SLUSC60B December 2017 – October 2019 UCC28064A
Resistor RA and RB are selected to activate brownout protection at ~75% of the specified minimum-operating input voltage. Resistor RA programs the brownout hysteresis comparator, which is selected to provide 17 V (~12 VRMS) of hysteresis. Calculations for RA and RB are shown in Equation 41 through Equation 44.
To meet voltage requirements, three 2.87-MΩ resistors were used in series for RA.
Select a standard value for RB.
In this design example, brownout becomes active (shuts down PFC) when the input drops below 67 VRMS for longer than 680 ms and deactivates (restarts with a full soft start) if, after that the tBORST time is elapsed, the input reaches 81 VRMS.