When an output voltage overshoot occurs during step-down load transients, the V_O feedback loop commands the UCC28782 to stop switching quickly by increasing I_FB, in order to prevent additional energy from aggravating the overshoot. Since V_VDD drops during this time, the typical way to prevent a controller from shutting down is to oversize the VDD capacitor (C_VDD) so as to hold V_VDD above V_VDD(OFF). Instead, UCC28782 is equipped with survival-mode operation to hold V_VDD above V_VDD(OFF) during a transient event. Therefore, the size of C_VDD can be significantly reduced and the PCB footprint for the auxiliary power can be minimized. Specifically, there is a ripple comparator to regulate V_VDD above a 13-V threshold, which is V_VDD(OFF) plus V_VDD(PCT) in the electrical table. The ripple regulator is enabled when the V_O feedback loop requests the UCC28782 to stop switching due to V_O overshoot.

The regulator initiates unlimited PWML pulses when V_VDD drops lower than 13 V, and stops switching after V_VDD rises above 13 V. Since V_VDD or V_BIN is lower than the reflected output voltage overshoot, most of the magnetizing energy is delivered to the auxiliary winding and brings V_BIN above 2.2 V or V_VDD above 13 V quickly. After V_O moves back to the regulation level, V_O feedback loop forces the UCC28782 to begin switching again by reducing I_FB, 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 should be considered:

1. For fixed output voltage applications where the switching bias regulator is not required, the normal V_VDD level under regulated V_O must be designed to be above the 13-V threshold by an appropriate turns count for the auxiliary winding.
2. C_VDD should not be over-sized, but designed just large enough to hold V_VDD > V_VDD(OFF) under the longest V_O soft-start time.
3. For variable output voltage applications where the bias regulator is used, C_VDD voltage can be ramped up faster if the turns count of the auxiliary winding (N_AUX) can be increased, because the switching bias regulator can process more energy from its input, especially under the lowest output voltage condition. The design limitation on N_AUX is the maximum voltage rating of BIN and BSW pins under the highest output voltage condition.
4. The BIN-pin capacitor (C_BIN) provides energy storage for the bias regulator. Higher C_BIN value, such as >33 uF, can help avoid excess survival-mode operation and reduce the potential increase of V_O under no-load conditions.
5. When the bias regulator is not required, an auxiliary resistor in series with the auxiliary rectifier diode (D_AUX) should not be too large of value, because the lower series impedance can help the VDD capacitor to charge faster.
6. When the bias regulator is used, a series auxiliary resistor should not be used since it limits the energy transfer to C_BIN. When the resistor is removed, one effective way to prevent C_BIN from being overcharged by high leakage inductance or other potential energy source is to parallel a small 24-V TVS diode between the BIN pin and the AGND pin. If there is a layout limitation which forces the TVS diode connected to BGND pin instead, the impedance between BGND and AGND needs to be as small as possible, such that the TVS diode can clamp the voltage below the two pin ratings more effectively.
7. If the output voltage dynamic range is very wide, such as from 3.3 V to 20 V, low auxiliary winding resistance less than 0.1 Ω and a Schottky-type auxiliary diode are recommended, such that the majority of the survival mode energy can be transferred to C_BIN instead of diverting to the output capacitor under the lowest output voltage condition.
8. Ensure good coupling between the auxiliary winding (N_AUX) and the secondary winding (N_S) of the transformer.

When the control loop is inevitably stuck in the survival mode at no load, it is important to ensure that the high-side switch can be responsive to the PWMH signal of the survival mode switching pattern entirely, such that the survival mode energy can be diverted to the boost converter and the risk of output voltage drifting higher than the regulation level can be mitigated. It is essential to choose the high-side driver with short power-on delay less than 10 μs.