5.3 Input Capacitor Selection

The input capacitor for the power stage input should be selected for ripple current rating and voltage rating. Tantalum input capacitors may fail when subjected to high inrush currents, caused by turning the input supply on. A tantalum input capacitor’s voltage rating should be at least two times the maximum input voltage to maximize reliability. Aluminum electrolytic, OS-CON, and multilayer polymer film capacitors can handle the higher inrush currents without voltage derating. The input voltage ripple will primarily depend on the input capacitor’s ESR.

The peak input current is equal to the peak inductor current, so:

$∆V_{IN} = I_{L\left(PK\right)} \times ES{R}_{CIN}$

The input capacitor must be rated for the input current ripple. The RMS value of the input capacitor current is determined at the maximum output current. Assuming the peak-to-peak inductor current ripple is low:

$I_{CIN\left(RMS\right)} \approx I_{OUT\left(MAX\right)} \times \sqrt{D \times \left(1 - D\right)}$

The power dissipated in the input capacitor is:

$P_{DISS\left(CIN\right)} = {I_{CIN\left(RMS\right)}}^2 \times ES{R}_{CIN}$