The device requires an input capacitor to reduce the surge current drawn from the input supply and the switching noise from the device. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. For most applications, a 10-μF capacitor is enough. An additional 0.1-μF capacitor from VIN to GND is optional to provide additional high-frequency filtering. The input capacitor must have a voltage rating greater than the maximum input voltage and have a ripple-current rating greater than the maximum input-current ripple of the converter. The rms input-ripple current is calculated in Equation 7, where D is the duty cycle (output voltage divided by input voltage).
Use Equation 8 to calculate the input ripple voltage, where ESRCIN is the ESR of input capacitor. Ceramic capacitance tends to decrease as the applied dc voltage increases. This depreciation must be accounted for when calculating input ripple voltage.
In this design, a 10-µF, 35-V X7R ceramic capacitor, part number GRM32ER7YA106KA12L from muRata, is chosen. This yields around 70-mV input ripple voltage. The calculated rms input ripple current is 0.75 A, well below the ripple-current rating of the capacitor.