Why is my PLD laser diode driver current limit disabling the output even when the output is below the limit current level?


The PLD Series laser diode drivers are capable of modulation, so to protect the laser from over-current situations we built hysteresis into the current limit circuit. This hysteresis may cause the limit current to trip at a slightly lower current than expected. If you are operating the PLD close to the current limit setpoint this hysteresis may affect operation.

The control circuit hysteresis is 0.2 V, and you can adjust the current limit setting by adding an offset using the following method:

Hysteresis Offset = HO = (V+5V – VIMON) / 25.9

Example: PLD5000 with a current limit of 2.5 A operating from 5 V at Pin 10.
Use the above equation to calculate the hysteresis offset.

HO = (5 – 1.25) / 25.9 = 0.14 V
Then add the hysteresis offset to the limit voltage calculated from the transfer function. The limit current monitor transfer function for the PLD5000 is 2 A / V, as indicated in the datasheet.
VLIM = ILIM / (2 A / V) + HO = 2.5 / 2 V + 0.14 V = 1.39 V