[SI-LIST] Interference from planar magnetics
- From: "Bob Welte" <welte@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 24 Jan 2003 16:30:37 -0500
Hello (once again)
I would like to get some insight, (or references) on the effects of having
planar magnetics (from DC/DC converters) on a digital board in close
proximity to the digital traces.
Specifically, I need to determine the effects that the fields from the
planar magnetics of the DC/DC VRM might have on various types of digital
signals.
The digital signals fall into two classes: High frequency (1.25Ghz) which
are differential, and low voltage (1V amplitude), and low frequency (<200
Mhz), unbalanced, but with various voltages (some are 3.3V, 2.5, and 1.8V
signals).
The VRMs are relatively low voltage/ high current (2.5 and 1.8V at 10-25
Amps). The VRMs typically have switching frequencies not much greater than
a couple Mhz. The magnetics of the VRM are not "part of the board" (ie
board traces do not make up any portion of the inductor/transformer
windings).
Ideally, I would like to be able to route the digital signals near (and
even under) the VRM. I could, of course put a large copper shape (with
plenty of vias tied to ground planes below) beneath the VRM to block the
electric fields from penetrating into the lower wiring planes. While I
believe that this would affect the magnetic field (since it affects the
electric field), it will not block it!.
As voltages get lower, and VRMs need to be closer to the ASICs, where board
real estate is at a premium, I thought this topic would have wide interest.
Any thoughts?
Bob Welte
IBM Microelectronics
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