An ideal buck-boost switching regulator may be modeled as a pair of caps connected directly to the input and output, a coil, and some routing circuitry which can switch between three configurations (a buck-only, boost-only, or inverting circuit would only need two).
- The input connects to the output through the coil
- The coil is connected directly across the input
- The coil is connected directly across the output
Assume components behave in ideal fashion (no resistive or switching losses, etc.) the source caps sit at 10V, the output draws 1A, the switcher spends half its time in the first configuration, half in the third, and cycles fast enough that the cap voltages and coil current don't have a chance to change much during each cycle.
In "steady" state, subject to the above conditions, the coil will have one amp flowing through it all the time (since it will always be in series with a load drawing 1 amp). If the output cap is sitting at five volts, then half the time the coil will have +5V across it, and half the time it will have -5V, so on average its current will remain at 1 amp. Half the time the source cap will have one amp taken out of it (when it's connected to the coil), and half the time it will have none, so the source will see half an amp of current draw.
The simplest way to see how a switcher can draw less current from the source than the load draws from it is to look at where the electrons are flowing: half of the electrons which go through the load will come from the source, and half will be switched to bypass the source. Thus, the load will have twice as much current flowing through it as the source.