Dimming TFT screen brightness - LG LED screens

LG-E2242 - 55cm / 22inch LED LCD Monitor

LG LED monitor

This screen uses LED edge lights for screen illumination. It consists of 4 strings LEDs, each string working at ~49 volts, 60mA/~3 Watts (100% brightness). All strings together uses 240mA/12 Watts. PWM is used for brightness control, setting brightness to 0% equals in 30% PWM duty cycle (~20mA avg).
This monitor contains a chip from Novatek NT68652UFG 1153-BB. There is unfortunately very little documentation found about this "all in one" chip.



The Novatek implements a boost converter topology. Boosting the input voltage of 18 volts to 49 volts for the LED's. From the 4 LED"s strings, two LED strings are connected in parallel on the PCB board. Watch out that removing one strings only causes that the remaining string gets doubled power, the Novatek IC does not  sense this nor shutdown the outputs.

Internal PCB of the LG LED monitor

The only schematic about the LED driver part that I could find is the one below. It comes from a presentation file found on the internet. The schematic has many similarities with the PCB inside this LG screen. Use it as a reference.

Dimming using external regular PWM to LED's is not possible since the Novatek chip detects the external PWM-off period as defective LED's and shuts down the output power to the LED's. It turns the power back to the LED's after off/on of the LCD screen. Since there is no way to reprogram the Novatek IC and change the brightness values, there are only two options left for dimming:
(1)external boost converter to 50volts, then manually drive the LED's with PWM.


(2)external pwm driver that keeps the used current constant in the OFF period, regardless the brightness pwm to the LED's.

PWM duty at 0% setting

In my case, the smallest and cheapest solution was the second option. Tested this and I can now fully dim the brightness up to 0.1% duty cycle PWM!! So now in complete darkness the TFT monitor wont blind me anymore.

Schematic Novatec IC

Menu to set the internal brightness

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