7.3.1 Active Bridge Control

The TPS7A78 has an internally controlled, actively clamped, full-bridge rectifier between the AC+ and AC– pins that requires one of these pins to be connected in series with the high-voltage capacitor C_S and the surge resistor R_S. The active clamp for the bridge is designed to stabilize the rectified DC voltage at the SCIN pin to optimize performance given the LDO output voltage. The clamp circulates any excess AC charging current from the cap-drop capacitor C_S and surge resistor R_S through the AC+ or the AC– pins to the GND pins when the SCIN pin voltage surpasses its UVLO_SCIN rising threshold during startup. The clamp maintains the SCIN pin voltage higher than this threshold to support the targeted output voltage. This excess AC charging current is also referred to as the shunt current, I_SHUNT; see the Standby Power and Output Efficiency section for details on the shunt current.

A DC supply can also be used to provide power directly to the SCIN pin, which completely bypasses the bridge active-clamp circuit; see Table 1 for details on the DC supply mode.