I have a RGB LED for photo-detection in my design with each color controlled by a MOSFET to manage light intensity of each color. The green and blue LEDs work great, but my red LED appears to slowly fade out over time. I measured my forward voltage (Vf) to be 2.6V on the red LED at the beginning, but it drops to ~2.56V over the next few minutes (in spec).
Here's my schematic. The left is my tricolor LED and MOSFET set-up and the right is the PWM expander, controlled by I2C:
PCB:
Here are the specs for my tricolor LED (I circled the correct one):
When I measure the light intensity coming from the LED over time, I can see the red intensity dropping while green and blue stay steady. Can a solder joint affect the LED? I wouldn't have thought twice about this, but I have an extra RGB LED so I removed one of my old (dead) ones and connected the LED with wires instead of soldering to my board. I have the LED on 'red' and the intensity isn't going down.
This has happened to 4 LEDs now…
Best Answer
Just because the forward voltage is in spec it doesn't automatically mean the forward current is too. 2.6V across the LED leaves 2.4V across the resistor/MOSFET which could mean 96mA through the LED and hence 250mW dissipated, which is well above the absolute maximum of 150mW.
I think you're killing your red LED.
The forward voltage in the datasheet simply gives the value you would expect to see when passing a given current through it, I presume this is the 50mA it gives in the column header but it will say somewhere.