MAXIM MAX3261 User Manual

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_______________General Description

The MAX3261 is a complete, easy-to-program, single
+5V-powered, 1.25Gbps laser diode driver with com­plementary enable inputs and automatic power control
(APC). The MAX3261 accepts differential PECL inputs
and provides complementary output currents. A tem­perature-stabilized reference voltage is provided to
simplify laser current programming. This allows modu­lation current to be programmed up to 30mA and bias
current to be programmed up to 60mA with two exter­nal resistors.

Complementary enable inputs allow the MAX3261 to
interface with open-fiber-control architecture—a feature
not found in other 1.25Gbps laser diode drivers.

An APC circuit is provided to maintain constant laser power
in transmitters that use a monitor photodiode. Only two
external components are required to implement the APC
function.

The MAX3261’s fully integrated feature set includes a
TTL-compatible laser failure indicator and a program­mable slow-start circuit to prevent laser damage. The
slow-start is preset to 50ns and can be extended by
adding an external capacitor.

________________________Applications

Laser Diode Transmitters
531Mbps and 1062Mbps Fibre Channel
622Mbps SDH/SONET
Gigabit Ethernet

____________________________Features

♦ Rise Times Less than 250ps
♦ Differential PECL Inputs
♦ Single +5V Supply
♦ Automatic Power Control
♦ Temperature-Compensated Reference Voltage
♦ Complementary Enable Inputs

______________Ordering Information

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

________________________________________________________________

Maxim Integrated Products

1

TQFP

TOP VIEW

IMODSET
IBIASSET
IBIASFB
OSADJ
VREF1

IBIASOUT

ENB+

V

CC

B

GNDA

IPIN

SLWSTRT

GNDB

VREF2

GNDA

FAILOUT

IPINSET

V

CC

A

V

CC

A

OUT-

GNDA

OUT+

GNDA

V

CC

A

GNDA

VIN+

GNDB

V

CC

B

VIN-

GNDB

V

CC

B

GNDB

ENB-

16

15

14

13

12

11

10

9

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

8

7

6

5

4

3

2

1

MAX3261

__________________Pin Configuration

19-0323; Rev 4; 8/97

PART

MAX3261CCJ 0°C to +70°C

TEMP. RANGE PIN-PACKAGE

32 TQFP

MAX3261

+5V +5V

+5V

+5V

OUT+

OUT-

2.7k

IPIN

PECL

INPUTS

TO OPEN

FIBER

CONTROL

IBIASOUT

FAILOUT

IBIASFB

OSADJ

IMODSET

IPINSET

IBIASSET

VCCAVCCB

ZO = 25Ω

MICROSTRIP

VIN+

VIN-

ENB+

GND B

GND A

ENB-

SLWSTRT

VREF1
VREF2

LASER

PHOTO-

DIODE

__________Typical Operating Circuit

MAX3261ECJ -40°C to +85°C 32 TQFP
MAX3261E/D -40°C to +85°C Dice*

*

Dice are designed to operate over a -40°C to +140°C junction

temperature (Tj) range. Tested and guaranteed at Tj = +25°C.

For free samples & the latest literature: http://www.maxim-ic.com, or phone [phone].
For small orders, phone [phone] ext. 3468.

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

DC ELECTRICAL CHARACTERISTICS

(VCC= VCCA = VCCB = +4.75V to +5.25V, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at VCC= +5V and

TA= +25°C.) (Note 1)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

Terminal Voltage (with respect to GND)

Supply Voltages (V

CC

A, VCCB)...............................-0.3V to 6V

VIN+, VIN-, FAILOUT...............................................0V to V

CC_

OUT+, OUT-, IBIASOUT.......................................1.5V to V

CC_

ENB+, ENB-.......................V

CC_

or 5.5V, whichever is smaller

Differential Input Voltage (

|

VIN+ - VIN-|)...........................3.8V

Input Current

IBIASOUT............................................................0mA to 75mA

OUT+, OUT-........................................................0mA to 40mA

IBIASSET........................................................0mA to 1.875mA

IMODSET...............................................................0mA to 2mA

IPIN, IPINSET, OSADJ...........................................0mA to 2mA

FAILOUT..............................................................0mA to 10mA

IBIASFB................................................................-2mA to 2mA

