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 the 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 cause the TPS54678 to trip OCP, or excessive current draw from the input power supply may cause the input voltage rail to sag. Limiting the output voltage slew rate mitigates both of these issues.

The slow-start capacitor value can be calculated using Equation 25. For the example circuit, the slow-start time is not critical because the output capacitor value is 5 × 47 μF which does not require much current to charge to
1.2 V. The example circuit has the slow-start time set to an arbitrary value of 3.33 ms, which requires a 10-nF capacitor.

Equation 25.