19-2194; Rev 1; 2/02 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Features ♦ 9psP-P Deterministic Jitter ♦ 20-Pin QFN 4mm ✕ 4mm Package ♦ 3.0V to 5.5V Supply Voltage ♦ Automatic Power Control ♦ Integrated Safety Circuits ♦ 30mA Laser Modulation Current ♦ Temperature Compensation of Modulation Current ♦ Compliant with SFF and SFP MSA Ordering Information PART TEMP RANGE MAX3996CGP PIN-PACKAGE 0°C to +70°C 20 QFN Typical Application Circuit VCC OPTIONAL SHUTDOWN CIRCUITRY Applications VCC 1.8kΩ Fibre Channel Optical Transmitters 0.01µF VCC TX_DISABLE VCSEL Transmitters SHDNDRV 0.01µF Gigabit Ethernet Optical Transmitters FAULT OUT- ATM LAN Optical Transmitters 10 Gigabit Ethernet WWDM 0.01µF 0.01µF IN+ OUT+ MAX3996 L1* 0.01µF 25Ω INBIAS Pin Configuration appears at end of data sheet. PORDLY MD TC MODSET MON1 MON2 COMP GND CPORDLY RTC RMOD N.C. CCOMP RSET *FERRITE BEAD ________________________________________________________________ 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 MAX3996 General Description The MAX3996 is a high-speed laser driver for smallform-factor (SFF) fiber optic LAN transmitters. It contains a bias generator, a laser modulator, and comprehensive safety features. Automatic power control (APC) adjusts the laser bias current to maintain average optical power, regardless of changes in temperature or laser properties. The driver accommodates common anode or differential laser configurations. The output current range of the MAX3996 is appropriate for VCSELs and high-efficiency edge-emitting lasers. The MAX3996 operates up to 3.2Gbps. It can switch up to 30mA of laser modulation current and sink up to 60mA bias current. Adjustable temperature compensation is provided to keep the optical extinction ratio within specifications over the operating temperature range. The MAX3996 accommodates various laser packages, including low-cost TO-46 headers. Low deterministic jitter (9ps P-P ), combined with fast edge transitions, (65ps) provides excellent margins compared to industry-standard transmitter eye masks. This laser driver provides extensive safety features to guarantee single-point fault tolerance. Safety features include a transmit disable, redundant shutdown, and laser-bias monitoring. The safety circuit detects faults that could cause hazardous light levels and immediately disables the laser output. The MAX3996 safety circuits are compliant with SFF and small-form-factor pluggable (SFP) multisource agreements (MSA). The MAX3996 is available in a compact 4mm ✕ 4mm, 20-pin QFN package and operates over a temperature range of 0°C to +70°C. MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver ABSOLUTE MAXIMUM RATINGS Current into FAULT, SHDNDRV ..........................-1mA to +25mA Current into OUT+, OUT- ....................................................60mA Current into BIAS ..............................................................120mA Continuous Power Dissipation (TA = +70°C) 20-Pin QFN (derate 20mW/°C)...................................1600mW Operating Ambient Temperature Range .............-40°C to +85°C Operating Junction Temperature Range. ..........-40°C to +150°C Storage Temperature Range.... .........................-55°C to +150°C Supply Voltage at VCC...........................................-0.5V to +7.0V Voltage at TX_DISABLE, PORDLY, MON1, COMP, IN+, IN-, MD, BIAS, MODSET, TC..........-0.5V to (VCC + 0.5V) Voltage between COMP and MON2 .....................................2.3V Voltage between IN+ and IN- ..................................................5V Voltage at OUT+, OUT- .........................