Maxim MAX3869EHJ, MAX3869E-D Datasheet

General Description

The MAX3869 is a complete, single +3.3V laser driver
for SDH/SONET applications up to 2.5Gbps. The
device accepts differential PECL data and clock inputs
and provides bias and modulation currents for driving a
laser. A synchronizing input latch can be used (if a
clock signal is available) to reduce jitter.

An automatic power control (APC) feedback loop is
incorporated to maintain a constant average optical
power over temperature and lifetime. The wide modula­tion current range of 5mA to 60mA and bias current of
1mA to 100mA are easy to program, making this prod­uct ideal for use in various SDH/SONET applications.

The MAX3869 also provides enable control, two current
monitors that are directly proportional to the laser bias
and modulation currents, and a failure-monitor output to
indicate when the APC loop is unable to maintain the
average optical power. The MAX3869 is available in a
small 32-pin TQFP package as well as dice.

Applications

SONET/SDH Transmission Systems

Add/Drop Multiplexers

Digital Cross-Connects

Section Regenerators

2.5Gbps Optical Transmitters

Features

♦ Single +3.3V or +5V Power Supply

♦ 64mA Supply Current at +3.3V

♦ Programmable Bias Current from 1mA to 100mA

♦ Programmable Modulation Current from

5mA to 60mA

♦ Bias Current and Modulation Current Monitors

♦ 87ps Rise/Fall Time

♦ Automatic Average Power Control with Failure

Monitor

♦ Complies with ANSI, ITU, and Bellcore

SDH/SONET Specifications

♦ Enable Control

*EP = Exposed Paddle.
**Dice are designed to operate over this range, but are tested and

guaranteed at TA= +25°C only. Contact factory for availability.

Ordering Information

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

________________________________________________________________ Maxim Integrated Products 1

EVALUATION KIT

AVAILABLE

124Ω 124Ω

SERIALIZER

WITH

CLOCK GEN.

124Ω

23Ω

LD

25Ω

+3.3V

+3.3V

0.056µF

0.01µF

BIASMON

MODMON

+3.3V

1000pF

BIASMAX

LATCH

ENABLE

FAIL

MODSET

APCSET

APCFILT

CLK-

CLK+

DATA-

DATA+

FERRITE
BEAD

OUT+

BIAS

MD

OUT-

CAPC

124Ω

84.5Ω 84.5Ω 84.5Ω84.5Ω

MAX3869

MAX3890

Typical Application Circuit

19-1570; Rev 0; 12/99

Pin Configuration appears at end of data sheet.

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

Dice**

32 TQFP-EP*

PIN-PACKAGETEMP. RANGE

-40°C to +85°C

-40°C to +85°CMAX3869E/D

MAX3869EHJ

PART

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

2 _______________________________________________________________________________________

IMD= 1mA

IMD= 18µA

90

ENABLE = low (Note 4)

APC open loop

(Note 3)

900

(Note 2)

(Note 5)

Sourcing 50µA

ENABLE, LATCH

(Note 6)

