EP0972282B1 - Device for controlling a matrix display cell - Google Patents

Description

The present invention relates to a control device matrix, more particularly a matrix control device used in a flat screen such as a matrix type liquid crystal display active or other types of flat screen.

In the prior art, a matrix control device is used, for example, to control the cells of a flat screen such than liquid crystal cells. In this case, it is a screen to liquid crystals of the active matrix type also known by the abbreviation AM-LCD. Such an active matrix type liquid crystal screen is shown in Figure 1. In this case, the screen consists of a certain number of electro-optical cells each formed by an electrode and a counter-electrode enclosing the liquid crystal. These cells are referenced XL in said figure. The electro-optical cells are arranged in rows and columns, and each controlled by a switching circuit forming part of a control device matrix type. As shown in Figure 1, the circuit switching is carried out by a transistor T of which one of the electrodes is connected to a column Cj and the other electrode of which is connected to the XL electro-optical cell. On the other hand, the gate of transistor T is connected to one of the lines Li of the control device. most often the XL electro-optical cell is associated with a capacity of CP storage mounted in parallel on the capacity formed by the cell electro-optics at the output of transistor T. The assembly formed by the transistor T and the capacity CP form an elementary control circuit referenced Pij in Figure 1. On the other hand, each of the lines of selection Li is connected to a control circuit 2 or "line driver" which successively applies a pulse of control with a voltage typically varying between - 10 and + 20 volts. Likewise, each of the columns Cj or rows of data is connected to a column control circuit 3 or "column driver" which sends on the columns Cj an analog signal corresponding to the video signal representing more particularly a gray scale whose voltage typically varies between + and - 5 volts.

With this matrix control device, the control of a XL electro-optical cell is carried out as follows. when pulse is applied to a selection line Li, the transistor switching T becomes conducting. Therefore, the analog voltage applied to column Cj is transmitted across the electrodes of the XL electro-optical cell which displays a gray level corresponding to the data signal.

In general, a matrix control device of this type has switching transistors which are most often constituted by TFT thin film transistors for "Thin Film Transistor ". Such a device is generally made of amorphous silicon. On the other hand, the lines and columns control circuits 2,3 can be integrated on the substrate plate on which the flat screen is made or be done independently. When integrated on the plate substrate, they are also made using amorphous silicon.

One of the problems encountered with this type of device matrix control is a consumption problem due in particular to the amplitude of the signals applied to the rows and columns. This problem is all the more important that one uses for addressing lines of the matrix screen, the technique known as "inversion line ", the polarity reversal occurring on each line. In this case, you can get consumption of up to one watt for one line frequency of 30 KHz.

Another problem encountered with this structure when it is made with polycrystalline silicon or silicon transistors monocrystalline, relates to the leakage current of the switching transistor T in the blocked state which tends to discharge the elementary points or cells electro-optics XL.

The object of the present invention is to remedy the drawbacks cited above by proposing a matrix control device having a new structure for the control circuit elementary of each elementary point, this structure being particularly well suited to the use of polycrystalline silicon or monocrystalline for the production of transistors or other semiconductor circuits.

The present invention therefore relates to a device for matrix control comprising a set of control circuits arranged in rows and columns and each commanding an elementary point, the state of each elementary point being a function of first and second control signals applied to the control circuit respectively by rows and columns, characterized in that each control circuit is an electrical circuit provided with two independent inputs supplied by the first and the second signal, and with an output which generates the control signal for the elementary point (XL). impedance of this circuit between its output and that of one of its inputs which carries the first signal becomes weak following application of an adequate voltage pulse on this first signal, and this same impedance becomes very high following the application of an adequate voltage on the second signal.

In this case, the first signal is a signal which, in a first step, activates all line control circuits corresponding by making them passers-by then applying a ramp of voltage which is transmitted from the control circuit to the point corresponding elementary. According to a preferred embodiment, the first signal consists of a ramp-shaped signal preceded by a negative precharge pulse. According to an improvement, preferably, the instant of triggering of the ramp-shaped signal is adjusted from line to line to compensate for propagation delays on the columns.

