MAX3656ETG+T - Maxim Integrated

General Description

The MAX3656 is a burst-mode laser driver that oper-

ates at data rates from 155Mbps up to 2.5Gbps. The

laser driver accepts either positive-referenced emitter-

coupled logic (PECL) or current-mode logic (CML) data

inputs and provides bias and modulation current for the

laser diode. The device can switch the laser diode from

a completely dark (off) condition to a full (on) condition

(with proper bias and modulation currents) in less than

2ns. The MAX3656 incorporates DC-coupling between

laser driver and laser diode and operates with a single-

supply voltage as low as +3.0V.

A digital automatic power-control (APC) loop is provided

to maintain the average optical power over the full tem-

perature range and lifetime. The APC loop is functional

for a minimum burst on-time of 576ns and minimum

burst off-time of 96ns, with no limit on the maximum

burst on- or off-time. A fail monitor is provided to indi-

cate when the APC loop can no longer maintain the

average power. The MAX3656 can be configured for

nonburst-mode applications (continuous mode) by con-

necting burst enable (BEN) high. For power saving, the

MAX3656 provides enabling and disabling functionality.

The modulation current can be set from 10mA to 85mA

and the bias current can be set from 1mA to 70mA.

The MAX3656 is packaged in a small, 24-pin, 4mm ✕

4mm thin QFN package and consumes only 132mW

(typ), excluding bias and modulation currents.

Applications

Fiber-to-the-Home (FTTH) and Fiber-to-the-

Business (FTTB) Broadband Access Systems

Passive Optical Network (PON) Transmitters

APON, EPON, and GPON Upstream Transmitters

Features

♦Multirate Operation from 155Mbps to 2.5Gbps

♦Burst Enable/Disable Delay <2ns

♦Burst On-Time of 576ns to Infinity

♦Infinite Bias-Current Hold Time Between Bursts

♦DC-Coupled Operation with Single +3.3V Power

Supply

♦40mA Typical Supply Current

♦Programmable Bias Current from 1mA to 70mA

♦Programmable Modulation Current from 10mA

to 85mA

♦Automatic Average Power Control with Failure

Monitor (No CAPC Capacitor Needed)

♦APC Loop Initialization ≤3 Bursts

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

________________________________________________________________

Maxim Integrated Products

VCC 1

7 8 9 101112

192021222324

IN+

*EP

IN-

VCC

BEN+

BEN-

THIN QFN

GND

VCC

GND

LONGB

BIAS-

BIAS+

VCC

OUT+

OUT-

VCC

GND

VCC

BIASMAX

MODSET

APCSET

TOP VIEW

*EXPOSED PAD IS CONNECTED TO GND

MAX3656

FAIL

Pin Configuration

MAX3656

GND

FAIL

LONGB VCC

APCSET

MODSET

BIASMAX

+3.3V

IN-

IN+

SERIAL

DATA

SOURCE

130Ω130Ω

82Ω82Ω

+3.3V

BEN-

BEN+

BURST

CONTROL

130Ω130Ω

82Ω82Ω

OUT-

+3.3V

10nH

75Ω

+3.3V

10Ω

15Ω

OUT+

5.6Ω

BIAS+

5.6Ω

BIAS-

10Ω

27pF

Typical Application Circuit

19-2790; Rev 4; 9/10

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,

or visit Maxim’s website at www.maxim-ic.com.

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

EP = Exposed pad.

Ordering Information

PART TEMP RANGE PIN-PACKAGE

MAX3656ETG -40°C to +85°C 24 Thin QFN-EP*

MAX3656ETG+ -40°C to +85°C 24 Thin QFN-EP*

Functional Diagram appears at end of data sheet.

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

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

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

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

Current into FAIL ...............................................-10mA to +10mA

Voltage at IN+, IN-, BEN+, BEN-, EN,

LONGB...................................................-0.5V to (VCC + 0.5V)

Voltage at MODSET, APCSET, BIASMAX .............-0.5V to +3.0V

Voltage at OUT+, OUT-.............................+0.5V to (VCC + 1.5V)

Voltage at BIAS+, BIAS-............................+0.5V to (VCC + 0.5V)

Continuous Power Dissipation (TA= +85°C)

24-Lead Thin QFN

(derate 27.8mW/°C above +85°C).............................1805mW

Operating Ambient Temperature Range (TA)......-40°C to +85°C

Storage Ambient Temperature Range (TSTG) ...-55°C to +150°C

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

Soldering Temperature (reflow)

Lead(Pb)-free...............................................................+260°C

Containing lead(Pb) .....................................................+240°C

OPERATING CONDITIONS

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Voltage VCC 3.0 3.3 3.6 V

Supply Turn-On Time 10% to 90% 0.001 10 ms

Ambient Temperature -40 +85 °C

Data Mark Density Average 50 %

Consecutive Identical Digits CID 80 Bits

Data Rate 155 2500 Mbps

Monitor Diode Capacitance CMD For minimum burst on-time (Note 1) 15 pF

Laser-to-Monitor Diode Gain ALMD

Δ ( m oni tor cur r ent) /Δ ( l aser cur r ent ( ab ove IT H

) )

( N otes 2, 3) 0.005 0.050 A/A

Extinction Ratio re 10 log (P1/P0) (Note 3) 8.2 12.0 dB

Note 1: Larger MD capacitance increases the minimum burst on-time.

