EP0260747A1 - Display device - Google Patents

Abstract

The invention relates to a novel addressing scheme for passive matrices in which voltage losses across the electrodes are compensated. The row electrodes (3) of a matrix of display elements (12) are alternately driven from the left and from the right while simultaneously presenting data voltages to the column electrodes (4).

Description

• The invention relates to a display device comprising a liquid crystalline material between two parallel support plates having surfaces facing each other, a pattern of row electrodes being provided on the one surface and a pattern of column electrodes being provided on the other surface, the row electrodes crossing the column electrodes, thus constituting display elements at the area of the crossings, the device comprising a drive circuit for presenting data signals to the column electrodes, and a row scanning circuit for periodically scanning the row electrodes.
• Display devices of this type are known and are usually operated in multiplex drive with electrical voltages in accordance with the so-called rms-mode. The drive mode is described by Alt & Pleshko in I.E.E.E.. Trans. El. Dev., Vol. ED 21, 1974, pages 146-155 and is known as the most commonly used mode for driving liquid crystal display devices which are built up as a matrix of picture elements as described above and in which no active switch per picture element is used. The maximum number of rows N_max which can be driven with an acceptable contrast ratio by means of this mode is determined by the relation:

  

  in which V on represents the required rms voltage across a display element to switch it in the "on"- condition and V off represents the rms voltage at which the display cell is in the "off-condition. As V_an and V off are closer together, a larger number of rows can be driven. This of course requires a steep threshold in the transmission/voltage characteristic of the display element.
• The rms voltages across the separate display elements do not naturally follow from the selection and data voltages presented. Due to the resistance of the electrodes voltage losses occur so that the rms voltage across the elements may be lower than is required to cause them to switch. This leads to a decrease of the maximum number of rows to be multiplexed.
• A known solution to inhibit the effects of voltage losses across the row electrodes is that the selection voltage is presented simultaneously to both ends of the row electrodes during a selection time t_s. This results in a given reduction of the voltage losses.
• A display device according to the invention is characterized in that the row scanning circuit provides the two ends of each row electrode with signals in such a manner that first the one end and thereafter the other end is connected to the row scanning circuit.
• The invention is based on the recognition that the voltage losses can be reduced in rms-driven matrices by first presenting a selection voltage to one end of a row electrode during preferably substantially half the selection time and then presenting the selection voltage to the other end during approximately a same period, whilst of course the information to be displayed is presented simultaneously as data voltages to the column electrodes.
• The selection of the two ends does not necessarily have to be effected immediately after each other. More preferably, the row scanning circuit first provides all row electrodes with selection signals at one end and than at the other end because this can be more easily realised from a circuit-technical point of view.
• The invention will now be described in greater detail with reference to some embodiments and the drawing in which
• Fig. 1 diagrammatically shows in a cross-section a liquid crystal display device;
• Fig. 2 is a diagrammatic representation of such a device in which the picture elements are arranged in accordance with a matrix, together with a part of the drive circuits;
• Fig.3 shows graphs of
  

at a point halfway the row electrode as a function of V_D/V_thr for different drive modes.
• The display device shown in fig. 1 has twoglass support plates 1 and 2. Thesupport plate 1 is provided with a pattern of strip-shaped row electrodes 3 consisting of, for example, indium tin oxide. Thesupport plate 2 is also provided with a pattern of strip-shaped column electrodes 4 consisting of, for example, indium tin oxide. Theelectrodes 3 cross theelectrodes 4 and the crossings constitute the display elements which are thus arranged in accordance with a matrix. The surfaces of thesupport plates 1 and 2 provided with electrodes have orientation layers 6 and 7. A liquid crystal material 8 is present between the support plates. The distance between the plates is of the order of 10um which is maintained by spacers which are not shown in the drawing and are regularly spaced over the plate surfaces. A sealingedge 9 connects the support plates at their circumference. Thesupport plates 1 and 2 may further be each provided with a linear polariser, namely apolariser 10 and ananalyser 11, but this is not necessary. The display elements can be switched from a first condition to an optically different second condition by driving theelectrodes 3 and 4 in a suitable manner.
• Fig. 2 diagrammatically shows the matrix array of thedisplay elements 12 in such a device. The display elements are present at crossings ofrow electrodes 3 which are driven by row scanning circuits 13 °, 13^b andcolumn electrodes 4 which are driven by adrive circuit 14.
• According to the invention the row selection points are first supplied by the row scanning circuit 13^a and later by the row scanning circuit 13^b. The duration of these pulses is approximately equal. Preferably all rows are first activated successively by the row scanning circuit 13^a while simultaneously presenting information via thedrive circuit 14 and subsequently all rows are activated by the row scanning circuit 13^b. For the purpose of mutual synchronisation and timely take-over of circuit 13a by circuit 13^b the device has synchronisinglines 15 anddrive lines 16.
• With the device thus shown a higher multiplex ratio can be obtained than in the common bilateral drive when voltage losses along theelectrodes 3 are considered. This will be explained hereinafter.
• In the case of unilaterally driving therow electrodes 3, for example, from row scanning circuit 13a in Fig.2, the voltage decreases from, for example, α_oV_s at 13^ato V_s at row scanning circuit 13^b, (a_o >1). At the points (a), (b) it holds for separate drive with α_oV_s from both sides during

