WO2006008687A1

WO2006008687A1 - Display device

Info

Publication number: WO2006008687A1
Application number: PCT/IB2005/052258
Authority: WO
Inventors: Theodorus J. P. Van Den Biggelaar; Roy Van Dijk
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-07-13
Filing date: 2005-07-07
Publication date: 2006-01-26

Abstract

In a bistable high-voltage display device the display device has a driver for selecting a set of picture elements by supplying patterns of selection voltage via a capacitive coupling (15) between the row driving circuitry (13) and an electrode (11) selecting the set of picture elements.

Description

Display device

The invention relates to a display device having picture elements, each of the picture elements comprising on a substrate, a first electrode, a dielectric layer between said first electrode and a second electrode, the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element.

Wherever in this Patent Application reference is made to a picture element (pixel) it may either be a full picture element or a sub-pixel such as the red, green or blue sub-pixel in a picture element. Wherever in this Patent Application reference is made to a dielectric layer a layer is meant having such a high resistance that the mobility of the second electrode to move between the two positions, which positions, in the case of a display device, are related to electro-optical states of the display device (fully transmissive, fully reflecting or fully opaque (back)) is not influenced. The invention also relates to a display driver for driving such a display device.

The display device can be used, dependent on the pixel size in micro-projector applications, large screen applications such as wallpaper but also in window applications.

A display device of the kind mentioned above is known from e.g.

USP 5,519,565. The second electrode here is reliable in response to an electric field between a first position in which the rolled electrode is present at the edge region of the picture element and a second position in which the second electrode is unrolled and covers the further surface region of the picture element. One of the problems encountered in driving such a display is that due to the bistability (the difference between the voltages for switching on and off respectively) in such display pixels that are switched on will stay on and all pixel that are switched off stay switched off unless addressed a second time. With currently available display drivers providing voltages in the order of 10 V to 200 V, this can only be obtained with high voltage drivers leading to high driver costs.

The invention has as its purpose to overcome at least partly the above- mentioned problems. To this end in a display device according to the invention the display device has a driver for selecting a set of picture elements by supplying patterns of selection voltages which display device comprises a capacitive coupling between the row driving circuitry and an electrode selecting the set of picture elements.

By introducing a capacitive coupling voltage, voltages supplied in a lower voltage range are boosted to a range providing voltages in the order of 10 to 50 V. The capacitive coupling can de realized in different ways. In a first embodiment of the invention the display device has a driver for selecting rows of picture elements by supplying patterns of selection voltages and has capacitors between row driving outputs of the driver and row electrodes of the display device. Here the capacitive coupling is realized externally.

A further device according to the invention has a driver for selecting rows of picture elements by supplying patterns of selection voltages, the driver having capacitors between internal circuitry and a row driving output. This device can easily be realized by modifying existing, commercially available, low- voltage drivers such as LCD - drivers. A driver according to the invention can also be used in other high-voltage displays such as foil- displays

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:

Figure 1 schematically shows a part of a device according to the invention, Figure 2 shows a plan view of the part of the device of Figure 1, Figure 3 shows transmission voltage characteristics of the device of Figure 1, Figure 4 schematically shows a matrix of display elements in a device according to the invention,

Figure 5 shows driving forms for the device of Figure 4, while

Figure 6 shows a further embodiment of a device according to the invention. The Figures are diagrammatic and not to scale; corresponding components are generally denoted by the same reference numerals.

