For sourcing capabilities, this design uses the PP5V power path to supply a 5V source and the PPHV power path to source higher voltages. Therefore to keep a constant 5V system source readily available on VBUS for the PD controller and for VCONN support, a buck converter is needed. Due to the buck topology selection, a 2S-4S battery is needed to work with the TPS54531 to achieve a 5V output operation using the battery as the input. For this design, the TPS54531 is a good choice, the device features integrated FETs and a wide input voltage range. Since this converter is connected to the system rail of the battery charger, the converter can handle any input voltage in the range of 3.5V to 28V and provide a 5V output capable of delivering up to 5A. For a sink-only application, this device is not needed and a 5V rail can be omitted.

Figure 2-6 TPS54531 Buck Converter Schematic