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| 19-2194; Rev 1; 2/02 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 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 0C to +70C. o 9psP-P Deterministic Jitter o 20-Pin QFN 4mm 4mm Package o 3.0V to 5.5V Supply Voltage o Automatic Power Control o Integrated Safety Circuits o 30mA Laser Modulation Current o Temperature Compensation of Modulation Current o Compliant with SFF and SFP MSA Features MAX3996 Ordering Information PART MAX3996CGP TEMP RANGE 0C to +70C PIN-PACKAGE 20 QFN Typical Application Circuit VCC OPTIONAL SHUTDOWN CIRCUITRY VCC 1.8k Applications Fibre Channel Optical Transmitters VCSEL Transmitters Gigabit Ethernet Optical Transmitters ATM LAN Optical Transmitters 10 Gigabit Ethernet WWDM 0.01F IN+ 0.01F INTX_DISABLE VCC 0.01F SHDNDRV 0.01F FAULT OUT0.01F MAX3996 OUT+ L1* 25 BIAS Pin Configuration appears at end of data sheet. PORDLY MD TC CPORDLY RTC RMOD N.C. CCOMP RSET MODSET MON1 MON2 COMP GND *FERRITE BEAD ________________________________________________________________ Maxim Integrated Products 1 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. 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 ABSOLUTE MAXIMUM RATINGS 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 Current into FAULT, SHDNDRV ..........................-1mA to +25mA Current into OUT+, OUT- ....................................................60mA Current into BIAS ..............................................................120mA Continuous Power Dissipation (TA = +70C) 20-Pin QFN (derate 20mW/C)...................................1600mW Operating Ambient Temperature Range .............-40C to +85C Operating Junction Temperature Range. ..........-40C to +150C Storage Temperature Range.... .........................-55C to +150C 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 = 0C to +70C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA = +25C.) (Figure 1) PARAMETER Supply Current Data Input Voltage Swing TX_DISABLE Input Current TX_DISABLE Input High Voltage TX_DISABLE Input Low Voltage FAULT Output High Voltage FAULT Output Low Voltage BIAS GENERATOR Minimum Bias Current Maximum Bias Current MD Quiescent Voltage Monitor Resistance MD Input Current BIAS Current During Fault APC Time Constant POWER-ON RESET (POR) POR Threshold POR Delay POR Hysteresis SHUTDOWN ISHDNDRV = 10A, FAULT = high Voltage at SHDNDRV LASER MODULATOR Data Rate < 3.2 Gbps ISHDNDRV = 1mA, FAULT = low ISHDNDRV = 15mA, FAULT = low 0 VCC - 0.4 VCC - 2.4 VCC - 1.2 V tPORDLY Measured at VCC PORDLY = open (Note 3) CPORDLY = 0.001F (Note 3) 2.65 30 1.7 2.7 55 2.4 20 3.0 V s ms mV IBIAS_OFF CCOMP = 0.1F 35 IBIAS IBIAS VMD RMON Current into BIAS pin Current into BIAS pin APC loop is closed FAULT = high TX_DISABLE = high (Figure 4) FAULT = low, TX_DISABLE = low 9.3 -3 60 1.04 1.12 VCC - 0.73 VCC - 0.73 11 0.8 12.7 3 10 A A s V 1 mA mA VIH VIL VOH VOL IOH = -100A, 4.7k < RFAULT < 10k IOL = 1mA 2.4 0.4 SYMBOL ICC (Figure1) (Note 1) VID CONDITIONS VCC = 3.3V, IMOD = 15mA VCC = 5.5V, IMOD = 30mA, RMODSET = 2.37k Total differential signal (Figure 2) 0 < VPIN < VCC 200 -100 2.0 0.8 MIN TYP 47 52 75 2200 100 mA MVP-P A V V V V MAX UNITS 2 _______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver ELECTRICAL CHARACTERISTICS (continued) (VCC = 3.0V to 5.5V, TA = 0C to +70C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA = +25C.) (Figure 1) PARAMETER Minimum Modulation Current Maximum Modulation Current Accuracy of Modulation Current (Part-to-Part Variation) Edge Transition Time tr, tf SYMBOL iMOD iMOD RL 25 RMODSET = 2.37k (iMOD 30mAp-p into 25) iMOD = 5mA into 25, 20% to 80% (Note 3) iMOD = 10mA into 25, 20% to 80% (Note 3) iMOD = 30mA into 25, 20% to 80% (Note 3) iMOD = 5mA into 25 (Notes 2, 3) Deterministic Jitter Random Jitter Modulation Current During Fault Modulation Current Tempco Input Resistance Output Resistance Input Common-Mode Voltage SAFETY FEATURES (See Typical Operating Characteristics) MODSET and TC Pin Fault Threshold BIAS Pin Fault Threshold A fault will be triggered if VBIAS is less than this voltage A fault will be triggered if VMON2 exceeds this voltage t_off t_on Time from rising edge of TX_DISABLE to IBIAS = IBIAS_OFF and iMOD = iMOD_OFF (Note 3) Time from falling edge of TX_DISABLE to IBIAS and iMOD at 95% of steady state (Note 3) 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) Time from fault to FAULT = high, CFAULT < 20pF, RFAULT = 4.7k (Note 3) Time TX_DISABLE must be held high to reset FAULT (Note 3) 200 300 400 mV mV RIN ROUT iMOD_OFF Tempco = MAX, RMOD = open Tempco = MIN, RTC = open Differential Single ended; outputs to VCC 85 42 50 VCC - 0.3 iMOD = 10mA into 25 (Notes 2, 3) iMOD = 30mA into 25 (Notes 2, 3) (Note 3) 30 -10 54 55 65 17 14 9 2 15 4000 50 115 58 40 +10 100 125 130 35 22 20 8 200 psRMS AP-P ppm/C V psP-P ps CONDITIONS MIN TYP MAX 2 UNITS mAP-P mAP-P % MAX3996 Excessive Bias Current Fault TX Disable Time TX Disable Negate Time 400 0.06 37 440 5 500 mV s s Reset Initialization Time t_init 23 200 ms Fault Assert Time TX_DISABLE Reset t_fault t_reset 14 0.01 50 1 s s 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 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Typical Operating Characteristics (VCC = 3.3V, TA = +25C, unless otherwise noted.) ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 2.5Gbps) MAX3996 toc01 ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps) 25 LOAD MAX3996 toc02 OPTICAL EYE DIAGRAM (iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS, 2.5Gbps, 1870MHz FILTER) MAX3996 toc03 25 LOAD 120mV/ div 120mV/ div 64ps/div 52ps/div 57ps/div OPTICAL EYE DIAGRAM (iMOD = 15mA, 1310nm LASER, 27 - 1 PRBS, 2.5Gbps, 1870MHz FILTER) MAX3996 toc04 TRANSITION TIME vs. MODULATION CURRENT MAX3996 toc05 DETERMINISTIC JITTER vs. MODULATION CURRENT MAX3996 toc06 80 70 TRANSITION TIME (ps) 60 50 40 30 20 FALL TIME RISE TIME 30 DETERMINISTIC JITTER (psP-P) 25 20 15 TOTAL DJ 10 PWD 5 0 57ps/div 5 10 15 20 iMOD (mA) 25 30 35 5 10 15 20 iMOD (mA) 25 30 35 SUPPLY CURRENT vs. TEMPERATURE (iMOD = 15mA) 65 SUPPLY CURRENT (mA) 60 55 50 45 40 35 30 0 15 30 45 60 75 AMBIENT TEMPERATURE (C) 10 10p POR DELAY (s) 10m EXCLUDES IBIAS, iMOD 25 LOAD MAX3996 toc07 POR DELAY vs. CPORDLY MAX3996 toc08 70 1 100m 1m 100 100p 1n CPORDLY (F) 10n 100n 4 _______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Typical Operating Characteristics (continued) (VCC = 3.3V, TA = +25C, unless otherwise noted.) HOT PLUG WITH TX_DISABLE LOW MAX3996 toc09 MAX3996 STARTUP WITH SLOW RAMPING SUPPLY MAX3996 toc10 TRANSMITTER ENABLE VCC 3.3V 3.3V MAX3996 toc11 3.3V 0V VCC FAULT TX_DISABLE LOW LOW t_init = 23mS VCC FAULT TX_DISABLE 0V LOW LOW FAULT TX_DISABLE LOW HIGH t_on = 37s LOW LASER OUPUT