Output Current

VREF1, VREF2.....................................................0mA to 20mA

SLWSTRT ..............................................................0mA to 5mA

Continuous Power Dissipation (T

A

= +70°C)

TQFP (derate 10.2mW/°C above +70°C)......................816mW

Operating Temperature Ranges

MAX3261CCJ......................................................0°C to +70°C

MAX3261ECJ ...................................................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range.............................-55°C to +175°C

Processing Temperature (die).........................................+400°C

TA= +25°C

(Note 2)

CONDITIONS

mA12I

REF

Available Reference Current

V3.15 3.3 3.45V

REF

mA60I

BIAS

Range of Programmable Laser
Bias Current

Reference Voltage

V0.8V

IL

TTL Input Low

mA50I

VCC

Supply Current

VVCC- 1.165V

IH

PECL Input High

VVCC- 1.475V

IL

PECL Input Low

V2V

IH

TTL Input High

UNITSMIN TYP MAXSYMBOLPARAMETER

AC ELECTRICAL CHARACTERISTICS

(VCC= VCCA = VCCB = +4.75V to +5.25V, R

LOAD

(at OUT+ and OUT-) = 25Ωconnected to VCC, TA= -40°C to +85°C, unless other-

wise noted. Typical values are at VCC= +5V and TA= +25°C.) (Note 3)

Minimum differential input
swing is 1100mVp-p (Note 4)

CONDITIONS

mA30I

MOD

Range of Programmable
Modulation Current

UNITSMIN TYP MAXSYMBOLPARAMETER

I

BIAS

= 25mA, I

MOD

= 12mA, 4ns unit

interval; measured from 10% to 90%.

ps250tR, t

F

Modulation-Current Rise and
Fall Time

I

MOD

= 12mA,

TA= +25°C

%

±10

Aberrations, Rising and Falling
Edge

I

BIAS

= 25mA, I

MOD

= 12mA, 4ns unit interval ps80PWD

Modulation-Current Pulse­Width Distortion

±15

Loaded with 2.7kΩpull-up resistor to V

CC

V4.5V

OH

FAILOUT Output High

Loaded with 2.7kΩpull-up resistor to V

CC

V0.5V

OL

FAILOUT Output Low

Note 1: Dice are tested at TA= +25°C.
Note 2: I

VCC

= I

VCC

A + I

VCC

B, I

BIAS

= 60mA, I

MOD

= 30mA, and I

PIN

= 140µA.

Note 3: AC characteristics are guaranteed by design and characterization.
Note 4: An 1100mVp-p differential is equivalent to complementary 550mVp-p signals on VIN+ and VIN-.

MAX3261E/D

MAX3261ECJ

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

_______________________________________________________________________________________

3

-250mV

250.3mV

38.23ns 200ps/div

EYE DIAGRAM

(622Mbps, LOAD = 25Ω, NOT FILTERED)

MAX3261-1

50mV/div

40.23ns

-35mV

265mV

38.14ns 200ps/div

EYE DIAGRAM

(622Mbps, LOAD AT OUT- = 1300nm

LASER WITH 467MHz BESSEL FILTER)*

MAX3261-2

30mV/div

40.14ns

-250mV

250.3mV

38.13ns 117ps/div

EYE DIAGRAM

(1062Mbps, LOAD = 25Ω, NOT FILTERED)

