I have a question when I am working on MP2639A 2-cell LIPO charger (+8V4) from USB +5V. The recommend schematic uses P-MOSFET to protect reverse current from Load back to a USB plug.
The signal /ACOK is an open-drain output signal. It will pull-low when the USB-power is OK.
From my understanding to turn-on P-MOSFET Vgs > Vgs-threshold. However, when we use Vg and Vs have the same voltage ( +5V ) How could the USB power turn on MOSFET? even the Vgs-threshold is a negative value. If the P-MOSFET can turn on, the current that can flow from Drain to the Source is the minimum current ( near 0 )
I am not sure that I understand correctly about the P-MOSFET using in this situation