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

Francesco Anzalone
Prodigy 110 points
Part Number: LM3410

Hello,

For my applicartion, I'm using two identical LM3410s to drive 4 white LEDs in series (20mA, 3.1V) and one blue laser diode (90mA, 5.5V), in the configuration of the typical application 8.2.1 of the datasheet.

I'm using the DIM input in order to turn on/off the leds or the laser with a 30Hz frequency.

My issue is during the time when LEDs and laser should be off (DIM is low). I noticed that LEDs and laser do not completely turn off, as the voltage on C2 does not decrease under a certain value. (see attached picture in the laser case, DIM input in blue, VC2 in yellow). Why such voltage does not decrease any further in this time?

What happens to SW pin when DIM is low? Does it stay low so that D1 is off? In this case I would expect a slow but continuos decrease of voltage on C2, but instead I see it flat (4.7V in the picture).

Thank you.

  • 0
    TI__Mastermind 28655 points

    Hello Francesco,

    I'm assuming you have a 5V input?  A boost converter cannot drop below the input voltage minus the boost diode voltage drop.  If you look at the circuit (even remove the LM3410) the input voltage goes to the inductor which is a DC short then to the boost diode to the output.  The output voltage is Vin - Vdiode.

    "What happens to SW pin when DIM is low? Does it stay low so that D1 is off? In this case I would expect a slow but continuos decrease of voltage on C2, but instead I see it flat (4.7V in the picture)."  If SW stayed low nothing limits the current through L1 which would damage the LM3410.  L1 is a DC short.  The SW is open, voltage across L1 is zero.

    Best Regards,