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Topic: TLC5940 + high current (Read 11 times) previous topic - next topic

yopla

Hello, I thought the TLC5940 could drive each channel at 120mA, ended burning two chips before looking at the power dissipation equation in the datasheet and realizing that won't work if I want all my led strips on at the same time.

Now I'm looking to use transistors (TIP120) to switch the 120mA current. Here was my plan : TLC5940 OUTx to base of TIP120 and pull-up resistor (10K) :

Code: [Select]

       5v      12v
       |       |
       10k    LEDs (120mA)
       |      /
OUTx ---------||
              \
               v
               |
               GND


The problem is that I'm not able to completly light-off the LEDs when I set OUTx to 4095. There's a remaining voltage of 0.1v between the base and the emitter which is sufficent to let the current go through the transistor.

I use an IREF resistor of 1.8K (=> 20mA draw) and I'm not willing to draw more current (burning + consumption issues).

Does anyone know how I could solve this problem ?

Thanks for your answers.


big_mark_h

If you want to run multiple LEDs in series from each OUTx pin, this document from Texas Instruments might help. It gives details on using a MOSFET to accomodate higher LED supply voltages, and LEDs in series.

Grumpy_Mike

Quote
when I set OUTx to 4095. There's a remaining voltage of 0.1v between the base and the emitter which is sufficent to let the current go through the transistor.


4095 is normally the maximum brightness not the minimum one. You get this inversion because you are using a transistor and that inverts the signal and that is your problem. You are always going to get this with the way the circuit operates, you don't see it in normal operation because it is only one clock pulse away from being full on and you don't notice it is not full on. Here you do notice it is not full off.
I am not sure if there is any way round this with the setup you have.

yopla

@big_mark_h

I have seen this document. But with their setup the high current still goes through the TLC, which means that it will burn the TLC. The document wants to accomodate the chip with higher _voltage_ levels (which implies a higher number of leds in series). But not with higher _currents_. Or do I miss something ?

In my setup I have only 12V, for strips of 3 leds in series at 20mA and I need to draw a current for 6 stripes in parallel (120mA). So that won't help since I'm stuck with 12V.

@grumpy_mike

Thanks for the explanation. I didn't think about that. Grrr.


I thought about the following solution, but I'm not sure if it will work, but maybe someone more experienced knows :

Use four (what remains in my hands) TLC5940 in parallel drawing 30mA currents. All corresponding inputs of the TLCs are connected together aswell as all corresponding output pins.

Code: [Select]

       12V -------------------------
               |   |   |   |   |   |
               V   V   V   V   V   V
                |   |   |   |   |   |
               V   V   V   V   V   V
               |   |   |   |   |   |
               V   V   V   V   V   V
TLC1 OUTx --\   |   |   |   |   |   |
TLC2 OUTx ---\__|___|___|___|___|___|
TLC3 OUTx ---/
TLC4 OUTx --/


Do you think that would work ?

Oh and by the way knowing that all LEDs are the same should I still put a resistor in series on each strip or is it ok without (at least I'll put one in front of the OUTx connections to drop the remaining voltage and hence dissipate less power according to the equation) ?

Thanks.


yopla

And I forgot to ask would the first idea work actually with in a PNP setting ? Collector to GND, emitter to LEDs and base to TLC OUTx without pull-up ?


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