EP0260747B1 - Display device - Google Patents

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 (see for example "1984 SID INTERNATIONAL SYMPOSIUM, DIGEST OF TECHNICAL PAPERS", June 1984, pages 316-319) 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 V̂_on represents the required rms voltage across a display element to switch it in the "on"-condition and V̂_off represents the rms voltage at which the display cell is in the "off-condition. As V̂_on and V̂_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 ${\text{V}}_{\text{D}}{\text{/V}}_{\text{thr}}$

  

  for different drive modes.
• The display device shown in fig. 1 has two glass support plates 1 and 2. The support plate 1 is provided with a pattern of strip-shaped row electrodes 3 consisting of, for example, indium tin oxide. The support plate 2 is also provided with a pattern of strip-shaped column electrodes 4 consisting of, for example, indium tin oxide. The electrodes 3 cross the electrodes 4 and the crossings constitute the display elements which are thus arranged in accordance with a matrix. The surfaces of the support 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 10µm which is maintained by spacers which are not shown in the drawing and are regularly spaced over the plate surfaces. A sealing edge 9 connects the support plates at their circumference. The support plates 1 and 2 may further be each provided with a linear polariser, namely a polariser 10 and an analyser 11, but this is not necessary. The display elements can be switched from a first condition to an optically different second condition by driving the electrodes 3 and 4 in a suitable manner.
• Fig. 2 diagrammatically shows the matrix array of the display elements 12 in such a device. The display elements are present at crossings of row electrodes 3 which are driven by row scanning circuits 13ª, 13^b and column electrodes 4 which are driven by a drive circuit 14.
• According to the invention the row selection points are first supplied by the row scanning circuit 13ª 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 13ª while simultaneously presenting information via the drive 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 synchronising lines 15 and drive lines 16.
• With the device thus shown a higher multiplex ratio can be obtained than in the common bilateral drive when voltage losses along the electrodes 3 are considered. This will be explained hereinafter.
• In the case of unilaterally driving the row electrodes 3, for example, from row scanning circuit 13ª in Fig.2, the voltage decreases from, for example, ${\text{α}}_{\text{o}}{\text{V}}_{\text{s}}$

  

  at 13ª to V_s at row scanning circuit 13^b, (α_o >1). At the points (a), (b) it holds for separate drive with ${\text{α}}_{\text{o}}{\text{V}}_{\text{s}}$

  

  from both sides during ½t_s with N rows that

  

  Optimisation of this function with the boundary condition V̂_off= V_thr in which V_thr is the threshold voltage of the electro-optical effect leads to too low V̂_on-voltages between the points (a) and (b).
• In order to find the position (p) in which V̂_on is minimum, it is assumed by way of example that α(p) is linear so that
  $\text{α(p) =}$

  

  ${\text{α}}_{\text{o}}{\text{+(1-α}}_{\text{o}}\text{)p}$

  

  when addressing from the left
  ${\text{α(p) = 1+(α}}_{\text{o}}{\text{-1)}}_{\text{p}}$

  

  when addressing from the right.
  These assumptions, together with (1), (2), lead to the conclusion that $\text{α}{\hat{\text{V}}}_{\text{on}}$

  

  is minimum when p = ½, in other words the expression

  

  with

  

  must be optimalised with the boundary condition

  
• For given values of α_o and N combinations of ${\text{V}}_{\text{s}}{\text{/V}}_{\text{thr}}$

  

  and ${\text{V}}_{\text{D}}{\text{/V}}_{\text{thr}}$

  

  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 ${\text{V}}_{\text{D}}{\text{/V}}_{\text{tr}}$

  

  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
  ${\text{V}}_{\text{D}}{\text{/V}}_{\text{thr}}$

  

  = 0.74. The associated value of ${\text{V}}_{\text{s}}{\text{/V}}_{\text{thr}}$

  

  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

  
• 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 ${\hat{\text{V}}}_{\text{on}}{\text{/V}}_{\text{thr}}$

  

  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 α_o = 1.15 while for reversed scanning 1 : 256 multiplexing is possible.
• 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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