Connect with us

×

ARTICLES

DESIGN NEWS

PROJECTS

TECHNICAL ARTICLES

INDUSTRY ARTICLES Coming Soon

BY
CATEGORY

Forum

ALL FORUMS

CIRCUITS & PROJECTS

EMBEDDED & MICRO

MATH & SCIENCE

COMMUNITY

Education

Textbooks

Video Lectures

Worksheets

Webinars Coming
Soon

By
Category

IoT

ARDUINO

Power

Log In/Sign Up

Log
In

Sign
Up

Sign
in with

Member
Benefits

Log
In

Sign Up

Sign in with

Member
Benefits

Home

Technical Articles

Clean Power for Every IC, Part 2: Choosing and Using Your
Bypass Capacitors

Related Content

Clean Power for Every IC, Part 2: Choosing and
Using Your Bypass Capacitors

September 27, 2015
by Robert Keim

Proper component selection and careful PCB
layout are integral to power supply bypassing.

Articles That Provide Supporting Information

AC Capacitor Circuits

Capacitor Quirks

Previous Article in This Series

Clean Power for Every IC, Part 1: Understanding Bypass
Capacitors

Capacitance: How Much Is Enough?

At the end of the previous article, we introduced the idea that a particular
capacitor’s performance as part of a power-supply bypass network is dependent
upon two of its nonideal characteristics, namely equivalent series
resistance (ESR) and equivalent series inductance (ESL). Actually, it turns out
that the part’s exact capacitance is not particularly important in the context
of power-supply bypassing. This is why IC manufacturers can confidently offer
the same recommendation—“0.1 µF ceramic capacitor at each power pin”—for a wide
variety of analog and digital ICs. Why is the capacitance of relatively minor
importance? Well, recall that capacitance is simply the ratio of charge stored
on the capacitor’s plates to the voltage across the capacitor:

So capacitance tells you how much charge the capacitor can store per volt
across the capacitor. If a fully charged 10 µF and 0.1 µF capacitor are in
parallel between ground and a 5 V power rail, the larger capacitor has 50 ×
10-6 coulombs of charge (10 × 10-6 coulombs per
volt) and the smaller one has 0.5 × 10-6 coulombs (0.1 ×
10-6 coulombs per volt).

How much charge is this in relation to a power-supply bypassing application?
Let’s take a look: Current (in amperes) is defined as the amount of charge (in
coulombs) flowing through a conductor per unit time (seconds). Another way to
express this is with a derivative:

Current, then, is the rate of change of charge with respect to time. This
means that if we integrate current with respect to time, we have total
charge:

Now, let’s go back to the simulated power-supply disturbances discussed in
the previous article. The following current disturbance is generated in the
circuit with 8 inverters and 1 nH of parasitic inductance in series with the
power-supply source resistance:

LTSpice doesn’t give us the actual integration, but we can calculate it by
multiplying the average current (26.3 µA) by the interval (114 µs - 98 µs = 16
µs). Thus, the total charge required to compensate for this disturbance is 26.3
µA × 16 µs = 4.2 × 10-10 coulombs. This is about 1000 times
less charge than we have stored in our 0.1 µF capacitor.

This simulation is highly simplified—the amount of charge required would
depend on the number of inverters in the IC, the electrical characteristics of
the transistors, and so forth. Nonetheless, we can still conclude based on these
calculations that a single 0.1 µF capacitor can store much more charge than is
required to compensate for high-frequency current spikes generated by digital
switching behavior. And this, in turn, demonstrates why the exact capacitance of
a bypass capacitor is not particularly important: as long as the capacitor can
store enough charge, the capacitance value is acceptable. It turns out that 0.1
µF is a convenient value, but 1 µF or even 0.01 µF might be equally appropriate
in terms of capacitance.

So now we have another question to confront: Clearly a 10 µF capacitor would
provide more than enough charge storage for bypassing
requirements, so why bother with the 0.1 µF cap? This brings us back to our
discussion of ESR and ESL.

