Figure 2-10 and Figure 2-11 show the start-up waveforms for the TPS56921EVM-188. In Figure 2-10, the output voltage ramps up as soon as the input voltage reaches the UVLO threshold as set by the R1 and R2 resistor divider network. In Figure 2-11, the input voltage is initially applied and the output is inhibited by using a jumper at JP2 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 1.1 V. The input voltage for these plots is 12 V and the load is 1 Ω. Figure 2-12 shows a detailed view of the output voltage ramp up.

Figure 2-10 TPS56921EVM-188 Start-Up Relative to V_IN

Figure 2-11 TPS56921EVM-188 Start-Up Relative to Enable

Figure 2-12 TPS56921EVM-188 Start-Up Relative to V_IN Detail