Microcontroller open drain pin sustaining its power source mosfet gate negative charge

Thread Starter


Joined May 25, 2009
Hi guys,

For a project I'm working on, I wonder if this circuit would work.
The switch pulls low the gate of an PNP mosfet to allow current through to power the microcontroller. Then an initialization routine of the microcontroller would set an open drain pin low supposedly to keep the aforesaid mosfet gate low when the switch is released.
But, I have doubts, would saturation gradually loose. It's hard to explain I'm thinking of something perpetual here.
Sustaining PNP power source mosfet with microcontroller open drain pin.png


Joined Jan 8, 2017
The term PNP (And NPN) are use with transistors. Mosfets are either N channel or P channel. For your application you need a P channel device. The FQU13N10LTU is an N channel device so it would not work. I see two problems with your schematic. The first is that the push button would be shorting tho PIC output to ground if it was at a high level during the PIC was powering up. The other problem is that with no power on the PIC it may be partly pulling down the gate of the P channel mosfet which may be enough to turn the mosfet on. I would suggest using an NPN transistor or small N channel mosfet as an inverter between the PIC output and the gate of the P channel mosfet. That way you would need to set the PIC output high to hold the power on. As in the off state the PIC pin could not go high you would not have that problem.