LASER OUPUT LASER OUPUT 10.0ms/div 10.0ms/div 20.0s/div TRANSMITTER DISABLE MAX3996 toc12 RESPONSE TO FAULT MAX3996 toc13 FAULT RECOVERY TIME EXTERNAL FAULT REMOVED VTC FAULT OFF MAX3996 toc14 VCC 3.3V t_off = 60ns VMON2 EXTERNALLY FORCED FAULT ON t_fault = 14s FAULT LOW HIGH LOW IBIAS HIGH FAULT ELECTRICAL OUPUT 20.0ns/div 10.0s/div TX_DISABLE LOW LASER OUPUT 10.0s/div TX_DISABLE LASER OUPUT FREQUENT ASSERTION OF TX_DISABLE EXTERNALLY FORCED FAULT OV MAX3996 toc15 VTC FAULT TX_DISABLE LASER OUPUT 1.00ms/div _______________________________________________________________________________________ 5 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Pin Description PIN 1 2 3, 9 4 5 6, 16, 19 7 8 10 11 12 13 14 15 17 18 20 EP NAME TC FAULT GND TX_DISABLE PORDLY VCC IN+ INMON1 MON2 COMP MD SHDNDRV BIAS OUT+ OUTMODSET Exposed Pad 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. 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 Noninverting Data Input Inverting Data Input Attaches to the emitter of the bias driving transistor through a 10 resistor. See the Design Procedure section. This pin attaches to the emitter of the bias driving transistor. See the Design Procedure section. A capacitor connected from this pin to ground sets the dominant pole of the APC loop. See the Design Procedure section. Monitor Diode Connection. MD is used for automatic power control. Shutdown Driver Output. Provides a redundant laser shutdown. Laser Bias Current Output Positive Modulation-Current Output. Current flows from this pin when input data is high. Negative Modulation-Current Output. Current flows to this pin when input data is high. 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. 6 _______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 3.0V TO 5.5V ICC iOUT VCC VCC VOLTS VIN+ VIN- SINGLE-ENDED SIGNAL 100mVP-P MIN 1100mVP-P MAX DIFFERENTIAL SIGNAL MAX3996 ROUT ROUT OUT- FERRITE BEAD* VID = VIN+ - VINOUT+ iMOD 0.01F 0.01F 200mVP-P MIN 2200mVP-P MAX 0.01F IN+ VID 0.01F INMODULATION CURRENT GENERATOR TC *MURATA BLM11HA102SG MODSET RMOD RIN CURRENT iMOD 25 25 TIME Figure 2. Required Input Signal and Modulation-Current Polarity Bias Generator 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. 400mV BIAS FAULT 1 BIAS 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 SHDNDRV BIAS SMOOTH START 1.1V MD POWER-CONTROL AMPLIFIER BIAS FAULT 2 PORDLY TX_DISABLE POR CIRCUIT SAFETY CIRCUITRY BIAS ENABLE BIAS GENERATOR WITH SMOOTH START VCC MD COMP MON1 MON2 MON2 RMON(11) 400mV COMP MON1 MAX3996 50 50 OUTOUT+ LASER MODULATION MAX3996 BIAS DISABLE IN+ INPUT BUFFER Figure 4. Bias Circuitry 100 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 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Table 1. Typical Fault Conditions PIN MON2 BIAS TC, MODSET FAULT CONDITION VMON2 > 400mV VBIAS < 400mV VMODSET or VTC < 200mV IN+ INPUT BUFFER CURRENT SWITCH VCC 50 50 OUT+ OUT- MAX3996 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 = 0.1F. IN- 100 ENABLE CURRENT AMPLIFIER 96X 1.2V REFERENCE 4000ppm/C MODULATION CURRENT GENERATOR 1.2V REFERENCE 0ppm/C 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 25 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. TC RTC 200mV TC FAULT 200mV MODSET FAULT MODSET RMOD Figure 5. Modulation Circuitry 1.3 1.2 iMOD/(iMOD AT +52C) 1.1 1.0 0.9 0.8 0.7 0.6 0 10 20 30 40 50 60 70 80 90 100 110 JUNCTION