A power supply system has a power supply unit generating a power supply voltage signal, a voltage controller generating a first power supply control signal in response to the power supply voltage signal and a lamp-on/off signal, an inverter controller being driven in response to the first power supply control signal, and an inverter driving a lamp in response to a control signal from the inverter controller. The first power supply control signal prevents the inverter from being shut down. The voltage controller includes a comparator comparing a comparison voltage signal with a reference signal, a switch unit being turned on or off in response to the lamp-on/off signal and an output signal of the comparator, and a constant voltage generator providing a constant voltage signal as the first power supply control signal to the inverter controller. A liquid crystal display device has a timing controller, gate and data drivers, a display panel, a lamp unit, and the power supply system for providing electric power to the lamp unit.