Minimizing Switching Regulator Residue in Linear Regulator Outputs (Linear Technology)
| 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 / 1 — INTRODUCTION. Switching Regulator AC Output Content. Figure 2. Switching … |
| File Format / Size | PDF / 358 Kb |
| Document Language | English |
INTRODUCTION. Switching Regulator AC Output Content. Figure 2. Switching Regulator Output Contains Relitively Low
Text Version of Document
Application Note 101 July 2005 Minimizing Switching Regulator Residue in Linear Regulator Outputs Banishing Those Accursed Spikes Jim Williams
INTRODUCTION
include noise reduction and minimization of residual input- derived artifacts appearing at the regulators output. It is Linear regulators are commonly employed to post-regulate this last category–residual input-derived artifacts–that is of switching regulator outputs. Benefi ts include improved concern. These high frequency components, even though stability, accuracy, transient response and lowered output small amplitude, can cause problems in noise-sensitive impedance. Ideally, these performance gains would be video, communication and other types of circuitry. Large accompanied by markedly reduced switching regulator numbers of capacitors and aspirin have been expended in generated ripple and spikes. In practice, all linear regulators attempts to eliminate these undesired signals and their re- encounter some diffi culty with ripple and spikes, particu- sultant effects. Although they are stubborn and sometimes larly as frequency rises. This effect is magnifi ed at small seemingly immune to any treatment, understanding their regulator VIN to VOUT differential voltages; unfortunate, origin and nature is the key to containing them. because such small differentials are desirable to maintain effi ciency. Figure 1 shows a conceptual linear regulator
Switching Regulator AC Output Content
and associated components driven from a switching Figure 2 details switching regulator dynamic (AC) output regulator output. content. It consists of relatively low frequency ripple at the The input fi lter capacitor is intended to smooth the ripple and switching regulator’s clock frequency, typically 100kHz to spikes before they reach the regulator. The output capaci- 3MHz, and very high frequency content “spikes” associ- tor maintains low output impedance at higher frequencies, ated with power switch transition times. The switching improves load transient response and supplies frequency regulator’s pulsed energy delivery creates the ripple. Filter compensation for some regulators. Ancillary purposes capacitors smooth the output, but not completely. The , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. INPUT DC + RIPPLE SWITCHING SPIKES: HARMONIC CONTENT AND SPIKES FROM RIPPLE: TYPICALLY 100kHz to 3MHz APPROACHING 100MHz SWITCHING REGULATOR LINEAR PURE DC IN OUT REGULATOR OUTPUT FILTER GND FILTER CAPACITOR CAPACITOR AN101 F02 AN101 F01
Figure 2. Switching Regulator Output Contains Relitively Low Figure 1. Conceptual Linear Regulator and Its Filter Capacitors Frequency Ripple and High Frequency “Spikes” Derived From Theoretically Reject Switching Regulator Ripple and Spikes Regulators Pulsed Energy Delivery and Fast Transition Times
an101f AN101-1
INTRODUCTION
include noise reduction and minimization of residual input- derived artifacts appearing at the regulators output. It is Linear regulators are commonly employed to post-regulate this last category–residual input-derived artifacts–that is of switching regulator outputs. Benefi ts include improved concern. These high frequency components, even though stability, accuracy, transient response and lowered output small amplitude, can cause problems in noise-sensitive impedance. Ideally, these performance gains would be video, communication and other types of circuitry. Large accompanied by markedly reduced switching regulator numbers of capacitors and aspirin have been expended in generated ripple and spikes. In practice, all linear regulators attempts to eliminate these undesired signals and their re- encounter some diffi culty with ripple and spikes, particu- sultant effects. Although they are stubborn and sometimes larly as frequency rises. This effect is magnifi ed at small seemingly immune to any treatment, understanding their regulator VIN to VOUT differential voltages; unfortunate, origin and nature is the key to containing them. because such small differentials are desirable to maintain effi ciency. Figure 1 shows a conceptual linear regulator
Switching Regulator AC Output Content
and associated components driven from a switching Figure 2 details switching regulator dynamic (AC) output regulator output. content. It consists of relatively low frequency ripple at the The input fi lter capacitor is intended to smooth the ripple and switching regulator’s clock frequency, typically 100kHz to spikes before they reach the regulator. The output capaci- 3MHz, and very high frequency content “spikes” associ- tor maintains low output impedance at higher frequencies, ated with power switch transition times. The switching improves load transient response and supplies frequency regulator’s pulsed energy delivery creates the ripple. Filter compensation for some regulators. Ancillary purposes capacitors smooth the output, but not completely. The , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. INPUT DC + RIPPLE SWITCHING SPIKES: HARMONIC CONTENT AND SPIKES FROM RIPPLE: TYPICALLY 100kHz to 3MHz APPROACHING 100MHz SWITCHING REGULATOR LINEAR PURE DC IN OUT REGULATOR OUTPUT FILTER GND FILTER CAPACITOR CAPACITOR AN101 F02 AN101 F01
Figure 2. Switching Regulator Output Contains Relitively Low Figure 1. Conceptual Linear Regulator and Its Filter Capacitors Frequency Ripple and High Frequency “Spikes” Derived From Theoretically Reject Switching Regulator Ripple and Spikes Regulators Pulsed Energy Delivery and Fast Transition Times
an101f AN101-1
