The regulator supply voltage has a large 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 on-time. When the buck switch turns on, the current into the VIN pin steps to the lower peak of the inductor current waveform, ramps up to the peak value, then drops to zero at turn-off. The average current into VIN during the on-time is the load current. The input capacitance must be selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current rating necessary is I_RMS > I_OUT / 2.

Quality ceramic capacitors with a low ESR must be selected for the input filter. To allow for capacitor tolerances and voltage effects, one 1.0μF, 100V ceramic capacitor is used. If step input voltage transients are expected near the maximum rating of the LM5574, a careful evaluation of ringing and possible spikes at the device VIN pin must be completed. An additional damping network or input voltage clamp can be required in these cases.