NL8800704A - Picture display device e.g. for alphanumeric data - has electro-optical medium between support plates and non-linear switching devices connected between data and reference electrodes - Google Patents

Abstract

The display device has a system of row and column electrodes (11,8) supplying selection and data signals to provide a range of voltages dependent on the display medium across the picture elements (as 12). The picture electrode (7) on one of the support plates is connected electrically to the common point of two non-linear switching diodes (9,19a) arranged in series between a column electrode (8) for data signals and an auxiliary voltage supply electrode (18). This connection applies, prior to selection, an auxiliary voltage across the picture elements beyond, or on the limit of, the voltage range to be used for picture display. The diodes are p-n diodes, pin diodes or Schottky diodes.

Description

PHN 12,487 1 N.V. Philips' Incandescent light factories in Eindhoven.

Display device and method for controlling such a display device.

The invention relates to a display device comprising an electro-optical display medium between two support plates, a system of picture elements arranged in rows and columns, each picture element being formed by picture electrodes arranged on the facing surfaces of the support plates and a system of row and column electrodes for presenting selection and data signals by means of which an area of voltages dependent on the electro-optical display medium can be presented across the picture elements for the purpose of image display.

In addition, the invention relates to a method for controlling such a display device.

Such a display device is suitable for displaying alpha-numerical information and video information using passive electro-optical display media such as liquid crystal electrophoretic suspensions and electro-chrome materials.

A display device of the type mentioned in the preamble is known from Dutch Tervisielegging No. 8502863 to Applicant. In the device shown there, in an active matrix, diodes are used as non-linear switching elements, namely two diodes per picture element. Each row electrode is always common to two successive rows of picture elements. The manner of control is such that in television applications (for example in control according to the PAL or NTSC system) the information of two consecutive even and odd lines of varying polarity is offered for each picture element with the raster frequency. The information of a picture element is therefore determined by the average signal of two consecutive odd and even lines. Since two rows of image electrodes are always inscribed simultaneously, because two successive rows always have a row electrode 30 in common, such a device has little flexibility with regard to the choice of color flashes to be used. In fact, this choice is limited to strip-shaped color filters.

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In unpublished patent application no. 8701420 (PHN 12.154) of the Applicant describes an image display device of the type mentioned in the preamble, in which the row electrodes are not common and the rows of picture elements are driven separately without the expiry of common row electrodes leading to an increase in the number of connections.

This provides, among other things, a wide freedom of choice with regard to the color filters to be used. This is possible by giving the picture elements per row a specific setting, namely by charging or discharging the capacities associated with these picture elements after having first discharged or charged them (accurately or not).

In the aforementioned application this is done by applying an auxiliary voltage over the picture elements before selection outside or on the edge 15 of the voltage range to be used for image display, for instance with an auxiliary voltage (reference voltage) or reset voltage.

In a preferred embodiment of the device described there, a Zener diode is located between a picture element and a row or column electrode. When such a device is used, the breakdown behavior of the Zener diodes used partly determines the operating voltages to be used; in addition, the minimum Zener breakdown voltage limits the number of electro-optical media to be used. In addition, Zener diodes in the technology used for liquid crystal device circuits (amorphous silicon, polycrystalline silicon) are difficult to manufacture in a reproducible manner. The invention seeks to provide this by providing an alternative solution.

To this end, a device according to the invention is characterized in that the device between a picture electrode and a row or column electrode contains at least a first branch with at least one non-linear switching element in order to apply an auxiliary voltage outside or on the edge of the picture elements before selection over the picture elements. to provide an area to be used for image display and the device contains a second branch per picture element between the picture electrode and the row or column electrode with at least one non-linear switching element which is anti-parallel connected to the non-linear switching element in

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PHN 12.487 3 the first branch.

Preferably, the first branch comprises a series connection of non-linear switching elements, such as diodes.

The series connection or the anti-parallel switching element can be arranged such that a switching behavior analogous to that of a Zener diode is obtained, but just better reproducibility.

Preferably, the auxiliary voltage lies outside or on the edge of the transition region in the transmission voltage characteristic of the electro-optical medium.

The auxiliary voltage can now be a reset voltage, in which all picture elements in a row are first charged to a certain value negative or positive and then, depending on the data signals presented, are charged or discharged, respectively, to the correct signal value.