ENABLE, LATCH

I

BIAS

= 100mA

APC open loop

Figure 1

PECL compatible

Sinking 100µA

CONDITIONS

%

-15 15

Monitor-Diode Bias Absolute
Accuracy

ppm/°C

-480 50 480

Monitor-Diode Bias Setpoint
Stability

µA

18 1000

I

MD

Monitor-Diode DC Current Range

V

1.5

Monitor-Diode Reverse Bias
Voltage

V

0.1 0.44

TTL Output Low Voltage FAIL

µA

100

I

BIAS-OFF

Bias Off-Current

mA

1 100

I

BIAS

mA

64 112

I

CC

Supply Current

Bias Current Range

V

2.4 VCC- 0.3 V

CC

TTL Output High Voltage FAIL

V

0.8

TTL Input Low Voltage

V

2.0

TTL Input High Voltage

µA

-1 10

I

IN

Clock and Data Input Current

ppm/°C

230

Bias-Current Stability

%

-15 15

(Note 5)Bias-Current Absolute Accuracy

mVp-p

200 1600

V

ID

Differential Input Voltage

V

VCC- VCC- VCC-

1.49 1.32 V

ID

/4

V

ICM

Common-Mode Input Voltage

UNITSMIN TYP MAXSYMBOLPARAMETER

DC ELECTRICAL CHARACTERISTICS

(VCC= +3.14V to +5.5V, TA= -40°C to +85°C. Typical values are at VCC= +3.3V, I

MOD

= 30mA, I

BIAS

= 60mA, TA= +25°C, unless

otherwise noted.) (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.

Supply Voltage, VCC............................................. -0.5V to +7.0V

Current into BIAS ...........................................-20mA to +150mA

Current into OUT+, OUT- ................................-20mA to +100mA

Current into MD.....................................................-5mA to +5mA

Voltage at DATA+, DATA-, CLK+, CLK-, ENABLE,

LATCH, FAIL, BIASMON, MODMON .....-0.5V to (V

CC

+ 0.5V)

Voltage at APCFILT, CAPC, MODSET,

BIASMAX, APCSET ...........................................-0.5V to +3.0V

Voltage at OUT+, OUT-.............................+1.5V to (V

CC

+ 1.5V)

Voltage at BIAS .........................................+1.0V to (VCC+ 0.5V)

Continuous Power Dissipation (T

A

= +85°C)

32-Pin TQFP-EP (derate 22.2mW/°C above +85°C) ..1444mW

Storage Temperature Range .............................-65°C to +165°C

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

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

Lead Temperature (soldering, 10s) .................................+300°C

ABSOLUTE MAXIMUM RATINGS

I

BIAS/IBIASMON

A/A

37

A

BIAS

BIASMON to I

BIAS

Gain

I

MOD/IMODMON

A/A

29

A

MOD

MODMON to I

MOD

Gain

I

BIAS

= 1mA

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

_______________________________________________________________________________________ 3

AC ELECTRICAL CHARACTERISTICS

(V

CC

= +3.14V to +5.5V, load as shown in Figure 2, TA= -40°C to +85°C. Typical values are at VCC= +3.3V, I

MOD

= 30mA, TA= +25°C.)

(Note 7)

Note 1: Dice are tested at T

A

= +25°C only.

Note 2: Tested at R

MODSET

= 2.49kΩ, R

BIASMAX

= 1.69kΩ, excluding I

BIAS

and I

MOD

.

Note 3: Voltage on BIAS pin is (V

CC

- 1.6V).

Note 4: Both the bias and modulation currents will be switched off if any of the current set pins are grounded.
Note 5: Accuracy refers to part-to-part variation.
Note 6: Assuming that the laser to monitor-diode transfer function does not change with temperature. Guaranteed by design and

characterization.

Note 7: AC characteristics are guaranteed by design and characterization.
Note 8: Measured with 622Mbps 0-1 pattern, LATCH = high.
Note 9: PWD = (wider pulse - narrower pulse) / 2.
Note 10: See Typical Operating Characteristics for worst-case distribution.

I

MOD

= 5mA

300

78

LATCH = high, Figure 3

LATCH = high, Figure 3

20% to 80% (Note 8)

ps

69

(Note 8)

20% to 80% (Note 8)

ENABLE = low (Note 4)

I

MOD

= 60mA

(Note 5)

Jitter BW = 12kHz to 20MHz, 0-1 pattern

(Notes 8, 9)

CONDITIONS

t

R

Output Rise Time ps

ps

p-p

720

Jitter Generation

ps

14 50

PWDPulse-Width Distortion

mA

560

I

MOD

Modulation-Current Range

ps

100

t

H

ps

100

t

SU

Input Latch Setup Time

Input Latch Hold Time

bits

80

Maximum Consecutive Identical
Digits

ns

250

Enable/Start-Up Delay

%

±15

Output Aberrations

79

t

F

Output Fall Time

µA

200

I

MOD-OFF

Modulation-Off Current

ppm/°C

-480 -8 480

Modulation-Current Stability

%

-15 15

Modulation-Current Absolute
Accuracy

87 (Note 10)

