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Webench phase margin

Bill Evans1
Prodigy 10 points

I need a boost converter:  4~5 Volt in, 14 Volts out @ 100 mA for a battery powered application.  Webench suggested an LM2735X based circuit with an unshielded inductor and 45 degree phase margin.  I selected a very similar but slightly larger shielded inductor.  The phase margin went to 15 degrees.  I changed back to the original inductor and the phase margin went to 14 degrees!

The Bode plots seemed to be nonsense.  The original had only -20 dB of gain at 1 Hz.  with the shielded inductor.  With the new inductor the phase is between -100 and -200 degrees out to 100 kHz, then dived to -360 degrees and suddenly spiked to about 0 degrees at 900 kHz.

Is the simulation working correctly?  If so, how can I change to a shielded inductor and maintain an acceptable phase margin?

  • 0
    TI__Intellectual 1825 points

    Hi Bill,

    Can you share with me your design?   My email address is jonathan.arzadon@ti.com

    I ran a design with your input requirements (4V-5V in, 14V out, 0.1A out) in WEBENCH and the Bode Plots that generated for me were fine (for both the shielded and unshielded inductors).  I did find an error with the calculated phase margin specified in the OpVals window which i am currently working to fix.

    Regards,
    Jonathan Arzadon
    Applications Engineer
    WEBENCH Design Center
    Texas Instruments 

  • 0 in reply to Jonathan Arzadon
    TI__Intellectual 1825 points

    Hello Bill,

    Thank you for sharing your design with me.  I have fixed the issue with the phase margin calculation in the OpVals window so that the correct phase is being displayed.  Now you can change between shielded and unshielded inductors without this phase margin issue.

    I'm waiting for our team to push out this bug fix into WEBENCH®, so it may take a day or two before you see the issue fully resolved.

    Please let me know if you have any other questions or concerns or if you require any other assistance.

    Regards,
    Jonathan Arzadon
    Applications Engineer
    WEBENCH Design Center
    Texas Instruments