An input capacitor is necessary to limit the input ripple voltage while providing AC current to the buck power stage at every switching cycle. To minimize the parasitic inductance in the switching loop, position the input capacitors as close as possible to the VIN and GND pins of the LM5013-Q1. The input capacitors conduct a square-wave current of peak-to-peak amplitude equal to the output current. It follows that the resultant capacitive component of AC ripple voltage is a triangular waveform.

Along with the ESR-related ripple component, use Equation 14 to calculate the peak-to-peak ripple voltage amplitude.

Equation 14.

Use Equation 15 to calculate the input capacitance required for a load current, based on an input voltage ripple specification (ΔVIN).

Equation 15.

The recommended high-frequency input capacitance is 4.4 µF or higher. Ensure the input capacitor is a high-quality X7S or X7R ceramic capacitor with sufficient voltage rating for C_IN. Based on the voltage coefficient of ceramic capacitors, choose a voltage rating preferably twice the maximum input voltage. Additionally, some bulk capacitance can required for large input loop inductance or long wire harnesses used in the system. This capacitor provides parallel damping to the resonance associated with parasitic inductance of the supply lines and high-Q ceramics. See the Power Supply Recommendations for more detail.