7.4.2 Buck-Boost Operation

To regulate the output voltage at all possible input voltage conditions, the device automatically switches from step-down operation to boost operation and back as required by the configuration. It always uses one active switch, one rectifying switch, one switch permanently on, and one switch permanently off. Therefore, it operates as a step-down converter (buck) when the input voltage is higher than the output voltage, and as a boost converter when the input voltage is lower than the output voltage. There is no mode of operation in which all four switches are permanently switching. Controlling the switches this way allows the converter to maintain high efficiency at the most important point of operation when input voltage is close to the output voltage. The RMS current through the switches and the inductor is kept at a minimum to minimize switching and conduction losses. For the remaining two switches, one is kept permanently on and the other is kept permanently off, thus causing no switching losses.