FreeLists / si-list / [SI-LIST] Antwort: Re: Questions about interplane capacitance

[Andreas.Lenkisch@xxxxxxxxxx]

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
> 
> 
> 
>
> ------------------------------------------------------------------
> 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:
> http://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.net
>
> List archives are viewable at: 
>                                http://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:
http://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.net

List archives are viewable at: 
                                 http://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
 




.


Schroff GmbH 
Langenalber Str. 96-100
D-75334 Straubenhardt
Amtsgericht Mannheim, HRB 503549
Geschaeftsfuehrer: Peter B. Dessing, Walter Kritikos

Important Notice
The information contained in this message is confidential and is intended for 
the addressee only.
The unauthorised use, disclosure, copying or alteration of this message is 
prohibited and may be unlawful.
If you received this message in error or there are any problems please notify 
the originator immediately and contact postmaster@xxxxxxxxxxxxxxxxxxxxxx
This mail and any attachments have been scanned for viruses prior to leaving 
Pentair's network.
Pentair will not be liable for direct, special, indirect or consequential 
damages arising from alteration of the contents of this message by a third 
party or as a result of any virus being passed on.
All personal views stated in this message are my own and may not represent the 
position of, nor is this message binding upon, Pentair Inc. or any of its 
subsidiaries.
------------------------------------------------------------------
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:
http://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.net

List archives are viewable at:     
                http://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