When the system is first powered on, if the supercapacitor is uncharged, it must be charged from the main supply through the operation of the LDO. The TPS61381 IC can charge the supercapacitor from 0V to the set VBUB based on the application requirement.

At the start of the charging process, the TPS61381-Q1 detects system voltage with VOUT pin and automatically transition into boost mode when a power interruption of your system is detected. In this application involving automatic boost and standby mode, the device works in standby mode when Vout is normal. When the power failure occurs and Vout drops below than min{VBST_WAKEUP (Set by I2C bits BST_WAKE), VBST_STANDBY (BST_VOUT ×106%)}, the device enters boost mode to maintain the output voltage. When the 12V main battery recovers and Vout rises higher than VBST_STANDBY, the device enters standby mode again.

Figure 2-3 Supercapacitor Charging Behavior