EP0435750B1 - Addressing method for every column of a matrix LCD screen - Google Patents

Abstract

The present invention relates to an addressing method for every column (cl) of a matrix LCD screen comprising production of a control pulse by a transistor (3) driving the said column, the said pulse having a duration determined by the value of the sample of a video input signal, the said pulse acting on the conduction state of the said transistor in order to connect the said column to a supply terminal where a voltage ramp (2) is developed. In accordance with the method, two pulse durations are alternated, whose sum is predetermined and so that a given value of the sample of a video signal produces the same optical effect from one period to the next, differentiated excitation voltages are employed on at least one of the framing electrodes of the liquid crystal layer, namely the said column (cl) and its counter-electrode (CE). Application especially to active matrix LCD screens.

Description

The present invention relates to the control of the columns of a matrix type LCD screen, more particularly a method of addressing each column of a matrix type LCD screen and in particular of an active matrix LCD screen.

A matrix type LCD screen comprises a set of bus-lines and bus-columns which controls the voltage applied to electrodes located on the same side of a liquid crystal layer, the other side being occupied by a counter electrode which cooperates with the first electrode to electrically orient the molecules of the liquid crystal and achieve the modulation of a light beam by polarization rotation. The columns are controlled by supplying each column with a charge current of the capacity produced between the column conductor and the counter-electrode such that the electric voltage across this capacity represents a sample of video signal between two successive addresses. An example of a control circuit for an active matrix LCD screen is described in particular in European application EP-A-0 298 255.

On the other hand, to obtain this charge current as a function of the video signal, it has been proposed in particular in the case where the control circuits are integrated into the LCD screen, to use a column driver transistor whose gate receives a control pulse based on the video signal sample. To illustrate this case, there is shown in Figure 1 the block diagram of a known control for an active matrix LCD screen symbolized by a column cl, a line L, a point P, the counter electrode CE and a transistor in thin line selection film.

More specifically, it has been proposed, as shown in FIG. 1, to use a control circuit of a column comprising a voltage-duration converter 1 which receives on one of its inputs the sample E of video signal and on its other input a voltage ramp from a generator of ramps 2. This converter outputs a pulse I whose duration t reflects the amplitude of the input video signal sample. This pulse I is sent to the gate g of a driving effect transistor 3 having a field effect, one of the electrodes or drain d of which, in the embodiment shown, receives a voltage ramp from generator 2 and whose l The other electrode or source is connected to the column bus in question. With the circuit described above, as long as the voltage Vgs of transistor 3 remains greater than its threshold voltage Vt, the signal on the source s therefore follows the evolution of the voltage ramp applied to the drain d and the capacitance C which represents the equivalent capacity of the column, namely the liquid-crystal capacity, the parasitic capacities of the point P control transistors and the bus crossing capacity, is charged. As soon as the voltage Vgs becomes lower than the threshold voltage Vt, the transistor 3 is blocked and the signal on the column keeps for value the value of the voltage charged in the capacitor C. We therefore obtain on each column cl, a voltage, for example, proportional to the width of the control pulse I of transistor 3

In this circuit, the pulse I obtained at the output of converter 1 does not have a falling edge with a steep slope. As a result, the blocking of transistor 3 occurs at a time which depends on the value of the conduction threshold. Consequently, the charging voltage of the capacity changes with the displacement of the threshold. Indeed, the conduction threshold shifts due to the electrical stress or constraint undergone by the field effect transistor 3 which is used to switch the column. This stress can be defined as the product of the gate-source voltage by the time during which the voltage is applied. Thus, this stress is a function of the value of the input signal and therefore of the video signal since the duration of the pulse is a function of the video signal. To illustrate this problem, FIG. 2 shows the pulse I, the voltage ramp and the column voltages V1 and V2 obtained. respectively with threshold voltages T1 and T2. It can be seen that the rise in the threshold voltage of the transistor tends to reduce the column voltage Vsg. Thus, it can be seen that a low value of the video signal sample generates a gate-source voltage stress lower than the stress generated by a high value of the video signal sample, as shown diagrammatically by A and B on FIG. 3. In the first case, the offset of the threshold voltage will therefore be lower. This therefore results in a non-uniform shift of the threshold voltages of the various column switching transistors, resulting in non-uniformity of luminance on the LCD screen.