UNITSMIN TYP MAXSYMBOLPARAMETER

MAX3869EHJ

MAX3869E/D

MAX3869EHJ

MAX3869E/D

DATA+

DATA-

(DATA+) - (DATA-)

I

OUT

+

100mV MIN

800mV MAX

200mVp-p MIN

1600mVp-p MAX

I

MOD

Figure 1. Required Input Signal and Output Polarity

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

4 _______________________________________________________________________________________

CLK

DATA

t

CLK

= 402ps

t

SU

t

H

Figure 3. Setup/Hold Time Definition

0.056µF

OUT+

BIAS

OUT-

A

B

A

A, B ARE SMD FERRITE BEADS:
B = BLM11A601S MURATA ELECTRONICS
A = BLM21A102S MURATA ELECTRONICS

B

V

CC

50Ω

15Ω

OSCILLOSCOPE

50Ω

25Ω

0.056µF

V

CC

MAX3869

I

OUT

+

Figure 2. Output Termination for Characterization

Typical Operating Characteristics

(V

CC

= +3.3V, load as shown in Figure 2, TA= +25°C, unless otherwise noted.)

EYE DIAGRAM

(2.488Gbps, 1300nm FP LASER,

1.87GHz FILTER, 32 TQFP-EP)

MAX3869-01

48ps/div

MITSUBISHI ML725C8F LASER DIODE

0

5

15

10

20

25

84

83

87

32 TQFP-EP
I

MOD

= 30mA

MEAN = 87.3ps
σ = 1.6ps

88 90

89

8685

91

92

TYPICAL DISTRIBUTION OF FALL TIME

MAX3869-02

FALL TIME (ps)

PERCENT OF UNITS (%)

0

5

20

15

10

30

25

35

114.5

113

119

32 TQFP-EP
I

MOD

= 60mA

V

CC

= 3.14V

T

A

= +85°C

MEAN = 119.1ps
σ = 2.0ps

120.5 123.5

122

117.5116

125

126.5

DISTRIBUTION OF FALL TIME

(WORST-CASE CONDITIONS)

MAX3869-03

FALL TIME (ps)

PERCENT OF UNITS (%)

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

_______________________________________________________________________________________ 5

120

0

110

100

300

I

BIASMAX

vs. R

BIASMAX

MAX3869-07

R

BIASMAX

(kΩ

)

I

BIASMAX

(mA)

40

80

100

20

60

1.2

0

0.1 1 10 100

IMD vs. R

APCSET

0.4

MAX3869-09

R

APCSET

(kΩ

)

I

MD

(mA)

0.6

0.8

1.0

1.1

0.3

0.2

0.1

0.5

0.7

0.9

100

0

1 10 100

I

MOD

vs. R

MODSET

20

10

MAX3869-08

R

MODSET

(kΩ

)

I

MOD

(mA)

40

30

60

70

50

80

90

0

10

60

50

40

30

20

90

70

80

100

-40 -15 10 35 60 85

SUPPLY CURRENT vs. TEMPERATURE

(EXCLUDE I

BIAS

, I

MOD

, 25Ω LOAD)

MAX3869-10

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

VCC = +5.5V

VCC = +3.14V

0

10

30

20

40

50

-40 -15 10 35 60 85

BIAS-CURRENT MONITOR GAIN

vs. TEMPERATURE

MAX3869-11

TEMPERATURE (°C)

GAIN (I

BIAS

/I

BIASMON

)

I

BIAS

= 100mA, I

MOD

= 50mA

I

BIAS

= 10mA, I

MOD

= 10mA

ELECTRICAL EYE DIAGRAM

(I

MOD

= 30mA, 213-1 +80 CID, 32 TQFP-EP)

