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LV2862: Design not able to provide designed current level

John Piskulic1
Prodigy 20 points
Part Number: LV2862

Created a design via Webench, (18-60Vin / 12V@0.4A out) however actual build can only maintain 12V up to around 12mA after which it drops fairly precipitously, ie: 470ohm load drops to 3V. The layout is pretty close to what is recommended in the datasheet. I have tried increasing output C, tried a different diode, doubled Cboot to no effect. The switch waveform has a fair amount of ringing. I can provide all the documentation. Also, this is the Y versions of the LV2862.

  • 0
    TI__Mastermind 30430 points

    Hello John,

    Can you share the webench design as a picture in the forum?

    Please include both schematic and show the loop response.

    in addition, if you have them available, please share scope shots of vin, sw, and vout.

    On switch measurement, careful attention must be placed on ground connection. Tip and barrel method is recommended for probing.

  • 0 in reply to Marshall_Beck
    Prodigy 20 points

    Hey Marshal,

    I sure can. Let me try and give you as much info as I can. I have attached the following images:

    1. Page 1 of the design containing schematic

    2. Page 5 of the design containing the loop response

    3. The supply section of my schematic

    4. My layout

    5. The original SWX waveform

    6. I had noticed that on the eval board that the input cap, diode and output cap were all taken separately and directly to the ground plane. I cut my upper plane to separate these and took them directly to the ground plane and got the following: Vin, Swx and Vout

    I did use the tip and barrel method. I would have to assume that the swx ringing is the issue, but not sure what to try from here. I have also tried adding additional output and boost C and tried a different schottky diode, but all with no affect. It does seem that the swx ringing is not quite as bad with the grounding modified so it does seem that is the crux of the problem.

    Thanks

  • 0 in reply to John Piskulic1
    TI__Mastermind 30430 points

    Do you have persistence on your scope?

    Can you take the same waveform with it off?

    Might have to wiggle your trigger point around.

    It seems OK if the above is the case.

    The ring you are seeing is DCM ring (both FETs tri state and their individual patristics and output inductor resonate)

    The output cap does seem terribly small. May I suggest increasing to 22uF, 16VDC if not done so already.