The object of the present invention is therefore to remedy this drawback by proposing a method for addressing each column which prevents the switching threshold of the transistor from changing with the video signal samples.

The present invention also aims to propose a method for addressing each column which makes it possible to create conditions such that the gate-source stress of the transistor is on average independent of the video signal sample applied to the column.

Consequently, the subject of the present invention is a method of addressing each column of a matrix type LCD screen comprising the periodic production of pulses of period T as a function of the video signal to be displayed on the screen, the pulses controlling a drive transistor of said column and having a duration determined by the amplitude of the sample video signal at input, each pulse acting on the conduction state of said transistor to connect said column to a supply terminal where develop voltage ramps of period T, characterized in that the pulses are alternated periodically so that the sum of the durations of two alternating pulses corresponds to said period T of the pulses, and characterized in that so that a given amplitude of the video signal sample produces the same optical effect from one period to the next, is applied to said column either, in the case where the voltage applied to the liquid crystal counter electrode is not alternated from period T to period T, for all consecutive pairs of periods, a first of the voltage ramps followed by a second of the said voltage ramps offset with respect to the first ramp by a value at least equal to its maximum amplitude, that is, in the case where the voltage applied to the counter electrode is alternated from period T to period T, an identical voltage ramp from period T to period T.

According to a particular embodiment, before the production of the pulses, the video signal is periodically inverted so as to obtain, during a first period a pulse of duration t and during a second period a pulse of duration Tt, T being the duration of the period.

On the other hand, when an alternating voltage is applied periodically to the counter electrode, the same voltage ramp is applied to each period.

However, when a fixed voltage is applied to the counter electrode, the voltage ramp is offset symmetrically at each period with respect to the fixed voltage so as to apply a ramp varying between V and V 'during a first period and a ramp varying between -V 'and -V during a second period.

With this process, the average value of grid-source stresses is constant. Thus, the offset of the threshold voltages of the transistors controlling the columns is also constant, which leads to uniform visual degradation.

However, to compensate for this constant offset of the voltages on the columns with respect to normal, according to another characteristic of the present invention, a DC offset voltage is applied to the counter electrode compensating for the average offset of the threshold voltages of the switching transistors.

The present invention also relates to an LCD screen for implementing the method described above.

• la figure 1 déjà décrite est un schéma synoptique d'un circuit de commande d'une colonne d'un écran LCD à matrice active selon l'art antérieur ;
• la figure 2 est une courbe donnant la tension en fonction du temps expliquant les problèmes dûs au stress du transistor de commutation dans le circuit de la figure 1 ;
• la figure 3 représente deux courbes schématiques montrant le stress en fonction de la tension de l'échantillon vidéo ;
• la figure 4 est un schéma synoptique d'un circuit de commande pour la mise en oeuvre du procédé de la présente invention dans le cas où une tension alternative est appliquée sur la contre-électrode ;
• la figure 5 est un diagramme des temps expliquant le fonctionnement de la présente invention, et
• la figure 6 est un schéma simplifié expliquant le fonctionnement de la présente invention dans le cas d'un écran LCD à matrice active dont la contre-électrode est à un potentiel fixe.

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

• Figure 1 already described is a block diagram of a control circuit of a column of an active matrix LCD screen according to the prior art;
• Figure 2 is a curve giving the voltage as a function of time explaining the problems due to stress of the transistor switching in the circuit of FIG. 1;
• FIG. 3 represents two schematic curves showing stress as a function of the voltage of the video sample;
• Figure 4 is a block diagram of a control circuit for implementing the method of the present invention in the case where an AC voltage is applied to the counter electrode;
• FIG. 5 is a time diagram explaining the operation of the present invention, and
• Figure 6 is a simplified diagram explaining the operation of the present invention in the case of an active matrix LCD screen whose counter electrode is at a fixed potential.

