I'm thinking of Using 3W RGB high power LEDs for a project. I will only have one color on at a time so I'm pretty sure that it would be 1W of ussage at a time. (please correct me if I'm wrong) My question is how hot do they get? I'm planing to use heat sink but trying to gauge if this is realistic for my project or will these be too hot for what I want to use them for. Nothing I read talks about temperatures it just says "they get hot!"
How hot do High Power 1-3W LEDs get
ledtemperature
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:
Russell commented in a recent answer that efficiency(*) up to 20 % is possible. That's 4 or 5 times the efficiency of an incandescent bulb. So we're doing fine here. But of course, a 20 W LED lamp will still produce 16 W in heat (that bulb would have 19 W in heat). And if it comes with a heat sink, better leave it as it is; it's there for a reason.
(*) I'm using the word "efficiency" here because that's the word used for it in everyday language. Szymon correctly points out that "efficacy" would be better, expressed in lumen per watt. Actual efficiency as Szymon uses the word is always 100 %, because that's the ratio energy out/energy in, due to the laws of thermodynamics.
Best Answer
Since you don't have a datasheet, you could figure this out empirically. Run a known amount of current (start with very small currents to try and avoid self-heating) through each of the 3 diodes and measure the Vf and temperature (you can get close on temperature without measuring the junction since the thin dielectric of the MPCB has very low thermal impedance). From this you can calculate a rough thermal impedance. Repeat for various ambient temperatures (or just use self-heating to get to a different temperature) and currents. Your results won't be exact, but you should get close to the same number across several measurements because as temperature goes up, Vf goes down (and thereby power). Based on your application, you can then decide how much additional heat-sinking you may need.
Between the LED's thermal impedance and the MCPB, I would guess you will probably be in the 5 C/W range from past experience.