MAXIM MAX3738 Technical data

General Description

The MAX3738 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
MAX3738 for reduced component count and ease of
multirate operation.

Laser extinction ratio control (ERC) combines the features
of automatic power control (APC), modulation compensa­tion, and built-in thermal compensation. The APC loop
maintains constant average optical power. Modulation
compensation increases the modulation current in pro­portion to the bias current. These control loops, com­bined 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) mod­ulation 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 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 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 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 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

________________________________________________________________

Maxim Integrated Products

1

Ordering Information

19-3162; Rev 1; 7/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

Pin Configuration

Typical Application Circuit appears at end of data sheet.

+

Denotes lead-free package.

PART

MAX3738ETG -40°C to 85°C 24 Thin QFN T2444-3

MAX3738ETG+ -40°C to 85°C 24 Thin QFN T2444-3

TEMP

RANGE

PIN­PACKAGE

PKG
CODE

TOP VIEW

TH_TEMP

MODBCOMP

24 23 22 21 20 19

MODTCOMP

TX_DISABLE

THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY
GROUND ON THE CIRCUIT BOARD.
PIN1 INDICATED BY + ON LEAD-FREE PACKAGE.

1

V

2

CC

3

IN+

4

IN-

V

5

CC

6

7 8 9 10 11 12

PC_MON

MAX3738

BC_MON

MODSET

APCSET

GND

SHUTDOWN

APCFILT2

APCFILT1

18

MD

V

17

OUT+

16

OUT-

15

V

14

BIAS

13

GND

TX_FAULT

CC

CC

MAX3738

155Mbps to 2.7Gbps 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, I

BIAS

= 60mA, I

MOD

= 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 (V

CC

+ 0.5V)

OUT+, OUT-, BIAS Current.............................-20mA to +150mA

Continuous Power Dissipation (T

A

= +85°C)

24-Pin TQFN (derate 20.8mW/°C above +85°C) .......1805mW

Operating Junction Temperature Range...........-55°C to +150°C

24-Pin TQFN (derate 20.8mW/°C above +85°C) ........1805mW

Storage Temperature Range .............................-55°C to +150°C

POWER SUPPLY

Supply Current I

Power-Supply Noise Rejection PSNR f ≤ 1MHz, 100mA

I/O SPECIFICATIONS

Differential Input Swing V

Common-Mode Input V

LASER BIAS

Bias-Current-Setting Range 1 100 mA

Bias Off Current TX_DISABLE = high 0.1 mA

Bias-Current Monitor Ratio I

LASER MODULATION

Modulation Current-Setting
Range

Output Edge Speed

Output Overshoot/Undershoot With 1pF between OUT+ and OUT- ±6%

Random Jitter (Notes 6, 7) 0.62 1.3 ps

Deterministic Jitter (Notes 6, 8)

Modulation-Current Temperature
Stability

Modulation-Current-Setting Error

Modulation Off Current TX_DISABLE = high 0.1 mA

AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS

Monitor-Diode Input Current
Range

MD Pin Voltage 1.4 V

MD Current Monitor Ratio IMD / I

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

CC

I

MOD

I

MD

CM

(Note 3) 47 60 mA

DC-coupled, Figure 1 0.2 2.4 V

ID

/ I

BIAS

BC_MON

(Note 5) 5 85 mA

20% to 80%
(Notes 6, 7)

2.7Gbps, 5mA ≤ I

1.25Gbps, 5mA ≤ I

622Mbps, 5mA ≤ I

155Mbps, 5mA ≤ I

(Note 6)

15Ω load,
T

= +25°C

A

Average current into the MD pin 18 1500 µA

PC_MON

(Note 4) 33 dB

P-P

5mA ≤ I

≤ 85mA 18 40

MOD

≤ 85mA 20 41

MOD

≤ 85mA 24 46

MOD

≤ 85mA 45 100

MOD

5mA ≤ I

10mA ≤ I

5mA ≤ I

10mA < I

≤ 85mA 65 80 ps

MOD

≤ 10mA ±175 ±600

MOD

≤ 85mA ±125 ±480

MOD

≤ 10mA ±20

MOD

≤ 85mA ±15

MOD

V

-

1.7

68 79 95 mA/mA

0.85 0.93 1.15 mA/mA

CC

/ 4

V

ID

ps

ppm/°C

P-P

V

RMS

P-P

%

MAX3738

155Mbps to 2.7Gbps 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 I

BIAS

and I

MOD

. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.

