Backlight brightness cannot be changed

Hi! I have a Samsung galaxy (GT-S7580) that has a problem with its back-lightning, such that the brightness of it cannot be controlled anymore. I narrowed it down to a faulty component or connection on the motherboard as I had another working S7580 to test it with. But there ends my knowledge of electronics. Any help to troubleshoot it further would be most appreciated. What is the component that controls the brightness of the backlight?

Thank you!