MAX3869-04

100ps/div

250mV/div

ELECTRICAL EYE DIAGRAM

(I

MOD

= 60mA, 213-1 +80 CID, 32 TQFP-EP)

MAX3869-05

100ps/div

400mV/div

3.0

4.0

3.5

5.0

4.5

6.0

5.5

6.5

7.5

7.0

8.0

5 15202510 30 35 40 45 50

RANDOM JITTER vs. I

MOD

MAX3869-06

I

MOD

(mA)

RANDOM JITTER (psp-p)

Typical Operating Characteristics (continued)

(V

CC

= +3.3V, load as shown in Figure 2, TA= +25°C, unless otherwise noted.)

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

6 _______________________________________________________________________________________

Pin Description

Typical Operating Characteristics (continued)

(V

CC

= +3.3V, load as shown in Figure 2, TA= +25°C, unless otherwise noted.)

NAME FUNCTION

1, 4, 7 V

CC

1

Power Supply for Digital Circuits

2 DATA+ Noninverting PECL Input

PIN

3 DATA- Inverting PECL Input

8 LATCH TTL/CMOS Latch Input. High for latched data, low for direct data. Internal 100kΩ pull-up to VCC.

6 CLK- Negative PECL Clock Input. Leave unconnected if latch function is not used.

5 CLK+ Positive PECL Clock Input. Connect to VCCif latch function is not used.

14 APCFILT Connect a capacitor (C

APCFILT

= 0.1µF) from this pad to ground to filter the APC noise.

12 MODMON

Modulation Current Monitor. Sink current source that is proportional to the laser modulation
current.

11 BIASMON Bias Current Monitor. Sink current source that is proportional to the laser bias current.

10, 15 GND1 Ground for Digital Circuits

9 ENABLE

TTL/CMOS Enable Input. High for normal operation, low to disable laser bias and modulation
current. Internal 100kΩ pull-up to V

CC

.

19 OUT+ Positive Modulation-Current Output. I

MOD

flows through this pad when input data is high.

16, 18, 21 V

CC4

Power Supply for Output Circuitry

17 BIAS Laser Bias Current Output

13

FAIL

TTL/CMOS Failure Output. Indicates APC failure when low.

0.

5

15

10

20

25

52010 30 40 50 60

PULSE-WIDTH DISTORTION

vs. I

MOD

MAX3869-13

I

MOD

(mA)

PWD (ps)

V

CC

= +5V

VCC = +3.3V

0

5

10

15

20

25

30

35

40

-40 -15 10 35 60 85

MODULATION-CURRENT MONITOR GAIN

vs. TEMPERATURE

MAX3869-12

TEMPERATURE (°C)

GAIN (I

MOD

/I

MODMON

)

I

BIAS

= 100mA, I

MOD

= 50mA

I

BIAS

= 10mA, I

MOD

= 10mA

_______________Detailed Description

The MAX3869 laser driver consists of two main parts: a
high-speed modulation driver and a laser-biasing block
with automatic power control (APC). The circuit design
is optimized for both high-speed and low-voltage
(+3.3V) operation. To minimize the pattern-dependent
jitter of the input signal at speeds as high as 2.5Gbps,
the device accepts a differential PECL clock signal for
data retiming. When LATCH is high, the input data is
synchronized by the clock signal. When LATCH is low,
the input data is directly applied to the output stage.

The output stage is composed of a high-speed differential
pair and a programmable modulation current source.
Since the modulation output drives a maximum current
of 60mA into the laser with an edge speed of 100ps,
large transient voltage spikes can be generated (due to
the parasitic inductance). These transients and the
laser forward voltage leave insufficient headroom for
the proper operation of the laser driver if the modulation
output is DC-coupled to the laser diode. To solve this
problem, the MAX3869’s modulation output is designed
to be AC-coupled to the cathode of a laser diode. An
external pull-up 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 V

CC

. A simplified functional diagram

is shown in Figure 4.

