8.2.2.5 Input Capacitor

The TPS57140-EP requires a high-quality ceramic, type X5R or X7R, input decoupling capacitor of at least 3 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 TPS57140-EP. Calculate the input-ripple current using Equation 38.

The value of a ceramic capacitor varies significantly over temperature and the amount of dc bias applied to the capacitor. The designer can minimize the capacitance variations due to temperature by selecting a dielectric material that is stable over temperature. Designers usually select X5R and X7R ceramic dielectrics for power regulator capacitors because they have a high capacitance-to-volume ratio and are fairly stable over temperature. The designer must also take the dc bias into account for output capacitor selection. The capacitance value of a capacitor decreases as the dc bias across a capacitor increases.

This example design requires a ceramic capacitor with at least a 20-V voltage rating to support the maximum input voltage. Common standard ceramic capacitor voltage ratings include 4 V, 6.3 V, 10 V, 16 V, 25 V, 50 V, and 100 V, so select a 25-V capacitor. For this example, the selection is two 2.2-μF, 25-V capacitors in parallel. Table 2 shows a selection of high-voltage capacitors. The input capacitance value determines the input-voltage ripple of the regulator. Calculate the input-voltage ripple using Equation 39. Using the design example values, Ioutmax = 1.5 A, Cin = 4.4 μF, ƒ_SW = 1200 kHz, yields an input-voltage ripple of 71 mV and an RMS input-ripple current of 0.701 A.

Equation 38.

Equation 39.