© Application Notes | May 29, 2013

Appendix B

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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APPENDIX B - Inductors as High Frequency Filters

Inductors can sometimes be used for high frequency filtering instead of beads. Typically, values of 2µH to10µH are appropriate. Advantages include wide availability and better effectiveness at lower frequencies, e.g., ≤100kHz. Figure B1 shows disadvantages are increased DC resistance in the regulator path due to copper losses, parasitic shunt capacitance and potential susceptibility to stray switching regulator radiation.

The copper loss appears at DC, reducing effi ciency; parasitic shunt capacitance allows unwanted high frequency feedthrough. The inductors circuit board position may allow stray magnetic fields to impinge its winding, effectively turning it into a transformer secondary. The resulting observed spike and ripple related artifacts masquerade as conducted components, degrading performance.

Figure B2 shows a form of inductance based filter constructed from PC board trace. Such extended length traces, formed in spiral or serpentine patterns, look inductive at high frequency. They can be surprisingly effective in some circumstances, although introducing much less loss per unit area than ferrite beads.

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