Note 2: Laser-to-monitor gain equals the laser slope efficiency multiplied by the photodiode responsivity multiplied by the losses due

to laser-to-monitor diode coupling (ALMD = ηLASER ✕ρMONITORDIODE ✕LLASER-TO-MONITORDIODE).

where L = laser-to-monitor diode coupling loss. ALMD can also be calculated by:

where IMD, IMOD, and re(extinction ratio) are set externally.

Note 3: Operation outside this range degrades APC loop performance.

LMD

MOD

=×

⎛

⎝

⎜⎞

⎠

⎟+

⎛

⎝

⎜⎞

⎠

⎟

−2I r I

ELECTRICAL CHARACTERISTICS

(Typical values are at VCC = +3.3V, IBIAS = 20mA, IMOD = 25mA, extinction ratio = 10dB, and TA= +25°C, unless otherwise noted.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

POWER SUPPLY

Power-Supply Current ICC (Note 1) 40 70 mA

INPUT SPECIFICATIONS

Differential Input Voltage VIN, VBEN 0.2 1.6 VP-P

Common-Mode Input Voltage VCM VCC -

1.49

VCC -

1.32

VCC -

VIN/4 V

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Typical values are at VCC = +3.3V, IBIAS = 20mA, IMOD = 25mA, extinction ratio = 10dB, and TA= +25°C, unless otherwise noted.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Single-Ended Input Voltage VBEN+,

VBEN- 0.8 V

EN Input High Voltage VIH 2.0 V

EN Input Low Voltage VIL 0.8 V

FAIL Output High Voltage VOH Sourcing 50μA, VCC = 3.0V 2.4 V

FAIL Output Low Voltage VOL Sinking 100μA, VCC = 3.6V 0.4 V

BIAS GENERATOR

Bias-On Current Range IBIAS Voltage at BIAS pin 0.6V 1 70 mA

Bias-Off Current Range IBIAS-OFF EN = high or BEN = low, VBIAS  2.6V (Note 2) 5 100 μA

IBIAS = 70mA 148

Bias-Current Temperature

Stability APC open loop

IBIAS = 15mA 98 ppm/°C

Bias-Current Absolute

Accuracy APC open-loop IBIAS > 20mA (Note 3) -15 +15 %

BIASMAX Current-Setting Range 15 70 mA

APC LOOP

MD Reverse-Bias Voltage VMD With respect to VCC 1.6 V

IMD = 50μA -750 +750

MD Bias-Setting Stability

(Note 4) IMD = 1500μA -480 +480

ppm/°C

IMD = 50μA -25 +25

MD Bias-Setting Accuracy

(Note 3) IMD = 1500μA -15 +15 %

MD DC-Current Range IMD Average current into MD pin 50 1500 μA

Case 1 (Note 5) (LONGB = 0) 12

Case 2 (Note 6) (LONGB = 0) 2.12

APC Loop Initialization Time

(Note 4) tINIT

Case 3 (Note 7) (LONGB = 0) 1.60 1.92

μs

LASER MODULATOR

Data rate 1.25Gbps 10 85

Modulation ON Current Range IMOD Data rate >1.25Gbps 10 60

Modulation OFF Current IMOD-OFF EN = high or BEN = low, IN = low (Note 2) 16 150 μA

Modulation-Current Stability (Note 13) -480 +480 ppm/°C

Modulation-Current Absolute

Accuracy I

MOD > 15mA (Note 3) -15 +15 %

10mA  IMOD < 60mA 0.6

Instantaneous Voltage at

Modulator Output (OUT+) 60mA  IMOD  85mA 0.75 V

Modulation-Current Rise Time tR 10mA  IMOD  85mA (Notes 8, 13) 40 85 ps

Modulation-Current Fall Time tF10mA  IMOD  85mA (Notes 8, 13) 40 85 ps

Output Over-/Undershoot 20 %

155Mbps to 1.25Gbps, 10mA  IMOD  85mA 17 45

Deterministic Jitter

(Notes 9, 13) DJ 1.25Gbps to 2.5Gbps, 10mA  IMOD  60mA 17 40 psP-P

Random Jitter RJ (Note 13) 0.8 1.4 psRMS

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(Typical values are at VCC = +3.3V, IBIAS = 20mA, IMOD = 25mA, extinction ratio = 10dB, and TA= +25°C, unless otherwise noted.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

