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LM3509: Maximum LED current

Menaka Srinivasan
Intellectual 520 points
Part Number: LM3509
Other Parts Discussed in Thread: TPS61169, TPS61165

Hi,

I have planned to use LM3509 LED driver. 

I would like to connect 4 LEDs in series and get 40mA constant output current. Can you confirm if this is possible and what is the maximum LED current 

Thanks,

Menaka

  • 0
    TI__Mastermind 22745 points

    Hi Menake,

         Yes, you can driver four LEDs with 40mA, the total output current depends on the internal current limit(Isw maximum current).

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Intellectual 520 points

    Hi Sean,

    Thanks for the reply!

    Will Rset= 5.9k suit my application to get constant 40mA with 4LEDs connected in series, can you please confirm

    I would like to use one channel only, so can I connect 4 LEDs in series to the MAIN pin? Should I handle SUB/FB pin or can I leave it floating? Can you please clarify this

    Thanks,

    Menaka

  • 0 in reply to Menaka Srinivasan
    TI__Mastermind 22745 points

    Hi Menaka,

        for 40mA, Rset=192*1.244/40mA=5.9712kohm;

        Yes, you connect 4 LEDs sreies to the MAIN pin;

        Yes, you can left it open and Write a 0 to ENS (bit 2) of the General Purpose register to turn off the SUB/FB current sink and force SUB/FB high impedance if you don't want to use SUB/FB. But we usually suggest to use this, you can also combine MAIN and SUB/FB together to driver total 40mA for the LED string.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Intellectual 520 points

    Hi Sean,

    Thanks for the confirmation!

    Can you please explain why you suggest to combine MAIN and SUB/FB together to drive the LED, is there any specific reason or something?

    The LED that I'm driving has forward voltage of 3.1V. Can you please suggest me how to set the output voltage ( 4 x 3.1 V ) to drive 4 LEDs in series with Vf = 3.1 V each?

    Awaiting your reply

    Thanks,

    Menaka

  • 0 in reply to Menaka Srinivasan
    TI__Mastermind 22745 points

    Hi Menaka,

        It is better not to float the SUB/FB pin so I suggest you use the MAIN and SUB/FB pin together to driver 40mA. For our LED driver, it outputs constant current so you don't need to set the output voltage and the voltage is adjustable.

        Actualy you can also use our TPS61169 and TPS61165 which is the single channel LED driver.

    Regards

    Sean