TEMPERATURE (C) RTC = OPEN TEMPCO = 50ppm/C RTC 1.9k RMOD = OPEN TEMPCO = 4000ppm/C 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 8 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 TEMPCO (ppm/C) 3500 3000 2500 2000 1500 1000 500 iMOD = 30mA RMOD (k) 17.1 8.04 5.20 3.81 2.98 2.44 2.05 RTC (k) 1.85 2.19 2.68 3.42 4.64 7.08 14.4 34.4 16.3 10.6 7.86 6.21 5.12 4.34 iMOD = 15mA RMOD (k) RTC (k) 3.94 4.64 5.62 7.08 9.53 14.4 29.1 104 49.5 32.4 24.1 19.1 15.9 13.5 iMOD = 5mA RMOD (k) RTC (k) 12.3 14.4 17.4 21.8 29.1 43.8 87.8 Table 3. Circuit Responses to Various Single-Point Faults PIN NAME TC FAULT TX_DISABLE PORDLY IN+, INMON1 MON2 COMP CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC Does not affect laser power. Does not affect laser power. Modulation and bias current are disabled. Does not affect laser power. Does not affect laser power. Fault state* occurs. Fault state* occurs. 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. 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. Does not affect laser power. Fault* state may occur. CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Fault state* occurs. Does not affect laser power. Normal condition for circuit operation. Modulation and bias current are disabled. Does not affect laser power. Does not affect laser power. Does not affect laser power. 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. Does not affect laser power. Fault state* occurs. If the shutdown circuitry is used, bias current is shut off. Does not affect laser power. Fault state* occurs. MD SHDNDRV BIAS OUT+, OUTMODSET *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. Shutdown 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. Fault Detection 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 _______________________________________________________________________________________ 9 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 VCC STARTUP DELAY VBG Programming Modulation Current PORDLY SHDNDRV BIAS ENABLE TX_DISABLE MODULATOR ENABLE 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 () of the laser, use the following equation: 70 - 25 LASER _ TEMPCO = x 106 25 (70C - 25C) [ppm / C] For example, if a laser has a slope efficiency 25 = 0.021mW/mA, which reduces to 70 = 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: R TC = RMOD = 0.22 Tempco / 106 x iMOD - FAULT LATCH RQ BIAS FAULT 1 BIAS FAULT 2 TC FAULT MODSET FAULT FAULT S 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. 250 250 Tempco / 106 (R TC + 250) 52 0.19 - 48 x Tempco / 106 - 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. 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. iMOD (T) = 77 x 50 1.15 + 50 + RL RMOD + 250 1.06 x 0.004(T - 25C) Amps R TC + 250 iMOD (70C) = iMOD (25C) + iMOD (25C) x (70C - 25C)Amps 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 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Determine Modulator Configuration 1000 MAX3996 500ppm 1000ppm 1500ppm 2000ppm 2500ppm 3000ppm 3500ppm 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. RTC (k) 10 Designing the Bias Filter and Output Pullup Beads 5mA 10mA 15mA 20mA 25mA 30mA 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 >100 between f = 10MHz and f = 2GHz, and a DC resistance < 3; Maxim recommends the Murata BLM11HA102SG. These inductors are also desirable for connecting the OUT+ and OUT- pins