Decoupling Capacitors – What Are They and Why Are They Used?

decoupling capacitors

Introduction

So im not whether or not you have heard of decoupling capacitors. If you have you must be wondering why do you even need them. I mean it feels like they sometimes are a waste of space right?  Well in this email im going to tell you what they are and why they used in circuit design.

decoupling capacitor
Decoupling capacitors on an IC.

What are decoupling capacitors?

So a decoupling or smoothing capacitor is a capacitor that suppressed high-frequency noise in power supply signals. They take tiny voltage ripples, which could otherwise be harmful to delicate ICs, out of the voltage supply. The way to think of it is as a temporary power source which can temporarily provide power to the IC should the voltage drop (which happens quite often when high switching occurs).  The picture below shows decoupling capacitors around a microcontroller IC.

220px-CPU-Beschaltung-mit_MLCC-P1040239-d

What values of capacitance do I require?

So you now know what they are and what their purpose is but now you want to know how to choose the values of the capacitors and how many you should use for your particular project. Typical values used around microcontrollers such as Arduino are 0.1uF and 10uF (polarized capacitor), but for other applications you would need to calculate this value. The equation you may have seen is the one as follows:

i=C(dv/dt)

Which can be simplified to

C=it/V

where C=needed capacitance, i=output current, t=rise time of output, and V=voltage ripple on the bus [1]. A really good article I came across on how to understand and calculate the decoupling capacitance value can be found here at this link explains it in detail:

http://bit.ly/decouplingCapacitors

Placement

So once you have the values and the number of bypass capacitors, you need to know where to place the capacitors. The best place to attach your decoupling caps are right near your IC pins as possible. Layout is critical for quality bypassing.

Capacitor Type

So in selecting the type of capacitor for decoupling. Ceramic is commonly used on PCBs because they are cheap, have a high voltage rating and provide robust performance[2].  You can read about the trade offs between different capacitor types at the link below:

http://bit.ly/capcitormaterial

Summary

So I hope you enjoy this article, and understand what decoupling capacitors are and why they are used. To learn more on PCB design. Check more articles on my website.

www.ArduinoStartups.com

[1] EDN Access 09.01.95 Don't let rules of thumb set decoupling-capacitor value – EDN Staff http://www.edn.com/design/components-and-packaging/4351231/EDN-Access-09-01-95-Don-t-let-rules-of-thumb-set-decoupling-capacitor-value
[2] Choosing and Using Bypass Capacitors – Intersil, Tamara Schmitz and Mike Wong https://www.intersil.com/content/dam/Intersil/documents/an13/an1325.pdf

 

Ritesh
administrator
Ritesh Kanjee has over 7 years in Printed Circuit Board (PCB) design as well in image processing and embedded control. He completed his Masters Degree in Electronic engineering and published a paper for IEEE called Vision-based adaptive Cruise control using Pattern matching (on Google Scholar). His work was implemented in LabVIEW. He works as an Embedded Electronic Engineer in defence research. He has experience in FPGA design with programming in both VHDL and Verilog.