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 switch steps to the valley of the inductor current waveform, ramps up to the peak value, and then drops to zero at turnoff. The input capacitors must be selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current rating is I_RMS > I_OUT / 2.

Quality ceramic capacitors with a low ESR were selected for the input filter. To allow for capacitor tolerances and voltage rating, four 2.2-µF, 100-V ceramic capacitors were used for the typical application circuit. With ceramic capacitors, the input ripple voltage will be triangular and peak at 50% duty cycle. Taking into account the capacitance change with DC bias, the input ripple voltage is approximated as:

Equation 21.

When the converter is connected to an input power source, a resonant circuit is formed by the line impedance and the input capacitors. If step input voltage transients are expected near the maximum rating of the LM5116, a careful evaluation of the ringing and possible overshoot at the device VIN pin must be completed. To minimize overshoot make C_IN > 10 ✕ L_IN. The characteristic source impedance and resonant frequency are:

Equation 18.

The converter exhibits a negative input impedance which is lowest at the minimum input voltage:

Equation 19.

The damping factor for the input filter is given by:

Equation 20.

where

• R_IN is the input wiring resistance
• ESR is the series resistance of the input capacitors

The term Z_S / Z_IN will always be negative due to Z_IN.

When δ = 1, the input filter is critically damped. This can be difficult to achieve with practical component values. With δ < 0.2, the input filter will exhibit significant ringing. If δ is zero or negative, there is not enough resistance in the circuit and the input filter will sustain an oscillation. When operating near the minimum input voltage, an aluminum electrolytic capacitor across C_IN can be needed to damp the input for a typical bench test setup. Any parallel capacitor must be evaluated for its RMS current rating. The current will split between the ceramic and aluminum capacitors based on the relative impedance at the switching frequency.