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WEBENCH® Tools/LM5164: Fly-buck with negative output and coupled inductor selection

Ilya Dontsov
Prodigy 230 points
Part Number: LM5164

Tool/software: WEBENCH® Design Tools

1. I'm planning to get split rail (+/-19V at 0.4A each) from a single positive supply (+40V). The ground should be common to both input and output. I wonder if the following concept will work:

2. The second question is about the coupled inductor selection. I'm making the design in the WEBENCH for a simple LM5164 buck converter. Can I take the same inductance value and put in a copled inductor with the same parallel inductance value? What about DC current rating in this case?

3. Another question is regarding inductor DC current. In the WEBENCH design the software shows suggested inductor with DC current of 2.5A or higher. Although L Ipp is just 0.2A. Why do I need such huge inductor?

Hope I'm not asking too many stupid questions.

Thanks!

  • 0
    TI__Mastermind 25590 points

    Ilya,

    1. Yes, that's an option. You can put GND anywhere on an isolated output.

    2 and 3. This app note should answer your questions and give more information on designing a Flybuck.

    No stupid questions here :)

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 230 points

    Sam,

    Thanks for the answers. According to the app note I get the following figures for my case:

    Vin(max) = 45V
    Vout = 19V
    IL1 = 0.4A
    IL2 = 0.4A
    Fsw = 300 kHz
    iSwPeak = 1.25A

    Calculations give me the following:

    dIL1 = 0.9A, suggested L1 = 40.65uH. 

    For the selected 220uH inductor I get dIL = 0.166A. So the total current is 0.4A + 0.166A = 0.566A. I read in other papers that it's better to choose an inductor rated for maximum current limit for the IC, which in my case is 1.25A. However, WEBENCH insists that the inductor should be 2.5A or more. Is there an error in the software or is there a mistake in my calculations?

  • 0 in reply to Ilya Dontsov
    TI__Mastermind 25590 points

    Ilya,

    The max high-side current limit is 1.75A (1.5A typicial) so it should be rated for 1.75A in the case of a soft short.

    WEBENCH is a little more conservative, plus it suggests existing parts which meet or exceed the requirement.

    -Sam