When the system voltage falls 180 mV (VBAT > VSYSMin) or 45 mV (VBAT < VSYSMin) below the battery voltage, the BATFET turns on and the BATFET gate is regulated the gate drive of BATFET so that the minimum BATFET VDS 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 RDSON until the BATFET is in full conduction. At this point onwards, the BATFET VDS linearly increases with discharge current. Figure 9-9 shows the V-I curve of the BATFET gate regulation operation. BATFET turns off to exit supplement mode when the battery is below battery depletion threshold.

Figure 9-9 BAFET V-I Curve