The Secret Life of a Capacitor

As the following equivalent circuit demonstrates, there is a lot more going
on inside a capacitor than mere capacitance:

For this discussion we don’t need to worry about Rpar (which
accounts for leakage current through the dielectric) or RDA and
CDA (which together account for dielectric absorption). Thus, we
have this simplified equivalent circuit:

The problem here should be readily apparent. Our bypass capacitor is intended
to rapidly supply current during transient disturbances on the power line, yet
now we have two components that impede the flow of current: a
resistor, which presents a fixed impedance regardless of frequency, and an
inductor, which presents higher impedance as frequency increases. At this
juncture it is important to understand that ESR and ESL are determined primarily
by a capacitor’s “type” (ceramic, tantalum, polymer, etc.) and package. Ceramic
caps are the most popular for bypassing because they exhibit low ESR and ESL
(they are also inexpensive). Next in line are tantalums; these offer moderate
ESR and ESL along with high capacitance-to-volume ratio, and thus they are used
for higher-value bypass capacitors intended to compensate for lower-frequency
variations in the power line. For both ceramic and tantalum caps, larger
packages generally correspond to higher ESL. The following table, taken from a
technical report published by AVX Corporation, lists ESL values for different
surface-mount packages:

Case Size

Inductance (pH)

0603 (ceramic)

850

0805 (ceramic)

1050

1206 (ceramic)

1250

1210 (ceramic)

1020

0805 (tantalum)

1600

1206 (tantalum)

2200

1210 (tantalum)

2250

2312 (tantalum)

2800

Incorporating ESR considerations into the design process is fairly
straightforward: small-value capacitors intended to deal with high-frequency
power-line noise must have low ESR. The ESL factor, though, is somewhat more
complicated. The following plot shows the impedance of a 0.1 µF, 0603 ceramic
capacitor with 850 pH of ESL and 50 mΩ of ESR:

As discussed in the previous article, a bypass capacitor should provide a
low-impedance path that allows high-frequency noise to “pass by” the IC on its
way to the circuit’s ground node. An ideal capacitor would accomplish this
easily, since a capacitor’s impedance decreases with increasing frequency. But
the above plot tells a different story: at a certain frequency the ESL starts to
dominate the capacitance, such that the impedance actually begins to
rise with frequency. Now let’s imagine that instead of the ceramic
cap above we decided to use a 1 µF tantalum capacitor with 2200 pH of ESL and
1.5 Ω of ESR:

The impedance of the tantalum starts out lower than that of the ceramic
because of its higher capacitance, but the effect of higher ESR and ESL causes
the impedance to level out at around 100 kHz, such that the impedance of the
ceramic is actually 10 times lower than that of the tantalum at 10
MHz. So, if the circuit is susceptible to noise at frequencies around 10 MHz,
the ceramic capacitor would be far more effective than the tantalum capacitor,
even though the tantalum has higher capacitance. Furthermore, if we are dealing
with very high noise frequencies, even this ceramic cap might present too much
impedance. In such a case, we would need lower ESL, which means a smaller
package. This plot compares the original 0603 cap with a 0.01 µF ceramic with
only 500 pH of ESL (a value that might be achievable with the 0402 package):

At first glance it seems like we can’t win: the 0402 cap improves the
high-frequency performance, but the impedance is worse than that of the 0603
from low frequency all the way to 50 MHz. We can win, though: we
can put all three of these capacitors in parallel, and at any particular
frequency the overall impedance will be determined by the lowest impedance among
the three.

So now we have a bypass network that maintains relatively low impedance over
a very wide range of frequencies. The only surprise here is the peak at 50 MHz
where the overall impedance is higher than the individual impedances. This is
referred to as an anti-resonance peak, and you need to watch out for these
wherever a decreasing (i.e., capacitance-dominant) impedance intersects with an
increasing (i.e., inductance-dominant) impedance.

