[SI-LIST] Re: subscribe: bulk capacitor
- From: steve weir <weirsp@xxxxxxxxxx>
- To: pratap.singh@xxxxxx, si_datum@xxxxxxxxx
- Date: Fri, 28 Jan 2005 02:28:56 -0800
Pratap, the only justification for spreading bulk capacitors that I have
ever seen is if the spreading resistance is high. If you doubt this,
construct a bedspring model and then move the capacitors around with
respect to the VRM and the loads. In a properly designed bypass network,
the bulk capacitors support current only up to the low MHz range. The
device attached inductance is an order of magnitude greater than the
spreading inductance even if across the whole PCB, making it almost always
a non-factor between the bulk caps and the IC noise sources.. Spreading
resistance can be a different issue and depends on both the loads and the
plane topology.
If you spread the devices around then you place spreading resistance and
inductance between the caps and the VRM increasing VRM output switching
noise, but decreasing noise EMF across the spreading resistance of the
planes. In a forward topology, the VRM inductor current swing is typically
40% of the DC load. Whereas device load current variation is typically (
but not always ) much less. So if you spread the devices evenly, with the
VRM at one end of the board, you shunt the inductor swing current with an
RC ladder network. OTOH, if you put the caps all right at the switcher
output, there is very little R between that 40% inductor swing current and
the capacitors. If you use a flyback, or boost type topology, the
situation moves even further in favor of the caps at the VRM output.
Steve.
At 11:35 AM 1/28/2005 +0530, Pratap Narayan SINGH wrote:
>hi dave,
> It's better to spread the bulk capacitors on the board.
>It will
>construct a multi supply transmission line which is more better to
>satisfy the high current requirements from the heavy loads.
>please see multi generator transmission lines for more details
>in this regard.
>
>Regards,
>Pratap
>
>dave_zhao wrote:
>
> >i'm confused by location of the bulk capacitors. I
> >shall place them near the voltage regulator module or
> >spread them around to supply enough charge for the
> >heavy loads. I use the ADI's tigersharc DSPs in my
> >design, each of them need to consume 3A current, so i
> >have to choose enough bulk capacitors for them. And
> >where should i locate those capacitors, near the power
> >chip or near the dsp chip?
> > best wishes
> >
> >=====
> >best regards
> > dave zhao
> >
> >
> >
> >__________________________________
> >Do you Yahoo!?
> >Yahoo! Mail - Find what you need with new enhanced search.
> >http://info.mail.yahoo.com/mail_250
> >------------------------------------------------------------------
> >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:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //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:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //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
>
The weirsp@xxxxxxxxxx e-mail address will terminate March 31, 2005.
Please update your address book with weirsi@xxxxxxxxxx
------------------------------------------------------------------
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:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//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
Other related posts:
