1. Field of the Invention
The invention is related to a pixel circuit for liquid crystal display, wherein a digital circuit is installed at a pixel of the liquid crystal display for processing static image. The digital circuit works with an analogue circuit for lowering the power consumption so as to accomplish power saving function of a pixel circuit for liquid crystal display.
2. Description of the Prior Art
Liquid crystal display (LCD) is widely used in notebook computers and various apparatus with display functions. An image pixel driving circuit used in the LCD is an analogue circuit. Among prior art LCD elements, passive or active matrix liquid crystals such as thin film transistor (TFT) and twisted nematic (TN) are used. A schematic view of exemplary circuit of a prior art pixel circuit is shown in the
In the prior art, a surface stabilized ferroelectric liquid crystal (SSFLC) is also used to form a LCD. The SSFLC has spontaneous polarization. When an external electric field is applied, the direction of the spontaneous polarization reverses and such direction is then retained. As a result, when the LCD displays static image, it's no longer required to continually writing signals into pixels, neither is required to continually charge/discharge data line, so as to reduce power consumption. The drawback of the method is that such display only shows black and white. A gray level display requires complicated circuits such as pulse width modulation (PWM).
In order to resolve the aforementioned drawbacks of pixel circuit for liquid crystal display such as high power consumption or requirements to use complicated circuits, a digital circuit is employed at a pixel of the LCD in the present invention, such frequent display refresh is eliminated and the power consumption is reduced.
The invention is about a pixel circuit for liquid crystal display. A digital circuit is installed at a pixel of the liquid crystal display for processing static image. The digital circuit works with an analogue circuit for processing dynamic image. Traditionally, analogue pixels have better performance for gray level display. According to the present invention, a digital operation is provided, wherein the data line is not required to be charged/discharged, such that the power consumption is reduced. In addition, several multiplexers are provided to enhance the digital and analogue signal processing, for lowering the power consumption so as to accomplish power saving function of a pixel circuit for liquid crystal display.
The pixel circuit for liquid crystal display comprises a plurality of multiplexers for circuit switching, a TFT as a control switch of the circuit connected to a scanning line and a data line, a capacitor for storing voltage signals from the data line, and a switch device for isolating digital mode circuit from the analogue mode circuit to prevent interference between two modes.
The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings.
The present invention initializes a digital operation mode of dynamic memory to enable a static image display without continually refreshing the display, so as to reduce power consumption and save power.
Refer to
The TFT 201 of the liquid crystal unit circuit is a switch; the switch is used to receive signals from scanning line 203 to determine whether liquid crystal unit circuit should be switched on or off. When the TFT 201 is switched on, an analogue voltage value from the data line 205 into is written to the capacitor 207, and the analogue voltage value is output from the output terminal of the second multiplexer 204 to liquid crystal unit 209 for displaying gray level image. Due to the malfunction of TFT 201, a current leakage may occur and result in gray level loss. Therefore, the data line 205 is required to continually charge/discharge the capacitor 207. When dynamic image processing is on the first mode (mode=0), The first multiplexer does not function, that means the display operates exactly like the prior art does.
When the signals from the scanning line 203 switches on the liquid crystal circuit, and the mode control terminal 206 is enabled to receive digital signals for static image as the control signals, then the operation mode is on the second mode (mode=1) according to the embodiment of the present invention. On the second mode, the selection terminal sel of the second multiplexer 204 input the signals from mode control terminal 206 to the second multiplexer 204. In addition, the second mode terminal in1 receives a voltage value from the output terminals out of the first multiplexer 202. The output voltage value acts as a signal via scanning line 204 for switching on the liquid crystal circuit. Then data line 205 charges/discharges the capacitor 207 via the TFT 201 and writes the digital voltage signal into the capacitor 207. Because the selection terminal sel of the first multiplexer 202 is connected to the TFT 201 and the capacitor 207, the digital voltage signals stored in the capacitor 207 is used to determine the output status of general voltage terminal Vcom and the reference voltage terminal Vref. The voltage value of the general voltage terminal Vcom has the electrical potential status without extra voltage applied. Vcom and Vcom′ are two general voltage output terminals connected to the same voltage level. The voltage value of the reference voltage terminal Vref is used as a driving voltage. By switching the current between general voltage terminal Vcom and reference voltage terminal Vref and the voltage between liquid crystal unit 209 and the other general voltage terminal Vcom′, the status (bright or dim) of liquid crystal unit 209 is determined. When the digital voltage value of capacitor 207 from the data line 205 is a low voltage, then both terminals of the liquid crystal unit 209 are used as Vcom and Vcom′. When there is not any electrical field applied on the liquid crystal unit 209; the display mode operates as that two terminals of liquid crystal unit are served as a reference voltage terminal Vref on one terminal and general voltage terminal Vcom′ on the other terminal. When there is an electrical field applied on the liquid crystal unit 209, the voltage value of the reference voltage terminal Vref can be used for controlling the switching of polarization to extend the life of liquid crystals. According to the operation mentioned above, the liquid crystal unit 209 is enabled to render static image display of bright/dim status by switching current between reference voltage terminal Vref and general voltage terminal Vcom.
When the operation is on the second mode, the digital mode, the first switch device 302 is switched to on status, digital voltage values from the data line 205 are stored to the capacitor 207 via the TFT 201. The first switch device 302 connects to the liquid crystal unit 209 via the second mode terminal in1 of the first multiplexer 202, then separately connects to the general voltage terminal Vcom and reference voltage terminal Vref of the first multiplexer 202. Vcom and Vcom′ are two general voltage output terminals connected to the same voltage level. Such a second mode, which is a digital mode for display static image, uses the voltage status of the capacitor 207 to switch the current between the general voltage terminal Vcom and the reference voltage terminal Vref of the second multiplexer 204. The bias between two terminals of the liquid crystal unit 209 is used to perform bright/dim status of the image.
When the operation is on the second mode, the digital mode, the first switch device 302 is switched to on status, digital voltage values from the data line 205 are stored to the capacitor 207 via the TFT 201. The first switch device 302 connects to the liquid crystal unit 209 via the second mode terminal in1 of the first multiplexer 202, then separately connects to the general voltage terminal Vcom and reference voltage terminal Vref of the first multiplexer 202. Such a second mode, which is a digital mode for display static image, uses the voltage status of the capacitor 207 to switch the current between the general voltage terminal Vcom and the reference voltage terminal Vref of the second multiplexer 204. The bias between two terminals of the liquid crystal unit 209 is used to perform bright/dim status of the image.
Aforementioned display pixel with format as dynamic random access memory, chooses to use general voltage terminal Vcom or reference voltage terminal Vref to apply a bias on liquid crystal unit 209 based on the digital voltage value stored in the capacitor 207, so as to change the bright/dim display status. However, because the possible current leakage of TFT elements may result in changes of the digital voltage value level, under the circumstance, it is recommended to charge/discharge the capacitor 207 when required.
The above provides a detailed description of the embodiments according to the pixel circuit for liquid crystal display in the present invention. The present invention lowers the refresh rate of the display and the power consumption by implementing a plurality of multiplexers and analogue and digital pixel circuits for liquid crystal display composed of DRAM or SRAM.
The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.
Number | Date | Country | Kind |
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92107205 A | Mar 2003 | TW | national |
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Number | Date | Country | |
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20040189578 A1 | Sep 2004 | US |