Generally, in an actual system, PPS protocol is always running in the AP (Application Processor) or PMU (Power Management Unit) side, which means that the PPS protocol is independent of the BQ25970 device. All decoding processes about PPS protocol, such as PD3.0, take place in AP or PMU. To designers, it is very flexible, they can run any PPS protocols (standard or customized) in AP or PMU to realize fast charging functions. In Figure 2-1, the host device is a smartphone and the slave device is an AC adaptor. The smartphone sends a command to the AC adaptor, and the AC adaptor will response to the smartphone command and adjust the output voltage or output current.

Figure 2-1 PPS Working Process

The PPS protocol decoding unit is marked with a red line as Figure 2-1 shows. Of course, in a real smartphone system, this unit will be replaced by AP/PMU. In Figure 2-1, PPS protocol is PD3.0 and it is done using TI's MCU MSP430 The TUSB442 device, also from TI, is a USB Type-C™ PD controller that can communicate with an AC adaptor with PD3.0 protocol. When the PD AC adaptor is plugged into a smartphone, the AP will handshake with the AC adaptor by toggling the CC pin. After initial communication, the smartphone recognizes the AC adaptor and sends a PD command to the AC adaptor, after that the AC adaptor responds to the smartphone and the output target voltage and current as the smartphone sends requests to start the flash charging process. During the charging, the BQ25970 device will be always reporting battery voltage and charging current status to the AP. According to the BQ25970 continuous reporting, the AP will keep updating new commands to the AC adaptor to adjust the output voltage or current so as to keep constant current charging process until finishing the flash charging stage.