WO2000014713A1

WO2000014713A1 - Field sequential reflective liquid crystal display without external frame buffer

Info

Publication number: WO2000014713A1
Application number: PCT/US1999/020199
Authority: WO
Inventors: Gary W. Jones; Olivier F. Prache
Original assignee: Fed Corporation
Priority date: 1998-09-04
Filing date: 1999-09-02
Publication date: 2000-03-16

Abstract

The present invention is directed to a reflective active matrix liquid crystal display (10) device. The liquid crystal display device comprises a first substrate (11) having first electrical circuitry formed thereon, a second substrate (12) having second electrical circuitry formed thereon, and a liquid crystal composition interposed between the first and second substrates. The liquid crystal display device includes at least one pixel cell. The at least one pixel cell is defined by overlapping regions of the first and second electrical circuitry. Each of the at least one pixel cell includes at least one storage element for storing data associated with at least one color.

Description

FIELD SEQUENTIAL REFLECTIVE LIQUID CRYSTAL DISPLAY WITHOUT EXTERNAL FRAME BUFFER

CROSS REFERENCE TO RELATED APPLICATION

This application is related to and claims priority on Provisional Application Serial No. 60/099,293 filed September 4, 1998, entitled "Field Sequential Reflective Liquid Crystal Display Without External Frame Buffer." FIELD OF THE INVENTION

The invention relates to liquid crystal display devices. In particular, the present invention relates to a liquid crystal display in which the need for an external frame buffer is eliminated. BACKGROUND OF THE INVENTION

Flat panel displays comprise a wide variety of light emission devices, including liquid crystal, electroluminescent and light emitting diode type devices. These flat panel displays have found broad application. The wide use of these devices is particularly due to their light weights and small sizes, especially when compared to common cathode ray tube (CRT) devices. Flat panel displays have found use in many compact electronic appliances, such as lap top computers, pocket television systems, and other electronic display applications. Typically, flat panel displays are manufactured as individual components, separate from the electronic circuits to which they are connected. The electronic circuits are generally integrated circuits on a printed circuit board and may include circuitry for driving the display matrix, as well as microprocessors. The electronic circuitry may be consolidated into as few separate components as possible, with the main processor and drive circuitry being located on a single chip. A display matrix is connected to the printed circuit board to form the electro-optical display. The electrical connection is often made through an FPC (flexible print circuit) between the printed circuit board and the electrodes located around the periphery of the display matrix.

Contemporary miniature reflective active matrix liquid crystal displays are typically monochrome devices. In order to produce color images, a field sequential technique is used whereby the information is displayed one color field at a time in a rapid succession such that flicker is reduced to a minimum. The image source itself is built on a silicon substrate and typically incorporates driver circuitry and pixel cells. This standard approach mandates the use of an external storage medium or "frame buffer" to hold the three primary color fields. The use of an external memory for field sequential color displays has several disadvantages. The cost of the display is excessive due to the use of memory and memory control components. The data bandwidth must be extremely large in order to keep the overall image frame rate high enough to prevent flicker. There is also a substantial increase in power consumption, which is detrimental to wearable, battery operated, applications such as watches or pagers.

Wearable systems in particular require lower power electronic systems. Even though the image source may be low power, the fact that it is monochrome mandates the use of additional components and higher bandwidth, leading to an increase in power consumption. In addition, wearable devices need to be lightweight. The additional components associated with the external frame buffer increase weight and should be eliminated if possible. Finally, the additional components have a tendency to increase the cost of the system, when compared to a smaller integrated display.

U.S. Patent No. 5,130,833 to Mase, entitled "Liquid Crystal Device and Manufacturing Method Therefor," discloses a liquid crystal display device having a pair of glass substrate between which a liquid crystal layer is disposed. A pair of auxiliary substrates containing driving circuits are provided for connection to the electrode strips on the glass substrates to drive the display.

U.S. Patent No. 5,225,823 to Kanaly, entitled "Field Sequential Liquid Crystal Display With Memory Integrated Within The Crystal Panel" discloses a field sequential liquid crystal display having a backlight that provides different color fields and a liquid crystal display panel that has a matrix of cells. Data lines are coupled to the cells and provide gray level data to the cells. Select lines are coupled to the cells and enable the cells. The cells include integrated memories, located at each cell, coupled to the data lines, these integrated memories storing gray level data received via the data lines. The integration of the memories on the display panel allows all of the cells on the panel to be charged approximately simultaneously so that the time any one color is backlighting the entire display approaches an optimum fraction of a frame. The circuit described by Kanaly, however, is too large for use in miniature displays. In contrast, the present invention is suitable for use on miniature reflective liquid crystal displays on silicon. Furthermore, the circuit proposed by Kanaly provides a digital-based approach to gray-scale generation as opposed to analog based approach, set forth in the present invention, that has better dynamic characteristics.

