ZHCSNG6 November 2021 UCC28781-Q1
PRODUCTION DATA
The REF pin may be used as an external shutdown function by shorting this pin to AGND with a small-signal control switch. This provides an additional design flexibility for the control function extension with external circuitry. When the REF-pin voltage drops lower than its internal power good threshold (approximately 4.5 V), the switching action will be terminated and VVDD will drop to VVDD(OFF), and the controller begins the UVLO restart cycle.
As long as the REF pin is shorted to AGND continuously, the UVLO restart cycle will repeat and switching action is inhibited until the external pull-down on REF is released. During the switch-short condition on REF, the falling slope of VVDD will drop faster than normal case due to the 17-mA over-current limit of the REF regulator which discharges the VDD capacitor faster than the normal IVDD currents in the run and wait states.