It goes without saying that rechargeable battery voltages change with charge status. This presents a problem for circuits that operate somewhere in the mid-battery voltage range. Such applications require a dc-dc converter that can step up or step down the battery voltage—commonly called a buck-boost converter. Several topologies are capable of step-up and step-down operation, including SEPICs and other transformer-coupled circuits. But these circuits are relatively complex and expensive and consume an unacceptable amount of pc-board space for most portable applications.
Figure 1 shows a tiny, simple buck-boost circuit that requires very few components. A small SOT23 MOSFET (Q1) with a low gate charge (QG) is chosen for high-frequency operation at 1 MHz. Q1 and Schottky diode D1 operate like a boost converter stage. The LTC3412EFE (U1) buck converter's internal top and bottom MOSFET switches function as a typical buck converter. Inductor L1 stores energy when Q1 and U1's internal top MOSFET switch on during the first portion of the switching cycle. When Q1 and the MOSFET switch off, energy is transferred to the load through the D1 and the internal bottom MOSFET of the buck regulator. This circuit can deliver up to 1.2 A at 3.3 V from a 2.5- to 5.5-V input.
Figure 2 shows the circuit's efficiency for different output-current conditions. Operating at 1 MHz, this converter is designed for a very small footprint suitable for battery-powered, handheld applications. If high efficiency is more important than small circuit size, reduce the operating frequency and use a MOSFET with a lower RDS(ON) and low QG. The operating frequency of the circuit can be determined from:
ROSC =\{3.23 ↔ 1011/f(Hz)\}Ω − 10 kΩ
Unlike typical boost converters, this buck-boost circuit is short-circuit protected. When an output short circuit occurs, the internal MOSFETs are held off until the inductor current falls to a safe level.