Since this now takes place per separate row, without affecting a subsequent row or previous row, the image information can be adapted to a color filter to be used, which can for instance be built up from triplets, as described, for example, in Dutch Patent Application No. 861063 of Applicant, or for example has a diagonal structure.

Discharging or charging prior to the actual control with the image information can take place during the same line time in which the image information is presented, but also during the line time preceding it.

Because each row of image elements is now written separately, the voltage across these image elements can also be inverted per row, which leads to a higher surface flicker frequency and thus to a calmer image. Preferably diodes are chosen for the non-linear switching elements 30; it is advantageously possible to apply redundancy as described in the non-prepublished Patent Application No. 8800204 (PHN 12.409) of Applicant.

The first branch, which may contain a series connection of non-linear switching elements (or a single non-linear switching element, such as, for example, an MIM), can be arranged separately for each picture element.

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A preferred embodiment of a device according to the invention which hardly costs any additional components (in particular with larger numbers of rows) than that according to the aforementioned Patent Application No. 8701420, is characterized in that several (and preferably all) picture elements of a column form the first branch. at least partially in common. The common part can also be used for two (adjacent) columns.

A method according to the invention is characterized in that at least a first branch with at least one non-linear switching element and a second branch with at least one non-linear switching element connected anti-parallel thereto is connected between the picture elements and the row electrodes or the column electrodes 10. and the pixels are charged or discharged to a voltage outside or on the edge of the area to be used for image display by means of the first branch, before applying a data signal and then to the correct via the anti-parallel switching element. value are discharged or charged.

The invention will now be further elucidated with reference to some exemplary embodiments and the drawing, in which: Figure 1 shows a schematic representation of the device;

Figure 2 shows the associated voltage transmission characteristic;

Figure 3 shows a set of 25 control signals associated with the device;

Figure 4 shows a variant of the previous device;

Figure 5 shows another embodiment of a device according to the invention, and;

Figure 6 shows a number of variants for the device 30 according to Figure 1.

Figure 1 shows in schematic form the display device 1 according to the invention. Picture elements 12, which in a liquid crystal display device are for instance formed by opposing picture electrodes 6, 7 between which there is a layer of liquid crystal material, are here connected via the picture electrodes 7 to row electrodes 11, which are arranged in matrix form together with the column electrodes 8. The picture electrodes 6 of the 8 8 0 0 4 4 PHNf 12.487 picture elements 12 are connected via diodes 9 to column electrodes 8. At the same time they are connected via a series circuit 2 of diodes 19 to a column electrode 18.

Figure 2 schematically shows a transmission-voltage characteristic of a display cell such as that which occurs in the display of Figure 1, 2. Below a certain threshold voltage (Vj or vth>) the cell practically does not transmit light, while above a certain saturation voltage (V2 or vs) the cell is practically completely transmissive. The intermediate region forms the above-mentioned transition region and is indicated with a brace in Figure 3. 17. It should be noted that since such cells are usually operated with alternating voltage, the absolute value of the voltage is plotted along the abscissa.

For reading in information, during a selection time tg on a selection line 11, a first selection voltage Vg ^ is applied while simultaneously applying the information or data voltages to the column electrodes 8; this leads, for example, to a positive voltage across a picture element 12, which represents the information presented.

In order to prevent degradation of the liquid crystal and to be able to increase the so-called flat flicker frequency, information of varying sign is preferably presented over the picture element 12.

In a device according to the invention, a negative voltage across the picture element 12, which represents the information presented, is achieved by discharging (or too negatively charged) too far after the capacitance associated with the picture element 12 via the series circuit 2 of diodes 19. have to offer a second selection voltage Vgg while simultaneously offering inverted data voltages (-V ^).

Figure 3 shows how for a number of rows of picture elements 12 the control signals are chosen to write them with picture information which (for example in TV applications) changes sign during each frame.

From time tQ (see Figure 3a), during a selection time tg (which in this example is chosen equal to a line time for TV applications, namely 64 usec), a selection voltage Vg1 is applied to a row electrode 11 while simultaneously presenting information voltages or data voltages Vd on the column electrodes 8. After the time t ^, the associated row of picture elements 12 is no longer selected, because the row electrode 11 is supplied with a voltage Vns1. This voltage is maintained until just before the next selection of the row of picture elements 12. In this example this is done by just before reselecting the first row of picture elements 12, namely at a time t ^ tj-tg, in which t ^ a frame period the selection line 11a proposes to reset the voltage. The reset voltage can be selected such that the picture elements 12 of the series circuit 2 of diode 19 10 are charged so negatively that the voltage across each of the associated picture elements is outside the range to be used for picture display (up to a value of 1-VS t). In a subsequent selection period (from t ^) they are then charged up to the desired value, determined by data voltages -V ^, via diodes 9. The row electrodes are offered the voltage VS2 for this purpose and after the selection period has expired (after tg ) a non-selection voltage VnS2 · In this way, the voltage across the pixels is inverted every frame period.

