The TPS54020 is designed to start up into a pre-biased output. The output voltage is not discharged to ground at the beginning of the slow-start sequence, but only starts to increase towards regulation once the slow start voltage reaches the pre-bias bus voltage. Figure 2-10 shows the start-up waveform with the output voltage pre-biased to 1 V.

Figure 2-10 TPS54020EVM-082 Pre-Bias Startup