MAX3261-3

50mV/div

39.3ns

-50mV

450mV

38.74ns 117ps/div

EYE DIAGRAM

(1062Mbps, LOAD AT OUT- = 1300nm

LASER WITH 800MHz BESSEL FILTER)*

MAX3261-4

50mV/div

39.91ns

-35mV

265mV

38.15ns 200ps/div

MAX3261CCJ EYE DIAGRAM

(622Mbps, LOAD AT OUT- = 1300nm

LASER WITH 467MHz BESSEL FILTER)*

MAX3261-5

30mV/div

40.15ns

-66.2mV

513.7mV

37.78ns 117ps/div

MAX3261CCJ EYE DIAGRAM

(1062Mbps, LOAD AT OUT- = 1300nm

LASER WITH 800MHz BESSEL FILTER)*

MAX3261-6

58mV/div

38.95ns

100

0 500

R

PINSET

vs. MONITOR CURRENT

MAX3261-09

MONITOR CURRENT (µA)

R

PINSET

(Ω)

1000

10,000

1000

100,000

1,000,000

0

0 20 40

R

BIASSET

vs. BIAS CURRENT

5

MAX3261-07

I

BIAS

(mA)

R

BIASSET

(kΩ)

60

3

1

4

2

6

7

8

* LASER = EPITAXX EDL 1300RFC TO-STYLE HEADER

0

0 5 10 15 20 25

R

MODSET

vs. MODULATION CURRENT

8

MAX3261-08

MODULATION CURRENT (mAp-p)

R

MODSET

(kΩ)

30

6

4

2

10

12

DIFFERENTIAL INPUT
SWING = 1100 mVp-p

__________________________________________Typical Operating Characteristics

(MAX3261E/D, load at OUT+ and OUT- = 25Ω, VCC= VCCA = VCCB = +5V, TA= +25°C, unless otherwise noted.)

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

4 _______________________________________________________________________________________

____________________________Typical Operating Characteristics (continued)

(MAX3261E/D, LOAD at OUT+ = OUT- = 25Ω, VCC= VCCA = VCCB = +5V, TA = +25°C, unless otherwise noted.)

-2

-1

0

1

2

3

0 20 40

PERCENT CHANGE IN BIAS

CURRENT vs. TEMPERATURE

MAX3261-11

TEMPERATURE (°C)

% CHANGE (w.r.t. +25°C)

806010 30 7050

APC DISABLED

34

36

38

40

42

44

46

48

50

0 20 40

SUPPLY CURRENT

vs. TEMPERATURE

MAX3261-12

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

8060

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 20 40

PERCENT CHANGE IN MODULATION

CURRENT vs. TEMPERATURE

MAX3261-10

TEMPERATURE (°C)

% CHANGE (w.r.t. +25°C)

8060

0

0 5 10 15 20 25

ALLOWABLE R

OSADJ

vs. MODULATION CURRENT

8

MAX3261-13

MODULATION CURRENT (mAp-p)

ALLOWABLE R

OSADJ

(kΩ)

30

6

4

2

10

12

ALLOWABLE

RANGE

0

0 400 800 1200 1600

MAXIMUM MODULATION CURRENT

vs. MINIMUM DIFFERENTIAL

INPUT SIGNAL AMPLITUDE

25

MAX3261-14

MINIMUM DIFFERENTIAL

INPUT SIGNAL AMPLITUDE

(mVp-p)

MAXIMUM MODULATION CURRENT

(mAp-p)

2000

20
15

10

5

30

35

40

R

MODSET

= 1.2kΩ

R

OSADJ

= 2kΩ

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

_______________________________________________________________________________________ 5

______________________________________________________________Pin Description

NAME FUNCTION

1, 2, 25,

27, 29

GNDA Ground for Bias and Modulation Current Drivers

3 IPIN Monitor Photodiode Current Input. Connect IPIN to photodiode’s anode.

PIN

4 SLWSTRT

Slow-Start Capacitor Input. Connect capacitor to ground or leave unconnected to set start-up time,
t

STARTUP

= 25.4kΩ (C

SLWSTRT

+ 2pF).

5, 9,

11, 13

GNDB Ground for Voltage Reference and Automatic Power-Control Circuitry

8 FAILOUT

Failout Output. Active-low, open-collector TTL output indicates if automatic power-control loop is out
of regulation due to insufficient monitor-diode current (when V

IPIN

is below the 2.6V threshold).

Connect FAILOUT to V

CC_

through a 2.7kΩ pull-up resistor.

7 IPINSET

Monitor Photodiode Programming Input. Connect IPINSET to VREF1 or VREF2 through a resistor to
set the monitor current when using automatic power control (see

Typical Operating Characteristics

).

6 VREF2 Temperature-Compensated Reference Output. VREF2 is internally connected to VREF1.

19 VREF1 Temperature-Compensated Reference Output. VREF1 is internally connected to VREF2.

16 ENB- Inverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low.

14, 15, 18 VCCB

+5V Supply Voltage for Voltage Reference and Automatic Power-Control Circuitry. Connect VCCB to
the same potential as VCCA, but provide separate bypassing for VCCA and VCCB.

12 VIN- Inverting PECL Data Input

10 VIN+ Noninverting PECL Data Input

21 IBIASFB

Bias-Feedback Current Output. Output from automatic power-control circuit. Connect to I

BIASSET

when using APC.

20 OSADJ

Overshoot-Adjust Input. Connect to internal voltage reference through a resistor to adjust the over­shoot of the modulation output signal (see

Typical Operating Characteristics

).

22 IBIASSET

Laser Bias Current-Programming Input. Connect to internal voltage reference through a resistor to set
bias current (see

Typical Operating Characteristics

). I

IBIASOUT

= 40 x (I

IBIASSET

+ I

IBIASFB

).

23 IMODSET

Laser Modulation Current-Programming Input. Connect to internal voltage reference through a resistor
to set modulation current (see

Typical Operating Characteristics

). I

MOD

= 20 x I

IMODSET

.

24 IBIASOUT

Laser Bias Current Output. Connect to laser cathode through an R-L compensation network (see the

Bias Network Compensation

section).

26 OUT- Modulation Output. When VIN+ is high and VIN- is low, OUT- sinks I

MOD

.

30, 31, 32 VCCA

+5V Supply Voltage for Bias and Modulation Current Drivers. Connect VCCA to the same potential as
VCCB, but provide separate bypassing for VCCA and VCCB.

28 OUT+ Modulation Output. When VIN+ is low and VIN- is high, OUT+ sinks I

MOD

.

17 ENB+ Noninverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low.

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

6 _______________________________________________________________________________________

_______________Detailed Description

The MAX3261 laser driver has three main sections: a
reference generator with temperature compensation, a
laser bias block with automatic power control, and a
high-speed modulation driver.

The reference generator provides temperature-com­pensated biasing and a voltage-reference output. The
voltage reference is used to program the current levels
of the high-speed modulation driver, laser diode, and
PIN (p+, intrinsic, n-) monitor diode.

The laser bias block sets the bias current in the laser diode
and maintains it above the threshold current. A current-con­trolled current source (current mirror) programs the bias,
with IBIASSET as the input. The mirror’s gain is approxi­mately 40. Keep the output voltage of the bias stage above

2.2V to prevent saturation.
The modulation driver consists of a high-speed input

buffer and a common-emitter differential output stage. The
modulation current mirror sets the laser modulation current
in the output stage. This current is switched between the
OUT+ and OUT- ports of the laser driver. The modulation

MAX3261

VCCA

V

CC

B

GNDA

GNDB

V

CC

20 x I

IMODSET

40 x I

IBIASSET

IBIASOUT

IPIN

TRANSCONDUCTANCE

AMPLIFIER

1 X I

IPINSET

R

PINSET

IBIASSET

IBIASFB

IPINSET

R

BIASSET

IMODSET

R

MODSET

R

OSADJ

IOSADJ

VIN+

VIN-

ENB+

ENB-

SLWSTRT

VREF1 OR VREF2

LASER

PHOTO-

DIODE

LOOP-

STABILITY

CAPACITOR

1000pF

BIAS

COMPEN-

SATION

OUT+

OUT-

FAILOUT

COMPARATOR

+2.6V

+3V

MAIN

BIAS

GENERATOR

BANDGAP

REFERENCE

Figure 1. Functional Diagram

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

_______________________________________________________________________________________ 7

current mirror has a gain of approximately 20. Keep
the voltages at OUT+ and OUT- above 2.2V to prevent
saturation.

