Continuous-wave laser diodes in precision-instrument applications require constant-current sources to drive them. Proper design of such a driver must involve careful tackling of robustness, stability, noise, and other issues and is consequently costly and complicated (Reference 1). Figure 1 shows a compact, cathode-grounded laser-diode driver with protection against ESD (electrostatic-discharge) damage, start-up spikes, overshoot, and possible fluctuation arising from external optical feedback. An op amp, IC4, with an enable input drives PMOS FET Q1 and controls the output current. RS sets the current to the rated value for a 35-mW laser diode from Opnext. To prevent output from Q1 during start-up, comparator IC5A keeps IC4 off, and a 10-kΩ pullup resistor keeps Q1 off by linking Q1's gate to the supply of IC4 until the terminal supply, VB, reaches the designed value, approximately 6.5 V, and opens Q1 via IC4.
|Figure 1.||This laser-diode-drive circuit provides a constant current and protection against input
overvoltages and start-up transients.
The key point for protection against ESD damage and overshoot lies in the use of Q2, a depletion-mode NMOS FET. With power off, Q2 conducts, shunting any harmful ESD to ground. With power on, comparator IC5B outputs a negative voltage far below the gate-to-source off-state voltage. Hence, Q2 is off and has little effect upon the drive current unless the operating voltage at the laser's anode exceeds the maximum rating of 2.8 V in the figure. In this case, the operating voltage triggers IC5B to output high and thus turns on Q2, shunting the drive current to ground, as well. The circuit now introduces significant hysteresis to latch off the state of emergency. Considering the low on-resistance of Q2, this circuit provides better protection than the common method of relying on a paralleled zener diode for overshoot suppression.
Despite employing a split supply, this design requires no particular supply sequencing. You must cut off Q2 only at the beginning of start-up, so it would be better to turn on the -9 V external supply before enabling the driver. Despite the availability of substitutes for some ICs in this design, selection of the proper devices may be troublesome. For example, you can with slight modifications replace the Texas Instruments TLC070 with a Linear Technology LT1637; the two devices are not pin-compatible. However, the TLC070's superior ac performance, especially higher CMRR (common-mode-rejection ratio) over a wider bandwidth permits more effective protection against fluctuations in the operating voltage because of external optical feedback under some desired or undesired circumstances.
- Williams Jim, "Current sources for fiber-optic lasers: a compendium of pleasant current events," EDN, Aug 22, 2002, pg. 69.