On the other hand, the second signal is a switching signal of numerical type determining the duration during which the circuits of activated commands remain passers-by. According to one embodiment preferential, the second switching signal consists of PWM type pulses for "Pulse Width Modulation" in language English. Preferably, the moment of triggering of the pulses is adjusted from column to column to compensate for delays on the lines.

According to a preferred embodiment of the present invention, the control circuit consists of a first transistor connecting the elementary point to the corresponding line receiving the first signal and a second transistor of which a first electrode is connected to the gate of the first transistor, the gate of which is connected to the corresponding column receiving the second signal and the second of which electrode is connected to a reference potential.

According to an additional feature of this invention, the elementary control circuit further comprises a capacitance connected between the gate of the first transistor and the line corresponding. Likewise, the second electrode of the second transistor is connected to the previous line. According to another characteristic of the present invention, the circuits are made using silicon Polycrystalline.

• la figure 1 déjà décrite est une représentation schématique d'un dispositif de commande matriciel utilisé dans le cas d'un écran à cristaux liquides à matrice active associé à des circuits de commande lignes et colonnes conformément à l'art antérieur ;
• la figure 2 est une représentation schématique d'un dispositif de commande matriciel conforme à la présente invention dans le cas où le point élémentaire est constitué par une cellule à cristaux liquides, ce dispositif étant associé à des circuits de commande lignes et colonnes,
• la figure 3 représente la forme des différents signaux appliqués respectivement sur les lignes, les colonnes, au point A, au point B et la tension grille source du transistor MN2 dans le cas du circuit de commande élémentaire de la figure 2, et,
• la figure 4 est une représentation schématique d'un dispositif de commande matriciel conforme à la présente invention dans le cas où le point élémentaire est constitué par un matériau électroluminescent, ce dispositif étant associé à des circuits de commande lignes et colonnes.

Other characteristics and advantages of the present invention will appear on reading the description given below of a preferred embodiment, this description being made with reference to the attached drawings in which:

• Figure 1 already described is a schematic representation of a matrix control device used in the case of an active matrix liquid crystal screen associated with row and column control circuits according to the prior art;
• FIG. 2 is a schematic representation of a matrix control device in accordance with the present invention in the case where the elementary point is constituted by a liquid crystal cell, this device being associated with row and column control circuits,
• FIG. 3 represents the shape of the different signals applied respectively to the rows, the columns, at point A, at point B and the source gate voltage of the transistor MN2 in the case of the elementary control circuit of FIG. 2, and,
• Figure 4 is a schematic representation of a matrix control device according to the present invention in the case where the elementary point is constituted by an electroluminescent material, this device being associated with row and column control circuits.

In the figures, to simplify the description, the same elements have the same references. On the other hand, this invention will be described with reference to a liquid crystal display. However, it is obvious to those skilled in the art that the invention can apply to elementary points constituted by any circuit of storage of an electrical signal such as an electro-optical cell or the like.

In Figure 2, there is shown a control device matrix according to the present invention associated with a circuit control lines 20 and a column control circuit 30, said circuits which may or may not be integrated on the same substrate as the matrix control device. In the control device matrix of Figure 2, the elementary control circuit referenced P'ij has been modified to limit power consumption and thus allowing production in polycrystalline silicon. In a more specific, the elementary control circuit P'ij arranged in lines and in columns commands an elementary point made up, in the mode of realization represented, by an electro-optical cell XL, more particularly a liquid crystal cell. On this electro-optical cell a CP storage capacity is mounted in parallel, said cell itself playing the role of a capacitance and its optical properties being modified according to the value of the electric field which crosses liquid crystal.