Figure 3b shows the same voltage variation as Figure 20 3a, but shifted over a grid period plus a selection time (in this case a line time). This makes it possible to read two successive rows of picture elements relative to each other with inverse data voltages. Figure 3c is identical to Figure 3a but shifted over two selection times.

In half vertical resolution (television) images where the lines of the even and the odd grid are written on top of each other, this ensures that the image information is reversed and refreshed once per grid period. Although the line flicker frequency here is 25 Hz (30 Hz), the phase difference of 180 ° between successive rows introduced a reverse polarity phase of 50 Hz (60 Hz).

The selection voltages and VS2 can of course also be chosen shorter than a line time (64 psec). In that case, the application of the reset voltage can also take place during a part 35 of the line time during which selection takes place, provided that there is sufficient time left to charge the picture elements 12. The voltage variation on the electrodes 11 then takes place, for example, as indicated diagrammatically in Figure f p f Λ * * PHN 12.487 7 5a by means of the broken line 14.

The device shown is very suitable for applying a control method in which the average voltage across a picture element vc = Vs, ^ | + Vth 5 is chosen (see Figure 3), so that the absolute value of the voltage for image display over the picture pixels 12 remain practically limited to the region between Vth and Vsat.

A good effect with regard to gray scales is obtained if, depending on the data voltages Vd on the 10 column electrodes 8, the voltage values across the picture elements 12 are maximum vc + vdmax = vsat and minimum Vvdmax = vth.

Elimination of Vc yields: that is, .5 -) / 2ί% ΐ -%) ^ χ ^ 1/2 (ν ^ ί -%)

For example, to positively charge a row of picture elements 12, the associated row electrode 11 is provided with a selection voltage Vs1 = -VQn-1/2 (Vsat + Vth ^ where v0n is the forward voltage of the diode 9. The voltage across the picture element 12 is therefore V (j-von-vs-f; it moves between -1/2 <ν3 »ί -%) + 1/2 <% ί +%) =% and 1/2) vsat"%) + 1 / 2) vsat + vth) = vsat 'depending on V ^.

In order to negatively charge the same row of image elements 12 (in a subsequent raster or image period) in a subsequent selection with data voltages that are invented, these are first connected, via diodes 19, to a reference voltage, charged too far negatively with the aid of a reset voltage Vreset on the row electrode 11. Thereafter, the selected row electrode (in the same line time or in a subsequent one) receives a selection voltage Vs2 = -Von + 1/2 (Vsat + VtJl). The negatively charged picture elements 12 are now charged via the diodes 9 to ^ d “Von“ Vs2 them? 9 and to values between: -) / 2)) / sat “vth) -) / 2 / vsat ~ vth) = - ^ sat 35 and 1/2 (% ΐ -%) - 1/2 (ν5ϊχ -%) = -%.

so that information about the image elements 12 becomes the opposite sign t *: * ~ r.

f ’* * * V * * PHN 12.487 8 offered.

In the case of non-selection, the requirement must be met that neither diodes 9 nor diodes 19 can conduct, or conduct such a low current Iq ££ associated with a voltage that discharge via the 5 diodes 19 is practically negligible.

At a lowest non-selection voltage vns1, the voltage VA at the node 13 moves between values: vAmin = vns 1 + vth <1> and 10 vAmax = vns1 + vsat <2>

For the values VAm | n and VAmax, on the other hand: vAmax - vdmax + r voff ^ where r is the number of diodes 19 in the series circuit 2.

Discharge via diode 9 must also be prevented, so that the following applies: 15 VAmin ^ Vdmax-Voff <4>

Equations (1) and (4) lead to: + vdmax-voff-vns1 + vth or (with Vdmax = 1/2 (Vsat-Vth)).

