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LM20136: Inductor calculation from datasheet differs from WebBench and Design Spreadsheet

Brandon Moretz
Prodigy 10 points
Part Number: LM20136
Other Parts Discussed in Thread: TPS62812-Q1, TPS628512

I am using the formula on page 16 of the LM20136 datasheet to calculate the minimum inductor with the values below.  I am getting a minimum inductor value much larger (6.7uH) that what is recommended by the WEBBENCH and design spreadsheet (1uH).  Am I missing something?  I know my continuous output current (for this instance) is well below the maximum rated output current but seems like the equations should align between the tools.  

Vin=5V, Vout=1V, ripple current=0.3A, around 40% of max output current, switching freq= default (410kHz)

Thanks.

  • 0
    TI__Guru*** 142910 points

    Hi Brandon,

    Yes, you are encountering discrepancies because your output current is so much lower than what this 6A device can provide and which most designs would be for.

    Why do you use the old LM20136?  What features do you need in your application?

    If you need clock sync, for example, we have TPS62812-Q1 and TPS628512 for 2A devices.  Those are also rated up to 150 C Tj.  

    Thanks,

    Chris

  • 0
    Prodigy 10 points

    Chris, Thank you for the reply.  This is an existing design that is being analyzed to be used for a lower current application.  If we were to keep this design would you recommend using the inductor value that was recommended by the WEBBENCH/design spreadsheet or the equations from the datasheet.  Aside from the lower efficiency when operating at lower current, are there any other concerns operating this part around 0.75A?

    Thanks.

  • 0 in reply to Brandon Moretz
    TI__Guru*** 142910 points

    Hi Brandon,

    Thanks for explaining.  The D/S is the official document, so please follow it.

    But use 6A as your output current for purposes of calculating the passives.  This should then match the D/S and Webench much better.

    There is no issue operating most any power supply at lower load currents, which is what you are doing.

    Chris