Today's feature-laden portable electronics devices, typically running off a single Lithium-ion cell, often use one or more step-down DC/DC converters to power the core processor or other key functions. These fast transient, compact, power management ICs offer significantly higher efficiency than comparable linear alternatives. Moreover, as portable system designers have grappled with rapidly shrinking product footprints, power semiconductor designers have migrated to step-down converters using higher and higher switching frequencies to allow designers to take advantage of smaller external inductors and ceramic capacitors.
The move to step-down converters using higher switching frequencies has generated another problem for system designers however. If it isn't filtered, DC/DC converter input ripple and noise can reach levels high enough to interfere with other devices powered from the same source. Fortunately, a number of relatively simple methods are available to reduce input noise and its impact on other devices. This article will describe these sources of input noise and ripple and some basic methods to attenuate its occurrence.
Two noise sources
Input noise in a step-down DC/DC converter has two components. The first occurs at the fundamental switching frequency commonly referred to as ripple. The second noise component is associated with the very high frequency ringing that occurs during switching transitions. Figure 1 below shows a typical input ripple and noise waveform for a buck converter with both the saw-tooth ripple and high frequency ringing components.
Figure 1: Typical Input Ripple and Noise Waveform for Buck Converter