I have an LCD backlight driving design that I want to use to evaluate several different LCDs. I used the TL4242 with 5 volts at the IN input and the PWM input tied to the IN input. The "D" input has a .1uF cap tied to GND. There is a 5 ohm resistor from the REF input to GND. My calculations for the constant current are ".177V / 5ohms = .035A. I have attached a single LED (3v forward v-drop) and it looks like it works correctly. I then added a second LED and I expect the current to stay at .035A and it doesn't. The REF input drops from .177V at one LED to 0V with two LEDs. I have two boards that act the same way. Do you have any thoughts on what may be wrong here? The LCD backlight I eventually want to evaluate will have 7 LEDs in series with 3V-drops each.
Thanks for your time.
Wayne