[SI-LIST] Re: Bulk caps
- From: Larry Smith <Larry.Smith@xxxxxxx>
- To: ravindra@xxxxxxxxxx
- Date: Thu, 02 Sep 2004 17:36:55 -0700
Ravindra - The bulk capacitors have to supply the load current long
enough for the VRM to make a power transition. For example, if your
load makes a 30 amp transient and the VRM takes 10 uSec to respond,
the
charge that the bulk capacitance must deliver (or absorb) is 30amps x
10 uSec. You probably don't want the power supply to droop (or spike)
more than some amount (i.e. 5% of Vdd), so you can calculate the
number of bulk uF you need from Q=CV.
The above calculation will get you into the ball park. It is
pessimistic because the VRM is actually ramping up current during the
10 uSec. But it is optimistic because it does not take capacitor ESR
into account. This is a time domain calculation.
You can achieve the same results in the frequency domain if you choose
bulk capacitors such that the output impedance of the VRM crosses the
impedance of the bulk capacitors at the target impedance. There
should be enough resistance in the VRM, bulk capacitors, and/or power
planes to damp this inductive/capacitive crossing and prevent a high Q
circuit.
regards,
Larry Smith
Sun Microsystems
Ravindra wrote:
>
> Hi Gurus!
> How do i decide as to how many bulk caps should be used at the stage of
> schematic entry and also what values??
> I would like to know as to on what basis the placement of these bulk
> capacitors is done
> on board.Are there any related documents on this?
>
> Regards,
> Ravindra
>
> ------------------------------------------------------------------
> 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
Other related posts:
