The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying the ripple current and isolating switching noise from other circuits. The typical value for this capacitor is 2.2-μF. This capacitance must be rated for at least the maximum input voltage that the application requires, preferably twice the maximum input voltage. This capacitance can be increased to help reduce input voltage ripple and maintain the input voltage during load transients. In addition, a small case size 100-nF ceramic capacitor must be used at the input, as close a possible to the regulator. This provides a high frequency bypass for the control circuits internal to the device. For this example a 2.2-µF, 100-V, X7R (or better) ceramic capacitor is chosen. The 100 nF must also be rated at 100 V with an X7R dielectric.

Using an electrolytic capacitor on the input in parallel with the ceramics is desirable. This statement is especially true if long leads or traces are used to connect the input supply to the regulator. The moderate ESR of this capacitor can help damp any ringing on the input supply caused by the long power leads. The use of this additional capacitor also helps with voltage dips caused by input supplies with unusually high impedance.

Most of the input switching current passes through the ceramic input capacitor or capacitors. The approximate RMS value of this current can be calculated from Equation 11 and must be checked against the manufacturers' maximum ratings.

Equation 11.