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WEBENCH® Tools: LM5118

user6354596
Prodigy 35 points
Other Parts Discussed in Thread: LM5118, LM5121

Tool/software: WEBENCH® Design Tools

hello，

Is the input and output polarity of lm5118 opposite? For example, can I input - 48V and output + 9V? As a buck boost circuit, it should support the opposite polarity of input and output. Why can my circuit output normal + 9V only when the input voltage is positive? Please help to answer, thank you!

  • 0
    TI__Guru 65200 points

    Hello user6354596,

    LM5118 is a non-inverting buck-boost controller. This means that input and output voltage have the same polarity.

    As far as I recall, we do not have devices that support negative input voltage and create positive output voltage. So I would recommend to use a flyback converter with floating GND on the primary side (-48V connected to GND pin) and then use the transformer to invert the voltage.

    LM5121 could for example be used for such a design.

  • 0 in reply to Brigitte
    Prodigy 35 points

    Thank you very much for your reply.

    You mean isolated output through transformer, right?

    You recommend the chip of lm5121, this is boost topology。but my solution is - 48 input and + 9V output. is It should be buck topology？

  • 0 in reply to user6354596
    TI__Guru 65200 points

    Hello user6354596,

    Yes, I mean that you use some type of transformer to isolate the input voltage from the output voltage.

    A flyback topology has a low side power switch and such a switch can be driven as well by any boost controller. The bucking functionality will be created by the turns ratio of the transformer.

  • 0 in reply to Brigitte
    Prodigy 35 points

    That's a good idea. I can give it a try. Thanks again