19-3161; Rev 1; 7/04 KIT ATION EVALU E L B A AVAIL 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control Features ♦ Single +3.3V Power Supply ♦ 47mA Power-Supply Current ♦ 85mA Modulation Current ♦ 100mA Bias Current ♦ Automatic Power Control (APC) ♦ Modulation Compensation ♦ On-Chip Temperature Compensation ♦ Self-Biased Inputs for AC-Coupling ♦ Ground-Referenced Current Monitors ♦ Laser Shutdown and Alarm Outputs ♦ Enable Control and Laser Safety Feature Ordering Information PART TEMP RANGE PINPACKAGE PKG CODE MAX3646ETG MAX3646ETG+ -40°C to +85°C 24 Thin QFN T2444-1 -40°C to +85°C 24 Thin QFN T2444-1 +Denotes lead-free package. Pin Configuration Multirate OC-3 to OC-12 FEC Transceivers 125Mbps Ethernet SFP, GBIC, and 1 x 9 Transceivers MODSET APCSET APCFILT2 APCFILT1 Applications MODBCOMP TOP VIEW TH_TEMP The MAX3646 is offered in a 4mm x 4mm, 24-pin thin QFN package and operates over the extended -40°C to +85°C temperature range. 24 23 22 21 20 19 MODTCOMP 1 18 MD VCC 2 17 VCC IN+ 3 16 OUT+ MAX3646 IN- 4 15 OUT14 VCC VCC 5 13 BIAS 8 9 10 11 12 BC_MON GND TX_FAULT GND PC_MON 7 SHUTDOWN TX_DISABLE 6 THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND ON THE CIRCUIT BOARD. Typical Application Circuit appears at end of data sheet. ________________________________________________________________ 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 MAX3646 General Description The MAX3646 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 155Mbps to 622Mbps. Lasers can be DC-coupled to the MAX3646 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. The MAX3646 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, make the MAX3646 ideal for driving FP/DFB lasers in fiber optic modules. External resistors set the required laser current levels. The MAX3646 provides transmit disable control (TX_DISABLE), single-point fault tolerance, bias-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 MAX3646 is compliant with the SFF-8472 transmitter diagnostic and SFP MSA timing requirements. MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control ABSOLUTE MAXIMUM RATINGS Supply Voltage VCC...............................................-0.5V to +6.0V IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN, BC_MON, PC_MON, 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) 24-Pin QFN (derate 20.8mW/°C above +85°C) .........1805mW Operating Junction Temperature Range ...........-55°C to +150°C Storage Temperature Range .............................-55°C to +150°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, 100mAP-P (Note 4) 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 95 mA/mA 85 mA 200 ps 2.5 psRMS LASER BIAS Bias-Current-Setting Range Bias Off Current TX_DISABLE = high Bias-Current Monitor Ratio IBIAS / IBC_MON 68 (Note 5) 5 79 LASER MODULATION Modulation Current-Setting Range IMOD Output Edge Speed 20% to 80% (Notes 6, 7) Output Overshoot/Undershoot (Note 7) (with 2pF between OUT+ and OUT-) ±6 Random Jitter (Notes 6, 7) 1.1 Deterministic Jitter (Notes 6, 8) 5mA ≤ IMOD ≤ 85mA 100 % 622Mbps, 5mA ≤ IMOD ≤ 85mA 24 46 155Mbps, 5mA ≤ IMOD ≤ 85mA 45 100 Modulation-Current Temperature Stability (Note 6) Modulation-Current-Setting Error 15Ω load, TA = +25°C Modulation Off Current TX_DISABLE = high 5mA ≤ IMOD ≤ 10mA ±175 ±600 10mA < IMOD ≤ 85mA ±125 ±480 5mA ≤ IMOD ≤ 10mA ±20 10mA < IMOD ≤ 85mA ±15 psP-P ppm/°C % 0.1 mA 1500 µA 1.4 V 1.15 mA/mA AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS Monitor-Diode Input Current Range IMD Average current into the MD pin 18 MD Pin Voltage MD Current Monitor Ratio 2 IMD / IPC_MON 0.85 0.93 _______________________________________________________________________________________ 155Mbps to 622Mbps SFF/SFP 