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Propery Color Mixing with TLC

Franklin Cooper44479
Prodigy 10 points
Other Parts Discussed in Thread: TLC5940

I am developing a LED screen for a Senior Design project that I am working on. For my prototype I have a small led screen which has RGB leds for each pixel. I am having trouble properly coloring mixing the leds together to successfully achieve proper color mixing. I simply want to use only 8 bit per led thus achieving 24bit resolution per pixel. Right now my setup is incorporating 3 TLC5940. Each chip is driving leds that are all the same color. Right now I have the leds properly balanced that each pixel is producing white light.

However, other heavily mixed colors are unachievable using this setup such as brown. I verified the current and voltages coming out of each led is correct based on the duty cycle that I am sending to the driver.

From the various Application Notes by Micheal Day about this chip it says that these drivers can be used to create LED based video screens. Not just for the backlight but also for generating individual pixels. From my understanding dot correction is used to compensate for variation of led intensity due to age and temperature. So what variations to the actually PWM value do I need to apply to achieve the proper color mixing. I am hoping to achieve a full 24bit resolution and just don't know what I need to do to achieve this. Perhaps a simple explanation can be provided or a link to an Application Note that can help me with this project.

  • 0
    TI__Mastermind 36110 points

    Properly reproducing a color is actually a fairly complicated process.  Generating a pure red  color that is perceived by the human eye to be a pure red color requires driving all three LED colors.  A pure red pixel will be generated with the red LED almost fully on with the green and blue LED at very low duty cycles.  A true green contains mostly green with some red and blue.  Because of this, you end up with a 3x3x3 matrix of values to describe the TLC5940 PWM values required to generate the three basic colors.  It gets even more complicated when you start mixing colors.  I do not have the expertise in this area to be of any more assistance.  There are many references on the web that go into excruciating detail about color mixing theory.  I can tell you that the accepted minimum viewing distance for a display is 1meter of viewing distance for every 1mm of pixel to pixel spacing.  The farther you are from the LEDs, the better they will look.  From experience with the TLC5940 EVM, I have noticed that the colors look much better when the LEDs are driven with less than full current (adjusted with the dot correction).  Another area of consideration is that a brown color can be generated by mixing many other colors.  Take a magnifiying glass and look closely at the print in a magazine article or news paper.  The brown color is made from many pixels of many different colors.  Unfortunately I'm not able to provide much help in this area.  A little more research may lead you to the answer you need.  As you can see from images displayed on large screen video displays (jumbotrons), the three LED pixels can be driven to generate millions of colors to display very high quality video.