U.S. Patent No. 5,402,255 to Nakanishi et al., entitled "Liquid Crystal Panel Module and Tape Carrier Package For Liquid Crystal Drive IC," discloses a flat panel display having externally connected electronic circuitry. Nakanishi teaches a plurality of tape carrier packages arranged along the periphery of a liquid crystal panel.

U.S. Patent No. 5,471 ,225 to Parks, entitled "Liquid Crystal Display With Integrated Frame Buffer" discloses a liquid crystal display having bit lines and word lines which need not carry large buffered currents. Specifically, buffering or driving currents necessary to charge respective display electrodes are provided within each liquid crystal cell by a storage cell. The storage cell provides static random access memory as well as current and voltage drive to the display electrode load. By placing memory for each display electrode or pixel directly adjacent to the perspective electrode and within each cell, lesser current is needed within the bit lines since the bit lines themselves do not produce the drive. Drive current within the bit lines need only exceed that necessary to activate the storage cell. Once activated, the bit line current is buffered via storage cell/buffer and presented to the display electrode. The buffered output is remote from the cross-over word line locations thereby leaving less chance for capacitive coupling thereto. Parks, however, does not focus on field sequential color displays; rather, the focus is on a monochrome bi-level liquid crystal display.

Parks does not provide any means for gray scale display with integrated memory. Furthermore, because of its power dissipation through the use of the resistors, the liquid crystal displays of Parks are more applicable for use in low information content displays. U.S. Patent No, 5,563,619 to Hilbrink, entitled "Liquid Crystal Device with Integrated Electronics," liquid crystal display which has integrated electronics. The liquid crystal display includes a first liquid crystal plate member having a bottom surface and a plurality of substantially parallel conductors on the bottom surface, and a second liquid crystal plate member mounted below the first plate member and having a top surface and a plurality of substantially parallel conductors on the top surface. A flexible member coupled to the liquid crystal display includes a circuit having a driver circuit which drives the first and second plate members to form the liquid crystal display.

U.S. Patent No. 5,566,010 to Ishii et al., entitled "Liquid Crystal Display With Several Capacitors For Holding Information At Each Pixel" discloses a liquid crystal display that is intended to realize multicolor display or full-color display by placing a color variable filter variable in wavelength of transmitted light by a voltage applying assembly for monochromatic display, and varying the display pattern of the liquid crystal display in synchronism with the color change of the filter. Ishii focuses on direct view displays; rather than miniature displays. Furthermore, Ishii focuses on the use of specific driving voltages to discriminate between dichroic filters.

U.S. Patent No. 5,712,652 to Sato et al., entitled "Liquid Crystal Display Device" discloses liquid crystal display device of low power consumption for use with a portable data processing apparatus, in particular. The liquid crystal display device includes a switch element array substrate having a plurality of data lines and a plurality of scanning lines both arranged being intersected to each other in a matrix form so as to form matrix intersection points; a plurality of pixel electrodes each arranged for each matrix intersection point; and a plurality of first switching elements each arranged for each matrix intersection point and each turned on or off by the scanning line, for applying write voltage supplied from the data line to the pixel electrode, respectively when turned on; and a plurality of memory elements interposed between the corresponding first switching element and the corresponding pixel electrode, for holding the write voltage supplied through the data line as data, when the first switching element is turned on. U.S. Patent No. 5,798,746 to Koyama, entitled "Liquid Crystal Display Device" discloses a time gradation display type liquid crystal display. A voltage applied to a pixel corresponds to a binary value, one digital memory circuit is arranged for one pixel electrode and output of the digital memory circuit is connected with the pixel electrode. A voltage on a signal line is supplied to the digital memory circuit in accordance with a scanning line signal and is stored for a desired period of time. Since the pixel electrode is connected with the output of the digital memory circuit, a high voltage or a low voltage in the digital memory circuit is supplied to the pixel electrode while the digital memory circuit is in a storage state.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a liquid crystal display that eliminates the need for an external storage medium to hold primary color fields.

It is another object of the present invention to provide a liquid crystal display that eliminates the need for an external storage medium by incorporating the memory onto the liquid crystal display. It is another obj ect of the present invention to provide a liquid crystal display having at least one pixel having at least three storage elements built in for storing color field information.

