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LM3409 analog dimming issue

Cl Lin
Prodigy 20 points
Other Parts Discussed in Thread: LM3409

Hello,

On the LM3409 we are driving 4 series LEDs and are having difficulty getting the LEDs to dim far enough down using the IADJ pin (using the analog 0-1.24V control). At 0V, we still have about 13mA @ 10V flowing through the LEDs string and thus the LEDs are still lit. Looking more closely at the datasheet, we now see that the IADJ voltage only gets the output current close to zero, but does not actually take the output down to 0mA. What could we try to force the output current to 0mA when the input voltage is 0V? We could use another separate switch to control the enable pin, but it would take a major mechanical change for that. Another workaround would be to find a simple way to automatically shunt 13mA or so away from the LEDs, which would waste some power but would be better than lit LEDs in the "OFF" position.

Anybody's help is much appreciated!

-C

  • 0
    TI__Intellectual 2590 points

    Would need to see your schematic

  • 0 in reply to Mike Kosteva
    Prodigy 20 points

    It's the same schematic as Design Example #2 on page 29 of the datasheet, with the exception of the input voltage being in the range of 15-18V. For now we are just placing a 333Ohm resistor across the LED output and it seems to be doing the job with respect to dimming all the way to no light output.

    We have a related question - is there a maximum voltage on the IADJ pin? We are controlling this voltage with a slide potentiometer and due to imperfect contacts, the voltage may increase up to the input voltage (18V) momentarily. The current would be limited to 120uA in that situation though. (Not sure if this would cause the internal 1.24V diode to be damaged).

    Thanks