MAX3656ETG ,155Mbps to 2.5Gbps Burst-Mode Laser DriverApplicationsFiber-to-the-Home (FTTH) and Fiber-to-the-Functional Diagram appears at end of data she ..
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MAX3656ETG
155Mbps to 2.5Gbps Burst-Mode Laser Driver
General DescriptionThe 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.
ApplicationsFiber-to-the-Home (FTTH) and Fiber-to-the-
Business (FTTB) Broadband Access Systems
Passive Optical Network (PON) Transmitters
APON, EPON, and GPON Upstream Transmitters
FeaturesMultirate Operation from 155Mbps to 2.5GbpsBurst Enable/Disable Delay <2nsBurst On-Time of 576ns to InfinityInfinite Bias-Current Hold Time Between BurstsDC-Coupled Operation with Single +3.3V Power
Supply40mA Typical Supply CurrentProgrammable Bias Current from 1mA to 70mAProgrammable Modulation Current from 10mA
to 85mAAutomatic Average Power Control with Failure
Monitor (No CAPCCapacitor Needed)APC Loop Initialization ≤3 Bursts
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
Pin Configuration
Typical Application Circuit19-2790; Rev 1; 5/04
+ Denotes lead-free package.
*Dice are designed to operate from TJ= -40°C to +120°C, but
are only tested and guaranteed at TA= +25°C.
**EP = Exposed pad.
Ordering Information
Functional Diagram appears at end of data sheet.
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
ABSOLUTE MAXIMUM RATINGSStresses 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-EP
(derate 27.8mW/°C above +85°C).............................1805mW
Operating Ambient Temperature
Range (TA).......................................................-40°C to +85°C
Operating Junction Temperature
Range (TJ).....................................................-55°C to +150°C
Storage Ambient Temperature
Range (TSTG).................................................-55°C to +150°C
Processing Temperature (die).........................................+400°C
Lead Temperature (soldering,10s)..................................+300°C
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. ALMDcan also be calculated by:
where IMD, IMOD, and re(extinction ratio) are set externally.
Note 3:Operation outside this range degrades APC loop performance.
ELECTRICAL CHARACTERISTICS(Typical values are at VCC= +3.3V, IBIAS= 20mA, IMOD= 25mA, extinction ratio = 10dB, and TA= +25°C, unless otherwise noted.)
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
ELECTRICAL CHARACTERISTICS (continued)
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
ELECTRICAL CHARACTERISTICS (continued)(Typical values are at VCC= +3.3V, IBIAS= 20mA, IMOD= 25mA, extinction ratio = 10dB, and TA= +25°C, unless otherwise noted.)
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:
IBIASError: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 ENchanges 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:IBIASerror 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:IBIASerror 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:IBIASerror 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 DriverFigure 1. Enable and Disable Delay Times
Figure 2. Output Termination for Characterization
Typical Operating Characteristics(TA = +25°C, unless otherwise noted.)
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX3656
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
Pin Description
MAX3656
Detailed DescriptionThe 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.
The MAX3656 modulation output is optimized for dri-
ving a 15Ωload. The modulation current can swing up
to 85mA for data rates less than or equal to 1.25Gbps
and up to 60mA for data rates greater than 1.25Gbps
when the laser is DC-coupled. To interface with the
laser diode, a damping resistor (RD) is required for
impedance matching. The combined resistance due to
the series damping resistor and the equivalent series
resistance (ESR) of the laser diode should be equal to
15Ω. The OUT- pin should be connected with a 15Ω
resistor to VCC. To reduce optical output aberrations
and duty-cycle distortion caused by laser diode para-
sitic inductance, an RC shunt network is necessary.
The currents in the BIAS output switch at high speeds
when bursting. Therefore, the BIAS+ pin should be con-
nected directly through a resistor to the cathode of the
laser. The BIAS- pin should be connected to VCC
through a 15Ωresistor.
Automatic Power ControlTo 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 ENpin.
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 Proceduresection.
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
directlyset the laser current.
APC Failure MonitorThe MAX3656 provides an APC failure monitor (TTL) to
indicate an APC loop-tracking failure. FAILis 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 FAILis
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 FAILis asserted. FAILis also set low
during initialization.
Slow-StartFor 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 ControlThe MAX3656 features a chip-enable function. Whenis high, the bias and modulation currents are off and
the digital state of the APC loop is reset. When ENis
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 InitializationThe digital APC loop is reset whenever the power is
turned off and/or the ENinput is driven high. When
power is turned on or when ENis 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
155Mbps to 2.5Gbps Burst-Mode
Laser Driver