Figures 1 and 2 schematically show a part of a device 1 according to the invention, in this particular embodiment a transparent display device. A transparent substrate 2 is covered with transparent first electrodes 3 e.g. ITO -electrodes. The electrodes 3 are covered with a thin dielectric layer 4. A foil 6, which is covered with a conductive electrode part 5, forms together with said conductive electrode part 5 a second, Tollable electrode. The thin dielectric layer 4 electrically isolates the electrodes 3 (e.g. parts of column electrodes) from the rollable electrode parts 5 (e.g. parts of column electrodes). Figure 1 shows three (sub) picture elements, two of which are in an open, transparent state (rollable electrode 5, 6 rolled up to a first position), the other one being in a closed, opaque (black) state (rollable electrode 5, 6 unrolled to a second position). In this example the foil 6 is glued to the dielectric layer 4 on one side part 7 of every picture element. The rollable electrode 5, 6 are switchable between a transmissive (open) state and an opaque (closed) state, e.g. by choosing aluminum for the electrode parts 5. The device 1 further comprises e.g. driving means and for example a backlighting system. On the other hand, in a reflective device the foil 6 or the rollable electrode 5, 6 may be covered with a white layer to reflect the ambient light, while the substrate now is opaque by covering it with an opaque layer at one of its sides. A further possibility is making the substrate 2 reflective and the rollable electrode 5, 6 black.

It is assumed that in the device 1 three (or four) forces determine the switching behavior, an elastic force, an electrostatic force, and the "van der Waals" force and to a minor extend the gravitational force. The elastic force is the force present in the rollable electrode 5, 6 and is the result of e.g. shrinkage during manufacturing and this force is directed at rolling up the rollable electrode 5, 6 of a (sub) picture element or (sub) pixel. The electrostatic force is the attractive force between the conductive electrode part 5 and the (ITO) on the substrate by applying a voltage. The "van der Waals" force is the force between the (sub) pixel foil 6 and the substrate 2. This force depends on the distance between the two media, the roughness of the media and the material properties. The smaller the distance is, the larger the "van der Waals" force is. The electrostatic force depends strongly on the distance, the surface area, dielectric constant of the materials and the voltage difference between the foil and the substrate. The gravitational force acts upon the rolled up electrode 5, 6 which also depends on the orientation of this foil. It is very thin and has therefore a very low mass, so it is probably negligible.

The elastic force is directed at rolling up the rollable electrode 5, 6 , while the electrostatic force and the "van der Waals" force are directed at keeping the rollable electrode 5, 6 closed. To keep a picture element open (the left two picture elements in Figures 1, 2, the elastic force must be larger than the "van der Waals" force and residual electrostatic force (due to charging), since the picture element (pixel) in the device 1 is open if no or little electrostatic force is present. When a picture element or pixel is closed, the "van der Waals" force and the electrostatic force keep it closed, whereas the elastic force wants to open it. If no voltage is applied the rollable electrode 5, 6 is in a rolled up state, giving a transparent picture element in transmissive mode or a dark pixel in reflective mode. When applying a certain voltage V₂, in matrix-display devices the difference between the column voltages and the row voltages, the electrostatic forces rolls down the rollable electrode 5, 6 on to the substrate 2, covering the pixel area and creating a dark pixel in transmissive mode or a white pixel in reflective mode.

This switching behavior is shown by means of the transmission voltage characteristic of Figure 3, which shows the transmission T of the device of Figure 1 as a function of the voltage V. At a first threshold voltage Vi (which may be presented as a voltage difference between a row 11 and column 12 in a matrix display 1, see Figure 4) a picture element is opened (the rollable electrode 5, 6 rolls up), if it was not open already. At the second threshold voltage, V₂ a pixel is closed (the rollable electrode 5, 6 becomes flattened), if it was not already closed. The polarity of the voltages is not important, only the absolute value is important. In between these values a pixel that was open, will remain open and a pixel that was closed will remain closed. The threshold voltages are determined by the material parameters, i.e. the elastic forces, thickness of the foil, material properties, and surface properties, etcetera.

The (matrix) display 1 of Figure 4, having n rows and m columns is driven by a (schematically shown) row driver 13 and column driver 14, having picture element at the crossings of rows and columns. The row driver 13 selects rows 11, having row numbers j (j = 1...m), while the column driver 13 selects columns 12, having column numbers i (i = 1...n). Selection is not necessarily sequential. According to the invention row driving occurs via capacitive coupling, in this example via external capacitors 15 between row driver outputs 11 'and rows 11. The capacitive coupling alternatively may be realized inside the row driver 13. The rows are connected via resistors 17 to a hold voltage line 16 Figure 5 schematically shows driving forms for the device of Figure 4 during a row times selecting rows j. During the "Hold" period when there is no input signal (Input = OV, in this example), V_row, the voltages on the electrodes 11 are held on the hold voltage, in this example 175 Volt. Since the column voltages in this example vary between - 24 Volt and + 24 Volt the voltage difference between row and column stays between 151 Volt and 199 Volt, so between the threshold voltage Vl (150V in this example) and V2 (200V in this example). This means that the pixel doesn't change state, independent of the column signal.

