Inverting buck: issue about CIO

Wesley,

A buck (as well as an inverting buck-boost) converts a supply voltage to an output voltage. But in order to do so it must switch which generates ripples. These ripples are smoothed by the input capacitor and the output capacitor so you can think of a buck (and inverting buck-boost) as converting energy from the input capacitor into energy in the output capacitor.

A buck transfers energy from CIN to COUT easily because both are connected to the same GND. Current flows from CIN+ through the buck to COUT+ and the return current flows from COUT- to CIN- through the GND connection.

An inverting buck-boost on the other hand does not have this direct connection when we look at CIN. Energy is flowing from CIN+ through the buck to COUT+ which is the same as GND. COUT- needs a path back to CIN- so we add a capacitor which we call CIO as the new CIN to achieve this function. This means CIO is our effective CIN for an inverting buck-boost.

The start of the paragraph you mentioned says, "As we see from Figure 8 the input capacitor(s) of the buck become the CIO of the IBB." so this agrees with the article.

-Sam