7.3.9 Sequencing

Many of the common power supply sequencing methods can be implemented using the SS, EN and PWRGD pins. The sequential method can be implemented using an open drain or collector output of a power on reset pin of another device. Figure 25 shows the sequential method. The power good is coupled to the EN pin on theTPS54318 device which enables the second power supply once the primary supply reaches regulation.

Ratiometric start up can be accomplished by connecting the SS pins together. The regulator outputs ramp up and reach regulation at the same time. When calculating the soft-start time the pull up current source must be doubled in Equation 4. The ratiometric method is shown in Figure 27.

Figure 25. Sequencial Start-Up Schematic

Figure 26. Sequential Startup using EN and PWRGD

Figure 27. Ratiometric Start-Up Schematic

Figure 28. Ratiometric Start-Up Using Coupled SS Pins