Note 4: Power-supply noise rejection (PSNR) = 20log

10(Vnoise (on VCC

) / ΔV

OUT

). V

OUT

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).

ELECTRICAL CHARACTERISTICS (continued)

(VCC= +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC= +3.3V, I

BIAS

= 60mA, I

MOD

= 60mA, TA= +25°C, unless

otherwise noted.) (Notes 1, 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

APC Loop Time Constant C

APC Setting Stability (Note 6) ±100 ±480 ppm/°C

APC Setting Accuracy TA = +25°C ±15 %

I

Compensation-Setting

MOD

Range by Bias

I

Compensation-Setting

MOD

Range by Temperature

Threshold-Setting Range for
Temperature Compensation

LASER SAFETY AND CONTROL

Bias and Modulation Turn-Off
Delay

Bias and Modulation Turn-On
Delay

Threshold Voltage at Monitor Pins V

INTERFACE SIGNALS

TX_DISABLE Input High V

TX_DISABLE Input Low V

TX_DISABLE Input Current

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

K K = ΔI

TC TC = ΔI

T

TH

REF

HI

LO

APC_FILT

(Note 6) +10 +60 °C

C

APC_FILT

(Note 6)

C

APC_FILT

(Note 6)

Figure 5 1.14 1.3 1.39 V

R

PULL

VHI = V

V

LO

= 0.01µF, ΔIMD / ΔI

/ ΔI

MOD

= 45kΩ (typ) 0.8 V

= GND -70 -140

BIAS

/ ΔT (Note 6) 0 1.0 mA/°C

MOD

= 0.01µF, ΔIMD / ΔI

= 0.01µF, ΔIMD / ΔI

CC

BIAS

BIAS

BIAS

= 1/70 3.3 µs

= 1/80

= 1/80

0 1.5 mA/mA

5µs

600 µs

2.0 V

15

µA

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control

4 _______________________________________________________________________________________

Typical Operating Characteristics

(VCC= +3.3V, C

APC

= 0.01µF, I

BIAS

= 20mA, I

MOD

= 30mA, TA= +25°C, unless otherwise noted.)

OPTICAL EYE DIAGRAM

(2.7Gbps, 2

7

- 1 PRBS, 2.3GHz FILTER)

MAX3738 toc01

54ps/div

1310nm FP LASER

r

e

= 8.2dB

OPTICAL EYE DIAGRAM

(1.25Gbps, 2

7

- 1 PRBS, 940MHz FILTER)

MAX3738 toc02

116ps/div

1310nm FP LASER

r

e

= 8.2dB

ELECTRICAL EYE DIAGRAM

(I

MOD

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

MAX3738 toc03

52ps/div

75mV/div

1pF BETWEEN OUT+

AND OUT-

SUPPLY CURRENT (ICC) vs. TEMPERATURE

(EXCLUDES BIAS AND MODULATION CURRENTS)

MAX3738 toc04

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

80706050403020100-10-20-30

35

40

45

50

55

60

30

-40 90

3.63V

2.97V

3.3V

BIAS-CURRENT MONITOR RATIO

vs. TEMPERATURE

MAX3738 toc05

TEMPERATURE (°C)

I

BIAS

/I

BC_MON

(mA/mA)

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

72

74

76

78

80

82

84

86

88

90

70

-40 90

PHOTOCURRENT MONITOR RATIO

vs. TEMPERATURE

MAX3738 toc06

TEMPERATURE (°C)

I

MD

/I

PC_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. R

MODSET

MAX3738 toc07

R

MODSET

(kΩ)

I

MOD

(mA)

10

10

20

30

40

50

60

70

80

90

0

1 100

PHOTODIODE CURRENT vs. R

APCSET

MAX3738 toc08

R

APCSET

(kΩ)

I

MD

(mA)

101

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

0.1 100

DETERMINISTIC JITTER

vs. MODULATION CURRENT

MAX3738 toc09

I

MOD

(mA)

DJ (ps

P-P

)

807050 6020 30 4010

5

10

15

20

25

30

35

40

45

50

0

090

2.7Gbps

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

_______________________________________________________________________________________

5

Typical Operating Characteristics (continued)

(VCC= +3.3V, C

APC

= 0.01µF, I

BIAS

= 20mA, I

MOD

= 30mA, TA= +25°C, unless otherwise noted.)

vs. MODULATION CURRENT

2.0

1.8

1.6

1.4

)

1.2

RMS

1.0

RJ (ps

0.8

0.6

0.4

0.2

0

090

TEMPERATURE COMPENSATION vs.

