Trouble getting AL8861 LED Driver to illuminate LEDs


#1

I am trying to get a AL8861 LED driver hooked up based on the wiring diagram in PFC_EDU. Right now I’ve connected it to a single LED (this one) to test (see circuit below). I eventually intend to connect it to the DAC5578 Digital to Analog converter to communicate via I2C. For now, I’d like to test if the lighting circuit works.

AL8861 Wiring Diagram

LED

I’ve connected the components and supplied 24v. As soon as I plugged it in, the LED flashed and then went dark. I’ve also tried connecting VSET to ground, and it begins to glow slowly but remains very dim. I tried connecting VSET to +3v and +5v (probably a bad idea) and it flashed again. Now when I connect VSET to ground I get no glow.

I get some weird results when I check the test pads voltages. Here are the readings:

  • (+24v) 24.30v
  • (VSET) 23.93v
  • (CH1+) 24.30v
  • (CH1-) 24.27v

I thought VSET would float. When I connect it to ground, it has ~60mA of current running through it.

Is my circuit wrong in some way? I appreciate any help. Thank you!


#2

UPDATE: I found the primary issue - my board had an error with GND not being connected to the driver for that channel. Now I have light, but not very bright. Connecting VSET to GND or 3v3 doesn’t seem to do anything. CH1+ reads 2.54v and CH1- is 0v. Still stumped…


#3

I figured it out – it was two things:

Something in the board must have been soldered improperly, because I reflowed everything and tested all connections, plugged it back in and it was suddenly working as expected.

The LED isn’t shining as bright as I would have expected because of the RS2 resistor value. The formula in the datasheet is Current = .1/R. Right now my RS2 is 6.34ohms, which is 15mA. To run this much higher, I will need a resistor value between .1 and .15 ohms (I didn’t even know these existed).

Hope this helps anyone else out there with this chip!


#4

Hi Adrian,
I’m glad you were able to figure out your problem. I use sense resistor values of 0.68 ohms when driving midpower LEDs, at around 150mA. If you don’t have resistors in the appropriate values, you can always put multiple in parallel to reduce the resistance. This could be especially useful for testing while you wait for the 0.1 ohms to ship.
For reference, you calculate parallel resistance by: 1/Rtotal = 1/R1 + 1/R2 +1/R3…