Other Parts Discussed in Thread: BQ25798, BQ25792, BQ25672
There is a BATFET between the system (SYS) and the battery (BAT) in the integrated buck boost chargers BQ25790, BQ25792, and BQ25798 and the integrated buck charger BQ25672. The BATFET separates the system voltage and the battery voltage. This separation generates the so called power path, which allows the system to be regulated to the Minimum System Voltage (MINSYS) even if the battery is depleted. The charger can still power a system load while the battery is depleted.
When the battery voltage is below the MINSYS, the BATFET is operated in low dropout (LDO) mode. It is equivalent to an adjustable resistor and limits the current from the system voltage MINSYS to the battery. Once the battery voltage reaches the MINSYS, the BATFET is fully on and the voltage drop across the BATFET is just the Rdson of the BATFET multiplied by the charging current.
When the input power cannot fully support the system load and the charging current, the charger will enter Dynamic Power Management (DPM) operation. If the input current increases to or the input voltage drops below the preset current or voltage thresholds, the Input Current DPM or the Input Voltage DPM will regulate the input current or input voltage and the charging current will reduce to maintain the power balance. If the charge current drops to zero and the system current continues to increase, the system voltage will start to drop. When the system voltage drops below the battery voltage, the battery starts to discharge to provide the extra system current that the system load requires. This mode of operation is called Supplement Mode, and allows the system to be supported by both the input source and the battery.