The MAX3869 modulation output is optimized for driv­ing a 25Ω load; the minimum required voltage at OUT+
is 2.0V. Modulation current swings of 80mA are possi­ble, but due to minimum power-supply and jitter
requirements at 2.5Gbps, the specified maximum mod­ulation current is limited to 60mA. To interface with the
laser diode, a damping resistor (R

D

) is required for
impedance matching. An RC shunt network may also
be necessary to compensate for the laser-diode para­sitic inductance, thereby improving the optical output
aberrations and duty-cycle distortion.

At the data rate of 2.5Gbps, any capacitive load at the
cathode of a laser diode will degrade the optical output
performance. Since the BIAS output is directly connect­ed to the laser cathode, minimize the parasitic capaci­tance associated with this pin by using an inductor to
isolate the BIAS pin from the laser cathode.

Automatic Power Control

To maintain constant average optical power, the
MAX3869 incorporates an APC loop to compensate for
the changes in laser threshold current over temperature
and lifetime. A back-facet photodiode mounted in the

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

_______________________________________________________________________________________ 7

NAME FUNCTIONPIN

25 V

CC3

Power Supply for APC

24 MD

Monitor Diode Input. Connect this pad to a monitor photodiode anode. A capacitor to ground
is required to filter high-speed AC monitor photocurrent.

20 OUT- Negative Modulation-Current Output. I

MOD

flows through this pad when input data is low.

32 V

CC2

Power Supply for Internal Reference

31 BIASMAX

A resistor connected from this pad to ground sets the maximum bias current. The APC
function can subtract from this maximum value, but cannot add to it.

30 MODSET A resistor connected from this pad to ground sets the desired modulation current.

29 APCSET

A resistor connected from this pad to ground sets the desired average optical power.
Connect 100kΩ from this pad to ground if APC is not used.

26 CAPC

A capacitor connected from this pad to ground controls the dominant pole of the APC feed­back loop (C

APC

= 0.1µF).

Pin Description (continued)

22 GND4 Ground for Output Circuitry

23 GND3 Ground for APC

27 GND2 Ground for Internal Reference

28 N.C. No Connection. Leave unconnected.

MAX3869

laser package is used to convert the optical power into
a photocurrent. The APC loop adjusts the laser bias
current so that the monitor current is matched to a ref­erence current set by R

APCSET

. The time constant of
the APC loop is determined by an external capacitor
(C

APC

). To eliminate the pattern-dependent jitter asso­ciated with the APC loop-time constant, and to guaran­tee loop stability, the recommended value for C

APC

is

0.1µF.

When the APC loop is functioning, the maximum allow­able bias current is set by an external resistor, R

BIASMAX

.

An APC failure flag (FAIL) is set low when the bias current
can no longer be adjusted to achieve the desired aver­age optical power. To filter out the APC loop noise, use

an external capacitor at APCFILT with a recommended
value of 0.1µF.

APC closed-loop operation requires the user to set three
currents with external resistors connected between
ground and BIASMAX, MODSET, and APCSET. Detailed
guidelines for these resistor settings are described in
the Design Procedure section.

Open-Loop Operation

If necessary, the MAX3869 is fully operational without
APC. In this case, the laser current is directly set by two
external resistors connected from ground to BIASMAX
and MODSET. See the Design Procedure section for
more details on open-loop operation.

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

8 _______________________________________________________________________________________

LATCH

C

D

V

CC

1000pF

I

MD

R

APCSET

R

BIASMAX

R

MODSET

APCSET

CAPC

C

APC

BIASMAX

MD

MODSET

FAIL

BIAS

R

D

25Ω

I

MOD

I

BIAS

L

P1

V

CC

DATA

CLK

ENABLE

BIASMON

MOBMON

OUT+

OUT-

DQ

165x

FAILURE

DETECTOR

40x

5x

29

MAX3869

R

P

L

P2

I

MOD

37

I

BIAS

0

MUX

1

Figure 4. Functional Diagram

Optional Data Input Latch

To minimize input data pattern-dependent jitter, the dif­ferential clock signal should be connected to the data
input latch, which is selected by an external LATCH
control. If LATCH is high, the input data is retimed by
the rising edge of CLK+. If LATCH is low, the input data
is directly connected to the output stage. When this
latch function is not used, connect CLK+ to VCCand
leave CLK- unconnected.

