19-2818; Rev 3; 6/11 Multirate Laser Driver with Extinction Ratio Control TEMP RANGE PIN-PACKAGE MAX3737ETJ -40°C to +85°C 32 Thin QFN-EP* MAX3737ETJ+ -40°C to +85°C 32 Thin QFN-EP* MAX3737EGJ -40°C to +85°C 32 QFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. 26 APCFILT1 27 APCFILT2 28 APCSET 29 MODSET 30 MODBCOMP TOP VIEW 31 TH_TEMP 32 MODTCOMP Pin Configurations GND 1 24 MD TX_DISABLE 2 23 VCC VCC 3 22 OUT+ IN+ 4 21 OUT+ 20 OUT- 19 OUT- 15 16 GND *EP GND 8 14 7 BC_MON 13 PC_MON VBS 6 SHUTDOWN 5 12 INVCC MAX3737EGJ 11 Functional Diagram and Typical Application Circuit appear at end of data sheet. PART VCC 1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC Transceivers Ordering Information TX_FAULT Gigabit Ethernet SFF/SFP and GBIC Transceivers ♦ Ground-Referenced Current Monitors ♦ Laser Safety, Shutdown, and Alarm Outputs 10 Multirate OC-3 to OC-48 FEC Transceivers 100mA Bias Current Automatic Power Control (APC) Modulation Compensation On-Chip Temperature Compensation Self-Biased Inputs for AC-Coupling GND Applications ♦ ♦ ♦ ♦ ♦ 9 The MAX3737 accepts differential data input signals. The wide 5mA to 60mA (up to 85mA AC-coupled) modulation current range and up to 100mA bias current range makes the MAX3737 ideal for driving FP/DFB lasers in fiber-optic modules. External resistors set the required laser current levels. The MAX3737 provides transmit disable control (TX_DISABLE), single-point fault tolerance, bias-current monitoring, modulation-current monitoring, and photocurrent monitoring. The device also offers a latched failure output (TX_FAULT) to indicate faults, such as when the APC loop is no longer able to maintain the average optical power at the required level. The MAX3737 is compliant with the SFF8472 transmitter diagnostic and SFP MSA timing requirements. The MAX3737 is offered in a 5mm x 5mm 32-pin thin QFN and QFN package and operates over the -40°C to +85°C extended temperature range. ♦ Single 3.3V Power Supply ♦ 47mA Power-Supply Current ♦ 85mA Modulation Current MC_MON The MAX3737 is a 3.3V laser driver designed for multirate transceiver modules with data rates from 155Mbps to 2.7Gbps. Lasers can be DC-coupled to the MAX3737 for reduced component count and ease of multirate operation. Laser extinction ratio control (ERC) combines the features of automatic power control (APC), modulation compensation, and built-in thermal compensation. The APC loop maintains constant average optical power. Modulation compensation increases the modulation current in proportion to the bias current. These control loops combined with thermal compensation maintain a constant optical extinction ratio over temperature and lifetime. Features 18 VCC 17 BIAS 5mm x 5mm QFN *THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND TO ACHIEVE SPECIFIED PERFORMANCE. Pin Configurations continued at end of data sheet. ________________________________________________________________ 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 MAX3737 General Description 25 VMD KIT ATION EVALU E L B AVAILA MAX3737 Multirate Laser Driver with Extinction Ratio Control ABSOLUTE MAXIMUM RATINGS Supply Voltage VCC...............................................-0.5V to +6.0V IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN, MC_MON, BC_MON, PC_MON, VBS, VMD, APCFILT1, APCFILT2, MD, TH_TEMP, MODTCOMP, MODBCOMP, MODSET, and APCSET Voltage .......................................-0.5V to VCC + 0.5V OUT+, OUT-, BIAS Current.............................-20mA to +150mA Continuous Power Dissipation (TA = +85°C) QFN/TQFN (derate 21.2mW/°C above +85°C) ................1.3W Operating Junction Temperature Range ...........-55°C to +150°C Storage Temperature Range .............................