Vns1 ^ 1/2 (Vsat-Vth) -Voff-Vth (5) 20 Likewise, equations (2) and (4) lead to: “vdmax + r voff-vns1 + vsat or vnsli-1/2 (Vsat-Vth ) -vsat + r Vo £ f (6)

Combination of (5) and (6) results in: 25 -1/2 (Vsat-Vth) -Vsat + r Vo £ i2V „s1> '^ Vsat-Vthl-VoM-Vy, or <7>

If we choose the number of diodes r such that 30 ^ tt-1Vart-Vth) (8) then holds: W - '/ Wsat +' th ^ oM '9'

The picture elements 12 are then (see Figure 3a) discharged to a value Vsat by giving the row electrode 11a a sufficiently high reset voltage. For this applies: vreset-vdmax + vsat + r Von or, P C G 7 ί "PHN 12.487 9 vreset ^ ^ ^ Vsat-Vth ^ + vsat + r von

The picture elements are then charged accurately via the diodes 9. For this purpose, a selection voltage VS2 = 1/2 (Vsat + Vth) -Von is applied for this purpose, while simultaneously applying data voltages to the column electrodes 8 on the row electrode 11a.

Then, the row electrode 11a is supplied with a non-selection voltage vns2. For the voltages at node 14 the following now applies: VAmax_ ^ ns2 ~ vth and VAmin = vns2-vsat 10 With equations (3) and (4) and Vdjnax = 1/2 (Vsat-Vth) this leads to: vns2- ” vdmax + r voff ~ -1/2 (vSafvth> + vth + r vo «<10» and 15 \ s2zW-Vf + Vt = + 1'2 (vSat-vth> - <11>

Voff + Vsat

Combination of (10) and (11) leads to: 1/2 (Vsat-VthJ'Voff + Vsat ^ («) -1/2 (Vsat-Vth + r VQff 20 so that again applies: ¥ W <vsat-vth> '7')

If we choose r again such that: ^ Vf ^ sat-Vth) <8 ') then holds: 25 <13>

The applicability of this device is therefore determined by the minimum voltage VQff and the maximum width of the transition region, in the transmission voltage characteristic (indicated in figure 2 by the brace 17) of the relevant liquid crystalline liquid or other electro-optical medium. .

A similar limitation also applies to a display device in which the switching elements are manufactured from so-called diodes. With such a switching element, built up from a series connection of n diodes of two diodes, the condition is 35 n v0ffMvSat “vth ^ · 2n diodes are required per picture element.

For example, if Vsat-Vth = 1.8 Volts and V f-f = 0.4 Volts PHN 12,487 10, the minimum number of diodes when using diode rings is n = 10 while in the device of Figure 1 this is r + 1 = 9.

In the example of Fig. 1, the picture elements 12 are always charged or discharged via one diode 9 during writing. The spread in the forward voltage VQn of these diodes is approximately 20 mV or 1/90 of the range (Vsat-Vth) over an entire device. This achieves greater uniformity than in devices with multiple diodes, where the spread increases cumulatively with the number of diodes.

The number of diodes can be considerably reduced by making a part of the diode branch 2 for a number or more picture elements in common in one column. This is shown in figure 4. The common diode branch 3 now contains (r-1) diodes 19, while each picture electrode 6 is connected via a diode 19 'to the common electrode 4 connected to the common part 3' of the series circuit of (r -1) diodes 19. The series circuit 3 is now used during each line time for charging or discharging successive rows of picture elements. Compared to the device of figure 1, now (n-1). (R-1) fewer diodes suffice (n = number of 20 rows).

The diode branch 3 can moreover be used in common for two columns, as shown in figure 5. This figure is obtained by mirroring the right column about an axis parallel to the column direction, the electrodes 4 and 4 'being designed as a common electrode. Advantageously, a diode branch 3 'indicated by dashed lines can still be arranged in the right-hand column, so that it is in fact redundant.

Redundancy in the various elements can of course also be obtained by making diodes double (in series and / or parallel) as further described in Applicant's non-prepublished Application No. 8800204 (PHN 12.409).

The diodes in the device of Figure 1 may also be switched the other way around, or located between the column electrode 8 and the picture element 12, as shown schematically in Figure 6. 35 For such configurations, similar expressions for the (non) selection voltages, data voltages and reset voltage are derived as for the device of Figure 1, for example for .sect; ..

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> PHN 12.487 11, the device of Figure 6b has the same voltage values of the control signals, while the inverted voltage values apply to the circuits of Figures 6a and 6c.

