8.2.2.2 Input Capacitor Selection

A 22-µF or 47-µF high-quality dielectric (X5R, X7R) ceramic capacitor rated at twice the maximum input voltage is typically sufficient. The input capacitor must be placed as close as possible to the VIN pin and GND exposed pad to substantially eliminate the parasitic effects of any stray inductance or resistance on the PCB and supply lines.

Neglecting capacitor equivalent series resistance (ESR), the resultant input capacitor AC ripple voltage is a triangular waveform. The minimum input capacitance for a given peak-to-peak value (ΔV_IN) of V_IN is specified as follows:

Equation 7.

where

• the PWM duty cycle, D, is given by Equation 8:

Equation 8.

If ΔV_IN is 1% of V_IN, this equals to 50 mV and f_SW = 1 MHz.

Equation 9.

A second criteria before finalizing the C_in bypass capacitor is the RMS current capability. The necessary RMS current rating of the input capacitor to a buck regulator can be estimated by:

Equation 10.

Equation 11.

With this high AC current present in the input capacitor, the RMS current rating becomes an important parameter. The maximum input capacitor ripple voltage and RMS current occur at 50% duty cycle. Select an input capacitor rated for at least the maximum calculated I_Cin(RMS).

Additional bulk capacitance with higher ESR may be required to damp any resonance effects of the input capacitance and parasitic inductance.