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LM3409HVEVAL: Design 3 conflict with datasheet specs

felipe carvalho
Prodigy 150 points
Part Number: LM3409HVEVAL
Other Parts Discussed in Thread: LM3409HV

I bought a 3409hv evm and I'm changing it to work with a 2.8A Iled and 0.15A detailed. A design somewhat similar to design 3 of the evm,but with slightly higher output current. The issue I now encountered is that in the lm3409hv datasheet (8.3.4) it states that the inductor ripple current should be greater than 24mV/Rsns. A condition which both design 3, and consequently my design, does not meet.

Is design 3 incorrect or is the datasheet incorrect?

Thanks,

  • 0
    TI__Mastermind 28655 points

    Hello felipe,

    I will check the values and get back.  This doesn't hurt the operation just in extreme cases the offset can make the ripple frequency seem like half.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Dear Irwin

    Thank you for your reply. I found some information which my be relevant... What I pointed out seems to be true for the inductor current, but not the led current. Perhaps that's the difference. In any case, I'm interested to understand the effect of having such a low inductor p-p current because the current modifications I made seems to work fine so far.

  • 0 in reply to felipe carvalho
    TI__Mastermind 28655 points

    Hello Felipe,

    Design 3 I calculate 78 mV of ripple for Vcs, well below 24 mV.

    Did you mean Design 4?  I get 34 mV for that one.

    If you increased the current for Design 3 to 2.8A with 68 uH at 450 KHz it would be down to 22 mV but that isn't any of the designs for the EVM.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Hi Irwin,

    Perhaps I didn't make my question clear and that could be due to a lack of understanding from my part.

    Section 8.3.4 in the lm3409hv datasheet provides an equation for the minimum inductor current ripple.

    Design 3 of the lm3409hv Eval seems to break that rule, as would my design.

    I'm slightly confused by your calculations... 78mV lower than 24mV?

    Which calculations have you done?

    If they are related to my question then I would expect your results to be in mA (current).

    Regards,

  • 0 in reply to felipe carvalho
    TI__Mastermind 28655 points

    Hello,

    I am converting the ripple current of those designs back to ripple voltage on the current sense resistor.  The equation is calculating the minimum ripple current to maintain 24 mV of ripple on the current sense resistor.  I'm just taking the ripple current and returning back to voltage.  They are all higher than 24 mV which means they should all be okay.

    By using the equation for Design 3 the Lpkpk minimum would be 80 mA, 250 mA is much larger than this.  By using 250 mA there will be approximately 78 mV of voltage ripple on Ris.

    Best Regards,