JAJSH18A March 2019 – September 2019 TPS7A78
The TPS7A78 can monitor the rectified DC voltage at the SCIN pin to provide the application with an early warning via the power-fail (PF) pin if the main power fails. An external resistor-divider network connected to the VSCIN pin provides the input to the power-fail detect (PFD) analog input pin to monitor for an AC line supply failure. When the AC supply falls below its minimum level programmed by the resistor divider R1 and R2, as illustrated in Figure 14 and in Figure 15, the PF output is pulled low. If this feature is not used, omit R1 and R2 and connect PFD and PF pins to the device GND pins reference.
The PFD pin can also be used to monitor another DC rail within the application to provide an early warning via the PF pin. However, this DC rail must share the same GND reference with the TPS7A78 GND and the absolute maximum voltage of the PF pin must not be exceeded.