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 MIN APC Loop Time Constant CAPC_FILT = 0.01µF, ∆IMD / ∆IBIAS = 1/70 APC Setting Stability (Note 6) APC Setting Accuracy TA = +25°C IMOD Compensation-Setting Range by Bias K IMOD Compensation-Setting Range by Temperature Threshold-Setting Range for Temperature Compensation TYP MAX UNITS ±480 ppm/°C ±15 % 3.3 ±100 µs K = ∆IMOD / ∆IBIAS 0 1.5 mA/mA TC TC = ∆IMOD / ∆T (Note 6) 0 1.0 mA/°C 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 0.8 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 2.0 VHI = VCC Sinking 1mA, open collector Shutdown Output High Sourcing 100µA Shutdown Output Low Sinking 100µA Note 5: Note 6: Note 7: Note 8: 15 VLO = GND TX_FAULT Output Low Note 1: Note 2: Note 3: Note 4: V RPULL = 45kΩ (typical) -70 -140 0.4 VCC - 0.4 µA V V 0.4 V 23 AC characterization is performed using the circuit in Figure 2 using a PRBS 2 - 1 or equivalent 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) / ∆VOUT). 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 622Mbps. DJ includes pulse-width distortion (PWD). _______________________________________________________________________________________ 3 MAX3646 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) OPTICAL EYE DIAGRAM (622.08Mbps, 27 - 1 PRBS, 467MHz FILTER) ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 622.08MHz, 27 - 1 PRBS) OPTICAL EYE DIAGRAM (155Mbps, 27 - 1 PRBS, 117MHz FILTER) MAX3646 toc01 MAX3646 toc03 MAX3646 toc02 1310nm FP LASER re = 8.2dB 2pF BETWEEN OUT+ AND OUT- 1310nm FP LASER re = 8.2dB 75mV/div 270ps/div 1ns/div 320ps/div SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS) BIAS-CURRENT MONITOR RATIO vs. TEMPERATURE PHOTOCURRENT MONITOR RATIO vs. TEMPERATURE 45 2.97V 3.3V 40 86 84 82 80 78 76 1.15 1.00 0.95 70 0.80 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) MODULATION CURRENT vs. RMODSET PHOTODIODE CURRENT vs. RAPCSET DETERMINISTIC JITTER vs. MODULATION CURRENT 70 100 MAX3646 toc08 1.4 MAX3646 toc07 80 1.2 80 IMD (mA) 50 40 DJ (psP-P) 1.0 60 155mbps 90 MAX3646 toc09 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 90 0.8 0.6 70 60 50 30 0.4 40 20 0.2 10 30 0 0 1 10 RMODSET (kΩ) 4 1.05 0.85 72 30 1.10 0.90 74 35 MAX3646 toc06 MAX3646 toc05 1.20 IMD/IPC_MON (mA/mA) 3.63V 50 88 IBIAS/IBC_MON (mA/mA) 55 SUPPLY CURRENT (mA) 90 MAX3646 toc04 60 IMOD (mA) MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control 100 20 0.1 1 10 RAPCSET (kΩ) 100 0 10 20 30 40 50 IMOD (mA) _______________________________________________________________________________________ 60 70 80 90 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control 1.8 1.6 1.4 100 MAX3646 toc11 10 MAX3646 toc10 2.0 90 1 1.0 0.8 IMOD (mA) 1.2 0.1 0.6 RTH_TEMP = 12kΩ 80 K (mA/mA) RJ (psRMS) TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500Ω) COMPENSATION (K) vs. RMODBCOMP MAX3646 toc12 RANDOM JITTER vs. MODULATION CURRENT RTH_TEMP = 7kΩ 70 60 50 RTH_TEMP = 4kΩ RTH_TEMP = 2kΩ 0.4 40 0.2 0 0 10 20 30 40 50 60 70 80 90 IMOD (mA) 0.1 1 10 RMODBCOMP (kΩ) TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 10kΩ) HOT PLUG WITH TX_DISABLE LOW RTH_TEMP = 7kΩ 38 RTH_TEMP = 4kΩ 100 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (°C) TRANSMITTER ENABLE MAX3646 toc14 MAX3646 toc13 RTH_TEMP = 12kΩ 40 36 30 0.01 IMOD (mA) 44 42 0.01 0.001 VCC RTH_TEMP = 2kΩ 3.3V LOW t_init = 59.6ms FAULT 0V FAULT MAX3646 toc15 VCC 3.3V HIGH LOW TX_DISABLE TX_DISABLE t_on = 23.8µs LOW LOW 34 LASER OUTPUT LASER OUTPUT 32 30 20ms/div -10 0 10 20 30 40 50 60 70 80 90 100 10µs/div TEMPERATURE (°C) RESPONSE TO FAULT TRANSMITTER DISABLE VCC FAULT RECOVERY TIME MAX3646 toc17 MAX37646 toc16 VPC_MON 3.3V MAX3646 toc18 VPC_MON EXTERNALLY FORCED FAULT EXTERNALLY FORCED FAULT LOW