MAX3738ETG+ - Hytronics

General Description

The MAX3738 is a +3.3V laser driver designed for mul-

tirate transceiver modules with data rates from

155Mbps to 4.25Gbps. Lasers can be DC-coupled to

the MAX3738 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, com-

bined with thermal compensation, maintain a constant

optical extinction ratio over temperature and lifetime.

The MAX3738 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, make the MAX3738 ideal for driving FP/DFB

lasers in fiber optic modules. External resistors set the

required laser current levels. The MAX3738 provides

transmit disable control (TX_DISABLE), single-point

fault tolerance, bias-current monitoring, and photocur-

rent 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 MAX3738 is

compliant with the SFF-8472 transmitter diagnostic and

SFP MSA timing requirements.

The MAX3738 is offered in a 4mm x 4mm, 24-pin thin

QFN package and operates over the extended -40°C to

+85°C temperature range.

Applications

Multirate OC-3 to OC-48 FEC Transceivers

Gigabit Ethernet SFF/SFP and GBIC

Transceivers

1Gbps/2Gbps/4Gbps 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 Shutdown and Alarm Outputs

♦Enable Control and Laser Safety Feature

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

________________________________________________________________

Maxim Integrated Products

Ordering Information

19-3162; 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

MAX3738ETG -40°C to +85°C 24 Thin QFN-EP*

MAX3738ETG+ -40°C to +85°C 24 Thin QFN-EP*

BIAS

APCFILT2

VCC

TX_DISABLE

MODTCOMP

TH_TEMP

OUT+

APCFILT1

SHUTDOWN

GND

BC_MON

TX_FAULT

GND

VCC

MODSET

APCSET

7 8 9 10 11 12

24 23 22 21 20 19

TOP VIEW

*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY

GROUND ON THE CIRCUIT BOARD.

PIN1 INDICATED BY + ON LEAD-FREE PACKAGE.

IN+

IN-

PC_MON

OUT-

MODBCOMP

MAX3738

*EP

Pin Configuration

Typical Application Circuit appears at end of data sheet.

Denotes a lead(Pb)-free/RoHS-compliant package.

EP = Exposed pad.

MAX3738

155Mbps to 4.25Gbps SFF/SFP 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

otherwise 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,

BC_MON, PC_MON, 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= +70°C)

TQFN (derate 27.8mW/°C above +70°C) ..................2222mW

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, 100mAP-P (Note 4) 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

Output Edge Speed 20% to 80%

(Notes 6, 7) 5mA  IMOD  85mA 65 80 ps

Output Overshoot/Undershoot ±6 %

Random Jitter (Notes 6, 7) 0.62 1.3 psRMS

2.7Gbps, 5mA  IMOD  85mA 18 40

1.25Gbps, 5mA  IMOD  85mA 20 41

622Mbps, 5mA  IMOD  85mA 24 46

Deterministic Jitter (Notes 6, 8)

155Mbps, 5mA  IMOD  85mA 45 100

psP-P

5mA  IMOD  10mA ±175 ±600

Modulation-Current Temperature

Stability (Note 6) 10mA  IMOD  85mA ±125 ±480 ppm/°C

5mA  IMOD  10mA ±20

Modulation-Current-Setting Error 15 load,

TA = +25°C10mA < IMOD  85mA ±15 %

Modulation Off Current TX_DISABLE = high 0.1 mA

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

_______________________________________________________________________________________ 3

Note 1: AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent 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).

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

AUTOMATIC POWER AND 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 0.93 1.15 mA/mA

APC Loop Time Constant CAPC_FILT = 0.01μF, IMD / IBIAS = 1/70 3.3 μs

APC Setting Stability (Note 6) ±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 / T (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 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 = 45k (typ) 0.8 V

VHI = VCC 15

TX_DISABLE Input Current VLO = VGND -70 -140 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

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

otherwise noted.) (Notes 1, 2)

MAX3738

155Mbps to 4.25Gbps SFF/SFP 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.)

