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LM3429: SEPIC application details

FHR
Prodigy 40 points
Part Number: LM3429

Hi,

I was wondering if there are any design tools/equations one can use for calculation of values when using LM3429 in a SEPIC mode?
The datasheet contains a SEPIC schematic at the end with values, but doesn't provide any explanation as to how those values were calculated.

I'm trying to design a fairly simple SEPIC SLA charger, so I simply need a ~1A current limit and 13.8V voltage limit - figured this chip might work as it can do CC and CV.

Would be glad for any pointers.

Thanks,
Filip

  • 0
    TI__Mastermind 25500 points

    Hello Filip,

    I cannot explain the values from the datasheet per se, but I have recently been working on some SEPIC calculators. If you would like to give me your specs such as input voltage range, max output current, output voltage, and switching frequency (if you have a preference) I could generate a pdf with the equations and calculations that could help you. SEPICs are for sure a different animal. Let me know if that would interest you. Thanks.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 40 points
    Thanks for the response,
    here are my values:

    Input voltage: 10 - 19V
    Desired output current limit: 1A (doesn't have to be exact, +- few hundred mA is fine)
    Output voltage: 13.8V

    Don't have a preference for switching frequency, efficiency or inductor rating.
  • 0 in reply to FHR
    TI__Mastermind 25500 points

    Is this automotive? The specs look similar. So I'll assume a switching frequency around 350kHz to keep out of the AM band and base it on that. Give me a little time since it's in Mathcad and not very clear what is what. So I'll run the calcs and put some notes in there and then send it to you in pdf format. If you want to change any variables (including Fsw) after that you will have the equations. I'll try and get something to you by the end of tomorrow at the latest.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 40 points
    Not automotive, it's actually a part of a charging circuit for a backup power supply.
    It doesn't need an undervoltage protection or automotive grade parts.

    Output ripple also doesn't matter much, the battery should not care.

    Thanks,
    Filip
  • 0 in reply to FHR
    TI__Mastermind 25500 points

    Hello Filip,

    I'm having some issues today with my Mathcad license and haven't been able to use it. I will try and take care of this tomorrow morning if it is fixed.

    If it isn't fixed I will at least get you the equations to use one way or another tomorrow. So sorry for the delay.

    Regards,

    Clint

  • 0 in reply to FHR
    TI__Mastermind 25500 points

    Hello Filip,

    Here is the pdf with all of the equations you need to calculate the power components for a SEPIC converter. If you have Mathcad let me know and I can send the actual .mcd file so you can change the input parameters (in yellow). I already input the parameters you gave so maybe that is all you need. Note that this is set up for the level at which, if analog dimming, you enter DCM (Po(bndry)). But it's really the same as picking a maximum inductor current ripple. If you choose Po(bndry) as 1/3 as in this example it gives a peak-to-peak inductor current ripple of 66% (666mA), if you change it to 1/4 you will have a peak-to-peak inductor current ripple of 50% (500mA) and so on.

    Feel free to let me know if you have any questions. Thanks.

    Regards,

    Clint

    Mathcad - SEPIC.pdf

  • 0 in reply to Clinton Jensen
    Prodigy 40 points
    Thank you very much! Fantastic!
  • 0 in reply to FHR
    TI__Mastermind 25500 points

    Hello Filip,

    No problem. I did forget to highlight "Irip" and "Vrip" in yellow for the output/input capacitor sections. Those are inputs you can adjust for your specific application and will change the min values. I guess I need to clean it up a bit for the future.

    Regards,

    Clint

  • 0 in reply to FHR
    TI__Mastermind 25500 points

    Mathcad - SEPIC_calcs.pdfBy the way, here is a cleaned up version with all inputs highlighted in yellow and the correct inductor value entered as an input. This will have the correct numbers given your inputs.

    Regards,

    Clint