19-2790; Rev 4; 9/10 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 Ordering Information TEMP RANGE PIN-PACKAGE MAX3656ETG PART -40C to +85C 24 Thin QFN-EP* MAX3656ETG+ -40C to +85C 24 Thin QFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Functional Diagram appears at end of data sheet. Typical Application Circuit 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 +3.3V +3.3V +3.3V VCC MD GND 23 22 21 20 19 130 SERIAL DATA SOURCE IN+ 1 18 VCC IN+ 2 17 OUT- IN- 3 16 OUT+ 15 IN82 VCC BIASMAX BIASMAX 24 130 APCSET MODSET TOP VIEW APCSET Pin Configuration OUT5.6 82 OUT+ MAX3656 5.6 75 10 +3.3V BIAS+ +3.3V 5 BEN- 6 15 VCC 14 BIAS+ 13 BIAS- BIAS- 130 BEN+ BURST CONTROL BEN- THIN QFN 11 12 LONGB 10 GND EN 9 FAIL 8 VCC 7 GND EN *EP 130 82 LONGB BEN+ MAX3656 FAIL 4 27pF GND VCC 10nH VCC 10 MODSET 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 MD 82 *EXPOSED PAD IS CONNECTED TO GND ________________________________________________________________ Maxim Integrated Products 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. 1 MAX3656 General Description MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 (derate 27.8mW/C above +85C) .............................1805mW Operating Ambient Temperature Range (TA)......-40C to +85C Storage Ambient Temperature Range (TSTG) ...-55C to +150C Lead Temperature (soldering,10s) ..................................+300C Soldering Temperature (reflow) Lead(Pb)-free...............................................................+260C Containing lead(Pb) .....................................................+240C 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 SYMBOL CONDITIONS VCC Supply Turn-On Time 10% to 90% Ambient Temperature Data Mark Density Consecutive Identical Digits MIN TYP MAX UNITS 3.0 3.3 3.6 V 0.001 10 ms -40 +85 C 80 Bits 2500 Mbps 15 pF 0.005 0.050 A/A 8.2 12.0 dB Average 50 CID Data Rate 155 Monitor Diode Capacitance Laser-to-Monitor Diode Gain Extinction Ratio CMD For minimum burst on-time (Note 1) ALMD (monitor current)/ (laser current (above ITH)) (Notes 2, 3) re 10 log (P1/P0) (Note 3) % 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 SYMBOL CONDITIONS MIN TYP MAX UNITS 40 70 mA 1.6 VP-P VCC 1.32 VCC VIN/4 V POWER SUPPLY Power-Supply Current ICC (Note 1) INPUT SPECIFICATIONS Differential Input Voltage Common-Mode Input Voltage 2 VIN, VBEN 0.2 VCM VCC 1.49 _______________________________________________________________________________________ 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 Single-Ended Input Voltage SYMBOL CONDITIONS MIN VBEN+, VBEN- EN Input High Voltage VIH EN Input Low Voltage VIL TYP MAX 0.8 V 2.0 FAIL Output High Voltage VOH Sourcing 50A, VCC = 3.0V FAIL Output Low Voltage VOL Sinking 100A, VCC = 3.6V IBIAS Voltage at BIAS pin 0.6V UNITS V 0.8 V 0.4 V 70 mA 100 A 2.4 V BIAS GENERATOR Bias-On Current Range Bias-Off Current Range 1 IBIAS-OFF EN = high or BEN = low, VBIAS 2.6V (Note 2) 5 IBIAS = 70mA 148 IBIAS = 15mA 98 Bias-Current Temperature Stability APC open loop Bias-Current Absolute Accuracy APC open-loop IBIAS > 20mA (Note 3) BIASMAX Current-Setting Range ppm/C -15 +15 % 15 70 mA APC LOOP MD Reverse-Bias Voltage VMD MD Bias-Setting Stability (Note 4) MD Bias-Setting Accuracy (Note 3) MD DC-Current Range IMD With respect to VCC 1.6 IMD = 50A -750 +750 IMD = 1500A -480 +480 IMD = 50A -25 +25 IMD = 1500A -15 +15 Average current into MD pin 50 Case 1 (Note 5) (LONGB = 0) APC Loop Initialization Time (Note 4) t INIT V 1500 ppm/C % A 12 Case 2 (Note 6) (LONGB = 0) 2.12 Case 3 (Note 7) (LONGB = 0) 1.60 s 1.92 LASER MODULATOR Modulation