8.4.4 Fault Mode

A separate overcurrent comparator within the UCCx813-x devices handles operation into a short-circuited or severely overloaded power supply output. This overcurrent comparator has a 1.5-V threshold and is also gated by the leading-edge-blanking signal to prevent false triggering. When the fault is first detected, the internal soft-start capacitor instantly discharges and stays discharged until the fault clears. At the same time, the PWM output is turned off and held off. This is often referred to as hiccup. This delay time is used to significantly reduce the input and dissipated power of the main converter and switching components. Full-cycle soft-start insures that there is a predictable delay of greater than 3 milliseconds between successive attempts to operate during fault. When the fault clears, the capacitor slowly charges and allows the error amp output (COMP) to rise. When COMP gets high enough to enable the output, another fault occurs, latching off the PWM output, but the soft-start capacitor still continues to rise to 4 V before being discharged and permitting start of a new cycle. This means that for a severe fault, successive retries are spaced by the time required to fully charge the soft-start capacitor.