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LM5122: Design Review

Deepak R
Prodigy 20 points
Part Number: LM5122

Hello,

I am designing a LM5122 based dc dc boost converter.  The input voltage varies from 9 - 18v and output voltage required is 24v, 10A. I used Webench tool and LM5122 datasheet to design this converter. I need your help in determining whether the PCB layout follows the guidelines for boost converter. I have referred the LM5122 and 5123 datasheet, TI's guideline for power converters and the PCB layout generated by Webench. Please provide your feedback and suggestions if there are design improvements to be made. I have attached the my schematic and pcb layout for reference. Also, providing the Webench's data link:  webench.ti.com/.../SDP.cgi

Thanks,
Deepak 

5756.Schematic.pdf6327.Top layer.pdf8484.bottom layer.pdf

  • 0
    TI__Genius 10385 points

    Hi Deepak,

    Thank you for reaching out, I will look into your schematic and layout. Please allow me some time to get back to you as my bandwidth currently is very narrow.

    I shall have something for you latest by end of next week.

    Thank you

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Prodigy 20 points

    Hi Haroon,

    Thanks for taking time to address my query. I realized that I did not attach the Eagle files of PCB design. I am attaching these files just in case you need it.

    Thank You,

    Deepak

    PT 100 522 v48.brdPT_100_522_v20.sch

  • 0 in reply to Deepak R
    TI__Genius 10385 points

    Hi Deepak,

    Thank you for the files.

    I will get back to you as soon as possible.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    TI__Genius 10385 points

    Hi Deepak,

    Sorry the delayed response, regarding the review:

    - The design does not follow TI suggestions on the CBST or C15 capacitors capacitance and also there are no filters on the RS as suggested in the datasheet, it would be better to have those in the schematic.

    - Your inductor seems ok, but the ripple ration of the inductor current is 17%, normally we suggest 30 to 20%. I would make the inductor a little bit smaller like 4.4uH or 3.7uH. In any case make sure that your inductor has a rated current of around 30A or so and a saturation current above 35A or so.

    Apart from that I would also change the compensation circuit to the following: Rcomp = 60k ohms, Ccomp = 36nF and CHF = 700pF.

    Hope this helps.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Prodigy 20 points

    HI Haroon,

    Thanks for reviewing my design.

    -As suggested, I have added the current sense filters. The datasheet suggests Rcsp and Rcsn are both 100 Ω and Ccs is 0.1uF. However, what will be wattage of both the resistor?. I want to calculate for max Vin of 18V.

    About the Cbst value, section 7.3.8 in datasheet suggests Cbst  should be greater than 0.1uF. Cbst as per calculation method provided in the typical application is around 0.22uF. The high side mosfet is CSD1751QA which has Qg of 32nC @7.6v. Selection of 1uF should meet the above recommendation?? What am I missing?

    How did you calculate the values of compensation circuit? I followed the method suggested in section 8.2.2.19 but they are nowhere close to yours.

    BR,

    Deepak

  • 0 in reply to Deepak R
    TI__Genius 10385 points

    Hi Deepak,

    Thank you for using E2E forums, regarding your query please allow me some time to get back to you on this, at the moment our bandwidth is quite narrow.

    BR,

    Haroon

  • 0 in reply to Deepak R
    TI__Guru 59435 points

    Hello Deepak,

    The wattage of the resistors is low as the input impedance of the sense amplifier is relatively high, so the standard 1/8th W should be sufficient.

    1uF for CBST should be ok.

    The 2nF capacitor in parallel to the RC compensation network seems to be somewhat high, but you can try with whichever compensation is sufficient for your application.

    Best regards,
    Brigitte