When the system voltage falls below the battery voltage, the BATFET turns on and the BATFET gate is regulated so that the minimum BATFET V_DS stays at 30 mV when the current is low. This prevents oscillation from entering and exiting the supplement mode.

As the discharge current increases, the BATFET gate is regulated with a higher voltage to reduce R_DSON until the BATFET is in full conduction. At this point onwards, the BATFET V_DS linearly increases with discharge current. The BATFET turns off to exit supplement mode when the battery is below battery depletion threshold.