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WEBENCH® Tools/LM2733: Unstable output. Webench not working. Webench export to Tina screwed up

David Gustavson50
Prodigy 170 points
Part Number: LM2733

Tool/software: WEBENCH® Design Tools

I am trying to use the LM2733 as a boost converter for a low load, < 20mA.  +5V input, +15V output.  The output is unstable and has a >1V oscillation at about 1KHz.  

Webench for this part seems completely broken.  Here is a screen shot:

If I export Webench into Tina, note how the part pin names seem to be in the wrong places:

Please help.  Other posts I see about stability issues with this part are unresolved.

  • 0
    TI__Intellectual 2655 points

    Hi David,
        I download TINA file from Webench, the pin is in the wrong place, too. I think Webench has some issue, I will tell this to Webench team.
        But you could change the wire connection and do the simulation.  I think your output unstable because of output cap model is not right. You could change it in the TINA. Please find my SCH and waveform as below.

  • 0 in reply to Jing Ji
    Prodigy 170 points

    Hi Jing,

    Thanks for you response.  My actual circuit uses a 220uH inductor, since the current is so low.  I usually try to keep switching regulators in continuous conduction mode.  The problem is the actual circuit is unstable, the simulation doesn't show any problem, even with the transient response to a step in current.

    I changed the inductor in the actual circuit to 15uH, and that is stable.  The datasheet doesn't say anything about a maximum inductor value, but that seems to be the case.

    Thanks,

    Dave Gustavson

  • 0 in reply to David Gustavson50
    TI__Intellectual 2655 points
    Hi David,
    You don't need to keep it in CCM. LM2733 could work at DCM. 10uH or 15uH are both OK.
  • 0 in reply to Jing Ji
    Prodigy 170 points
    I am up and running, but I think TI should address 2 issues:

    1) The LM2733 is not stable with larger inductors. The datasheet should be changed to give guidelines on inductor selection.
    2) TINA models should be improved to show unstable behavior with larger inductors. The whole point of a simulator is to show problems BEFORE you build a physical circuit.

    Thanks,
    Dave Gustavson
  • 0 in reply to David Gustavson50
    TI__Intellectual 2655 points
    Thanks David,
    I will update your feedback.