I am running my board (barebones Arduino) at 3.30V.
I have these RGB LEDs from Cree; you can see the datasheet here: http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/HB/Data%20Sheets/CLV1AFKB(874).pdf
The Blue and Green "parts" of this LED have a forward voltage specification of 3.20V (Typical) and 4.00V (Maximum) which is stated for a forward current of 20 mA.
I am planning to use a TLC5940 (LED driver with constant-current sinks) at VCC = 3.30V.
Note that I will be running only a current of around 2 mA through the LEDs (very small current because these LEDs are extremely, almost painfully, bright!)
With the above in mind, would the 3.30 V supply of my board (being less than the Maximum-case of the LEDs' forward voltage) be an issue, or should this setup work reliably?
I am still trying to learn about LED operation/the forward-voltage concept.
"would the 3.30 V supply (being less than the Maximum-case of the LEDs' forward voltage) be an issue"
Yes. Green & Blue may not turn on at all.
V Out Low is provided by an NPN resistor with some voltatge drop across it. All the references show Vo as 1V.
3.3V - 3.2v - 1V <0, so the LEDs may not turn on at all.
forward voltage - an LED is just a diode that gives off light as it conducts.
When the anode voltage exceeds the cathode voltage by the designed amount (determined by the device's material properties) it acts like a switch and begins to conduct current - as much as the supply can provide it (hence current limit resistors) with the voltage from anode to cathode as specified (forward voltage).
Robert, The simplified explanation of the LED operation clarifies a lot!
But that said, since the forward voltage of an LED appears to be lower at smaller forward currents (even looking at the curve on the datasheet), isn't it thus true that the Green and Blue LEDs would still light up (given that, in my case, I'm planning to do a constant current of only 2 milliamps) ?
Try it. I can't say for your specific parts.
I can only say that 3.3V - 1V = 2.3V and the curve starts around 2.9V. Will green/blue turn on? I doubt it.
Indeed. Upon multiple tests on various LEDs (of same model), I noticed: (1) The "turn-on" voltage varied a bit among them and (2) The LEDs did turn on at around 3V, but only when the current-limiting resistor was large enough to ensure a pretty small current (which in this LED's case was still enough to give me indication-sufficient light)