Figure 3-26 and Figure 3-27 show the start-up and shutdown waveforms for U2 with EN. In Figure 3-26, the input voltage is initially applied and the output is inhibited by pulling EN to GND using an external function generator. When the EN voltage is increased above the enable-threshold voltage, the start-up sequence begins and the output voltage ramps up to the externally set value. In Figure 3-27, the external function generator pulls EN to ground and the TPS543820 shuts down.

Figure 3-28 shows EN start-up with a 0.2-Ω resistor load. Figure 3-29 shows the BP5 internal LDO start-up relative to the EN pin.

A shunt on the ENOFF_U1 jumper or RDIV_VIN can be used to test the EN start-up of U1 and U2, respectively. When the shunt is removed from ENOFF_U1, EN is released and the start-up sequence begins for U1. When the shunt is placed on RDIV_VIN, EN is pulled to the input voltage through the resistor divider and the start-up sequence begins for U2.

Figure 3-26 U2 Start-up with EN – No Load

Figure 3-28 U2 Start-up with EN – 0.2-Ω Load

Figure 3-27 U2 Shutdown with EN – No Load

Figure 3-29 U2 Startup with EN – No Load and Measuring BP5