It is another object of the present invention to provide a simple decoding circuit that connects the storage element on the pixel pad synchronously with an active color field. It is another object of the present invention to provide a liquid crystal display which does not require the use of an external frame buffer.

It is another object of the present invention to provide a liquid crystal display having reduced power requirements.

It is another object of the present invention to provide a more economical liquid crystal display.

It is still another object of the present invention to provide a liquid crystal display with fewer component parts. It is yet a further object of the present invention to provide a liquid crystal display with a more compact design.

It is another object of the present invention to provide a liquid crystal display having analog storage using capacitors. It is another object of the present invention to provide a liquid crystal display that is light weight.

It is another object of the present invention to provide a liquid crystal display that may be incorporated into a wearable system.

It is another object of the present invention to provide a liquid crystal display that may be incorporated into a body mountable display device.

It is another object of the present invention to provide a liquid crystal display that is capable of displaying data without any increase in operating frequency.

Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.

SUMMARY OF THE INVENTION

The present invention is directed to a reflective active matrix liquid crystal display device. The liquid crystal display device comprises a first substrate having first electrical circuitry formed thereon, a second substrate having second electrical circuitry formed thereon, and a liquid crystal composition interposed between the first and second substrates.

The liquid crystal display device includes at least one pixel cell. The at least one pixel cell is defined by overlapping regions of the first and second electrical circuitry. Each of the at least one pixel cell includes at least one storage element for storing data associated with at least one color. In accordance with the present invention, each pixel cell may comprise a plurality of storage elements such that there is one storage element for each color displayed on the liquid crystal display device. In accordance with the present invention, the at least one storage element may be formed as part of one of the first and second electrical circuitry. Furthermore, the at least one storage element is formed on one of the first substrate and the second substrate.

In accordance with the present invention, at least one of the first and second electronic circuitry may be operatively coupled to the at least one storage element. Furthermore, at least one of the first and second electronic circuitry includes at least one switching element operatively connected to the at least one storage element. The at least one of the first and second electronic circuitry may include a switching element operatively connected to each of the at least one storage element. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:

Fig. 1 illustrates a basic composition of a liquid crystal display according to the present invention;

Fig. 2 illustrates a pixel cell for a liquid crystal cell having an integrated memory constructed in accordance with the present invention; and

Fig. 3 illustrates the timing sequence for a liquid crystal display constructed in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a miniature reflective active matrix liquid crystal display 10, as shown in Fig. 1. The reflective type liquid crystal display element 10 is an active matrix type liquid crystal display element having a first substrate 11, a second substrate 12 and liquid crystal 13 interposed between the first substrate 11 and the second substrate 12. The first substrate 11 is preferably formed from a transparent material to permit the transmission of light there through. The second substrate 12 is preferably formed from silicon. The present invention, however, is not limited to the use of silicon; rather, other materials having similar properties are considered to be well within the scope of the present invention. It is preferred that the second substrate 12 also include a reflective material or layer to direct the transmission of light towards the first substrate 11. The first substrate 11 includes electronic circuitry formed thereon. The electronic circuitry faces the second substrate 12. The second substrate 12 also includes electronic circuitry formed thereon. The electronic circuitry faces the first substrate 11. The overlapping regions of the electronic circuitry on the first and second substrates 11 and 12 form a pixel cell. In a preferred embodiment, the liquid crystal display 10 includes a plurality of pixel cells. A driving circuit 14 supplies a driving voltage to the electronic circuitry on the first and second substrates 11 and 12. Each pixel cell 20 of the display 10 includes at least one storage element 211, 212, and 213.

The at least one storage element 211, 212, and 213 and the electronic circuitry 22 associated therewith will be described in connection with Fig. 2. The storage elements 211, 212 and 213 store specific color information for each pixel cell. In a preferred form, a separate storage element is provided for each color (e.g., red, green and blue). For example, a first storage element 211 is provided for storing data associated with displaying the color green; a second storage element 212 is provided for storing data associated with displaying the color blue; and a third storage element 213 is provided for storing data associated with displaying the color red. Each storage element 211, 212 and 213 preferably provides for analog storage using, for example, a capacitor. The present invention, however, is not limited to the use of capacitors; rather, other storage elements that permit the storage of information at each pixel cell 20 is considered to be well within the scope of the present invention.