BURST-MODE SPECIFICATIONS

Burst Enable Delay APC closed loop (Notes 10, 11, 13) 2.3 ns

Burst Disable Delay APC closed loop (Notes 10, 12, 13) 2.0 ns

155Mbps 2881

622Mbps 720

Burst On-Time (Note 13) tB-ON

1.25Gbps, 2.5Gbps 576

155Mbps 192

622Mbps 96

Burst Off-Time (Note 13) tB-OFF

1.25Gbps, 2.5Gbps 96

OPTICAL EVALUATION

155.52Mbps 45

622.08Mbps 40

1.24416Gbps 31

Optical Eye Diagram Mask

Margin

ExceLight

SLT2886-LR

laser diode

(or equivalent) 2.48832Gbps 24

Note 1: Excludes IBIAS and IMOD. Maximum value is specified at IMOD = 85mA, IBIAS = 70mA, and IMD = 1.5mA.

Note 2: For safety purposes, both the bias and modulation currents are switched off if any of the current set pins (BIASMAX, MODSET)

are grounded.

Note 3: Accuracy refers to part-to-part variation.

Note 4: APC loop initialization definitions:

IBIAS Error: IBIAS - IBIASSET, where IBIAS = the actual bias current and IBIASSET = the level of bias current set by the RAPCSET

resistor.

Initialization Case 1: Continuous Mode Power-Up. In this case, EN = low, BEN = high, and then VCC is ramped up from

0V to ≥3.0V.

Initialization Case 2: Chip-Enable Reset. In this case, 3.0V ≤VCC ≤3.6V, BEN = high, and then EN changes from high to low.

Initialization Case 3: Burst-Mode Startup. In this case, 3.0V ≤VCC ≤3.6V, EN = low, and then BEN changes from low to high.

Note 5: IBIAS error is less than 3.8mA (for an extinction ratio of 10dB and IMD = 1500µA) within 12µs from the time that VCC ≥3.0V.

Note 6: IBIAS error is less than 3.8mA (for an extinction ratio of 10dB and IMD = 1500µA) within 2.1µs (typ) from the time that EN < 0.8V.

Note 7: IBIAS error must be less than 3.8mA (for an extinction ratio of 10dB and IMD = 1500µA) at or before the end of the third

burst following the transition of BEN from low to high. For the shortest burst on- and off-time (576ns and 96ns), this trans-

lates to 1.92µs from when BEN toggles from low to high for the first time after startup.

Note 8: Rise and fall times are measured as 20% to 80% of the output amplitude with a repeating 0000011111.

Note 9: Deterministic jitter is measured with a continuous data pattern (no bursting) of 27- 1 PRBS + 80 consecutive ones + 27- 1

PRBS + 80 consecutive zeros.

Note 10: Measured electrically with a resistive load matched to the laser driver output.

Note 11: Enable delay is measured between (1) the time at which the rising edge of the differential burst enable input signal reach-

es the midpoint of the voltage swing, and (2) the time at which the combined output currents (bias and modulation) reach

90% of the final level set by RAPCSET, RBIASMAX, and RMODSET (after all transients such as overshoot, ringing, etc., have

settled to within 10% of their final values). See Figure 1. Measurement done for 10mA ≤IMOD ≤85mA and 4mA ≤IBIAS ≤

70mA.

Note 12: Disable delay is measured between (1) the time at which the falling edge of the differential burst enable input signal reaches

the midpoint of the voltage swing, and (2) the time at which the combined output currents (bias and modulation) fall below

10% of the bias on current (after transients have settled). See Figure 1. Measurement done for 10mA ≤IMOD ≤85mA and

4mA ≤IBIAS ≤70mA.

Note 13: Guaranteed by design and characterization.

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

_______________________________________________________________________________________ 5

Figure 1. Enable and Disable Delay Times

BEN+

BEN-

ENABLE DELAY DISABLE DELAY

BEN ±MIDPOINT

IFINAL × 110%

IFINAL

IFINAL × 90%

10% OF IBIAS

IBIAS

IMOD

Figure 2. Output Termination for Characterization

OUT-

OUT+

BIAS+

BIAS-

VCC

22.1Ω

49.9Ω

50Ω

22.1Ω

26.7Ω

50Ω

121Ω

OSCILLOSCOPE

26.7Ω

35.7Ω

IMOD

IBIAS

MAX3656

Z0 = 50Ω

Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

OPTICAL EYE DIAGRAM

(155.52Mbps, 117MHz FILTER,

PATTERN = 223 - 1 PRBS)