Enable Control

The MAX3869 incorporates a laser driver enable func­tion. When ENABLE is low, both the bias and modulation
currents are off. The typical laser enable time is 250ns,
and the typical disable time is 25ns.

Current Monitors

The MAX3869 features bias- and modulation-current
monitor outputs. The BIASMON output sinks a current
equal to 1/37 of the laser bias current (I

BIAS

/ 37). The
MODMON output sinks a current equal to 1/29 of the
laser modulation current (I

MOD

/ 29). BIASMON and
MODMON should be connected through a pull-up resis­tor to VCC. Choose a pull-up resistor value that ensures a
voltage at BIASMON greater than VCC- 1.6V and a volt­age at MODMON greater than VCC- 1.0V.

Slow-Start

For laser safety reasons, the MAX3869 incorporates a
slow-start circuit that provides a delay of 250ns for
enabling a laser diode.

APC Failure Monitor

The MAX3869 provides an APC failure monitor
(TTL/CMOS) to indicate an APC loop tracking failure.
FAIL is set low when the APC loop can no longer adjust
the bias current to maintain the desired monitor current.

Short-Circuit Protection

The MAX3869 provides short-circuit protection for the
modulation, bias, and monitor current sources. If either
BIASMAX, MODSET, or APCSET is shorted to ground,
the bias and modulation output will be turned off.

Design Procedure

When designing a laser transmitter, the optical output is
usually expressed in terms of average power and extinc­tion ratio. Table 1 gives the 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%.

Programming the Modulation Current

For a given laser power P

AVG

, slope efficiency η, and

extinction ration re, the modulation current can be calcu­lated using Table 1. See the I

MOD

vs. R

MODSET

graph in
the Typical Operating Characteristics and select the
value of R

MODSET

that corresponds to the required cur-

rent at +25°C.

Programming the Bias Current

When using the MAX3869 in open-loop operation, the
bias current is determined by the R

BIASMAX

resistor. To
select this resistor, determine the required bias current
at +25°C. See the I

BIASMAX

vs. R

BIASMAX

graph in the
Typical Operating Characteristics and select the value
of R

BIASMAX

that corresponds to the required current at

+25°C.

When using the MAX3869 in closed-loop operation, the
R

BIASMAX

resistor sets the maximum bias current avail­able to the laser diode over temperature and life. The
APC loop can subtract from this maximum value but
cannot add to it. See the I

BIASMAX

vs. R

BIASMAX

graph
in the Typical Operating Characteristics and select the
value of R

BIASMAX

that corresponds to the end-of-life

bias current at +85°C.

Programming the APC Loop

When the MAX3869’s APC feature is used, program the
average optical power by adjusting the APCSET resistor.
To select this resistor, determine the desired monitor cur­rent to be maintained over temperature and life. See the
IMDvs. R

APCSET

graph in the Typical Operating

Characteristics and select the value of R

APCSET

that cor-

responds to the required current.

Interfacing with Laser Diodes

To minimize optical output aberrations caused by signal
reflections at the electrical interface to the laser diode, a
series damping resistor (R

D

) is required (Figure 4).

Additionally, the MAX3869 outputs are optimized for a
25Ω load. Therefore, the series combination of R

D

and

RL(where RLrepresents the laser-diode resistance)

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

_______________________________________________________________________________________ 9

PARAMETER SYMBOL RELATION

Average Power P

AVGPAVG

= (P0+ P1) / 2

Extinction Ratio r

e

re= P1/ P

0

Optical Power High P

1

P1= 2P

AVG

· r

e

/ (re+ 1)

Optical Power Low P

0

P0= 2P

AVG

/ (re+ 1)

