DS1863; DS1865; DS1875: How to Interface to MAX3736 Compact Laser Driver
The ControllersThe DS1863, DS1865, and DS1875 controllers provide two outputs for controlling the laser driver: BIAS and MOD. BIAS is a 13-bit current DAC with a full-scale range of 1.2mA. The MOD output is an 8-bit voltage DAC with a full-scale range of 1.25V.
The Compact Laser DriverThe MAX3736 laser driver has two inputs for laser control: BIASSET and MODSET. Both inputs should be connected to a current-sink controller with 1.2mA full-scale range in order to utilize the laser driver's full output range.
Interface Between Controller and Laser DriverThe controller's BIAS output provides the correct range for the BIASSET input of the laser driver. The MOD output is voltage mode, so must be adapted to interface to the laser driver (Figure 1).
Voltage-to-Current ConversionAn operational amplifier and NPN transistor are used to create a 1.2mA full-scale output from the MOD voltage-DAC output. The MOD output ranges from 0V to 1.25V. The NPN needs to sink current from the MODSET pin, which is maintained at 1.2V. R1 is chosen to keep the voltage at the NPN's emitter to less than 0.3V, and this ensures that the NPN does not saturate at full-scale current. The maximum full-scale current sunk by the NPN is set by the equation: IMAX = R2 × 1.25V/(R1 × (R3 + R2)).
Figure 1. Controller-to-laser-driver interface.