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LMR33640: +/- Step-down Output with Sepic Design

User4699
Prodigy 70 points
Part Number: LMR33640

Tool/software:

Dear Experts Greetings from Istanbul.
I need advice abouth my An inverting and non-inverting step-down output design.

Due to my limited pcb space and component count I need to get both + and - output from a single step down DC/DC Converter.
Q1- Is this design technicaly correct ?
Q2- If not, i would be grateful for your valued information on how to fix and improve it ?
Q3- I would also be great if you could help me to choose correct components.
Best Regards

  • 0
    Prodigy 70 points

    Hi again;
    As an alternative, i would like to share my other circuit inspired by these TI'S split-rail buck converter topology;slyt380
    Which of these two circuits is more suitable for my application?
    I will be waiting for your valuable thoughts and comments.
    Best Regards

  • +1 in reply to User4699
    TI__Mastermind 18685 points

    Hello,

    The second design is closer. A couple things need to be changed. Refer to Figure 2-2 of the below application note.

    Remove D1 from the circuit. If that is a reverse current protection diode, it needs to be placed after the output capacitors of the buck.

    I recommend using these application note as a guide

    There is a section on inverting designs.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 70 points

    Ridge
    Thank you very much for your quick reply and valuable comments. I made the change you specified. Finally, after your short answers, my problem will be solved.
    Q1- Is 1Mhz design suitable? Or i can able to re-design as 400kHz thanks to LMR33640 settings
    Q2- I do not need to isolate the ground connection in my circuit. Does it cause a problem if I connect it directly to the common ground?
    Q3- According to my output voltages/current consumptions, does the coupled inductor I choose meet my needs?
    Thank you.

  • +1 in reply to User4699
    TI__Mastermind 18685 points

    Hello,

    I have listed my answers to correspond with the questions.

    Q1: Either 1MHz or 400kHz is OK

    Q2: Connecting the to the same GND is OK. As you stated, it will not be an isolated output.

    Q3: That inductor is very large for this application. I calculate 8.2uH using Equations 16 and 17 in the application note linked in the above post, assuming Fsw=1MHz. Peak current mode devices require a certain amount of current ripple to regulate properly. Target 20%-40% current ripple.

    Best regards,

    Ridge