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1  Forum 2005-2010 (read only) / Interfacing / Re: TLC5940 + high current on: April 06, 2009, 06:26:09 am
The chip needs a Vcc of 5V, but the maximum allowed voltage at the OUTx pins is from -0.3V to 18V, thus I don't think there's a problem of using 12V for the LEDs.

For the power dissipitation actually the right number for me seems to be 1572@70° (dip package). So four chips seem needed for my 120ma. What do you mean by if you can keep it a 70°, you think it actually may raise a lot above this ?

Thanks.

P.S. Tried with two TLC @20ma in parallel to draw 40mA and seems to work.  


2  Forum 2005-2010 (read only) / Interfacing / Re: TLC5940 + high current on: April 04, 2009, 03:18:09 pm
Regarding leds in parallel it seems safer to put resistors on each strip.

And for the PNP vs NPN you'd still need a pull up to be able to sink current in the base, but other than that wouldn't it work ?

Now back to the idea of TLCs in parallel,  I'm not sure I fully understand the power dissipation equation of the TLC5940 :

P = (Vcc * Icc) + (Vout * Imax * (DCn / 63) * d * n)

where

Vcc: device supply voltage
Icc: device supply current
Vout: TLC5940 OUTn voltage when driving LED current
Imax: LED current adjusted by IREF Resistor
DCn: maximum dot correction value for OUTn
n: numberof OUTn driving LED at the same time
d: duty cycle defined by BLANK pin or GSpwm value

I don't really understand what Vout stands for is it the voltage at the OUTn pin after the voltage drop of the LEDs or the voltage applied before the LEDs (12V) ?

I think it is the former and in my case by using resistors in series with the LED I have a max. remaining voltage of 2.5V at a given OUTn pin. Thus sinking 30mA when everyhting is on results in a power dissipation of :

Vcc = 5v
Icc = 60mA (max according to dsheet)
Vout = 2.5
Imax = 30ma
DCn = 63
N = 16
d = 100%

P = (5 * 60e-3) + (2.5 * 30e-3 * 1 * 1 * 16) = 1.5W

which is well below the max of 2.4W.

Actually I could even run them at 40ma (P = 1.9W) and use only three TLC in parallel to get to my 120ma but maybe I prefer to err on the safe side.

Can anyone confirm I'm using the right numbers ?
3  Forum 2005-2010 (read only) / Interfacing / Re: TLC5940 + high current on: April 03, 2009, 08:34:16 am
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 ?

4  Forum 2005-2010 (read only) / Interfacing / Re: TLC5940 + high current on: April 03, 2009, 08:10:09 am
@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:
       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.

5  Forum 2005-2010 (read only) / Interfacing / TLC5940 + high current on: April 02, 2009, 12:30:50 pm
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:
       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.

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