.75V for currents from 60mA to 85mA. Operationabove 60mA can be accomplished by AC-coupling orwith sufficient voltage at the laser to meet the driveroutput voltage requirement.

To interface with the laser diode, a damping resistor (RD)is required. The combined resistance due to the seriesdamping resistor and the equivalent series resistance(ESR) of the laser diode should equal 15Ω. To furtherdamp aberrations caused by laser diode parasitic induc-tance, an RC shunt network may be necessary. Refer toApplication Note 274: HFAN-02.0: Interfacing MaximLaser Drivers with Laser Diodes for more information.

At data rates of 2.7Gbps, any capacitive load at thecathode of a laser diode degrades optical output perfor-mance. Because the BIAS output is directly connectedto the laser cathode, minimize the parasitic capacitanceassociated with the pin by using an inductor to isolatethe BIAS pin parasitics from the laser cathode.

Extinction Ratio ControlThe extinction ratio (re) is the laser on-state powerdivided by the off-state power. Extinction ratio remainsconstant if peak-to-peak and average power are heldconstant:

re = (2PAVG + PP-P) / (2PAVG - PP-P)

Average power is regulated using APC, which keepsconstant current from a photodiode coupled to thelaser. 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 = η x IMOD

PI

AVGMD

MON=

ρ


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

VOLTAGESUPPLY

C10.1μF

C30.1μF

C210μF

L11μH OPTIONAL

TO LASERDRIVER VCC

OPTIONAL

SOURCENOISE

HOST BOARDFILTER 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 biascurrent increases with temperature. Refer to MaximApplication Note 1119: HFAN-02.2.1: Maximizing theExtinction Ratio of Optical Transmitters Using K-FactorControl for details:

This provides a first-order approximation of the currentincrease needed to maintain peak-to-peak power. Slopeefficiency decreases more rapidly as temperatureincreases. The MAX3737 provides additional tempera-ture compensation as temperature increases past auser-defined threshold (TTH).

Safety CircuitryThe safety circuitry contains a disable, input (TX_DIS-ABLE), a latched fault output (TX_FAULT), and faultdetectors (Figure 5). This circuitry monitors the opera-tion of the laser driver and forces a shutdown if a faultis detected (Table 1). The TX_FAULT pin should bepulled high with a 4.7kΩ to 10kΩ resistor to VCC asrequired by the SFP MSA. A single-point fault can be ashort to VCC or GND. See Table 2 to view the circuitresponse to various single-point failures. The transmitfault condition is latched until reset by a toggle ofTX_DISABLE or VCC. The laser driver offers redundantlaser diode shutdown through the optional shutdowncircuitry as shown in the Typical Operating Circuit. Thisshutdown transistor prevents a single-point fault at thelaser from creating an unsafe condition.

KIIMOD

BIAS=

ΔΔ

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MAX3737

DATAPATH

X1/2

X1

RPULL = 7.5kΩ

PC_MON

IN-

SHUTDOWN

TX_FAULTTX_DISABLE

IN+

IMD1

VCC

VCC

INPUT BUFFER

IMOD ENABLE

IMOD

OUT-

SHUTDOWN

OUT+

BIAS

RD

IBIAS ENABLESAFETY LOGICAND POWERDETECTOR

BC_MON

IBIAS82

MC_MON

IMOD268

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

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Safety Circuitry Current MonitorsThe 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, orBC_MON result in a fault state. For example, connecting a


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 photocurrentexceed 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 theprogrammed threshold.

Table 1. Typical Fault Conditions

PINCIRCUIT RESPONSE TO OVERVOLTAGE

OR SHORT TO VCC

CIRCUIT RESPONSE TO UNDERVOLTAGEOR 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 occursif VPC_MON exceeds the threshold. The APC loopresponds by decreasing the bias current.

The optical average power decreases and the APC loopresponds by increasing the bias current. A fault stateoccurs if VBC_MON exceeds the threshold voltage.

IN-The optical average power decreases and the APC loopresponds by increasing the bias current. A fault stateoccurs if VBC_MON exceeds the threshold voltage.

The optical average power increases and a fault occursif VPC_MON exceeds the threshold. The APC loopresponds by decreasing the bias current.

MD This disables bias current. A fault state occurs.The APC circuit responds by increasing bias currentuntil a fault is detected, then a fault* state occurs.

SHUTDOWNDoes not affect laser power. If the shutdown circuitry isused, laser current is disabled.

Does not affect laser power.

BIASIn this condition, laser forward voltage is 0V and no lightis emitted.

Fault state* occurs. If the shutdown circuitry is used,laser current is disabled.

OUT+The APC circuit responds by increasing the bias currentuntil 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.

APCFILT1IBIAS increases until VBC_MON exceeds the thresholdvoltage.

IBIAS increases until VBC_MON exceeds the thresholdvoltage.

