When an input source is present at VBUS, the device checks the input source voltage to turn on REGN LDO and all the bias circuits. It detects and sets the input current limit before the buck-boost converter is started. The power up sequence from input source is as listed below:

1. The device begins the POR sequence when V_VBUS > V_{VBUS_UVLOZ} if there is a direct path from input to VBUS, or otherwise when V_AC1 or V_AC2 > V_{AC_PRESENT}.
2. 5 ms (typical) after a valid voltage is first present at either VBUS or VAC1/VAC2 pins, the charger starts the ACFET-RBFET detection, reads the resistance at PROG pin, and then configures the charger power on reset (POR) default register settings accordingly.
3. If ACFET-RBFET are detected on the input source pathway, the corresponding ACDRV turns on the pair.
4. 20 ms (typical) after valid voltage voltage presence, the I²C registers become accessible to the host.
5. As soon as V_VBUS > V_{VBUS_UVLOZ} (in step 1 or after step 3), VBUS_PRESENT_STAT is set to 1. 150 ms (typical) later, REGN is turned on.
6. Once REGN is on, the charger starts the poor source detection. After a good input source is detected (typically 30 ms if there is no retry), PG_STAT is set to 1, then the ADC reads the ILIM_HIZ pin voltage and VBUS voltage and updates the IINDPM and VINDPM registers accordingly.
7. When the poor source detection completes, the charger performs D+/D- detection and updates the VBUS_STAT and IINDPM registers accordingly.
8. 30 ms (typical) after the D+/D- detection completes, the converter starts switching to power SYS and charge the battery.