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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 3740
Keywords: step-down, buck, transformer, flyback, SEPIC
APPLICATION NOTE 3740
How to Generate Auxiliary Supplies from a
Positive Buck DC-DC Converter
Mar 23, 2006
Abstract: Many applications require a low-power supply in addition to the main supply. For reasons of
cost, inventory management, or electromagnetic compatibility (EMC), a separate converter may not be
appropriate. Consequently, another means of providing extra power rails from the main supply is needed.
This application note shows how to use a step-down IC converter's switching action to derive one or
more outputs, isolated or non-isolated, quasi-regulated or unregulated.
Introduction
Many applications require a low-power supply in addition to the main supply. A typical example is when
an analog front-end amplifier needs ±5V, while the main digital circuitry requires +5V only. For reasons
of cost, inventory management, or EMC, a separate -5V converter may not be appropriate.
Consequently, another means of providing extra power rails from the main supply is needed.
As a solution to this problem, a step-down IC converter's switching action can be used to derive one or
more outputs, isolated or non-isolated, quasi-regulated or unregulated. Auxiliary output currents of 10%
to 30% of the main output are perfectly possible. This application note will illustrate this technique using
the MAX5035 DC-DC converter.
Step-Down Waveforms
A review of the waveforms found in a working step-down converter will identify the voltage and currents
that can be used to generate additional outputs. See Figure 1 below and Example 1 waveforms at the
end of this article.
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