1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a driving method thereof, and more particularly, it relates to an LCD designed to stably operate at a low temperature and a driving method thereof.
2. Description of the Related Art
In recent years, personal computers and televisions have been designed to be light-weight and flat, and consequentially display devices are being designed to be thinner and more light-weight. Thus, flat panel displays including liquid crystal displays (LCDs) have been developed as an alternative to traditional cathode ray tubes (CRT).
An LCD display utilizes two substrates and a liquid crystal material with an anisotropic dielectric constant solution injected therebetween. An electric field is applied to the liquid crystal material. The amount of light transmitted through the substrates from an external light source is controlled by the intensity of the electric field to obtain a desired image signal.
Such an LCD is the most common type of flat panel display. A thin film transistor-LCD (TFT-LCD) using a TFT as a switch is a common embodiment of such an LCD.
In the TFT-LCD, electrodes (pixel electrodes and a common electrode) are arranged opposite to each other with a liquid crystal between the opposite electrodes thereby defining pixels. Accordingly, each pixel of the TFT-LCD can be modeled as a liquid crystal capacitor. FIG. 1 is an equivalent pixel circuit illustrating such a pixel in the LCD.
As shown in FIG. 1, each pixel of the LCD includes a TFT 10 having a source electrode and a gate electrode respectively coupled to a data line Dm and a scan line Sn, a liquid crystal capacitor Cl coupled between a drain electrode and a common voltage Vcom of the TFT 10, and a storage capacitor Cst coupled to the drain electrode of the TFT 10.
The TFT 10 applies a data voltage Vd to a pixel voltage Vp through a data line Dm to each pixel (not shown) in response to a scan signal applied through the scan line Sn. An electric field corresponding to a difference between the pixel voltage Vp and a common voltage Vcom applied to a common electrode (not shown) is applied to a liquid crystal (equivalently shown as a liquid crystal capacitor in FIG. 1). The liquid crystal in turn controls light transmittance corresponding to the intensity of the electric field. Accordingly, the storage capacitor Cst maintains the pixel voltage applied to the liquid crystal Cl until the next data voltage Vd is applied so as to allow light to transmit through the liquid crystal.
In general, methods for driving an LCD can be classified into a color filter method and a field sequential method.
An LCD employing the color filter method forms a color filter layer having three primary colors (red, green, and blue) on an upper substrate of a panel, and adjusts the amount of light transmitted through the color filter layer to express a desired color. The LCD employing the color filter method adjusts the amount of light transmitted through color filters of red, green, and blue from a single light source and combines the red, green, and blue lights to display a desired color.
Because such an LCD displays colors using a single light source and three color filter layers, it thus requires more than triple the number of pixels needed to display a monochrome image, which pixels respectively correspond to red, green, and blue color areas. Accordingly, sophisticated manufacturing technology is required to produce a color device which provides a high resolution image. Moreover, the red, green, and blue color filters are separately formed on the upper substrate of the liquid crystal display which further complicates the manufacturing process of the liquid crystal display; light transmittance of the color filters must be considered as well.
On the other hand, an LCD employing the field sequential driving method periodically and sequentially turns on and off independent red, green, and blue light sources, and synchronously applies a corresponding pixel voltage Vp to each pixel in accordance with the periodic switching to thereby obtain a color image. In other words, the field sequential driving method uses persistence of vision, i.e., the after image effect on the eye, to display a colored image by way of outputting the red, green, and blue lights from RGB light sources (backlights), and time-divides the red, green, and blue lights to sequentially display the time-divided red, green, and blue lights as a pixel instead of dividing the pixel into three separate red, green, and blue colored pixels.
Accordingly, an LCD using the field sequential method drives one frame by dividing it into R, G, and B fields, and so the response speed of the liquid crystal must be faster than that of an LCD using the color filter method. However, a liquid crystal in a portable device such as a cellular phone may be frequently exposed to low temperatures which slow the device's response speed. Because the response speed of a liquid crystal slows down at low temperatures, color reproducibility is degraded when the field sequential method is employed.