During the Addr(ess) period the row signal is "active". The result is that the voltage difference between row and column also changes. The voltage difference exceeds the Vl or V2 threshold dependent of the column signal). When a pixel must be closed the column voltage is brought to such a voltage that the voltage difference between the row and column exceeds the threshold level |V2|, thus the pixel closes. This is independent whether that pixel was closed or not before. When the pixel must be open (whether is was already open or closed), the column is brought to such a voltage that the voltage difference becomes smaller than |V1| and a pixel will open (whether is was already open or not before). This means that there is no requirement for a separate reset phase. The row signals (Input) are applied to the individual rows via a capacitor 15. For a 3 millisecond pixel-speed the value of the resistors 17 is about 1MΩ and for the capacitor 15 the value is about 1OnF. The resistors only dissipate during the time a row is selected. Due to the capacitive coupling rows 11 are brought at a higher or a lower row voltage V_row (indicated in Figure 5 as Level 1 (180V in this example) and level 2 (170V in this example). Dependent on the column voltages the pixel voltages (V_pi_xei in Figure 5) switch e.g. to a(n absolute) voltage above |V1| for pixels that must be opened or to a(n absolute) below |V2| for pixels that must remain closed. The values of the resistors 17 and the capacitors 15 have been chosen such that these values are maintained during such a time that switching actually occurs.

In an alternative way of driving an alternating hold voltage is applied to the pull-up resistors so charging of the dielectric can be prevented. Depending on the closing an opening of switches 18, 19 the voltages on the electrodes 11 are held on a positive or negative hold voltage (± Vhoid), as shown in Figure 6.

The invention is not restricted to the embodiments shown. The rollable electrodes 5, 6 have two sides. In a reflective display device the top side is reflective (white, red, green or blue). The bottom side is black. When a pixel is open (rollable electrodes 5, 6 unrolled) the topside is shown over the full pixel (except for the black matrix, which is obtained by using said black substrate). When a pixel is closed, the pixel rolls up and shows the bottom side of the roll and simultaneously, the black substrate is shown. As mentioned in the introduction a driver according to the invention can also be used in other high- voltage displays such as foil-displays. The invention resides in each and every novel characteristic feature and each and every combination of features. Reference numerals in the claims do not limit the protective scope of these claims. The use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. The use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A display device (1) having picture elements, each of the picture elements comprising on a substrate (2) a) a first electrode (3) b) a dielectric layer (4) between said first electrode and c) a second electrode (5), the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element which display device has a driver for selecting a set of picture elements by supplying patterns of selection voltages which display device comprises a capacitive coupling between the row driving circuitry and an electrode selecting the set of picture elements .

2. A display device according to Claim 1, in which the display device has a driver for selecting at leas one row (11) of picture elements by supplying patterns of selection voltages, the driver having capacitors between internal circuitry and a row driving output.

3. A display device according to Claim 1, in which the display device has a driver (13) for selecting rows of picture elements by supplying patterns of selection voltages and has capacitors (15) between row driving outputs (11 ') of the driver and row electrodes (11) of the display device.

4. A display device according to Claim 1 in which resistors (17) are provided between the row electrodes and a hold voltage.

5. A display device according to Claim 4 in which the hold voltage is switchable between different values.

6. A display driver for driving a display device having hysteresis in its picture elements driving characteristics comprising two switches in series between a first voltage and a second voltage at the common point of which a part of the voltages of the patterns of selection voltages is supplied by the driver, the driver having capacitors between internal circuitry and a row driving output.

7. A display driver according to Claim 6, in which resistors are provided between the row electrodes and a hold voltage.

8. A display device according to Claim 7 in which the hold voltage is switchable between different values.