MAX3656 toc01

EXCELIGHT SLT2886-LR LASER

AVERAGE OPTICAL POWER = -6dBm

EXTINCTION RATIO = 15dB

MASK MARGIN = 45%

OPTICAL EYE DIAGRAM

(622.08Mbps, 467MHz FILTER,

PATTERN = 223 - 1 PRBS)

MAX3656 toc02

EXCELIGHT SLT2886-LR LASER

AVERAGE OPTICAL POWER = -6dBm

EXTINCTION RATIO = 15dB

MASK MARGIN = 40%

OPTICAL EYE DIAGRAM

(1.24416Gbps, 933MHz FILTER,

PATTERN = 223 - 1 PRBS)

MAX3656 toc03

EXCELIGHT SLT2886-LR LASER

AVERAGE OPTICAL POWER = -6dBm

EXTINCTION RATIO = 12dB

MASK MARGIN = 31%

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

OPTICAL EYE DIAGRAM

(2.48832Gbps, 2.3GHz FILTER,

PATTERN = 223 - 1 PRBS)

MAX3656 toc04

EXCELIGHT SLT2886-LR LASER

AVERAGE OPTICAL POWER = -6dBm

EXTINCTION RATIO = 10dB

MASK MARGIN = 24%

ELECTRICAL EYE DIAGRAM

(2.5Gbps, IMOD = 30mA,

PATTERN = PRBS 27 - 1 + 80 CID)

MAX3656 toc05

100ps/div

-40 -20 -10-30 0 1020304050607080

SUPPLY CURRENT vs. TEMPERATURE

(EXCLUDES IBIAS, IMOD, 15Ω LOAD)

MAX3656 toc06

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

10 30 4020 50 60 70 80

DETERMINISTIC JITTER vs. IMOD

MAX3656 toc07

IMOD (mA)

DETERMINISTIC JITTER (psP-P)

DATA RATE = 2.5Gbps

PATTERN = 27 - 1PRBS + 80 CID

VIN = 200mVP-P

200 800 1000400 600 1200 1400 1600

DETERMINISTIC JITTER

vs. INPUT AMPLITUDE

MAX3656 toc08

INPUT AMPLITUDE (mVP-P)

DETERMINISTIC JITTER (psP-P)

IMOD = 30mA

PATTERN = 27 - 1PRBS + 80 CID

DATA RATE = 2.5Gbps

0.5

0.7

0.6

0.9

0.8

1.1

1.0

1.2

1.4

1.3

1.5

10 30 40

20 50 60 70 80

RANDOM JITTER vs. IMOD

MAX3656 toc09

IMOD (mA)

RANDOM JITTER (psRMS)

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

_______________________________________________________________________________________

10,000

0.1 10 100

IMD vs. RAPCSET

100

1000

MAX3656 toc10

RAPCSET (kΩ)

IMD (μA)

100

1 10 100

IMOD vs. RMODSET

MAX3656 toc11

RMODSET (kΩ)

IMOD (mA)

100

1 10 100

IBIASMAX vs. RBIASMAX

MAX3656 toc12

RBIASMAX (kΩ)

IBIASMAX (mA)

TIMING DIAGRAM, BURST ON

MAX3656 toc13

500ps/div

VMOD+

VBIAS+

BURST-ENABLE

SIGNAL

TIMING DIAGRAM, BURST OFF

MAX3656 toc14

1ns/div

VMOD+

VBIAS+

BURST-DISABLE

SIGNAL

Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

8 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

1, 4, 9,

15, 18, 21 VCC Power-Supply Voltage

2 IN+ Noninverting Data Input with On-Chip Biasing

3 IN- Inverting Data Input with On-Chip Biasing

5 BEN+ Noninverting Burst-Enable Input with On-Chip Biasing

6 BEN- Inverting Burst-Enable Input with On-Chip Biasing

7EN TTL/CMOS Enable Input. Low for normal operation. Pull high or leave unconnected to disable laser

bias and modulation currents.

8, 11, 19 GND Power-Supply Ground

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

12 LONGB TTL/CMOS Long Burst (See the Setting the LONGB Input Pin Section)

13 BIAS- Inverting Laser Bias-Current Output. Connect through 10 resistor and switching diode to VCC.

14 BIAS+

Noninverting Laser Bias-Current Output. Bias current flows into this pin when BEN is high. Minimize

capacitance on this pin.

16 OUT+

Noninverting Laser Modulation-Current Output. Modulation current flows into this pin when BEN and IN

are high.

17 OUT-

Inverting Laser Modulation-Current Output. Connect through 15 resistor and switching diode to laser

diode anode.

20 MD

Monitor Diode Input. Connect this pin to the anode of the monitor diode. Leave unconnected for open-

loop operation. Minimize capacitance on this pin.

