8.2.2.5 Input Filter

A power supply input typically has a relatively high source impedance at the switching frequency. Good-quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the buck switch on-time. When the buck switch turns on, the current drawn from the input capacitor steps from zero to the valley of the inductor current waveform, then ramps up to the peak value, and then drops to the zero at turnoff.

Average input current can be calculated from the total input power required to support the load at V_OUT:

Equation 31.

The efficiency (η) is assumed to be 83% for this design example, yielding a total input power:

Equation 32.

Equation 33.

Equation 34.

The input capacitors should be selected with sufficient RMS current rating and the maximum voltage rating.

Equation 35.

Equation 36.