SLUSD12A October 2017 – February 2018 UCC28780
When the output voltage overshoot occurs during step-down load transients, the VO feedback loop commands the UCC28780 to stop switching quickly through increasing IFB, in order to prevent additional energy from aggravating the overshoot. Since VVDD keeps dropping during this time, the conventional way to prevent a controller from shutting down is to oversize the VDD capacitor so as to hold VVDD above VVDD(OFF). Instead, UCC28780 is equipped with the survival-mode operation to hold VVDD above VVDD(OFF) during the transient event, so the size of VDD capacitor can be significantly reduced and the PCB footprint for the auxiliary power can be minimized. Specifically, there is a ripple comparator to regulate VVDD above a 11-V threshold, which is VVDD(OFF) plus VVDD(PTC) in the electrical table. The ripple regulator is enabled when the VO feedback loop requests the UCC28780 to stop switching due to VO overshoot.
The regulator initiates unlimited PWML pulses when VVDD drops lower than 11 V, and stops switching after VVDD rises above 11 V. Since VVDD is lower than the reflected output voltage overshoot, most of the magnetizing energy is delivered to the auxiliary winding and brings VVDD above the 11-V threshold quickly. After VO moves back to the regulation level, VO feedback loop forces the UCC28780 to begin switching again by reducing IFB, and the PWML and PWMH pulses are then controlled by the normal operating mode.
To prevent the controller from getting stuck in survival mode continuously or toggling between SBP and survival mode at zero load, some guidelines on the auxiliary power delivery path to VDD can be considered: