ACTIVE MATRIX CELL FOR AC OPERATION

Abstract: An active matrix liquid crystal display cell (20) for AC operation including a first field effect transistor switch (22) for switching video information to a first storage capacitor (24) and a second field effect transistor switch (62) for switch­ ing the voltage of the first storage capacitor (24) to one side of a liquid crystal cell (20). The second transistor (62) is con­ nected to be switched on by a common signal, for example, a vertical sync signal, and the top plate voltage applied to the liquid crystal cell (20) is alternated in synchronism with the common signal, resulting in a doubling of the effective poten­ tial applied across the liquid crystal cell (20).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The subject invention relates to liquid crystal displays and more particularly to improved AC activation circuitry for an active matrix addressed liquid crystal display. 2. Description of Related Art Presently, all active matrix addressed liquid crystal displays use AC activation. Present AC displays use one transistor per liquid crystal cell to switch a stored control voltage across the cell. Such structures achieve AC biasing of the liquid crystal cells by driving the pixel voltage symmetrically about a fixed potential which is applied to the top plate electrode. This AC drive technique limits the maximum applied RMS voltage to one-half of the available DC voltage range. Since the new information on each pixel appears immediately after that pixel is updated, it is impossible to switch the top plate potential in such a way that the effects are uniform for each pixel. Limiting the applied RMS voltage to one-half the available DC voltage is disadvantageous because it restricts the liquid crystal effects which may be used with AC drive. For example, AC dynamic scattering liquid crystal materials cannot be driven by present drive structures. In addition, other scattering liquid crystal effects which incorporate encapsulated liquid crystal materials also require higher voltages than are presently available. The long chain organic molecules of which liquid crystal materials are composed also can be degraded by various chemical contaminants. AC activated displays are more desirable than DC displays in this respect since the applied AC potential tends to limit electrochemical degradation of the liquid crystal material. It would therefore be desirable to continue use of AC activated displays, while eliminating the present limitations on the operating voltage of such displays. It has occurred to the inventor that placement of additional transistors in the display cell structure might be used to eliminate the limitation on the operating voltage of the display cells in an AC activated display. Display cells with multiple transistors have been proposed in the prior art, generally to construct a static random access memory (RAM) cell. To the inventor's knowledge, additional transistors have not been proposed as part of a technique to extend the dynamic operating voltage of the liquid crystal cells. SUMMARY OF THE INVENTION It is therefore an object of the invention to improve liquid crystal displays; It is another object of the invention to provide an improved structure for driving an AC activated liquid crystal display; It is another object of the invention to increase the dynamic operating voltage of the liquid crystal cells in an A.C. activated display. According to the invention, there is provided a liquid crystal display comprising a plurality of liquid crystal display cells having a common top plate electrode and means to address a first control signal to each display cell, each display cell comprising a signal storage means for storing a display signal and a first switch means for switching the display signal to the storage means in response to the first control signal, characterised in that each liquid crystal display cell further comprises a second switch means for switching the display signal stored by the signal storage means to the liquid crystal cell in response to a second control signal, whereby the plurality of the liquid crystal display cells having a common top plate electrode may be activated simultaneously by a common second control signal. The vertical sync signal is a convenient signal for use as the second signal, although another signal could be used. Use of the commonly activated plurality of second switch means permits the top plate voltage to be alternated, rather,than remaining fixed as the prior art. Alternation of the top plate voltage according to the invention permits doubling of the voltage applied to the liquid crystal material. In one embodiment, the first and second switch means comprise first and second transistors, such as, for example, field effect transistors. By using the second transistors to switch the voltage on all of the pixels of the display in parallel, one can switch the voltage applied to the top plate of the display in concert with the