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LM5158-Q1: Two inductors or a transformer

Garrett Schmuelgen
Prodigy 40 points
Part Number: LM5158-Q1
Other Parts Discussed in Thread: LM5157

I am trying to use this part in the SEPIC topology. In the data sheet it looks like there are two inductors, but when using the quickstart calculator provided in the data sheet I noticed that it looks like they're actually the two windings of a transformer because of the dot on the symbol. Which one is correct? Does it matter which one I choose to use?

Thanks

  • +1
    TI__Genius 16195 points

    Hello Garrett,

    Thanks for reaching out to us via e2e.

    1)
    When you look at the product page of the LM5157, under Design tools & simulation, you will find three different Quick Start Calculators for three different topologies.
    Please make sure that you are using the correct version.
    www.ti.com/.../LM5157

    2)
    A SEPIC needs to use two identical inductors. Those can be two separate parts but they can also be combined on one core.
    Which option to choose is more a question of space and cost.
    When they are combined, it is important to connect them in the right way, therefore the dots.

    In the end the two combined SEPIC inductors are similar to a Flyback transformer, with the exception that both windings must be the same, whereas the flyback can use different turns ratios.

    Best regards,
    Harry