SMPS noise

Sounds like they're trying to phase-lock, or they're heterodyning, or something. Some switchers are sync'able to avoid this, but this one isn't. Try pushing the frequencies far apart; that might help.

Moving them farther apart might help. Of if you can change the layout, just use syncable switchers and lock them all to some common clock available on the board. We just did a board with an LTC3407 dual syncable switcher (powering a big fpga, 1.8 and 3.3 volts) and locked it to the master clock on the board.

John

I have seen this very problem, you are correct it is a beat note between two or more of the switching frequencies and it will vary with temperature and unit to unit becasue the switching frequencies are not well controlled. It may be very difficult to provide sufficent isolation to prevent this. A better approach would be to synchronize the switching frequencies.

Hello QQ,

It is possible to see "motor boating" when a regulator becomes unstable. Duty cycles over 50% can cause RHP instability. However, if you have closely followed the data sheet, app notes and good layout practices this may be a lesser concern in your case, as you had said.

What concerns me is that you mentioned split grounds. In such complex designs I have rarely seen that work. If I also count the EMC failures that clients with split ground design have experienced I could almost say that I have never seen it work. Try to connect the two planes tightly via short bridges every inch or so. These bridges should be less than 1/4" long or you'd have to use copper tape and solder. Then look at the noise again.

That looks like the regulators are, to an extent, fighting with each other for load. A split ground can cause that and so can shared loads such as level translators. Problem is, in a partially digital design loads are not constant at all and that can cause modulation. Large RAM banks are often the dominant contributors and you can usually find their read/write patterns right on the supply nodes. If mounting large low-ESR caps on all the rails doesn't significantly help then the ground structure would become even more of a suspect.

Do you have even more grounds than AGND and DGND? I'd try to tie the all together to see how the noise behaves then. "Thin nets" are in fact inductors.

Again, controlled ground return paths in systems with several grounds are next to impossible unless you can guarantee that there will be no other paths than that single point return. However, this would mean that the system couldn't have any outside connections or each would have to loop through a substantial ferrite core.

10nsec jitter is still pretty gross. Somehow that tells me there must be lots of cross contamination between circuits that aren't supposed to "see" each other.

If this problem becomes stubborn and deadlines loom you might need the help of a consultant who knows about crosstalk, noise etc. It's not so bad, lots of companies use this kind of help in the same way that they use attorneys to vet their business deals. Even large ones do. But don't wait too long because worst case you may have to perform a re-layout.

Regards, Joerg

I read in sci.electronics.design that QQ wrote (in ) about 'SMPS noise', on Thu, 24 Feb 2005:

If you run eight, or even three, power oscillators close together, you should be more astonished that ALL of them don't show beat-note effects. Why one (two if you count the G5 one as well) shows the effect and not the others is probably due to the exact layout or maybe you have a slightly different circuit for the lowest voltage unit?

I don't know how you solve the problem; SMPS are not my field. I suppose you can't run them all synchronized to a master clock?

--
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Sounds like crosstalk to me. If you could sync all the PWM chips it might help.

Ground noise is a possible source of the problem.

Graham

In addition to the other pertinent comments, what about magnetic coupling between your chokes? if possible, use closed magnetic cores (eg toroids, pot cores or shielded drum cores). orienting adjacent magnetics at right angles to one another will also help. Many smps chokes have huge air gaps - ie use drum cores. Most of these are also available as shielded designs - a ferrite sleeve is placed around the assembly, spaced far enough away that it doesnt wreck the NIsat

Cheers Terry