BIAS-CURRENT MONITOR RATIO

vs. TEMPERATURE

MAX3738 toc07

TEMPERATURE (°C)

IBIAS/IBC_MON (mA/mA)

807050 60-10 0 10 20 30 40-30 -20

-40 90

PHOTOCURRENT MONITOR RATIO

vs. TEMPERATURE

MAX3738 toc08

TEMPERATURE (°C)

IMD/IPC_MON (mA/mA)

807050 60-100 10203040-30 -20

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0.80

-40 90

MODULATION CURRENT vs. RMODSET

MAX3738 toc09

RMODSET (kΩ)

IMOD (mA)

1 100

OPTICAL EYE DIAGRAM

(2.7Gbps, 27 - 1 PRBS, 2.3GHz FILTER)

MAX3738 toc01

54ps/div

1310nm FP LASER

re = 8.2dB

OPTICAL EYE DIAGRAM

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

MAX3738 toc02

116ps/div

1310nm FP LASER

re = 8.2dB

ELECTRICAL EYE DIAGRAM

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

MAX3738 toc03

52ps/div

75mV/div

1pF BETWEEN OUT+

AND OUT-

ELECTRICAL EYE DIAGRAM

(IMOD = 30mA, 3.125Gbps, 27 - 1 PRBS)

MAX3738 toc04

46ps/div

ELECTRICAL EYE DIAGRAM

(IMOD = 30mA, 4.25Gbps, 27 - 1 PRBS)

MAX3738 toc05

34ps/div

SUPPLY CURRENT (ICC) vs. TEMPERATURE

(EXCLUDES BIAS AND MODULATION CURRENTS)

MAX3738 toc06

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

80706050403020100-10-20-30

-40 90

3.63V

2.97V

3.3V

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

_______________________________________________________________________________________

Typical Operating Characteristics (continued)

(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless otherwise noted.)

PHOTODIODE CURRENT vs. RAPCSET

MAX3738 toc10

RAPCSET (kΩ)

IMD (mA)

101

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.1 100

DETERMINISTIC JITTER

vs. MODULATION CURRENT

MAX3738 toc11

IMOD (mA)

DJ (psP-P)

807050 6020 30 4010

090

2.7Gbps

RANDOM JITTER

vs. MODULATION CURRENT

MAX3738 toc12

IMOD (mA)

RJ (psRMS)

807050 6020 30 4010

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

090

COMPENSATION (K) vs. RMODBCOMP

MAX3738 toc13

RMODBCOMP (kΩ)

K (mA/mA)

1010.10.01

0.1

0.01

0.001 100

TEMPERATURE COMPENSATION vs.

RTH_TEMP (RMODTCOMP = 500Ω)

MAX3738 toc14

TEMPERATURE (°C)

IMOD (mA)

80706050403020100

100

-10 90

RTH_TEMP = 12kΩ

RTH_TEMP = 7kΩ

RTH_TEMP = 4kΩ

RTH_TEMP = 2kΩ

TEMPERATURE COMPENSATION vs.

RTH_TEMP (RMODTCOMP = 10kΩ)

MAX3738 toc15

TEMPERATURE (°C)

IMOD (mA)

80706050403020100

-10 10090

RTH_TEMP = 12kΩ

RTH_TEMP = 7kΩ

RTH_TEMP = 4kΩ

RTH_TEMP = 2kΩ

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

6 _______________________________________________________________________________________

