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Author Topic: Looking for: Serial / SPI LED SOURCE driver with PWM  (Read 1355 times)
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Essentially, I'm driving 50 10W RGB LEDs for an architectural application and would like to be able to individually control the brightness on each.   

The options I would use for LOWER powered LEDS include:
TLC5940 (serial sink)
WS2801 (serial sink)
CAT4109 (PWM sink controlled by MCU)

The issue is that I don't really need a driver here.  I already have drivers that are doing a fantastic job.  They just need a PWM signal on their OE pins to control brightness of each channel.    My Arduino, when carted off to each one will do a FINE job of this, it's just not really... possible.

IDEALLY, I would find a SOURCE version of the WS2801 or TLC5940.  These can be chained along to provide as many outputs as I like and since the current is not a concern, as we are just bringing a pin high or low, there are no real driving issues.  I would be using them as PWM sources, but that could be easily controlled / programmed for.

I know I can use p-channel transistors, and that I can reverse my code, mapping everything in reverse, it's just not as ideal as finding a chip that does precisely what I want.   

I asked TI, ON SEMI and MAXIM for options but each of their serial sources doesn't support PWM, only their sinks.

short version: Need a serial / chainable PWM outputting SOURCE.  As few as 3 channels per chip is fine.

Thanks in advance!
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Why can't TLC5940 with an logic level inverter (equivalent of F241 function for example) be used?
F-logic can output 24mA, altho it sounds like you just neec the levels inverted.
Am I missing something?
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I wondered the same thing.  TI sent this response:

"You will experience instable(SIC) operation and unexpected results if you use the TLC5940 in this mannor (SIC)"

Potentially I got the B team?  I've always gotten pretty good support from them in the past...
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Definitely the B team, I have helped several people use the TLC5940 and TLC5947 with inverters to control higher power LED drivers. You just need resistors and inverter chips, good to go. Pity that you lose the current sinks and have to use it just as a PWM generator, though.
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Ok.  Might be a valid answer, depending on the question asked.

Per the datasheet:
"Imax must be set between 5 mA and 120 mA. The output current may be unstable if Imax is set lower than 5 mA.  Output currents lower than 5 mA can be achieved by setting Imax to 5 mA or higher and then using dot correction."

Add a pullup resistor to +5V to put a load on the pins so they can think there is LED current switching going on.
Set the output for 6mA.
Connect the outputs to a inverter to get High's out during the normal LED on-time.
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You've got me convinced.  I'm happy to try it at the very least!

For reference the original question was:
"Can the TLC5940 be utilized as a serial PWM source when paired with a HEX inverter in order to provide PWM to a second, high power LED driver?"

Thanks again!
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I'd say you got a bum answer then. They could have said "No, but if you do this ..." to help you out.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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