[SI-LIST] Re: Antwort: Re: Questions about interplane capacitance
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: Doug Brooks <doug@xxxxxxxxxx>
- Date: Thu, 13 Mar 2008 08:58:43 -0400
Doug,
Yes, we agree that the charge that makes up the transition must be (and
is) within a given radius,
assuming that we have distributed PDN and not a hypothetical lumped
source. My point was
that when we have a matched transmission medium, this radius simply does
not matter, because
looking back from the load we have a frequency independent resistive
source impedance. This
means charge will move towards the load as needed, and the disturbance
is I(t)*R.
Regards,
Istvan Novak
SUN Microsystems
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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