To simplify the description, in the figures the same elements have the same references.

FIG. 4 shows an active matrix LCD screen. This screen has been represented diagrammatically by a single point or pixel P at the intersection of a column bus cl and a line bus L. In the embodiment shown, the coupling line L-column cl is produced by a transistor T in thin film (TFT) which receives on its grid the voltage applied to the line L and on an electrode the voltage applied to the column, the other electrode being connected to the liquid crystal electrode forming with the CE counter electrode the capacity C. The liquid crystal is therefore equivalent to a capacity C with a resistance not shown. In the embodiment of FIG. 4, the CE counter-electrode receives an alternating voltage from a rectangular voltage generator 4. As shown in FIG. 5, the CE counter-electrode voltage alternately goes to a level of, for example 5 Volts during a first period then at a level of, for example, 0 Volts during a second period. On the other hand, the column control circuit comprises, like the column control circuit of FIG. 1, a voltage-duration converter 1 which receives as input a sample of video signal and on another input of a ramp from a ramp generator 2. According to the present invention, the video signal sample E comes from a circuit 5 complementing the video signal. The circuit 5 is controlled by the rectangular voltage generator 4 so as to apply for a first period the video signal itself and for a second period the complement of the video signal. Thus, at the output of the voltage-duration converter 1, a pulse I is obtained whose duration is a function of the amplitude of the video signal, namely a pulse I having, for example, a duration t during the first period and a duration Tt during the second period, T representing the duration of the period, namely preferably a frame duration. More generally, the alternation of the two pulse durations is equal to a predetermined sum.

With the above circuit, when the pulse I has a duration t, one obtains on the source s of the transistor 3 a voltage V ′ as represented in FIG. 5. In this case the voltage across the terminals of the cell of liquid crystal is equal to (5 Volts - V ′) in the embodiment shown and corresponds to a high voltage allowing for example the display of black. During the second period, the pulse I has a duration T-t and corresponds to a voltage V on the source. In this case, the voltage across the terminals of the liquid crystal cell becomes equal to 0-V, this voltage is also a high voltage corresponding to the display of black. As shown in Figure 5, the rectangular voltage between 0 and 5V and the voltages V and V ′ are chosen so that (5-V ′) = (V-0). A zero mean voltage is therefore applied across the terminals of the liquid crystal cell. The same conclusions would be obtained for the display of whites or grays.

In practice, the threshold voltages of the control transistors of the different lines undergo an offset so that the voltages corresponding to white and black respectively are no longer V and V ′ but in general V-DV and V′-DV. To remedy these shifts in voltages at the point, in accordance with the present invention, an offset voltage of the same level is applied to the counter-electrode. However, this compensation can also be carried out at other places, in particular by decoding the ramp or at the level of the video signal itself.

A description will now be given with reference to FIG. 6, an embodiment of the addressing method according to the present invention in the case where the CE counter-electrode receives a fixed voltage, for example a voltage of 0 Volt. According to the present invention, in this case, not only the video signal E is alternately transmitted or supplemented at each period, but the ramp applied to one of the electrodes of the switching transistor 3 is offset in voltage at each period. Thus, one applies either a ramp varying, for example, between 0 and 5 Volts during a first period, or a ramp varying between -5 Volts and 0 Volt during a second period, as shown in FIG. 5 which relates to an example of coding with a black dot. In the embodiment shown, the pulse applied to the gate of the transistor 3 has a duration t0 which corresponds to maximum stress. The voltage across the pixel P is therefore equal to 5 Volts (maximum value of the ramp) -0 Volt (voltage applied to the counter-electrode), or 5 Volts which corresponds to a minimum luminance of the point. During the second period, the pulse applied to the gate of transistor 3 has a duration T-to corresponding to minimal stress. The voltage across the pixel is equal to - 5 Volts (minimum value of the ramp) - 0 Volt (value of the counter-electrode voltage) or - 5 Volts. The luminance of the point is still minimal in this case. Thus, by adapting the voltage variations of the ramp applied to the electrode of transistor 3, it is possible to standardize the stress applied to all the transistors of the control circuit by inverting the video signal from one period to another.

