[SI-LIST] Re: Antwort: Re: Questions about interplane capacitance
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: Andreas.Lenkisch@xxxxxxxxxx
- Date: Wed, 12 Mar 2008 20:16:24 -0400
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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