We will now describe an embodiment of a circuit of elementary command P'ij whose main characteristic is to have an output signal following the input signal when activated by a first signal, namely that applied to the lines L'i, and of which the impedance between the input and the output becomes very large under the effect of a second signal, namely the signal applied to the columns C'j. In the case of FIG. 2, the control circuit P'ij consists of essentially by a switching device MN2 consisting of preferably by a thin film transistor or TFT. An electrode of the transistor MN2 is connected to an electrode of the electro-optical cell XL while its other electrode is connected to a line L'i. On the other hand, the gate of transistor MN2 is connected to an electrode of a second transistor MN1, the other electrode of which is connected to ground in the embodiment shown and the grid of which is connected to a column C'j. As shown in Figure 2, a capacity referenced CB is connected between the gate of the switching transistor MN2 and the line L'i. The connection point between the capacitor CB and the gate of the transistor MN2 is referenced A while the connection point between the electrode of the transistor MN2 and the electrode of the electro-optical cell XL is referenced B. The lines L'i are connected to a line control circuit 20 which provides on the lines a data signal constituted by a signal which, initially, activates all the elementary circuits of command of the corresponding line by making them passers-by, then then apply a voltage ramp which is transmitted at the output of the elementary control circuit at the XL cell. Likewise, the whole columns C'j is connected to a column control circuit 30 which provides, on each column, a second signal consisting of a signal digital type switching, more particularly pulses of PWM type determining the duration during which the P'ij command activated remain passers-by.

We will now explain, with reference to Figure 3, the operation of the control circuit shown in Figure 2. As represented in FIG. 3, the signal applied to the lines L'i, L'i + 1 is consisting of a negative pulse enabling all the elementary control circuits of a line followed by a ramp including the amplitude typically varies between - 5 volts and + 10 volts preferably. The duration T of the signal L'i corresponds to a line time. On the line L'i + 1, the same signal is applied but offset by a time T as shown on figure 3. On the other hand, on the columns C'j is applied a signal of switching constituted by PWM type pulses to modulate the pulses in width, the signal having levels included typically between 0 and 2 volts, in the case of a silicon realization polycrystalline or monocrystalline silicon.

When the line L'i is not addressed, the control circuit elementary consisting mainly of the two transistors MN1 and MN2 works as follows. As shown in Figure 2, the second electrode of transistor MN1 is at a reference potential at know either to ground in the embodiment shown or to potential of the previous line which is itself at a voltage of reference, because it is not addressed. When a pulse is applied on column C'j, namely on the gate of transistor MN1, the transistor MN1 becomes conducting and point A, namely the gate of transistor MN2 goes to the reference potential. At this point, the source gate voltage Vgs of transistor MN2 is zero and the current "off" of transistor MN2 is minimum. As a result, the electro-optical cell XL does not not discharge.

When the line L'i is addressed, i.e. when it applies a signal as represented by L'i in FIG. 3, the line L'i undergoes all first a negative voltage drop - V. Point A, due to the CB capacity, undergoes the same instantaneous voltage drop. Column C'j receiving a positive pulse, as shown in Figure 3, the transistor MN1 is conducting and, therefore, the potential of point A is reduced to the level of the reference potential, namely to ground or zero, in the case of the embodiment shown. Source grid voltage Vgs of transistor MN2 becomes positive and goes to a value corresponding to the voltage drop on the line L'i which makes the MN2 transistor on. Immediately afterwards, the voltage applied to the column C'j drops to zero, causing the transition to the "off" or high state impedance of transistor MN1. The source gate voltage Vgs of the transistor MN2 remains constant due to the capacity CB. When the ramp of voltage is applied to the line L'i, the transistor MN2 being on, the voltage at point B copies the voltage of the ramp until a new positive pulse on the column returns the transistor MN1 passing, which has the effect of reducing the voltage at point A to the potential reference. At this moment, the transistor MN2 becomes non-conducting and the voltage at point B remains constant as shown in Figure 3.

The new elementary control circuit above allows therefore to display gray levels corresponding to the duration during which the ramp is applied at point A. For use in a flat liquid crystal display, the voltage of each elementary cell P'ij can therefore reach any value in the range of variation of the ramp provided by the first signal. The polarity of each cell can therefore be chosen independently of that of its neighbors provided that the counter electrode voltage is adjusted to a value close to the half of the maximum voltage reached by the first signal.

The control circuit described above makes it possible to effectively reduce consumption. Indeed, the consumption is given by ½ f CV ₂ , f being the line frequency, V the amplitude of the applied signal and C the capacities.