(VCC - 2V) to (VCC + 2V) Voltage between MON1 and MON2 .....................................1.5V Voltage between BIAS and MON2...........................................4V 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 = 3.0V to 5.5V, TA = 0°C to +70°C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA = +25°C.) (Figure 1) PARAMETER Supply Current Data Input Voltage Swing SYMBOL ICC (Figure1) (Note 1) VID TX_DISABLE Input Current TX_DISABLE Input High Voltage TX_DISABLE Input Low Voltage CONDITIONS MIN TYP VCC = 3.3V, IMOD = 15mA 47 VCC = 5.5V, IMOD = 30mA, RMODSET = 2.37kΩ 52 MAX 75 UNITS mA Total differential signal (Figure 2) 200 2200 MVP-P 0 < VPIN < VCC -100 100 µA VIH 2.0 V VIL 0.8 FAULT Output High Voltage VOH IOH = -100µA, 4.7kΩ < RFAULT < 10kΩ FAULT Output Low Voltage VOL IOL = 1mA Minimum Bias Current IBIAS Current into BIAS pin Maximum Bias Current IBIAS Current into BIAS pin MD Quiescent Voltage VMD 2.4 V V 0.4 V 1 mA BIAS GENERATOR APC loop is closed Monitor Resistance RMON MD Input Current BIAS Current During Fault 60 mA 1.04 1.12 FAULT = high VCC - 0.73 TX_DISABLE = high VCC - 0.73 (Figure 4) 9.3 11 12.7 Ω FAULT = low, TX_DISABLE = low -3 0.8 3 µA 10 µA IBIAS_OFF APC Time Constant V CCOMP = 0.1µF 35 µs POWER-ON RESET (POR) POR Threshold POR Delay Measured at VCC tPORDLY 2.65 2.7 PORDLY = open (Note 3) 30 55 CPORDLY = 0.001µF (Note 3) 1.7 2.4 ms 20 mV POR Hysteresis 3.0 V µs SHUTDOWN ISHDNDRV = 10µA, FAULT = high Voltage at SHDNDRV VCC - 0.4 ISHDNDRV = 1mA, FAULT = low ISHDNDRV = 15mA, FAULT = low VCC - 2.4 0 V VCC - 1.2 LASER MODULATOR Data Rate 2 < 3.2 _______________________________________________________________________________________ Gbps 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver (VCC = 3.0V to 5.5V, TA = 0°C to +70°C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA = +25°C.) (Figure 1) PARAMETER SYMBOL Minimum Modulation Current iMOD Maximum Modulation Current iMOD Accuracy of Modulation Current (Part-to-Part Variation) Edge Transition Time tr, tf Deterministic Jitter Random Jitter CONDITIONS MIN TYP RL ≤ 25Ω 30 40 RMODSET = 2.37kΩ (iMOD ≈ 30mAp-p into 25Ω) -10 Input Resistance mAP-P +10 100 55 125 iMOD = 30mA into 25Ω, 20% to 80% (Note 3) 65 130 iMOD = 5mA into 25Ω (Notes 2, 3) 17 35 iMOD = 10mA into 25Ω (Notes 2, 3) 14 22 iMOD = 30mA into 25Ω (Notes 2, 3) 9 20 85 Single ended; outputs to VCC 42 Input Common-Mode Voltage ps psP-P 2 8 psRMS 200 µAP-P ppm/°C 50 Differential % 15 4000 Tempco = MIN, RTC = open ROUT mAP-P 54 Tempco = MAX, RMOD = open RIN Output Resistance 2 iMOD = 10mA into 25Ω, 20% to 80% (Note 3) iMOD_OFF Modulation Current Tempco UNITS iMOD = 5mA into 25Ω, 20% to 80% (Note 3) (Note 3) Modulation Current During Fault MAX 50 115 Ω 58 Ω VCC - 0.3 V SAFETY FEATURES (See Typical Operating Characteristics) MODSET and TC Pin Fault Threshold 200 mV BIAS Pin Fault Threshold A fault will be triggered if VBIAS is less than this voltage Excessive Bias Current Fault A fault will be triggered if VMON2 exceeds this voltage 400 440 mV 300 400 mV TX Disable Time t_off Time from rising edge of TX_DISABLE to IBIAS = IBIAS_OFF and iMOD = iMOD_OFF (Note 3) 0.06 5 µs TX Disable Negate Time t_on Time from falling edge of TX_DISABLE to IBIAS and iMOD at 95% of steady state (Note 3) 37 500 µs Reset Initialization Time t_init From power ON or negation of FAULT using TX_DISABLE. Time to set FAULT = low, iMOD = 95% of steady state and IBIAS = 95% of steady state (Note 3) 23 200 ms Fault Assert Time t_fault Time from fault to FAULT = high, CFAULT < 20pF, RFAULT = 4.7kΩ (Note 3) 14 50 µs TX_DISABLE Reset t_reset Time TX_DISABLE must be held high to reset FAULT (Note 3) 0.01 1 µs Note 1: Supply current excludes bias and modulation currents. Note 2: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern 00111110101100000101. Note 3: AC characteristics guaranteed by design and characterization. _______________________________________________________________________________________ 3 MAX3996 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VCC = 3.3V, TA = +25°C, unless otherwise noted.) 