R

TH_TEMP

44

42

R

TH_TEMP

40

R

TH_TEMP

R

38

TH_TEMP

(mA)

R

= 2kΩ

TH_TEMP

MOD

I

36

34

32

RANDOM JITTER

I

(mA)

MOD

(R

MODTCOMP

= 12kΩ

= 7kΩ

= 4kΩ

= 10kΩ)

807050 6020 30 4010

MAX3738 toc10

MAX3738 toc13

TX_DISABLE

COMPENSATION (K) vs. R

10

1

K (mA/mA)

0.1

0.01

0.001 100

HOT PLUG WITH TX_DISABLE LOW

3.3V

V

CC

0V

LOW

FAULT

LOW

LASER

OUTPUT

R

MODBCOMP

t_init = 59.6ms

(kΩ)

MODBCOMP

1010.10.01

MAX3738 toc14

MAX3738 toc11

TX_DISABLE

TEMPERATURE COMPENSATION vs.

TH_TEMP

R

TH_TEMP

TH_TEMP

= 2kΩ

R

(R

MODTCOMP

= 12kΩ

TH_TEMP

= 7kΩ

= 4kΩ

TEMPERATURE (°C)

R

100

90

80

70

(mA)

MOD

I

R

60

R

TH_TEMP

50

40

30

-10 90

TRANSMITTER ENABLE

V

CC

3.3V

LOW

FAULT

HIGH

t_on = 23.8μs

LASER

OUTPUT

= 500Ω)

MAX3738 toc12

80706050403020100

MAX3738 toc15

LOW

V

FAULT

TX_DISABLE

LASER

OUTPUT

30

-10 10090
TEMPERATURE (°C)

TRANSMITTER DISABLE

CC

3.3V

LOW

HIGH

91.2ns

LOW

20ns/div

80706050403020100

MAX3738 toc16

V

PC_MON

FAULT

TX_DISABLE

LASER

OUTPUT

20ms/div

RESPONSE TO FAULT

EXTERNALLY

FORCED FAULT

400ns/div

MAX3738 toc17

t_fault = 160ns

V

PC_MON

FAULT

TX_DISABLE

LASER

OUTPUT

10μs/div

FAULT RECOVERY TIME

EXTERNALLY
FORCED FAULT

HIGH

t_init = 58ms

HIGH

40ms/div

MAX3738 toc18

LOW

LOWLOW

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control

6 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature

1 MODTCOMP

2, 5, 14,

17

3 IN+ Noninverted Data Input

4 IN- Inverted Data Input

6 TX_DISABLE

7 PC_MON

8 BC_MON

9 SHUTDOWN

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. I

16 OUT+ Noninverted Modulation-Current Output. I

18 MD

19 APCFILT1

20 APCFILT2 (See pin 19)

21 APCSET A resistor connected from this pin to ground sets the desired average optical power.

22 MODSET

23 MODBCOMP

24 TH_TEMP

EP Exposed Pad

V

CC

coefficient of the modulation current when above the threshold temperature. Leave open for zero
temperature compensation.

+3.3V Supply Voltage

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.

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.

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.

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

flows into this pin when input data is low.

MOD

flows into this pin when input data is high.

MOD

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.

Connect a capacitor (C
pole of the APC feedback loop.

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

Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.
Mirrors I

Threshold for Temperature Compensation. A resistor at this pin programs the temperature above
which compensation is added to the modulation current.

Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical
performance.

through an external resistor. Leave open for zero-coupling.

BIAS

) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant

APC

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 opti­mized for high-speed, low-voltage (+3.3V) operation
(Figure 4).

