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19-2790; Rev 0; 3/03
155Mbps to 2.5Gbps Burst-Mode Laser Driver
General Description
The MAX3656 is a burst-mode laser driver that operates at data rates from 155Mbps up to 2.5Gbps. The laser driver accepts either positive-referenced emittercoupled 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 singlesupply voltage as low as +3.0V. A digital automatic power-control (APC) loop is provided to maintain the average optical power over the full temperature 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 indicate when the APC loop can no longer maintain the average power. The MAX3656 can be configured for nonburst-mode applications (continuous mode) by connecting 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.
Features
o Multirate Operation from 155Mbps to 2.5Gbps o Burst Enable/Disable Delay <2ns o Burst On-Time of 576ns to Infinity o Infinite Bias-Current Hold Time Between Bursts o DC-Coupled Operation with Single +3.3V Power Supply o 40mA Typical Supply Current o Programmable Bias Current from 1mA to 70mA o Programmable Modulation Current from 10mA to 85mA o Automatic Average Power Control with Failure Monitor (No CAPC Capacitor Needed) o APC Loop Initialization 3 Bursts
MAX3656
Ordering Information
PART MAX3656E/D MAX3656ETG TEMP RANGE -40C to 85C -40C to 85C PIN-PACKAGE Dice* 24 Thin QFN-EP** PKG CODE -- T2444-1
*Dice are designed to operate from -40C to +85C, but are only tested and guaranteed at TA = +25C. **EP = Exposed pad. Functional Diagram appears at end of data sheet.
Applications
Fiber-to-the-Home (FTTH) and Fiber-to-theBusiness (FTTB) Broadband Access Systems Passive Optical Network (PON) Transmitters APON, EPON, and GPON Upstream Transmitters
Typical Application Circuit
Pin Configuration
VCC
GND
VCC
24 VCC IN+ INVCC BEN+ BEN1 2 3 4 5 6 7 EN
23
22
21
20
19 18 17 16 VCC OUTOUT+ VCC BIAS+ BIAS-
IN+ SERIAL DATA SOURCE 100 IN-
BIASMAX
APCSET
MODSET
VCC OUTOUT+ BIAS+ VCC BIASLONGB MD
VCC
MD
TOP VIEW
BIASMAX
MODSET
APCSET
MAX3656
BEN+ BURST CONTROL 100 BEN-
MAX3656
15 14 13
GND
GND
GND
THIN QFN **EXPOSED PAD IS CONNECTED TO GND
________________________________________________________________ MAXIM Integrated Products
LONGB
VCC
FAIL
EN
8
9
10
11
12
FAIL
1
For pricing, delivery, and ordering information, please contact MAXIM/Dallas Direct! at 1-888-629-4642, or visit MAXIM's website at www.MAXIM-ic.com.
155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
ABSOLUTE MAXIMUM RATINGS
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 = +85C) 24-Lead Thin QFN-EP (derate 27.8mW/C above +85C) .............................1805mW Operating Ambient Temperature Range (TA).......................................................-40C to +85C Operating Junction Temperature Range (TJ) .....................................................-55C to +150C Storage Ambient Temperature Range (TSTG) .................................................-55C to +150C Processing Temperature (die) .........................................+400C Lead Temperature (soldering,10s) ..................................+300C
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.
OPERATING CONDITIONS
PARAMETER Supply Voltage Supply Turn-On Time Ambient Temperature Data Mark Density Consecutive Identical Digits Data Rate Monitor Diode Capacitance Laser-to-Monitor Diode Gain Extinction Ratio CMD ALMD re For minimum burst on-time (Note 1) d (monitor current)/d (laser current (above ITH)) (Notes 2, 3) P1/P0 (Note 3) 0.005 6.6 CID 155 Average SYMBOL VCC 10% to 90% CONDITIONS MIN 3.0 0.001 -40 50 80 2500 15 0.050 16.0 TYP 3.3 MAX 3.6 10 +85 UNITS V ms C % Bits Mbps pF
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 = LASER MONITORDIODE LLASER-TO-MONITORDIODE). where L = laser-to-monitor diode coupling loss. ALMD can also be calculated by:
2 xI r - I MD e ALMD = IMOD re + I
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 = +25C, unless otherwise noted.)