The overshoot mirror sets the bias in the input buffer
stage (Figure 2). Reducing this current slows the input
stage and reduces overshoot in the modulation signal.
At the same time, the peak-to-peak output swing of the
input buffer stage is reduced. Careful design must be
used to ensure that the buffer stage can switch the out­put stage completely. The input swing required to com­pletely switch the output stage depends on both
R

OSADJ

and the modulation current. See Allowable

R

OSADJ

Range vs. Modulation Current and Modulation

Current vs. Differential Input Signal graphs in the

Typical Operating Characteristics

.

Failure to ensure that the output stage switches com­pletely results in a loss of modulation current (and
extinction ratio). In addition, if the modulation port does
not switch completely off, the modulation current will
contribute to the bias current, and may complicate
module assembly.

Automatic Power Control

The automatic power control (APC) feature allows an
optical transmitter to maintain constant power, despite
changes in laser efficiency with temperature or age. The
APC feature requires the use of a monitor photodiode.

The APC circuit incorporates the laser diode, the moni­tor photodiode, the PIN set current mirror, a transcon­ductance amplifier, the bias set current mirror, and the
laser fail comparator (Figure 1). Light produced by the
laser diode generates an average current in the monitor
photodiode. This current flows into the MAX3261’s IPIN
input. The PIN set current mirror draws current away
from the IPIN node. When the current into the IPIN node
equals the current drawn away by IPINSET, the node
voltage is set by the 3/5x VCCreference of the transcon­ductance amplifier. When the monitor current exceeds
IPINSET, the IPIN node voltage will be forced higher. If
the monitor current decreases, the IPIN node voltage is
decreased. In either case, the voltage change is ampli­fied by the transconductance amplifier, and results in a
feedback current at the IBIASFB node. Under normal
APC operation, IBIASFB is summed with IBIASSET, and
the laser bias level is adjusted to maintain constant out­put power. This feedback process continues until the
monitor-diode current equals IPINSET.

If the monitor-diode current is sufficiently less than
IPINSET (i.e., the laser stops functioning), the voltage on

V

CC

OUTPUTS

280Ω 280Ω

9Ω

400Ω

9Ω

2(I

IOSADJ

) I

IMODSET

2(I

IOSADJ

)

INPUT BUFFER OUTPUT STAGE

MAX3261

INPUTS

Figure 2. MAX3261 Modulation Driver (Simplified)

ENB+

DATA OUT
(LOAD = 1300nm
LASER AT OUT-)

2µs/div

Figure 3. Enable/Disable Operation

Table 1. MAX3261 Truth Table

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

8 _______________________________________________________________________________________

the IPIN node will drop below 2.6V. This will trigger the
failout comparator, which provides a TTL signal indicat­ing laser failure. The FAILOUT output asserts only if the
monitor-diode current is low, not in the reverse situation
where the monitor current exceeds IPINSET. FAILOUT is
an open-collector output that requires an external pull-up
resistor of 2.7kΩ to VCC.

The transconductance amplifier can source or sink cur­rents up to approximately 1mA. Since the laser bias
generator has a gain of approximately 40, the APC
function has a limit of approximately 40mA (up or
down) from the initial set point. To take full advantage
of this adjustment range, it may be prudent to program
the laser bias current slightly higher than required for
normal operation. However, do not exceed the I

BIASOUT

absolute maximum rating of 75mA.
To maintain APC loop stability, a 1000pF bypass capaci-

tor may be required across the photodiode. If the APC
function is not used, leave IBIASFB unconnected.

