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'[OT] Bypass capacitors'
1999\11\11@230134 by Robert A. LaBudde

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Having found some more information, I thought I'd revive this issue.

See the attached figure showing capacitor impedance vs. frequency.

Consider the power regulation issue again:

If the supply is, say, 5 V, and the ripple is to be kept less than 1% = 50
mV, and the package is going to pull 100 mA, then an impedance of 50 /100 =
0.5 ohm can be tolerated. If only a 10 mA load, then this increases to 5 ohms.

A 1 uF tantalum has a flat impedance curve below 1 ohm from 100 kHz up to
10+ MHz.

A 0.1 uF monolithic ceramic has sharpness (Q) in its impedance curve, so
its impedance is below 1 ohm only between 2 and 10 MHz. It acts as a notch
filter at about 1 MHz.

A 1000 pF poly film cap has < 1 ohm impedance only from 50 MHz to 1 GHz,
and acts as a notch filter at about 100 MHz.

If a PIC circuit operates < 20 MHz, why would you prefer a smaller ceramic
over the 1 uF tantalum, when it's got a better bandwidth on low impedance?
Particularly since at 20 MHz the tantalum is probably lower impedance than
the smaller capacitance ceramic?

If op-amps with frequencies below 1 MHz are driven by the supply, higher
capacitance is a more important factor in impedance than type is.

So I ask again: Why use a small capacitor to bypass power on these types of
circuits? Why isn't the 1 uF tantalum a good solution, albeit a few cents more?

It still seems to me you only need a very large capacitor at the input of
the regulator, with 1-100 uF at the output, and 1 uF tantalums at the chip
level to control switching noise.

As I said before, I'm not an EE, so perhaps I'm missing something in the
spec sheets here.

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Attachment converted: wonderland:capacitor impedance vs freq.gif (GIFf/JVWR) (0000F601)
<x-flowed>================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: spam_OUTralTakeThisOuTspamlcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================
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1999\11\12@030733 by Michael Rigby-Jones

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<P><FONT SIZE=2 FACE="Arial">See the attached figure showing capacitor impedance vs. frequency.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Consider the power regulation issue again:</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">If the supply is, say, 5 V, and the ripple is to be kept less than 1% = 50</FONT>
<BR><FONT SIZE=2 FACE="Arial">mV, and the package is going to pull 100 mA, then an impedance of 50 /100 =</FONT>
<BR><FONT SIZE=2 FACE="Arial">0.5 ohm can be tolerated. If only a 10 mA load, then this increases to 5 ohms.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">A 1 uF tantalum has a flat impedance curve below 1 ohm from 100 kHz up to</FONT>
<BR><FONT SIZE=2 FACE="Arial">10+ MHz.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">A 0.1 uF monolithic ceramic has sharpness (Q) in its impedance curve, so</FONT>
<BR><FONT SIZE=2 FACE="Arial">its impedance is below 1 ohm only between 2 and 10 MHz. It acts as a notch</FONT>
<BR><FONT SIZE=2 FACE="Arial">filter at about 1 MHz.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">A 1000 pF poly film cap has &lt; 1 ohm impedance only from 50 MHz to 1 GHz,</FONT>
<BR><FONT SIZE=2 FACE="Arial">and acts as a notch filter at about 100 MHz.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">If a PIC circuit operates &lt; 20 MHz, why would you prefer a smaller ceramic</FONT>
<BR><FONT SIZE=2 FACE="Arial">over the 1 uF tantalum, when it's got a better bandwidth on low impedance?</FONT>
<BR><FONT SIZE=2 FACE="Arial">Particularly since at 20 MHz the tantalum is probably lower impedance than</FONT>
<BR><FONT SIZE=2 FACE="Arial">the smaller capacitance ceramic?</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">If op-amps with frequencies below 1 MHz are driven by the supply, higher</FONT>
<BR><FONT SIZE=2 FACE="Arial">capacitance is a more important factor in impedance than type is.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">So I ask again: Why use a small capacitor to bypass power on these types of</FONT>
<BR><FONT SIZE=2 FACE="Arial">circuits? Why isn't the 1 uF tantalum a good solution, albeit a few cents more?</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Tantalums have their uses, but in general they aren't nice components.&nbsp; They have very little, if any overload resistance.&nbsp; A small voltage surge exceeding the working voltage will likely cause the device to go short circuit, or at least reduce it's life.&nbsp; If you have any requirements to build a circuit with very high MTBF's then avoid tants.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Electroylitic caps are also large.&nbsp; When board space is important, and you simply don't need a 1uF cap, then why use it?</FONT></P>
<UL>
<P><FONT SIZE=2 FACE="Arial">It still seems to me you only need a very large capacitor at the input of</FONT>
<BR><FONT SIZE=2 FACE="Arial">the regulator, with 1-100 uF at the output, and 1 uF tantalums at the chip</FONT>
<BR><FONT SIZE=2 FACE="Arial">level to control switching noise.</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Reasonably large caps on the input are important, especialy if the regulator is a fair distance from the PSU reservoir caps.&nbsp; Without this cap, the regulator can turn into a pretty good oscillator!</FONT></P>
<UL>
<P><FONT SIZE=2 FACE="Arial">As I said before, I'm not an EE, so perhaps I'm missing something in the</FONT>
<BR><FONT SIZE=2 FACE="Arial">spec sheets here. &lt;&lt; File: capacitor impedance vs freq.gif &gt;&gt;&nbsp; &lt;&lt; File: ATT427318.txt &gt;&gt;</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">A device running at 20MHz (usually) produces harmonics that extend far beyond the base frequency.&nbsp; At 100MHz a tantalum will be as usefull as a chocolate fireguard.&nbsp; I'm guesing you based your figures on leaded components?&nbsp; SMD ceramic caps have a tiny inductance and good for far more than 10MHz</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Regards</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Mike Rigby-Jones</FONT>
</P>
<BR>

