19-2790; Rev 1; 5/04 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ 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 PART TEMP RANGE MAX3656E/D MAX3656ETG — PKG CODE PIN-PACKAGE Dice* — -40°C to +85°C 24 Thin QFN-EP** MAX3656ETG+ -40°C to +85°C 24 Thin QFN-EP** T2444-1 — + 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. Applications Fiber-to-the-Home (FTTH) and Fiber-to-theBusiness (FTTB) Broadband Access Systems Passive Optical Network (PON) Transmitters Functional Diagram appears at end of data sheet. Typical Application Circuit APON, EPON, and GPON Upstream Transmitters Pin Configuration VCC IN+ VCC 1 18 VCC IN+ 2 17 OUT- IN- 3 16 OUT+ 15 VCC VCC MAX3656 4 BEN+ 5 BEN- 6 14 BIAS+ 13 BIAS- SERIAL DATA SOURCE OUT+ IN- MAX3656 BIAS+ VCC BEN+ BURST CONTROL BIAS100Ω EN 12 LONGB THIN QFN 11 GND GND 10 FAIL EN 9 VCC 8 OUT- 100Ω BEN7 VCC 19 LONGB 20 BIASMAX GND 21 FAIL MD 22 GND VCC 23 APCSET BIASMAX 24 MODSET MODSET TOP VIEW APCSET VCC MD **EXPOSED PAD IS CONNECTED TO GND ________________________________________________________________ Maxim Integrated Products 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. 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 = +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 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 Average 50 CID Data Rate 155 Monitor Diode Capacitance CMD For minimum burst on-time (Note 1) Laser-to-Monitor Diode Gain ALMD d (monitor current)/d (laser current (above ITH)) (Notes 2, 3) Extinction Ratio re P1/P0 (Note 3) % 80 Bits 2500 Mbps 15 pF 0.005 0.050 6.6 16.0 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: 2 ×I 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 = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 40 70 mA 1.6 VP-P 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 VCC 1.32 _______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver (Typical values are at VCC = +3.3V, IBIAS = 20mA, IMOD = 25mA, extinction ratio = 10dB, and TA = +25°C, unless otherwise noted.) PARAMETER Single-Ended Input Voltage SYMBOL CONDITIONS MIN VBEN+, VBEN- TYP MAX 0.8 EN Input High Voltage VIH EN Input Low Voltage VIL FAIL Output High Voltage VOH Sourcing 50µA, VCC = 3.0V FAIL Output Low Voltage VOL Sinking 100µA, VCC = 3.6V IBIAS Voltage at BIAS pin ≥ 0.6V UNITS V 2.0 V 0.8 V 0.4 V 70 mA 100 µA 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 With respect to VCC 1.6 V MD Bias-Setting Stability (Note 4) IMD = 50µA -750 +750 IMD = 1500µA -480 +480 MD Bias-Setting Accuracy (Note 3) IMD = 50µA -25 +25 IMD = 1500µA -15 +15 Average current into MD pin 50 MD DC-Current Range APC Loop Initialization Time (Note 4) IMD tINIT 1500 Case 1 (Note 5) (LONGB = 0) 12 Case 2 (Note 6) (LONGB = 0) 2.12 Case 3 (Note 7) (LONGB = 0) 1.60 ppm/°C % µA µs 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 µA 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 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.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 2.3 ns 2.0 ns BURST-MODE SPECIFICATIONS Burst Enable Delay APC closed loop (Notes 10, 11, 13) Burst Disable Delay Burst On-Time (Note 13) Burst Off-Time (Note 13) APC closed loop (Notes 10, 12, 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 SLT3120-DN laser diode (or equivalent) 155.52Mbps 42 622.08Mbps 30 1.25Gbps 23 2.48832Gbps 18 % 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 translates 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 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 22.1Ω BEN+ BEN ±MIDPOINT BEN- OUT- IFINAL × 110% 49.9Ω VCC IFINAL IBIAS + IMOD IFINAL × 90% MAX3656 22.1Ω 10% OF IBIAS ENABLE DELAY IMOD OSCILLOSCOPE Z0 = 50Ω OUT+ DISABLE DELAY VCC 50Ω 26.7Ω IBIAS Figure 1. Enable and Disable Delay Times Z0 = 50Ω BIAS+ 121Ω VCC 50Ω BIAS26.7Ω 35.7Ω Figure 2. Output Termination for Characterization Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) OPTICAL EYE DIAGRAM (155.52Mbps, 117MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID) OPTICAL EYE DIAGRAM (622.08Mbps, 467MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID) MAX3656 toc01 EXCELIGHT SLT3120-DN LASER AVERAGE OPTICAL POWER = -4dBm EXTINCTION RATIO = 15.2dB MASK MARGIN = 42% OPTICAL EYE DIAGRAM (1.25Gbps, 933MHz FILTER, PATTERN = PRBS 27 - 1 + 80 CID) MAX3656 toc02 EXCELIGHT SLT3120-DN LASER AVERAGE POWER = -4dBm EXTINCTION RATIO = 15.1dB MASK MARGIN = 30% MAX3656 toc03 EXCELIGHT SLT3120-DN LASER AVERAGE