[SI-LIST] Re: Bypass Capacitor Selection
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Steve Weir" <weirsp@xxxxxxxxxx>, "David Anthony" <x2y@xxxxxxx>,jmartinson@xxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx,si-list@xxxxxxxxxxxxx
- Date: Fri, 2 Jan 2004 14:44:16 -0800
Steve,
As i mentioned, we measured all sorts of land patterns and their effect on
the impedance vs. frequency that one sees on a PCB power supply. Figure
34.10 shows this with an array capacitor, 2 each 0603 capacitors with for
vias and an 0612 with twelve vias.
From this data, it can be seen that the array capacitor is only marginally
better than a pair of 0603 capacitors at a much higher overall cost.
To get real value from these very low inductance capacitors it is necessary
to make a significant reduction in mounting inductance. That can happen on
very thin substrates, such as BGA packages.
The rest of the time, sticking with standard components is a good
compromise between performance and cost.
I'm sure this news will not be appreciated by those making the array style
capacitors, but that's how the measurements work out , but that's why we
made all of the measurements. We needed to know just what we were going to
get from the various types of parts.
Lee
> [Original Message]
> From: steve weir <weirsp@xxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>; David Anthony <x2y@xxxxxxx>;
<jmartinson@xxxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 1/2/2004 9:17:45 AM
> Subject: Re: [SI-LIST] Re: Bypass Capacitor Selection
>
> Lee,
>
> I have become a big believer in the X2Y parts over the past several
> years. While, the land patterns naturally suit themselves to multiple
vias
> to mitigate the effects of attachment inductance, the presence of the
G1/G2
> plates make these devices perform much better than anything else I have
> seen, ( as well as offering some applications regular caps can't do at
all
> ). I love these things for SMPS and decoupling. The limited sources of
> supply used to make them costly and hard to get, but that has improved
> quite a bit in the past 24 months, to the point where they are both
> performance and cost effective.
>
> One of the really amazing characteristics of these devices is the very
well
> behaved and lower impedance floor. Since we are limited largely by how
> many vias we attach with, these devices are about as close to ideal as we
> can get. I think it would be well worth your while to reevaluate
> decoupling to any particular target impedance using X2Ys versus any
> alternative. I believe you will be very pleasantly surprised.
>
> Steve.
>
>
> At 08:40 AM 1/2/2004 -0800, Lee Ritchey wrote:
> >This is a good subject to explore. However, focusing on the parasitic
> >inductance of the capacitor itself is too narrow a view. What counts is
> >the total inductance, including the mounting pads and vias. On
multilayer
> >PCBs, the mounting inductance dominates the picture. When this had been
> >taken into account, the ultralow inductance capacitors turn out to be not
> >much better than the standard two terminal devices.
> >
> >While I haven't published any papers specifically on this topic, nor
have I
> >seen anyone else do so, we did treat this topic in great detail with many
> >lab measurements in the book I published late last year. Don't mean to
> >push the book in this reply, but want to make sure the information is
> >visible to those who are curious.
> >
> >On my web site, there is a list of articles, one by Micheal Grime and one
> >by the engineers at UMR which sheds some light on the overall topic of
> >capacitor selection. site is www.speedingedge.com
> >
> >
> >Lee
> >
> >
> > > [Original Message]
> > > From: David Anthony <x2y@xxxxxxx>
> > > To: <jmartinson@xxxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>;
> ><si-list@xxxxxxxxxxxxx>
> > > Date: 12/30/2003 2:27:02 PM
> > > Subject: [SI-LIST] Re: Bypass Capacitor Selection
> > >
> > > Jerry,
> > >
> > > Here are links to comparisons of standard discretes (also reverse
aspect
> > > caps) vs. an X2Y cap. The 4-terminal X2Y's internal design promotes
> > > cancellation of mutual inductance. There have been discussions on the
list
> > > as to what's the best test board for comparing vendor components and
we
> >look
> > > forward to a standard emerging. We choose a PCB similar to one used
by UMR
> > > in a paper they presented at the 2002 EMC Symposium. Bart Bouma of
> > > Yageo/Phycomp also compared X2Y to standard discretes on a similar
board.
> > >
> > > Links to data (watch for URL wrap):
> > >
> > > (1) X2Y vs.(5)different valued discretes(same size caps):
> > >
>
>http://www.x2y.com/cube/x2y.nsf/(files)/X2YPCB121203.pdf/$FILE/X2YPCB121203
.
> > > pdf
> > >
> > >
> > > Phycomp data: X2Y vs. standard discretes and low L reverse aspect caps
> > > (0306):
> > >
>
>http://www.x2y.com/cube/x2y.nsf/(files)/092703X2YReverse.pdf/$FILE/092703X2
Y
> > > Reverse.pdf
> > >
> > > regards,
> > > Dave
> > >
> > >
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jerry Martinson
> > > Sent: Tuesday, December 30, 2003 2:18 PM
> > > To: si-list@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Bypass Capacitor Selection
> > >
> > >
> > > Martin,
> > >
> > > To address your 3rd question, I'm a bit of a maverick as I believe
that
> > > using surface mount capacitor arrays are an often-overlooked trick for
> > > having better decoupling performance at a reasonable cost. With most
> > > designs, the via inductance is a large part of the impedance, cap
> > > inductance is also a larger but smaller component. To reduce this, it
> > > makes sense to try to cram the vias for the two voltages close
together
> > > and it also makes sense to try to have more vias and caps in parallel.
