Patent ID: 8102361

Claim:
A liquid crystal display for adjusting a brightness of a backlight of a liquid crystal display panel in accordance with an illuminance of external light, comprising: an external light sensing circuit including: a photosensor; a capacitor connected in parallel to the photosensor; and a writing switch connected to the photosensor and the capacitor through an output node to charge and discharge the capacitor, and for varying a level of an output voltage applied to the output node in accordance with the illuminance of the external light; and a pulse width modulation (PWM) duty controller for detecting the illuminance of the external light by using a time taken for the output voltage to exceed a predetermined reference voltage, and controlling a duty ratio of a pulse width modulation signal used for controlling the brightness of the back light in accordance with a detected illuminance of the external light, wherein a control signal applied to a gate electrode of the photosensor is generated at a first logic level during a sensing permitting period from a starting point of a write-ON period for charging the capacitor until a specific time point within a write-OFF period for discharging the capacitor, and the control signal is generated at a second logic level for detrapping electric charges trapped in a dielectric layer in the gate electrode due to the first logic level during a sensing blocking period from the specific time point until a finishing point of the write-OFF period, wherein an amplitude of the control signal and a length of a maintenance period of the second logic level of the control signal are varied according to an amount of electric charges trapped in the dielectric layer in the gate electrode and a sensing speed and a sensing sensitivity for sensing the illuminance of external light.