Enable Inputs

The MAX3261 provides complementary enable inputs
(ENB+, ENB-) for interfacing with open-fiber-control
architecture. The laser is disabled by reducing the ref­erence voltage outputs (VREF1, VREF2). Only one logic
state will enable laser operation (Table 1).

With a 1000pF stability capacitor, the MAX3261 modula­tion and bias can be enabled and disabled within 5µs
(Figure 3). This timing satisfies the requirements of the
Open Fiber Control system used in Fibre Channel
networks.

Temperature Considerations

The MAX3261 output currents are programmed by cur­rent mirrors. These mirrors each have a 2VBEtemperature
coefficient. The reference voltage (VREF) is adjusted
2VBEso these changes largely cancel, resulting in output
currents that are very stable with respect to temperature
(see

Typical Operating Characteristics

).

Wire Bonding Die

For reliable operation, the MAX3261 has gold metalliza­tion. Make connections to the die with gold wire only,
using ball bonding techniques. Wedge bonding is not
recommended. Pad size is 4mils.

__________________Design Procedure

Interfacing Suggestions

Use high-frequency design techniques for the board
layout of the MAX3261 laser driver. High-speed inter­faces often require fixed-impedance transmission lines
(Figure 5). Adding some damping resistance in series
with the laser raises the load impedance, making the
transmission line more realizable, and it also helps
reduce power consumption (see the section

Reducing

Power Consumption

). Minimize any series inductance
to the laser, and place a bypass capacitor as close to
the laser’s anode as possible.

Power connections labeled VCCA are used to supply the
laser modulation and laser bias circuits. VCCB connec­tions supply the bias-generator and automatic-power
control circuits. For optimum operation, isolate these sup­plies from each other by independent bypass filtering.

VCCA, VCCB, GNDA, and GNDB all have multiple pins.
Connect all pins to optimize the MAX3261’s high­frequency performance. Ground connections between
signal lines (VIN+, VIN-, OUT+, OUT-) improve the quali­ty of the signal path by reducing the impedance of the
interconnect. Multiple connections, in general, reduce
inductance in the signal path and improve the high­speed signal quality. GND pins should be tied to the
ground plane with short runs and multiple vias. Avoid
ground loops, since they are a source of high-frequency
interference.

The MAX3261 data inputs accept PECL input signals,
which require 50Ωtermination to (VCC- 2V). Figure 4
shows alternative termination techniques. When a ter­mination voltage is not available, use the Thevenin­equivalent termination. When interfacing with a
non-PECL signal source, use one of the other alterna­tive termination methods shown in Figure 4.

Bias Network Compensation

When driving the laser diode with transmission lines, it is
important to maintain a constant load impedance in
order to minimize aberrations due to reflections. The
inductive nature of laser packages will cause the laser
impedance to increase with frequency, and the parasitic
capacitance of the laser driver bias output (IBIASOUT)
has some loading effects at high frequency. Of these
two effects, the loading due to the laser lead induc­tance dominates. Impedance variation must be com­pensated for high-frequency operation. One possible
approach is to use a shunt R-C network in parallel with
the laser diode to compensate for the laser impedance
(Figures 5 and 6). Add an R-L circuit in series with the
bias output to compensate for the IBIASOUT capaci­tance (Figures 5 and 7).

ENB-

0 1

0 0

ENABLED

1

ENB+ OUTPUT CURRENTS

0 DISABLED

DISABLED

1 1 DISABLED

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

_______________________________________________________________________________________ 9

Figure 4. Alternative PECL Data-Input Terminations

THIS SYMBOL REPRESENTS
A TRANSMISSION LINE
WITH CHARACTERISTIC
IMPEDANCE Z

o

= 50Ω.

