I’ve been designing a device with dual power supplies in mind, 3.3V for the logic and 5V to power the LEDs. But I’m wondering if this is an unnecessary excess. The forward voltage of the LEDs I’m using is ~2.5V for the red segment and ~3V for the green and blue segments. Is there any brightness or efficiency benefit to running these at 5V with appropriate resistors, vs. say running a single power supply at 3.5V with smaller resistors? (The logic is rated at 3.0-3.6V.)
i.e. is there any reason to give a Vf=3.0V LED 5 volts, or is a half-volt headroom enough to get full performance?
Craig,
Well according to this non-detailed datasheet here…
…the table called “LED characteristic parameter” shows that Green has a “Voltage” of 3.0-3.2v and Blue has a “Voltage” of 3.2-3.4. The deceiving part of this number is that it is just taking a snapshot @ 20mA of the true Current vs Voltage curve for the infividual LED color chips… in other words, LEDs are non-linear devices where the current rises non-linearly with voltage…the above snapshot numbers are just snapshots of the typical voltage when the current is at a nominal 20mA. Not all LEDs are created equal from a physics and chemistry standpoint…this is where LED binning comes into play (and who knows what bins Ray Wu is peddling?).
Also, there is also a voltage drop across whatever circuit you are using for current regulation whether that be a series resistor or the little current sources built into our favourite digital pixels. Anyway, to answer your question, in my opinion, if you run an RGB pixel from 3.0v, it might light the Blues and Greens, but they will never acheive full brightness and as a result your colour mixing will always be crippled.
…and cutting off a good portion of the Green/Blue side…or something like that. Just my opinion, curious what others will say.
thx!
-frenchy http://www.voltvision.com
To clarify I’m using plain common anode rgb leds. No addressable or anything else fancy. The forward voltages I measured myself as 2.5V for red and 3.0V for blue and green.
LEDs will run fine off 3v, but your MCU probably won’t be able sink/source the current, so use a transistor, and a current limiting resistor for the led