-55°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Soldering Temperature (reflow) Lead(Pb)-free...............................................................+260°C Containing lead(Pb) .....................................................+240°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. ELECTRICAL CHARACTERISTICS (VCC = 2.97V to 3.63V, TA = -40°C to +85°C. Typical values are at VCC = 3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25°C, unless otherwise noted.) (Notes 1, 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS (Note 3) 47 60 mA f 1MHz, 100mVP-P (Notes 4, 6) 33 POWER SUPPLY Supply Current Power-Supply Noise Rejection ICC PSNR dB I/O SPECIFICATIONS Differential Input Swing Common-Mode Input VID DC-coupled, Figure 1 VCM 0.2 2.4 VP-P 1.7 VCC VID/4 V 1 100 mA 0.1 mA 90 mA/mA 85 mA LASER BIAS Bias-Current Setting Range Bias Off Current TX_DISABLE = high Bias-Current Monitor Ratio IBIAS/IBC_MON 62 (Note 5) 5 76 LASER MODULATION Modulation-Current Setting Range IMOD Output Edge Speed 20% to 80% (Notes 6, 7) Output Overshoot/Undershoot (Note 7) Random Jitter (Notes 6, 7) 2.7Gbps 1.25Gbps Deterministic Jitter (Notes 6, 8) 622Mbps 155Mbps Modulation-Current Temperature Stability 2 (Note 6) 5mA IMOD 10mA 71 80 10mA < IMOD 85mA 52 80 ±6 % 0.65 1.3 25.6 40 10mA < IMOD 85mA 16 40 5mA IMOD 10mA 32 41 10mA < IMOD 85mA 15 41 5mA IMOD 10mA 39 46 10mA < IMOD 85mA 21 46 5mA IMOD 10mA 65 100 10mA < IMOD 85mA 46 70 ±150 ±480 5mA IMOD 10mA _______________________________________________________________________________________ ps ps psP-P ppm/°C Multirate Laser Driver with Extinction Ratio Control (VCC = 2.97V to 3.63V, TA = -40°C to +85°C. Typical values are at VCC = 3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25°C, unless otherwise noted.) (Notes 1, 2) PARAMETER SYMBOL CONDITIONS Modulation-Current Setting Error 15 load, TA = +25°C Modulation Off Current TX_DISABLE = high Modulation-Current Monitor Ratio IMOD/IMC_MON MIN TYP MAX 5mA IMOD 10mA ±20 10mA < IMOD 85mA ±15 223 262 UNITS % 0.1 mA 302 mA/mA 1500 μA EXTINCTION RATIO CONTROLS Monitor-Diode Input Current Range IMD Average current into the MD pin 18 MD Pin Voltage MD-Current Monitor Ratio IMD/IPC_MON APC Loop Time Constant CAPC_FILT = 0.01μF, IMD/IBIAS = 1/70 0.85 1.0 V 1.15 mA/mA ±480 ppm/°C ±15 % 3.3 APC Setting Stability ±100 APC Setting Accuracy 1.4 TA = +25°C μs IMOD Compensation Setting Range by Bias K K = IMOD/IBIAS 0 1.5 mA/mA IMOD Compensation Setting Range by Temperature TC TC = IMOD/ (Note 6) 0 1.0 mA/°C Threshold Setting Range for Temperature Compensation TTH (Note 6) 10 60 °C LASER SAFETY AND CONTROL Bias and Modulation Turn-Off Delay CAPC_FILT = 0.01μF, IMD/IBIAS = 1/80 (Note 6) 5 μs Bias and Modulation Turn-On Delay CAPC_FILT = 0.01μF, IMD/IBIAS = 1/80 (Note 6) 600 μs 1.39 V Threshold Voltage at Monitor Pins VREF Figure 5 1.14 1.3 INTERFACE SIGNALS TX_DISABLE Input High VHI TX_DISABLE Input Low VLO TX_DISABLE Input Current TX_FAULT Output Low 2.0 RPULL = 7.5k 0.8 VHI = VCC 15 VLO = GND -450 Sinking 1mA, open collector Shutdown Output High Sourcing 100μA Shutdown Output Low Sinking 100μA Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Note 8: V -800 0.4 VCC 0.4 V μA V V 0.4 V AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent test pattern. Specifications at -40°C are guaranteed by design and characterization. Excluding IBIAS and IMOD. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open. Power-supply noise rejection (PSNR) = 20log10(Vnoise (on VCC)/ΔVOUT). VOUT is the voltage across the 15Ω load when IN+ is high. The minimum required voltage at the OUT+ and OUT- pins is +0.75V. Guaranteed by design and characterization. Tested with 00001111 pattern at 2.7Gbps. DJ includes pulse-width distortion (PWD). _______________________________________________________________________________________ 3 MAX3737 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) 1310nm FP LASER Er = 8.2dB OPTICAL EYE DIAGRAM (155Mbps, 27 - 1PRBS, 117MHz FILTER, CAPC = 0.1μF) ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 2.7Gbps, 27 - 1PRBS) MAX3737 toc04 116ps/div MAX3737 toc03 54ps/div MAX3737 toc02 1310nm FP LASER Er = 8.2dB OPTICAL EYE DIAGRAM (1.25Gbps, 27 - 1PRBS, 940MHz FILTER) MAX3737 toc01 OPTICAL EYE DIAGRAM (2.7Gbps, 27 - 1PRBS, 2.3GHz FILTER) 75mV/div 920ps/div 52ps/div SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS) VCC = 3.63V VCC = 3.3V 50 45 VCC = 2.97V 40 86 84 82 80 78 76 70 -20 0 20 40 TEMPERATURE (°C) 4 60 80 1.10 1.05 1.00 0.95 0.85 72 -40 1.15 0.90 74 35 MAX3737 toc07 88 IMD/IPC_MON (mA/mA) 55 1.20 MAX3737 toc06 60 90 IBIAS/IBC_MON (mA/mA) IMOD = 60mA IBIAS = 60mA PHOTO-CURRENT MONITOR GAIN vs. TEMPERATURE BIAS-CURRENT MONITOR GAIN vs. TEMPERATURE MAX3737 toc05 65 SUPPLY CURRENT (mA) MAX3737 Multirate Laser Driver with Extinction Ratio Control 0.80 -40 -15 10 35 TEMPERATURE (°C) 60 85 -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 Multirate Laser Driver with Extinction Ratio Control MODULATION CURRENT vs. RMODSET 280 80 70 270 1.6 MAX3737 toc09 290 1.4 1.2 60 250 240 1.0 IMD (mA) IMOD (mA) 260 50 40 0.8 0.6 30 230 220 20 210 10 200 0 -15 10 35 60 0.2 0 1 85 10 TEMPERATURE (°C) 0.1 100 10 1 100 RAPCSET (kΩ) RMODSET (kΩ) RANDOM JITTER vs. MODULATION CURRENT DETERMINISTIC JITTER vs. MODULATION CURRENT 45 40 MAX3737 toc12 2.0 MAX3737 toc11 50 1.5 35 30 RJ (psRMS) DJ (psP-P) 25 20 15 1.0 0.5 10 5 0 0 10 20 30 40 50 60 70 80 0 90 20 40 60 80 IMOD (mA) IMOD (mA) COMPENSATION (K) vs. RMODBCOMP TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500Ω) 10 100 RTH_TEMP = 12kΩ 90 1 IMOD (mA) 80 RTH_TEMP = 7kΩ 70 RTH_TEMP = 4kΩ 100 MAX3737 toc14 0 MAX3737 toc13 -40 0.4 K (mA/mA) IMOD/IMC_MON (mA/mA) 90 MAX3737 toc08 300 PHOTODIODE CURRENT vs. RAPCSET MAX3737 toc10 MODULATION-CURRENT MONITOR GAIN vs. TEMPERATURE 60 RTH_TEMP = 2kΩ 0.1 50 40 0.01 30 0 0.1 1 RMODBCOMP (kΩ) 10 100 -20 0 20 40 60 80 100 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX3737 Typical Operating Characteristics (continued) (VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VCC = 3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 10kΩ) HOT PLUG WITH TX_DISABLE LOW MAX3737 toc15 MAX3737 toc16 44 RTH_TEMP = 12kΩ 42 3.3V VCC RTH_TEMP = 7kΩ 40 IMOD (mA) MAX3737 Multirate Laser Driver with Extinction Ratio Control 0V LOW RTH_TEMP = 4kΩ 38 FAULT RTH_TEMP = 2kΩ 36 TX_DISABLE LOW t_init = 60ms 34 32 LASER OUTPUT 30 -20 0 20 40 60 80 20ms/div 100 TEMPERATURE (°C) TRANSMITTER ENABLE TRANSMITTER DISABLE MAX3737 toc17 VCC MAX3737 toc18 3.3V 3.3V VCC FAULT LOW TX_DISABLE HIGH FAULT LOW t_on = 75μs LOW TX_DISABLE t_off = 134ns HIGH LOW LASER OUTPUT LASER OUTPUT 20μs/div 40ns/div RESPONSE TO FAULT FAULT RECOVERY TIME MAX3737 toc19 VPC_MON EXTERNALLY FORCED FAULT t_fault = 0.9μs FAULT TX_DISABLE LOW HIGH LOW MAX3737 toc20 VPC_MON EXTERNAL FAULT REMOVED FAULT LOW HIGH TX_DISABLE HIGH LOW LASER OUTPUT LASER OUTPUT 1μs/div 6 LOW t_init = 68ms 100ms/div _______________________________________________________________________________________ Multirate Laser Driver with Extinction Ratio Control PIN NAME 1, 10, 15, 16 GND FUNCTION 2 TX_DISABLE 3, 6, 11, 18, 23 VCC 4 IN+ Noninverted Data Input 5 IN- Inverted Data Input 7 PC_MON Photodiode-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the monitor-diode current. 8 BC_MON Bias-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the bias current. 9 MC_MON Modulation-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the modulation current amplitude. 12 TX_FAULT Ground Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left unconnected. The laser output is enabled when this pin is asserted low. 3.3V Supply Voltage Open-Collector Transmit Fault Indicator (Table 1) Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. 