NL8602328A - DISPLAY DEVICE. - Google Patents

Description

% * * Λ ΡΗΝ 11,868 1 N.V. Philips * Incandescent light factories in Eindhoven.

Display device.

The invention relates to a display device containing a liquid crystal material between two parallel support plates with facing surfaces, one surface of which contains a pattern of line electrodes and the other surface of which contains a pattern of column electrodes, the line electrodes crossing the column electrodes and thus in situ display elements are formed from the crossings and the device comprises a control circuit for applying data signals to the column electrodes and a line scanning circuit for periodically scanning the line electrodes.

Such displays are known and are usually operated in multiplex control with electrical voltages in the so-called r.m.s. mode. The method of control is described by Alt and Pleshko in I.E.E.E. Trans. El. Dev.f Vol. ED 21, 1974, p.

146-155 and is the most common way of driving liquid crystal display devices constructed as a matrix of picture elements as described above, with no active switch being used per picture element. The maximum number of lines N_aw that can be driven with an acceptable contrast ratio with this method ηαλ 20 is determined by the relationship: N _ Von + Voff 2_ / s2 + 1 \ 2 s_ ^ on W '~ 1 ° «25 where VQn is the required rms represents voltage across a display element to switch it to the "on" state and the rms represents voltage with the display cell being in the "off" state. As VQn and Toff are closer together, a greater number of lines can be driven Obviously, this requires a steep threshold in the transmission / voltage characteristic of the display element.

The r.m.s. voltages across the individual 8602328 _ PHN 11.868 2 '* display elements do not necessarily follow from the selection and data voltages offered. Due to the resistance of the electrodes, voltage drops occur, so that the r.m.s.voltage across the elements may be less than is necessary to make them switch. This leads to a reduction in the maximum number of lines to be multiplexed.

A known solution to counteract the effects of voltage losses across the line electrodes is to select the selection voltage during a selection time t_

S

simultaneously at both ends of the line electrodes.

As a result, a certain reduction of the voltage losses is obtained.

A display device according to the invention is characterized in that the line scanning circuit signals the two ends of each of the line electrodes such that first one end 15 and then the other end are connected to the line scanning circuit. The invention is based on the insight that the voltage losses can be reduced by first applying a selection voltage to one end of a line electrode during rms-controlled matrices, preferably during practically half the selection time, and then, for approximately the same period, to apply the selection voltage to the other end. which, of course, simultaneously presents the information to be displayed as data voltages on the column electrodes.

The selection of both ends need not take place immediately one after the other. Preferably even the line scanning circuit first supplies all line electrodes at one end and then at the other end with selection signals, because this is simpler to realize switching technology.

The invention will now be further elucidated with reference to some exemplary embodiments and the drawing, in which Figure 1 schematically shows a liquid crystal display device in cross section;

Figure 2 shows a schematic representation of such a device, wherein the picture elements are arranged in a matrix, together with a part of the control circuits; , Figure 3 shows graphs of on 2 at a point midway through the 1 8602328 thr t PHN 11.868 3 line electrode as a function of Vn / V ^ r in different modes of driving.

The display device shown in figure 1 comprises two glass support plates 1 and 2. The support plate 1 is provided with a pattern 5 of strip-shaped line 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 intersections form the display elements, which are thus arranged according to a matrix. Orientation layers 6 and 7 are provided over the electrodes of the support plates 1 and 2. Between the support plates there is a liquid crystal material 8. The distance between the plates is of the order of 10 µm, which is shown in the drawing. spacers not shown, which are regularly distributed over the plate surfaces, are maintained. A sealing edge 9 connects the support plates at their periphery. The support plates 1 and 2 can further each be provided with a linear polarizer, namely a polarizer 10 and an analyzer 11, but this is not necessary. The display elements can be switched from a first state to an optically different second state therefrom by suitably driving the electrodes 3 and 4.

Figure 2 schematically illustrates the matrix arrangement of the display elements 12 in such an arrangement. The display elements are located at intersections of line electrodes 3 controlled by line sensing circuits 13a, 13 * 3 and column electrodes 4 controlled by a control circuit 14.

According to the invention, the line selection points are now first output from the line scan circuit 13a and later by the line scan circuit 13 * 3. The duration of these pulses is approximately the same.

Preferably, all lines are hereby first activated successively by the line scanning circuit 13a, while simultaneously supplying information via the control circuit 14, and then all lines are activated by the line scanning circuit 13a. For the purpose of mutual synchronization and timely transfer of circuit 13a by circuit 13b, the device has synchronization lines 15 and control lines 16.

With the device thus shown, a higher 8602328 t PHN 11.868 4 multiplex degree can be obtained than with the normal two-sided control, when voltage losses along the electrodes 3 are taken into account. This is explained in more detail below.

With one-sided driving of the line electrodes 3, 5, for example from line scanning circuit 13a in Figure 2, the voltage decreases from, for example, aQVs at 13a to Vs at line scanning circuit 13 * (aQ> 1). In points (a), (b) now applies with separate control with aQVs from both sides during Λ 2 tg, with N lines 10 7on <a'b> - J »<« oW2 +! II, (W2 + Tr VD < «» Off <a'b »=>« toVvD> 2 + liB (VVD) 2 + T: VD (2). · 15 Optimizing this function with the precondition Vofj = Vfchr, in which Vfchr the threshold voltage of the electro-optical between the points (a) and (b) leads to too low VQn voltages.

