Voltage regulators can be broadly divided into two types—switching and nonswitching. Nonswitching types, like the linear regulator and the shunt regulator, have the advantages of simplicity and low noise, but the limitation of an input voltage that must always be higher than the output voltage. In contrast, switching regulators are more complex and noisy, but their input voltage can be either lower or higher than the output voltage. And, they are usually more efficient.
Two of the most common topologies for a switching regulator are the step-down converter (a) and the step-up converter (b). The step-down configuration produces an output voltage that is lower than the input voltage, while the step-up (also known as boost) configuration generates an output voltage that is higher than the input voltage. For the boost configuration, in a shutdown situation, the output voltage equals the input voltage minus a diode voltage drop. This diode drop does not permit a true zero-current condition, so the load is never quite completely disconnected and continues to drain the battery even during shutdown.
Another common switching regulator (c) is the single-ended primary inductance converter (SEPIC), which can act as both a step-up and a step-down converter. It accepts an input voltage that is either lower or higher than the output voltage. The SEPIC configuration employs a capacitor (CP) between two inductors, L1 and L2. This capacitor inhibits any dc current flow from input to output whether the output voltage is above or below the input voltage. Furthermore, the capacitor helps to isolate the load and completely disconnect it in shutdown, eliminating any battery drain and extending battery life. For lower parts count, the SEPIC's two inductors can be wired over the same core with the same number of turns.