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LM5574 Frequency "Glitch"

Brian Mohlman
Prodigy 70 points
Other Parts Discussed in Thread: LM5574

Hello All,

I have prototypes of a LM5574 design that has 7 - 41 V input and outputting 3.3 Volts. (I used WebBench to design it) It appears to be converting OK, but when I look at the recirculating diode side of the inductor with a scope I see small spikes (about 5 volts peak) between the square waves and was wondering what would cause this? I have a LM5574 eval board and it doesn't appear to have these spikes. Any ideas what is causing this or how to mitigate? The device I am powering pulls from 20  - 250mA.

Thank you

Brian

  • 0
    TI__Expert 3950 points

    Hello Brian,

    what can cause the spikes are stray inductance of the PCB layout.

    I would like to take a look at your PCB layout and please share your webench design and some scope shots of the problem.

    Regards,

    Giuseppe

  • 0 in reply to Giuseppe Pinto
    Prodigy 70 points

    Giuseppe,

    Thanks for your reply, I have attached the webench file, the top and bottom layer and also a scope trace of what I am seeing. The only thing I noticed is this occurs when the switcher is lightly loaded, ie. 18 - 21 mA, but if I put loading resistor on the 3.3 Volt side the spike goes away. My problem is I don't want to have to load it falsely to get it to work.

    Thank You,

    0728.LM5574.zip.zip

  • 0 in reply to Brian Mohlman
    TI__Expert 3950 points

    Hello Brian,

    It looks to me just simple ringing due to discontinuous current mode therefore that's not a problem. The ringing happens when both internal MOSFET and diode are off since the inductor's current goes to zero.

    Furthermore I suggest you to create a ground plane that will join both the ground side of the input caps and the IC ground, this will allow to reduce the AC current loops of the converter hence less noise.

    Anyway, like i said it looks just like DCM ringing, can you take a scope shot of the inductor's current?

    Regards,

    Giuseppe

  • 0 in reply to Giuseppe Pinto
    Prodigy 70 points

    Giuseppe,

        Thanks for your help, I will see when I get back to the lab on Monday if I can get a snapshot of the inductor current. I am sending the top layer again with the addition of the vias tie the input caps and the IC to the bottom ground plane, is there something more I need to do?

     

  • 0 in reply to Brian Mohlman
    TI__Expert 3950 points

    Hello Brian,

    i think that's it for now.

    Regards,

    Giuseppe