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Topic: What will happen if I drive LEDs in parallel from a TLC5947? (Read 1 time) previous topic - next topic

scswift


Well only because he is running the LEDs at less than half the rated current so there is little point in having two anyway. The fact remains putting two LEDs in parallel is just plane stupid design.


Spoken like a true engineer. :)

It may not be efficient to put two LEDs in parallel attached to a constant current source only capable of sourcing 20mA, but the consideration here is for aesthetics, not efficiency.


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And there is some kind of reflection issue in the data lines when I have more than six modules in series, where the LEDs on the last few modules will be messed up.

Yes signal integrity problems are common with this chip as people tend to think you can string them together without considering the signals. You might be lucky but just R & Cs do not normally fix things. You need proper layout and proper buffering to the signal lines.[/quote]

I don't understand.  What do you mean by proper buffering and layout?  TI's datasheets make no mention of any special layout other than keeping the capacitors near the pins, and properly heat sinking, nor do they mention any need for buffering.

Also, I'm not familiar with buffering, but if by that you mean put another chip on the line, that seems like it would be way too expensive, and I don't understand why it would be necessary.  If the signal successfully reached one of the modules, and the LED driver picked it up, and passed it along to the next one, why would I need to worry about signal degradation?  Shouldn't each chip emit a new clean signal?

Actually though, now that I think about it, while each chip does output data to the next, the clock signal used for that data isn't passed along in that manner.  That's just on its own separate clock line.  So maybe the data is reaching the end of my chain just fine, but the clock signal is no longer in sync with it, or reflected signals are messing it up?

Assuming adding buffering is too expensive, what would I do if I just wanted to try to reduce reflected signals in the clock line to try to extend its reach from 6 modules to 10?

Grumpy_Mike



These LEDs are attached to a constant current driver.  So there's no worries about that happening.

He's got you there, Mike...

Well only because he is running the LEDs at less than half the rated current so there is little point in having two anyway. The fact remains putting two LEDs in parallel is just plane stupid design.

Quote
And there is some kind of reflection issue in the data lines when I have more than six modules in series, where the LEDs on the last few modules will be messed up.

Yes signal integrity problems are common with this chip as people tend to think you can string them together without considering the signals. You might be lucky but just R & Cs do not normally fix things. You need proper layout and proper buffering to the signal lines.

scswift

Btw, if you're interested in the project I'm using these chips in, you can check that out here:
http://arduino.cc/forum/index.php/topic,139819.0.html

There are a couple minor issues with the LED modules I designed.  I should have put a 10K resistor on them to pull down the enable pin so they don't flash when you apply power.  And there is some kind of reflection issue in the data lines when I have more than six modules in series, where the LEDs on the last few modules will be messed up.  It isn't a serious problem since six modules is a whole lot of LEDs, but I'd like to fix that eventually.  I assume I need to put some resistors or caps on the data lines to solve that issue, though I haven't really had time to research possible solutions.

scswift

I did take the power dissipation into account.  I have used an appropriate resistor on the Iref pin to limit the chips to 20mA per channel, which I calculated to be below what they could handle with all outputs on.  I also followed TI's recommendations closely for the thermal vias.  They only get slightly warm and I have been testing them for the last two months and have never had any of them go into thermal shutdown.

I used these chips and set them up this way so that the boards would be idiot proof.  They're designed for people who don't know anything about electronics to play with, so I wanted them to be able to just plug in any old LED and have it just work.  I also did not want them to have to solder resistors onto the LEDs.  (Nor did I want to have to do that myself when assembing kits.)

dc42

If you are using the TLC5947 to drive a lot of LEDs, then you need to take the power dissipation of the chip into account. For each channel, it is the voltage at the output pin (which is the supply voltage less the LED forward voltage) times the output current. Then multiply by the number of channels you are using.

You may find that you need to reduce the voltage at the output pin to keep the power dissipation within limits. One way of doing this is to use series resistors - which gives you the opportunity to use the same series resistors for balancing the current between the LEDs.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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