Re: [SI-LIST] : Medium range capacitors

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From: Shayle Hirschman (shayle@mho.net)
Date: Wed Mar 08 2000 - 03:35:44 PST


Dear Larry (Smith) and Steve,

I gather from your comments and the reference articles which I examined
that the bulk capacitance covers the range of frequency of current
transients, needed by the RS-232 transmissions (and even sequences of
software activities by processors as mentioned in one of the references),
up to the effective range of the ceramics (which deal with edge rates), and
that, at those speeds, the coverage from the single 47uF tantalum covers
the board.

Thank you for your help and insights.

Shayle

At 08:59 AM 3/8/00 -0800, you wrote:
>I have seen several references to this alleged "Larry Miller at Sun."
>I think you probably mean Larry Smith. Ray Anderson, Tanmoy Roy and
>myself have worked hard to develop a decoupling capacitor methodology.
>It has been published in the IEEE Transactions on Advanced Packaging in
>August 1999 and is available at:
>
> http://www.qsl.net/wb6tpu/si_documents/docs.html
>
>(Sorry for the multiple plugs on this list..) The method involves the
>use of many capacitor values with controlled inductance and ESR to meet
>a target impedance over a broad frequency range. Steve has a good
>summary of it below. We find that this is the most effective way to
>meet the power needs of modern CMOS technology that demands low
>impedance over a broad frequency range.
>
>best regards,
>Larry Smith
>Sun Microsystems
>
>
>> X-Sender: weirsp@a.crl.com
>> Date: Tue, 07 Mar 2000 19:21:50 -0800
>> To: si-list@silab.eng.sun.com
>> From: sweir <weirsp@a.crl.com>
>> Subject: Re: [SI-LIST] : Medium range capacitors
>> Mime-Version: 1.0
>>
>> Shayle,
>>
>> Sure, if you use enough of them, 100nF caps work ok to high freq, but they
>> are acting as parallel inductors at high freq. The papers delivered by
>> Larry Miller and co. of SUN show that for demanding applications, careful
>> choice of a wide range of capacitor values provide a lower impedance at a
>> lower component count than is possible either with many 100nF or similar,
>> or capacitors on decade or century boundaries.
>>
>> The mid frequency performance is just a matter of impedance
>> calculation. From 1KHz up, the 47uF capacitor starts life at
approximately
>> 3.5 ohms, and will bottom out at the combined ESR, ESL, which depending on
>> the part will vary from the 10's to 100's of milliohms. RS-232 currents
>> depend on the voltage swing. The typical today is +/-5V or less. That
>> will yield about 100mA max on a mark-space excursion. If your noise
budget
>> is say 100mV, then one line requires a bus impedance of an ohm or
>> less. How far down the spectra extends depends to some extent on how long
>> the RS-232 lines are. Even if you have 100' of cable, the current will
>> drop-off within one round trip at about 500nS, so even the low frequency
>> stuff is still in the hundreds of KHz. After that, it is literally the
>> difference in DC load, and RS-232 lightly loads, so this is very small.
>>
>> Over the space of a 3 inch board a wave will propagate across the whole
>> board in less than 1 nS. Bulk capacitors respond on the order of 100's of
>> nS, so distribution doesn't buy anything. The ceramic capacitors and the
>> planes are doing all the work. If I were your customer, I would pay a lot
>> more attention to the PLD than the 232 drivers.
>>
>>
>> Regards,
>>
>>
>> Steve.
>>
>>
>> At 08:44 PM 3/7/00 +0000, you wrote:
>> >Thanks, Steve, for your answers.
>> >
>> >I have used 0.1 uF successfully in speeds to 66 MHz with fast edge rate
>> >logic (terminated when needed) and I suspect smaller values won't really
>> >help with higher frequencies unless a smaller, or less inductive, package
>> >is used instead of the 0805 of the 0.1 uF caps.
>> >
>> >I think you hit upon the crux of my client's concern, which is the voltage
>> >regulator's output impedence at the mid range frequencies of the RS-232
>> >transmissions. You mention that the bulk capacitor at the entry point of
>> >the power input would help with this. I think my client would then ask
>> >about that cap's influence elsewhere in the board, which is why he
>> >mentioned "sprinkling" a few around.
>> >
>> >But my real question is, aren't we interested in edge rates (edge
>> >transition times), covered by the 0.1 uF decoupling caps, or do the slower
>> >(assume periodic) frequencies of the RS-232 transmissions have an affect
>> >which is different than and in addition to edge rate EMI and which needs
>> >mid-range filtering such as the 6.8 uF caps (and I assume at those
>> >frequencies the extra inductance is not a factor)?
>> >
>> >I welcome more input from Steve and others on this issue of mid range
>> >capacitors and how they are effective.
>> >
>> >Thanks,
>> >Shayle
>> >
>> >
>> >
>> >
>> >
>> >At 05:42 PM 3/7/00 -0800, you wrote:
>> > >If you are sadistic, they will drive your purchasing crazy, as tant's
are
>> > >hard to come by these days.
>> > >
>> > >The PLD edge rates need to be watched. You can optionally filter the
>> > >outputs depending on the application, with anything from 10 ohm series
>> > >resistors, to RC networks, should the edge rates pose a noise issue.
>> > >
>> > >The tantalum caps will filter only up to a couple of MHz. So the
question
>> > >that arises is, how much filtering do you need from at least a couple of
>> > >KHz where your power-supply regulator impedance goes up to a couple of
>> > >MHz? If you are really only running some RS-232 then the one 47 uF
cap s/b
>> > >plenty. The 100nF caps will go inductive around 8-15MHz ( mounted ).
 So,
>> > >you will either need lots of them in parallel, or other smaller
values to
>> > >cover at least up to 80MHz. You can refer to recent postings on this
>> > >thread for more details.
>> > >
>> > >Regards,
>> > >
>> > >
>> > >Steve.
>> > >At 06:20 PM 3/7/00 +0000, you wrote:
>> > >>Hi,
>> > >>
>> > >>I have been asked to sprinkle 6.8 uF tantalum or electrolytic
capacitors
>> > >>evenly around the PC board of a new design.
>> > >>
>> > >>As usual, I have the 0.1 uF 805 ceramics at the power pins of chips
and a
>> > >>bulk storage cap. of 47 uF where power enters. The board is 3x4
inches,
>has
>> > >>a few chips, slow edge rates. Mainly RS-232 (up to 1 mb/s) and the
>> > >>internals of a PLD clocked at 16 MHz.
>> > >>
>> > >>Can anyone tell me what the benefit of these 6.8 tantalums sprinkled
>around
>> > >>would be?
>> > >>
>> > >>Thanks,
>> > >>Shayle
>> > >>
>> > >>
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