switching of the pixel voltage. This allows the applied RMS voltage to equal the full D.C. voltage range as described in further detail herein. BRIEF DESCRIPTION OF THE DRAWINGS The just summarised invention will now be described in detail in conjunction with the drawings of which: FIG. 1 is a schematic circuit diagram illustrating a prior art A.C. activated liquid crystal display; FIG. 2 is a circuit diagram of a display cell of the prior art; FIG. 3 is a timing diagram illustrating operation of a matrix of display cells wherein each cell is constructed as shown in FIG. 2; FIG. 4 is a circuit diagram of the display cell of the preferred embodiment; FIG. 5 is a timing diagram illustrating operation of an AC activated liquid crystal display employing display cells of the preferred embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a typical prior art liquid crystal display panel configuration and addressing format which can be used for presenting conventional television type imagery. In FIG. 1, the individual liquid crystal cells are arranged in a matrix of M rows and N columns. Each cell 20 employs liquid crystal material 28 sandwiched between a first transparent electrode 32 and a second reflective electrode 36. The transparent electrode 32 of each cell 20 is connected to a fixed reference voltage level as indicated by the arrowhead extending therefrom. Each liquid crystal cell 20 also has associated therewith a field effect transistor (FET), which is designated generally by a reference numeral 22 and a capacitor which is designated generally by reference numeral 24. The transparent electrode 32 of each cell is typically formed as a transparent top plate electrode common to all cells. The capacitor 24 and FET 22 form an elemental sample and hold circuit which functions to "stretch" addressing pulses, which could be 30 microseconds in duration, for example, to pulses of one millisecond duration needed to effectively energize the liquid crystal material. When the gate of a FET 22 is driven negative, the FET 22 turns "on" and the associated capacitor 24 is connected to a column electrode bus, such as column electrode bus 48, for example. Since the gates of all of the FETS 22 in any one row are connected together by a gate electrode bus, such as bus 42, for example, the placement of an enable signal on any given gate electrode bus causes all the storage capacitors 24 for that given row to be loaded with video data that is then present on their corresponding column electrode buses. When the gate drive voltage is removed (the gate to drain voltage returns to zero or becomes positive), all of the capacitors 24 of the corresponding row are disconnected from the column bus by the high "off" resistance of the field effect transistors 22. In this manner, each of the capacitors 24 in any given column is separated from the other capacitors 24 in that column. The synchronous line-at-a-time loading of new video information into all the elements in a given row of the display requires the video information to be present in parallel synchronous form on all the column electrode buses; i.e., the video cannot be scanned or switched from one column to the next because the video on all columns must be simultaneously valid and independent of video on the other columns. It is noted that contrary to a conventional cathode ray tube system wherein each element is addressed by an electron beam that scans in two directions, the liquid crystal display addressing format of FIG. 1 permits scanning in only one direction. As shown in FIG. 1, a vertical sweep multiplexer 46 supplied with vertical and horizontal sync signals V s , H s provides the ordered scan by driving the gate electrode buses, e.g. bus 42 with gate enable signals, e.g. GE A , GE B , GE C . The multiplexer 46 may be implemented by driving the gate electrode buses from the parallel outputs of a shift register, for example. The video signals are applied in the format required for conventional television scanning by means of a dual serial to parallel video converter 52. Additional details of such addressing circuitry are disclosed in U.S. Patent No. 3,862,360, assigned to Hughes Aircraft Company. Additional detail of a prior art active matrix cell 20 is shown in FIG. 2. As in FIG. 1, the cell 20 contains one transistor 22 and a storage capacitor 24. As in FIG. 1, the storage capacitor 24 stores a voltage PV A representing the applied video signal between refresh cycles. The ungrounded terminal 23 of the storage capacitor 24 connects the capacitor voltage PV A to one side of a liquid crystal cell LC, which has associated therewith a capacitance C LC , shown in phantom in FIG. 2. The opposite side or top plate electrode of the liquid crystal cell LC is connected to a fixed top plate voltage V TP . FIG. 3 shows the timing of the signals of three isolated rows or lines A, B, C of a display such as that of FIG. 1, which are located at the beginning (top), middle, and end (bottom) of the display, respectively. The