The table below shows the difference in consumption between the control device in FIG. 1 and in FIG. 2 for a liquid crystal screen comprising 600 rows and 2400 columns on a diagonal of the order of 30 cm. COLUMNS APPLIED SIGNALS LINE FREQUENCY POWER prior art analog: +/- 5 V 30 KHz ≈ ½ W invention PWM: 0 - 1 V 30 KHz 20 mW LINES APPLIED SIGNALS LINE FREQUENCY POWER prior art digital: 40 V 30 KHz
1 line at a time ≈ 10 mW invention analog data ramp: 15 V 30 KHz
1 line at a time ≈ 2 mW

On the other hand, when using polycrystalline silicon produced on glass or monocrystalline silicon to make the device control, the MN2 transistors operate with a gate-source voltage Vgs controlled, which gives a lower "off" current.

Another advantage of this invention is that the "drivers column »30 have a numerical function only, and operate at low voltage, which facilitates their design and reduces their cost.

FIG. 4 presents a variant of the invention where the outlet of the elementary control circuits P'ij identical to those represented on Figure 3 is no longer connected to a liquid crystal element, but to the gate of a transistor MN3 whose role is to deliver, to a material electroluminescent, an excitation current controlled by this voltage.

Claims (13)

1. Matrix drive device including a set of control circuits (P'ij) arranged in rows and columns and each controlling an elementary point (XL), the state of each elementary point being a function of first and second control signals (L'i, C'j) applied to the control circuit (P'ij) respectively via the rows (L'i) and columns (C'j), characterized in that each control circuit (P'ij) is an electrical circuit provided with two independent inputs, supplied by the first signal and the second signal, and with one output which generates the signal for controlling the elementary point (XL), the impedance of this electrical circuit between the output and that one of its inputs which is carrying the first signal becoming low following the application of an adequate voltage pulse on this first signal (L'i), and in that this same impedance becomes very high following the application of an adequate voltage on the second signal.
2. Device according to Claim 1, characterized in that the first signal (L'i) is a signal which, in a first stage, makes it possible to activate all the control circuits (P'ij) of the corresponding row (L'i) by turning them on, then to apply a voltage ramp which is sent as the output of the control circuit (P'ij) to the corresponding elementary point.
3. Device according to Claim 2, characterized in that the first signal (L'i) consists of a ramp-shaped signal preceded by a negative precharge pulse.
4. Device according to either of Claims 2 and 3, characterized in that the triggering of the ramp-shaped signal is adjusted from row to row so as to compensate for the propagation delays on the columns (C'j).
5. Device according to Claim 1, characterized in that the second signal (C'j) is a switching signal of digital type determining the duration for which the activated control circuits (P'ij) remain on.
6. Device according to Claim 5, characterized in that the second switching signal (C'j) consists of pulses of PWM (Pulse Width Modulation) type.
7. Device according to either of Claims 5 and 6, characterized in that the triggering of the pulses is adjusted from column to column in order to compensate for the delays on the rows (L'i).
8. Device according to any one of Claims 1 to 7, characterized in that the control circuit (P'ij) consists of a first transistor (MN2) connecting the elementary point (XL) to the corresponding row (L'i) receiving the first signal, and a second transistor (MN1) a first electrode of which is connected to the gate of the first transistor, the gate of which is linked to the corresponding column (C'j) receiving the second signal and the second electrode of which is connected to a reference potential.
9. Device according to Claim 8, characterized in that it further includes a capacitor (CB) connected between the gate of the first transistor (MN2) and the corresponding row (L'i).
10. Device according to Claim 8, characterized in that the second electrode of the second transistor (MN1) is connected to the preceding row (L'i-1).
11. Device according to any one of the preceding claims, characterized in that the circuits are produced using polycrystalline silicon, amorphous silicon or single-crystal silicon.
12. Device according to any one of the preceding claims, characterized in that the elementary points are electro optic cells.
13. Device according to any one of the preceding claims, characterized in that the output of the elementary points (P'ij) serves to modulate the excitation current of an electroluminescent material.

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