The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying ripple current and isolating switching noise from other circuits. An effective value of at least 14 µF is normally sufficient for the input capacitor. If the main input capacitor or capacitors cannot be placed close to the module, then a small 10 nF to 100 nF capacitor should be placed directly at the module across the supply and ground pins.

Many times, it is desirable to use an electrolytic capacitor on the input, in parallel with the ceramics. This is especially true if long leads/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 long power leads. This method can also help to reduce voltage spikes that can exceed the maximum input voltage rating of the LMZ20501. The use of this additional capacitor will also help with voltage dips caused by input supplies with unusually high impedance.

Most of the switching current passes through the input ceramic capacitor or capacitors. The approximate RMS value of this current can be calculated with Equation 2 and should be checked against the manufactures maximum ratings.

Equation 2.