POWER = -3.5dBm EXTINCTION RATIO = 14dB MASK MARGIN = 23% _______________________________________________________________________________________ 5 Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES IBIAS, IMOD, 15Ω LOAD) 75 70 65 60 55 50 45 40 40 30 25 20 15 35 30 DETERMINISTIC JITTER vs. INPUT AMPLITUDE 25 20 30 40 50 60 70 80 IMD vs. RAPCSET 1.4 1.3 1.2 10,000 1000 1.1 IMD (µA) 30 20 IMOD (mA) MAX3656 toc08 IMOD = 30mA PATTERN = 27 - 1PRBS + 80 CID DATA RATE = 2.5Gbps RANDOM JITTER (psRMS) 35 10 RANDOM JITTER vs. IMOD 1.5 MAX3656 toc07 40 10 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 TEMPERATURE (°C) 100ps/div DATA RATE = 2.5Gbps PATTERN = 27 - 1PRBS + 80 CID VIN = 200mVP-P 35 MAX3656 toc09 SUPPLY CURRENT (mA) DETERMINISTIC JITTER vs. IMOD MAX3656 toc05 80 DETERMINISTIC JITTER (psP-P) MAX3656 toc04 MAX3656 toc06 ELECTRICAL EYE DIAGRAM (2.5Gbps, IMOD = 30mA, PATTERN = PRBS 27 - 1 + 80 CID) DETERMINISTIC JITTER (psP-P) 1.0 0.9 100 0.8 0.7 15 0.6 10 200 400 600 800 1000 1200 1400 1600 10 0.5 10 INPUT AMPLITUDE (mVP-P) 20 30 40 50 60 IMOD (mA) 70 80 IBIASMAX vs. RBIASMAX 90 80 70 IBIASMAX (mA) 70 60 50 40 60 50 40 30 30 20 20 10 10 0 0 1 10 RMODSET (kΩ) 6 MAX3656 toc11 80 10 1 100 MAX3656 toc10 90 0.1 RAPCSET (kΩ) IMOD vs. RMODSET 100 IMOD (mA) MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 100 1 10 RBIASMAX (kΩ) _______________________________________________________________________________________ 100 100 155Mbps to 2.5Gbps Burst-Mode Laser Driver TIMING DIAGRAM, BURST OFF TIMING DIAGRAM, BURST ON MAX3656 toc13 MAX3656 toc12 VMOD+ VMOD+ VBIAS+ VBIAS+ BURST-ENABLE SIGNAL BURST-DISABLE SIGNAL 1ns/div 500ps/div Pin Description PIN NAME 1, 4, 9, 15, 18, 21 FUNCTION VCC 2 IN+ Noninverting Data Input with On-Chip Biasing 3 IN- Inverting Data Input with On-Chip Biasing 5 BEN+ 6 BEN- Power-Supply Voltage 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. 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 15Ω 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 15Ω to VCC. 20 MD 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 50kΩ resistor to ground for APC open-loop operation. EP TTL/CMOS Long Burst (See the Setting the LONGB Input Pin Section) Monitor Diode Input. Connect this pin to the anode of the monitor diode. Leave floating for open-loop operation. Minimize capacitance on this pin. Exposed Pad Ground. This pad must be soldered to ground. _______________________________________________________________________________________ 7 MAX3656 Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 15Ω 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 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 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 15Ω resistor. 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- 8 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 50kΩ 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 2.1µs (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 _______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 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 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 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. 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 (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. Table 1. Optical Power Definition PARAMETER SYMBOL RELATION Average power PAVG Extinction ratio re r e = P1 / P 0 Optical power high P1 P1 = 2PAVG × re / (re + 1) Optical power low P0 P0 = 2PAVG / (re + 1) Optical amplitude PP-P Laser slope efficiency PAVG = (P0 + P1) / 2 PP-P = P1 - P0 η η = PP-P / IMOD Modulation current IMOD IMOD = PP-P / η Laser-to-monitor diode gain ALMD (2 x IMD / IMOD)((re - 1) / (re + 1)) _______________________________________________________________________________________ 9 MAX3656 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 MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 50kΩ 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 +85°C. 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 VCC 16kΩ 5kΩ IN+ Setting the LONGB Input Pin VCC Set the LONGB pin according to Table 2 to optimize APC loop operation. 