SLUSDF9 June 2020 BQ25790
To meet the maximum current limit in USB spec and avoid over loading the adapter, the device features Dynamic Power Management (DPM), which continuously monitors the input current and input voltage. When the input power at the VBUS pin is too low to support the load from SYS pin and the battery charge current from BAT pin, the charger engages either IINDPM to limit its current or VINDPM to prevent further reduction in VBUS pin voltage.
When the system voltage is regulated at VSYSMIN, the charger could be in trickle charge, pre-charge or fast charge stages, the SYS voltage drops lower than VSYSMIN, the VSYSMIN loop takes over and reduces the trickle charge, pre-charge or fast charge current, so that the SYS voltage remains at the VSYSMIN level.
If the charge current falls to zero, but the input source is still overloaded, the SYS voltage will drop. Once the SYS voltage falls below the battery voltage, the device automatically enters Supplement Mode in which the BATFET turns on. The battery starts discharging so that the system is supported from both the input source and battery. In supplement mode, the battery FET is operated in ideal diode mode in which the charger regulates the battery FET gate voltage to keep the BATFET minimum VDS to approximately 25 mV when the current is low. This prevents SYS voltage oscillations from entering and exiting the supplement mode. As the discharge current increases, the charger regulates the BATFET gate to a higher voltage, in order to reduce the battery FET RDSON until the MOSFET is in full turn-on stage. At this point onwards, the VDS of the battery FET linearly increase with the discharge current. The figure below shows the V-I curve of the BATFET gate regulation operation. The BATFET turns off to exit Supplement Mode when the battery is below battery depletion threshold.
During DPM mode, the status register bits VINDPM_STAT (VINDPM) and/or IINDPM_STAT (IINDPM) go high. The figure below shows the DPM response with 5V/3A adapter, 6.4V battery, 1.5A charge current and 6.8V minimum system voltage setting.