I am designing a circuit board for a small mobile robot and there is a problem with the power supply. The circuit is powered by a 1-cell Li-ion battery (VBAT) and includes a charger (MCP73831/2), a pMOS high side switch (SI2369DS), a booster (TPS61232) and a simple dual Schottky voltage selector because it can also be powered through USB (5VUSB). The relevant part of the schematics with all component part numbers is below.
The problem The pMOS high side switch does not close properly. When open, everything works as expected (green voltages). When closed (red voltages), the drain voltage is approx. 0.3V and the whole circuit is draining about 80mA. In all cases, the charger is off (jumper JP1 not present).
Things that I tried so far, without success:
- Changed the pMOS to SI2399DS which has a lower Vgs, same behaviour.
- Held a 330R resistor in parallel to the R14 pull-up, same behaviour.
- Tried with both, a battery, and lab power supply, same behaviour.
- Tried on 5 boards, behaviour is the same on all of them, so probably not a soldering problem.
- Measured VON on oscilloscope and it is steady.
- The pMOS footprint was checked and it is fine.
What could be the source of the problem?