ON Current Range Modulation OFF Current IMOD Data rate 1.25Gbps 10 85 Data rate >1.25Gbps 10 60 IMOD-OFF EN = high or BEN = low, IN = low (Note 2) 16 mA 150 A Modulation-Current Stability (Note 13) -480 +480 ppm/C Modulation-Current Absolute Accuracy IMOD > 15mA (Note 3) -15 +15 % Instantaneous Voltage at Modulator Output (OUT+) 10mA IMOD < 60mA 0.6 60mA IMOD 85mA 0.75 Modulation-Current Rise Time tR 10mA IMOD 85mA (Notes 8, 13) Modulation-Current Fall Time tF 10mA IMOD 85mA (Notes 8, 13) Output Over-/Undershoot V 40 85 ps 40 85 ps 20 Deterministic Jitter (Notes 9, 13) DJ Random Jitter RJ % 155Mbps to 1.25Gbps, 10mA IMOD 85mA 17 45 1.25Gbps to 2.5Gbps, 10mA IMOD 60mA 17 40 (Note 13) 0.8 1.4 psP-P psRMS _______________________________________________________________________________________ 3 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 = +25C, 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 Burst On-Time (Note 13) Burst Off-Time (Note 13) tB-ON tB-OFF 155Mbps 2881 622Mbps 720 1.25Gbps, 2.5Gbps 576 155Mbps 192 622Mbps 96 1.25Gbps, 2.5Gbps 96 ns ns OPTICAL EVALUATION Optical Eye Diagram Mask Margin ExceLight SLT2886-LR laser diode (or equivalent) 155.52Mbps 45 622.08Mbps 40 1.24416Gbps 31 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 = 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 _______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656 VCC 22.1 BEN+ BEN MIDPOINT BEN- OUT- IFINAL x 110% 49.9 VCC IFINAL IBIAS + IMOD IFINAL x 90% MAX3656 10% OF IBIAS 22.1 IMOD ENABLE DELAY DISABLE DELAY OSCILLOSCOPE Z0 = 50 OUT+ VCC 50 26.7 IBIAS Figure 1. Enable and Disable Delay Times Z0 = 50 BIAS+ 121 VCC 50 BIAS26.7 35.7 Figure 2. Output Termination for Characterization Typical Operating Characteristics (TA = +25C, unless otherwise noted.) OPTICAL EYE DIAGRAM (155.52Mbps, 117MHz FILTER, PATTERN = 223 - 1 PRBS) OPTICAL EYE DIAGRAM (622.08Mbps, 467MHz FILTER, PATTERN = 223 - 1 PRBS) MAX3656 toc01 EXCELIGHT SLT2886-LR LASER AVERAGE OPTICAL POWER = -6dBm EXTINCTION RATIO = 15dB MASK MARGIN = 45% OPTICAL EYE DIAGRAM (1.24416Gbps, 933MHz FILTER, PATTERN = 223 - 1 PRBS) MAX3656 toc02 EXCELIGHT SLT2886-LR LASER AVERAGE OPTICAL POWER = -6dBm EXTINCTION RATIO = 15dB MASK MARGIN = 40% MAX3656 toc03 EXCELIGHT SLT2886-LR LASER AVERAGE OPTICAL POWER = -6dBm EXTINCTION RATIO = 12dB MASK MARGIN = 31% _______________________________________________________________________________________ 5 Typical Operating Characteristics (continued) (TA = +25C, unless otherwise noted.) ELECTRICAL EYE DIAGRAM (2.5Gbps, IMOD = 30mA, PATTERN = PRBS 27 - 1 + 80 CID) SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES IBIAS, IMOD, 15 LOAD) MAX3656 toc05 MAX3656 toc04 80 MAX3656 toc06 OPTICAL EYE DIAGRAM (2.48832Gbps, 2.3GHz FILTER, PATTERN = 223 - 1 PRBS) 75 SUPPLY CURRENT (mA) 70 65 60 55 50 45 40 35 30 EXCELIGHT SLT2886-LR LASER AVERAGE OPTICAL POWER = -6dBm EXTINCTION RATIO = 10dB MASK MARGIN = 24% DETERMINISTIC JITTER vs. INPUT AMPLITUDE 25 20 15 30 25 20 MAX3656 toc09 1.4 1.3 RANDOM JITTER (psRMS) 30 35 IMOD = 30mA PATTERN = 27 - 1PRBS + 80 CID DATA RATE = 2.5Gbps RANDOM JITTER vs. IMOD 1.5 MAX3656 toc08 DATA RATE = 2.5Gbps PATTERN = 27 - 1PRBS + 80 CID VIN = 200mVP-P 40 DETERMINISTIC JITTER (psP-P) MAX3656 toc07 40 35 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 TEMPERATURE (C) 100ps/div DETERMINISTIC JITTER vs. IMOD DETERMINISTIC JITTER (psP-P) MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 1.2 1.1 1.0 0.9 0.8 0.7 15 0.6 10 10 10 20 30 40 50 IMOD (mA) 6 60 70 80 0.5 200 400 600 800 1000 1200 1400 1600 INPUT AMPLITUDE (mVP-P) 10 