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I'm planning to drive multiple RGB LEDs using a TLC5940 with PWM. These LEDs have the following specs:

Voltage Red: 1.8V
Voltage Green: 3.0V
Voltage Blue: 3.3V
Current per color: 20mA
Common anode

Since the TLC5940 is a current sink, how would I make sure each led gets the correct voltage. Hook up a resistor between each cathode and the TLC5940?

Please forgive me for asking such a simple question, I'm still learning :).

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Since this is a constant-current sink IC, no resistors are needed in series with the LEDs.

The current is programmed using a reference resistor connected to one of the pins that sets the reference current used to drive the LEDs, and it is the driver chip's responsibility to make sure the correct voltage and current is applied to the LED. In practice, I have found this works well.

You could add a resistor in series, but this defeats the purpose of using this chip. It will increase your part count, increase your minimum VDD requirement, and if you need to increase VDD, then you will waste additional power.

Below is the figure used to determine your reference current for each resistor from the datasheet: enter image description here

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  • \$\begingroup\$ Ah ok, that's neat! But do the three different colors need a different voltage (and can this be done), or am I misunderstanding the current/voltage relation? \$\endgroup\$ – wvteijlingen Mar 12 '11 at 19:51
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    \$\begingroup\$ @Rengers All you need to do is make sure that you have enough voltage applied to the anode of the device so that your highest voltage LED will turn on. Looking at the datasheet, your anode voltage should be at least 3.6V. If you apply 20mA to your LED, then you will have your forward voltage drop - they are related. \$\endgroup\$ – W5VO Mar 12 '11 at 20:25
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    \$\begingroup\$ The TLC5940 forces the correct current through the LEDs. As long as the correct current goes through the LEDs, the LEDs will set their own voltage -- there's nothing you can do about that voltage, even if you wanted to. \$\endgroup\$ – davidcary Mar 13 '11 at 1:04
  • \$\begingroup\$ Thanks, I think I get it now. Since my LEDs require 20mA current, I would need a 1.92k Ohm reference resistor (as stated in the datasheet). \$\endgroup\$ – wvteijlingen Mar 13 '11 at 1:22

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