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Generate -15V from LM27313

Paul Nathan
Prodigy 30 points
Other Parts Discussed in Thread: LM27313, TPS54620

I have used the LM27313 to boost a 5V input to +15V without problem, now I'm wondering whether the same part can be used to generate -15V without any additional components. Can I simply swap the Vout and GND connections as I see done on some other converters (e.g. TPS54620)? Or is there something about the LM27313 that prevents this? Many thanks.

  • 0
    TI__Guru 57555 points

    If your load and input supply are fully isolated, then you can define the 15V output with any polarity.

    FD

  • 0
    TI__Genius 10095 points

    If the input and output are not isolated then you can use a similar approach to making a negative rail as I used in the following post.

    http://e2e.ti.com/support/power_management/simple_switcher/f/858/t/355460.aspx

  • 0 in reply to Marc Davis-Marsh
    Prodigy 30 points

    Ah, excellent! Not isolated, so what you have shown in that post is actually exactly what I was looking to do, a bipolar boosted output using just one switcher. Your schematic leads me to a new question: When I remove the parts that make the negative voltage and compare it with the schematic found for the boost converter example on page 1 of the LM27313 datasheet, I notice the output in the datasheet comprises just a straight Schottky diode without the in-line capacitor and the diode to ground as you have it. Please could you elucidate the role and necessity of these extra components. Many thanks so far though!

  • 0 in reply to Paul Nathan
    TI__Genius 10095 points

    The cap and extra diode are left over from another circuit where I was voltage doubling the output of the boost converter so that it could exceed the output voltage rating of the part.

    They are not needed for your design.

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Prodigy 30 points

    Hello Marc, right. I've been playing with this circuit and indeed the extra capacitor and diode are not required for the +15V output but they are required for the -15V output. Without the capacitor and the diode clamping to <=0 there is no way for a negative voltage to be generated ahead of the reversed diode. So it would seem that the voltage doubler circuit is necessary after all! Any thoughts?

    I have just now found an example in the datasheet of the LT3579 (page 36) which matches your circuit topology.

    Thanks for the help, it has got me off to a good start,

    Paul