I would like to use a 30W RGB LED that is in common-anode configuration. The issue is the voltage ranges for the LEDs are different, 20-24V for red, and 30-36V for green and blue. Is it possible to run the red led at the same time as the blue or green? If using PWM w/mosfets you still get the full 34V across the red LED I'm assuming that's not going to work.
Electrical – Driving High-Power RGB LEDs
ledled-driver
Related Solutions
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:
Without a schematic, based on your question, you have Common Cathode Led with TIP120 transistors? I assume you have them connected backwards from normal.
simulate this circuit – Schematic created using CircuitLab
The TIP120 is a NPN darlington pair transistor. It normally expects to be on the low side of the load. You are using them for High Side Switching, which won't work. If your led was common anode, you could swap them around, and it would work (WITH THE RESISTORS).
But in this case, you have 1) A different signal level from the led power level (3.3v vs 5v) so Direct PNP transistor won't work, and 2) The Raspberry PI GPIO cannot source a lot of current. 16mA at max. So You need both NPN and PNP transistors. Some 2n3904 and 2n3906 Common npn and pnp transistors, but any similar would work
I only show the blueled (5v source voltage - 3.2v forward voltage - 0.2v VCE drop / 20mA = 80Ω), do the same for the green led, and use a 140Ω or higher resistor for the red led.
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Best Answer
One way is to use a single 36 V power supply, then essentially a small switching power supply for each LED to regulate the current thru each LED separately, but driven from PWM.
Here is what the circuit for each LED would be:
When the switch is closed, current builds up linearly thru L1 and therefore thru the LED. When the switch is opened, the current continues thru the LED, L1, but now thru D2. The current will now ramp down linearly.
With fast enough PWM there is little difference between the maximum current at the end of the switch closed time, and the minimum current at the end of the switch open time. The PWM duty cycle then controls the average current thru the LED with minor ripple at the PWM frequency.
You probably want to use a PWM frequency of a MHz or more. For extra filtering, add a ceramic capacitor from the LED cathode to ground.