SLUSA75B July 2010 – January 2020 BQ24650
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 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 Equation 14:
A low ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and must be placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET. The voltage rating of the capacitor must be higher than the normal input voltage level. A 25-V rating or higher capacitor is preferred for a 20-V input voltage. A 20-μF capacitance is suggested for a typical 3-A to 4-A charging current.