High-Speed Modulation Driver

The output stage is composed of a high-speed differ­ential 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 induc­tance, an RC shunt network may be necessary. Refer to
Maxim Application Note:

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 perfor­mance. 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 form 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 = (2P

AVG

+ P

P-P

) / (2P

AVG

- P

P-P

)

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

_______________________________________________________________________________________ 7

Figure 1. Required Input Signal and Output Polarity

Figure 2. Test Circuit for Characterization

Figure 3. Supply Filter

VOLTAGE

(V

) - (V

IN+

CURRENT

V

V

I

OUT+

IN+

IN-

)

IN-

SINGLE ENDED

DIFFERENTIAL

TIME

100mV (min)

1200mV (max)

200mV (min)

2400mV (max)

I

MOD

OUT-

MAX3738

OUT+

V

CCVCC

30Ω 30Ω

0.5pF

I

OUT+

Z

= 30Ω

0

30Ω

= 30Ω Z0 = 50Ω

Z

0

75Ω 50Ω

OSCILLOSCOPE

HOST BOARD

FILTER DEFINED BY SFP MSA

SOURCE

NOISE

VOLTAGE

SUPPLY

C1

0.1μF

C2
10μF

MODULE

L1

1μH

C3

0.1μF

OPTIONAL

OPTIONAL

TO LASER

DRIVER V

CC

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 compen­sating the modulation current for reduced slope effi­ciency (η) of laser over time and temperature:

P

AVG

= I

MD

/

ρ

MON

P

P-P

= η x I

MOD

Modulation compensation from bias increases the mod­ulation current by a user-selected proportion (K) need­ed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note

HFAN-02.21

for details:

K = ΔI

MOD

/ ΔI

BIAS

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 MAX3738 provides additional tem­perature compensation as temperature increases past
a user-defined threshold (TTH).

155Mbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control

8 _______________________________________________________________________________________

Figure 4. Functional Diagram

SHUTDOWN

MAX3738

R

PC_MON

R

BC_MON

IN+

IN-

SHUTDOWN

TX_FAULT

TX_DISABLE

PC_MON

BC_MON

= 45kΩ

R

PULL

I

MD

1

I

BIAS

82

INPUT BUFFER

SAFETY LOGIC

AND

POWER DETECTOR

V

CC

T > T

TH

T

I

MOD

ENABLE

I

BIAS

ENABLE

DATA
PATH

x268

OUT-

R

R

APCSET

I

D

BIAS

I

MD

OUT+

I

MOD

BIAS

V

CC

V

APCSET

x1/2

I

APCSET

x1

BG

MD

I

BIAS

xKxTC

V

CC

C

MD

V

BG

TH_TEMP

R

TH_TEMP

MODTCOMP

R

MODTCOMP

MODSET

R

MODSET

MODBCOMP

APCFILT1 APCFILT2

R

MODBCOMP

C

APC

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

_______________________________________________________________________________________ 9

Table 1. Typical Fault Conditions

Figure 5. Simplified Safety Circuit

V

CC

TX_DISABLE

POR AND COUNTER

60ms DELAY

I

MOD

ENABLE

I

MD

1

PC_MON

R

PC_MON

I

BIAS

82

BC_MON

R

BC_MON

COUNTER

60ms DELAY

V

CC

V

CC

MOD CURRENT

V

REF

V

REF

EXCESSIVE

APC CURRENT

SETPOINT

EXCESSIVE

SETPOINT

COMP

COMP

I

100ns DELAY

R

S

Q

RS

LATCH

BIAS

ENABLE

CMOS

TTL

OPEN COLLECTOR

SHUTDOWN

TX_FAULT

If any of the I/O pins are shorted to GND or V

1

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.

No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the

4

programmed threshold.

(single-point failure; see Table 2), and the bias current or the photocurrent

CC

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 4.7kΩ to 10kΩ resistor to VCCas
required by the SFP MSA. A single-point fault can be a
short to VCCor 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 Applications 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 (I

BIAS

) and photocurrent (IMD). The
monitors are realized by mirroring a fraction of the cur­rents and developing voltages across external resistors
connected to ground. Voltages greater than V

REF

at
PC_MON or BC_MON result in a fault state. For exam­ple, connecting a 100Ω resistor to ground at each mon­itor output gives the following relationships:

V

BC_MON

= (I

BIAS

/ 82) x 100Ω

V

PC_MON

= IMDx 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.

155Mbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control

10 ______________________________________________________________________________________

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.

PIN

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+

IN-

MD This disables bias current. A fault state occurs.

SHUTDOWN

BIAS

OUT+

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

APCFILT2

MODSET Does not affect laser power. Fault state* occurs.

APCSET Does not affect laser power. Fault state* occurs.

CIRCUIT RESPONSE TO OVERVOLTATGE OR

SHORT TO V

The optical average power increases, and a fault occurs
if V
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 V

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

In this condition, the laser forward voltage is 0V and no
light is emitted.

The APC circuit responds by increasing the bias current
until a fault is detected; then a fault state* occurs.

I

BIAS

voltage.

I

BIAS

voltage.

exceeds the threshold. The APC loop

PC_MON

BC_MON

increases until V

increases until V

exceeds the threshold voltage.

BC_MON

BC_MON

CC

exceeds the threshold

exceeds the threshold

CIRCUIT RESPONSE TO UNDERVOLTAGE OR

SHORT TO GROUND

The optical average power decreases, and the APC loop
responds by increasing the bias current. A fault state
occurs if V

The optical average power increases and a fault occurs
if V
responds by decreasing the bias current.

The APC circuit responds by increasing the bias current
until a fault is detected; then a fault* state occurs.

Does not affect laser power.

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

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

I

BIAS

voltage.

I

BIAS

voltage.

BC_MON

PC_MON

increases until V

increases until V

exceeds the threshold voltage.

exceeds the threshold. The APC loop

BC_MON

BC_MON

exceeds the threshold

exceeds the threshold

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 opti­cal waveform are 50%.

For a desired laser average optical power (P

AVG

) and
optical extinction ratio (re), the required bias and modu­lation currents can be calculated using the equations in

Table 3. Proper setting of these currents requires

knowledge of the laser to monitor transfer (ρ

MON

) 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:

P

AVG

= I

MD

/ ρ

MON

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. R

APCSET

graph in the

Typical Operating Characteristics

and

select the value of R

APCSET

that corresponds to the

required current at +25°C.

IMD= 1/2 x V

REF

/ R

ACPSET

The laser driver automatically adjusts the bias to main­tain the constant average power. For DC-coupled
laser diodes:

I

AVG

= I

BIAS

+ I

MOD

/ 2

Programming the Modulation Current with

Compensation

Determine the modulation current from the laser slope
efficiency:

I

MOD

= 2 x P

AVG

/ η x (re- 1) / (r

e+

+ 1)

The modulation current of the MAX3738 consists of a
static modulation current (I

MODS

), a current proportion-

al to I

BIAS

, and a current proportional to temperature.

The portion of I

MOD

set by MODSET is established by
an internal current regulator, which maintains the refer­ence voltage of V

REF

across the external programming

resistor. See the Modulation Current vs. R

MODSET

graph in the

Typical Operating Characteristics

and

select the value of R

MODSET

that corresponds to the

required current at +25°C:

I

MOD

= I

MODS

+ K x I

BIAS

+ I

MODT

I

MODS

= 268 x V

REF

/ R

MODSET

I

MODT

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

TH

I

MODT

= 0 | T <

T

TH

An external resistor at the MODBCOMP pin sets current
proportional to I

BIAS

. Open circuiting the MODBCOMP

pin can turn off the interaction between I

BIAS

and I

MOD

:

K = 1700 / (1000 + R

MODBCOMP

) ±10%

If I

MOD

must be increased from I

MOD1

to I

MOD2

to
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:

K = (I

MOD2

- I

MOD1

) / (I

BIAS2

- I

BIAS1

)

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 + R

TH_TEMP

)°C ±10%

The temperature coefficient of thermal compensation
above TTHis set by R

MODTCOMP

. Leaving the
MODTCOMP pin open disables additional thermal
compensation:

TC = 1 / (0.5 + R

MODTCOMP

(kΩ)) mA/°C ±10%

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

______________________________________________________________________________________ 11

Table 3. Optical Power Relations

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

PARAMETER SYMBOL RELATION

Average Power P

Extinction Ratio r

Optical Power of a One P

Optical Power of a Zero P

Optical Amplitude P

Laser Slope Efficiency ηη = P

Modulation Current I

Threshold Current I

Bias Current
(AC-Coupled)

