The BQ2461x automatically switches adapter or battery power to the system load. The battery is connected to the system by default during power up or during SLEEP mode. The battery is disconnected from the system and then the adapter is connected to the system 30 ms after exiting SLEEP. An automatic break-before-make logic prevents shoot-through currents when the selectors switch.
The ACDRV is used to drive a pair of back-to-back P-channel power MOSFETs between the adapter and ACP with sources connected together and to VCC. The FET connected to the adapter prevents reverse discharge from the battery to the adapter when turned off. The P-channel FET with the drain connected to the adapter input provides reverse battery discharge protection when off; and also minimizes system power dissipation with its low rDS(on), compared to a Schottky diode. The other P-channel FET connected to ACP separates the battery from the adapter, and provides a limited dI/dt when connecting the adapter to the system by controlling the FET turnon time. The BATDRV controls a P-channel power MOSFET placed between BAT and the system.
When the adapter is not detected, ACDRV is pulled to VCC to keep ACFET off, disconnecting the adapter from system. BATDRV stays at ACN-6V to connect the battery to the system.
Approximately 30 ms after the device comes out of SLEEP mode, the system begins to switch from the battery to the adapter. The break-before-make logic keeps both ACFET and BATFET off for 10 µs before ACFET turns on. This prevents shoot-through current or any large discharging current from going into the battery. BATDRV is pulled up to ACN and the ACDRV pin is set to VCC-6V by an internal regulator to turn on P-channel ACFET, connecting the adapter to the system.
When the adapter is removed, the system waits until VCC drops back to within 200 mV above SRN to switch from the adapter back to the battery. The break-before-make logic still keeps 10 μs dead time. The ACDRV is pulled up to VCC and the BATDRV pin is set to ACN-6V by an internal regulator to turn on P-channel BATFET, connecting the battery to the system.
Asymmetrical gate drive (fast turnoff and slow turnon) for the ACDRV and BATDRV drivers provides fast turnoff and slow turnon of the ACFET and BATFET to help the break-before-make logic and to allow a soft start at turnon of either FET. The soft-start time can be further increased by putting a capacitor from gate to source of the P-channel power MOSFETs.