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Discussion in 'LEDs and Optoelectronics' started by AchillesZA, Sep 6, 2012.

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  1. AchillesZA


    Sep 6, 2012
    Hi Guys

    Ive got a newb question here.

    Im planning on running 10 x TLC5940 daisy chained to run a bunch of RGB led's.

    Now each TLC have its own resistor to set the constant current sink for each pin. But now my question is this:

    Can i run different voltages to the LED's on the 16pins? Red led's +- 2v each, Blue led's +-3v each, but the constant current sink set at 20ma?
  2. CocaCola


    Apr 7, 2012
    LEDs are current driven not voltage, if the driver chip is constant current just run the LEDs off whatever the supply is, as long as it's within the driver chip's operating voltage range..

    I would just use the 3.3V or 5V rail you are using to power the rest of the circuit...
  3. AchillesZA


    Sep 6, 2012
    thanx alot, so whats the point of LED's having a forward voltage then?

    im gonna run 2 LED's in series one each pin, meaning 20ma, then push like 6v into it? should be fine yes
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The TLC5940 is a current sink, so yeah, in principle, you can drive the LEDs from a lower voltage source.

    However you'll have to note that the output driver is bipolar (fig 1) and figures 5 and 6 indicate that you can have up to 1V across the output transistor.

    I probably wouldn't run a blue LED in this case from much under 5V.

    In any case, you need at least Vf @ the operating current plus Vo at the operating current (plus a bit of margin to allow for tolerances and component variation).
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    That's a property of LEDs. Like anything that emits energy (in this case light) you have to input this energy to the emitter (LED). Energy = Power*time. Power = V*I. If you have V=0 (no forward voltage) then P=0 and Energy=0 => no light.

    The forward voltage of an LED is a property that you will find in the datasheet as a function of current. Du to the non-linear characteristics of an LED it is advisable to drive it by a controlled current. The forward voltage is a consequence of the current then. Do not rely on an exact value for the forward value for a given current. It will vary greatly due to tolerances of the LEDs. Therefore it is not advisable to control LEDs by voltage.

    This is inverse to an incandescent lamp where you apply a voltage and the current is the consequence.

    In theory you could drive an incandescent lamp by a controlled current as well as an LED by a controlled voltage. But it is impractical - you'd need to put a lot of effort in designing a controller that is precise enough while at the same time being tolerant to the tolerances of the components (LED, lamp).
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