25Ω LOAD 120mV/ div MAX3996 toc03 MAX3996 toc02 25Ω LOAD OPTICAL EYE DIAGRAM (iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS, 2.5Gbps, 1870MHz FILTER) ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps) MAX3996 toc01 ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 2.5Gbps) 120mV/ div 64ps/div 57ps/div 52ps/div OPTICAL EYE DIAGRAM (iMOD = 15mA, 1310nm LASER, 27 - 1 PRBS, 2.5Gbps, 1870MHz FILTER) TRANSITION TIME vs. MODULATION CURRENT DETERMINISTIC JITTER vs. MODULATION CURRENT TRANSITION TIME (ps) FALL TIME 60 RISE TIME 50 40 25 20 15 TOTAL DJ 10 PWD 5 20 57ps/div MAX3996 toc06 70 30 DETERMINISTIC JITTER (psP-P) MAX3996 toc05 MAX3996 toc04 80 30 0 5 10 15 20 25 30 35 5 10 15 iMOD (mA) 1 MAX3996 toc08 EXCLUDES IBIAS, iMOD 25Ω LOAD 65 25 POR DELAY vs. CPORDLY MAX3996 toc07 70 20 iMOD (mA) SUPPLY CURRENT vs. TEMPERATURE (iMOD = 15mA) 100m 60 POR DELAY (s) SUPPLY CURRENT (mA) MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 55 50 45 40 10m 1m 100µ 35 10µ 30 0 15 30 45 60 AMBIENT TEMPERATURE (°C) 4 75 10p 100p 1n 10n CPORDLY (F) _______________________________________________________________________________________ 100n 30 35 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver STARTUP WITH SLOW RAMPING SUPPLY VCC VCC FAULT LOW FAULT 0V MAX3996 toc11 3.3V VCC 3.3V LOW FAULT LOW TX_DISABLE LOW HIGH t_init = 23mS LOW TX_DISABLE TX_DISABLE LASER OUPUT LASER OUPUT RESPONSE TO FAULT t_off = 60ns LOW FAULT RECOVERY TIME EXTERNALLY FORCED FAULT ON FAULT 20.0µs/div MAX3996 toc13 TRANSMITTER DISABLE MAX3996 toc12 10.0ms/div VMON2 LOW LASER OUPUT 10.0ms/div 3.3V VCC t_on = 37µs t_fault = 14µs MAX3996 toc14 0V TRANSMITTER ENABLE MAX3996 toc10 3.3V MAX3996 toc09 HOT PLUG WITH TX_DISABLE LOW EXTERNAL FAULT REMOVED VTC FAULT IBIAS OFF HIGH HIGH LOW TX_DISABLE FAULT LASER OUPUT TX_DISABLE LOW ELECTRICAL OUPUT 20.0ns/div LASER OUPUT 10.0µs/div 10.0µs/div VTC MAX3996 toc15 FREQUENT ASSERTION OF TX_DISABLE EXTERNALLY FORCED FAULT OV FAULT TX_DISABLE LASER OUPUT 1.00ms/div _______________________________________________________________________________________ 5 MAX3996 Typical Operating Characteristics (continued) (VCC = 3.3V, TA = +25°C, unless otherwise noted.) 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Pin Description PIN NAME 1 TC 2 FAULT 3, 9 GND FUNCTION Temperature Compensation Set. The resistor at TC programs the temperature-increasing component of the laser-modulation current. Fault Indicator. See Table 1. Ground Transmit Disable. Laser output is disabled when TX_DISABLE is high or left unconnected. The laser output is enabled when this pin is asserted low. 4 TX_DISABLE 5 PORDLY 6, 16, 19 VCC 7 IN+ Noninverting Data Input 8 IN- Inverting Data Input 10 MON1 11 MON2 This pin attaches to the emitter of the bias driving transistor. See the Design Procedure section. COMP A capacitor connected from this pin to ground sets the dominant pole of the APC loop. See the Design Procedure section. 12 6 13 MD 14 SHDNDRV Power-On Reset Delay. A capacitor connected between PORDLY and GND can be used to extend the delay for the power-on reset circuit. See the Design Procedure section. Supply Voltage Attaches to the emitter of the bias driving transistor through a 10Ω resistor. See the Design Procedure section. Monitor Diode Connection. MD is used for automatic power control. Shutdown Driver Output. Provides a redundant laser shutdown. 