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LM3409 analog dimming to 0A

Patrick Szabo
Prodigy 160 points
Other Parts Discussed in Thread: LM3409

Hi,

I have a similar issue as described in this post:
e2e.ti.com/.../246125

As soon as I enable the LM3409 with 0V on Pin IAdj, the LED turns on with some % of current. But I want to be able to dimm the LED over the whole range from 0 to 100%.

In the link above the solution was to place a 333Ohm resistor "across the LED output". Where should it be placed exactly? Or is there another solution to that problem?

Below is my circuit. I am generating an analog voltage out of a PWM. We need the analog dimming because we are looking to the light of the LED with a camera with variable shutter speed and dimming with PWM would generate interferences. R40 is not assembled.

Thanks!

Regards,
Patrick

  • 0
    TI__Mastermind 25500 points
    A resistor can be used to shunt the current a low IADJ, of so it should be connected from LED+ to ground (across C18). Other options would be to use a shunt FET across C18 to cause the current to go to zero, or you can pull the UVLO pin to ground which will turn off the LED current but keep the part active.
  • 0 in reply to Clinton Jensen
    Prodigy 160 points
    Thanks for your answer. I have mounted a resistor (330 Ohms) across C18 but I now have the problem that the LED only turns on when the Voltage on IAdj is around 50% or higher. But when the LED is on it can be dimmed below 50%, but still not to 0. Is the value of the resistor correct? Or how to determine the value?
  • 0 in reply to Patrick Szabo
    TI__Mastermind 25500 points
    I know someone in the past had suggested a resistor as a possible option, and it can work. But it isn't necessarily the best option. You need to size it so that the voltage across it at minimum output current is just below the voltage at which the LED conducts enough current to light up. Since the minimum current can vary with circuit tolerances and the Vf of LEDs can vary from part to part and over temp this may not be the best way to go. If zero current is needed I would recommend one of the other two methods I mentioned. But you can check the LED properties and try the resistor, it looks like 330ohm is not the correct value. But that depends on the minimum current and the LED current vs Vf curve.
  • 0 in reply to Clinton Jensen
    Prodigy 160 points

    It seems that the resistor does not work for me. I tried various values considering the few information I can get from the datasheet of the LED but it either has no effect or the LED only turns on when a certain % of output current is reached.

    I want to try the shunt FET you have mentioned. Can you provide me some more details on that? How does it fit into my circuit (where to connect Soruce, Drain, Gate)?

    What about the third option? When is the pin UVLO to be tied to GND?

    Thanks!

  • 0 in reply to Patrick Szabo
    TI__Mastermind 25500 points

    Shunt FET dimming is described in the datasheet, but it does require more components. It would be easier to just pull the UVLO pin to ground when you need zero current. That will turn off the PFET until the UVLO pin is released again and will only require an extra signal rather than a FET plus a couple of diodes and resistors along with an extra signal.

  • 0 in reply to Patrick Szabo
    Prodigy 220 points
    Increasing the inductor will decrease the ripple current which in turn will lower the peak current which is what is turning your led on at lower currents.
    Keeping the input voltage close to the output voltage (as high duty cycle as possible) will also help to minimize ripple current.