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

Grumpy_Mike

That as drawn would not work.

Actually I would use N channel FETs and something like a 74LS04 to invert the logic signal from the TLC to the gate of the FET. You need a pull up on the TLC output and make sure you use a logic level FET that switches on with 5V.

You can parallel up the outputs of the TLC do a search for application note
SLVA280 Using TLC5940 With Higher LED Supply Voltages and
Series LEDs
and
SLVA253 LED Driver - Paralleled Outputs
Provide High-Current Outputs

bensaddiction

Yeah thanks mate, I was afraid somebody was going to tell me that,

Oh well - back to the drawing board.


Using 74LS04s or something similar and some n-channels then.

Is there a way to calculate value for pull ups though by the way?

Would 10k be the right amout? (only I seem to hear that value being given every now and then

Grumpy_Mike

Quote
Is there a way to calculate value for pull ups though by the way?


Yes, it depends on what you are trying to drive.

The TLC5940 has a constant current supply so any resistor will be pulled down with this amount of current if possible with 5V across it maximum. If you are going into TTL you need to pull this down to less than 0.8V. So the voltage across the resistor needs to be 5 - 0.8V, therefore:-
R = 4.2 / I where I is the current setting of the TLC5940.
If this is set at 5mA then the resistor is 840R, if set at 20mA it is 210R
As you are not doing anything with this current it is best if it is as low as possible.

ill_switch

Quote
You can parallel up the outputs of the TLC do a search for application note
SLVA280 Using TLC5940 With Higher LED Supply Voltages and
Series LEDs
and
SLVA253 LED Driver [ch8211] Paralleled Outputs
Provide High-Current Outputs


What's the possibility for combining these two techniques, i.e. driving several high-current LEDs?

Let's say I have LEDs with 3.5V forward voltage and ~500mA current (Luxeon Rebels, Cree XR-E, LEDs like that. . . ) If the TLC can do 120mA per pin, I understand from the second app note you posted that I could parallel 4 of the outputs to drive one of these LEDs, correct?

And, the second app note describes using a transistor on an output pin if you need higher voltages than the chip can handle (17v).

What if I wanted to drive a string of 6 of these LEDs, in series? Could I use an N-channel MOSFET on the 4 paralleled output pins?

If so, then one tlc plus 4 MOSFETs could drive 24 LEDs (6 leds per 4 pins). This would be a big deal, since within the next year I'm going to be trying to drive ~150 - 200 HB LEDs. Eight or ten TLC5940 chips, plus a handful of transistors, would be cheaper and simpler than what I had been planning.

Grumpy_Mike

Quote
Could I use an N-channel MOSFET on the 4 paralleled output pins?


Yes that would work to get the current up with a higher supply voltage, but ....

OK on the 120mA per pin but look at the power dissipation of the overall chip, you are bound to fry it if you don't watch out.

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