8.1.4 Input Capacitor Selection (C_IN)

Input capacitors are necessary to limit the input ripple voltage while supplying much of the switch current during the high-side MOSFET on-time. It is generally recommended to use ceramic capacitors at the input as they provide both a low impedance and a high RMS current rating. It is important to choose a stable dielectric for the ceramic capacitor such as X5R or X7R. A quality dielectric provides better temperature performance and also avoids the DC voltage derating inherent with Y5V capacitors. Place the input capacitor as close as possible to the drain of the high-side MOSFET and the source of the low-side MOSFET. Non-ceramic input capacitors must be selected for RMS current rating, minimum ripple voltage, and to provide damping. A good approximation for the required ripple current rating is given by the relationship of Equation 6:

Equation 6.

The highest requirement for RMS current rating occurs for D = 0.5. When D = 0.5, the RMS ripple current rating of the input capacitor must be greater than half the output current. Low ESR ceramic capacitors can be placed in parallel with higher valued bulk capacitors to provide optimized input filtering for the regulator. The input voltage ripple is calculated using Equation 7:

Equation 7.

The minimum amount of input capacitance as a function of desired input voltage ripple is estimated using Equation 8:

Equation 8.