Simple Stepdown Regulator Generates Multiple Voltage Rails With Good Efficiency and Regulation

by Vijay Choudhary, National Semiconductor, Phoenix, Ariz, How2Power Today, May 27 2011

Focus:
Among the various ways of generating multiple voltage rails, one approach uses a synchronous buck regulator together with coupled inductors to generate auxiliary outputs. This approach is simple and can provide both isolated and nonisolated outputs. However, the efficiency degrades at lower values of secondary output voltage because of diode rectification. In addition, the flyback action of the diode causes poor regulation at no load. This article describes an approach employing coupled inductors together with a constant-on-time (COT) buck regulator with low-side gate drive (the LM5006). This design uses synchronous rectification for both rails resulting in better efficiency and regulation. And since the COT topology does not need compensation, less design effort is needed. Here the example of a dual-output buck converter is presented along with plots of its regulation and efficiency. But this technique can be extended to generate any number of additional voltage rails. The article explains circuit operation, limits on secondary output voltage range, and a cost-saving circuit modification.

What you’ll learn:

• How to build a simple, multi-output buck converter with high efficiency and good regulation

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