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Appendix A

Linear regulators are commonly employed to post-regulate switching regulator outputs. Benefits include improved stability, accuracy, transient response and lowered output impedance.

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Linear Technology - AN 101

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APPENDIX A - About Ferrite Beads A ferrite bead enclosed conductor provides the highly desirable property of increasing impedance as frequency rises. This effect is ideally suited to high frequency noise fi ltering of DC and low frequency signal carrying conductors. The bead is essentially lossless within a linear regulator’s passband. At higher frequencies the bead’s ferrite material interacts with the conductors magnetic fi eld, creating the loss characteristic.
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Various ferrite materials and geometries result in different loss factors versus frequency and power level. Figure A1’s plot shows this. Impedance rises from 0.01Ω at DC to 50Ω at 100MHz. As DC current, and hence constant magnetic fi eld bias, rises, the ferrite becomes less effective in offering loss. Note that beads can be “stacked” in series along a conductor, proportionally increasing their loss contribution. A wide variety of bead materials and physical confi gurations are available to suit requirements in standard and custom products.
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