At start-up, the TPS25750D PD controller sends the initial I²C commands to set up the BQ25798 battery charger for charging, number of cells and so on. These initial commands are based on the answers provided on the Web-based Application Customization Tool for proper functionality. When a 5-V sink PDO Contract is negotiated, the PD controller sets up the buck-boost battery charger BQ25798 for a V_IN of 5 V and charging the battery at
1 A based on this particular configuration.

Similarly, when a 20 V Sink PDO contract is negotiated, the same behavior can be seen where the PD controller sets up V_IN at 20 V this time, while maintaining charging at 1 A.

When the TPS25750D negotiates a source PDO contract, the device configures the BQ25798 to operate in OTG mode so that the battery can source the power needed for the port partner. In this case, the BQ25798 needs to boost up the battery voltage up to 5 V to source power to the other device.

The same behavior can be observed when the port partner and the TPS25750D negotiate a 20 V contract using the TPS25750D as a Source.

One of the features for USB Type-C PD Controllers, such as the TPS25750D is the ability to perform Power Role Swaps depending on when certain conditions change. In the corresponding figure, initially the sourcing can be observed as VBUS is at 20 V but the charging current is at 0 A, then a reset or renegotiation occurs where the PD controller needs to become a Sink and both devices accept this change. Therefore, VBUS goes to 0 V and then negotiates a 20 V Sink PDO contract. Once the Power Role Swap is done, the TPS25750D configures the BQ25798 for charging mode to charge at 1 A, which can be seen at the end of the capture.

Similarly, the same can be done in reverse order, going from the TPS25750D being a Sink and charging the battery, to renegotiating a Sourcing contract with the port partner and supplying the 20 V using the BQ25798.