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LM3489-Q1: Inductor ripple formula clarification

Part Number: LM3489-Q1
Other Parts Discussed in Thread: LM3485, LM3489

Reference the latest LM3489-Q1 datasheet: SNVS443C –MAY 2006–REVISED DECEMBER 2016

On page 15, The equations for the inductor ripple as a function of output current is:

 

(1) My question is if the equation for Iout < 2A is correct?

The reason I ask is that the equation for the LM3485 (similar device) is different.

Reference the latest LM3485datasheet: SNVS178H –JANUARY 2002–REVISED DECEMBER 2015

(2) Is there a formula for Cff?

*** As a side note, there is a missing equation on page 16 of the LM3489-Q1 datasheet. See changes below:

  • Hi Michael,

    The hysteretic architecture is not very controllable, the on-time depends on the time it takes FB voltage to reach hysteretic threshold, which is a function of things including output capacitance, inductor current, and load current.

    As these variables change depending on application and use, the equation shown is only an approximation to try and select an appropriate inductor for the output current. The true resulting ripple current will vary from the equations with the variables mentioned before.

    If you are using the LM3489-Q1 use the equation associated with it. 

    There is no formula for Cff, the datasheet guidelines state "A good starting point is 470-pF ceramic at 100- kHz decreasing linearly with increased operating frequency."

    Hope this helps, let me know if you have further questions

    -Orlando

  • Hi, Orlando -

    Thanks for the quick reply!

    I understand the formulas are a "rule of thumb" approach. Due to the nature of this particular converter, lots of things have an influence on each other!

    So I will use the equation shown in the LM3489 datasheet.

    BTW - You didn't make any mention of the LM3485 equation. I assume this is a mistake?

    Regards...

  • Michael, 

    I would not say the equation is incorrect. Perhaps the automotive part is designed with higher ripple current for the trade-off associated with it (faster response but more output ripple).

    Let me know if you have issues working with this!

    -Orlando