22 BIASMAX

Maximum Bias Current Set. A resistor connected from this pin to ground sets the maximum bias

current. The bias current cannot exceed this level. The APC loop controls the bias current up to the

level of the BIASMAX. For APC open-loop operation, this pin sets the laser bias current.

23 MODSET Modulation Current Set. A resistor connected from this pin to ground sets the desired modulation current.

24 APCSET

Average Power Control Set. A resistor connected from this pin to ground sets the desired average

optical power. Connect a 50k resistor to ground for APC open-loop operation.

— EP Exposed Pad. Ground. This pad must be soldered to ground.

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

_______________________________________________________________________________________ 9

Detailed Description

The MAX3656 laser driver has three main parts: a high-

speed modulator, a high-speed bias driver, and a laser-

biasing block with automatic power control (see the

Functional Diagram

). Both the bias and modulation output

stages are composed of differential pairs with program-

mable current sources. The circuit design is optimized for

high-speed, low-voltage (3.3V), DC-coupled operation.

The device is ideal for burst-mode operation with turn-on

and turn-off times less than 2ns. The MAX3656 can be

configured for nonburst-mode applications (continuous

mode) by connecting BEN high.

Automatic Power Control

To maintain constant average optical power, the

MAX3656 incorporates a digital automatic power-con-

trol (APC) loop to compensate for the changes in laser

threshold current over temperature and lifetime. A

back-facet photodiode mounted in the laser package

converts the optical power into a photocurrent. The

APC loop adjusts the laser bias current so the monitor

current is matched to a reference current set by RAPCSET.

At startup, the APC loop traverses through a pseudo-

binary search algorithm to set the proper monitor current

that translates to the proper bias current. When BEN is

high, the APC loop maintains constant optical power by

digitally controlling the bias current. When BEN is low,

the APC loop digitally stores the bias current value of the

previous burst. The APC loop is reset in two ways, either

power cycling or toggling the EN pin.

An external resistor (RBIASMAX) sets the maximum allow-

able bias current during closed-loop operation and sets

the bias current during open-loop operation. An APC fail-

ure flag (FAIL) is set low during initialization and when

the bias current cannot be adjusted to achieve the

desired average optical power.

APC closed-loop operation requires that the user set

three currents with external resistors connected between

GND, BIASMAX, MODSET, and APCSET pins. Detailed

guidelines for these resistor settings are described in the

Design Procedure

section.

If necessary, the MAX3656 is fully operational without

APC. To operate the MAX3656 open loop, connect a

50kΩresistor from APCSET to ground and leave the

MD pin unconnected. In this case, two external resis-

tors connected from BIASMAX and MODSET to GND

directly set the laser current.

APC Failure Monitor

The MAX3656 provides an APC failure monitor (TTL) to

indicate an APC loop-tracking failure. FAIL is set low

when the APC loop cannot adjust the bias current to

maintain the desired monitor current. For example, the

laser diode requires more bias current (to maintain a

constant optical output) than maximum bias current set

by RBIASMAX. The bias current is limited and FAIL is

asserted. In an alternate example, assume that a circuit

failure causes the cathode of the laser diode to be short-

ed to GND, thereby causing an uncontrolled high optical

output. In this case, the APC loop cannot decrease the

user current, and FAIL is asserted. FAIL is also set low

during initialization.

Slow-Start

For safety reasons, at initial power-up or after toggling

EN, the MAX3656 incorporates a slow-start circuit that

provides a typical delay of 450ns during the beginning

of APC loop initialization.

Enable Control

The MAX3656 features a chip-enable function. When

EN is high, the bias and modulation currents are off and

the digital state of the APC loop is reset. When EN is

toggled from a high to a low, the APC loop begins ini-

tialization. The initialization time is typically 2.1µs

(LONGB = low) and 3.72µs (LONGB = high).

APC Loop Initialization

The digital APC loop is reset whenever the power is

turned off and/or the EN input is driven high. When

power is turned on or when EN is toggled low, the APC

loop automatically performs an initialization routine that

quickly adjusts the bias current from its reset level to its

initialized level. The initialized bias current level is

defined to be within 3.8mA of the final bias current level

set by the APCSET resistor. Once initialized, the APC

loop enters its fine-adjustment mode of operation and

adjusts the bias current to match the level set by the

APCSET resistor. There are three different cases in

which the APC loop starts initialization, and each has a

unique initialization time. These cases are defined

as follows:

•Continuous-Mode Power-Up

In continuous-mode power-up, the chip is enabled

(EN = low) and the burst-enable input is high (BEN =

high) when power is applied to the laser driver. APC

loop initialization begins when the power-supply volt-

age rises above the minimum specified limit of +3.0V.

The BEN input remains high indefinitely and the laser

driver operates in continuous (nonbursting) mode. In

this case, the initialization time is 12µs (typ).

