SLVSAF1E October 2010 – May 2019 TPS54388-Q1
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. Slow start is useful if a load requires a controlled rate of voltage slew. Another use for slow start is if the output capacitance is large and would require large amounts of current to charge the capacitor quickly to the output-voltage level. The large current necessary to charge the capacitor may make the TPS54388-Q1 device 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.
Calculate the slow-start capacitor value using Equation 32. For the example circuit, the slow-start time is not too critical because the output-capacitor value is 44 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 the TPS54388-Q1 device, I(SS/TR) is 2.2 μA and Vref is 0.8 V.