first line of FIG. 3 illustrates standard vertical sync pulses, which synchronize or time the provision of each successive frame of the display. On the second line of the timing diagram, below the vertical sync pulses are shown the horizontal sync pulses, each one of which synchronizes the activation of a particular row of the display during the vertical sync period by triggering generation of corresponding "Gate Enable" pulses for each of the rows of the display. Below the respective gate enable signals, Gate Enable A (GE A ), Gate Enable B (GE B ) and Gate Enable C (GE C ), the pixel voltage PV A , PV B , PV C is shown for each of the rows A, B, C, in the example under discussion. The respective pixel voltages PV A , PV B , PV C are each sampled and held by respective storage capacitors 24 at the beginning of application of the respective gate enable pulse GE A , GE B , GE C . The top plate voltage V TP remains at a fixed level, -15 volts, throughout operation of the display. From FIG. 3, it may be observed that there is no time at which the top plate voltage signal V TP could be changed symmetrically for all three of the rows A, B, C of the display to which the gate enable signals GE A , GE B , GE C are respectively applied. While the top plate is at -15 volts, the applied video signals PV A , PV B , PV C vary between 0 and -30 volts at different times. Thus, the display exhibits a maximum potential of 15 volts RMS across the liquid crystal LC. The display cell 60 of the preferred embodiment of the invention is shown in FIG. 4. In this cell 60, a second field effect transistor 62 is added. This transistor 62 is located between the storage capacitor 24 of the FIG. 2 design and the liquid crystal element LC. The source of the second transistor 62 is connected to the capacitor terminal 23, and the drain D of the second transistor 62 is connected to the liquid crystal cell LC. The gate of the second transistor 62 is connected to a common line 64. The common top plate electrode of the liquid crystal element LC is connected to a voltage source V TP which provides an alternating voltage V TP as will be described in further detail in connection with FIG. 5. The liquid crystal element LC of FIG. 4 again has a capacitance C LC associated therewith. It will be apparent that a display cell 60 may be substituted for each display cell 20 in a matrix structure such as that of FIG. 1. In a display such as that of FIG. 1, the gate of each second transistor 62 in each display cell is connected to a common line and is activated in common by, for example, the vertical sync signal. The purpose of the second transistor 62 is to allow the charge stored on the first capacitor 24 to be transferred to the capacitance C LC associated with the liquid crystal element LC in parallel for all of the liquid crystal elements of the display. Thus, the first transistor 22 and the first capacitor 24 are used as in prior art displays to store the video information line by line, and the second transistor 62 and capacitance C LC of the liquid crystal cell are used to apply that stored information in parallel to the liquid crystal cells LC. One requirement of this design is that the capacitance C 1 of the first capacitors 24 be much greater than the capacitance C LC of the liquid crystal cells LC, i.e. C 1 >> C LC . This constraint limits the display cell size. Such a constraint might be overcome by adding additional capacitance through higher dielectric constant insulators or vertical integration of the capacitor. The capacitance associated with a 79 micron square liquid crystal element 10 microns thick is approximately 0.3 pF; the storage capacitance of one present LCMOS display layout (employing an approximately 45 x 55 micron capacitor with a 1400 angstrom thick SiO 2 dielectric) is approximately 7.5 pF, or 25 times larger than the calculated liquid crystal capacitance. Integration of the second transistor 62 would reduce this capacitance by approximately a factor of two or so, but would still permit a C 1 to C LC ratio of ten. FIG. 5 shows the timing of the signals for three lines or rows A, B, C of a display which includes display cells according to the preferred embodiment. The diagram of FIG. 5 is similar to that of FIG. 3 in that the rows A, B and C are rows at the top, middle and bottom of a display as shown in FIG. 1. The vertical sync and horizontal sync signals shown are identical to those of FIG. 3, as are the gate enable signals, again illustrated as Gate Enable A (GE A ), Gate Enable B (GE B ) and Gate Enable C (GE C ). Three new timing lines appear in FIG. 5, however, in order to show the presence of signals C A , C B , C C stored respectively on the capacitors 24 of each display cell 60 of each row A, B, C of cells according to the preferred embodiment. Since the second transistors 62 of each display cell 60 are activated by the vertical sync signal, the respective pixel voltages PV A , PV B , PV C are established and applied at the same time, namely, the