The input capacitor should have enough ripple current rating to absorb input switching ripple current. The worst-case RMS ripple current is half of the charging current when the duty cycle is 0.5. If the converter does not operate at 50% duty cycle, then the worst-case capacitor RMS current ICIN occurs where the duty cycle is closest to 50% and can be estimated by the following equation:
A low-ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be placed to the drain of the high-side MOSFET and source of the low-side MOSFET as close as possible. The voltage rating of the capacitor must be higher than the normal input voltage level. A 25-V or higher-rating capacitor is preferred for 20-V input voltage. 10-µF to 20-µF capacitance is suggested for typical of 3-A to 4-A charging current.