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tlc5940

Karl Bates
Prodigy 20 points
Other Parts Discussed in Thread: TLC5940

Ok I have a tlc 5940, I am interested in lighting a room with it. I was planning on having 5 5mm Super-Bright RGB LEDs in 12 wall sconces. I would prefer to keep the circuit as simple as possible. 

Here are the LEDs I'm using http://cgi.ebay.com/100Pcs-5mm-4-PIN-Common-Anode-RED-GREEN-BLUE-RGB-LED-/250467944182?pt=LH_DefaultDomain_0&hash=item3a510d86f6

And here is the circuit hoping to use.



Would the tlc5940 work with this circuit?

  • 0
    TI__Guru 65200 points

    Hello Karl,

    TLC5940 would work in this application as long as you program it accordingly. The sink current is then divided by 5 per LED because you parallel 5 LEDs. Dependent on the current one LED needs to enlight, TLC5940 might be able to sink enough current or not. I could not find the current capability of the LEDs you mention.

    Dependent on your requirements, there might be the possibility to ease your design with another IC from our portfolio. TLC5940 is able to support video screens with high quality brightness control of every single LED. Do you need the brightness control of all LED blocks or do you just want to turn the LEDs on and off? If you need brightness control, how many different brightness steps do you need?

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 20 points

    I would prefer a chip with 256 steps or better in brightness control. 

     

    Also the LEDs in the link have a forward current of 20mA. Im worried about the heat dissapation capabilities of the tlc5940, if I reduce the voltage going into the LEDs to around 2.5V would that be acceptable?

  • 0 in reply to Karl Bates
    TI__Guru 65200 points

    Hello Karl,

    Do you want to dim all LEDs at the same time or do you want to dim them independently? If you want to dim all LEDs at the same time, I would recommend an IC that just turns on or off the LEDs and use either global brightness control or the external current limiting resistor to dim the LEDs. This makes the system design easier.

    The closer you are with the input voltage to the forward voltage of the diodes plus the voltage drop inside TLC5940, the lower the power dissipation inside the IC. So reducing the LED anode voltage is for sure a good solution to reduce the power dissipation.

    I think if you reduce the voltage to 2.5V instead of 6V, then the LEDs might not turn on at all.

    In addition, the input voltage of TLC5940 has to be above 3.6V, if you want to sink 100mA. The device will not be able to sink 100mA with below 3.6V input voltage.

    Best regards,
    Brigitte

  • 0
    Prodigy 70 points

    Hi there.

    With your existing circuit, you appear to have 15 parallel strings, each with 5 LEDs connected in parallel.

    Unless the forward voltage of the LEDs is very closely matched, you will probably end up with one of the 5 in each string being much brighter than the others (the one with the lowest forward voltage will start conducting first and hog most of the current). If you can connect them in series instead (needs a higher voltage and gets closer to the limit of the driver), they will all share the same current and be (in theory) the same brightness. Alternatively you can connect a small series resistor for each LED and connect them in parallel as you have.

    Nick