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
1
VCC 1
2
3
4
5
6
7 8 9 101112
13
14
15
16
17
18
192021222324
IN+
*EP
IN-
VCC
BEN+
BEN-
THIN QFN
GND
VCC
GND
LONGB
BIAS-
BIAS+
VCC
OUT+
OUT-
VCC
GND
MD
VCC
BIASMAX
MODSET
APCSET
TOP VIEW
*EXPOSED PAD IS CONNECTED TO GND
MAX3656
EN
FAIL
Pin Configuration
MAX3656
EN
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
+3.3V
10Ω
15Ω
OUT+
5.6Ω
BIAS+
MD
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.
AI
LMD
MD
MOD
e
e
=×
⎛
⎝
⎜⎞
⎠
⎟+
⎛
⎝
⎜⎞
⎠
⎟
−2I r I
rI
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
mA
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
ns
155Mbps 192
622Mbps 96
Burst Off-Time (Note 13) tB-OFF
1.25Gbps, 2.5Gbps 96
ns
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
VCC
VCC
VCC
22.1Ω
49.9Ω
50Ω
22.1Ω
26.7Ω
50Ω
121Ω
OSCILLOSCOPE
26.7Ω
35.7Ω
IMOD
IBIAS
MAX3656
Z0 = 50Ω
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
80
75
70
65
60
55
50
45
40
35
30
-40 -20 -10-30 0 1020304050607080
SUPPLY CURRENT vs. TEMPERATURE
(EXCLUDES IBIAS, IMOD, 15Ω LOAD)
MAX3656 toc06
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
10
15
20
25
30
35
40
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
10
20
15
30
25
35
40
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
_______________________________________________________________________________________
7
10,000
10
0.1 10 100
IMD vs. RAPCSET
100
1000
MAX3656 toc10
RAPCSET (kΩ)
IMD (μA)
1
100
90
80
70
60
50
40
30
20
10
0
1 10 100
IMOD vs. RMODSET
MAX3656 toc11
RMODSET (kΩ)
IMOD (mA)
100
90
80
70
60
50
40
30
20
10
0
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
r
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
VCC
VCC
5kΩ
VCC
5kΩ
16kΩ
24kΩ
IN+
IN-
VCC
VCC
VCC
5kΩ
VCC
5kΩ
16kΩ
24kΩ
BEN+
BEN-
MAX3656
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
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
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
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-
MD
RMODSET
MAX3656
EN FAIL
DAC DSP ASP
VCC
LP
RCOMP
VCC
RP
RD
RD
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 ____________________
15
© 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
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