15 BIAS 17 OUT+ Laser Bias Current Output Positive Modulation-Current Output. Current flows from this pin when input data is high. 18 OUT- Negative Modulation-Current Output. Current flows to this pin when input data is high. 20 MODSET EP Exposed Pad A resistor connected from this pin to ground sets the desired modulation current. Ground. This must be soldered to the circuit board ground for proper thermal and electrical performance. See the Layout Considerations section. _______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 3.0V TO 5.5V VOLTS VIN+ ICC VCC iOUT ROUT 100mVP-P MIN 1100mVP-P MAX VIN- VCC MAX3996 SINGLE-ENDED SIGNAL DIFFERENTIAL SIGNAL FERRITE BEAD* ROUT 200mVP-P MIN 2200mVP-P MAX VID = VIN+ - VINOUTOUT+ 0.01µF IN+ CURRENT iMOD iMOD 0.01µF 0.01µF RIN VID 0.01µF IN- 25Ω 25Ω MODULATION CURRENT GENERATOR TC TIME Figure 2. Required Input Signal and Modulation-Current Polarity MODSET Bias Generator RMOD *MURATA BLM11HA102SG Figure 1. Output Load for AC Specification Detailed Description The MAX3996 contains a bias generator with automatic power control and smooth start, a laser modulator, a power-on reset (POR) circuit, and safety circuitry (Figure 3). VCC FAULT Figure 4 shows the bias generator circuitry that contains a power-control amplifier, smooth-start circuitry, and two bias-fault sensors. The power-control amplifier combined with an internal NPN transistor provides DC laser current to bias the laser in a light-emitting state. The APC circuitry adjusts the laser bias current to maintain average power over temperature and changing laser properties. The smooth-start circuitry prevents current spikes to the laser during power-up or enable, ensuring compliance with safety requirements and extending the life of the laser. SHDNDRV PORDLY SAFETY CIRCUITRY POR CIRCUIT BIAS ENABLE BIAS GENERATOR WITH SMOOTH START MD COMP MON1 MON2 MON2 POWER-CONTROL AMPLIFIER BIAS FAULT 2 MAX3996 50Ω BIAS MD VCC MAX3996 BIAS FAULT 1 1.1V BIAS TX_DISABLE 400mV SMOOTH START BIAS DISABLE RMON(11Ω) 400mV MON1 50Ω COMP IN+ INPUT BUFFER OUTOUT+ LASER MODULATION Figure 4. Bias Circuitry 100Ω INMODULATION ENABLE MODULATION FAULT MODULATION CURRENT GENERATOR TC MODSET The MD input is connected to the anode of a monitor diode, which is used to sense laser power. The BIAS output is connected to the cathode of the laser through an inductor or ferrite bead. The power-control amplifier drives a transistor to control the laser’s bias current. In a fault condition (Table 1), the base of the bias-driving transistor is pulled low to ensure that bias current is turned off. Figure 3. Laser Driver Functional Diagram _______________________________________________________________________________________ 7 Table 1. Typical Fault Conditions PIN MON2 VMON2 > 400mV BIAS VBIAS < 400mV TC, MODSET VCC FAULT CONDITION 50Ω OUT+ VMODSET or VTC < 200mV 100Ω IN- ENABLE Safety Circuitry The safety circuitry contains a disable input, a fault latch, and fault detectors (Figure 7). This circuitry monitors the operation of the laser driver and forces a shutdown if a single-point fault is detected. A single-point fault can be a short to VCC or GND, or between any two CURRENT AMPLIFIER 96X MODULATION CURRENT GENERATOR Σ 1.2V REFERENCE 4000ppm/°C 200mV Modulation Circuitry The modulation circuitry consists of an input buffer, a current mirror, and a high-speed current switch (Figure 5). The modulator drives up to 30mA of modulation current into a 25Ω load. Many of the modulator performance specifications depend on total modulator current. To ensure good driver performance, the voltage at either OUT+ or OUT- must not be less than VCC - 1V. The amplitude of the modulation current is set with resistors at the MODSET and temperature coefficient (TC) pins. The resistor at MODSET (RMOD) programs the temperature-stable portion of the modulation current, and the resistor at TC (RTC) programs the temperatureincreasing portion of the modulation current. Figure 6 shows modulation current as a function of temperature for two extremes: RTC is open (the modulation current has zero temperature coefficient), and RMOD is open (the modulation temperature coefficient is 