Minimizing Switching Regulator Residue in Linear Regulator Outputs (Linear Technology) - 9
| Authors | Jim Williams |
| Manufacturer | Linear Technology |
| Description | Application Note 101. Linear regulators are commonly employed to post-regulate switching regulator outputs. Benefits include improved stability, accuracy, transient response and lowered output impedance. Ideally, these performance gains would be accompanied by markedly reduced switching regulator generated ripple and spikes. In practice, all linear regulators encounter some difficulty with ripple and spikes, particularly as frequency rises. This publication explains the causes of linear regulators' dynamic limitations and presents board level techniques for improving ripple and spike rejection. A hardware based ripple/spike simulator is presented, enabling rapid breadboard testing under various conditions. Three appendices review ferrite beads, inductor based filters and probing practice for wideband, sub-millivolt signals. |
| Pages / Page | 12 / 9 — APPENDIX A. About Ferrite Beads. Figure A1. Impedance vs. Frequency at … |
| File Format / Size | PDF / 358 Kb |
| Document Language | English |
APPENDIX A. About Ferrite Beads. Figure A1. Impedance vs. Frequency at Various DC Bias Currents
Text Version of Document
Application Note 101
APPENDIX A About Ferrite Beads
60 A ferrite bead enclosed conductor provides the highly desir- 0A 50 0.1A able property of increasing impedance as frequency rises. 0.2A This effect is ideally suited to high frequency noise fi lter- ) Ω 40 ing of DC and low frequency signal carrying conductors. 0.5A 30 The bead is essentially lossless within a linear regulator’s passband. At higher frequencies the bead’s ferrite material IMPEDENCE ( 20 interacts with the conductors magnetic fi eld, creating the loss characteristic. Various ferrite materials and geometries 10 result in different loss factors versus frequency and power 0 level. Figure A1’s plot shows this. Impedance rises from 1 10 100 1000 FREQUENCY (MHz) 0.01Ω at DC to 50Ω at 100MHz. As DC current, and hence DC = 0.01Ω AN101 FA1 constant magnetic fi eld bias, rises, the ferrite becomes less effective in offering loss. Note that beads can be “stacked”
Figure A1. Impedance vs. Frequency at Various DC Bias Currents
in series along a conductor, proportionally increasing their
for a Surface Mounted Ferrite Bead (Fair-Rite 2518065007Y6).
loss contribution. A wide variety of bead materials and
Impedance is Essentially Zero at DC and Low Frequency, Rising Above 50
Ω
Depending on Frequency and DC Current. Source:
physical confi gurations are available to suit requirements
Fair-Rite 2518065007Y6 Datasheet.
in standard and custom products.
APPENDIX B
unwanted high frequency feedthrough. The inductors circuit board position may allow stray magnetic fi elds to
Inductors as High Frequency Filters
impinge its winding, effectively turning it into a transformer Inductors can sometimes be used for high frequency fi lter- secondary. The resulting observed spike and ripple related ing instead of beads. Typically, values of 2µH to10µH are artifacts masquerade as conducted components, degrad- appropriate. Advantages include wide availability and better ing performance. effectiveness at lower frequencies, e.g., ≤100kHz. Figure Figure B2 shows a form of inductance based fi lter con- B1 shows disadvantages are increased DC resistance in structed from PC board trace. Such extended length traces, the regulator path due to copper losses, parasitic shunt formed in spiral or serpentine patterns, look inductive at capacitance and potential susceptibility to stray switch- high frequency. They can be surprisingly effective in some ing regulator radiation. The copper loss appears at DC, circumstances, although introducing much less loss per reducing effi ciency; parasitic shunt capacitance allows unit area than ferrite beads. STRAY MAGNETIC TERMINAL ACCESSABLE WITH PC VIA. PARASITIC FIELD CAPACITANCE USER USER TERMINAL TERMINAL PARASITIC PARASITIC AN101 FB2 RESISTANCE RESISTANCE AN101 FB1
Figure B1. Some Parasitic Terms of an Inductor. Parasitic Figure B2. Spiral and Serpentine PC Patterns are Sometimes Resistance Drops Voltage, Degrading Effi ciency. Unwanted Used as High Frequency Filters, Although Less Effective Than Capacitance Permits High Frequency Feedthrough. Stray Ferrite Beads Magnetic Field Induces Erroneous Inductor Current
an101f AN101-9
APPENDIX A About Ferrite Beads
60 A ferrite bead enclosed conductor provides the highly desir- 0A 50 0.1A able property of increasing impedance as frequency rises. 0.2A This effect is ideally suited to high frequency noise fi lter- ) Ω 40 ing of DC and low frequency signal carrying conductors. 0.5A 30 The bead is essentially lossless within a linear regulator’s passband. At higher frequencies the bead’s ferrite material IMPEDENCE ( 20 interacts with the conductors magnetic fi eld, creating the loss characteristic. Various ferrite materials and geometries 10 result in different loss factors versus frequency and power 0 level. Figure A1’s plot shows this. Impedance rises from 1 10 100 1000 FREQUENCY (MHz) 0.01Ω at DC to 50Ω at 100MHz. As DC current, and hence DC = 0.01Ω AN101 FA1 constant magnetic fi eld bias, rises, the ferrite becomes less effective in offering loss. Note that beads can be “stacked”
Figure A1. Impedance vs. Frequency at Various DC Bias Currents
in series along a conductor, proportionally increasing their
for a Surface Mounted Ferrite Bead (Fair-Rite 2518065007Y6).
loss contribution. A wide variety of bead materials and
Impedance is Essentially Zero at DC and Low Frequency, Rising Above 50
Ω
Depending on Frequency and DC Current. Source:
physical confi gurations are available to suit requirements
Fair-Rite 2518065007Y6 Datasheet.
in standard and custom products.
APPENDIX B
unwanted high frequency feedthrough. The inductors circuit board position may allow stray magnetic fi elds to
Inductors as High Frequency Filters
impinge its winding, effectively turning it into a transformer Inductors can sometimes be used for high frequency fi lter- secondary. The resulting observed spike and ripple related ing instead of beads. Typically, values of 2µH to10µH are artifacts masquerade as conducted components, degrad- appropriate. Advantages include wide availability and better ing performance. effectiveness at lower frequencies, e.g., ≤100kHz. Figure Figure B2 shows a form of inductance based fi lter con- B1 shows disadvantages are increased DC resistance in structed from PC board trace. Such extended length traces, the regulator path due to copper losses, parasitic shunt formed in spiral or serpentine patterns, look inductive at capacitance and potential susceptibility to stray switch- high frequency. They can be surprisingly effective in some ing regulator radiation. The copper loss appears at DC, circumstances, although introducing much less loss per reducing effi ciency; parasitic shunt capacitance allows unit area than ferrite beads. STRAY MAGNETIC TERMINAL ACCESSABLE WITH PC VIA. PARASITIC FIELD CAPACITANCE USER USER TERMINAL TERMINAL PARASITIC PARASITIC AN101 FB2 RESISTANCE RESISTANCE AN101 FB1
Figure B1. Some Parasitic Terms of an Inductor. Parasitic Figure B2. Spiral and Serpentine PC Patterns are Sometimes Resistance Drops Voltage, Degrading Effi ciency. Unwanted Used as High Frequency Filters, Although Less Effective Than Capacitance Permits High Frequency Feedthrough. Stray Ferrite Beads Magnetic Field Induces Erroneous Inductor Current
an101f AN101-9