PARAMETER POWER SUPPLY Power-Supply Current INPUT SPECIFICATIONS Differential Input Voltage Common-Mode Input Voltage VIN, VBEN VCM 0.2 VCC 1.49 VCC 1.32 1.6 VCC VIN/4 VP-P V ICC (Note 1) 40 70 mA SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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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 = +25C, unless otherwise noted.)
PARAMETER Single-Ended Input Voltage EN Input High Voltage EN Input Low Voltage FAIL Output High Voltage FAIL Output Low Voltage BIAS GENERATOR Bias-On Current Range Bias-Off Current Range Bias-Current Temperature Stability Bias-Current Absolute Accuracy BIASMAX Current-Setting Range APC LOOP MD Reverse-Bias Voltage MD Bias-Setting Stability (Note 4) MD Bias-Setting Accuracy (Note 3) MD DC-Current Range APC Loop Initialization Time (Note 4) LASER MODULATOR Modulation ON Current Range Modulation OFF Current Modulation-Current Stability Modulation-Current Absolute Accuracy Instantaneous Voltage at Modulator Output (OUT+) Modulation-Current Rise Time Modulation-Current Fall Time Output Over-/Undershoot Deterministic Jitter (Notes 9, 13) Random Jitter DJ RJ 155Mbps to 1.25Gbps, 10mA IMOD 85mA 1.25Gbps to 2.5Gbps, 10mA IMOD 60mA (Note 13) tR tF IMOD Data rate 1.25Gbps Data rate >1.25Gbps (Note 13) IMOD > 15mA (Note 3) 10mA IMOD < 60mA 60mA IMOD 85mA 10mA IMOD 85mA (Notes 8, 13) 10mA IMOD 85mA (Notes 8, 13) 10 10 16 -480 -15 0.6 0.75 40 40 20 17 17 0.8 45 40 1.4 85 85 85 60 150 +480 +15 mA A ppm/C % V ps ps % psP-P psRMS IMD tINIT VMD With respect to VCC IMD = 50A IMD = 1500A IMD = 50A IMD = 1500A Average current into MD pin Case 1 (Note 5) (LONGB = 0) Case 2 (Note 6) (LONGB = 0) Case 3 (Note 7) (LONGB = 0) 1.6 -750 -480 -25 -15 50 12 2.12 1.60 1.92 s +750 +480 +25 +15 1500 V ppm/C % A IBIAS Voltage at BIAS pin 0.6V IBIAS = 70mA IBIAS = 15mA -15 15 1 5 148 98 +15 70 70 100 mA A ppm/C % mA IBIAS-OFF EN = high or BEN = low, VBIAS 2.6V (Note 2) APC open loop SYMBOL VBEN+, VBENVIH VIL VOH VOL Sourcing 50A, VCC = 3.0V Sinking 100A, VCC = 3.6V 2.4 0.4 2.0 0.8 CONDITIONS MIN TYP 0.8 MAX UNITS V V V V V
MAX3656
APC open-loop IBIAS > 20mA (Note 3)
IMOD-OFF EN = high or BEN = low, IN = low (Note 2)
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155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
ELECTRICAL CHARACTERISTICS (continued)
(Typical values are at VCC = +3.3V, IBIAS = 20mA, IMOD = 25mA, extinction ratio = 10dB, and TA = +25C, unless otherwise noted.)
