8.4.3 System Power Selection

The BQ24725A automatically switches adapter or battery power to system. The battery is connected to system at POR if battery exists. The battery is disconnected from system and the adapter is connected to system after default 150ms delay (first time, the next time default is 1.3s and can be changed to 150ms) if ACOK goes HIGH. An automatic break-before-make logic prevents shoot-through currents when the selectors switch.

The ACDRV drives a pair of common-source (CMSRC) n-channel power MOSFETs (ACFET and RBFET) between adapter and ACP (see Figure 18 for details). The ACFET separates adapter from battery or system, and provides a limited di/dt when plugging in adapter by controlling the ACFET turn-on time. Meanwhile it protects adapter when system or battery is shorted. The RBFET provides negative input voltage protection and battery discharge protection when adapter is shorted to ground, and minimizes system power dissipation with its low R_DS(on) compared to a Schottky diode.

When the adapter is not present, ACDRV is pulled to CMSRC to keep ACFET and RBFET off, disconnecting adapter from system. BATDRV stays at V_SRN + 6V to connect battery to system if all the following conditions are valid:

• V_VCC > UVLO;
• V_SRN > UVLO;
• V_ACN < 200mV above V_SRN (ACN_SRN comparator);

Approximately 150ms (first time; the next time default is 1.3s and can be changed to 150ms) after the adapter is detected (ACDET pin voltage between 2.4V and 3.15V), the system power source begins to switch from battery to adapter if all the following conditions are valid:

• Not in LEARN mode or in LEARN mode and V_SRN is lower than battery depletion threshold;
• ACOK high

The gate drive voltage on ACFET and RBFET is V_CMSRC + 6V. If the ACFET/RBFET have been turned on for 20ms, and the voltage across gate and source is still less than 5.9V, ACFET and RBFET will be turned off. After 1.3s delay, it resumes turning on ACFET and RBFET. If such a failure is detected seven times within 90 seconds, ACFET/RBFET will be latched off and an adapter removal and system shut down is required to force ACDET < 0.6V to reset the IC. After IC reset from latch off, ACFET/RBFET can be turned on again. After 90 seconds, the failure counter will be reset to zero to prevent latch off. With ACFET/RBFET off, charge is disabled.

To turn off ACFET/RBFET, one of the following conditions must be valid:

• In LEARN mode and V_SRN is above battery depletion threshold;
• ACOK low

To limit the in-rush current on ACDRV pin, CMSRC pin and BATDRV pin, a 4kΩ resistor is recommended on each of the three pins.

To limit the adapter inrush current when ACFET is turned on to power system from adapter, the Cgs and Cgd external capacitor of ACFET must be carefully selected. The larger the Cgs and Cgd capacitance, the slower turn on of ACFET will be and less inrush current of adapter. However, if Cgs or Cgd is too large, the ACDRV-CMSRC voltage may still go low after the 20ms turn on time window is expired. To make sure ACFET will not be turned on when adapter is hot plugged in, the Cgs value should be 20 times or higher than Cgd. The most cost effective way to reduce adapter in-rush current is to minimize system total capacitance.