In order to find the position (p) where Von is minimal, it is assumed, for example, that a (p) is linear such that 20 α (ρ) = a0 + (1-aQ) p when addressing the left ot (p) = t + ( a0 ~ 1) p when addressing from the right.

Together with (1), (2) these assumptions lead to the conclusion that α VQn Λ is minimal if p = j, in other words the expression 25 Von (p = b = N (a'Vs + VD) 2+ (3) a +1 with a '= must be optimized, with as 9 9 boundary condition VQff (a, b) = V £ hr either, with (2) 30> ia0VsWD) 2 + jN (Vs «D) 2 + ^ i v2 -v2hr (4)

For given words of aQ and N, combinations of Vs / Vthr and VD / Vtlir can now be determined that satisfy (4). For each of these combinations 35, v (p s 4 ·) on'y 2 '- ^ vthr 860 2 32 8 ί t ΡΗΝ 11,868 5 can be calculated with equation (3). Figure 3 shows (broken line) the course of dtp ° b 5 1thr as a function of Vjj / Vtr for an electrode set of a liquid crystal display device for which N = 128 and 0o = 1 «10. The figure shows that the maximum value of V2 10 -9n - approx. 1.19 for

Vthr VD ^ 1thr = 0.74. The corresponding value of Vg / Vtjir is approx.

7.95. In this case it is therefore necessary to choose an electro-optical medium with a threshold steepness S for a good operation, wherein at least 15 applies that V2 S2 = -80-voff is at least 1.19; according to the Alt & 20 Pleshko relations, this steepness corresponds to an N = v of approximately 130. Of course, electro-optical & optical media with an even steeper threshold voltage characteristic are also suitable.

The same calculation can be made for simultaneously controlling (two-sided control) the left and right of the selection electrodes. In that case, the boundary condition V2ff (a, b) = V2hr leads to the expression 1. Ρθνκ ~ νρΙ2 + itly2 (5)

vfhr N N

30 tiir

For an electrode system with again N = 128 and c0 = 1.10 this leads to the drawn curve in Figure 3. This shows that for the same electrode system, the ratio is lower for the same electrode system than for the individual control ("reversed scanning"). In this example, this ratio is approximately 1.14. The steepness of an electro-optical medium for which o V2 sz = -SS- ca, 1J4 is 5 nhr corresponds according to the Alt & Pleshko relations with an Nmax of approx. 215.

Thus, with a constant electrode configuration, the two-sided or simultaneous drive requires electro-optical media 10 for which Nme is higher than the media that can be used in the max method according to the invention (reversed scan). To further illustrate this, Table 1 shows some comparative figures for electrode systems with 128 and 256 lines, respectively, and various values for tr0. The table shows the corresponding maximum 15 ^ Qn / Vthr ratio and the corresponding values of Nmax according to Alt & Pleshka correspondence.

Multiplex ratio 1: 128 Reserved scan Two-sided

I

20 v max N V2 / V2 max N

ui on thr max Q / on thr max o o (p = h) opt. eff. (P =%). ° pt. eff. .

1.00 1.1939 128 1.00 1.1939 128 1.03 1.1938 129 1.03 1.1772 151 25 1.05 1.1935 129 1.05 1.1674 168 1.10 1.1924 130 1.10 1.1465 215 1.15 1.1908 132 1.15 1.1300 269 * 1.20 1.1868 135 1.20 1.1168 329 .......

Multiplex ratio 1,256 30

Reserved scan Two-sided 1 8602328

2 / V2 max N tf2 / V2 max N

06 on thr max on thr max (p =%) opt. eff. . (p =%) opt. eff. ; 35 * 1.00 1.1333 256 1.00 1.1333 256 1.05 1.1330 258 1.05 1.1089 376; 1.10 1.1320 261 1.10 1.0909 530 i _1.15 ___ 1.1304 .. 267 *. 1.15 1.0776. 717 i *>

V

PHN 11.868 7 5 Among other things, it appears from this data (see *) that, for example, a medium to which Nfflax 268 applies can be operated with 1: 128 multiplexing with two-sided control in a system for which aQ = 1.15 while with reversed scanning 1 : 256 multiplexing is possible.

The invention is of course not limited to the example described here, but various variations are possible for the skilled person within the scope of the invention. For example, the shown mode of actuation can be combined with the provision of variable data and selection voltages, as described in the patent application no. (PHN 11.867) from Applicant.

8602328

Claims (3)

A display device containing a liquid crystalline material between two parallel support plates with facing surfaces of which one surface contains a pattern of line electrodes and the other surface contains a pattern of 5 column electrodes, the line electrodes crossing the column electrodes and thus at the location of the crossings display elements are formed and the device includes a control circuit for applying data signals to the column electrodes as well as a line scan circuit for periodically scanning the line electrodes 10, characterized in that the line scan circuit signals the two ends of each of the line electrodes such that first one end is connected to the line scan circuit and then the other end is connected to the line scan circuit. Display device according to claim 1, characterized in that the line scanning circuit first signals all line electrodes at one end and then all line electrodes at the other end. 3. Display device according to claim 1 or 2, characterized in that the signals presented from both ends have a practically identical duration. 8602328