FAULT t_fault = 160ns FAULT HIGH 91.2ns TX_DISABLE LOW HIGH FAULT LASER OUTPUT LASER OUTPUT LOW HIGH TX_DISABLE TX_DISABLE t_init = 58ms LOW LOW LASER OUTPUT 20ns/div 400ns/div 40ms/div _______________________________________________________________________________________ 5 MAX3646 Typical Operating Characteristics (continued) (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646 Pin Description 6 PIN NAME FUNCTION 1 MODTCOMP Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature coefficient of the modulation current when above the threshold temperature. Leave open for zero temperature compensation. 2, 5, 14, 17 VCC 3 IN+ Noninverted Data Input 4 IN- Inverted Data Input 6 TX_DISABLE 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. 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 SHUTDOWN Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. 10, 12 GND 11 TX_FAULT +3.3V Supply Voltage Ground Open-Collector Transmit Fault Indicator (Table 1) 13 BIAS Laser Bias-Current Output 15 OUT- Inverted Modulation-Current Output. IMOD flows into this pin when input data is low. 16 OUT+ Noninverted Modulation-Current Output. IMOD flows into this pin when input data is high. 18 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. 19 APCFILT1 Connect a capacitor (CAPC) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant pole of the APC feedback loop. 20 APCFILT2 (See Pin 19) 21 APCSET A resistor connected from this pin to ground sets the desired average optical power. 22 MODSET A resistor connected from this pin to ground sets the desired constant portion of the modulation current. 23 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. 24 TH_TEMP Threshold for Temperature Compensation. A resistor at this pin programs the temperature above which compensation is added to the modulation current. EP Exposed Pad Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical performance. _______________________________________________________________________________________ 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control SINGLE ENDED VIN+ VCC 100mV (min) VIN- 1200mV (max) VCC 30Ω Z0 = 30Ω OUT- DIFFERENTIAL (VIN+) - (VIN-) 30Ω MAX3646 VOLTAGE 200mV (min) 30Ω 2400mV (max) MAX3646 0.5pF OSCILLOSCOPE IOUT+ CURRENT Z0 = 30Ω OUT+ IOUT+ Z0 = 50Ω 75Ω IMOD 50Ω TIME Figure 1. Required Input Signal and Output Polarity Figure 2. Test Circuit for Characterization HOST BOARD FILTER DEFINED BY SFP MSA L1 1µH SOURCE NOISE VOLTAGE SUPPLY MODULE C1 0.1µF C2 10µF OPTIONAL C3 0.1µF TO LASER DRIVER VCC OPTIONAL Figure 3. Supply Filter Detailed Description The MAX3646 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 MAX3646 is optimized for driving a 15Ω load. The minimum instantaneous voltage required at OUT- is 0.7V for modulation currents 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 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 Maxim Application Note HFAN 0.0: Interface Maxim’s Laser Driver to Laser Diode for more information. 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 form 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) _______________________________________________________________________________________ 7 MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control VCC SHUTDOWN MAX3646 INPUT BUFFER IN+ DATA PATH IN- OUTOUT+ IMOD ENABLE SHUTDOWN SAFETY LOGIC AND POWER DETECTOR TX_FAULT TX_DISABLE RD IMOD IBIAS ENABLE BIAS VCC IBIAS RPULL = 45kΩ VCC IMD 1 VBG IBIAS APCSET RAPCSET PC_MON x1/2 RPC_MON xTC xK x268 IAPCSET IBIAS 82 MD T > TTH BC_MON IMD CMD RBC_MON T x1 VBG TH_TEMP RTH_TEMP MODTCOMP RMODTCOMP MODSET MODBCOMP RMODSET APCFILT1 APCFILT2 RMODBCOMP CAPC Figure 4. Functional Diagram 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 (h) of laser over time and temperature: PAVG = IMD/ρMON PP-P = η x IMOD 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 HFAN-02.21 for details: 8 K = ∆IMOD / ∆IBIAS 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 MAX3646 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH). _______________________________________________________________________________________ 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646 VCC POR AND COUNTER 60ms DELAY IMOD ENABLE TX_DISABLE COUNTER 60ms DELAY 100ns DELAY IBIAS ENABLE VCC IMD 1 VREF PC_MON Q R COMP VCC RPC_MON IBIAS 82 RS LATCH VREF BC_MON S SHUTDOWN COMP RBC_MON CMOS EXCESSIVE APC CURRENT SETPOINT EXCESSIVE MOD CURRENT SETPOINT TX_FAULT TTL OPEN COLLECTOR Figure 5. Simplified Safety Circuit Table 1. Typical Fault Conditions 1 If any of the I/O pins are shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent exceeds 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. _______________________________________________________________________________________ 9 MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control Table 2. Circuit Responses to Various Single-Point Faults PIN TX_FAULT CIRCUIT RESPONSE TO OVERVOLTATGE OR SHORT TO VCC CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Does not affect laser power. Does not affect 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 the bias current until a fault is detected, then a fault* state occurs. SHUTDOWN Does not affect laser power. If the shutdown circuitry is used, the laser current is disabled. Does not affect laser power. BIAS In this condition, the laser forward voltage is 0V and no light is emitted. Fault state* occurs. If the shutdown circuitry is used, the 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, the laser current is disabled. OUT- TX_DISABLE 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. 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. 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. 10 ______________________________________________________________________________________ 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control SYMBOL RELATION Average power PARAMETER PAVG PAVG = (P0 + P1) / 2 Extinction ratio re r e = P1 / P 0 Optical power of a one P1 P1 = 2PAVG x re / (re + 1) Optical power of a zero P0 P0 = 2PAVG / (re + 1) 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 IBIAS IBIAS ≥ ITH + IMOD / 2 Laser slope efficiency Bias current (AC-coupled) Laser to monitor IMD / PAVG ρMON transfer Note: Assuming a 50% average input duty cycle and mark density. 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 failure. The transmit fault condition is latched until reset by a toggle or TX_DISABLE or VCC. The laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the Typical Operation Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition. Safety Circuitry Current Monitors The MAX3646 features monitors (BC_MON, PC_MON) for bias current (IBIAS) and photocurrent (IMD). The monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than VREF at PC_MON or BC_MON result in a fault state. For example, connecting a 100Ω resistor to ground at each monitor output gives the following relationships: VBC_MON = (IBIAS / 82) x 100Ω VPC_MON = IMD x 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 shows 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 MAX3646 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 monitor diode current. An internal current regulator establishes the APCSET current in the same manner as the MODSET pin. See the IMD vs. RAPCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current at +25°C: IMD = 1/2 x VREF / RACPSET The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: IAVG = IBIAS + IMOD / 2 Programming the Modulation Current with Compensation Determine the modulation current form the laser slope efficiency: IMOD = 2 x PAVG / η x (re - 1)/(re+ + 1) The modulation current of the MAX3646 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 the I MOD vs. R MODSET graph in the Typical Operating Characteristics and select the value ______________________________________________________________________________________ 11 MAX3646 Table 3. Optical Power Relations MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control of RMODSET that corresponds to the required current at +25°C: IMOD = IMODS + K x IBIAS + IMODT IMODS = 268 x VREF / RMODSET IMODT = TC x (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 / (1000 + RMODBCOMP) +10% If I MOD must be increased from I MOD1 to I MOD2 to maintain the extinction ratio at elevated temperatures, the required compensation factor is: K = (IMOD2 - IMOD1) / (IBIAS2 - IBIAS1) A threshold for additional temperature compensation can be set with a programming resistor at the TH_TEMP pin: TTH = -70°C + 1.45MΩ / (9.2kΩ + RTH_TEMP)°C +10% The temperature coefficient of thermal compensation above T TH is set by R MODTCOMP . Leaving the MODTCOMP pin open disables additional thermal compensation: TC = 1 / (0.5 + RMODTCOMP(kΩ)) mA/°C +10% 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: VOUT+ = VCC - VDIODE - IMOD x (RD + RL) IBIAS x RL ≥ 0.7V Current Compliance (IMOD > 60mA), AC-Coupled For applications requiring modulation current greater than 60mA, headroom is insufficient from proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3646’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 (VCC). When AC-coupled, the MAX3646 modulation current can be programmed up to 85mA. Refer to Maxim Application Note HFAN 02.0: Interfacing VCC VCC MAX3646 PACKAGE 16kΩ PACKAGE VCC 0.7nH OUT- 0.7nH IN+ 0.11pF 0.11pF 5kΩ 0.7nH 0.11pF VCC 5kΩ 0.7nH IN0.11pF MAX3646 24kΩ Figure 6. Simplified Input Structure 12 OUT+ Figure 7. Simplified Output Structure ______________________________________________________________________________________ 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control 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 MAX3646 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements. 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: CAPC(µF) ≅ 68 / (f3dB(kHz) x (η x ρMON)1.1 Layout Considerations To minimize loss and crosstalk, keep the connections between the MAX3646 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. High-frequency noise can be filtered with an additional cap, CMD, from the MD pin to ground: CMD ≅ CAPC / 4 The MAX3646 is designed so turn-on time is faster than 1ms for most laser gain values (η x ρMON). Choosing a smaller value of CAPC reduces turn-on time. Careful Typical Application Circuit +3.3V OPTIONAL SHUTDOWN CIRCUITRY 0.01µF +3.3V VCC 15Ω OUT- 0.1µF 10Ω IN- OUT+ RMODBCOMP MAX3646 MODBCOMP BIAS RMODTCOMP FERRITE BEAD MODTCOMP MD RTH_TEMP CMD BC_MON PC_MON RPC_MON APCFILT2 APCFILT1 CAPC RBC_MON RAPCSET RMODSET APCSET MODSET TH_TEMP GND CDR SHUTDOWN IN+ TX_FAULT 0.1µF TX_DISABLE +3.3V REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE. ______________________________________________________________________________________ 13 MAX3646 Maxim’s Laser Drivers to Laser Diodes for more information on AC-coupling laser drivers to laser diodes. For compliance: VOUT+ = VCC - IMOD / 2 x (RD + RL) ≥ 0.75V Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) 24L QFN THIN.EPS MAX3646 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm 21-0139 C 1 2 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. 14 ____________________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.