Circuit Design Guide for DC/DC Converters(5/7)

Selecting the CIN

Although its influence on output stability is not as significant as CL, CIN also has a large capacity, and the smaller the ESR is, the more the output is stabilized and the smaller the ripple voltage becomes. Increasing CIN to some extent will reduce the effect of minimizing the output ripple. In order to prevent EMI on the input side, the CIN value should start with about half that of the CL value. With CIN, even if ESR is too small, the output will not oscillate. Therefore, using capacitors with ESR as small as possible is recommended.