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Original question:

LM3410: LEDs does not completely turn off when DIM input is low

LM3410: LEDs will not completely turn off when DIM input is low

Erik Bakke
Prodigy 10 points
Part Number: LM3410

This question has been asked before, but the answer did not offer a solution to the problem.  I have a similar situation:  LM3410 is driving white leds in parallel:

Vin: 5V

Vf (entire chain): 6.14V

Max current set at 0.5A

Dimming works well with PWM, but even when the DIM pin on LM3410 is set to GND, there is still current flowing - the leds are turned on at a very low level.

I've used the LM3410 in different similar solutions with higher Vf output, and it worked fine.  Any ideas on how to solve this problem?  Thanks in advance.

Note: my circuit is basically this one with fewer serial leds:

  • 0
    TI__Genius 17385 points

    Hi Erik,

    Since I don't have the LM3410 EVM board on our office, I can't reply you right now. But I have applied for several EVM boards. I will do some bench test when I get EVM and will reply ASAP if there is any progress.

    Best Regards,

    Jared

  • 0 in reply to Jared Zhou
    Prodigy 10 points

    Thanks for the quick response, Jared!  I'll continue testing here to verify it isn't an issue with the board.  Previous successful designs with the part had higher output voltages (8-10V output with 5V input). This design's Vf is only 1.14V over Vin.  Not sure if this could cause the issue.

  • 0 in reply to Erik Bakke
    TI__Genius 17385 points

    Hi Erik,

    Thanks for your reminder, I also will test this case on our EVM board when I receive it.

    Best Regards,

    Jared

  • +1 in reply to Jared Zhou
    TI__Genius 17385 points

    Hi Erik,

    I reproduce the behavior on EVM as you said, Vin=5V, Vf=6V, Iout=50mA. When I power on Vin and set the DIM pin to GND, there is still current flowing and the LEDs are turned on at a very low level.

    When DIM pin is low, L1 can be seen as a DC short, so Vsw=Vin and the output voltage can be calculated as Vout=Vsw-Vdiode=Vin-Vdiode. It's about 5-0.4V=4.6V. This voltage may meet the minimum forward voltage of the Vled. Because in this case, I measured the Vfb is about 0.34 mV.

    The main reason that there is still current flowing is because there is still a voltage drop on FB pin. Hope this can help you. Thanks!

    If you want to know how to fix it, I will discuss with our design and reply to you later.

    Best Regards,

    Jared

  • +1 in reply to Jared Zhou
    TI__Genius 17385 points

    Hi Erik,

    I find a way to solve it, could you please share your email address for me? I can elaborate in the email. Thanks!

    Best regards,

    Jared

  • 0 in reply to Jared Zhou
    Prodigy 10 points

    Hi Jared, thank you for the response and thorough explanation.  It makes sense - I tested with a higher voltage Vf led board with the same circuit and it works fine.  If you have a suggestion how to make the lower Vf led work in this circuit, I'd appreciate it.  I'll change to higher Vf for production boards but would like to make the prototype boards functional.  Email: lrex@luminousrex.com

    Thanks again for your help.

  • 0 in reply to Erik Bakke
    TI__Genius 17385 points

    Hi Erik,

    Glad to help you!Blush

    Best Regards,

    Jared