I would guess that if the display is not connected to the laptop video output the display shutsdown/ power save mode => switch off the LCD voltages => I havent checked the TP11 but I guess it goes to the LCD or LEDs.
if you read the issue:
[post|169538]
You will find an an extensive description on the LED background not lighting. If you have any questions just ask and I ll see what I can find out but please read the thread carefully, I think you ll find your answer there.