Since RDM and RTZ pins are the critical programming pins for ZVS control, UCC28781 offers both open-circuit and short-to-GND protections for those pins. At initial start-up when V_VDD reaches V_VDD(ON) and before switching begins, a fixed voltage level is applied to each pin and the corresponding current level flowing out of the pin is sensed to detect a pin-fault condition. As a result, too small of a current represents the pin-open state, and too large of a current represents the pin-short state where the short-circuit current level is self-limited.

In general, maintain 2 kΩ < R_RDM < 500 kΩ and 20 kΩ < R_RTZ < 1.1 MΩ with ample margins to avoid triggering one of these faults. When a pin-fault condition is identified, no switching is allowed and one UVLO cycle of VDD is triggered as the protection response. The normal start-up sequence will proceed on the next VDD cycle after the fault condition is removed.