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1999\11\12@112053 by Robert A. LaBudde

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<x-flowed>At 08:06 AM 11/12/99 +0000, Mike Rigby-Jones wrote:

>Tantalums have their uses, but in general they aren't nice
>components.  They have very little, if any overload resistance.  A small
>voltage surge exceeding the working voltage will likely cause the device
>to go short circuit, or at least reduce it's life.  If you have any
>requirements to build a circuit with very high MTBF's then avoid tants.

This is a good practicality issue. But doesn't it depend on the likelihood
of such a surge? Perhaps specific types of applications should avoid them.

>Electroylitic caps are also large.  When board space is important, and you
>simply don't need a 1uF cap, then why use it?

A 1 uF tantalum doesn't require more space than a 0.1 uF tantalum or a
ceramic most of the time. This may be a lot different when surface mount
technology is used.


>A device running at 20MHz (usually) produces harmonics that extend far
>beyond the base frequency.  At 100MHz a tantalum will be as usefull as a
>chocolate fireguard.  I'm guesing you based your figures on leaded
>components?  SMD ceramic caps have a tiny inductance and good for far more
>than 10MHz

Again, I'm not an EE, so perhaps I'm misunderstanding this issue. But a
square-wave has odd harmonics with power dropping as 1/f^2. If the
impedance of a capacitor rises proportional to f, then the product still
drops.

The graph I found was from Rohm, who was describing the capacitor product
lines they make. What types of leads were testing, I don't know, but it
wouldn't seem as if they would want to portray their own products poorly.

I follow the arguments on the capacitor trade-offs, but I can't see how
they translate quantitatively into the right down-selection.

If size is the premier issue, then small capacity surface mounts would seem
a good start. But how do you prove that they will meet bypassing requirements?

If over-voltage survival is the key issue, then high voltage capacitors
would seem the starting point. But won't the filter caps on the regulator
blow too? Won't the chip the bypass is protecting blow instead? Where are
these power spikes or surges post-regulation coming from? If we've got
them, doesn't it mean the onboard regulation isn't working?

This simple issue is an opportunity for me to understand how engineering
requirement analysis should be performed to down-select design decisions.
In the past, I've just taking the word of others on what the standard
practices are.

