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LM34936: Please help review this schematic

Clas Sivertsen
Prodigy 45 points
Part Number: LM34936
Other Parts Discussed in Thread: LM5177, LM51772

Hi, can you please help review this design

lm34936_sch_01.pdf

  • 0
    TI__Expert 3260 points

    Hi Clas,

    Thanks for reaching out. If this is a new design, I would like to point you to the new four-switch-buck-boost converters we have in our portfolio: the LM5177 and the even newer LM51772.

    I've taken a look at your schematic and compared it to the proposal by Webench. I see you've chosen a smaller inductor, which would influence the inductor current ripple.
    Furthermore, your compensation circuit looks a little bit different, I'd like to point you to Switch-mode power converter compensation made easy for related questions. As long as you keep in mind that the chosen components in the compensation loop can only be approximations, and the actual tuning of said circuit should be done in a lab to keep the phase margin above 60 degrees. Which would assure system stability across component variation, drift and temperature changes.

    For further considerations, please also look at section 8 of the LM34936 datasheet.

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    Prodigy 45 points

    Hi Bryan,

    Thank you for your feedback! I appreciate the recommendation of the newer parts, unfortunately neither of these two are safe to spec in from an availability standpoint, as they are not readily available in DigiKey/Mouser and other places listed at octopart.com. 

    I pretty much used the design from Webench. Then, i took the LM34936 datasheet and your suggested document slup340.pdf and used ChatGPT to calculate the values for the 3 compensation components. I have actually made a Custom GPT specifically for DC-DC converter design, and then just attach my schematics and let it work from that. Below you can see the result, which is quite a bit different from the values calculated by Webench. 

    Could you please review this data and output from ChatGPT and from Webench and suggest to me which values i should build with as a starting point before we do the lab validation of the results?

    Clas

  • 0 in reply to Clas Sivertsen
    TI__Guru 58641 points

    Hi Clas,

    please use the attached quickstart calculatorLM34936 Buck-Boost Quickstart Tool_V1_0_0.zip for the design - this has been targeted for this device.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 45 points

    Thank you so much Stefan! I have used the excel macro tool you suggested, and here is the circuit now.

    Could you take a look and check if there is something i need to change?

    Clas

    LM34936 Buck-Boost Quickstart Tool 2024 FEB 25.pdf

  • 0 in reply to Clas Sivertsen
    TI__Guru 58641 points

    Hi Clas,

    looks OK from my side but as only a pdf is attached please also check the stability (Bode plot) over the whole operation range.

    Also check the loss over the full operation range of Vin.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 45 points

    Thank you so much Stefan,

    Can you please review the attached pdf with the bode plot and loss?

    Clas

    WEBBENCH_LM34936 Schematic2.pdf

  • +1 in reply to Clas Sivertsen
    TI__Guru 58641 points

    Hi Clas,

    looks OK from my side on the in the pdf shown operation point.

    Also check the stability and loss over the full operation range of Vin.

    Best regards,

     Stefan