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LM3410XMFLEDEV: Looking for way to calculate Inductance in App notes

Jeff Sylvester55
Prodigy 70 points

Part Number: LM3410XMFLEDEV

I am looking at the formulas for calculating the part values in the App notes section for typical Boost Application driving one string of 5 leds in series,  when It comes to calculating the Inductance, the formulas seem to have circular reference,  one formula uses unknown value from anther formula, which its result depends on the first unknown formula.    Delta current Inductor,  = Vin / 2L * DTS   and  inductance L = Vin / 2 Delta current Inductor * DTS.    so my question is what am I missing,  it there a constant, or guess I should be using, later on it says use 20% - 50% ripple current?   so for example section 8.2 Typical Application, I can not see how these part values were derived / calculated

  • 0
    TI__Mastermind 25500 points

    Hello Jeff,

    Yes, that could be explained a little bit better. But the general idea is to set the inductor ripple current to around 20% to 50% of its average current at the lowest input voltage. It's a rule of thumb, you can go outside of those bounds. But for example using the same application you mention in the datasheet:

    -You can assume efficiency of approximately 90%.

    -Pout is 50mA times 14.6V which is 0.73W. Pin would then be about 0.73W/0.9 which is 0.811W.

    -Iin (average inductor current) is then 0.811W/2.7V which is around 300mA.

    -Calculate the inductor value using a peak-to-peak ripple current of 20% to 50% of that, or 60mA to 150mA.

    I hope that helps.

    Regards,

    Clint