For sourcing capabilities, this design uses the PP5V power path to supply a 5-V source and the PPHV power path to source higher voltages. Therefore to keep a constant 5-V 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 5-V 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, it can handle any input voltage in the range of 3.5 V to 28 V and provide a 5-V output capable of delivering up to 5 A. For a sink-only application, this device is not needed and a 5-V rail can be omitted.

Figure 2-6 TPS54531 Buck Converter Schematic