PARAMETER Burst Enable Delay Burst Disable Delay Burst On-Time (Note 13) tB-ON SYMBOL CONDITIONS APC closed loop (Notes 10, 11, 13) APC closed loop (Notes 10, 12, 13) 155Mbps 622Mbps 1.25Gbps, 2.5Gbps 155Mbps Burst Off-Time (Note 13) OPTICAL EVALUATION Optical Eye Diagram Mask Margin ExceLight SLT3120-DN laser diode (or equivalent) 155.52Mbps 622.08Mbps 1.25Gbps 2.48832Gbps 42 30 23 18 % tB-OFF 622Mbps 1.25Gbps, 2.5Gbps 2881 720 576 192 96 96 ns ns MIN TYP MAX 2.3 2.0 UNITS ns ns
BURST-MODE SPECIFICATIONS
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 = 1500A) within 12s from the time that VCC 3.0V. Note 6: IBIAS error is less than 3.8mA (for an extinction ratio of 10dB and IMD = 1500A) within 2.1s (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 = 1500A) 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 translates to 1.92s 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 reaches 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.
4
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155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
VCC 22.1
BEN+ BENIBIAS + IMOD ENABLE DELAY BEN MIDPOINT IFINAL x 110% IFINAL IFINAL x 90% 10% OF IBIAS DISABLE DELAY
OUT+ VCC 50 26.7 IBIAS OUT49.9 VCC
MAX3656
22.1 IMOD Z0 = 50 OSCILLOSCOPE
Figure 1. Enable and Disable Delay Times
BIAS+ 121 BIAS-
Z0 = 50 VCC 26.7 50
35.7
Figure 2. Output Termination for Characterization
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (155.52Mbps, 117MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID)
MAX3656 toc01
OPTICAL EYE DIAGRAM (622.08Mbps, 467MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID)
MAX3656 toc02
OPTICAL EYE DIAGRAM (1.25Gbps, 933MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID)
MAX3656 toc03
EXCELIGHT SLT3120-DN LASER AVERAGE OPTICAL POWER = -4dBm EXTINCTION RATIO = 15.2dB MASK MARGIN = 42%
EXCELIGHT SLT3120-DN LASER AVERAGE POWER = -4dBm EXTINCTION RATIO = 15.1dB MASK MARGIN = 30%
EXCELIGHT SLT3120-DN LASER AVERAGE POWER = -3.5dBm EXTINCTION RATIO = 14dB MASK MARGIN = 23%
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155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
ELECTRICAL EYE DIAGRAM (2.5Gbps, IMOD = 30mA, PATTERN = PRBS 27 - 1 + 80 CID)
MAX3656 toc04
SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES IBIAS, IMOD, 15 LOAD)
MAX3656 toc05
75 70 SUPPLY CURRENT (mA) 65 60 55 50 45 40 35 30 100ps/div -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 TEMPERATURE (C)
DETERMINISTIC JITTER (psP-P)
35 30 25 20 15 10 10
DATA RATE = 2.5Gbps PATTERN = 27 - 1PRBS + 80 CID VIN = 200mVP-P
20
30
40
50
60
70
80
IMOD (mA)
DETERMINISTIC JITTER vs. INPUT AMPLITUDE
MAX3656 toc07
RANDOM JITTER vs. IMOD
1.4 1.3 RANDOM JITTER (psRMS) 1.2 1.1 1.0 0.9 0.8 0.7 0.6
MAX3656 toc08
IMD vs. RAPCSET
MAX3656 toc09
40 35 30 25 20 15 10
DETERMINISTIC JITTER (psP-P)
IMOD = 30mA PATTERN = 27 - 1PRBS + 80 CID DATA RATE = 2.5Gbps
1.5
10,000
1000 IMD (A)
100
200
400
600
800
1000 1200 1400 1600
0.5 10 20 30 40 50 60 IMOD (mA) 70 80
10 0.1 1 RAPCSET (k) 10 100
INPUT AMPLITUDE (mVP-P)
IMOD vs. RMODSET
MAX3656 toc10
IBIASMAX vs. RBIASMAX
90 80 70 IBIASMAX (mA) 60 50 40 30 20 10 0
MAX3656 toc11
100 90 80 70 IMOD (mA) 60 50 40 30 20 10 0 1 10 RMODSET (k)
100
100
1
10 RBIASMAX (k)
100
6
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MAX3656 toc06
80
DETERMINISTIC JITTER vs. IMOD
40
155Mbps to 2.5Gbps Burst-Mode Laser Driver
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
TIMING DIAGRAM, BURST ON
MAX3656 toc12
MAX3656
TIMING DIAGRAM, BURST OFF
MAX3656 toc13
VMOD+
VMOD+
VBIAS+
VBIAS+
BURST-ENABLE SIGNAL
BURST-DISABLE SIGNAL
500ps/div
1ns/div
Pin Description
