> 4.1 | Post Filtering

> With post filtering an extra inductor and an extra capacitor are added after the components included in the DC/DC design.
> The dimensions of these extra components depend on how well filtered the output power from the DC/DC switch is. It might be enough to use a small inductor of 1uH. The inductor must of course be able to handle the current (2…3A) and it should not have too much resistance so as not to create too big a voltage drop.
> A switched DC/DC regulator has a feedback line from the output side that is used by the circuit to regulate the output voltage. It would be nice, if the feedback line could be taken from the output of the extra LC filter. The regulator could then compensate for the voltage drop over the second inductor as well. In most cases this is not possible. An LC filter creates a phase shift. The regulation IC expects this phase shift from the ordinary output filter. By adding an extra phase shift in the feedback line, it’s likely to add malfunction of the regulation IC.
> However, the output capacitor in the extra LC filter can be calculated according to what was previously mentioned in this application note.