4000ppm/°C). Intermediate temperature coefficient values of the modulation current can be obtained as described in the Design Procedure section. Table 2 is the RTC and RMOD selection table. OUT- CURRENT SWITCH INPUT BUFFER IN+ Smooth-Start During startup, the laser does not emit light, and the APC loop is not closed. The smooth-start circuit pulls the MD pin to approximately 2.5V during the POR delay and while TX_DISABLE is high. This causes the powercontrol amplifier to shut off the bias transistor. When POR delay is over and TX_DISABLE is low, the MD pin is released and pulled to GND by RSET because there is no laser power and thus no monitor diode current. The output voltage of the power-control amplifier then begins to increase. A capacitor attached to COMP (CCOMP) slows the slew rate and allows a controlled increase in bias current (Figure 11). Maxim recommends CCOMP = 0.1µF. 8 50Ω MAX3996 1.2V REFERENCE 0ppm/°C 200mV TC FAULT TC MODSET FAULT MODSET RTC RMOD Figure 5. Modulation Circuitry 1.3 1.2 iMOD/(iMOD AT +52°C) MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver RTC ≥ 1.9kΩ RMOD = OPEN TEMPCO = 4000ppm/°C 1.1 1.0 0.9 RTC = OPEN TEMPCO = 50ppm/°C 0.8 0.7 0.6 0 10 20 30 40 50 60 70 80 90 100 110 JUNCTION TEMPERATURE (°C) Figure 6. Modulation Current vs. Temperature for Maximum and Minimum Temperature Coefficient _______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Table 2. RTC and RMOD Selection Table iMOD = 30mA iMOD = 15mA iMOD = 5mA TEMPCO (ppm/°C) RMOD (kΩ) RTC (kΩ) RMOD (kΩ) RTC (kΩ) 3500 17.1 1.85 34.4 3.94 104 12.3 3000 8.04 2.19 16.3 4.64 49.5 14.4 2500 5.20 2.68 10.6 5.62 32.4 17.4 2000 3.81 3.42 7.86 7.08 24.1 21.8 1500 2.98 4.64 6.21 9.53 19.1 29.1 1000 2.44 7.08 5.12 14.4 15.9 43.8 500 2.05 14.4 4.34 29.1 13.5 87.8 RMOD (kΩ) RTC (kΩ) Table 3. Circuit Responses to Various Single-Point Faults PIN NAME CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND TC Does not affect laser power. Fault state* occurs. FAULT Does not affect laser power. Does not affect laser power. Modulation and bias current are disabled. Normal condition for circuit operation. TX_DISABLE PORDLY Does not affect laser power. Modulation and bias current are disabled. IN+, IN- Does not affect laser power. Does not affect laser power. Fault state* occurs. Does not affect laser power. MON2 Fault state* occurs. Does not affect laser power. COMP A fault is detected at either the collector or the emitter of the internal bias transistor, and a fault state* occurs. If the shutdown circuitry is used, bias current is shut off. Disables bias current. Disables bias current. The APC circuit responds by increasing bias current until a fault is detected at the emitter or collector of the bias transistor, and then a fault* state occurs. MON1 MD SHDNDRV BIAS OUT+, OUTMODSET Does not affect laser power. If the shutdown circuitry is used, bias current is shut off. In this condition, laser forward voltage is 0V and no light is emitted. Does not affect laser power. Fault state* occurs. If the shutdown circuitry is used, bias current is shut off. Does not affect laser power. Does not affect laser power. Fault* state may occur. Fault state* occurs. Does not affect laser power. *A fault state asserts the FAULT pin, disables the modulator outputs, disables the bias output, and asserts the SHDNDRV pin. IC pins. See Table 3 to view the circuit response to various single-point failures. The shutdown condition is latched until reset by a toggle of TX_DISABLE or VCC. Applications Information for more information on laser safety. Fault Detection The laser driver offers redundant bias shutdown. The SHDNDRV output drives an optional external transistor. The bias and modulation drivers have separate internal disable signals. All critical nodes are monitored for safety faults, and any node