Michael Fisch
EDN
Laser diodes are sensitive to ESD, rapid turn-on currents, and overvoltage conditions. To address those problems, the simple laser-diode controller in Figure 1 has several functions. The first part of the circuit comprises an 8.2 V zener diode, D1 that forms the heart of a constant-voltage source for the laser diode. Next, IC1A, half of a dual FET-input op amp, forms an inverting integrator to slow the turn-on time. To turn on the laser diode, IC1B, the other half of the op-amp IC, triggers the base of Q2. This transistor forms a constant-current source for the laser diode. You can monitor the laser-diode supply voltage and the sense-diode current and voltage. You use these parameters as inputs to the differential amplifier, IC2A, the first half of another dual FET-input op amp. When an overvoltage condition occurs, the difference amplifier detects the condition, and its output drives IC2B, configured as an open-loop comparator. You set the threshold by using the potentiometer, R1. Zener diode D2 provides a constant-voltage source for that threshold setting. When the voltage reaches the threshold, the output triggers the base of Q1, which instantly shuts down IC1B, which in turn shuts down the laser diode.
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| Figure 1. | Constant voltage and current and slow turn-on time are the keys to laser diodes’ survival. | |
