Selecting the output capacitance for the converter involves a number of considerations. Closed loop stability and load transient response are the two main goals. From the perspective of loop stability, the impedance of the output capacitance must take into account not only the value of the capacitor, but the ESR and ESL as well. The ESR provides a zero to the filter transfer function at

and the ESL provides a second zero at

In selecting the output capacitance of the output filter, often two or three types of capacitors are used.

The selection of output capaciatance to support load transient response is the next consideration. The capacitance to minimize an output voltage overshoot is calculated as

This is the energy in the inductor being transferred to the energy difference between the nominal output voltage and the peak overshoot output voltage.

In many cases, with a 3% output voltage overshoot requirement, the resulting LC corner frequency is on the order of 1% to 1.5% of the switching frequency.

The fast load transient response of D-CAP4 largely addresses the case where there is a load increase and a voltage undershoot. However, there are cases where, during a load step increase, the minimum OFF time of the converter limits the ability of the converter to increase the switching frequency fast enough to maintain regulation. This can be true for higher output voltages and higher nominal switching frequencies.