As we see from Figure 5-1 the input capacitor(s) of the buck become the C_IO of the IBB. Typically this will be one or two ceramic capacitors in parallel with a small case size high frequency by-pass capacitor. To size these capacitors, use the recommendations in the buck data sheet; they can also be increased if desired. C_IO can help with load transients by providing a path from input to output for the load current transient. Remember that the C_IO bank will see a voltage of V_IN + |V_OUT| , and must have a voltage rating in excess of this voltage to help mitigate the voltage derating effect of the ceramic capacitors. As a first pass, the output capacitor can be sized based on the buck data sheet recommendations. Although the first pass should be stable, the output capacitors will probably need to be increased to get the best performance. Finally, the "new" input capacitor from V_IN to system ground helps to provide a low impedance path at the input for the IBB. This can be a ceramic or a large value aluminum electrolytic.

The RMS current in C_IO is calculated as for an ordinary buck, while the RMS current in C_O is calculated as that for a boost converter. The currents and duty cycle found in the above equations are used for these calculations.