I eventually ended up with chaining a current limiter to the boost converter. I used a mosfet transistor couple with a bunch of resistors in order to design a variable current limiter with 0.7v voltage drop-off driven by an arduino (protected with a zener diode). The circuit is pretty simple but the boost converter i made is quite inefficient (5 to 21v reaches ~50% efficiency which is orribile). I ordered an off the shelf on ebay, hoping for improvements.salami738 wrote: ↑Fri Dec 08, 2017 6:27 amI assume you use the display "LQ035NC111"?
According to the datasheet the backlight needs 18.6V - 21.0V and 20mA.
In my opinion you need a constant current, not a constant voltage source (boost converter). You can build it yourself or get a ready made lcd backlight driver chip, like: http://www.ti.com/product/tps61160. 1-output-channel, Input: 2,7-18V, Output 2,7-26V 20mA Current. Maybe @kite, or @ZeroBoy can confirm that.
The brightness is then controlled via PWM (f.e. from an analog output from your arduino) or via 1-wire protocol (arduino digital pin). See: http://www.ti.com/product/TPS61160/data ... LVS7915277
I also tried using an LM317 used as a constant current regulator but the voltage drop off is about 3v so for now the current limiter seems the best option apart from buying the smd part as you mentioned.