PIN 1, 4, 9, 15, 18, 21 2 3 5 6 7 8, 11, 19 10 12 13 14 16 17 20 NAME VCC IN+ INBEN+ BENEN GND FAIL LONGB BIASBIAS+ OUT+ OUTMD Power-Supply Voltage Noninverting Data Input with On-Chip Biasing Inverting Data Input with On-Chip Biasing Noninverting Burst-Enable Input with On-Chip Biasing Inverting Burst-Enable Input with On-Chip Biasing TTL/CMOS Enable Input. Low for normal operation. Float or pull high to disable laser bias and modulation currents. Power-Supply Ground TTL/CMOS Failure Output. Indicates APC failure when low. TTL/CMOS Long Burst (See the Setting the LONGB Input Pin Section) Inverting Laser Bias-Current Output. Connect through 15 to VCC. Noninverting Laser Bias-Current Output. Bias current flows into this pin when BEN is high. Minimize capacitance on this pin. Noninverting Laser Modulation-Current Output. Modulation current flows into this pin when BEN and IN are high. Inverting Laser Modulation-Current Output. Connect through 15 to VCC. Monitor Diode Input. Connect this pin to the anode of the monitor diode. Leave floating for open-loop operation. Minimize capacitance on this pin. 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. Modulation Current Set. A resistor connected from this pin to ground sets the desired modulation current. 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. FUNCTION
22 23 24 EP
BIASMAX MODSET APCSET
Exposed Pad Ground. This pad must be soldered to ground.
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7
155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Detailed Description
The MAX3656 laser driver has three main parts: a highspeed modulator, a high-speed bias driver, and a laserbiasing block with automatic power control (see the Functional Diagram). Both the bias and modulation output stages are composed of differential pairs with programmable 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 driving a 15 load. 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 15. The OUT- pin should be connected with a 15 resistor to VCC. To reduce optical output aberrations and duty-cycle distortion caused by laser diode parasitic 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 connected directly through a resistor to the cathode of the laser. The BIAS- pin should be connected to V CC through a 15 resistor. 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 50k resistor from APCSET to ground and leave the MD pin unconnected. In this case, two external resistors 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 shorted 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.
Automatic Power Control
To maintain constant average optical power, the MAX3656 incorporates a digital automatic power-control (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 pseudobinary 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 allowable bias current during closed-loop operation and sets the bias current during open-loop operation. An APC fail-
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 initialization. The initialization time is typically 2.1s (LONGB = low) and 3.72s (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
8
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155Mbps to 2.5Gbps Burst-Mode Laser Driver
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 voltage 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 12s (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 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 2.1s (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 1.6s (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. 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 minimum burst length is limited to the burst on-time listed in the Electrical Characteristics table. The MAXIMum burston time is unlimited. When BEN is switched off, the bias and modulation currents fall below the specified bias-off and modulationoff currents within the MAXIMum burst disable delay time of 2.0ns. For stable APC loop operation, the minimum burst off-time is limited to the value listed in the Electrical Characteristics table. The MAXIMum burst offtime is unlimited.
MAX3656
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 waveform are 50%.
Programming the Modulation Current
For a given laser power (PAVG), slope efficiency (), 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 +25C.
Table 1. Optical Power Definition
PARAMETER Average power Extinction ratio Optical power high Optical power low Optical amplitude Laser slope efficiency Modulation current Laser-to-monitor diode gain SYMBOL PAVG re P1 P0 PP-P IMOD ALMD RELATION PAVG = (P0 + P1) / 2 r e = P1 / P 0 P1 = 2PAVG x re / (re + 1) P0 = 2PAVG / (re + 1) PP-P = P1 - P0 = PP-P / IMOD IMOD = PP-P / (2 x IMD / IMOD)((re - 1) / (re + 1))
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.