13 SHUTDOWN 14 VBS Bias Voltage Sense. Isolated tap (3k ±15%) on the bias output reduces component count when a precision bias sense resistor is used. 17 BIAS Laser Bias-Current Output 19, 20 OUT- Inverted Modulation-Current Output (Connect Pins 19 and 20 Together). IMOD flows into this pin when input data is low. 21, 22 OUT+ Noninverted Modulation-Current Output (Connect Pins 21 and 22 Together). IMOD flows into this pin when input data is high. 24 MD Monitor Photodiode Input. Connect this pin to the anode of a monitor photodiode. A capacitor to ground is required to filter the high-speed AC monitor photocurrent. 25 VMD 26 APCFILT1 27 APCFILT2 Monitor Photodiode Voltage Sense. Isolated tap (3k ±15%) on the MD input reduces component count when a precision photodiode current-sense resistor is used. Connect a capacitor (CAPC) between pin 26 (APCFILT1) and pin 27 (APCFILT2) to set the dominant pole of the APC feedback loop. (See Pin 26.) The maximum capacitance allowed on this pin is 10pF. 28 APCSET A resistor connected from this pin to ground sets the desired average optical power. The maximum capacitance allowed on this pin is 10pF. 29 MODSET A resistor connected from this pin to ground sets the desired constant portion of the modulation current. 30 MODBCOMP Modulation-Current Compensation from Bias. Couples the bias current to the modulation current. Mirrors IBIAS through an external resistor. Leave open for zero coupling. 31 TH_TEMP Threshold for Temperature Compensation. A resistor at this pin programs the temperature, above which compensation is added to the modulation current. 32 — Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature MODTCOMP coefficient of the modulation current when above the threshold temperature. Leave open for zero temperature compensation. EP Exposed Pad. Solder the exposed pad to the circuit board ground for specified thermal and electrical performance. _______________________________________________________________________________________ 7 MAX3737 Pin Description MAX3737 Multirate Laser Driver with Extinction Ratio Control VCC VCC VOLTAGE SINGLE ENDED VIN+ VIN- 100mV (MIN) 1200mV (MAX) 30Ω 30Ω MAX3737 OUT- DIFFERENTIAL (VIN+) - (VIN-) Z0 = 30Ω OUT- 200mVP-P (MIN), 2400mV (MAX) 30Ω 0.5pF OUT+ CURRENT IOUT+ Z0 = 30Ω OUT+ Z0 = 50Ω IOUT+ 75Ω IMOD TIME OSCILLOSCOPE Figure 1. Required Input Signal and Output Polarity SOURCE NOISE VOLTAGE SUPPLY 50Ω Figure 2. Test Circuit for Characterization HOST BOARD FILTER DEFINED BY SFP MSA L1 1μH C1 0.1μF C2 10μF MODULE TO LASER DRIVER VCC OPTIONAL C3 0.1μF OPTIONAL Figure 3. Supply Filter Detailed Description The MAX3737 laser driver consists of three main parts: a high-speed modulation driver, biasing block with ERC, and safety circuitry. The circuit design is optimized for high-speed, low-voltage (3.3V) operation (Figure 4). High-Speed Modulation Driver The output stage is composed of a high-speed differential pair and a programmable modulation current source. The MAX3737 is optimized for driving a 15Ω load. The minimum instantaneous voltage required at OUT+ is 0.7V for modulation current up to 60mA and 0.75V for currents from 60mA to 85mA. Operation above 60mA can be accomplished by AC-coupling or with sufficient voltage at the laser to meet the driver output voltage requirement. To interface with the laser diode, a damping resistor (RD) is required. The combined resistance due to the series damping resistor and the equivalent series resistance (ESR) of the laser diode should equal 15Ω. To further damp aberrations caused by laser diode parasitic inductance, an RC shunt network may be necessary. Refer to Application Note 274: HFAN-02.0: Interfacing Maxim Laser Drivers with Laser Diodes for more information. 