1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device that performs a gradation display by a combination of a plurality of sub frames according to a gradation level represented by a plurality of bits.
2. Description of the Related Art
In the past, a sub frame drive scheme has been known as one of half-tone display schemes used in an LCD device. In the sub frame drive scheme which is a kind of a time axis modulation scheme, a certain period of time (for example, a frame which is a display unit of an image in a case of a moving image) is divided into a plurality of sub frames, and the sub frames are combined according to a gradation to be displayed so as to drive pixels. The gradation to be displayed is decided according to a ratio of a pixel drive period of time occupied in the certain period of time. The ratio of the pixel drive period of time occupied in the certain period of time is decided according to a combination of the divided sub frames.
As an LCD device employing the sub frame drive scheme, known is an LCD device including pixels each of which includes a master latch, a slave latch, an LCD element, and three switching transistors of first to third switching transistors as disclosed in, for example, Japanese Translation of PCT International Application Publication No. JP-T-2001-523847 is known. In this case, in each pixel, first data of one bit is applied to one of two input terminals of the master latch through the first switching transistor; and second data of one bit having a complementary relation with the first data is applied to another input terminal through the second switching transistor. Then, when a target pixel is selected as a row selection signal applied through a row scanning line, the first switching transistor and the second switching transistor are turned on, and the first data is written. When the first data has a logical value of “1” and the second data has a logical value of “0,” the pixel performs a display based on data.
After each data is written in all pixels by the above-described operation within a certain sub frame period of time, the third switching transistors of all pixels are turned on within the sub frame period of time. Then, the data written in the master latch is read out to the slave latch with a certain time difference. Then, the data latched in the slave latch is applied to a pixel electrode of the LCD element. The above-described series of operations are repeated at intervals of sub frames, and a desired gradation display is performed based on combinations of all sub frames within one frame period of time.
In other words, in the LCD device employing the sub frame drive scheme, certain display periods of time which are the same as or different from each other are allocated to all sub frames present within one frame period of time. Further, each pixel performs a white display in all sub frames at the time of a maximum gradation display (displayed), but does not perform a white display in all sub frames at the time of a minimum gradation display (not displayed, that is, displayed black). In the case other than at the time of the maximum gradation display or the minimum gradation display, a sub frame to be displayed white is selected according to a gradation to be displayed white. In addition, in the LCD device according to the related art, data to be input is digital data representing a gradation, and a digital drive scheme of a two-stage latch configuration is used.
However, in the LCD device according to the related art, each of the two latches in each pixel is configured with a so-called static random access memory (SRAM), and thus the number of transistors configuring the circuit becomes large. Thus, there is a problem in that it is difficult to reduce the size of a pixel. Further, data written in the two-stage master latches is read out to the slave latch at the same time, and the data latched in the slave latch is applied from the slave latch to the pixel electrode of the LCD element. At this time, however, when switching is performed on all pixels at the same time (at one time), all pixels are read out and thus a consumption current at the moment becomes extremely high. The occurrence of an instantaneous peak of consumption current results in a drop in a power voltage or an increase in a GND voltage, which seriously influences the drive operation of the whole LCD device.