Hi Claire, and thanks for the reply,
OK, using the PG pin to drain caps is definitely not an option
My circuit is powering a LED lamp (ca. 300mA @12V) controlled by a NE555 chip. The chip is there to provide low voltage disconnect for the battery.
A normal resistive load would of course drain the caps in a flash, but the threshold voltage of the LEDs prevent the caps from further discharge once their voltage reaches about 8V. The trigger of the NE555 doesn't reset until the voltage falls below around 4V. So if I try to restart the circuit before that, it will not respond.
I'm really new to MOSFETS or transistors in general, but I have done something similar to what you suggest (I think).
A J175 P-channel JFET is connected with source/drain between regulator ouptut and ground. The gate is connected to the 18V regulator input, providing about 6V of bias (VGS) wit a fresh battery. Once the 18 supply is cut off, the JFET will open and drain the caps.
The solution works, but only just. As the battery discharges, this bias shrinks to just a couple of volts and the JFET starts to "leak" considerably.
I'm also worried that the JFET may cause unexpected behaviour during start-up as it will conduct intermittently until the input voltage stabilizes to approximately 18V (there is a considerable amount of switch bounce at startup).
So, I was looking for a more elegant solution. But with my limited understanding of MOSFETs, I cannot see a way to use the PG pin to properly bias a MOSFET when the VDS voltage varies all the way from 12V down to 0V during the discharge process...