HOT PLUG WITH TX_DISABLE LOW

MAX3738 toc16

20ms/div

VCC

FAULT

TX_DISABLE

LASER

OUTPUT

3.3V

t_init = 59.6ms

LOW

TRANSMITTER ENABLE

MAX3738 toc17

10μs/div

VCC

FAULT

TX_DISABLE

LASER

OUTPUT

3.3V

t_on = 23.8μs

LOW

HIGH

LOW

TRANSMITTER DISABLE

MAX3738 toc18

20ns/div

VCC

FAULT

TX_DISABLE

LASER

OUTPUT

3.3V

91.2ns

LOW

HIGH

LOW

RESPONSE TO FAULT

MAX3738 toc19

400ns/div

VPC_MON

FAULT

TX_DISABLE

LASER

OUTPUT

t_fault = 160ns

EXTERNALLY

FORCED FAULT

FAULT RECOVERY TIME

MAX3738 toc20

40ms/div

VPC_MON

FAULT

TX_DISABLE

LASER

OUTPUT

t_init = 58ms

EXTERNALLY

FORCED FAULT

LOW

LOWLOW

HIGH

Typical Operating Characteristics (continued)

(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless otherwise noted.)

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

_______________________________________________________________________________________ 7

PIN NAME FUNCTION

1 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.

2, 5, 14, 17 VCC +3.3V Supply Voltage

3 IN+ Noninverted Data Input

4 IN- Inverted Data Input

6 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.

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 SHUTDOWN

Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown

circuitry.

10, 12 GND Ground

11 TX_FAULT Open-Collector Transmit Fault Indicator (Table 1)

13 BIAS Laser Bias-Current Output

15 OUT- Inverted Modulation-Current Output. IMOD flows into this pin when input data is low.

16 OUT+ Noninverted Modulation-Current Output. IMOD flows into this pin when input data is high.

18 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.

19 APCFILT1

Connect a capacitor (CAPC) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant

pole of the APC feedback loop.

20 APCFILT2 (See pin 19)

21 APCSET

A resistor connected from this pin to ground sets the desired average optical power. The total

capacitive load at the APCSET pin should be no more than 10pF. Minimize metal resistance for

ground connections.

22 MODSET

A resistor connected from this pin to ground sets the desired constant portion of the

modulation current. The total capacitive load at the MODSET pin should be no more than 10pF.

Minimize metal resistance for ground connections.

23 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.

24 TH_TEMP

Threshold for Temperature Compensation. A resistor at this pin programs the temperature above

which compensation is added to the modulation current.

— EP

Exposed Pad. Solder the exposed pad to the circuit board ground for specified thermal and

electrical performance.

Pin Description

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

8 _______________________________________________________________________________________

Detailed Description

The MAX3738 laser driver consists of three main parts:

a high-speed modulation driver, biasing block with

ERC, and safety circuitry. The circuit design is opti-

mized 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 MAX3738 is optimized for driving a 15Ω

load. The minimum instantaneous voltage required at

OUT- is 0.7V for modulation currents 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 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 high data rates, any capacitive load at the cathode of

a laser diode degrades optical output performance.

Because the BIAS output is directly connected to the

laser cathode, minimize the parasitic capacitance asso-

ciated 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)

VIN+

VIN-

200mV (min)

2400mV (max)

100mV (min)

1200mV (max)

IMOD

VOLTAGE

CURRENT

TIME

SINGLE ENDED

DIFFERENTIAL

(VIN+) - (VIN-)

IOUT+

Figure 1. Required Input Signal and Output Polarity

MAX3738

OUT+

OUT-

OSCILLOSCOPE

BIAS-T

21Ω

50Ω50Ω21Ω

1μF

Figure 2. Test Circuit for Characterization

1μH

0.1μF

10μF

VOLTAGE

SUPPLY

SOURCE

NOISE OPTIONAL

OPTIONAL

FILTER DEFINED BY SFP MSA

HOST BOARD MODULE

TO LASER

DRIVER VCC

Figure 3. Supply Filter

MAX3738

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 laser over time and temperature:

PAVG = IMD / ρMON

PP-P = ηx IMOD

Modulation compensation from bias increases the mod-

ulation current by a user-selected proportion (K) needed

to maintain peak-to-peak laser power as bias current

increases with temperature. Refer to Application Note

1119:

HFAN-02.2.1: Maintaining the Extinction Ratio of

Optical Transmitters Using K-Factor Control

for details:

K = ΔIMOD / ΔIBIAS

This provides a first-order approximation of the current

increase needed to maintain peak-to-peak power.

