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Original question:

Calculating L1 for TPS50601 using example.

TPS50601-SP: Calculating L1 for TPS50601 using example

Mark Kien
Prodigy 60 points
Part Number: TPS50601-SP

Please reference issue, "Calculating L1 for TPS50601 using example"

The datasheet still seems to have this factor of 2 error in subsection 9.2.2.2 for the output inductor, L1, calculation. 

Can anyone comment on why the output inductance calculates to twice what is given as the solution (5.4uH as apposed to 2.7uH) for this design example.  I speculate the Kind refers to the ratio between PEAK ripple current (not PEAK to PEAK) and the RMS output current.  

Regards

  • 0
    TI__Mastermind 28905 points

    Hey Mark,

    Actually it seems the value stated in the datasheet is simply miscalculated.


    I double checked our excel calculator and it shows the correct value of 5.46 uH
    Power Stage Designer also gives a value of 5.46 uH

    I will put this down to fix for the next update of the datasheet.

    Thanks,
    Daniel

  • 0 in reply to Daniel Hartung
    Prodigy 60 points

    5.4uH is what everyone calculates using the given formula.  When the ripple current is calculated I get ~606mApp.  This conflicts with direction in section 8.3.9 where the minimum peak to peak inductor current is supposed to be 1A.

    Going back to fundamentals for this type of converter.  If I do KVL on the converter during the charge cycle, D.  Using the datasheet example:  Vinmax = 6.3V, Vout=3.3V, fsw=480kHz, D=Vout/Vinmax, L=5.4uH

    Vl=Ldi/dt, Vg=Vl+Vout >>> di=(Vinmax-Vout)*(D/fsw)/L

    Given that the di or the peak to peak ripple current calculates to 606mA with an inductance of 5.4uH, I think think the selection of 2.7uH is the correct thing to do with respect to Section 8.3.9 resulting in a ripple of 1.2App.

    The use of "Kind = 0.1" is suspect here.  You might need to dig a little deeper to ensure the example is valid.

    I saw this mistake years ago but never mentioned it as I was forced to use another converter at the time...sorry.

    Thanks,

    Mark

  • 0 in reply to Mark Kien
    TI__Mastermind 28905 points

    Hey Mark,

    Of note:
    The 1 A "minimum" is more of a recommended minimum and not a hard number.
    If you go through your design and find a smaller number is appropriate that is fine.

    A conservative number is given in order for you not to run into the noise of the current sensing portion of the high side current sense.
    1 A is perhaps much too conservative however.

    Thanks,
    Daniel