•Chip-Enable Reset

In chip-enable reset, the power-supply voltage is

within the specified limits and BEN is high. The chip-

enable input (EN) is initially high (chip disabled and

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

10 ______________________________________________________________________________________

APC loop reset), and then it is driven low (chip-

enabled). In this case, APC loop initialization begins

when the voltage at EN drops below the specified

EN input low voltage of 0.8V. After initialization

begins, the laser driver can be operating in burst

mode (BEN toggling high and low) or continuous

mode (BEN = high). In this case, the initialization

time is 2.1µs (typ).

•Burst-Mode Startup

In burst-mode startup, the power-supply voltage is

within the specified limits and the chip is enabled

(EN = low). The burst-enable input is low (BEN =

low) and has not been in the high state since the

APC loop was reset. APC loop initialization begins

when the BEN input is driven high. After initialization

begins, the laser driver can be operating in burst

mode (BEN toggling high and low) or continuous

mode (BEN = high). In this case, the initialization

time is 1.6µs (typ).

In each of the three cases listed, initialization is complete

within three bursts (bursts must comply with specified

burst on- and burst off-time) or the time specified in the

Electrical Characteristics

table, whichever comes first.

Burst-Mode Operation

The bias and modulation outputs (BIAS+ and OUT+)

can be switched on and off quickly using the differential

burst-enable inputs (BEN+ and BEN-). Once the APC

loop has initialized, the bias and modulation outputs are

switched on when BEN+ = high and BEN- = low and are

switched off when BEN+ = low and BEN- = high.

When BEN is switched on, the laser driver sinks the bias

and modulation currents set by the APCSET, BIASMAX,

and MODSET resistors within the maximum BEN delay

time of 2.3ns. For stable APC loop operation, the mini-

mum burst length is limited to the burst on-time listed in

the

Electrical Characteristics

table. The maximum burst-

on time is unlimited.

When BEN is switched off, the bias and modulation cur-

rents fall below the specified bias-off and modulation-

off currents within the maximum burst disable delay

time of 2.0ns. For stable APC loop operation, the mini-

mum burst off-time is limited to the value listed in the

Electrical Cha

acteristics

table. The maximum burst off-

time is unlimited.

Short-Circuit Protection

The MAX3656 provides short-circuit protection for the

modulation and bias-current sources. If BIASMAX or

MODSET is shorted to ground, the bias and modulation

outputs are 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 shows the relationships 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 wave-

form are 50%.

Programming the Modulation Current

For a given laser power (PAVG), slope efficiency (η),

and extinction ratio (re), the modulation current can be

calculated using Table 1. See the IMOD vs. RMODSET

graph in the

Typical Operating Characteristics

, and

select the value of RMODSET that corresponds to the

required current at +25°C.

Programming the Bias Current

When the MAX3656 is used in open-loop operation, the

RBIASMAX resistor determines the bias current. To

select this resistor, determine the required bias current.

See the IBIASMAX vs. RBIASMAX graph in the

Typical

Operating Characteristics

, and select the value of

RBIASMAX that corresponds to the required current. For

open-loop operation, connect a 50kΩresistor from

RAPCSET to GND, and leave the MD pin open.

When using the MAX3656 in closed-loop operation, the

RBIASMAX resistor sets the maximum bias current avail-

able to the laser diode over temperature and lifetime.

The APC loop can subtract from this maximum value,

but cannot add to it. See the IBIASMAX vs. RBIASMAX

graph in the

Typical Operating Characteristics

and

select the value of RBIASMAX that corresponds to the

end-of-life bias current at +85°C. The RBIASMAX resistor

should not be set less than 5kΩ.

Table 1. Optical Power Definition

PARAMETER SYMBOL RELATION

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

Extinction ratio rere = P1 / P0

Optical power high P1P1 = 2PAVG × re / (re + 1)

Optical power low P0P0 = 2PAVG / (re + 1)

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

Laser slope

efficiency ηη = PP-P / IMOD

Modulation current IMOD IMOD = PP-P / η

Laser-to-monitor

diode gain ALMD (2 x IMD / IMOD)((re - 1) /

(re + 1))

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

______________________________________________________________________________________ 11

Programming the APC Loop

When using the MAX3656’s APC feature, program the

average optical power by adjusting the APCSET resis-

tor. To select this resistor, determine the desired moni-

tor current to be maintained over temperature and

lifetime. See the IMD vs. RAPCSET graph in the

Typical

Operating Characteristics

and select the value of

RAPCSET that corresponds to the required current.

Setting the LONGB Input Pin

Set the LONGB pin according to Table 2 to optimize

APC loop operation.

