Figure 2-13 and Figure 2-14 show the start-up waveforms for the TPS54A20EVM-770. In Figure 2-13, the output voltage ramps up as soon as the input voltage reaches the UVLO threshold as set by the R2 and R3 resistor divider network. In Figure 2-14, the input voltage is initially applied and the output is inhibited by using a jumper at J2 to tie EN to GND. When the jumper is removed, EN is released. When the EN voltage reaches the enable-threshold voltage, the start-up sequence begins and the output voltage ramps up to the externally set value of 1.2 V. The input voltage for these plots is 12 V and the load is 1 Ω.

Figure 2-13 TPS54A20EVM-770 Start-Up Relative to V_IN

Figure 2-14 TPS54A20EVM-770 Start-Up Relative to Enable