time of occurrence of the vertical sync signal. Thus, the voltage which is actually applied to the liquid crystal LC is now changed at the same moment for all of the rows of the display, including the three rows A, B, C. Thus, the top plate voltage V TP can be changed at the same moment as the voltage applied to the liquid crystal cells LC, resulting in symmetrical signals for all elements of the display. The top plate voltage V TP is therefore illustrated in FIG. 5 as being changed between 0 and -30 volts, and the line outputs again vary between 0 and -30 volts. This operation results in a theoretical 30v RMS signal for the liquid crystal matrix or twice the RMS signal level of the prior art. Losses in the charge transferred between capacitors will reduce this range somewhat. An improved AC activated liquid crystal display cell has thus been disclosed, which permits operation at approximately double the applied potential available in the prior art. Such operation greatly expands the number of liquid crystal effects which may be used in AC activated displays. Those skilled in the art will appreciate that modifications and various adaptations of the just disclosed preferred embodiment can be made without departing from the scope of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims: A liquid crystal display comprising a plurality of liquid crystal display cells (60) having a common top plate electrode and means to address a first control signal to each display cell (60), each display cell (60) comprising a signal storage means (24) for storing a display signal and a first switch means (22) for switching the display signal to the storage means (24) in response to the first control signal, characterised in that each liquid crystal display cell (60) further comprises a second switch means (62) for switching the display signal stored by the signal storage means (24) to the liquid crystal cell in response to a second control signal, whereby the plurality of the liquid crystal display cells (60) having a common top plate electrode may be activated simultaneously by a common second control signal. A liquid crystal display according to claim 1 wherein the said common second control signal is a vertical sync signal. A liquid crystal display according to claim 1 or 2 wherein a plurality of said second switch means (62) are connected to a common control signal line (64) for supplying the said common second control signal. A liquid crystal display according to claim 1, 2 or 3 wherein the first switch means (22) comprises a first transistor having a source connected to receive the display signal, a drain connected to the signal storage means (24), and a gate connected to receive the first control signal. A liquid crystal display according to any preceding claim wherein the second switch means (62) comprises a second transistor having a source connected to charge storage means (24), a drain connected to the liquid crystal cell, and a gate connected to receive the second control signal. A liquid crystal display according to claim 4 or 5 wherein the first and/or the second transistor is a metal oxide silicon (MOS) transistor. A liquid crystal display according to any preceding claim wherein the signal storage means (24) comprises a capacitor. A liquid crystal display according to any preceding claim wherein all of the liquid crystal display cells (60) share a common top plate electrode. A liquid crystal display according to any preceding claim wherein means are provided for supplying a top plate voltage to the common top plate electrode and for alternating the top plate voltage between first and second voltage levels. An A.C. activated liquid crystal display according to claim 9 wherein the means for supplying the top plate voltage is adapted to alternate the top plate voltage in synchronism with the common second control signal. A method of controlling a liquid crystal display according to any preceding claim comprising the steps of: storing display signals in the signal storage means (24) of the plurality of liquid crystal display cells (60); and simultaneously applying a common second control signal to the second switch means (62) of the plurality of liquid crystal display cells (60). A method of controlling a liquid crystal display according to claim 11 wherein a voltage is supplied to the common top plate electrode in synchronism with the said common second control signal. A method of controlling a liquid crystal display according to claim 12 wherein a plurality of sets of display signals are successively stored and successively applied, each set being applied in response to a respective one of a succession of common second control signals, and wherein the voltage supplied to the common top plate electrode is successively alternated in synchronism with the succession of common second control signals.

Inventors: SHIELDS, STEVEN, E.
Applicants: HUGHES AIRCRAFT COMPANY
Application year: 1988
Main classification: G09F9
IPC codes: G02F1, G09G3, H04N5