Interfacing with Laser Diodes

The laser modulation current output OUT+ is optimized

to drive a 15Ωload and must be DC-coupled. A series

damping resistor, RD, provides impedance matching to

the laser diode. The combined value of the series

damping resistor and the laser diode equivalent series

resistance should be close to 15Ω. An RC shunt com-

pensation network, RCOMP/CCOMP, connected between

the laser diode cathode and ground should be provided

to reduce optical output aberrations and duty-cycle dis-

tortion caused by laser diode parasitic inductance. The

values of RCOMP and CCOMP can be adjusted to match

the laser diode and PC board layout characteristics for

optimal optical eye performance (refer to Application

Note 274:

HFAN-02.0: Interfacing Maxim’s Laser Drivers

with Laser Diodes

). The OUT- pin is connected through

a 15Ωresistor and switching diode to the laser diode

anode. The switching diode at OUT- improves the opti-

cal output eye and burst-enable delay by better match-

ing the laser diode characteristics.

For data rates greater than 1Gbps, a parallel RL peak-

ing network, RP/LP, connected between the laser diode

anode and VCC is recommended. This network creates

a differential drive for the laser diode to improve

rise/fall times and reduce jitter. The values of RPand

LPare also adjusted to match the laser diode and PC

board layout characteristics for optimal optical eye

performance.

Current in the BIAS output switches at high speed

when bursting; therefore, the BIAS+ pin should be con-

nected directly through a resistor, equal to RDas

determined above, to the laser diode cathode. The

BIAS- pin is connected through a 10Ωresistor and

switching diode to VCC.

Input Termination Requirements

The MAX3656 data and BEN inputs are internally biased.

Although the inputs are compatible with LVPECL signals,

it is not necessary to drive the MAX3656 with a standard

LVPECL signal. While DC-coupled, the MAX3656 oper-

ates properly as long as the specified common-mode

voltage and differential voltage swings are met.

Because of the on-chip biasing network (Figure 3), the

MAX3656 inputs self-bias to the proper operating point

to accommodate AC-coupling.

See Figures 4 and 5 for connecting to PECL or CML

data outputs.

Table 2. Setting the LONGB Input Pin

LONGB CONDITION

0 Burst on-time ≤1.2µs

0 or 1 Burst on-time >1.2µs or

continuous mode operation

1 Data rates of 155Mbps

Figure 3. MAX3656 Internal Biasing

VCC

5kΩ

VCC

5kΩ

16kΩ

24kΩ

IN+

IN-

VCC

5kΩ

VCC

5kΩ

16kΩ

24kΩ

BEN+

BEN-

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

12 ______________________________________________________________________________________

Design Example

Select Laser

Select a communication-grade laser for the proper data

rate. Assume the laser output average power is PAVG =

0dBm, the operating temperature is -40°C to +85°C,

and the laser diode has the following characteristics:

wavelength: λ= 1.3µm, threshold current: ITH = 22mA

at +25°C, threshold temperature coefficient: βTH =

1.3%/°C, laser-to-monitor transfer: ρMON = 0.2A/W

(ρMON = ρMONITORDIODE x LLASER-TO-MONITORDIODE),

and laser slope efficiency: η= 0.05mW/mA at +25°C.

Determining RAPCSET

The desired monitor diode current is estimated by IMD =

PAVG ✕ρMON = 200µA. The IMD vs. RAPCSET graph in the

Typical Operating Characteristics

shows RAPCSET at 12kΩ.

Determining RMODSET

Assuming re= 10 and an average power of 0dBm (1mW),

the peak-to-peak optical power PP-P = 1.64mW (Table 1).

The required modulation current is 1.64(mW)/0.05(mW/mA)

= 32.8mA. The IMOD vs. RMODSET graph in the

Typical

Operating Characteristics

shows RMODSET at 9kΩ.

Determining RBIASMAX

Determine the maximum threshold current (ITH(MAX)) at

TA= +85°C and end of life. Assuming (ITH(MAX)) =

50mA, the maximum bias current should be:

IBIASMAX = ITH(MAX)

In this example, IBIASMAX = 50mA. The IBIASMAX

vs. RBIASMAX graph in the

Typical Operating Charact-

eristics

shows RBIASMAX at 8kΩ.

