8.2.3.1.3 Start-Up and Shutdown Waveforms

Figure 41 and Figure 42 show the start-up waveforms for the TPS54678EVM-155. In Figure 41, 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 42, the input voltage is initially applied and the output is inhibited by using a jumper at JP1 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 TPS54678 is designed to start up into prebiased outputs. Figure 43 shows the output voltage start-up waveform when the output is prebiased with 550 mV at no load.

Figure 44 and Figure 45 show the shutdown waveforms for the TPS54678EVM-155. In Figure 44, the output voltage ramps down as soon as the input voltage falls below the UVLO stop threshold as set by the R1 and R2 resistor divider network. At the point of shutdown, the input voltage rises slightly due to the resistive drop in the input feed impedance. In Figure 45, the output is inhibited by using a jumper at JP1 to tie EN to GND.