Slope efficiency decreases more rapidly as tempera-

ture increases. The MAX3738 provides additional tem-

perature compensation as temperature increases past

a user-defined threshold (TTH).

MAX3738

IN+

IN-

OUT-

OUT+

INPUT BUFFER

DATA

PATH

IMOD

VREF

RMODSET

MODSET

VREF RAPCSET

APCSET

IAPCSET

CAPC

APCFILT1 APCFILT2

IBIAS

BIAS

VCC

IBIAS

ENABLE

TX_FAULT

TX_DISABLE

SAFETY LOGIC

AND

POWER DETECTOR

RBC_MON

BC_MON

VCC

RPC_MON

PC_MON

VCC

IMD

CMD

SHUTDOWN

x268

RPULL = 45kΩ

xKxTC

T > TTH

IBIAS

IMOD

ENABLE

x1/2

MODTCOMP

RMODTCOMP

TH_TEMP

RTH_TEMP

MODBCOMP

RMODBCOMP

IBIAS

IMD

Figure 4. Functional Diagram

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

_______________________________________________________________________________________ 9

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

10 ______________________________________________________________________________________

1If any of the I/O pins are shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent

exceeds 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

RBC_MON

BC_MON

VCC

RPC_MON

PC_MON

COMP

VREF

TTL

OPEN COLLECTOR

CMOS

SHUTDOWN

TX_FAULT

LATCH

COUNTER

60ms DELAY

POR AND COUNTER

60ms DELAY

100ns DELAY IBIAS

ENABLE

IMOD

ENABLE

VCC

TX_DISABLE

COMP

EXCESSIVE

APC CURRENT

SETPOINT

EXCESSIVE

MOD CURRENT

SETPOINT

IMD

IBIAS

Figure 5. Simplified Safety Circuit

MAX3738

Safety Circuitry

The safety circuitry contains a disable input

(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). 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 failure. The transmit

fault condition is latched until reset by a toggle or

TX_DISABLE or VCC. The laser driver offers redundant

laser diode shutdown through the optional shutdown

circuitry as shown in the

Typical Application Circuit

This shutdown transistor prevents a single-point fault at

the laser from creating an unsafe condition.

Safety Circuitry Current Monitors

The MAX3738 features monitors (BC_MON, PC_MON)

for bias current (IBIAS) and photocurrent (IMD). The

monitors are realized by mirroring a fraction of the cur-

rents and developing voltages across external resistors

connected to ground. Voltages greater than VREF at

PC_MON or BC_MON result in a fault state. For exam-

ple, connecting a 100Ωresistor to ground at each mon-

itor output gives the following relationships:

VBC_MON = (IBIAS / 82) x 100Ω

VPC_MON = IMD x 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.

PIN CIRCUIT RESPONSE TO OVERVOLTATGE OR

SHORT TO VCC

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 This disables bias current. A fault state occurs. The APC circuit responds by increasing the bias current

until a fault is detected; then a fault* state occurs.

SHUTDOWN Does not affect laser power. If the shutdown circuitry is

used, the laser current is disabled. Does not affect laser power.

BIAS In this condition, the 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, the

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.

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.

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

______________________________________________________________________________________ 11

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

12 ______________________________________________________________________________________

Design Procedure

When designing a laser transmitter, the optical output is

usually expressed in terms of average power and

extinction ratio. Table 3 shows 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 modu-

lation currents can be calculated using the equations in

Table 3. Proper setting of these currents requires

knowledge of the laser to monitor transfer (ρMON) and

slope efficiency (η).