Figure 4. Connecting to PECL Outputs

MAX3656

82Ω

130Ω

ZO = 50Ω

VCC

IN+

82Ω

130Ω

ZO = 50Ω

VCC

IN-

82Ω

130Ω

ZO = 50Ω

VCC

BEN+

82Ω

130Ω

ZO = 50Ω

VCC

BEN-

Figure 5. Connecting to CML Outputs

MAX3656

100Ω

ZO = 50ΩIN+

ZO = 50ΩIN-

100Ω

ZO = 50Ω

BEN+

BEN-

Figure 6. Single-Ended Biasing for Burst Enable

IN+

IN-

BEN+

BEN-

VCMBEN + (100mV to 800mV)

VCMBEN - (100mV to 800mV)

VCMBEN

VCMBEN = +2.0V

3.3V

R1 = 1.65kΩ

R2 = 2.54kΩ

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

______________________________________________________________________________________ 13

Applications Information

Running Burst Enable Single-Ended

With PECL signal levels, for single-ended operation of

burst enable, connect the BEN+ to the burst-enable

control. Connect a resistor (R1) from VCC to BEN- and

resistor (R2) from BEN- to ground. The parallel combi-

nation of R1and R2should be less than 1kΩ. Choose

the values of R1and R2to set the common-mode volt-

age in the range defined in the

Electrical

Characteristics

table (see Figure 6).

With LVTTL or LVCMOS signal levels, for single-ended

operation of burst enable connect a 3kΩ(R4) resistor

from the burst-enable signal to BEN+. Connect a 5kΩ

(R3) resistor from VCC to BEN+. Connect a 5kΩresistor

(R5) from VCC to BEN- and a 9kΩresistor (R6) from

BEN- to ground. For typical LVTTL or LVCMOS specifi-

cations of VCC to 2.0V for a high and 0.8V to 0V for a

low, the LVTTL or LVCMOS sources a maximum of

75µA and sinks a maximum of 500µA. See Figure 7 for

setting up the single-ended LVTTL or LVCMOS biasing

for burst enable.

Layout Considerations

To minimize inductance, keep the connections between

the MAX3656 output pins and laser diode as close as

possible. Place a bypass capacitor as close as possi-

ble to each VCC connection. Take extra care to mini-

mize stray parasitic capacitance on the BIAS and MD

pins. Use good high-frequency layout techniques and

multilayer boards with uninterrupted ground planes to

minimize EMI and crosstalk.

Laser Safety and IEC825

Using the MAX3656 laser driver alone does not ensure

that a transmitter design is compliant with IEC825. The

entire transmitter circuit and component selections

must be considered. Each user must determine the

level of fault tolerance required by the application, rec-

ognizing that Maxim products are neither designed nor

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 in which the failure of a Maxim product

could create a situation where personal injury or death

can occur.

Figure 7. Single-Ended LVTTL or LVCMOS Biasing for Burst

Enable

IN+

IN-

BEN+

BEN-

LVTTL OR LVCMOS HIGH

LVTTL OR LVCMOS LOW VCC

R5 = 5kΩ

R4 = 3kΩ

R6 = 9kΩ

VCC

R3 = 5kΩ

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

14 ______________________________________________________________________________________

Functional Diagram

IN+

IN-

BEN+

BEN-

APC

RBIASMAX LONGB RAPCSET

IMOD

IBIAS

OUT-

OUT+

BIAS+

BIAS-

RMODSET

MAX3656

EN FAIL

DAC DSP ASP

VCC

RCOMP

VCC

CCOMP

Chip Information

TRANSISTOR COUNT: 8153

PROCESS: SiGe BIPOLAR

Package Information

For the latest package outline information and land patterns,

go to www.maxim-ic.com/packages. Note that a “+”, “#”, or

“-” in the package code indicates RoHS status only. Package

drawings may show a different suffix character, but the drawing

pertains to the package regardless of RoHS status.

PACKAGE

TYPE

PACKAGE

CODE

OUTLINE

NO.

LAND

PATTERN NO.

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

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

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

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

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

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 3/03 Initial release —

1 5/04 Added the lead-free package to the Ordering Information table 1

2 10/04

Added the package code for the lead-free package to the Ordering Information table;

updated the Operating Conditions table; updated the Running Burst Enable Singled-

Ended section; added the Design Example section; updated Figure 7; added the Burst

Enable Delay section; updated the Chip Topography diagram

1, 2, 11,

12, 14

3 7/10

Removed the dice and package code information from the Ordering Information table;

replaced the Typical Application Circuit; added the soldering temperature to the Absolute

Maximum Ratings section; updated the optical eye mask margins in the Electrical

Characteristics table; updated the optical eye diagrams and added 2.488Gbps eye

diagram in the Typical Operating Characteristics section; updated the descriptions for

pins 13 and 17 with diodes in the Pin Description table; removed redundant text in the

Detailed Description; replaced the Interfacing with Laser Diodes section; deleted the

bonding coordinates (Table 3 and Burst Enable Delay and Wire-Bonding Die sections);

replaced the Functional Diagram; deleted the Chip Topography section; added the

Package Information table

1, 2, 5–9,

11, 13, 14

4 9/10

Removed the operating junction temperature range from the Absolute Maximum Ratings

section; added a sentence stating the minimum RBIASMAX value to the Programming the

Bias Current section

2, 10