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LM3509: String length?

Steve Fenwick
Prodigy 40 points
Part Number: LM3509

I'm looking at using the SPMWHT541ML5XA (attached) with the LM3509SD. I want to run strings of 6 LEDs; we  want a specific light dispersion. Vf (max) is 2.9V. Current draw is expected to be 30mA per string, although it looks like the part might support higher current.

Since the LM3509SD can support up to 21.2V per string, is there any reason not to use 6 LEDs each 2.9V, total 17.4V, for MAIN and SUB?

Also, with a 10uH inductor and assuming 85% efficiency (5V nominal input), it looks the the internal switch can support up to 0.12A (Peak Current Limit, equation 1). Is that total for the part or per string?

Thanks!

Steve

lm561c_rev.7.0-1.pdf

  • 0
    TI__Genius 9630 points

    hi Steve

    not sure I fully understand question #1. Do you mean any performance difference between Vout=21.2V and 17.4V?

    For question #2, the current limit means total for the part. BTW, current limit for this device should be 0.77A

    Also have some questions from my side if you do not mind sharing.

    #1 Is there any specific reason to choose LM3509 as this is a quite old device? we still have other 2-channel devices available.

    #2 regarding "light dispersion", any specific spec of LED driver is intended to pay more attention to? what is the application here? thanks 

  • 0 in reply to Mason Wang
    Prodigy 40 points

    Let me clarify question 1. The LM3509 spec only lists up to 5 LEDs per string, but it seems like, as long as the total Vf of the string is under 21V, it should work. Is that correct? In our case we want to use 6 LEDs for a total max Vf of 17.4V.

    We are using this part because I2C control is important; I don't have a handy PWM source. I can get generate an analog voltage, 0-1V or maybe higher, if that would open up alternatives. Component availability for immediate prototype builds and later production is very important.

    Light dispersion: this is for a light bar application. We don't want to use a single large LED, even though that could provide the same number of lumens.

    Thanks!

    Steve

  • 0 in reply to Steve Fenwick
    TI__Genius 9630 points

    hi Steve

    yes, your understanding is correct. 6 LEDs with 17.4V string voltage is available. I have evaluated the operation condition with Vin 5V, Vled 17.4V, Iled 30mA*2, 10uH inductance, the peak current is within current limit. 

  • 0 in reply to Mason Wang
    Prodigy 40 points

    Thanks!

    With regards to your comment about this being an older part, is there concern about it becoming unavailable?

  • 0 in reply to Steve Fenwick
    TI__Genius 9630 points

    hi Steve

    May I ask the volume of this project and MP schedule? I will confirm the condition of this device internally. BTW, what is the end equipment of this device?

  • 0 in reply to Steve Fenwick
    TI__Genius 9630 points

    hi Steve

    no concern for this device as no EOL plan. thanks