LDO as a constant current LED driver

Question

I am doing some research about using a LDO as a constant current LED driver and For this project, it is imperative that the LEDs remain as stable as possible. I also need to be able to easily adjust the brightness of the LEDs. I have a few clarification questions about the setup.

I would like to use an LT3085 as a constant current LED driver as described here and in the image below. The LEDs that I am planning on using are red Osram LRW5SM, which have a forward voltage of 2.0-2.6V and a forward current of 100-1000 mA. I am planning on using one LDO to control one LED.

My questions/clarifications:

1. To achieve a 2.2V voltage across the LED, I should place a 220k resistor at the SET pin, correct?
2. In order to adjust the current across the LED from 100mA to 500mA (max output of this LDO) with a 220k resistor at the SET pin, I would need a ~4.4-22 Ohm potentiometer (this seems pretty low) at the OUT pin, correct? [LED current = 10uA * (R1/R2)]
3. What should I power this LDO with? Would a lab power supply work, or should I use a battery? What voltage?
4. Is there a better LDO option altogether for what I'm trying to do?

Answer 1

A better idea would be to use the LT3085 in constant current mode, with a potentiometer it could be adjustable. At 50k this would give you the full 500mA, 10k would give you 100mA ect. Place the diode on the output after the 1 ohm resistor going to ground.

Answer 2

1) That's incorrect. First of all, you don't want to set a constant voltage of 2.2V across the LED, you want a constant current of 100mA. You must choose the ratio of R1:R2 to be equal to the ratio of the desired LED current to 10 microampere

2) You could use a potentiometer either for R1 or R2, but your stability will suffer if you use a mechanical potentiometer.

3) A lab supply or battery would work. The voltage needs to be at least 1.6V more than the maximum LED forward voltage plus the voltage across R2. Sounds like you will want something in the vicinity of 6V to 10V. Be very careful about thermal design...make sure that the heatsinking is good or the device would go into thermal shutdown.

4) Possibly, but we don't recommend specific parts here. If you really want "as stable as possible" then you will probably need to spend a lot more money.