Now what about the output capacitor? Its purpose is to filter the inductor’s ripple current and make a nice clean dc voltage on the output. So it needs to store enough charge to hold the output voltage constant over the inductor’s charge-discharge cycle. Another way of looking at the output capacitor is that it bypasses the inductor’s ripple current to ground, so its impedance (its reactance Xc plus its equivalent series resistance, ESR and its equivalent series inductance, ESL) must be low enough that the ripple current times the impedance is less than the desired output voltage ripple