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 15Ω load. Therefore, the series combination of RD and RL (where RL represents the laser-diode resistance) should equal 15Ω. Typical values for RD are 8Ω to 13Ω. For best performance, a bypass capacitor (0.01µF 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 = 56Ω in series with C = 10pF. Adjust these values experimentally until the 5kΩ IN- 24kΩ MAX3656 VCC VCC VCC 16kΩ 5kΩ BEN+ VCC 5kΩ Table 2. Setting the LONGB Input Pin 10 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 BEN- 24kΩ Figure 3. MAX3656 Internal Biasing ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 1kΩ. 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 4kΩ (R4) resistor from the burst-enable signal to BEN+. Connect a 1kΩ (R3) resistor from VCC to BEN+. Connect a 1kΩ resistor (R5) from VCC to BEN- and a 9kΩ resistor (R6) from BEN- to ground. The parallel combination of R5 and R6 should be less than 1kΩ. 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 720µA ((3.6V to 0V) / 5kΩ). See Figure 7 for setting up the single-ended LVTTL or LVCMOS biasing for burst enable. 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 -40°C to +85°C, and the laser diode has the following characteristics: IN+ ZO = 50Ω VCC 100Ω 130Ω ZO = 50Ω IN- ZO = 50Ω BEN+ IN+ ZO = 50Ω 82Ω MAX3656 VCC 100Ω MAX3656 130Ω ZO = 50Ω BEN- IN- ZO = 50Ω 82Ω Figure 5. Connecting to CML Outputs VCC IN+ 130Ω IN- BEN+ ZO = 50Ω VCMBEN + (100mV to 800mV) 82Ω VCMBEN VCC BEN+ VCMBEN - (100mV to 800mV) 130Ω 3.3V BEN- BEN- ZO = 50Ω 82Ω R1 = 1.65kΩ MAX3656 VCMBEN = +2.0V R2 = 2.54kΩ Figure 4. Connecting to PECL Outputs Figure 6. Single-Ended Biasing for Burst Enable ______________________________________________________________________________________ 11 MAX3656 Running Burst Enable Single Ended MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver Table 3. Pad Locations VCC LVTTL OR LVCMOS HIGH IN+ IN- R3 = 1kΩ BEN+ LVTTL OR LVCMOS LOW R4 = 4kΩ VCC BENR5 = 1kΩ MAX3656 R6 = 9kΩ PAD NAME 1 VCC COORDINATES (microns) 51.2 1146.0 2 IN+ 51.2 1003.2 3 IN- 51.2 856.2 4 VCC 51.2 709.2 5 BEN+ 51.2 198.2 6 BEN- 51.2 51.2 7 GND 142.2 -111.2 8 EN 282.2 -111.2 9 GND 423.6 -111.2 10 VCC 608.4 -111.2 -111.2 11 FAIL 1569.6 Figure 7. Single-Ended LVTTL or LVCMOS Biasing for Burst Enable 12 GND 1738.2 -111.2 13 LONGB 1881.0 -111.2 wavelength: λ = 1.3µm, threshold current: ITH = 22mA at +25°C, threshold temperature coefficient: β TH = 1.3%/°C, laser-to-monitor transfer: ρ MON = 0.2A/W (ρMON = ρMONITORDIODE x LLASER-TO-MONITORDIODE), and laser slope efficiency: η = 0.05mW/mA at +25°C. 14 GND 2023.8 -111.2 15 BIAS- 2257.6 87.6 16 BIAS+ 2257.6 236.0 17 VCC 2257.6 453.0 18 OUT+ 2257.6 626.6 19 OUT+ 2257.6 768.0 20 OUT- 2257.6 931.8 21 OUT- 2257.6 1073.2 22 VCC 2257.6 1217.4 23 GND 2039.2 1242.6 24 MD 1893.6 1242.6 25 GND 1749.4 1242.6 26 VCC 1603.8 1242.6 27 GND 1461.0 1242.6 28 BIASMAX 700.8 1242.6 29 MODSET 555.2 1242.6 30 APCSET 412.4 1242.6 31 GND 262.6 1242.6 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 (I TH(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 8kΩ. 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 (102.4µm) square, and die size is 98 mils ✕ 65 mils (2489.2µm ✕ 1651µm). 12 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. ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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. Functional Diagram VCC IMOD OUT+ RD RCOMP MAX3656 CCOMP VCC OUT- IN+ IN- BEN+ IBIAS BIAS+ VCC BENBIAS- APC DAC RMODSET RBIASMAX DSP EN FAIL ASP LONGB MD RAPCSET ______________________________________________________________________________________ 13 MAX3656 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 155Mbps to 2.5Gbps Burst-Mode Laser Driver MAX3656 Chip Topography MODSET GND APCSET GND BIASMAX VCC GND MD GND VCC VCC IN+ OUT- IN- VCC OUT+ 0.065" (1.651mm) VCC BIAS+ BEN+ BIAS- BEN- (0,0) GND EN GND VCC FAIL GND LONGB GND 0.098" (2.489mm) Chip Information TRANSISTOR COUNT: 8153 SUBSTRATE: Electrically Isolated DIE SIZE: 2489.2µm X 1651µm DIE THICKNESS: 12 mils PROCCESS: SiGe Bipolar 14 ______________________________________________________________________________________ 155Mbps to 2.5Gbps Burst-Mode Laser Driver 24L QFN THIN.EPS PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm 21-0139 C 1 2 ______________________________________________________________________________________ 15 MAX3656 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.) MAX3656 155Mbps to 2.5Gbps Burst-Mode Laser Driver 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 THIN QFN, 4x4x0.8mm 21-0139 C 2 2 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 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.