When the input power source is removed, there are circumstances where stress may occur on the power components.

For example, situations where the output voltage capacitor may be able to back drive its secondary-side sync MOSFET after the flyback converter switching has stopped completely, either temporarily or not. Such situation could apply at power down or next soft-start.

To address this case, once the LINEUV voltage falls below V_LIUVF, the TPS23731 transitions to soft-stop mode. It turns on temporarily the startup to maintain V_VCC, then uses the SST control to ramp down the switching MOSFET peak current. As a result, the converter output is discharged in a controlled way, the energy of the output capacitor being sent back to the input bulk capacitor. Also note that at beginning of soft-stop, the TPS23731 temporarily forces the peak current to a low value (V_CS maximum at approximately 50 mV), until SST voltage becomes low enough to decrease it further. This advanced feature allows the soft-stop to immediately start discharging the output capacitor, regardless of the output load level, minimizing any system tradeoffs for optimum switching MOSFETs protection. See Figure 8-13.

Figure 8-13 Soft-Stop Operation

Once the soft-stop operation has been completed and to avoid subsequent oscillations caused by the impact of the energy transfer on the LINEUV voltage, an internal load (~7 mA) is applied on VDD for approximately 160 ms, before the converter is allowed to restart.