8 At data rates of 2.7Gbps, any capacitive load at the cathode of a laser diode degrades optical output performance. Because the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the BIAS pin parasitics from the laser cathode. Extinction Ratio Control The extinction ratio (r e ) is the laser on-state power divided by the off-state power. Extinction ratio remains constant if peak-to-peak and average power are held constant: re = (2PAVG + PP-P) / (2PAVG - PP-P) Average power is regulated using APC, which keeps constant current from a photodiode coupled to the laser. Peak-to-peak power is maintained by compensating the modulation current for reduced slope efficiency (η) of the laser over time and temperature: PAVG = IMD ρMON PP-P = η x IMOD _______________________________________________________________________________________ Multirate Laser Driver with Extinction Ratio Control MAX3737 VCC SHUTDOWN MAX3737 IN+ INPUT BUFFER OUTDATA PATH IN- RD OUT+ IMOD IMOD ENABLE SAFETY LOGIC AND POWER IBIAS ENABLE DETECTOR SHUTDOWN TX_FAULT TX_DISABLE RPULL = 7.5kΩ IMD 1 BIAS VCC IBIAS VBG PC_MON xTC RMDMON IBIAS VCC x268 xK RAPCSET X1/2 IBIAS 82 IAPCSET T > TH BC_MON APCSET MD IMD RBC_MON X1 T IMOD 268 CMD MC_MON VBG RMC_MON TH_TEMP RTH_TEMP MODTCOMP RMODTCOMP MODSET RMODSET MODBCOMP RMODBCOMP APCFILT1 APCFILT2 CAPC Figure 4. Functional Diagram Modulation compensation from bias increases the modulation current by a user-selected proportion (K) needed to maintain peak-to-peak laser power as bias current increases with temperature. Refer to Maxim Application Note 1119: HFAN-02.2.1: Maximizing the Extinction Ratio of Optical Transmitters Using K-Factor Control for details: K= ΔIMOD ΔIBIAS This provides a first-order approximation of the current increase needed to maintain peak-to-peak power. Slope efficiency decreases more rapidly as temperature increases. The MAX3737 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH). Safety Circuitry The safety circuitry contains a disable, input (TX_DISABLE), a latched fault output (TX_FAULT), and fault detectors (Figure 5). This circuitry monitors the operation of the laser driver and forces a shutdown if a fault is detected (Table 1). The TX_FAULT pin should be pulled high with a 4.7kΩ to 10kΩ resistor to VCC as required by the SFP MSA. A single-point fault can be a short to VCC or GND. See Table 2 to view the circuit response to various single-point failures. The transmit fault condition is latched until reset by a toggle of TX_DISABLE or VCC. The laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the Typical Operating Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition. _______________________________________________________________________________________ 9 MAX3737 Multirate Laser Driver with Extinction Ratio Control Table 1. Typical Fault Conditions 1 If any of the I/O pins is shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent exceed the programmed threshold. 2 End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold. 3 Laser cathode is grounded and photocurrent exceeds the programming threshold. 4 No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the programmed threshold. Table 2. Circuit Responses to Various Single-Point Faults PIN TX_FAULT CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Does not affect laser power. Does not effect laser power. Modulation and bias currents are disabled. Normal condition for circuit operation. IN+ The optical average power increases and a fault occurs if VPC_MON exceeds the threshold. The APC loop responds by decreasing the bias current. The optical average power decreases and the APC loop responds by increasing the bias current. A fault state occurs if VBC_MON exceeds the threshold voltage. IN- The optical average power decreases and the APC loop responds by increasing the bias current. A fault state occurs if VBC_MON exceeds the threshold voltage. The optical average power increases and a fault occurs if VPC_MON exceeds the threshold. The APC loop responds by decreasing the bias current. MD This disables bias current. A fault state occurs. The APC circuit responds by increasing bias current until a fault is detected, then a fault* state occurs. SHUTDOWN Does not affect laser power. If the shutdown circuitry is used, laser current is disabled. Does not affect laser power. BIAS In this condition, laser forward voltage is 0V and no light is emitted. Fault state* occurs. If the shutdown circuitry is used, laser current is disabled. OUT+ The APC circuit responds by increasing the bias current until a fault is detected, then a fault state* occurs. Fault state* occurs. If the shutdown circuitry is used, laser current is disabled. OUT- Does not affect laser power. Does not affect laser power. PC_MON Fault state* occurs. Does not affect laser power. BC_MON Fault state* occurs. Does not affect laser power. MC_MON Fault state* occurs. Does not affect laser power. APCFILT1 IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. APCFILT2 IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. TX_DISABLE MODSET Does not affect laser power. Fault state* occurs. APCSET Does not affect laser power. Fault state* occurs. *A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin. Safety Circuitry Current Monitors The MAX3737 features monitors (MC_MON, BC_MON, PC_MON) for modulation current (IMOD), bias current (IBIAS), and photocurrent (IMD). The monitors are realized 10 by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than VREF at MC_MON, PC_MON, or BC_MON result in a fault state. For example, connecting a ______________________________________________________________________________________ Multirate Laser Driver with Extinction Ratio Control VMC_MON = (IMOD / 268) ✕ 100Ω VBC_MON = (IBIAS / 82) ✕ 100Ω VPC_MON = IMD ✕ 100Ω External sense resistors can be used for high-accuracy measurement of bias and photodiode currents. On-chip isolation resistors are included to reduce the number of components needed to implement this function. Design Procedure When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 3 gives relationships that are 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%. For a desired laser average optical power (PAVG) and optical extinction ratio (re), the required bias and modulation currents can be calculated using the equations in Table 3. Proper setting of these currents requires knowledge of the laser to monitor transfer (ρMON) and slope efficiency (η). Programming the Monitor Diode Current Set Point The MAX3737 operates in APC mode at all times. The bias current is automatically set so average laser power is determined by the APCSET resistor: PAVG = IMD ρMON The APCSET pin controls the set point for the monitordiode current. An internal current regulator establishes the APCSET current in the same manner as the MODSET pin. See the I MD vs. R APCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current at +25°C: IMD = 1 VREF × 2 RAPCSET The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: I IAVG = IBIAS + MOD 2 Programming the Modulation Current with Compensation Determine the modulation current from the laser slope efficiency: IMOD = 2 × PAVG r -1 × e η re + 1 The modulation current of the MAX3737 consists of a static modulation current (IMODS), a current proportional to IBIAS, and a current proportional to temperature. The portion of IMOD set by MODSET is established by an internal current regulator, which maintains the reference voltage of VREF across the external programming resistor. See to 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 