In addition to topology, voltage, and current considerations, there are likely to be other application characteristics that direct the choice of DC-DC IC. (See the section, MOSFET gate capacitance below.) Most DC-DC controllers specify a maximum gate capacitance that they can drive. The switch wants to spend as little time as possible in transition between its on and off states because that is when power loss is greatest. The ability to quickly charge and discharge a MOSFET gate is critical in achieving high-efficiency conversion. DC-DC converters designed for driving external power switches are usually called "controllers." These ICs include drivers for quickly charging and discharging an external MOSFETs' gate capacitance. High-power or high-voltage applications that exceed the abilities of internal-MOSFET devices will need external MOSFET switches. An internal-switch device, if available, is usually preferred, both for overall simplicity and (often) for lower total cost. The load current capability of newer internal MOSFET DC-DC ICs can handle up to 25A (e.g., the MAX8655 and MAX8686). Most such ICs include MOSFETs, but some employ bipolar transistors. Many DC-DC load requirements can be met by DC-DC converter ICs that include integral power switches. It is said to be "in dropout" when that happens. Similarly, a buck converter cannot provide the desired output when the input voltage is less than that output. Note that a boost DC-DC converter output will rise with input voltage when the input exceeds what has been set for the output voltage. Lastly, if the output voltage is negative, an inverting topology is used. If the input voltage ranges above and below the output voltage, a buck-boost converter or a SEPIC converter is needed. If the input voltage is less than the output voltage, choose a boost (i.e., step-up) configuration. If the input voltage is greater than the output voltage, choose a buck (i.e., step-down) topology. The desired DC-DC topology will narrow this choice. Once the initial specs of a DC-DC design are selected (e.g., input voltage range, output voltage, output current), the first step is to select a converter IC. It is the result of the author's failures and successes with scores of power-supply circuits. This article fills in information gaps for a first DC-DC power supply design. ![]() Less information is available to guide the overall design of integrated-circuit-based DC-DC converters from start to finish. Data sheets for DC-DC converters give specific formulas and some layout information. ![]() Engineering textbooks discuss control theory, loop compensation, and other highly detailed analytical methods. There are few comprehensive sources of DC-DC design information. In addition, they are sensitive to board layout and component parasitics (i.e., the characteristics of a component that are not ideal, such as resistance in a capacitor or capacitance in a MOSFET switch). Even when simplified by highly integrated ICs, they still require extensive component calculations and thoughtful selection of the controller IC. That may sound like a gloomy assessment, but it nevertheless reflects the realities of switch-mode power-supply design.
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