[SI-LIST] Re: Connect SGND and PGND on Digital DC/DC Converter?
- From: "Fasig, Jonathan" <fasig.jonathan@xxxxxxxx>
- To: "Tomas Carlsson" <tomas.l.carlsson@xxxxxxxxx>
- Date: Mon, 12 Dec 2011 09:59:45 -0600
Tomas: To start, look at Figure 3 in the ZL2106 datasheet (the second
link from your note). Here you see that the PGND pins are connected
internally to the bottom MOSFET of the power switch. This MOSFET
conducts during the part of the cycle when the inductor current is
ramping down. To minimize switching losses the power MOSFETs are
usually made to switch as fast as possible so the di/dt can be large and
substantial voltage spikes can be generated across any inductances in
series with the current path. Such inductances include the vias
connecting the PGND pins to your internal ground planes. To avoid
coupling these voltage spikes directly into the regulator control
circuit, the designers typically provide a separate, "more-quiet" ground
pin and the intent is to provide a connection to the reference plane
which does not include these large voltage transients.
To be effective it is essential to ensure that PGND currents do not flow
through the SGND via, so design your footprint and board layout
accordingly. Be especially careful using dynamic shape fill in Allegro.
Hope this helps.
Regards,
jf
Jonathan Fasig, P.E.
Mayo Clinic - SPPDG
4001 NW 41st Street
Rochester, MN 55901
Tel: 507-538-5464
fasig.jonathan@xxxxxxxx
www.mayo.edu/sppdg
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Tomas Carlsson
Sent: Monday, December 12, 2011 9:02 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Connect SGND and PGND on Digital DC/DC Converter?
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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