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 (plus system current there is any system load) when 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 5:

Equation 5.

Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be placed in front of RAC current sensing and as close as possible to the power stage half bridge MOSFETs. Capacitance after RAC before power stage half bridge should be limited to10μF+10nF+1nF referring to Figure 8-3 diagram. Voltage rating of the capacitor must be higher than normal input voltage level, 35 V rating or higher capacitor is preferred for 28 V input voltage. Minimum 10 pieces of 10-µF 0603 size capacitors are suggested for 28V/140 W adapter design. 50-V rating or higher capacitor is preferred for 36-V input voltage. Minimum 10*10μF 0805 capacitors are needed when power reaches 36 V/180 W. Under different input voltage the minimum input capacitance requirement is summarized in Table 8-7, Table 8-3 and Table 8-4. For quasi dual phase it is recommended to spread the MLCC caps 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 may lead to a significant capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's data sheet about the derating performance with a dc bias voltage applied. It may be necessary to choose a higher voltage rating or nominal capacitance value in order to get the required capacitance value at the operating point. Tantalum capacitors (POSCAP) can avoid dc-bias effect and temperature variation effect which is recommended especially for 28-V and 36-V higher power application.