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9
155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Programming the Bias Current
When the MAX3656 is used in open-loop operation, the R BIASMAX 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 50k resistor 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 available 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 +85C. optical output waveform is optimized (refer to Application Note HFAN 3.0: Interfacing MAXIM's Laser Drivers with Laser Diodes).
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 operates 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.
Programming the APC Loop
When using the MAX3656's APC feature, 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 lifetime. See the IMD vs. RAPCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current.
VCC VCC 16k 5k IN+
VCC
Setting the LONGB Input Pin
Set the LONGB pin according to Table 2 to optimize APC loop operation.
IN-
VCC 5k
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 (RD) is required (see the Functional Diagram). Additionally, the MAX3656 outputs are optimized for a 15 load. Therefore, the series combination of RD and RL (where RL represents the laser-diode resistance) should equal 15. Typical values for RD are 8 to 13. For best performance, a bypass capacitor (0.01F typical) should be placed as close as possible to the anode of the laser diode. An RC shunt network between the OUT+ pin and ground helps minimize optical output aberrations. Starting values for most coaxial lasers are R = 56 in series with C = 10pF. Adjust these values experimentally until the
24k
MAX3656
VCC VCC VCC 16k 5k BEN+
VCC 5k
Table 2. Setting the LONGB Input Pin
LONGB 0 0 or 1 1 CONDITION Burst on-time 1.2s Burst on-time >1.2s or continuous mode operation Data rates of 155Mbps
BEN-
24k
Figure 3. MAX3656 Internal Biasing
10
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155Mbps to 2.5Gbps Burst-Mode Laser Driver
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 combination of R1 and R2 should be less than 1k. Choose the values of R1 and R2 to set the common-mode voltage 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 4k (R4) resistor from the burst-enable signal to BEN+. Connect a 1k (R3) resistor from VCC to BEN+. Connect a 1k resistor (R5) from VCC to BEN- and a 9k resistor (R6) from BEN- to ground. The parallel combination of R5 and R6 should be less than 1k. For typical LVTTL or LVCMOS specifications of VCC to 2.8V for a high and 0.4V to 0V for a low, the LVTTL or LVCMOS sources zero current and sinks a MAXIMum of 720A ((3.6V to 0V) / 5k). See Figure 7 for setting up the single-ended LVTTL or LVCMOS biasing for burst enable.
MAX3656
Applications Information
This section provides an example of how to set up the MAX3656.
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 -40C to +85C, and the laser diode has the following characteristics:
VCC 130
ZO = 50 100 ZO = 50
IN+
IN-
ZO = 50 82 VCC
IN+ ZO = 50 BEN+ 100 ZO = 50 IN82 BEN-
MAX3656
MAX3656
130 ZO = 50
Figure 5. Connecting to CML Outputs
VCC 130 ZO = 50 82 VCC 130 ZO = 50 82 BENVCMBEN = +2.0V R2 = 2.54k BEN+ VCMBEN + (100mV to 800mV) VCMBEN VCMBEN - (100mV to 800mV) BEN+ 3.3V BENR1 = 1.65k IN+ IN-
MAX3656
Figure 4. Connecting to PECL Outputs
Figure 6. Single-Ended Biasing for Burst Enable 11
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155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Table 3. Pad Locations
VCC R3 = 1k IN+ IN-
PAD 1 2 3 4 5 6 7 8 9 10 11
NAME VCC IN+ INVCC BEN+ BENGND EN GND VCC FAIL GND LONGB GND BIASBIAS+ VCC OUT+ OUT+ OUTOUTVCC GND MD GND VCC GND BIASMAX MODSET APCSET GND
COORDINATES (microns) 51.2 51.2 51.2 51.2 51.2 51.2 142.2 282.2 423.6 608.4 1569.6 1738.2 1881.0 2023.8 2257.6 2257.6 2257.6 2257.6 2257.6 2257.6 2257.6 2257.6 2039.2 1893.6 1749.4 1603.8 1461.0 700.8 555.2 412.4 262.6 1146.0 1003.2 856.2 709.2 198.2 51.2 -111.2 -111.2 -111.2 -111.2 -111.2 -111.2 -111.2 -111.2 87.6 236.0 453.0 626.6 768.0 931.8 1073.2 1217.4 1242.6 1242.6 1242.6 1242.6 1242.6 1242.6 1242.6 1242.6 1242.6
LVTTL OR LVCMOS HIGH
BEN+ LVTTL OR LVCMOS LOW R4 = 4k VCC BENR5 = 1k
MAX3656
R6 = 9k
Figure 7. Single-Ended LVTTL or LVCMOS Biasing for Burst Enable
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
wavelength: = 1.3m, threshold current: ITH = 22mA at +25C, threshold temperature coefficient: TH = 1.3%/C, laser-to-monitor transfer: MON = 0.2A/W (MON = MONITORDIODE x LLASER-TO-MONITORDIODE), and laser slope efficiency: = 0.05mW/mA at +25C.
