LT8614, A discrapancy between the datasheet and LTpowerCAD

Hello, in page 15, equation 6 in the datasheet of LT8614 there is "A good first choice for the inductor value". This value is about 10 times larger than the recomendation in LTpowerCAD. I suspect that equation (6) is wrong. I would like to know a real good first choice for the inductor and it would be great for it to come with some explanation.

Another thing, in page 16 you say:

"Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation. See Analog Devices Application Note 19",

but I could not find this application note.

Thanks in advance for the help,

Best regards.

  • 0
    •  Analog Employees 
    on Apr 19, 2021 12:53 PM

    Hello,

    Do you have examples for the different inductor values from the datasheet and LTPowerCAD?

    For your 2nd question:

    https://www.analog.com/media/en/technical-documentation/application-notes/an19fc.pdf

    kind regards

    Markus

  • I lost acces to dgm2029 so I write from this account.
    1) An example- LTpowerCAD suggests 35uH and the datasheet 3uH. By the way I noticed that the units in page 15, equation 6 are not inductance units.
    2) Thanks, it is helpful, but I have a question. In page 73 (AN19) they take Sx=2*10^5A/sec. How do I get Sx for lt8614? I need to know the minimal inductance I need to avoid subharmonic oscillations.

    Best regards.

  • 0
    •  Analog Employees 
    on May 3, 2021 3:56 PM in reply to Dgm2030

    Hello,

    I can only check your stated values for the inductor size (example in the ds and given from LTPowerCAD)  if you deliver your LTPowerCAD file and refer to the example in the ds.

    The formula you mention refers direct to the LT1070 parameters. For the LT8614 you should use the stated formula in the LT8614 datasheet, even if the physical unit is not correct (it results in Vs not Vs/A). Don't feel irritated as soon as we have the LTPowerCAD file resp. your intended setup things will be pretty clear and easy.

  • LT8614_01.zip

    I uploaded the LTpowerCAD file and a picture of the design (marked the suggested inductance) and a screen shot from the datasheet and the calculation.
    We can see that LTpowerCAD suggests 3.07uH and the datasheet suggests 35.5uH.

    About the other topic, there is no Sx value in LT8614 and its datasheet says to look in AN19.

    You said the formula in AN19 is for LT1070, so how do I calculate the minimal inductance to avoid harmonic oscillations for LT8614.

    Note: I want to be sure you understood me. I am writing here about two totally different isues:

    1) Calculating the suggested ("good starting value") inductance in LT8614.

    2) Calculating the minimal inductance to avoid harmonic oscillations in LT8614.