There are many situations where a power rail needs to be generated from an input voltage that may be lower or higher than the desired output voltage, and a smooth transition between buck mode (lowering input voltage) and boost mode (increasing input voltage) is required. A SEPIC is essentially a boost converter followed by a buck-boost stage, therefore it is similar to a traditional buck-boost converter but has advantages of having non-inverted output (the output has the same voltage polarity as the input), using a series capacitor to couple energy from the input to the output, and being capable of true shutdown: when the switch is turned off, its output drops to 0 V.
Because the input and output rails are separated by a capacitor, the output voltage rail is short circuit protected by design. In case of failure of the switch or other overload conditions, the load is can be easily protected using a simple TVS diode, since the capacitor is an open circuit for DC. SEPICs are useful in applications in which a battery voltage can be above and below that of the regulator's intended output. For example, a single lithium ion battery typically discharges from 4.2 volts to 3 volts; if other components require 3.3 volts, then the SEPIC would be effective.
Although datasheets of DC/DC converters rarely mention the capability, most off-the-shelf boost controllers are capable of controlling a SEPIC converter, including those with an integrated MOSFET switch. The SX1308 from SuoSemi is an ultra low cost, 1.5 MHz boost controller with up to 2 A switch current, and an input voltage from 2 V to ca. 20 V, and can be used to control a SEPIC converter. In the current implementation, the output voltage is set to 5.0 V with an external voltage divider, but knowing the feedback voltage of the SX1308 is 0.6 V, any output voltage can be set by recalculating the resistor values.
The coupling capacitor is decisive for the efficiency of the SEPIC converter, and a type with ultra low ESR is recommendable. Ceramic multi layer capacitors are recommended, and only bipolar types can be used because the voltage polarity inverts every switching cycle.