Self-biased indicator lamp driver circuits for high reliability applications

In some systems, it is necessary to turn on a low side driver when the load is properly connected to a power supply, even if the normal gate drive power supply is off or disconnected. This necessitates deriving the gate supply voltage for the output device from its own drain voltage. A current mirror method of this drain powered gate drive is explained including theory and simulation. Several additional features possible with the gate drive are also explained. Finally, results are compared for two recent BiCMOS/DMOS (BCD) processes.