8.2.1.2.7 Slow-Start Capacitor

The slow-start capacitor determines the minimum amount of time it will take 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 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 TPS54260 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 time must be long enough to allow the regulator to charge the output capacitor up to the output voltage without drawing excessive current. Equation 40 can be used to find the minimum slow-start time, tss, necessary to charge the output capacitor, Cout, from 10% to 90% of the output voltage, Vout, with an average slow-start current of Issavg. In the example, to charge the effective output capacitance of 72.4 µF up to 3.3 V while only allowing the average output current to be 1 A would require a 0.19-ms slow-start time.

Once the slow-start time is known, the slow-start capacitor value can be calculated using Equation 6. For the example circuit, the slow-start time is not too critical since the output capacitor value is 2 x 47 μF which does not require much current to charge to 3.3 V. The example circuit has the slow-start time set to an arbitrary value of 3.5 ms which requires a 8.75-nF slow-start capacitor. For this design, the next larger standard value of 10 nF is used.

Equation 40.