Design Idea:
Inductorless Step-Down DC/DC Converter Generates
High Efficiency 2V Output

by Sam Nork

There are two common methods for generating a regulated DC power supply that has an output voltage lower than the input voltage. The first approach is to use a low dropout linear regulator (LDO). LDOs are small, simple and inexpensive but all of 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 very efficient but they tend to be more complex, costly and area consuming than their LDO counterparts. Now, there is another option that retains the simplicity and small size of an LDO while enjoying the high efficiency previously reserved for inductor-based circuits.

The LTC1503-2 uses switched capacitor techniques to achieve high efficiency step-down conversion without the need for an inductor. The part produces a regulated 2V output with up to 100mA of load current capability. The V_IN range of 2.4V to 6V allows the LTC1503-2 to be powered directly from either a single Li-Ion cell or a 3 cell NiMH battery. Figure 1 illustrates the simplicity of the LTC1503-2; Figure 2 shows its high efficiency.

Figure 1. High efficiency 2V supply requires no inductor

The part uses fractional conversion techniques to achieve efficiencies typically more than 25% higher than that of an LDO. Internal control circuitry ensures that the part operates with the optimal step-down ratio as the input voltage and output load conditions vary. Only four small ceramic capacitors are needed to make a complete step-down supply. Extremely low quiescent current (I_Q = 25µA typical) and the small MSOP-8 package size make this part ideal for handheld applications such as cellular phones and PDAs.