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.
The TL4242 is not a boost converter type LED Driver. The input voltage (I pin) must be higher than the forward voltages of all of the LEDs plus the voltage on the R(REF) plus the Dropout voltage of the TL4242 (VDR).
To drive 3 LEDs at 3V forward drop each at 35mA, the input must be at least 3*3V + 0.177V + 0.35V. The VDR will vary with the current. 0.35V is the typical VDR at 300mA so the 35mA will be lower.
Plus, the variation of the forward voltages must be considered. For example, if the VF is 3V +/- 250mV, the Input voltage must account for 3.25V per LED.
In reply to Dick Stacey:
Another thing to look at is the Vf vs current chart in your LEDs datasheet. Also there is variation vs temp as well - although not usually as much as for current.
In my experience, only the newer high brightness LEDs or some pure green or pure blues have drops of 3V+ and can handle static currents of 35mA. So you may want to make sure that your LED can handle this as well as any PWM duty cycle range you may use with this part.
Just swaging it, you'll need a ~24VDC supply for 7 LEDs in series with this part.
GL - Matt
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