Laser to Monitor
Transfer

MOD

I

BIAS

ρ

MON

AVG

e

1

0

P-P

TH

P

AVG

P1 = 2P

P0 = 2P

P

I

MOD

I

BIAS

= (P0 + P1) / 2

re = P1 / P

AVG

P-P

P0 at I ≥ I

≥ ITH + I

IMD / P

0

x re / (re + 1)

/ (re + 1)

AVG

= P1 - P

P-P

= P

/ I

P-P

AVG

0

MOD

/ η

TH

MOD

/ 2

MAX3738

Current Compliance (I

MOD

≤≤

60mA),

DC-Coupled

The minimum voltage at the OUT+ and OUT- pins
is 0.7V.

For:

V

DIODE

= Diode bias point voltage (1.2V typ)

R

L

= Diode bias point resistance (5Ω typ)

R

D

= Series matching resistor (20Ω typ)

For compliance:

V

OUT+

= VCC- V

DIODE

- I

MOD

x (RD+ RL) -

I

BIAS

x RL≥ 0.7V

Current Compliance (I

MOD

> 60mA),

AC-Coupled

For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper opera­tion 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 sup­ply voltage (VCC). When AC-coupled, the MAX3738
modulation current can be programmed up to 85mA.
Refer to Maxim Application Note

HFAN-02.0: Interfacing

Maxim’s Laser Drivers with Laser Diodes

for more infor-

mation on AC-coupling laser drivers to laser diodes.

For compliance:

V

OUT+

= VCC- I

MOD

/ 2 x (RD+ RL) ≥ 0.75V

Determine C

APC

The APC loop filter capacitor (C

APC

) 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:

C

APC

(µF) ≅ 68 / (f

3dB

(kHz) x (η x

ρ

MON

)

1.1

)

High-frequency noise can be filtered with an additional
cap, CMD, from the MD pin to ground.

CMD≅ C

APC

/ 4

The MAX3738 is designed so turn-on time is faster than
1ms for most laser gain values (η x ρ

MON

). Choosing a

smaller value of C

APC

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 cir­cuits for the MAX3738 laser driver. If dice are used,
replace package parasitic elements with bondwire par­asitic elements.

155Mbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control

12 ______________________________________________________________________________________

Figure 6. Simplified Input Structure

Figure 7. Simplified Output Structure

V

V

CC

MAX3738

PACKAGE

0.7nH

IN+

0.11pF

0.7nH

IN-

0.11pF

16kΩ

V

CC

5kΩ

V

CC

5kΩ

24kΩ

CC

PACKAGE

0.7nH OUT-

0.11pF

0.7nH OUT+

0.11pF

MAX3738

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, 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 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

HFAN-08.1: Thermal Considerations

of QFN and Other Exposed-Paddle Packages

at

www.maxim-ic.com for additional information.

Chip Information

TRANSISTOR COUNT: 1884

PROCESSS: SiGe/Bipolar

MAX3738

155Mbps to 2.7Gbps SFF/SFP Laser Driver with

Extinction Ratio Control

______________________________________________________________________________________ 13

Typical Application Circuit

+3.3V

OPTIONAL SHUTDOWN

+3.3V

CIRCUITRY

CDR

CC

0.1μF

IN+

100Ω

0.1μF

IN-

R

MODBCOMP

R

MODTCOMP

R

TH_TEMP

REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.

MODBCOMP

MODTCOMP

TH_TEMP

TX_DISABLE

GND

MODSET

R

TX_FAULT

MODSET

APCSET

R

MAX3738

APCSET

V

SHUTDOWN

OUT-

OUT+

BIAS

MD

APCFILT1

C

APC

BC_MON

APCFILT2

BC_MON

R

PC_MON

PC_MON

R

+3.3V

15Ω

10Ω

FERRITE BEAD

C

MD

0.01μF

MAX3738

155Mbps to 2.7Gbps 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.

14

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

© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages

.)

PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm

21-0139

1

E

2

24L QFN THIN.EPS

PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm

21-0139

2

E

2