APCFILT2IBIAS increases until VBC_MON exceeds the thresholdvoltage.

IBIAS increases until VBC_MON exceeds the thresholdvoltage.

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.

100Ω resistor to ground at each monitor output gives thefollowing relationships:

VMC_MON = (IMOD / 268) ✕ 100ΩVBC_MON = (IBIAS / 82) ✕ 100Ω

VPC_MON = IMD ✕ 100ΩExternal sense resistors can be used for high-accuracymeasurement of bias and photodiode currents. On-chipisolation resistors are included to reduce the number ofcomponents needed to implement this function.

Design ProcedureWhen designing a laser transmitter, the optical output isusually expressed in terms of average power andextinction ratio. Table 3 gives relationships that arehelpful in converting between the optical averagepower and the modulation current. These relationshipsare valid if the mark density and duty cycle of the opti-cal waveform are 50%.

For a desired laser average optical power (PAVG) andoptical extinction ratio (re), the required bias and modula-tion currents can be calculated using the equations inTable 3. Proper setting of these currents requires knowl-edge of the laser to monitor transfer (ρMON) and slopeefficiency (η).

Programming the Monitor Diode CurrentSet Point

The MAX3737 operates in APC mode at all times. Thebias current is automatically set so average laser poweris determined by the APCSET resistor:

The APCSET pin controls the set point for the monitor-diode current. An internal current regulator establishesthe APCSET current in the same manner as the MODSETpin. See the IMD vs. RAPCSET graph in the TypicalOperating 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 maintainthe constant average power. For DC-coupled laser diodes:

Programming the Modulation Current withCompensation

Determine the modulation current from the laser slopeefficiency:

The modulation current of the MAX3737 consists of astatic modulation current (IMODS), a current proportionalto IBIAS, and a current proportional to temperature. Theportion of IMOD set by MODSET is established by aninternal current regulator, which maintains the referencevoltage of VREF across the external programming resis-tor. See to the IMOD vs. RMODSET graph in the TypicalOperating Characteristics and select the value of RMOD-SET that corresponds to the required current at +25°C:

IP r

rMODAVG e

e= × ×

+2

11

η

-

I II

AVG BIASMOD= +

2

IV

RMDREF

APCSET

= ×12

PI

AVGMD

MON=

ρ

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______________________________________________________________________________________ 11

PARAMETER SYMBOL RELATION

Average power PAVG PAVG = (P0 + P1) / 2

Extinction ratio re re = P1 / P0

Optical power of a 1 P1 P1 = 2PAVG re / (re + 1)

Optical power of a zero P0 P0 = 2PAVG / (re + 1)

Optical amplitude PP-P PP-P = P1 - P0

Laser slope efficiency η η = PP-P / IMOD

Modulation current IMOD IMOD = PP-P / ηThreshold 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.

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An external resistor at the MODBCOMP pin sets currentproportional to IBIAS. Open circuiting the MODBCOMPpin can turn off the interaction between IBIAS and IMOD:

If IMOD must be increased from IMOD1 to IMOD2 tomaintain the extinction ratio at elevated temperature,the required compensation factor is:

A threshold for additional temperature compensationcan be set with a programming resistor at theTH_TEMP pin:

The temperature coefficient of thermal compensationabove 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 is0.7V.

For:

VDIODE—Diode bias point voltage (1.2V typ)

RL—Diode bias point resistance (5Ω typ)

RD—Series matching resistor (20Ω typ)

For compliance:

Current Compliance (IMOD > 60mA), AC-Coupled

For applications requiring modulation current greaterthan 60mA, headroom is insufficient for proper opera-tion of the laser driver if the laser is DC-coupled. Toavoid this problem, the MAX3737’s modulation outputcan be AC-coupled to the cathode of a laser diode. Anexternal pullup inductor is necessary to DC-bias themodulation output at VCC. Such a configuration isolateslaser forward voltage from the output circuitry andallows the output at OUT+ to swing above and belowthe supply voltage (VCC). When AC-coupled, theMAX3737 modulation current can be programmed upto 85mA. Refer to Application Note 274: HFAN-02.0:Interfacing Maxim Laser Drivers with Laser Diodes formore information on AC-coupling laser drivers to laserdiodes.

For compliance:

Determine CAPCThe APC loop filter capacitor CAPC must be selected tobalance the requirements for fast turn-on and minimalinteraction with low frequencies in the data pattern. Thelow-frequency cutoff is:

High-frequency noise can be filtered with an additionalcap CMD from the MD pin to ground:

The MAX3737 is designed so that turn-on time is fasterthan 1ms for most laser gain values (η ✕ ρMON).Choosing a smaller value of CAPC reduces turn-ontime. Careful balance between turn-on time and low-fre-quency cutoff may be needed at low data rates forsome values of laser gain.

