National Semiconductor has a good online tutorial on using and designing voltage regulator circuits based on their chips. It can be found at http://www.national.com/appinfo/power/webench. The types of regulators we will look at are termed DC-DC converters. They take an unregulated DC voltage (often over a range of possible voltages) and provide a constant DC voltage output of a fixed value. There are three types of DC-DC converters: linear regulators, which produce lower voltages than the supply voltage; switching regulators that can step up (boost), step down (buck), or invert the input voltage; and charge pumps, which can also step up, step down, or invert the supply voltage, but with limited current drive capability. (Note that not all charge pumps provide regulated voltage.) The conversion process of any regulator is not 100% efficient. The regulator itself uses current (known as >quiescent current), and this is sourced from the input supply. The greater the quiescent current, the more power (and therefore heat) the regulator must dissipate. In choosing a regulator, select one that can supply the appropriate output voltage and the required current needed by your embedded system, yet has the lowest quiescent current. Linear regulators are small, cheap, low-noise, and very easy to use. The basic circuit for a linear regulator is shown in Figure 5-2. The inputs and outputs are filtered using decoupling capacitors, but beyond that, no other external components are needed.