[SI-LIST] Re: Antwort: Re: Questions about interplane capacitance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Joel Brown <joel@xxxxxxxxxx>
- Date: Thu, 13 Mar 2008 14:56:58 -0700
Joel, it is transmission line theory. Think about an infinitely long
signal transmission line for a moment. At any instant a driver sees a
constant impedance across frequency. The voltage to current relation is
independent of prior history. What Istvan describes is a very low
impedance transmission structure for power.
Sadly, a lot of IC vendors are still playing catch-up in terms of power
delivery. If they had their game together, they would be telling you:
The actual voltage tolerances at the die, the current spectrum at the
die, and the parasitics of the die and package. From that you could
engineer your PDN. Instead often what we see are partial recipes like
you describe. On a good day the recipes cost extra money. On a bad
day, they result in failures.
One can build a resistive networks several ways. One is to add discrete
resistance by any number of methods, another is to use the FDTIM method
that Larry Smith has long championed. And even though resistive
networks have attractive qualities, reactive networks may still be
cheaper and equally effective. It all depends on the circumstances. It
is somewhat akin to surface finishes: there is no ideal one. But there
are several that when used properly work very well. One just needs to
respect the limitations of each method.
Part of the problem figuring this stuff out is having sufficient
experience to judge trade-offs early in the design process. That's just
something that has to be learned. As more training materials come out
on power delivery, it will probably get easier. In the shameless plug
department, we ( Teraspeed ) do very nice jobs optimizing power delivery
systems for customers. If you've got a design you want advice on we can
get you squared-up pretty quickly.
Best Regards,
Steve.
Joel Brown wrote:
> Steve,
>
> So even though each cap has a relatively high ESR (1.6 Ohms) the PDN as a
> whole has a relatively low impedance which will result in low noise on the
> PDN. This goes against intuition and previous thinking that an IC needs a
> local bypass with low ESR and ESL to supply the needed charge during
> switching transients. I am starting to see that mathematically a resistive
> PDN lowers noise compared to one that is the inductive (you did a good job
> explaining that). The thing that I am having trouble grasping or visualizing
> is that why is charge propagation velocity not a factor when the PDN is
> purely resistive? Is the PDN model simply a resistor in series with the load
> and distance has no effect? Perhaps there is an analogy that would make this
> concept easier to understand? Also, when I see app notes from IC vendors
> that recommend using a 0.1uF and 1000pF cap on each supply pin and then
> instead I use distributed high ESR capacitors I feel like I am doing
> something quite different and contrary from the recommended and when I query
> the vendors on how they arrived at recommendations in the app note the
> answer I get is "we recommend that you do it exactly as shown in the app
> note, we know it works that way and if you don't follow it it may not work".
> Also I am wondering how all this relates to X2Y caps, I suppose they could
> be used with series resistors but that would somewhat defeat the purpose by
> adding inductance. Its not clear to me what approaches I should attempt on
> future designs.
>
> Joel
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Wednesday, March 12, 2008 11:26 PM
> To: Doug Brooks
> Cc: Istvan Novak; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Antwort: Re: Questions about interplane capacitance
>
> Doug, Istvan's representations are analytically exact. When the
> characteristic impedance of the transmission structure is high, and/or the
> rise times are slow then capacitors can be placed in close enough proximity
> that they load the transmission structure so as to make it appear a lower
> impedance with some little bumps. For example if one had a 10" x 10" 4 mil
> er4.0 board = 22.5nF and loaded it with bypass every square inch of 150pH,
> then for slow enough signals, the distributed impedance would look like
> 80mOhms. If the network were constructed from 200 capacitors, an ESR value
> of 1.6Ohms / cap would make that impedance uniform down to the RC knee of
> the parts. Assume that were matched by an AVP regulator of 80mOhms and the
> entire thing looks like 80mOhms from DC to over 1GHz. 80mOhms would support
> a 32 bit transition w/ about 5% droop. The impedance scales with dielectric
> height and the inverse square root of eR. Scale the dielectric down to
> 0.1mils and now 320 lines can switch simultaneously in one direction with 5%
> droop, and at arbitrary edge rates.
>
> Istvan has shown using analysis with the reverse pulse technique an
> inductive PDN shunt impedance acts like a noise high pass filter ( See DC
> papers from at least as far back as DC East 2005 ). Put in a square wave
> noise pulse ( load current ) and the leading edge changes by Vdelta =
> -L*di/dt below the baseline. Allow the pulse to persist long enough and the
> system recovers back to the baseline which would be -I*Rpdn.
> Return the load current to zero, and now the energy stored in the inductance
> kicks back -L*di/dt. The p-p noise is then 2*Ldi/dt - (Imax-Imin)*Rpdn. If
> Rpdn is very small then it approximates 2*Ldi/dt.
