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LM22675 design questions

Wm Rubenstein
Prodigy 20 points

Here's the design I've implemented -- www.stubbylatheusa.com/pdf/webench_design_3519623_8.pdf.  This is the supply for a low volume commercial product.  I'm not really an engineer but I've had a pretty good run of successes designing working boards so...

I've used it as shown except that I have added a 100pf cap in parallel with Cout (physically close) and have a few more of them scattered around the board.  I've populated several boards and the results are always the same.  When I look at the 5v output, I see a spike and ringing at the switching frequency.   The spike is about 200mV and the ringing settles down in about 100 nsecs.  While the design says 150 ma, the actual load is probably about 35 ma.

Here's the part where I get confused.  I've jerry-rigged a 100 uF cap in parallel with the 10 uF Cout and NOTHING CHANGES.  The board layout is not identical to the suggested layout but it is compact, reasonable, (a 2 layer board) and there are ground planes top and bottom in the power supply area.

Everything works, everything is stable --the microprocessor, the com chip, the LCD display -- but I would certainly like to feel more confident before I commit to a production board run.  I'm looking at the results with an ancient Tek 2465A scope (13pf 10x probe) but for its time, it was a good one,  it works fine and I don't think it is the problem.  Should I fix something, should I pretend I never saw it, ...   Any help would be appreciated.

Bill

  • 0
    TI__Intellectual 2930 points

    8750.Capacitors Selection for High Frequency Noise Mitigation.ppt

    Hi Bill,

    The high frequency ringing you are seeing is going to be a result of the layout and the output caps ESR and ESL. The 100uF cap you added in parallel with the 10uF probably had little effect because the bigger cap has higher ESR and ESL, so at high frequency it looks like a high impedance node. You may want to instead try adding some smaller caps in parallel that will have low impedance at the ringing frequency. 

    Attached is a presentation that should give you some helpful information on Cap selection for high frequency noise.

    Hope this helps,
    Tommy 

     

  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    Prodigy 20 points

    Thanks.  I forgot to say -- I tried another 10 uF cap (soldered right on top of the one on the board) and the difference was minimal.  The caps I'm using are the exact ones called out in the design so I think that I'd have to add a bunch to get the ringing down.  There is some slight ringing on the input side of the choke -- is this normal?

    I'd be happy to have somebody look at my layout -- I think it makes sense.  Or, can I safely ignore the ringing? 

    Bill

  • 0 in reply to Wm Rubenstein
    TI__Intellectual 2930 points

    Hi Bill,

    Whether or not the ringing is acceptable is up to you and your application. You say everything that's relying on its power seems to be working, so we know its not terrible. But if you are running any high frequency digital stuff (transievers and anything with clocks), from it you might want to make sure its not degrading their performance. Often times noise sensitive pins on these other devices will have suggested decoupling filters (either just really small caps or also ferrite beads) so that this noise doesn't get through.

    From what you said your ringing seems like its probably between 10MHz-100Mhz, so you'd probably be looking at putting some caps between 1nF-10nF in parallel with your Cout, but best to look at there impedance characterizations an find caps whose minimum is close to your ringing frequency. 

    You are welcome to attach your layout here for review. Or you can email me at Tommy.Jewell@ti.com if you don't want to post it in the forum.

    -Tommy