[SI-LIST] Re: Connect SGND and PGND on Digital DC/DC Converter?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 12 Dec 2011 07:50:50 -0800
Follow the current. Your first line of defense against unwanted noise
in common impedance paths is placement. The second line of defense is
selective moating. Moating is only applicable when specific currents
would traverse a common impedance path that would result in an
unacceptably high voltage disturbance without them. Moating is a lot
like ferrite beads. There are times when a little bit of voided etch
can do very nice things. They are not a panacea. They are every bit as
capable of introducing problems as fixing them. Just like there are a
lot of appnotes that call for overuse and misuse of ferrite beads, there
are lots of appnotes that call for moats where they are not appropriate
or are even harmful. If you plan your placement, particularly of the
bypass components carefully, you shouldn't need moats with those
modules. If you are lining up a bunch of those modules side by side,
selective moating to keep thepin 1/21 ... 17 side of one part's common
separated from the adjacent part's pin 10/11...15 side is a reasonable
idea. If you have a bunch of signal traces running underneath these
things then you may wish to rethink your placement, or get more creative.
Steve.
On 12/12/2011 7:02 AM, Tomas Carlsson wrote:
> Hello,
> This is a question on the popular topic whether to stitch different grounds
> (analog / digital / power) together or not.
>
> I am looking in the datasheet and app notes for a couple of Digital DC/DC
> Converter Modules (17A or 6A):
> http://www.intersil.com/data/fn/fn7914.pdf
> http://www.intersil.com/data/fn/fn6852.pdf
>
> It is recommended to connect the analog/signal ground (SGND) island to the
> power ground (PGND) in *one place only*.
>
> I have a 12 layer stack-up with 4-5 ground planes. I plan to stitch all
> ground points (regardless SGND / PGND / DGND) together through vias close
> to each ground pad to avoid creating plane cuts. This will in effect stitch
> SGND to PGND and make the design simpler. What are your thoughts on this?
> Is it a good or bad idea to make the entire DC/DC module see the same
> ground?
>
> Thanks,
> Tomas Carlsson
>
>
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
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