voltage that differs significantly from its expected value results in a fault (Table 1). When a fault condition is detected, the laser is shut down. See Shutdown _______________________________________________________________________________________ 9 MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Programming Modulation Current VCC PORDLY STARTUP DELAY VBG SHDNDRV BIAS ENABLE TX_DISABLE MODULATOR ENABLE FAULT LATCH FAULT R Q BIAS FAULT 1 BIAS FAULT 2 TC FAULT MODSET FAULT S Resistors at the MODSET and TC pins set the amplitude of the modulation current. The resistor RMOD sets the temperature-stable portion of the modulation current, and the resistor (R TC) sets the temperatureincreasing portion of the modulation current. To determine the appropriate temperature coefficient from the slope efficiency (η) of the laser, use the following equation: η70 − η25 LASER _ TEMPCO = × 106 η 70 − 25 ° C ° C ( ) 25 [ppm / °C] For example, if a laser has a slope efficiency η25 = 0.021mW/mA, which reduces to η70 = 0.018mW/mA. Using the above equation will produce a laser tempco of -3175ppm/°C. To obtain the desired modulation current and tempco for the device, the following equations can be used to determine the required values of RMOD and RTC: Figure 7. Safety Circuitry Functional Diagram Latched Fault Output An open-collector FAULT output is provided with the MAX3996. This output is latched until the power is switched off, then on, or until TX_DISABLE is switched to HIGH and then LOW. R TC = RMOD = 0.22 Tempco / 106 × iMOD − 250Ω Tempco / 106 (R TC + 250Ω) 52 0.19 − 48 × Tempco / 106 − 250Ω Power-On Reset The MAX3996 contains an internal power-on reset delay to reject noise on VCC during power-on or hotplugging. Adding capacitance to the PORDLY pin can extend the delay. The POR comparator includes hysteresis to improve noise rejection. Design Procedure Select Laser Select a communications-grade laser with a rise time of 260ps or better for 1.25Gbps or 130ps or better for 2.5Gbps applications. To meet the MAX3996’s AC specifications, the voltage at both OUT+ and OUTmust remain above VCC - 1V at all times. Use a high-efficiency laser that requires low modulation current and generates a low voltage swing. Trimming the leads can reduce laser package inductance. Typical package leads have inductance of 25nH per inch (1nH/mm); this inductance causes a large voltage swing across the laser. A compensation filter network also can be used to reduce ringing, edge speed, and voltage swing. 10 where tempco = -laser tempco, 0 < tempco < 4000ppm/°C, and 2mA < iMOD < 30mA. Figure 8 shows a family of curves derived from these equations. The straight diagonal lines depict constant tempcos. The curved lines represent constant modulation currents. If no temperature compensation is desired, leave TC open, and the equation for iMODsimplifies considerably. The following equations were used to derive Figure 8 and the equations at the beginning of this section. 50 1.15 + 50 + RL RMOD + 250Ω 1.06 × 0.004(T − 25°C) Amps R TC + 250Ω iMOD (T) = 77 × iMOD (70°C) = iMOD (25°C) + iMOD (25°C) × (70°C − 25°C)Amps ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 1000 500ppm 1000ppm 2000ppm 1500ppm RTC (kΩ) 2500ppm 3000ppm 3500ppm Designing the Bias Filter and Output Pullup Beads 10 5mA To reduce deterministic jitter, add a ferrite bead inductor (L1) between the BIAS pin and the cathode of the laser. Select L1 to have an impedance >100Ω between f = 10MHz and f = 2GHz, and a DC resistance < 3Ω; Maxim recommends the Murata BLM11HA102SG. These inductors are also desirable for connecting the OUT+ and OUT- pins to VCC. 10mA 15mA 20mA 25mA 30mA RL = 25Ω 1 1 100 10 1000 RMOD (kΩ) Figure 8. RTC vs. RMOD for Various Conditions Programming Laser Power and Bias Fault Threshold The IC is designed to drive a common anode laser with a photodiode. A servo-control loop is formed by the internal NPN bias-driving transistor, the laser diode, the monitor diode (RSET), and the power-control amplifier (Figure 11). The voltage at MD is stabilized to 1.1V. The VCC VCC OPTIONAL SHUTDOWN CIRCUITRY VCC VCC L2* 1.8kΩ VCC VCC TX_DISABLE SHDNDRV TX_DISABLE 0.01µF L2* FAULT VCC 0.01µF 0.01µF 0.01µF IN+ MAX3996 L3* IN+ 0.01µF MAX3996 0.01µF 0.01µF OUT+ 0.01µF OUT- FAULT SHDNDRV VCC L1* OUT- 0.01µF OUT+ L1* IN- IN25Ω PORDLY PORDLY TC MODSET MON1 MON2 COMP GND TC MODSET MON1 MON2 COMP GND BIAS MD CPORDLY RTC CPORDLY RTC N.C. N.C. RMOD CCOMP *FERRITE BEAD BIAS MD RSET RSET CCOMP RMOD *FERRITE BEAD Figure 9. Large Modulation Current Figure 10. Differential Configuration ______________________________________________________________________________________ 11 MAX3996 Determine Modulator Configuration The MAX3996 can be used in several configurations. For modulation currents less than 20mA, Maxim recommends the configuration shown in the Typical Application Circuit. Outputs greater than 20mA could cause the voltage at the modulator output to be less than VCC - 1V, which might degrade laser output. For large currents, Maxim recommends the configuration in Figure 9. A differential configuration is in Figure 10. MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver VCC OPTIONAL SHUTDOWN CIRCUITRY SHDNDRV VCC MAX3996 SMOOTH START MONITOR DIODE LASER SHUTDOWN CIRCUIT L1* 1.1V BIAS IBIAS MD MON2 POWER-CONTROL AMPLIFIER RSET ID *FERRITE BEAD 11Ω BIAS DISABLE CCOMP 0.1µF MON1 COMP Figure 11. APC Loop monitor photodiode current is set by ID = VMD/RSET. Determine the desired monitor current (ID), and then select RSET = 1.1V/ID. A bias stabilizing capacitor (CCOMP) must be connected between the COMP pin and ground to obtain the desired APC loop time constant. This improves powersupply and ground noise rejection. A capacitance of 0.1µF usually is sufficient to obtain time constants of up to 35µs. The degeneration resistance between MON2 and ground determines the bias current that causes a fault and affects the APC time constant. Select RMON (the total resistance between MON2 and ground) = 400mV/(maximum bias current). A degeneration resistance of 10Ω can be obtained by grounding MON1. Increasing RMON increases the APC time constant. The discrete components for use with the common anode with photodiode configuration are: 12 RSET = 1.1V/ID CCOMP = 0.1µF (typ) L1 = ferrite bead, see the Bias Filter section RMON = 400mV/(maximum bias current) Programming POR Delay A capacitor can be added to PORDLY to increase the delay when powering up the part. The delay will be approximately: t= CPORDLY 1.4 × 10 −6 sec onds See the Typical Operating Characteristics section. Designing the Laser-Compensation Filter Network Laser package inductance causes the laser impedance to increase at high frequencies, leading to ringing, overshoot, and degradation of the laser output. A lasercompensation filter network can be used to reduce the laser impedance at high frequencies, thereby reducing output ringing and overshoot. ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver UNCOMPENSATED POWER CORRECTLY COMPENSATED product could create a situation where personal injury or death may occur. Layout Considerations The MAX3996 is a high-frequency product whose performance largely depends upon the circuit board layout. Use a multilayer circuit board with a dedicated ground plane. Use short laser-package leads placed close to the modulator outputs. Power supplies must be capacitively bypassed to the ground plane, with surface-mount capacitors placed near the power-supply pins. The dominant pole of the APC circuit normally is at COMP. To prevent a second pole in the APC that can lead to oscillations, ensure that parasitic