The TPS65400 implements a UVLO function that prevents startup when the voltage at VIN (terminal 22) is below 4 V. In most applications, VIN and all of the power rails (PVIN1, PVIN2, PVIN3, and PVIN4) are tied to the same source and this single UVLO function is sufficient. However, in some applications, the power rails may be tied to different input voltages, and there is the possibility that the TPS65400 may attempt to startup a switcher even when its associated PVINx rail has not reached a high-enough voltage. In these cases, the precision enable threshold on each ENSWx can be used to precisely set the startup threshold for each individual switcher with a simple resistor divider to PVINx.

In cases where a single UVLO threshold is needed for all four switchers, but at a different level than 4 V, the TPS65400 can be configured for single-terminal enable (PMBus register D2h, bits 0:1 = 10) where the ENSW1/ENSEQ terminal is used as a sequence enable terminal. Then, a resistor divider to the appropriate PVINx rail can be used to set a precise UVLO threshold that applies to all four switchers.