> > > There are limits to how much of this you can achieve for DFM, routing,
> > > cost, and other reasons.
> > >
> > > For DFM reasons, you'll likely need to space your 0402 and 0603
> > > components further apart from each other than you'd ideally like.
This
> > > will limit your cap and via density. You'll also have some DFM
> > > resistance to putting the vias as close together and as close to the
SMT
> > > pads as you'd ideally like. Cap arrays can partially solve these
> > > problems. Here are some points about using the cap arrays:
> > >
> > > 1. They effectively put 4 0402 or smaller parts in the same area as
an
> > > 1206 or smaller. This means you can get a higher number of caps and
> > > vias in a given area. =20
> > > 2. Interdigitate so that much of the inductance works for you instead
> > > of against you. Alternate power and ground so that the power isn't
all
> > > on one side and the ground is on the other. There is a paper from AVX
> > > on using expensive IDC cap arrays that show this. I think the IDC cap
> > > arrays are nice but expensive. Using regular cap arrays on pads
> > > designed for the AVX IDC cap arrays, you can get the beneficial
> > > interdigitated mutual inductance in your vias, which are the larger
part
> > > of the inductance. Another benefit of this is that if your decoupling
> > > is found to be inadequate after your boards are made, you can use
these
> > > AVX IDC cap arrays without spinning your board.
> > > 3. You may not be able to have the vias for the two voltages point
> > > inward from the pads like you might have been able to with discretes.
> > > So you'll have to look at your board's DFM rules. This may be a
> > > disadvantage of using the cap arrays.
> > > 4. The purchase cost of the cap arrays is higher than 4 discretes.
> > > Average placement, etc... costs for each discrete are usually a couple
> > > US cents. These costs per discrete always exist but may not be
> > > internalized in the assembly pricing you see. You'll have to look at
> > > the economic ramifications of this. The situations I've seen slightly
> > > favor an array versus four discretes so you can actually _SAVE_ money
> > > and have better decoupling.
> > > 5. You can only use one value for the four elements in an array.
This
> > > may conflict with your other decoupling goals.
> > > 6. You can't spread an array around like you can with discretes.
> > >
> > > I must caution you that I have not been able to do very good
> > > quantitative measurements comparing the cap array trick's
effectiveness
> > > versus the discrete caps effectiveness in perfect apples to apples
> > > tests. However based on the differences I've seen in similar boards
> > > where I used discretes on one and arrays on another, I'm pretty sure
> > > that using cap arrays has given me substantially less noise. So using
> > > cap arrays are more art than science right now. I wish I had hard
data
> > > showing how good arrays as a function of X, Y, Z compared to
discretes,
> > > but I don't have the time or resources to do this personally.
> > >
> > > I'd sure be interested in hearing if anybody else has tried this or
has
> > > hard data.
> > >
> > > -Jerry
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> > > On Behalf Of SI List
> > > Sent: Tuesday, December 30, 2003 5:21 AM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Bypass Capacitor Selection
> > >
> > > Dear All,
> > >
> > > After studying many books, the bottom line for high frequency bypass
> > > capacitor selection seems to be the following:
> > >
> > > Use the smallest possible package and then take the largest valued
> > > capacitor available in that package.
> > >
> > > Although 0201 capacitors are available, they are kind of hard to
> > > assemble. So in our design, we are thinking of using 0402 X5R parts.
> > >
> > > In a 0402 package, the largest value available seems to be 1 uF.
> > >
> > > My frequency range of interest is DC to 500 MHz. In addition to the
> > > bypass capacitors, I know that there must be a pair of closely spaced
> > > VCC/GND reference planes.
> > >
> > > My questions:
> > > 1. Does it make sense to only use 1 uF 0402 capacitors for bypassing?
> > > Other people often use a combination of 1 nF/10nF/100nF. 2. I have
been
> > > looking at the frequency characteristic of 100 nF 0402 X5R capacitors
> > > given in the data sheets of different manufacturers. The impedance vs.
> > > frequency plot often looks quite different from manufacturer to
> > > manufacturer. Should I choose the manufacturer with the best frequency
> > > characteristic or are all such parts almost equivalent? 3. Does it
make
> > > sense to use surface mount capacitor arrays for bypassing?
> > >
> > > Thanks for all your expert feedback.
> > >
> > > Best regards,
> > > Martin Heimlicher, heimlicher__at__enclustra//dot//com
> > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > > List technical documents are available at:
> > > http://www.si-list.org
> > >
> > > List archives are viewable at: =20
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > > http://www.qsl.net/wb6tpu
> > > =20
> > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > > List technical documents are available at:
> > > http://www.si-list.org
> > >
> > > List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > > http://www.qsl.net/wb6tpu
> > >
> > >
> > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > > List technical documents are available at:
> > > http://www.si-list.org
> > >
> > > List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > > http://www.qsl.net/wb6tpu
> > >
> >
> >
> >
> >------------------------------------------------------------------
> >To unsubscribe from si-list:
> >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //www.freelists.org/archives/si-list
> >or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> >Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