  

  with N rows that

  

  
• Optimisation of this function with the boundary condition V off= V_tn, in which V_thr is the threshold voltage of the electro-optical effect leads to too low V on-voltages between the points (a) and (b).
• In order to find the position (p) in which V_on is minimum, it is assumed by way of example that a(p) is linear so that
• a(p) = α_o+(1-α_o)p when addressing from the left
• a(p) = 1 +(α_o-1)p when addressing from the right.
• These assumptions, together with (1), (2), lead to the conclusion that a V_on is minimum when

  

  in other words the expression

  

  with

  

  must be optimalised with the boundary condition

  
• For given values of α_o and N combinations of V_s/V_thr and V_p/V_thr can be determined which comply with (4). For each of these combinations

  

  can be calculated with equation (3). Fig. 3 (broken line) shows the variation of

  

  as a function of V_D/V_tr for a set of electrodes of a liquid crystal display device for which it holds that N = 128 and α_o = 1.10. It appears from the Figure that the maximum value of

  

  is approximately 1.19 for V_D/V_thr = 0.74. The associated value of V_s/V_thr is approximately 7.95. For a satisfactory operation an electro-optical medium is thus to be chosen with such a threshold slope S at which it certainly holds that

  

  is at least 1.19; in accordance with the Alt & Pleshko-relations this slope corresponds to an N_max of approximately 130. Electro-optical media having a still steeper threshold voltage characteristic are of course also suitable.
• The same calculation can be made for simultaneously driving (bilateral driving) of left and right of the selection electrodes. In that case the boundary condition
  V̂²_off (a,b) = Vt_hr leads to the expressior

  
• For an electrode system with N = 128 and α_o = 1.10 again this leads to the solid-line curve of Fig.3. It is apparent therefrom that for the same electrode system in the case of simultaneous drive the ratio

  

  is lower than in the case of separate drive ("reversed scanning"). This ratio is approximately 1.14 in this example. The slope of an electro-optical medium for which it holds that

  

  is approximately 1.14 corresponds to an N_max of approximately 215 in accordance with the Alt & Pleshko relations.
• In the case of an equal electrode configuration the bilateral or simultaneous rive thus requires electro-optical media for which N_max is higher than the media which can be used in the method according to the invention (reversed scan). For further elucidation Table 1 shows some comparative figures for electrode systems with 128 and 256 rows and various values for α_o. The Table shows the associated maximum V_on/V_thr ratio and the values of N_max corresponding thereto in accordance with Alt % Pleshko.

  

  
• Some of these data show inter alia (see^*) that, for example, a medium for which it holds that N_max = 268 can still just be driven in the case of bilateral drive with 1 : 128 multiplexing in a system for which a_o = 1.15 while for reversed scanning 1 : 256 multiplexing is possible.
• The invention is of course not limited to the embodiment described, but several variations within the scope of the invention are possible to those skilled in the art. For example, the drive mode shown may be combined, for example, with the presentation of variable data and selection voltages as described in the patent application No. (PHN.11.867) simultaneously filed in the name of the Applicant.

Claims (3)

1. A display device comprising a liquid crystalline material between two parallel support plates having surfaces facing each other, a pattern of row electrodes being provided on the one surface and a pattern of column electrodes being provided on the other surface, the row electrodes crossing the column electrodes, thus constituting display elements at the area of the crossings, the device comprising a drive circuit for presenting data signals to the column electrodes, and a row scanning circuit for periodically scanning the row electrodes, characterized in that the row scanning circuit provides the two ends of each row electrode with signals in such a manner that first the one end is connected to the row scanning circuit and thereafter the other end is connected to the row scanning circuit.

2. A display device as claimed in Claim 1, characterized in that the row scanning circuit first provides all row electrodes at one end with signals and thereafter provides all row electrodes at the other end with signals.

3. A display device as claimed in Claim 1 or 2, characterized in that the signals presented to the both ends have a substantially identical duration.

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