Programming the Monitor-Diode Current

Set Point

The MAX3738 operates in APC mode at all times. The

bias current is automatically set so average laser power

is determined by the APCSET resistor:

PAVG = IMD / ρMON

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 Photodiode Current vs. RAPCSET

graph in the

Typical Operating Characteristics

and

select the value of RAPCSET that corresponds to the

required current at +25°C.

IMD = 1/2 x VREF / RAPCSET

The laser driver automatically adjusts the bias to main-

tain the constant average power. For DC-coupled

laser diodes:

IAVG = IBIAS + IMOD / 2

Programming the Modulation Current with

Compensation

Determine the modulation current from the laser slope

efficiency:

IMOD = 2 x PAVG / ηx (re- 1) / (re + 1)

The modulation current of the MAX3738 consists of a

static modulation current (IMODS), a current proportion-

al 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 refer-

ence voltage of VREF across the external programming

resistor. See the Modulation Current vs. RMODSET

graph in the

Typical Operating Characteristics

and

select the value of RMODSET that corresponds to the

required current at +25°C:

IMOD = IMODS + K x IBIAS + IMODT

IMODS = 268 x VREF / RMODSET

IMODT = TC x (T - TTH) | T > TTH

IMODT = 0 | T < TTH

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:

K = 1700 / (1000 + RMODBCOMP) ±10%

If IMOD must be increased from IMOD1 to IMOD2 to

maintain the extinction ratio at elevated temperatures,

the required compensation factor is:

K = (IMOD2 - IMOD1) / (IBIAS2 - IBIAS1)

A threshold for additional temperature compensation

can be set with a programming resistor at the

TH_TEMP pin:

TTH = -70°C + 1.45MΩ/ (9.2kΩ+ RTH_TEMP)°C ±10%

The temperature coefficient of thermal compensation

above TTH is set by RMODTCOMP. Leaving the

MODTCOMP pin open disables additional thermal

compensation:

TC = 1 / (0.5 + RMODTCOMP(kΩ)) mA/°C ±10%

PARAMETER SYMBOL RELATION

Average Power PAVG P

AVG = (P0 + P1) / 2

Extinction Ratio re r

e = P1 / P0

Optical Power of a One P1 P

1 = 2PAVG x re / (re + 1)

Optical Power of a Zero P0 P

0 = 2PAVG / (re + 1)

Optical Amplitude PP-P P

P-P = P1 - P0

Laser Slope Efficiency  = PP-P / IMOD

Modulation Current IMOD I

MOD = PP-P / 

Threshold Current ITH P

0 at I ITH

Bias Current

(AC-Coupled) IBIAS I

BIAS I

TH + IMOD / 2

Laser to Monitor

Transfer MON IMD / PAVG

Table 3. Optical Power Relations

Note: Assuming a 50% average input duty cycle and mark

density.

MAX3738

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:

VOUT+ = VCC - VDIODE - IMOD x (RD+ RL) -

IBIAS x RL≥0.7V

Current Compliance (IMOD > 60mA),

AC-Coupled

For applications requiring modulation current greater

than 60mA, headroom is insufficient from proper opera-

tion of the laser driver if the laser is DC-coupled. To

avoid this problem, the MAX3738’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 sup-

ply voltage (VCC). When AC-coupled, the MAX3738

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 informa-

tion on AC-coupling laser drivers to laser diodes.

For compliance:

VOUT+ = VCC - IMOD / 2 x (RD+ RL) ≥0.75V

Determine CAPC

The APC loop filter capacitor (CAPC) must be selected

to balance the requirements for fast turn-on and mini-

mal interaction with low frequencies in the data pattern.

The low-frequency cutoff is:

CAPC(µF) ≅68 / (f3dB(kHz) x (ηx ρMON)1.1)

High-frequency noise can be filtered with an additional

cap, CMD, from the MD pin to ground.

CMD ≅CAPC / 4

The MAX3738 is designed so turn-on time is faster than

1ms for most laser gain values (ηx ρMON). Choosing a

smaller value of CAPC reduces turn-on time. Careful

balance between turn-on time and low-frequency 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 MAX3738 laser driver. If dice are used,

replace package parasitic elements with bondwire par-

asitic elements.

