The input capacitor has enough ripple current rating to absorb input switching ripple current. The worst-case RMS ripple current is half of the charging current (plus system current if there is any system load) when the duty cycle is 0.5 in buck mode. If the converter does not operate at 50% duty cycle, then the worst case capacitor RMS current occurs where the duty cycle is closest to 50% and can be estimated by Equation 7:

Low ESR ceramic capacitor, such as X7R or X5R, is preferred for input decoupling capacitor and must be placed in front of RAC current sensing and as close as possible to the power stage half bridge MOSFETs. Capacitance after RAC, before the power stage half bridge, must be limited to 10μF+10nF+1nF as shown in the Figure 8-4 diagram. The voltage rating of the capacitor must be higher than the normal input voltage level. A 35V rating or higher capacitor is preferred for a 28V input voltage. A minimum of 10 pieces of 10µF 0603 size capacitors are suggested for the 28V/140W adapter design. A 50V rating or higher capacitor is preferred for 36V input voltage. A minimum of 10×10μF 0805 capacitors are needed when power reaches 36V/180W. Under different input voltage, the minimum input capacitance requirement is summarized in the following corresponding table. For quasi-dual phase, spread the MLCC capacitors before RAC_A and RAC_B. 1×10nF+1nF 0402 package MLCC capacitors (EMI filter purpose) are recommended to be placed as close as possible to both phase A and phase B half bridge MOSFETs.

Ceramic capacitors show a DC-bias effect. This effect reduces the effective capacitance when a DC-bias voltage is applied across a ceramic capacitor, as on the input capacitor of a charger. The effect can lead to a significant capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's datasheet about the derating performance with a DC-bias voltage applied. Selecting a higher voltage rating or nominal capacitance value can be necessary to get the required capacitance value at the operating point. Tantalum capacitors (POSCAP) can avoid DC-bias effect and temperature variation affect, which is recommended especially for a 28V to 48V higher power application.