19-3162; Rev 0; 1/04 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control The MAX3738 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 1Gbps to 2.7Gbps. Lasers can be DC-coupled to the MAX3738 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 MAX3738 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 MAX3738 ideal for driving FP/DFB lasers in fiber optic modules. External resistors set the required laser current levels. The MAX3738 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 MAX3738 is compliant with the SFF-8472 transmitter diagnostic and SFP MSA timing requirements. 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 MAX3738ETG -40°C to 85°C 24 Thin QFN T2444-1 MODSET APCSET APCFILT2 APCFILT1 Multirate OC-24 to OC-48 FEC Transceivers MODBCOMP Applications TH_TEMP Pin Configuration TOP VIEW 1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC Transceivers PKG CODE Typical Application Circuit appears at end of data sheet. The MAX3738 is offered in a 4mm x 4mm, 24-pin thin QFN package and operates over the extended -40°C to +85°C temperature range. Gigabit Ethernet SFF/SFP and GBIC Transceivers PINPACKAGE 24 23 22 21 20 19 MODTCOMP 1 18 MD VCC 2 17 VCC IN+ 3 16 OUT+ MAX3738 IN- 4 15 OUT14 VCC VCC 5 13 BIAS 7 8 9 10 11 12 PC_MON BC_MON SHUTDOWN GND TX_FAULT GND TX_DISABLE 6 THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND ON THE CIRCUIT BOARD. ________________________________________________________________ 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 MAX3738 General Description MAX3738 1Gbps to 2.7Gbps 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 TQFN (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 VID Common-Mode Input VCM DC-coupled, Figure 1 0.2 2.4 VP-P 1.7 VCC VID / 4 V 1 100 mA 0.1 mA 95 mA/mA 85 mA 80 ps LASER BIAS Bias-Current-Setting Range Bias Off Current TX_DISABLE = high Bias-Current Monitor Ratio IBIAS / IBC_MON 68 79 LASER MODULATION Modulation Current-Setting Range IMOD (Note 5) 5 Output Edge Speed 20% to 80% (Notes 6, 7) Output Overshoot/Undershoot With 1pF between OUT+ and OUT- Random Jitter 5mA ≤ IMOD ≤ 85mA 65 ±6 (Notes 6, 7) Deterministic Jitter (Notes 6, 8) % 0.62 1.3 2.7Gbps, 5mA ≤ IMOD ≤ 85mA 18 40 1.25Gbps, 5mA ≤ IMOD ≤ 85mA 20 41 5mA ≤ IMOD ≤ 10mA ±175 ±600 10mA ≤ IMOD ≤ 85mA ±125 ±480 Modulation-Current Temperature Stability (Note 6) Modulation-Current-Setting Error 15Ω load, TA = +25°C Modulation Off Current TX_DISABLE = high 5mA ≤ IMOD ≤ 10mA ±20 10mA < IMOD ≤ 85mA ±15 psRMS psP-P ppm/°C % 0.1 mA 1500 µA 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 _______________________________________________________________________________________ 1.4 V 1.15 mA/mA 1Gbps to 2.7Gbps 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 RPULL = 45kΩ (typ) 0.8 V VHI = VCC 15 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 VLO = GND TX_FAULT Output Low 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 -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 2.7Gbps. DJ includes pulse-width distortion (PWD). _______________________________________________________________________________________ 3 MAX3738 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 (2.7Gbps, 27 - 1 PRBS, 2.3GHz FILTER) ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 2.7Gbps, 27 - 1 PRBS) OPTICAL EYE DIAGRAM (1.25Gbps, 27 - 1 PRBS, 940MHz FILTER) MAX3738 toc01 MAX3738 toc03 MAX3738 toc02 1310nm FP LASER re = 8.2dB 1pF BETWEEN OUT+ AND OUT- 1310nm FP LASER re = 8.2dB 75mV/div 54ps/div 116ps/div 52ps/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 1.2 IMD (mA) 60 50 40 30 50 40 1.0 35 0.8 30 0.6 2.7Gbps 45 DJ (psP-P) 80 MAX3738 toc08 MAX3738 toc07 1.4 MAX3738 toc09 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 90 25 20 15 0.4 20 10 0.2 10 0 5 0 1 10 RMODSET (kΩ) 4 1.05 0.85 72 30 1.10 0.90 74 35 MAX3738 toc06 MAX3738 toc05 1.20 IMD/IPC_MON (mA/mA) 3.63V 50 88 IBIAS/IBC_MON (mA/mA) 55 SUPPLY CURRENT (mA) 90 MAX3738 toc04 60 IMOD (mA) MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control 100 0 0.1 1 10 RAPCSET (kΩ) 100 0 10 20 30 40 50 IMOD (mA) _______________________________________________________________________________________ 60 70 80 90 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control 1.8 1.6 1.4 100 MAX3738 toc11 10 MAX3738 toc10 2.0 90 1 1.0 0.8 IMOD (mA) 1.2 RTH_TEMP = 7kΩ 70 60 0.1 0.6 RTH_TEMP = 12kΩ 80 K (mA/mA) RJ (psRMS) TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500Ω) COMPENSATION (K) vs. RMODBCOMP MAX3738 toc12 RANDOM JITTER vs. MODULATION CURRENT 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 100 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (°C) TRANSMITTER ENABLE MAX3738 toc15 MAX3738 toc14 MAX3738 toc13 RTH_TEMP = 12kΩ 40 RTH_TEMP = 7kΩ 38 RTH_TEMP = 4kΩ 36 30 0.01 IMOD (mA) 44 42 0.01 0.001 VCC 3.3V VCC FAULT 0V FAULT RTH_TEMP = 2kΩ 3.3V LOW t_init = 59.6ms HIGH LOW TX_DISABLE t_on = 23.8µs TX_DISABLE LOW LOW 34 LASER OUTPUT LASER OUTPUT 32 30 -10 0 10 20 30 40 50 60 70 80 90 100 10µs/div 20ms/div TEMPERATURE (°C) TRANSMITTER DISABLE RESPONSE TO FAULT MAX3738 toc16 VCC VPC_MON 3.3V FAULT RECOVERY TIME MAX3738 toc17 LOW EXTERNALLY FORCED FAULT FAULT MAX3738 toc18 VPC_MON EXTERNALLY FORCED FAULT t_fault = 160ns FAULT HIGH 91.2ns TX_DISABLE HIGH FAULT LOW LASER OUTPUT LASER OUTPUT LOW HIGH TX_DISABLE TX_DISABLE t_init = 58ms LOW LOW LASER OUTPUT 20ns/div 400ns/div 40ms/div _______________________________________________________________________________________ 5 MAX3738 Typical Operating Characteristics (continued) (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control Pin Description 6 PIN NAME 1 MODTCOMP 2, 5, 14, 17 VCC 3 IN+ Noninverted Data Input 4 IN- Inverted Data Input 6 TX_DISABLE 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 10, 12 GND 11 TX_FAULT FUNCTION 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. +3.3V Supply Voltage 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. Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. 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 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. 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. _______________________________________________________________________________________ 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control VCC VCC SINGLE ENDED VIN+ 100mV (min) VIN- 30Ω 1200mV (max) 30Ω Z0 = 30Ω OUT- DIFFERENTIAL (VIN+) - (VIN-) MAX3738 VOLTAGE 200mV (min) 30Ω 0.5pF MAX3738 2400mV (max) OSCILLOSCOPE IOUT+ Z0 = 30Ω OUT+ CURRENT Z0 = 50Ω 75Ω IOUT+ 50Ω IMOD 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 MAX3738 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 MAX3738 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. 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 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 MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control VCC SHUTDOWN MAX3738 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 x268 xK 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 MAX3738 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH). _______________________________________________________________________________________ 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738 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 MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control Table 2. Circuit Responses to Various Single-Point Faults PIN TX_FAULT TX_DISABLE 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- Does not affect laser power. Does not affect laser power. Fault state* occurs. Does not affect laser power. PC_MON 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. Safety Circuitry Safety Circuitry Current Monitors 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 Applications Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition. The MAX3738 features monitors (BC_MON, PC_MON) for bias current (I BIAS) and photocurrent (I MD). The monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than V REF 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 10 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. ______________________________________________________________________________________ 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control PARAMETER Average Power SYMBOL RELATION PAVG PAVG = (P0 + P1) / 2 Extinction Ratio re r e = P1 / P0 Optical Power of a One P1 P1 = 2PAVG x re / (re + 1) P0 P0 = 2PAVG / (re + 1) PP-P PP-P = P1 - P0 Optical Power of a Zero Optical Amplitude 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. 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 MAX3738 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 from the laser slope efficiency: IMOD = 2 x PAVG / η x (re - 1) / (re+ + 1) The modulation current of the MAX3738 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 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% ______________________________________________________________________________________ 11 MAX3738 Table 3. Optical Power Relations MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control 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 MAX3738’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 MAX3738 modulation current can be programmed up to 85mA. Refer to Maxim Application Note HFAN 02.0: Interfacing VCC 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 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) High-frequency noise can be filtered with an additional cap, CMD, from the MD pin to ground. CMD ≅ CAPC / 4 The MAX3738 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 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 MAX3738 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements. VCC MAX3738 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 MAX3738 24kΩ Figure 6. Simplified Input Structure 12 OUT+ Figure 7. Simplified Output Structure ______________________________________________________________________________________ 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control port 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. Exposed-Pad (EP) Package The exposed pad on the 24-pin TQFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3738 and should be soldered to the circuit board ground for proper thermal and electrical performance. Refer to Maxim Application Note HFAN-08.1: Thermal Consideration for QFN and Other Exposed-Pad Packages at www.maxim-ic.com for additional information. Laser Safety and IEC 825 Using the MAX3738 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 sup- Chip Information TRANSISTOR COUNT: 1884 PROCESSS: SiGe/Bipolar Typical Application Circuit +3.3V OPTIONAL SHUTDOWN CIRCUITRY 0.01µF +3.3V VCC 15Ω OUT- 0.1µF 10Ω IN- OUT+ RMODBCOMP MAX3738 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 MAX3738 Layout Considerations To minimize loss and crosstalk, keep the connections between the MAX3738 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. 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 MAX3738 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 B 1 2 PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 B 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.