Claims (8)

1. Method for addressing each column (cl) of a matrix type LCD panel comprising the periodic production of pulses (I) of period T depending on the video signal (E) to be displayed on the panel, the pulses (I) controlling a driver transistor (3) for the said column (cl) and having a duration (t) determined by the amplitude of the input video signal sample, each pulse (I) acting on the state of conduction of the said transistor (3) in order to connect the said column (cl) to a power supply terminal of which voltage ramps of period T develop, characterized in that the pulses (I) are periodically alternated in such a way that the sum of the durations (t, T-t) of two alternate pulses corresponds to the said period T of the pulses (I), and characterized in that so that a given amplitude of the video signal sample should produce the same optical effect from one period to the next, there is applied to the said column (cl) either, in the case where the voltage applied to the counter-electrode (CE) of the liquid crystal is not alternated from period T to period T, for all the consecutive pairs of periods, a first of the said voltage ramps followed by a second of the said voltage ramps, shifted with respect to the first ramp by a value equal to at least the maximum amplitude thereof, or, in the case where the voltage applied to the counterelectrode (CE) is alternated from period T to period T, a voltage ramp which is identical from period T to period T.
2. Addressing method according to Claim 1, characterized in that before the production of pulses, the video signal is reversed periodically so as to obtain a pulse with a duration t during a first period and a pulse with a duration T-t during a second period, T being the duration of the period.
3. Method according to Claim 1, characterized in that when an AC voltage is applied periodically to the counter-electrode (CE), the same voltage ramp is applied at each period.
4. Method according to Claim 1, characterized in that when a fixed voltage is applied to the counterelectrode (CE), the voltage ramp is shifted symmetrically at each period in relation to the fixed voltage so that a ramp varying between V and V' is applied during a first period and a ramp varying between -V' and -V is applied during a second period.
5. Method according to any one of Claims 1 to 4, characterized in that the period T corresponds to a frame.
6. Method according to any one of Claims 1 to 5, characterized in that the LCD panel is an active matrix panel.
7. Method according to any one of Claims 1 to 6, characterized in that a DC shift voltage, compensating for the mean shift in the threshold voltages of the switch-over transistors, is applied to the counterelectrode.
8. LCD panel comprising addressing columns (cl) each associated with a transistor (3) for driving the said column so as to implement the method according to any one of Claims 1 to 7, the LCD panel including a column control circuit comprising a voltage-duration converter (1) and a ramp generator (2), the converter (1) receiving the video signal at one of its inputs and voltage ramps of period T coming from the ramp generator (2) at the other input, the converter (1) delivering, for each period T, a pulse (I) whose duration (t) is a function of the amplitude of a sample (E) of the video signal, this pulse being sent to the gate of the said transistor (3), one of the electrodes of which receives the said voltage ramps and the other electrode of which is linked to a column (cl) characterized in that it includes a circuit (5) for complementing the video signal connected at the input of the voltage-duration converter (1), this circuit being controlled by a square-wave voltage generator (4) which also controls the voltage applied to the counterelectrode in such a way that, in the case where the voltage applied to the counter-electrode (CE) of the liquid crystal is not alternated from period T to period T, for all the consecutive pairs of periods, a first of the said voltage ramps followed by a second of the said voltage ramps, shifted with respect to the first ramp by a value equal to at least the maximum amplitude thereof, are applied to the said electrode receiving the voltage ramp, or in the case where the voltage applied to the counter-electrode (CE) is alternated from period T to period T, a voltage ramp which is identical from period T to period T is applied to the said electrode receiving the voltage ramp.

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