No, it's not turning off. it's oscillating. There is a vibration on lcd and it's hard to recognize linux logo. I installed zImage_N43 (for 4.3" lcd) kernel from www.friendlyarm.net
then boot it.
This time, lcd is stable but defined lcd dimensions is not compatible. Qtopia is not fit in lcd correctly.
I have to use one of the images: zImage_N35, zImage_T35, zImage_X35 according to dvd (came with board) and friendlyarm.net
I think, 3.5" lcd on my board is a new type and it's not compatible with kernel images mentioned above