MAX3261

5V

120Ω

82Ω82Ω

5V

PECL

SIGNAL SOURCE

VIN+

VIN-

120Ω

a) THEVENIN-EQUIVALENT TERMINATION

MAX3261

68Ω

50Ω

50Ω

50Ω

5V

NON-PECL

SIGNAL SOURCE

VIN+

VIN-

180Ω

c) SINGLE-ENDED NON-PECL TERMINATION

MAX3261

1.8k

680Ω

50Ω

5V

NON-PECL

SIGNAL SOURCE

VIN+

VIN-

5V

1.8k

680Ω

50Ω

b) DIFFERENTIAL NON-PECL TERMINATION

MAX3261

5V

3.6k

1.3k1.3k

OV

ECL

SIGNAL SOURCE

-2V

VIN+

VIN-

3.6k

50Ω

50Ω

-2V

d) ECL TERMINATION

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

10 ______________________________________________________________________________________

Reducing Power Consumption

The laser driver typically consumes 40mA of current
for internal functions. Typical load currents, such as
12mA of modulation current and 20mA of bias cur­rent, bring the total current requirement to 72mA. If
this were dissipated entirely in the laser driver, it
would generate 360mW of heat. Fortunately, a sub­stantial portion of this power is dissipated across the
laser diode. A typical laser diode will drop approxi­mately 1.6V when forward biased. This leaves 3.4V at
the MAX3261’s OUT- terminal. It is safe to reduce the
output terminal voltage even further with a series
damping resistor. Terminal voltage levels down to

2.2V can be used without degrading the laser driver’s
high-frequency performance. Power dissipation can
be further reduced by adding a series resistor on the
laser driver’s OUT+ side. Select the series resistor so
the OUT+ terminal voltage does not drop below 2.2V
with the maximum modulation current.

__________Applications Information

Programming the MAX3261 Laser Driver

Programming the MAX3261 is best explained by an
example. Assume the following laser diode characteris­tics:

Wavelength λ 780nm
Threshold Current I

TH

20mA at +25°C

(+0.35mA/°C temperature variation)

Monitor Responsivity ρ

mon

0.1A/W (monitor current /
average optical power into the
fiber)

Modulation Efficiency η 0.1mW/mA (worst case)
Now assume the communications system has the fol-

lowing requirements:
Average Power P

AVE

0dBm (1mW)
Extinction Ratio Er 6dB (Er = 4)
Temperature Range Tr 0°C to +70°C

1) Determine the value of IPINSET:
The desired monitor-diode current is (P

AVE

)(ρ

mon

) =

(1mW)(0.1A/W) = 100µA. The R

PINSET

vs. Monitor

Current graph in the

Typical Operating Characteristics

shows that R

PINSET

should be 18kΩ.

2) Determine R

MODSET

:

The average power is defined as (P1 + P0) / 2, where
P1 is the average amplitude of a transmitted “one”
and P0 is the average amplitude of a transmitted
“zero.” The extinction ratio is P1/P0. Combining these

equations results in P1 = (2 x P

AVE

x Er) / (Er + 1) and

P0 = (2 x P

AVE

) / (Er + 1). In this example, P1 = 1.6mW
and P0 = 0.4mW. The optical modulation is 1.2mW. The
modulation current required to produce this output is

1.2mW / η= (1.2mW) / (0.1mA/mW) = 12mA. The

Typical Operating Characteristics

show that R

MODSET

= 3.9kΩ yields the desired modulation current.

3) Determine the value of R

OSADJ

:

Using the Allowable R

OSADJ

vs. Modulation Current

graph in the

Typical Operating Characteristics

, a 5.6kΩ
resistor is chosen for 12mA of modulation current. The
maximum ROSADJ values given in the graph minimize
aberrations in the waveform and ensure that the driver
stage operates fully limited.

4) Determine the value of R

BIASSET

:

The automatic power control circuit can adjust the bias
current 40mA from the initial setpoint. This feature
makes the laser driver circuit reasonably insensitive to
variations of laser threshold from lot to lot. The bias set­ting can be determined using one of two methods:

a) Set the bias at the laser threshold.
b) Set the bias at the midpoint of the highest and low-

est expected threshold values.

Method A is straightforward. In the second method, it is
assumed that the laser threshold will increase with age.
The lowest threshold current occurs at 0°C, when the
laser is new. The highest threshold current occurs at
+70°C, at the end of the product’s life. Assume the
laser is near the end of life when its threshold reaches
two-times its original value.