Interface ModelsFigures 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

MDAPC≈4

C Ff kHzAPC

DBMON( )

( ) ( ) .μ η ρ≈ × ×68

3

1 1

V VI

R R VOUT C

MODD L+ = × +( ) ≥

C -

20 75 .

V V V I R R I R VOUT CC DIODE MOD D L BIAS L+ = × +( ) × ≥

- - - .0 7

TCR k

mACMODTCOMP

=+ °

±10 5

10. ( )

%Ω

T CM

k RCTH

TH TEMP= ° +

+° ±

..

%_

-701 45

9 210

ΩΩ

KI II IMOD MOD

BIAS BIAS= 2 1

2 1

- -

KRMODBCOMP

=+

±17001000

10%

I TC T T T T

I T TMODT TH TH

MODT TH

= × ( ) >

= ≤

|

|

-

0

IV

RMODSREF

MODSET

= ×268

I I K I IMOD MODS BIAS MODT= + × +


12 ______________________________________________________________________________________

Layout ConsiderationsTo minimize loss and crosstalk, keep the connectionsbetween the MAX3737 output and the laser diode asshort as possible. Use good high-frequency layouttechniques and multilayer boards with uninterruptedground plane to minimize EMI and crosstalk. Circuitboards should be made using low-loss dielectrics. Usecontrolled-impedance lines for data inputs, as well asthe module output.

Laser Safety and IEC 825Using the MAX3737 laser driver alone does not ensurethat a transmitter design is IEC 825 compliant. The entiretransmitter circuit and component selections must beconsidered. Each customer must determine the level offault tolerance required by their application, recognizingthat Maxim products are not designed or authorized foruse as components in systems intended for surgicalimplant into the body, for applications intended to supportor sustain life, or for any other application where the fail-ure of a Maxim product could create a situation wherepersonal injury or death may occur.

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______________________________________________________________________________________ 13

PC_MON

BC_MON

MC_MON

RPC_MON

RBC_MON

RMC_MON

VCC

VCC MAX3737

COMP

R Q

S

RSLATCH

CMOS

SHUTDOWN

TX_FAULT

TX_DISABLE

TTLOPEN

COLLECTOR

COMP

COMP

EXCESSIVEMODULATION

CURRENT

VCC

IMD1

VCC

VREF

VREF

VREF

IBIAS82

IMOD268

IMODENABLE

IBIASENABLE

COUNTER60ms DELAY

100nsDELAY

POR AND COUNTER60ms 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

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7 Exposed-Pad (EP) PackageThe exposed-pad on the 32-pin QFN provides a very lowthermal resistance path for heat removal from the IC. Thepad is also electrical ground on the MAX3737 and shouldbe soldered to the circuit board ground for proper ther-mal and electrical performance. Refer to Application Note862: HFAN-08.1: Thermal Considerations of QFN andOther Exposed-Paddle Packages at www.maxim-ic.comfor additional information.


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

MOD

TCOM

P

TH_T

EMP

MOD

BCOM

P

MOD

SET

APCS

ET

APCF

ILT2

APCF

ILT1

25VM

D

9 10 11 12 13 14 15

MC_

MON GN

D

V CC

TX_F

AULT

SHUT

DOW

N

VBS

GND

16GN

D

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 5mmTHIN QFN

*EP

Pin Configurations (continued)

Chip InformationPROCESS: SiGe/BIPOLAR

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______________________________________________________________________________________ 15

IN+

IN-

REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE

V CC

SHUT

DOW

N

3.3V

OPTIONAL SHUTDOWNCIRCUITRY

+3.3V15Ω

10Ω

OUT-

OUT+

BIAS

MD

BC_M

ON

MC_

MON

APCF

ILT1

APCF

ILT2

GND

APCS

ET

MOD

SET

TX_D

ISAB

LE

TX_F

AULT

3.3V

CMD

CDR

CAPC

0.01μF0.1μF

0.1μF

FERRITE BEAD

PC_M

ON

RMODSETRAPCSET RMC_MON RBC_MON RPC_MON

MODBCOMP

MODTCOMP

TH_TEMP

RMODBCOMP

RMODTCOMP

RTH_TEMP

MAX3737

Typical Operating Circuit

Package InformationFor 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 pertainsto 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

http://pdfserv.maxim-ic.com/package_dwgs/21-0140.PDF

http://pdfserv.maxim-ic.com/package_dwgs/21-0091.PDF

http://pdfserv.maxim-ic.com/land_patterns/90-0001.PDF

http://pdfserv.maxim-ic.com/land_patterns/90-0279.PDF

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Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses areimplied. 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 HistoryREVISION NUMBER

REVISION DATE

DESCRIPTIONPAGES

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

EVALUATION KIT Multirate Laser Driver with Extinction Ratio Control

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