circuit for floating load
Constant-output-power drive circuit needs to be operated with an absolute current limit to prevent any thermal runaway problem caused by uncontrolled increase in drive current. Laser diode modules with an integral photodiode (Fig. 3) facilitate constant-output-power operation, though noise intrinsic to the integral photodiode manifests itself in the form of noisy and unstable output in the case of constant-output-power mode of operation. The device usually has three terminals, including either anode or cathode of laser diode, either anode or cathode of photodiode and a common terminal obtained by connecting the remaining terminals of laser diode and photodiode. In some cases, all four terminals, two of laser diode and two of photodiode, are brought out.
Laser-diode drive circuit: constant-current mode
Fig. 4 shows the basic constant-current source circuit for grounded-laser-diode configuration. Laser-diode current is sensed differentially by measuring voltage across RSENSE wired in series with laser diode. Laser diode current in this case can be computed from IO = VIN×R4/(R3×RSENSE), where R1 = R3 and R2 = R4. C1 is the compensation capacitor connected for stable operation. VIN may be derived from a band-gap reference. In case the control voltage VIN is of negative polarity, it is connected to R1 and R3 and is grounded instead. A digitally-controlled current source may use a voltage output digital-to-analogue converter to generate VIN.
Similar circuit for a floating load is shown in Fig. 5. Laser-diode current in this case can be computed from IO = VIN×R2/(R1×RSENSE).
diode driver for OEM applications
The voltage-controlled constant- current sources discussed so far provide constant-current drive to the laser diode, provided the DC supply voltage, the voltage at the emitter terminal and the resistance in the emitter lead were all constant. It would be a reasonably good assumption if the source voltage were derived from a precision band-gap reference and the resistors used had stability specifications equal to or better than the desired level of current stability. But when it comes to achieving a higher level of current stability, say ±10ppm or better, a feedback loop that in situ samples the diode current and applies a correction in the case of drift in drive current becomes essential. A negative feedback loop of this kind would reduce the drift or error in the drive current by a factor that equals the loop gain.
One such circuit is shown in Fig. 6. The basic circuit topology is similar to the one used in driver circuits described in the previous pages. The only change is inclusion of a junction FET (JFET) connected in series with the sense resistor and wired as a voltage variable resistor. A small variation in drive current is compensated by an appropriate variation in the drain-source resistance of the JFET.
Initially, at the nominal value of drive current, circuit parameters are so adjusted as to ensure that JFET with the feedback loop closed gets a negative gate voltage to keep it nearly in the middle of its VVR characteristics. This is done to fully exploit the voltage-dependent resistance range. Precision constant-current laser diode drivers are commercially available today from a host of manufacturers for original equipment manufacturer (OEM) applications. A representative photograph is shown in Fig. 7.
Protection features, such as slow start, immunity to fast transients and overcurrent limit, as outlined earlier, are essential features for every laser diode driver circuit to have. Fig. 8 shows a constant-current laser diode drive circuit with modulation capability and in-built above-mentioned protection features.
Let us first talk about protection features provided by the drive circuit. The Schmitt comparator at the input provides a delay of the order of a few tens of milliseconds after the switch-on to offer protection against switch-on transients. The time delay is decided by the R1C1 time constant. R2 and C2 provide the desired slow or soft start and decay during switch on and switch off. R2C2 time constant is of the order of one to two seconds. R11 and C3 provide additional protection against transients; D1 protects the laser diode against reverse voltages.