Determining RAPCSET
The desired monitor diode current is estimated by IMD = PAVG MON = 200A. The IMD vs. RAPCSET graph in the Typical Operating Characteristics shows RAPCSET at 12k.
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 9k.
Determining RBIASMAX
Determine the MAXIMum threshold current (ITH(MAX)) at TA = +85C and end of life. Assuming (ITH(MAX)) = 50mA, the MAXIMum bias current should be: IBIASMAX = ITH(MAX) In this example, I BIASMAX = 50mA. The I BIASMAX vs. RBIASMAX graph in the Typical Operating Characteristics shows RBIASMAX at 8k.
Layout Considerations
To minimize inductance, keep the connections between the MAX3656 output pins and laser diode as close as possible. Optimize the laser diode performance by placing a bypass capacitor as close as possible to the laser anode. Take extra care to minimize stray parasitic capacitance on the BIAS and MD pins. Use good highfrequency layout techniques and multilayer boards with uninterrupted ground planes to minimize EMI and crosstalk.
Wire-Bonding Die
For high-current density and reliable operation, the MAX3656 uses gold metalization. Make connections to the die with gold wire only, using ball-bonding techniques. Die-pad size is 4.03 mils (102.4m) square, and die size is 98 mils 65 mils (2489.2m 1651m).
12
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155Mbps to 2.5Gbps Burst-Mode Laser Driver
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, recognizing 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.
MAX3656
Functional Diagram
VCC
IMOD
OUT+
RD
RCOMP
MAX3656
OUTIN+
CCOMP VCC
IN-
BEN+
IBIAS BIAS+ VCC BIAS-
BEN-
APC DAC DSP ASP MD
RMODSET
RBIASMAX
EN
FAIL
LONGB
RAPCSET
______________________________________________________________________________________
13
155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Chip Topography
MODSET GND APCSET BIASMAX GND
VCC
GND
MD
GND
VCC VCC
IN+ INOUT-
VCC
OUT+ 0.065" (1.651mm)
VCC
BEN+ BEN-
BIAS+ BIAS-
(0,0) GND EN GND
VCC
0.098" (2.489mm)
FAIL
GND LONGB GND
Chip Information
TRANSISTOR COUNT: 8153 SUBSTRATE: Electrically Isolated DIE SIZE: 2489.2m X 1651m DIE THICKNESS: 12 mils PROCCESS: SiGe Bipolar
14
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155Mbps to 2.5Gbps Burst-Mode Laser Driver
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.MAXIM-ic.com/packages.) 24L QFN THIN.EPS
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
MAX3656
21-0139
A
______________________________________________________________________________________
15
155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.MAXIM-ic.com/packages.)
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
A
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.
16 ____________________MAXIM Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2003 MAXIM Integrated Products Printed USA is a registered trademark of MAXIM Integrated Products.

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