SLVSDV8 July 2017 TPS54424
The soft-start capacitor determines the amount of time it takes for the output voltage to reach its nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This is also used if the output capacitance is very large and would require large amounts of current to quickly charge the capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the TPS54424 reach its current limit or cause the input voltage rail to sag due excessive current draw from the input power supply. Limiting the output voltage slew rate solves both of these problems. The soft-start capacitor value can be calculated using Equation 25. For the example circuit, the soft-start time is not critical because the output capacitor value of 100 μF does not require much current to charge to 1.8 V. The example circuit has the soft-start time set to an arbitrary value of 1 ms which requires a 8.2-nF capacitor.