capacitance at MD is minimized (10pF). OVERCOMPENSATED TIME Figure 12. Laser Compensation Using External Shutdown To achieve single-point fault tolerance, Maxim recommends an external shutdown transistor (Figure 11). In the event of a fault, SHDNDRV asserts high, placing the shutdown transistor in cutoff mode and thereby shutting off the bias current. Applications Information Laser Safety and IEC825 The International Electrotechnical Commission (IEC) determines standards for hazardous light emissions from fiber optic transmitters. IEC 825 defines the maximum light output for various hazard levels. The MAX3996 provides features that facilitate compliance with IEC825. A common safety precaution is singlepoint fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. When this laser driver is used, as shown in the Typical Application Circuit, the circuits respond to faults as listed in Table 3. Using this laser driver alone does not ensure that a transmitter design is compliant with IEC825. The entire transmitter circuit and component selections must be considered. Customers must determine the level of fault tolerance required by their applications, 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 Common Questions Laser output is ringing or contains overshoot. Inductive laser packaging often causes this. Try reducing the length of the laser leads. Modify the filter components to reduce the driver’s output edge speed (see the Design Procedure section). Extreme ringing can be caused by low voltage at the OUT± pins. This might indicate that pullup beads or a lower modulation current are needed. Low-frequency oscillation on the laser output. This is more prevalent at low temperatures. The APC might be oscillating. Try increasing the value of CCOMP or add additional degeneration by placing some resistance from MON1 to GND. Ensure that the parasitic capacitance at the MD node is kept very small (<10pF). The APC is not needed. Connect BIAS to VCC, leave MD open, and connect MON2 and COMP to ground. The modulator is not needed. Leave TC and MODSET open. Connect IN+ to V CC , IN- to ground through 750Ω, and leave OUT+ and OUT- open. Interface Models Figures 13–17 show typical models for the inputs and outputs of the MAX3996, including package parasitics. MAX3996 4kΩ FAULT NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT Figure 13. FAULT Output ______________________________________________________________________________________ 13 MAX3996 The compensation components (RF and CF) are most easily determined by experimentation. For interfacing with edge-emitting lasers, refer to application note HFAN-2.0, Interfacing Maxim Laser Drivers with Laser Diodes. Begin with RF = 50Ω and CF = 2pF. Increase CF until the desired transmitter response is obtained (Figure 12). MAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver VCC MAX3996 10kΩ 550Ω 60Ω SHDNDRV Figure 14. SHDNDRV Output VCC VCC PACKAGE PACKAGE 50Ω 1.1nH 50Ω OUT- OUT+ 1.1nH 0.15pF 0.15pF 1pF 1pF MAX3996 Figure 15. Modulator Outputs 14 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 VCC VCC MAX3996 PACKAGE MAX3996 BIAS VCC 1.1nH IN+ VCC 0.15pF 1pF MON2 1.1nH VCC 11Ω VCC IN- MON1 0.15pF 1pF Figure 16. Data Inputs Figure 17. BIAS Output VCC OUT- OUT+ VCC TOP VIEW MODSET Pin Configuration 20 19 18 17 16 TC 1 15 BIAS FAULT 2 14 SHDNDRV GND 3 13 MD TX_DISABLE 4 12 COMP PORDLY 5 11 MON2 9 10 MON1 IN+ 8 GND 7 IN- 6 VCC MAX3996 Chip Information TRANSISTOR COUNT: 1061 PROCESS: SILICON BIPOLAR QFN* *EXPOSED PAD IS CONNECTED TO GND ______________________________________________________________________________________ 15 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 12,16,20, 24L QFN.EPS MAX3996 Package Information 16 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX3996 Package Information (continued)