However, I've noticed over the years that the 'standard' has shifted. 25
years ago, you were supposed to put a 0.01 uF ceramic across power on TTL
packages that generated incredible noise and used incredible power. Then
the recommendation shifted to 0.1 uF ceramic or tantalum. Now we should
have a combination of a high value and a small value. People are including
large filter capacitors on the outputs of regulators.

If board space and cost are key issues, why include multiple capacitors? If
package noise and power usage have dropped dramatically, why require more
bypassing than 25 years ago? A 7805 regulator was only good to +/- 5% 25
years ago, but now its +/- 1%. Why do we then need more, not less, bypassing?

Even if I wasn't a 'rocket scientist' I'd realize there's something wrong
with these 'creeping requirements'.



================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam@spam@lcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

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1999\11\12@181538 by Russell McMahon

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<META content=text/html;charset=iso-8859-1 http-equiv=Content-Type><TITLE>RE: [OT] Bypass capacitors</TITLE><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<BLOCKQUOTE
style="BORDER-LEFT: #000000 solid 2px; MARGIN-LEFT: 5px; PADDING-LEFT: 5px">
   <UL>
       <P><FONT face=Arial size=2>So I ask again: Why use a small capacitor to
       bypass power on these types of</FONT> <BR><FONT face=Arial
       size=2>circuits? Why isn't the 1 uF tantalum a good solution, albeit a
       few cents more?</FONT> </P></UL></BLOCKQUOTE>
<P>&nbsp;</P>
<P><FONT color=#000000 size=2>Good analysis.</FONT></P>
<P><FONT color=#000000 size=2>Monolithic Ceramic costs less, won't catch fire,
scream, smell, explode, short (no fun at all :-)).</FONT></P>
<P><FONT color=#000000 size=2>Now, if that was a solid aluminium cap you were
recommending I may agree :-)</FONT></P>
<P><FONT color=#000000 size=2></FONT>&nbsp;</P>
<P>&nbsp;</P>
<P><FONT size=2>RM</FONT></P>
<P><FONT size=2></FONT>&nbsp;</P>
<P>&nbsp;</P>
<P>&nbsp;</P></BODY></HTML>

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1999\11\13@073511 by steve

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> Again, I'm not an EE, so perhaps I'm misunderstanding this issue. But a
> square-wave has odd harmonics with power dropping as 1/f^2. If the
> impedance of a capacitor rises proportional to f, then the product still
> drops.

It's not a square wave but a series of current impulses that occur at
intervals of 1/f, the duration of which are a function of the
rise/fall times. The capacitor has to do its thing when the gates
change from one state to another. This is when the current is drawn
from the capacitor/supply.

> The graph I found was from Rohm, who was describing the capacitor product
> lines they make. What types of leads were testing, I don't know, but it
> wouldn't seem as if they would want to portray their own products poorly.

You are thinking along the right lines in that ceramic caps are
fastest, tantalums quite fast and electrolytics, relatively slow. The
balancing act is in cost and capacity.
A 10nF ceramic cap may suitable for an unloaded PIC and be able to
supply the needed current during transitions. But if you now put a
few loads on the pins (like track capacitance), the little 10nF runs
out of steam very quickly.
You may decide that you require 1uF to cover this load. A 1uF ceramic
cap is still fast but it is also bulky and expensive.
A Tantalum here gives fairly good high frequency response and will
give you the capacity. You could use a wet electrolytic and probably
get more capacitance for the same price. Problem is they are slow and
won't do the switching transitions so you can use the ceramic to take
up that slack.

Rohm have the advantage of their circuit only consisting of a
capacitor. More complex designs require compromise.

> If size is the premier issue, then small capacity surface mounts would seem
> a good start. But how do you prove that they will meet bypassing requirements?

If you bring surface mount vs. through-hole into it, you are adding
another twist. You can safely generalise that surface mount anything
is better from a high frequency viewpoint because leads are inductive
and you want the inductance to be as low as possible.