Optical Amplitude Pp-p Pp-p = 2P

AVG(re

- 1) / (re+ 1)

Laser Slope
Efficiency

η

η = Pp-p / I

MOD

Modulation Current I

MOD

I

MOD

= Pp-p / η

Table 1. Optical Power Definition

MAX3869

should equal 25Ω. Typical values for RDare 18Ω to
23Ω. For best performance, a bypass capacitor (0.01µF
typical) should be placed as close as possible to the
anode of the laser diode. Depending on the exact char­acteristics of the laser diode and PC board layout, a
resistor (RP) of 20Ω to 70Ω in parallel with pull-up induc­tor LP1can be useful in damping overshoot and ringing
in the optical output.

In some applications (depending on laser-diode para­sitic inductance characteristics), an RC shunt network
between the laser cathode and ground will help mini­mize optical output aberrations. Starting values for most
coaxial lasers are R = 75Ω in series with C = 3.3pF.
These values should be experimentally adjusted until
the optical output waveform is optimized.

Pattern-Dependent Jitter

When transmitting NRZ data with long strings of con­secutive identical digits (CIDs), LF droop can occur
and contribute to pattern-dependent jitter (PDJ). To
minimize this PDJ, three external components must be
properly chosen: capacitor C

APC

, which dominates the
APC loop time constant; pull-up inductor LP; and AC­coupling capacitor CD.

To filter out noise effects and guarantee loop stability,
the recommended value for C

APC

is 0.1µF. This results
in an APC loop bandwidth of 10kHz or a time constant
of 16µs. As a result, the PDJ associated with an APC
loop time constant can be ignored.

The time constant associated with the output pull-up
inductor (L

P

≈

LP2), and the AC-coupling capacitor (CD)
will also impact the PDJ. For such a second-order net­work, the PDJ due to the low frequency cutoff will be
dominated by LP. For a data rate of 2.5Gbps, the rec­ommended value for CDis 0.056µF. During the maxi­mum CID period t,it is recommended to limit the peak
voltage droop to less than 12% of the average (6% of
the amplitude). The time constant can be estimated by:

12% = 1 - e

-t

/

τ

L

P

τ

LP

= 7.8t

If τLP= LP/ 25Ω, and t = 100UI = 40ns, then LP= 7.8µH.
To reduce the physical size of this element (LP), use of
SMD ferrite beads is recommended (Figure 2).

Input Termination Requirement

The MAX3869 data and clock inputs are PECL compat­ible. However, it is not necessary to drive the MAX3869
with a standard PECL signal. As long as the specified
common-mode voltage and the differential voltage
swings are met, the MAX3869 will operate properly.

Calculating Power Consumption

The junction temperature of the MAX3869 dice must be
kept below +150°C at all times. The total power dissipa­tion of the MAX3869 can be estimated by the following:

P = V

CC

· I

CC

+ (VCC- Vf) · I

BIAS

+ I

MOD(VCC

- 25Ω · I

MOD

/ 2)

where I

BIAS

is the maximum bias current set by R

BIAS-

MAX

, I

MOD

is the modulation current, and Vfis the typi-

cal laser forward voltage.

Junction Temperature = P(W) · 45 (°C/W)

___________Applications Information

An example of how to set up the MAX3869 follows.

Select Laser

A communication-grade laser should be selected for

2.488Gbps applications. Assume the laser output aver­age power is P

AVG

= 0dBm, minimum extinction ratio is
re= 6.6 (8.2dB), the operating temperature is -40°C to
+85°C, and the laser diode has the following character­istics:

Wavelength: λ = 1.3µm

Threshold Current: ΙTH= 22mA at +25°C

Threshold Temperature
Coefficient: βTH= 1.3%/°C

Laser to Monitor Transfer: ρ

MON

= 0.2A/W

Laser Slope Efficiency: η = 0.05mW/mA

at +25°C

Determine R

APCSET

The desired monitor diode current is estimated by
IMD= P

AVG

·

ρ

MON

= 200µA. The IMDvs. R

APCSET

graph in the Typical Operating Characteristics shows
that R

APCSET

should be 6.0kΩ.