Lowest Bias Current:

ITH+ ∆ITH= 20mA + (0.35mA/°C)(-25°C) = 11.25mA

Highest Bias Current:

2 x ITH+ ∆ITH= 40mA + (0.35mA/°C)(+45°C) = 55.8mA

In this case, set the initial bias value to 34mA (which is
the midpoint of the two extremes). The adjustment
range of the MAX3261 maintains the average laser
power at either extreme.

The

Typical Operating Characteristics

show that

R

BIASSET

= 1.8kΩ delivers the required bias current.

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

______________________________________________________________________________________ 11

-35mV

265mV

200ps/div

EYE DIAGRAM WITH R-C AND R-L COMPENSATION

(622Mbps, LOAD AT OUT- = 1300nm

LASER WITH 467MHz BESSEL FILTER)*

MAX3261-FG07

30mV/div

*EPITAXX EDL 1300 RFC, TO-STYLE HEADER

Figure 7. Eye Diagram with R-C and R-L Compensation
(LOAD at OUT- = 1300nm Laser)

-35mV

265mV

200ps/div

EYE DIAGRAM WITH R-C COMPENSATION

(622Mbps, LOAD AT OUT- = 1300nm

LASER WITH 467MHz BESSEL FILTER)*

MAX3261-FG06

30mV/div

*EPITAXX EDL 1300 RFC, TO-STYLE HEADER

Figure 6. Eye Diagram with R-C Compensation (LOAD at
OUT- = 1300nm Laser)

MAX3261

+5V

OUT+

OUT-

IPIN

1000pF

IBIASOUT

25Ω

25Ω

18Ω

200Ω

ZO = 25Ω

MICROSTRIP

LASER

SERIES R-L

SHUNT RC

PHOTO­DIODE

0.01µF

0.01µF
AS CLOSE TO THE
LASER ANODE AS
POSSIBLE

AS CLOSE TO THE
LASER CATHODE AS
POSSIBLE

100pF

47µH

Figure 5. Typical Laser Interface with Bias Compensation

Laser Safety and IEC 825

Using the MAX3261 laser driver alone does not ensure
that a transmitter design is compliant with IEC 825. The
entire transmitter circuit and component selections
must be considered. Each customer must determine
the level of fault tolerance required by their application,
recognizing that Maxim products are not designed or
authorized for use as components in systems intended
for surgical implant into the body, for applications
intended to support or sustain life, or for any other
application where the failure of a Maxim product could
create a situation where personal injury or death may
occur.

MAX3261

Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

____________________________________________________________Chip Topography

IBIASOUT
IMODSET
IBIASSET
IBIASFB
OSADJ
VREF1
VCCB
VCCB
ENB+
ENB-

V

CC

A

V

CC

A

V

CC

A

GNDA

OUT+

OUT+

GNDA

OUT-

OUT-

GNDA

GNDB

VIN+

VIN+

GNDB

VIN-

VIN-

GNDB

N.C.

V

CC

B

V

CC

B

0.080"

(2.032mm)

0.080"

(2.032mm)

GNDA

GNDA

IPIN

SLWSTRT

GNDB
GNDB

VREF2
IPINSET
FAILOUT

N.C.

TRANSISTOR COUNT: 197
SUBSTRATE CONNECTED TO GNDA AND GNDB

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.

12

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

© 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Maxim makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Maxim assume any lia­bility arising out of the application or use of any product or circuit and specifically disclaims any and all liability, without limitation, consequential or inciden­tal damages. “Typical” parameters can and do vary in different applications. All operating parameters, including “typicals” must be validated for each
customer application by customer’s technical experts. Maxim products are not designed, intended or authorized for use as components in systems intend­ed for surgical implant into the body, or for other applications intended to support or sustain life, or for any other application in which the failure of the
Maxim product could create a situation where personal injury or death may occur.