> This behavior is apparent in the transient response plots of virtually any
> non-AVP VRM.
>
> Now, suppose that the VRM and PDN can be made to appear resistive right
> through Fknee. Then the response to a current pulse of I is simply Vdelta =
> -I*Rpdn. There is no component of di/dt, and so the total p-p noise is just
> (Imax - Imin)*Rpdn. AVP schemes position the DC operating point
> intentionally high so that at Imin they are at their margined high limits
> and at Imax they are at their low limits. This allows increasing Rpdn while
> still meeting the same current and voltage specifications.
>
> Best Regards,
>
>
> Steve.
>
>
> Doug Brooks wrote:
>
>> Istvan,
>>
>> With all due respect, I would modify your argument a little bit. In
>> very simplistic terms, suppose we need x amount of charge to
>> transition from a zero to a one in one ns. That amount of charge (I
>> suggest) must be within
>> 6 inches of the need (what I think we are referring to as the service
>> radius). If not, it takes a little longer to reach the logical one state.
>> I look at it, not from the standpoint of a dip in the rail, as much as
>> the ability to satisfy the rise time requirement (unless you are
>> referring to a dip in the rail that occurs during the rise time
>> itself.) In the slightly longer term, the charge will replenish fairly
>> quickly, but not, perhaps fast enough to meet the rise time requirement.
>>
>> Doug Brooks
>>
>>
>>
>>
>>
>>> Andreas,
>>>
>>> Yes and no. It is true that charge moves with finite speed, so for
>>> any given time duration the charge has to come from locations closer
>>> than the ratio of distance over speed. BUT the whole notion of
>>> service radius is based on the assumption that as you deplete the
>>> charge available in the immediate vicinity of the active device, you
>>> have to wait for replenishment, otherwise you get a big dip on the
>>> supply rail.
>>>
>>> Having a matched
>>> transmission medium to deliver power to the active device, the charge
>>> moves without interruption, and as you deplete the planes close to
>>> the device, it gets replenished on the fly from areas further away,
>>> so the service area concept is pretty much meaningless in this
>>> scenario. Current flows without interruption.
>>> The bucket brigade of infinitesimally small inductive and capacitive
>>> elements of the transmission line transmits the power continuously.
>>> If the load current changes, for any I(t) time function of load
>>> current, the transient noise at the load point will be I(t)*Zo, where
>>> we assume that Zo is the resistive and frequency independent
>>> characteristic impedance of the transmission medium. This is a very
>>> simplistic one-dimensional model, but it gives a good insight of why
>>> the service radius matters only on PDNs where the network is not
>>> matched.
>>>
>>> Regards,
>>>
>>> Istvan Novak
>>> SUN Microsystems
>>>
>>>
>>>
>>>
>>>
>>> Andreas.Lenkisch@xxxxxxxxxx wrote:
>>>
>>>
>>>> Istvan,
>>>> I'm wodering a little about your comments to the service radius.
>>>> Independant if the impedance is resistive, we have still a
>>>> propagation time which would limit the service radius from my
>>>>
> understanding.
>
>>>> Do I'm wrong?
>>>>
>>>> regards
>>>> Andreas
>>>>
>>>>
>>>>
>>>> Istvan Novak <istvan.novak@xxxxxxxxxxx> Gesendet von:
>>>> si-list-bounce@xxxxxxxxxxxxx
>>>> 11.03.2008 13:14
>>>>
>>>> An
>>>> Joel Brown <joel@xxxxxxxxxx>
>>>> Kopie
>>>> si-list@xxxxxxxxxxxxx
>>>> Thema
>>>> [SI-LIST] Re: Questions about interplane capacitance
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Joel,
>>>>
>>>> Just one quick comments to the good summary from Steve:
>>>>
>>>> While considering planes and bypass capacitors in terms of effective
>>>> capacitances and inductances is a valid approach, we need to keep in
>>>> mind that focusing on the capacitive or inductive nature of parts
>>>> without looking at the wider picture misses a very important and
>>>> useful class of solutions, namely that of matched transmission
>>>> lines. As it was pointed out earlier several times on the SI list,
>>>> the best
>>>> (self) impedance for a power distribution network is a resistive
>>>> one, neither capacitive, nor inductive.
>>>> We can get resistive impedance from a matched transmission line,
>>>> regardless of its capacitance and inductance, and in such cases the
>>>> notion of 'service area' of parts become meaningless: you can put
>>>> bypass components further away from the active devices without
>>>> sacrificing performance.
>>>>
>>>> Regards,
>>>>
>>>> Istvan Novak
>>>> SUN Microsystems
>>>>
>>>> Joel Brown wrote:
>>>>
>>>>
>>>>
>>>>> Interplane capacitance is frequently cited as the only effective
>>>>> bypass capacitance on a PCB at frequencies above 200 MHz.
>>>>> I am currently working on a design which brings up some questions
>>>>>
>>>>>
>>>>>
>>>> regarding
>>>>
>>>>
>>>>
>>>>> interplane capacitance.
>>>>>
>>>>> 1. Power planes normally carry "standard" voltage rails that are
>>>>> used throughout a board such as +5V and +3.3V.
>>>>> High speed ICs usually have core voltages that are local to the IC
>>>>> and
>>>>>
>>>>>
>>>>>
>>>> are
>>>>
>>>>
>>>>
>>>>> provided by a local regulator which converts the standard rail to
>>>>> the
>>>>>
>>>>>
>>>>>
>>>> core
>>>>
>>>>
>>>>
>>>>> voltage (example 3.3 to 1.8V).
>>>>> The local core voltage is distributed on a plane area that is local
>>>>> to
>>>>>
>>>>>
>>>>>
>>>> the
>>>>
>>>>
>>>>
>>>>> IC and therefore is small in area (0.25 sq in or less) which
>>>>> results in
>>>>>
>>>>>
>>>>>
>>>> a
>>>>
>>>>
>>>>
>>>>> very small amount of interplane capacitance.
>>>>> Is this very small amount of capicitance effective for bypassing
>>>>> the IC?
>>>>>
>>>>>
>>>>>
>>>> I
>>>>
>>>>
>>>>
>>>>> am sure it depends somewhat on the current waveform being drawn by
>>>>> the
>>>>>
>>>>>
>>>>>
>>>> IC
>>>>
>>>>
>>>>
>>>>> but this can only be estimated because semiconductor manufacturers
>>>>> do
>>>>>
>>>>>
>>>>>
>>>> not
>>>>
>>>>
>>>>
>>>>> provide current consumption profile as a function of frequency. To
>>>>> make matters worse, some ICs have several different VCC pins which
>>>>> the manufacturer recommends connecting to separate networks of
>>>>> bypass caps
>>>>>
>>>>>
>>>>>
>>>> and
>>>>
>>>>
>>>>
>>>>> ferrite beads. This cuts the power distributuion up even more
>>>>> resulting
>>>>>
>>>>>
>>>>>
>>>> in
>>>>
>>>>
>>>>
>>>>> less (practically zero) interplane capacitance. It is somewhat
>>>>> ironic
>>>>>
>>>>>
>>>>>
>>>> that
>>>>
>>>>
>>>>
>>>>> the the voltages such as +5V and +3.3V which are required at points
>>>>>
>>>>>
>>>>>
>>>> across
>>>>
>>>>
>>>>
>>>>> the whole board and therefore have the most interplane capacitance
>>>>> are
>>>>>
>>>>>
>>>>>
>>>> also
>>>>
>>>>
>>>>
>>>>> the voltages which have least requirement for interplane
>>>>> capacitance
>>>>>
>>>>>
>>>>>
>>>> because
>>>>
>>>>
>>>>
>>>>> they do not directly supply high speed rails.
>>>>>
>>>>> 2. There has been a lot of emphasis on reducing the mounted
>>>>> inductance
>>>>>
>>>>>
>>>>>
>>>> of
>>>>
>>>>
>>>>
>>>>> bypass capacitors. Even with this reduced inductance they are still
>>>>> only effective up to several hundereds of MHz at which point the
>>>>> interplane capacitance becomes the only bypass capacitance
>>>>> mechanism. However there
>>>>>
>>>>>
>>>>>
>>>> is
>>>>
>>>>
>>>>
>>>>> inductance between the connection of the IC to the planes. This
>>>>>
>>>>>
>>>>>
>>>> inductance
>>>>
>>>>
>>>>
>>>>> consists of vias and package inductance. I did look for some
>>>>> numbers for package inductance and did not find much, it seems to
>>>>> be a closely held secret. Also it is unknown how much bypass
>>>>> capacitnace is internal to
>>>>>
>>>>>
>>>>>
>>>> the IC
>>>>
>>>>
>>>>
>>>>> package. Just for example if we assume 250pH for the vias and 500
>>>>> pH for
>>>>>
>>>>>
>>>>>
>>>> the
>>>>
>>>>
>>>>
>>>>> package, then the impedance at 500 MHz would be 2.36 Ohms. This
>>>>> seems
>>>>>
>>>>>
>>>>>
>>>> rather
>>>>
>>>>
>>>>
>>>>> high for the interplane capacitance to be of much benefit.
>>>>>
>>>>> In summary how much interplane capacitance is needed to be
>>>>> beneficial,
>>>>>
>>>>>
>>>>>
>>>> and
>>>>
>>>>
>>>>
>>>>> why is it beneficial given the inductance in the vias and package?
>>>>>
>>>>> Thanks - Joel
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>
>
> --
> Steve Weir
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
>
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--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(408) 884-3985 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
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http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
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