1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving method of the liquid crystal display device for reducing the electric power consumption.
2. Discussion of the Related Art
A liquid crystal display device (or “LCD”) represents video and/or picture image by controlling the light transmittance ratio of the liquid crystal layer using an electric field applied to the liquid crystal layer corresponding to the video signal. The liquid crystal display device is a flat panel display device having merits of small size, thin thickness and low electric power consumption. Therefore, it is used in portable computers such as notebook PC, office automation appliances, audio/video devices, and so on. An active matrix type liquid crystal display device has a switching element at each liquid crystal cell that controls the liquid crystal cell actively.
For the switching element used in the active matrix type LCD device, a thin film transistor (or “TFT”), as shown in FIG. 1, is typically used. With respect to FIG. 1, the active matrix type LCD supplies the digital video data to the data line (DL) after converting the digital video data into the analogue data voltage based on the gamma reference voltage. At the same time, it supplies the scan pulse to the gate line (GL) to charge the data voltage to the liquid crystal cell (Clc). To do this, the gate electrode of the TFT is connected to the gate line (GL), the source electrode is connected to the data line (DL), and the drain electrode of the TFT is connected to the pixel electrode of the liquid crystal cell (Clc) and one electrode of the storage capacitor (Cst1). The common voltage (Vcom) is supplied to the common electrode of the liquid crystal cell (Clc), the counter electrode of the pixel electrode. The storage capacitor (Cst1) charges the data voltage supplied from the data line (DL) when the TFT turns on to keep the voltage of the liquid crystal cell (Clc) constant. When the scan pulse is applied to the gate line (GL), the TFT turns on so that a channel is formed between the source electrode and the drain electrode to supply the voltage on the data line (DL) to the pixel electrode. At this time, the liquid crystal molecules of the liquid crystal cell (Clc) are re-arranged by the electric field formed between the pixel electrode and the common electrode so that the incident light is modulated.
As the LCD device is not a self-luminescent display device, it requires a light source such as the backlight unit. There are two types of backlight units for LCD device, i.e., the direct type and the edge type. For the edge type, the light source is disposed around the LCD panel and the light from the light source is guided to the front surface of the LCD panel using a transparent light guide. For the direct type, the light source is dispose on the rear surface of the LCD panel so that the light from the backlight source is directly radiated to the LCD panel. Compared with the edge type, the direct type can provide brighter luminescence by disposing more light sources. Further, the direct type has an advantage of a larger light irradiating surface. Therefore, for an LCD TV requiring a large size LCD panel, the direct type is generally used.
However, the direct type liquid crystal display device has the following drawbacks. First, the LCD device according to the related art has high electric power consumption at the backlight unit as the LCD panel becomes larger and has a higher resolution. Second, the issue of high power consumption remains even when a video image is displayed only in portions of the overall LCD panel. This is because the light sources included in the backlight unit are driven simultaneously according to the power supply regardless of the area in which video image is displayed. Therefore, even if a video image is displayed in only portions of the LCD panel, the electric power consumption for driving the LCD panel is substantially identical. For the large and high resolution LCD TV according to the related art, due to the high power consumption problem, when the LCD panel is not in a normal driving mode as shown in FIG. 2a, the back light unit (BLU) of the LCD panel is always off, thereby entering a black mode as shown in FIG. 2b. Therefore, electric power efficiency is not maximized when the LCD TV is operating in an interior mode, in which only portions of the LCD panel displays an image.