8.2.2.6 Step Five: Choose the Soft-Start Capacitor

The slow-start capacitor determines the minimum 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 TPS54618 reach the current limit or excessive current draw from the input power supply may cause the input voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.

The slow-start capacitor value can be calculated using Equation 32. For the example circuit, the slow-start time is not too critical because the output capacitor value is 110 μF which does not require much current to charge to 1.8 V. The example circuit has the slow-start time set to an arbitrary value of 4 ms which requires a 10-nF capacitor. In TPS54618, Iss is 2.2 μA and Vref is 0.799 V.

Equation 32.