Trying to use the LM3432B to control LCD back light using 5 sink pins (all but IOUT6) @ 20ma each. R(iref) is 54.9K (should be 5x20ma). Iout6 is grounded. Iout1-5 are all tied together and "sink" the back light. Mode, Agnd, Pgnd, and Thermal_Pad pins are all grounded. Cvcc has a 1uF ceramic cap (not tied to anything else). Cvin also has a 1uF ceramic cap.
Vin is +12VDC (through a pull-up resistor that I have varied from about 20 to 70 ohms as I have tried solutions). The back light voltage is specified as 4 to 4.4 VDC at the nominal operating current of 100 mA.
Initially I tied the Vin and the anode of the back light together pulled up to the 12VDC through the resistor just described. I soon realized that I was forcing the chip to regulate the current in the back light at a Vin that was less than 6 volts (remember the Vlcd is about 4.2 volts) and the internal regulator (Vcc) was not working (the input was less than 6V).
By adding either a resistor (10 or 20 ohms) in line with the back light OR a zener diode (about 2.4 volts) I was able to "break" the connection between Vin and the Vlcd which allowed the internal regulator to have 6+ volts (the Vlcd plus the voltage drop across the resistor OR the zener). The part that I don't understand is that the voltage at the Iout1 to 5 pins (still tied together to the cathode of the back light) is always very small (<.02 volts) and I would expect it to be higher if the chip was actually regulating the current. I don't know why.
My conclusion is that it is NOT regulating the current for some reason even though the internal regulator seems to be working (Vcc = 4.9 volts). In addition NEITHER the enable OR the DIM inputs have any effect on the operation of the circuit (another clue?). I can see the enable pin input changing from Off to On and the chip ignores it (back light is always on).
I realize I am probably doing something "stupid", but have not been able to figure it out yet. Any help or clues would be appreciated. I think the PWM circuit is probably not working because our PWM frequency is pretty slow. Regardless I can't understand why the enable pin is not working.