3. Optical Power Relations PARAMETER SYMBOL RELATION Average power PAVG PAVG = (P0 + P1) / 2 Extinction ratio re r e = P1 / P 0 Optical power of a 1 P1 P1 = 2PAVG re / (re + 1) P0 P0 = 2PAVG / (re + 1) Optical power of a zero Optical amplitude PP-P PP-P = P1 - P0 η η = PP-P / IMOD Modulation current IMOD IMOD = PP-P / η Threshold current ITH P0 at I ≥ ITH Bias current (AC-coupled) IBIAS IBIAS ≥ ITH + IMOD / 2 Laser to monitor transfer ρMON IMD / PAVG Laser slope efficiency Note: Assuming a 50% average input duty cycle and mark density. ______________________________________________________________________________________ 11 MAX3737 100Ω resistor to ground at each monitor output gives the following relationships: MAX3737 Multirate Laser Driver with Extinction Ratio Control IMOD = IMODS + K × IBIAS + IMODT VREF IMODS = 268 × RMODSET IMODT = TC × (T - TTH ) | T > TTH IMODT = 0 | T ≤ TTH An external resistor at the MODBCOMP pin sets current proportional to IBIAS. Open circuiting the MODBCOMP pin can turn off the interaction between IBIAS and IMOD: K= 1700 ± 10% 1000 + RMODBCOMP If I MOD must be increased from I MOD1 to I MOD2 to maintain the extinction ratio at elevated temperature, the required compensation factor is: - I I K = MOD2 MOD1 IBIAS2 - IBIAS1 A threshold for additional temperature compensation can be set with a programming resistor at the TH_TEMP pin: 1.45MΩ TTH = - 70°C + °C ± 10% 9.2kΩ + RTH _ TEMP The temperature coefficient of thermal compensation above TTH is set by RMODTCOMP. Leaving the MODTCOMP pin open disables additional thermal compensation: TC = mA 1 ± 10% 0.5 + RMODTCOMP (kΩ) °C Current Compliance (IMOD ≤ 60mA), DC-Coupled The minimum voltage at the OUT+ and OUT- pins is 0.7V. For: VDIODE—Diode bias point voltage (1.2V typ) RL—Diode bias point resistance (5Ω typ) RD—Series matching resistor (20Ω typ) For compliance: ( Current Compliance (IMOD > 60mA), AC-Coupled For applications requiring modulation current greater than 60mA, headroom is insufficient for proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3737’s modulation output can be AC-coupled to the cathode of a laser diode. An external pullup inductor is necessary to DC-bias the modulation output at VCC. Such a configuration isolates laser forward voltage from the output circuitry and allows the output at OUT+ to swing above and below the supply voltage (V CC ). When AC-coupled, the MAX3737 modulation current can be programmed up to 85mA. Refer to Application Note 274: HFAN-02.0: Interfacing Maxim Laser Drivers with Laser Diodes for more information on AC-coupling laser drivers to laser diodes. For compliance: VOUT+ = VCC - IMOD 2 ( × RD + RL ≥ 0.75V Determine CAPC The APC loop filter capacitor CAPC must be selected to balance the requirements for fast turn-on and minimal interaction with low frequencies in the data pattern. The low-frequency cutoff is: C APC (μF) ≈ 68 × (η × ρMON )1.1 f3DB (kHz) High-frequency noise can be filtered with an additional cap CMD from the MD pin to ground: CMD ≈ CAPC 4 The MAX3737 is designed so that turn-on time is faster than 1ms for most laser gain values (η ✕ ρ MON ). Choosing a smaller value of C APC reduces turn-on time. Careful balance between turn-on time and low-frequency cutoff may be needed at low data rates for some values of laser gain. Interface Models Figures 6 and 7 show simplified input and output circuits for the MAX3737 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements. ) VOUT+ = VCC - VDIODE - IMOD × RD + RL - IBIAS × RL ≥ 0.7V 12 ) ______________________________________________________________________________________ Multirate Laser Driver with Extinction Ratio Control MAX3737 POR AND COUNTER 60ms DELAY VCC MAX3737 IMOD ENABLE TX_DISABLE 100ns DELAY COUNTER 60ms DELAY IBIAS ENABLE VCC PC_MON R IMD 1 Q VREF SHUTDOWN COMP VCC RPC_MON RS LATCH CMOS BC_MON IBIAS 82 TX_FAULT S COMP VCC RBC_MON MC_MON VREF IMOD 268 TTL OPEN COLLECTOR VREF COMP RMC_MON EXCESSIVE MODULATION CURRENT Figure 5. Simplified Safety Circuit Layout Considerations VCC MAX3737 16kΩ VCC PACKAGE 0.83nH 0.11pF 5kΩ VCC 5kΩ 0.83nH 0.11pF 24kΩ Figure 6. Simplified Input Structure To minimize loss and crosstalk, keep the connections between the MAX3737 output and the laser diode as short as possible. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground plane to minimize EMI and crosstalk. Circuit boards should be made using low-loss dielectrics. Use controlled-impedance lines for data inputs, as well as the module output. Laser Safety and IEC 825 Using the MAX3737 laser driver alone does not ensure that a transmitter design is IEC 825 compliant. The entire transmitter circuit and component selections must be considered. Each customer must determine the level of fault tolerance required by their application, recognizing that Maxim products are not designed or 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 where the failure of a Maxim product could create a situation where personal injury or death may occur. ______________________________________________________________________________________ 13 25 VMD 26 APCFILT1 27 APCFILT2 28 APCSET 29 MODSET 30 MODBCOMP 31 TH_TEMP 1 24 MD 2 23 VCC VCC 3 22 OUT+ IN+ 4 21 OUT+ 20 OUT- 19 OUT- 18 VCC 17 BIAS 10 GND 16 9 OUT- GND *EP 15 8 14 7 BC_MON VBS PC_MON GND 6 13 5 12 INVCC MAX3737ETJ TX_FAULT 0.82nH GND TX_DISABLE 11 PACKAGE TOP VIEW VCC VCC Pin Configurations (continued) 32 MODTCOMP Exposed-Pad (EP) Package The exposed-pad on the 32-pin QFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3737 and should be soldered to the circuit board ground for proper thermal and electrical performance. Refer to Application Note 862: HFAN-08.1: Thermal Considerations of QFN and Other Exposed-Paddle Packages at www.maxim-ic.com for additional information. MC_MON 0.11pF 0.82nH OUT+ 0.11pF SHUTDOWN MAX3737 Multirate Laser Driver with Extinction Ratio Control 5mm x 5mm THIN QFN *THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND TO ACHIEVE SPECIFIED PERFORMANCE. Chip Information MAX3737 PROCESS: SiGe/BIPOLAR Figure 7. Simplified Output Structure 14 ______________________________________________________________________________________ Multirate Laser Driver with Extinction Ratio Control 3.3V OPTIONAL SHUTDOWN CIRCUITRY IN+ VCC SHUTDOWN 0.1μF TX_FAULT TX_DISABLE 3.3V 0.01μF +3.3V 15Ω OUT10Ω CDR 0.1μF OUT+ INBIAS RMODBCOMP MODBCOMP FERRITE BEAD MAX3737 MD RMODTCOMP CMD MODTCOMP RTH_TEMP RMODSET RAPCSET CAPC RMC_MON RBC_MON PC_MON BC_MON MC_MON APCFILT2 APCFILT1 APCSET GND MODSET TH_TEMP RPC_MON REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE Package Information For the latest package outline information and land patterns (footprints), 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. 32 TQFN-EP T3255-3 21-0140 90-0001 32 QFN-EP G3255-1 21-0091 90-0279 ______________________________________________________________________________________ 15 MAX3737 Typical Operating Circuit MAX3737 Multirate Laser Driver with Extinction Ratio Control Revision History REVISION NUMBER 3 REVISION DATE DESCRIPTION PAGES CHANGED 6/11 Added lead and soldering temperature information to the Absolute Maximum Ratings; changed the Bias-Current Monitor Ratio parameter specs from 68mA/mA (min), 82mA/mA (typ), 95mA/mA (max) to 62mA/mA (min), 76mA/mA (typ), 90mA/mA (max) in the Electrical Characteristics table; updated the APCFILT2 and APCSET pin functions in the Pin Description table; added the Package Information table 2, 7, 15 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 © 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.