SLVSAF1E October 2010 – May 2019 TPS54388-Q1
The TPS54388-Q1 device requires a high-quality ceramic, type X5R or X7R, input decoupling capacitor with at least 4.7 μF of effective capacitance, and in some applications a bulk capacitance. The effective capacitance includes any dc bias effects. The voltage rating of the input capacitor must be greater than the maximum input voltage. The capacitor must also have a ripple-current rating greater than the maximum input-current ripple of the TPS54388-Q1 device. Calculate the input ripple current using Equation 30.
The value of a ceramic capacitor varies significantly over temperature and the amount of dc bias applied to the capacitor. Minimize the capacitance variations due to temperature by selecting a dielectric material that is stable over temperature. X5R and X7R ceramic dielectrics are the usual selection for power regulator capacitors because they have a high capacitance-to-volume ratio and are fairly stable over temperature. The output-capacitor selection must also take dc bias into account. The capacitance value of a capacitor decreases as the dc bias across that capacitor increases.
This example design requires a ceramic capacitor with at least a 10-V voltage rating to support the maximum input voltage. For this example, the selection is one 10-μF 10-V and one 0.1-μF 10-V capacitor in parallel. The input capacitance value determines the input ripple voltage of the regulator. Calculate the input voltage ripple using Equation 31. Using the design example values, IO(max) = 3 A, C(IN) = 10 μF, and f(SW) = 1 MHz, yields an input voltage ripple of 76 mV and an rms input ripple current of 1.47 A.