I'm using a QVGA display with an MCB2460 board. When I connect the cable from the Keil Ulink2 to my PC the Display lights up with a grey backlight. When I connect my USB A cable supplying power for my board from my PC the Display darkens. I can use the display as normal but once power is removed from the board WITH the Ulink still in place the display returns to Grey backlight.
Maybe, read the user manual of LPC24xx, see Chapter 9: LPC24XX Pin connect, check the PINSEL4 and PINSEL10 related stuff.
or add the below code to your main(), and see what will happen.
PINSEL10 = 0x00;