Determine R

MODSET

To achieve a minimum extinction ratio (re) of 6.6dB over
temperature and lifetime, calculate the required extinc­tion ratio at +25°C. Assuming re= 20, the peak-to-peak
optical power P

p-p

= 1.81mW, according to Table 1. The
required modulation current is 1.81(mW) / 0.05(mW/mA)
= 36.2mA. The I

MOD

vs. R

MODSET

graph in the Typical

Operating Characteristics shows that R

MODSET

should

be 4.8kΩ.

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

10 ______________________________________________________________________________________

Determine R

BIASMAX

Calculate the maximum threshold current (I

TH(MAX)

) at

TA= +85°C and end of life. Assuming I

TH(MAX)

=

50mA, the maximum bias current should be:

I

BIASMAX

= I

TH(MAX)

+ I

MOD

/2

In this example, I

BIASMAX

= 68.1mA. The I

BIASMAX

vs.

R

BIASMAX

graph in the Typical Operating Characteristics

shows that R

BIASMAX

should be 3.2kΩ.

Modulation Currents Exceeding 60mA

With a +5V power supply, the headroom voltage for the
MAX3869 is significantly improved. In this case, it is
possible to achieve a modulation current of more than
60mA with AC-coupling, if the junction temperature is
kept below 150°C. The MAX3869 can also be DC-cou­pled to a laser diode when operating with a +5V sup­ply; the voltage at OUT+ should be ≥2.0V for proper
operation.

Wire Bonding Die

For high current density and reliable operation, the
MAX3869 uses gold metalization. Make connections to
the die with gold wire only, using ball-bonding tech­niques. Wedge bonding is not recommended. Die-pad
size is 4 mils (100µm) square, and die thickness is 12
mils (300µm) square.

Layout Considerations

To minimize inductance, keep the connections between
the MAX3869 output pins and LD as close as possible.
Optimize the laser diode performance by placing a
bypass capacitor as close as possible to the laser
anode. Use good high-frequency layout techniques
and multilayer boards with uninterrupted ground planes
to minimize EMI and crosstalk.

Laser Safety and IEC 825

Using the MAX3869 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.

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

______________________________________________________________________________________ 11

Chip Information

TRANSISTOR COUNT: 1561

SUBSTRATE CONNECTED TO GND

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

12 ______________________________________________________________________________________

Pin Configuration Chip Topography

MAX3869

TOP VIEW

32 28

293031

25

26

27

BIASMAX

MODSET

APCSET

N.C.

V

CC2

GND2

CAPC

V

CC3

10

13

15

14

1611 12

9

ENABLE

BIASMON

GND1

FAIL

MODMON

GND1

APCFILT

V

CC4

17

18

19

20

21

22

23

GND3

24 MD

GND4

V

CC4

OUT-

OUT+

V

CC4

BIAS

2

3

4

5

6

7

8LATCH

V

CC1

CLK-

CLK+

V

CC1

DATA-

DATA+

1V

CC1

LATCH

GND1

V

CC1

CLK+ GND1 DATA- V

CC1

CLK- V

CC1VCC1

DATA+ GND1

ENABLE

GND1

GND1
BIASMON

MODMON

FAIL

GND4

N.C.

APCFILT

GND4

V

CC4

MDGND4N.C.OUT+V

CC4

OUT-N.C. N.C. GND3V

CC4

BIAS

V

CC2

GND2

BIASMAX

MODSET

GND2
APCSET

N.C.
GND3

N.C.

GND3
N.C.

CAPC
V

CC3

GND3

0.083"

(2.108mm)

0.070"

(1.778mm)

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

______________________________________________________________________________________ 13

Package Information

32L,TQFP.EPS

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

14 ______________________________________________________________________________________

Package Information (continued)

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

______________________________________________________________________________________ 15

NOTES

MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC

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, including 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 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.

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

© 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

NOTES