A backlight unit includes; a light source, an inverter which provides the light source with an input voltage, and a printed circuit board (“PCB”) connected to the light source, wherein the PCB includes a protection circuit which detects an open-lamp-protection voltage which varies according to a change of the input voltage and changes a reference voltage according to the change of the input voltage, wherein the protection circuit turns off the inverter when the detected open-lamp-protection voltage is higher than the changed reference voltage.