> If over-voltage survival is the key issue, then high voltage capacitors
> would seem the starting point. But won't the filter caps on the regulator
> blow too? Won't the chip the bypass is protecting blow instead? Where are
> these power spikes or surges post-regulation coming from? If we've got
> them, doesn't it mean the onboard regulation isn't working?

The main problem is the failure mode. Tantalums fail short circuit
with a great deal of heat. They sit there burning with a hot flame. So
when they do go, they take out the rest of your circuit and make a
mess of your board. Other types of electrolytics have some degree of
self healing although they will go bang if you hit them hard enough.

It's back to compromise and cost-effectiveness. Larger capacitors
cost more and the unwanted parameters (inductance, resistance,
physical size) also go up. Like everything else, we can only reduce
the probability and severity of external influences like surges. Do
you design to survive lightning 10 miles away, 1 mile or a direct
hit ?

> This simple issue is an opportunity for me to understand how engineering
> requirement analysis should be performed to down-select design decisions.
> In the past, I've just taking the word of others on what the standard
> practices are.

You can go through the calculations yourself if you like. It's
similar to selecting a value for a pullup resistor. You can calculate
till you are blue in the face and come to the conclusion that
absolute values are relatively unimportant but that this parameter
is more important than that one and a particular value will suffice
in most instances

{Quote hidden}

CMOS switches from rail to rail very quickly, requiring a short pulse
of current. CMOS inputs are also referenced to the supply voltage so
if the supply drops because of something switching, so does the
logic0/1 threshold. TTL on the other hand, has thresholds set by
transistor junctions so it is less susceptable to this.

As the switching speeds increase, the need to get that current pulse
out of the cap becomes more important. Big caps just can't do it so a
compromise is required. The little caps can supply the needs of the
chip but also need to be charged up again and fairly quickly. The
next capacitor up the chain can be larger and a bit slower to provide
that function. Those caps in turn, need to be topped up and if the
regulator isn't fast enough, yet another cap can bridge the gap.

> Even if I wasn't a 'rocket scientist' I'd realize there's something wrong
> with these 'creeping requirements'.

That's because the technology is creeping and the decoupling rules
have to keep up.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebspamKILLspamtla.co.nz      fax +64 9 820-1929
======================================================

1999\11\14@074540 by Tom Handley

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  Robert, in addition to comments from Dennis and Steve Baldwin, it is
common to use parallel 0.1uf caps to provide more capacity and lower ESR.
This is a common technique when designing CPLD's and FPGAs. Search through
previous articles in the comp.arch.fpga news group for more info.

  As far as tantalum caps, there is a good reason that the market for them
is `huge'. They provide capacity and low ESR in a small space but they cost
more than monolithic solutions. While they do have a rather `dramatic'
failure mode, they are used in critical systems that operate in land, sea,
air, and space environments. It boils down to good engineering. If you
specify a 10uf/6V tantalum cap in a 5V environment where there may be large
transients, it's going to fail but so will the chips on the 5V rail...
I often specify a tantalum and a 0.1uf in parallel. The tantalum provides
the capacity and the other the fast response to transients. Again, this
depends on the environment.

  - Tom

At 10:59 PM 11/11/99 -0500, Robert A. LaBudde wrote:
>At 09:35 AM 11/10/99 +1100, Dennis wrote:
>> >So the output capacitor should be no smaller than 0.1 uF and no larger
than
>> >200 uF. Usually the breakpoint is in the cost of the capacitor. Since you
>> >can buy 0.47-1.0 uF tantalum caps for the same price as 0.1 uF, using a
>> >0.47 uF or 1.0 uF tantalum is the best choice.
>><snip>
>> >Larger capacitors provide no advantage at low frequency, since the output
>> >impedance of the regulator is very low (e.g., 1 mOhm). At high
frequencies,
{Quote hidden}

ohms.
{Quote hidden}

more?
>
>It still seems to me you only need a very large capacitor at the input of
>the regulator, with 1-100 uF at the output, and 1 uF tantalums at the chip
>level to control switching noise.
>
>As I said before, I'm not an EE, so perhaps I'm missing something in the
>spec sheets here.


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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