Don’t Ruin a Good Design with a Bad Layout

Proper PCB layout is a critical aspect of bypass design—for example, engineers at Texas Instruments found that extending the
distance between a 0.1 µF cap and the IC’s power pin from 0.3 inches to 1 inch
increased the power-line ringing amplitude from 250 mV to 600 mV. Fortunately,
the rules for laying out bypass capacitors are simple: minimize resistance,
minimize inductance. This is accomplished by placing the capacitor as close to
the power pin as possible and using the shortest possible traces for all
connections. Ideally, both the ground and the power rail can be accessed through
vias to planes:

Bypass Cap Recap

We now have enough information to formulate a concise set of guidelines for
successful bypassing:

When in doubt, give each power pin a 0.1 µF ceramic cap, preferably size
0805 or smaller, in parallel with a 10 µF tantalum or ceramic.

You can probably omit the 10 µF cap, or replace it with something smaller,
if you are concerned only about high-frequency noise.

If you need to compensate for long-term supply deviations that will
require large amounts of stored charge, you may need to give each IC an
additional larger capacitor, say 47 µF.

If your design includes very high frequencies or particularly sensitive
circuitry, use a simulator to analyze the AC response of your bypass network.
(It might be difficult to find solid specs for ESR and ESL, especially
considering that capacitor ESR can vary significantly with frequency—just do
the best you can.) If necessary, include lower-ESL ceramic caps to improve the
high-frequency impedance characteristics.

Locate the high-frequency ceramic caps as close to the power pin as
possible, and use short traces and vias to minimize parasitic inductance and
resistance. The location of larger capacitors intended for low-frequency
bypassing is not quite as critical, but these also should be close to the
IC—within a half-inch or so.

Next Article in Series: Clean
Power for Every IC, Part 3: Understanding Ferrite Beads

Tags:

types
of capacitor System
Noise decoupling
bypass
capacitor Signal
and Power Integrity equivalent
series resistance equivalent
series inductance

Share

Share

Share

Share

Recommended Articles

Intro to Magnetic Devices and the Science Behind
Them

Magnetic devices shape our world: they power our homes,
tools, toys, and they even store our data. Quantum physics provides insights
needed to understand magnetism and its relationship to the...

Marie
Christiano

July 07, 2015

Clean Power for Every IC, Part 3: Understanding
Ferrite Beads

Ferrite beads used in conjunction with bypass capacitors
can provide improved power-supply filtering and decoupling.

Robert
Keim

3
days ago

Measuring Resistance, In Circuit and Out

This article explains how to measure a resistance value,
even if the resistor cannot be removed from its circuit.

Robert
Keim

June 21, 2015

Clean Power for Every IC, Part 1: Understanding Bypass
Capacitors

A thorough understanding of bypass capacitors will help you
to properly incorporate these critical components into your designs.

Robert
Keim

September 21, 2015

Superposition and What it Means For You

This article describes superposition and its simple
applications.

Tristan
Cornell

May 11, 2015

Comments

1 CommentLogin

Please login before you can comment.

roderick young 2015-09-30

Great article, thanks!  Another choice for capacitor position would be
on the other side of the board, directly under the IC, if that’s an
option.

Reply

Please login before you can reply to this comment.

Close

Other Content

Beyond the DMM: Components and Circuits for Measuring
Current and Voltage
Technical Article

High Temperature Sensors Take the Heat
News

Capacitor Types
Textbook

Specific Resistance of Conductors
Worksheet

← Articles Index

WHO WE ARE

More
about us

Content

News

Projects

Technical
Articles

Textbook

Downloads

Video
Lectures

Worksheets

Forum

Categories

Latest

Arduino

Power

Embedded

IoT

Sensors

Digital
ICs

Test &
Measurement

Connect with Us

Facebook

Twitter

Contact Us

YouTube

LinkedIn

Sign Up

Register

© EETech Media, LLC. All
rights reserved

Privacy
Policy · Terms of
Service · User
Agreement