This section discusses the pre-bias start up with a presupposed condition that OVP is not triggered during start up. This could occur if the output voltage ramps up slowly, such as when the output capacitor is charged through the load path. This could also happen if the target output voltage is higher than the minimum required input voltage and the converter finishes start up before the OVP is triggered.

However, if the load resistance is very small, the output voltage ramps up very fast as the input voltage ramps up and finally equals to the input voltage before the converter is enabled. This results in an OVP event once the converter is enabled. Then, the converter will discharge the output voltage and restart again. Next, the output capacitor is charged through the load path and OVP will be triggered again. In the worst case, OVP events repeatedly appear. This causes the converter to hiccup and get stuck in the start-up state.

The following sections will analyze the behaviors of the TPS548D22 during start up based on bench test waveforms. Also, a solution to solve start up stuck issue will be also discussed.