Building a DC-DC power supply that works.pdf

Maxim App Notes Power-supply circuits Prototyping and PC-board layout Keywords: DC-DC, power supply, DC-DC converter, MOSFET, MOSFET gate capacitance, load and line regulation capacitance, total gate Sep 22, 2010 charge, PWM, PFM, DC resistance, DCR, load regulation APPLICATION NOTE 1897 Building a DC-DC power supply that works Abstract: This article discusses important considerations when designing a DC-DC power supply. Topics include choosing the right DC-DC converter for the application; MOSFET gate capacitance; high switching frequencies and component size; equations and calculations; selecting peripheral components; component placement and trade-offs; grounding; load and line regulation; temperature sensitivity. Also see: Application note 2031, DC-DC Converter Tutorial Power Supply Cookbook Introduction A designers first DC-DC converter circuit generally has one thing in common with first attempts in any other field: it has a slim chance of working satisfactorily when first powered. That may sound like a gloomy assessment, but it nevertheless reflects the realities of switch-mode power-supply design. DC-DC converters are complex systems. Even when simplified by highly integrated ICs, they still require extensive component calculations and thoughtful selection of the controller IC. 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). There are few comprehensive sources of DC-DC design information. Engineering textbooks discuss control theory, loop compensation, and other highly detailed analytical methods. Data sheets for DC-DC converters give specific formulas and some layout information. Less information is available to guide the overall design of integrated-circuit-based DC-DC converters from start to finish. This article fills in information gaps for a