SLUSAH0F October 2011 – November 2019 BQ25504
To prevent rechargeable batteries from being deeply discharged and damaged, and to prevent completely depleting charge from a capacitive storage element, the undervoltage (VBAT_UV) threshold must be set using external resistors. The VBAT_UV threshold voltage when the battery voltage is decreasing is given by Equation 2:
The sum of the resistors is recommended to be no higher than 10 MΩ that is, RUV1 + RUV2 = 10 MΩ. Spreadsheet SLURAQ1 provides help on sizing and selecting the resistors.
The undervoltage threshold when the battery voltage is increasing is VBAT_UV plus an internal hysteresis voltage denoted by VBAT_UV_HYST. For the VBAT_UV feature to function properly, the load must be connected to the VSTOR pin while the storage element should be connected to the VBAT pin. Once the VSTOR pin voltage goes above VBAT_UV plus VBAT_UV_HYST threshold, the VSTOR pin and the VBAT pins are effectively shorted through an internal PMOS FET. The switch remains closed until the VSTOR pin voltage falls below the VBAT_UV threshold. The VBAT_UV threshold should be considered a fail safe to the system. The system load should be removed or reduced based on the VBAT_OK threshold which should be set above the VBAT_UV threshold.