Display stopped working. Touch still works. Please help

Hello everybody,

Briefly will describe the problem:

It is iPhone 5 64GB. the problem started a year ago when updated to iOS 8. At the beginning the display started to turn itself black for couple of seconds. In the next weeks this becomes for minutes and ended with hours. The touch was working fine as I can hear the sound when messaging or typing. I have changed the display and the digitizer with another but the problem persist. Also I am still able to answer calls but it is still dead black. I found that the display can work on on low brightness and turned off the automatically adjustment of it. It last like this for couple of hours and used it for 3-4 mounts. But now it is very hard.

I will appreciate any help and advise how to fix this.

I have done a little research and found that there is a chip on the logic board that controls the brightness. May be this could be the problem.

Will a replacement of the logic board will help ? Or something else.

Thanks in advance.