8.2.2.6 Slow-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 charge the capacitor quickly to the output-voltage level. The large currents necessary to charge the capacitor may make the TPS57140-EP 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. Use Equation 40 to find the minimum slow start time, t_SS, 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 47-μF output capacitor up to 3.3 V while only allowing the average input current to be 0.125 A would require a 1-ms slow-start time.

After the slow-start time is known, calculate the slow-start capacitor value using Equation 6. For the example circuit, the slow-start time is not too critical, because the output capacitor value is 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 1 ms, which requires a 3.3-nF capacitor.

Equation 40.