The invention is of course not limited to the examples shown here, but various variations are possible within the scope of the invention.

For example, non-linear switching elements other than diodes are also possible, such as bipolar transistors with short-circuited base-collector junction or MOS transistors whose gate is short-circuited with the drain zone. In particular, the series circuits 2, 2 'and 3, 3' of diodes can be replaced by a MIM element (metal insulator-metal), with a current voltage characteristic such that the picture elements 12 can be discharged by means of a reset voltage ( charged with 15 poled diodes the other way round). The diode 9 and the MIM element again exhibit a behavior analogous to one Zener diode. There are also various options for the diodes themselves. In addition to the diodes customary in display technology, this may include, for example, a pn diode, Schottky diode or pin diode made of monocrystalline, polycrystalline or amorphous silicon, CdSe or other semiconductor material, while the diodes can be used both vertically and laterally are carried out.

In addition, the availability of a reset voltage makes the described device and method particularly suitable for use in a ferroelectric display medium as described in the non-prepublished Dutch patent application no. 8700627 of Applicant.

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Claims (17)

1. Display device comprising an electro-optical display medium between two support plates, a system of picture elements arranged in rows and columns, each picture element being formed by picture electrodes arranged on the facing surfaces of the support plate and a system of row and column electrodes for the offering selection and data signals by means of which a range of voltages dependent on the electro-optical display medium can be presented across the picture elements for the purpose of image display, characterized in that the device 10 between at least one picture electrode and a row or column electrode first branch with at least one non-linear switching element to apply an auxiliary voltage over the picture elements before selection outside or on the edge of the area to be used for image display and the device for each picture element a second branch between the picture electrode and the row - or column electrode with te at least one non-linear switching element which is anti-parallel to the non-linear switching element in the first branch. 2. Display device according to claim 1, characterized in that the first branch comprises a series connection of non-linear switching elements. Display device according to claim 1 or 2, characterized in that the first branch is a series connection of diodes. Display device according to claim 1 or 2, characterized in that the first branch contains a metal insulator-metal element. Display device according to claims 1 to 4, characterized in that the device for each display element comprises a branch with at least one non-linear switching element and a branch with an anti-parallel switching element. Display device according to one of Claims 1 to 30 and 4, characterized in that for several pixels in a column, a part of the branch is common with at least one non-linear switching element. Display device according to claim 6, characterized in that for two columns of picture elements a part of the branch is common with at least one of the non-linear switching element. Display device according to claim 7, characterized in that, <, V ** i. V, »/ \ * Li» PHN 12,487 13 that each of the two columns has a common part of the branch with at least one non-linear switching element. 9. Display device as claimed in claims 1 to 8, characterized in that the auxiliary voltage is located outside or on the edge of the transition region in the transmission / voltage characteristic of the electro-optical medium. Display device according to any one of the preceding claims, characterized in that the electro-optical medium contains a liquid crystalline material. Display device according to claim 10, characterized in that the electro-optical medium contains ferroelectric liquid crystalline material. 12. Method for controlling a display device comprising an electro-optical display medium between two support plates, a system of picture elements arranged in rows and columns, each picture element being formed by picture electrodes arranged on the facing surfaces of the support plates and a system of row and column electrodes, during which at least part of a line time a row of picture elements 20 is selected via the row electrodes, while data signals are presented via the column electrode, characterized in that at least a first branch with at least one branch is provided between the picture elements and the row electrodes or the column electrodes. non-linear switching element and a second branch per picture element with at least one non-linear switching element connected in parallel thereto, and the picture elements are charged or discharged to a voltage outside or on the first branch, before the data signal is applied.the edge of the area to be used for image display and then discharged or charged via the anti-parallel switching element to the correct value. Method according to claim 12, characterized in that the charging or discharging and the presentation of the data signal takes place within the same line selection time for a row of picture elements. 14. A method according to claim 12, characterized in that the charging or discharging takes place during a line selection time prior to the line time in which a row of picture elements is selected. K * PHN 12,487 14 A method according to any one of claims 12 to 14, characterized in that the voltage across a row of picture elements is applied with invented polarity relative to the just inscribed row of picture elements. A method according to any one of claims 12 to 16, characterized in that the display device contains a liquid crystalline material. Method according to claim 16, characterized in that the display device contains a ferroelectric liquid crystalline material 10. .88? E; ; ^