The active matrix liquid crystal display 10 in accordance with the present invention is scanned on a line at a time basis. Every row in the display 10 is enabled via a row select signal 23. When row select signal 23 is active, red, green and blue analog data is transferred from the column driver to the storage elements 211, 212 and 213. As discussed above, a separate storage element is provided for data associated with each color. In a preferred form of the present invention, one of the storage elements 211, 212 and 213 is the pixel cell 20 itself. In the illustrated embodiment, the pixel cell 20 is tied to the red data line 243 (i.e., the pixel cell 20 functions as the storage element 213). It is contemplated, however, that the pixel cell 20 need not be connected to the red data line 243; rather, the pixel cell 20 may be tied to one of the green data line 241 and the blue data line 242. A plurality of switches 25, 26, 27 and 28 interconnect the data lines 241, 242, and 243 with the storage elements 211, 212 and 213. Operation of the switches 25, 26, 27 and 28 causes the pixel cell to cycle between the display of red, green and blue light.

The row select signal has a pulse width that is equivalent to one horizontal line period. The NSYNC period for the pixel cell is divided into three equal time periods, as shown in Fig. 3, each called color field (corresponding to red, blue and green). During each color field, the illumination source has a specific and distinct color (i.e., red, green or blue).

The operation of the pixel cell will now be described. During the first color field, switches

25 and 26 are closed such that red light is displayed. At the end of the first color field, switch

26 is opened and switch 27 is closed such that green light is now displayed during the second color field. The display of green light is accomplished through the transfer of color data stored at storage element 211 to the load cell 213. At the end of the second color field, switch 27 is opened and switch 28 is closed such that blue light is now displayed for the duration of the third color field. The display of blue light is accomplished through the transfer of color data stored at storage element 212 to the load cell 213.

A Vcom line is tied to an alternative signal required for proper operation of the liquid crystal display.

The present invention advantageously incorporates the memory for the three primary color files onto the image source itself through storage elements 211, 212, and 213. The inclusion of the storage elements 211, 212 and 213 on the image source is possible due to recent advances in microelectronic integration, which now allow for very dense circuitry within a relatively small package. For example, a .5 μm CMOS process can be used to implement an XGA color display (1024 x 768 x 3 cells) on a single integrated circuit or "chip." The present invention leverages the densities offered by semiconductor processes to include at each pixel three storage elements which include both gating and access transistors. The transistors should have enough holding power to retain their value throughout the frame time. The above described decoding circuit connects the storage elements 211, 212 and 213 to the pixel pad synchronously with the active color field. The image source integrated circuit may include multiple levels for routing all the necessary control signals. The display device 10 may further include peripheral drives if necessary.

The approach of the present invention eliminates the need to store the incoming data frame. The data can be transferred seamlessly from the host to the image source with minimum additional components and without any increase in operating frequency. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A liquid crystal display device comprising: a first substrate having first electrical circuitry formed thereon; a second substrate having second electrical circuitry formed thereon; and a liquid crystal composition interposed between said first and second substrates, wherein said liquid crystal display device includes at least one pixel cell, said at least one pixel cell being defined by overlapping regions of said first and second electrical circuitry, wherein each of said at least one pixel cell includes at least one storage element for storing data associated with at least one color.

2. The liquid crystal display device according to Claim 1, wherein said at least one storage element comprises a plurality of storage elements.

3. The liquid crystal display device according to Claim 1, wherein said at least one storage element comprises a storage element for each color displayed on said liquid crystal display device.

4. The liquid crystal display device according to Claim 1, wherein said at least one storage element is formed as part of one of said first and second electrical circuitry.

5. The liquid crystal display device according to Claim 4, wherein said at least one storage element comprises a plurality of storage elements.

6. The liquid crystal display device according to Claim 1, wherein said at least one storage element is formed on one of said first substrate and said second substrate.

7. The liquid crystal display device according to Claim 6, wherein said at least one storage element comprises a storage element for each color displayed on said liquid crystal display device.

8. The liquid crystal display device according to Claim 1, wherein said at least one storage element is a capacitor.

9. The liquid crystal display device according to Claim 1 , wherein said liquid crystal display is an active matrix display.

10. The liquid crystal display device according to Claim 1, wherein at least one of said first and second electronic circuitry form an active matrix.

11. The liquid crystal display device according to Claim 1 , wherein at least one of said first and second electronic circuitry is operatively coupled to said at least one storage element.

12. The liquid crystal display device according to Claim 11 , wherein said at least one of said first and second electronic circuitry includes at least one switching element operatively connected to said at least one storage element.

13. The liquid crystal display device according to Claim 12, wherein said at least one of said first and second electronic circuitry includes a switching element operatively connected to each of said at least one storage element.