VCC

0.11pF

0.7nH

IN+

0.11pF

0.7nH

IN-

PACKAGE

5kΩ

24kΩ

16kΩ

MAX3738

Figure 6. Simplified Input Structure

MAX3738

PACKAGE

0.7nH

OUT-

0.7nH

OUT+

0.11pF

VCC

Figure 7. Simplified Output Structure

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

______________________________________________________________________________________ 13

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

14 ______________________________________________________________________________________

Layout Considerations

To minimize loss and crosstalk, keep the connections

between the MAX3738 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 MAX3738 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, recogniz-

ing 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 sup-

port or sustain life, or for any other application where the

failure of a Maxim product could create a situation

where personal injury or death may occur.

Exposed-Pad (EP) Package

The exposed pad on the 24-pin TQFN provides a very

low thermal resistance path for heat removal from the

IC. The pad is also electrical ground on the MAX3738 and

should be soldered to the circuit board ground for proper

thermal and electrical performance. Refer to Maxim

Application Note 862:

HFAN-08.1: Thermal Considerations

of QFN and Other Exposed-Paddle Packages

www.maxim-ic.com for additional information.

Chip Information

PROCESS: SiGe/BiPOLAR

MAX3738

155Mbps to 4.25Gbps SFF/SFP Laser Driver

with Extinction Ratio Control

______________________________________________________________________________________ 15

MAX3738

IN+

IN-

REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.

VCC

SHUTDOWN

+3.3V

OPTIONAL SHUTDOWN

CIRCUITRY

+3.3V

15Ω

10Ω

OUT-

OUT+

BIAS

BC_MON

APCFILT1

APCFILT2

GND

APCSET

MODSET

TX_DISABLE

TX_FAULT

+3.3V

CDR

CAPC

RPC_MON

CMD

0.01μF

FERRITE BEAD

PC_MON

RBC_MON

RMODSET

RAPCSET

MODBCOMP

MODTCOMP

TH_TEMP

RTH_TEMP

RMODTCOMP

RMODBCOMP

0.1μF

0.1μF100Ω

Typical Application Circuit

PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.

24 TQFN-EP T2444-3 21-0139 90-0021

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.

MAX3738

155Mbps to 4.25Gbps SFF/SFP 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.

____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 1/04 Initial release. —

Changed the data sheet from 1Gbps to 2.7Gbps to 155Mbps to 2.7Gbps. All

Added the lead-free package to the Ordering Information table. 1

In the Electrical Characteristics table, added 622Mbps and 155Mbps conditions to the

deterministic jitter parameter. 2

1 7/05

Added the 100 resistor to the Typical Application Circuit. 13

Increased the maximum data rate 2.7Gbps to 4.25Gbps. All

Added 4Gbps to the Applications for Fibre Channel SFF/SFP and GBIC transceivers. 1

Changed the derate factor for continuous power dissipation in the Absolute Maximum

Ratings from 20.8mW/°C to 27.8mW/°C. 2

Added new TOCs 4 and 5 to the Typical Operating Characteristics section. 4

Changed VBG to VREF in Figure 4. 9

2 12/08

Changed the transistor count from 1184 to 3754 in the Chip Information section. 14

Added Lead Temperature and Soldering Temperature to the Absolute Maximum Ratings

section. 2

Changed Continuous Power Dissipation from TA = +85°C to +70°C and 1805mW to

2222mW in the Absolute Maximum Ratings section. 2

Changed Bias-Current Monitor Ratio minimum, typical, and maximum values from [68,

79, 95] to [62, 76, 90] respectively in the Electrical Characteristics table. 2

Removed the condition from the Output Overshoot and Undershoot line in the Electrical

Characteristics table. 2

Replaced Figure 2 with new figure. 8

3 6/11

Removed Transistor Count from Chip Information section. 14