Understand and reduce DC/DC switching-converter ground noise

Focus:

Dc-dc converters introduce noise onto a circuit’s electrical ground causing operational errors in analog and digital signals in the system and generating EMI. Using the familiar buck converter as an example, this article explains and models two sources of converter-induced ground noise. One source is changing current-loop areas in the power converter, which lead to changing magnetic flux areas, and induced voltages. This problem can be alleviated with careful routing of PCB traces and careful placement of bypass-capacitors. The second source is the parasitic-capacitance of the inductor. Here, voltage changes on the inductor-switch node couple across the buck-inductor’s parasitic capacitance and the buck-filter capacitor, and then across the converter’s parasitic ground inductors, leading to ground noise oscillations. This problem is alleviated by reducing the inductor’s parasitic capacitance either by selecting an inductor with the highest self-resonant frequency or by replacing the single inductor with two series-connected inductors.

What you’ll learn:

• How to reduce switching-converter induced ground noise in dc-dc converters

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Author & Publication:

Jeff Barrow, Senior Director of Analog IC Design, Integrated Device Technology, Inc., EE Times, Nov 01 2011

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