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MAX3996 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3996 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 17 page 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 power- control 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 recom- mends CCOMP = 0.1µF. 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 cur- rent into a 25 Ω 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 resis- tors 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 temperature- increasing 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 mod- ulation current can be obtained as described in the Design Procedure section. Table 2 is the RTC and RMOD selection table. Safety Circuitry The safety circuitry contains a disable input, a fault latch, and fault detectors (Figure 7). This circuitry moni- tors the operation of the laser driver and forces a shut- down if a single-point fault is detected. A single-point fault can be a short to VCC or GND, or between any two 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 8 _______________________________________________________________________________________ PIN FAULT CONDITION MON2 VMON2 > 400mV BIAS VBIAS < 400mV TC, MODSET VMODSET or VTC < 200mV Table 1. Typical Fault Conditions MAX3996 100 Ω IN+ IN- INPUT BUFFER 50 Ω 50 Ω VCC OUT+ OUT- 1.2V REFERENCE 0ppm/ °C MODSET FAULT 200mV MODSET RMOD 1.2V REFERENCE 4000ppm/ °C TC FAULT 200mV TC RTC Σ CURRENT AMPLIFIER 96X ENABLE MODULATION CURRENT GENERATOR CURRENT SWITCH Figure 5. Modulation Circuitry 0.6 0.8 0.7 1.0 0.9 1.2 1.1 1.3 020 30 40 10 50 60 70 80 90 100 110 JUNCTION TEMPERATURE ( °C) RTC ≥ 1.9kΩ RMOD = OPEN TEMPCO = 4000ppm/ °C RTC = OPEN TEMPCO = 50ppm/ °C Figure 6. Modulation Current vs. Temperature for Maximum and Minimum Temperature Coefficient |
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