Inductorless converter provides high efficiency

Sam Nork, Linear Technology Corp, Milpitas, CA -- EDN, 2/17/2000

Two common methods exist for generating a regulated dc output voltage that is lower than the input voltage. The first approach is to use a low-dropout (LDO) regulator. LDO regulators are small, easy to use, and inexpensive, but all the output current must also flow through the input; hence, they exhibit low efficiency. The second approach is to use an inductor-based switching regulator. Inductor-based switchers can be efficient, but they tend to be more complex, costly, and area-consuming than their LDO-regulator counterparts. A third option retains the simplicity and size of an LDO regulator but enjoys the high efficiency usually reserved for inductor-based circuits. The circuit in Figure 1 uses switched-capacitor techniques to achieve high-efficiency step-down conversion without an inductor.

The circuit produces a regulated 2V output with as much as 100 mA of load-current capability. IC₁, an LTC1503-2, has an input range of 2.4 to 6V, allowing the IC to take power from either a single Li-ion cell or a three-cell NiMH battery. IC₁ uses fractional-conversion techniques to achieve efficiencies typically more than 25% higher than that of an LDO regulator (Figure 2). Internal control circuitry ensures that the device operates with the optimal step-down ratio as the input voltage and load conditions vary. You need only four small ceramic capacitors to make a complete step-down supply. Quiescent current of 25 µA typical and the small MSOP-8 package make the circuit ideal for handheld devices. (DI #2485)