20 30 40 50 60 IMOD (mA) _______________________________________________________________________________________ 70 80 155Mbps to 2.5Gbps Burst-Mode Laser Driver IMD vs. RAPCSET IBIASMAX vs. RBIASMAX IMOD vs. RMODSET 90 80 90 80 70 IMOD (mA) IMD (A) 100 IBIASMAX (mA) 70 1000 60 50 40 60 50 40 30 30 20 20 10 10 0 0 10 0.1 10 1 1 100 MAX3656 toc12 100 MAX3656 toc11 100 MAX3656 toc10 10,000 10 1 100 10 TIMING DIAGRAM, BURST ON 100 RBIASMAX (k) RMODSET (k) RAPCSET (k) TIMING DIAGRAM, BURST OFF MAX3656 toc14 MAX3656 toc13 VMOD+ VMOD+ VBIAS+ VBIAS+ BURST-ENABLE SIGNAL 500ps/div BURST-DISABLE SIGNAL 1ns/div _______________________________________________________________________________________ 7 MAX3656 Typical Operating Characteristics (continued) (TA = +25C, unless otherwise noted.) MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver Pin Description PIN NAME 1, 4, 9, 15, 18, 21 VCC Power-Supply Voltage 2 IN+ Noninverting Data Input with On-Chip Biasing 3 IN- 5 BEN+ 6 BEN- 8 FUNCTION 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. Pull high or leave unconnected to disable laser bias and modulation currents. 7 EN 8, 11, 19 GND Power-Supply Ground 10 FAIL TTL/CMOS Failure Output. Indicates APC failure when low. 12 LONGB 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 openloop 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 TTL/CMOS Long Burst (See the Setting the LONGB Input Pin Section) Exposed Pad. Ground. This pad must be soldered to ground. _______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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. 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 failure 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. 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 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 chipenable input (EN) is initially high (chip disabled and _______________________________________________________________________________________ 9 MAX3656 Detailed Description MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver APC loop reset), and then it is driven low (chipenabled). 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. 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 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. 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. 10 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. 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. The RBIASMAX resistor should not be set less than 5k. Table 1. Optical Power Definition PARAMETER Average power SYMBOL PAVG RELATION PAVG = (P0 + P1) / 2 Extinction ratio re r e = P1 / P 0 Optical power high P1 P1 = 2PAVG x re / (re + 1) Optical power low P0 P0 = 2PAVG / (re + 1) Optical amplitude PP-P Laser slope efficiency PP-P = P1 - P0 = PP-P / IMOD Modulation current IMOD IMOD = PP-P / Laser-to-monitor diode gain ALMD (2 x IMD / IMOD)((re - 1) / (re + 1)) ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 15 load 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 15. An RC shunt compensation network, RCOMP/CCOMP, connected between the laser diode cathode and ground should be provided to reduce optical output aberrations and duty-cycle distortion 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 15 resistor and switching diode to the laser diode anode. The switching diode at OUT- improves the optical output eye and burst-enable delay by better matching the laser diode characteristics. For data rates greater than 1Gbps, a parallel RL peaking 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 RP and LP are 