Choosing and Using Bypass Capacitors

Posted Nov 24th 2010
decoupling capacitor

Bypass capacitors are found in every working piece of electronic equipment. Most engineers know that systems, circuits, and individual chips need to be bypassed. The methods for choosing bypass capacitors typically follow decisions of tradition instead of optimizing for any particular circuit. This application note aims to bring the design aspect back to this seemingly simple component. After discussing the motivation for using bypass caps, we form a vocabulary around the basics; equivalent circuit, dielectrics and types of available capacitors. The next step is identifying the primary function and environment of the bypass capacitor. Circuits that exhibit large current spikes have different bypassing needs than ones that solely operate at high frequencies. A few special options are discussed, like scaling multiple bypass capacitors, as well as the importance of board layout. Finally, we present four application examples. These represent circuits with high and low currents as well as those with high and low frequencies.

The application note answer’s these three questions.

  • What size of capacitor do we need?
  • Where do we place the capacitors?
  • What type of capacitor will work best?
  • What type of capacitor should I use?

This application note covers:

  • Capacitor Basics
    • Dielectrics
    • Equivalent Circuit Model
  • Common Types of Capacitors and Trade-offs
  • Selecting the Package for the Bypass Capacitor
    • Sizing Bypass Capacitors
    • Bypassing a System with Wide Bandwidth
    • Bypass Capacitor Application Examples
    • Example 1 (Low Current/Low Frequency):Real-Time Clock-Calendar With EEPROM
    • Example 2 (Low Current/High Frequency):Voltage Reference
    • Example 3 (High Current/Low Frequency DC/DC Converter
    • Example 4 (High Current/High Frequency): DSL Amplifier
Download PDF: choosing-bypass-capacitors.pdf

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