to VCC. RL = 25 1 1 10 RMOD (k) 100 1000 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 Figure 8. RTC vs. RMOD for Various Conditions VCC OPTIONAL SHUTDOWN CIRCUITRY VCC 1.8k VCC TX_DISABLE VCC SHDNDRV 0.01F 0.01F FAULT 0.01F IN+ 0.01F IN25 L3* 0.01F L1* OUTVCC VCC VCC L2* TX_DISABLE VCC SHDNDRV OUT+ FAULT 0.01F IN+ 0.01F 0.01F L2* MAX3996 OUTL1* MAX3996 0.01F IN- OUT+ PORDLY PORDLY TC CPORDLY RTC N.C. RMOD CCOMP RSET MODSET MON1 MON2 COMP GND BIAS MD TC MODSET MON1 MON2 COMP GND CPORDLY RTC RMOD N.C. BIAS MD RSET CCOMP *FERRITE BEAD *FERRITE BEAD Figure 9. Large Modulation Current Figure 10. Differential Configuration 11 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 OPTIONAL SHUTDOWN CIRCUITRY SHDNDRV VCC VCC MONITOR DIODE SMOOTH START 1.1V MAX3996 SHUTDOWN CIRCUIT LASER L1* BIAS IBIAS MD POWER-CONTROL AMPLIFIER MON2 11 BIAS DISABLE MON1 RSET ID *FERRITE BEAD CCOMP 0.1F 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 0.1F usually is sufficient to obtain time constants of up to 35s. 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 10 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: RSET = 1.1V/ID CCOMP = 0.1F (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 x 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. 12 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 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 = 50 and CF = 2pF. Increase CF until the desired transmitter response is obtained (Figure 12). UNCOMPENSATED product could create a situation where personal injury or death may occur. MAX3996 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). CORRECTLY COMPENSATED POWER OVERCOMPENSATED 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 750, and leave OUT+ and OUT- open. 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 Interface Models Figures 13-17 show typical models for the inputs and outputs of the MAX3996, including package parasitics. MAX3996 4k FAULT NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT Figure 13. FAULT Output ______________________________________________________________________________________ 13 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 VCC MAX3996 10k 550 60 SHDNDRV Figure 14. SHDNDRV Output VCC PACKAGE VCC PACKAGE 50 1.1nH OUT- 50 OUT+ 1.1nH 0.15pF 1pF 1pF 0.15pF MAX3996 Figure 15. Modulator Outputs 14 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 VCC VCC PACKAGE VCC 1.1nH IN+ VCC 0.15pF 1pF MON2 VCC 1.1nH IN11 VCC MAX3996 MAX3996 BIAS MON1 1pF 0.15pF Figure 16. Data Inputs Figure 17. BIAS Output Pin Configuration VCC OUT+ OUTVCC Chip Information TRANSISTOR COUNT: 1061 PROCESS: SILICON BIPOLAR TOP VIEW 20 MODSET 19 18 17 16 TC FAULT GND TX_DISABLE PORDLY 1 2 3 4 5 15 14 BIAS SHDNDRV MD COMP MON2 MAX3996 13 12 11 6 VCC 7 IN+ 8 IN- 9 GND 10 MON1 QFN* *EXPOSED PAD IS CONNECTED TO GND ______________________________________________________________________________________ 15 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver MAX3996 Package Information 12,16,20, 24L QFN.EPS 16 ______________________________________________________________________________________ 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver Package Information (continued) MAX3996 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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. |
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