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 connected directly through a resistor, equal to R D as determined above, to the laser diode cathode. The BIAS- pin is connected through a 10 resistor 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 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. VCC VCC VCC 16k 5k IN+ VCC 5k IN- 24k MAX3656 VCC VCC VCC 16k 5k BEN+ VCC 5k Table 2. Setting the LONGB Input Pin LONGB CONDITION 0 Burst on-time 1.2s 0 or 1 Burst on-time >1.2s or continuous mode operation 1 Data rates of 155Mbps BEN- 24k Figure 3. MAX3656 Internal Biasing ______________________________________________________________________________________ 11 MAX3656 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. MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 -40C to +85C, and the laser diode has the following characteristics: 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. IN+ ZO = 50 VCC 100 130 ZO = 50 IN- ZO = 50 BEN+ IN+ ZO = 50 82 MAX3656 VCC 100 MAX3656 130 ZO = 50 BEN- IN- ZO = 50 82 Figure 5. Connecting to CML Outputs VCC IN+ 130 IN- BEN+ ZO = 50 VCMBEN + (100mV to 800mV) 82 VCMBEN VCC BEN+ VCMBEN - (100mV to 800mV) 130 3.3V BEN- BEN- ZO = 50 82 R1 = 1.65k MAX3656 VCMBEN = +2.0V R2 = 2.54k Figure 4. Connecting to PECL Outputs 12 Figure 6. Single-Ended Biasing for Burst Enable ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver LVTTL OR LVCMOS HIGH IN- R3 = 5k BEN+ LVTTL OR LVCMOS LOW R4 = 3k VCC BEN- from the burst-enable signal to BEN+. Connect a 5k (R3) resistor from VCC to BEN+. Connect a 5k resistor (R5) from VCC to BEN- and a 9k resistor (R6) from BEN- to ground. For typical LVTTL or LVCMOS specifications of VCC to 2.0V for a high and 0.8V to 0V for a low, the LVTTL or LVCMOS sources a maximum of 75A and sinks a maximum of 500A. See Figure 7 for setting up the single-ended LVTTL or LVCMOS biasing for burst enable. Layout Considerations R5 = 5k MAX3656 R6 = 9k Figure 7. Single-Ended LVTTL or LVCMOS Biasing for Burst Enable 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 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 3k (R4) resistor 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 possible to each VCC connection. Take extra care to minimize 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, 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. ______________________________________________________________________________________ 13 MAX3656 VCC IN+ 155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656 Functional Diagram VCC RP LP OUT- MAX3656 IMOD OUT+ RD IN+ RCOMP CCOMP IN- IBIAS BEN+ BIAS+ BEN- RD VCC BIAS- APC DAC RMODSET RBIASMAX DSP EN FAIL Chip Information TRANSISTOR COUNT: 8153 PROCESS: SiGe BIPOLAR 14 ASP LONGB MD RAPCSET 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 ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver REVISION NUMBER REVISION DATE 0 3/03 1 5/04 2 10/04 DESCRIPTION PAGES CHANGED Initial release -- Added the lead-free package to the Ordering Information table 1 Added the package code for the lead-free package to the Ordering Information table; updated the Operating Conditions table; updated the Running Burst Enable SingledEnded section; added the Design Example section; updated Figure 7; added the Burst Enable Delay section; updated the Chip Topography diagram 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 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 1, 2, 11, 12, 14 1, 2, 5-9, 11, 13, 14 2, 10 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 (c) 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc. MAX3656 Revision History