DC-DC Power Supply 

A designer's first DC-DC converter circuit generally has one thing in common with other first attempts: It doesn't work! That may sound like a gloomy assessment, but it reflects the realities of design. DC-DC converters aren't easy. They require extensive component-value calculations and thoughtful selection of the controller IC. They are very sensitive to board layout and component parasitic. And there are few comprehensive sources of design information. Engineering textbooks discuss control theory, loop compensation, and other highly detailed analysis methods. Data sheets for DC-DC converters give specific formulas and some layout information. But little information is available to guide the design of integrated-circuit DC-DC converters from start to finish. This article provides complete information for a first DC-DC power supply design. It is the result of success and failure by the author on dozens of power-supply designs. Each topic is covered briefly, and is intended as the launching point for further research.

A proper choice of DC-DC converter IC depends primarily on the design objectives. If the input voltage is always greater than the output voltage, choose a buck-converter topology. If the input voltage is always less than the output voltage, choose a boost configuration. If the input voltage ranges above and below the output voltage, either the buck or boost converter can be used, but it must be adapted to the non-standard input voltage range. In general, output currents below 2A can be accommodated by DC-DC converter ICs with integral power switches. Most modern ICs include MOSFETs as their internal power switches, but some include bipolar transistors. Switches internal to some of the newer ICs can handle up to 5A, and that capability should continue to increase. A device with internal switches is usually a better choice, both for overall simplicity and (often) for lower total cost. DC-DC converters designed for driving external power switches are usually called controllers. They include drivers capable of delivering up to 2° (briefly) for charging the gate capacitance of external MOSFETs. The ability to quickly charge and discharge a MOSFET gate is critical in achieving high-efficiency conversion, and most DC-DC controllers specify a maximum for the gate capacitance they can drive. (See "MOSFET Gate Capacitance.")

MOSFET manufacturers provide many capacitance and switching parameters on their datasheets, but for DC-DC converters, the total gate charge (QG) is of primary interest in most cases. Choose MOSFETs for which QG is within the range recommended by the manufacturer of the DC-DC converter in question. Use the typical value for QG in most cases. The maximum value can be ignored. 
Another switching parameter of interest is the reverse transfer capacitance (CRSS). CRSS is used in calculating switching loss in the high-side n-Channel MOSFET of a buck converter, per the following equation:

where IGATE is the peak gate-source current and sink current, and fSW is the switching frequency. 
The most popular control scheme is pulse-width modulation (PWM). PWM converters maintain a constant switching frequency over a broad range of loads. That behavior is important, because switching noise from a DC-DC converter can interfere with other processes in a system. 
Confining the noise to a known band elimina tes most of those problems. The next most common control scheme is pulse-frequency modulation (PFM). Simplicity of implementation made PFM popular with some of the earliest DC-DC converters, and it is still used in some cases. PFM converters excel in applications that require low quiescent current. The switching frequencies of available DC-DC converters range from 20kHz to over 1MHz. 
You should avoid devices that operate below 100kHz; such frequencies are typical of old devices with poor efficiency. In general, higher switching frequency allows external components that are smaller in both size and value. (See "High Switching Frequency Reduces Component Size.") If smallest possible size is important in the application, then look for converters with switching frequencies at 1MHz and above. Otherwise, simply choose a device that meets the main criteria specified, and verify that its switching frequency does not interfere with other components in the system.