KR100692862B1 - Electro-Luminescence Display Device And Driving Method Thereof - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-luminescence display device and a driving method thereof in which a thin film transistor for driving an electro-luminescence cell is operated in an unsaturation region to prevent a deterioration in image quality by compensating a threshold voltage.

An electroluminescent display device according to an embodiment of the present invention is an electroluminescent cell connected between a first supply voltage source and a base voltage source and emits light by a current supplied from the first supply voltage source and gate lines. And a cell driver formed at each intersection of the data lines and a driving thin film transistor connected between the first supply voltage source and the electro-luminescence cell to control a current flowing in the pixel cell. And a pulse supply unit connected between the luminescence cell and the base voltage source and supplying a pulse width modulation signal to the electro-luminescence cell for operating the driving thin film transistor in an unsaturated region. .

The present invention can prevent deterioration in image quality due to the variation of the threshold voltage by reducing the variation in the threshold voltage generated between the driving thin film transistors due to the non-uniformity of the excimer laser irradiated when the driving thin film transistors are formed.

Description

Electro-Luminescence Display Device And Driving Method Thereof}             

1 is a block diagram showing a conventional electro-luminescence display.

FIG. 2 is a circuit diagram illustrating a pixel cell shown in FIG. 1.

FIG. 3 is a view illustrating operating characteristics of the driving thin film transistor illustrated in FIG. 2.

4 is a block diagram illustrating an electro-luminescence display device according to an exemplary embodiment of the present invention.

FIG. 5 is a circuit diagram illustrating a pixel cell, a data driver, and a pulse supply unit of the electroluminescent display device according to the first embodiment of the present invention shown in FIG. 4.

FIG. 6 is a waveform diagram showing a modulation data signal supplied to the switch element shown in FIG. 5 and a pulse width modulation signal supplied to the cathode electrode of the EL cell. FIG.

FIG. 7 is a view illustrating operating characteristics of a driving thin film transistor according to a first embodiment of the present invention illustrated in FIG. 5.

FIG. 8 is a driving waveform diagram illustrating 12 gray levels in a pixel cell shown in FIG. 5; FIG.

FIG. 9 is a circuit diagram illustrating a pixel cell, a data driver, and a pulse supply unit of an electroluminescent display device according to a second embodiment of the present invention shown in FIG. 4.

FIG. 10 is a view illustrating operating characteristics of a driving thin film transistor according to a second exemplary embodiment of the present invention shown in FIG. 9.

<Description of Symbols for Main Parts of Drawings>

20, 120: EL panel 22, 122: gate driver

24, 124, 224: data driver 26: gamma voltage generator

28, 128: pixel cells 30, 130: cell driver

140: pulse supply unit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-luminescence display device, and more particularly, to an electro-luminescence display in which a thin film transistor for driving an electro-luminescence cell is operated in an unsaturated region to compensate for a threshold voltage to prevent image degradation. A display device and a driving method thereof.

Recently, various flat panel display devices that can reduce weight and volume, which are disadvantages of cathode ray tubes, have emerged. Such a flat panel display includes a liquid crystal display, a field emission display, a plasma display panel, and an electroluminescence (hereinafter, EL). And display devices.

Among them, the EL display device is a self-luminous element that emits a phosphor by recombination of electrons and holes, and is roughly classified into an inorganic EL using an inorganic compound as a phosphor and an organic EL using an organic compound as a phosphor. Such EL display devices are expected to be the next generation display devices because they have many advantages such as low voltage driving, self-luminous, thin film type, wide viewing angle, fast response speed and high contrast.

The organic EL element is usually composed of an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer, and a hole injection layer stacked between a cathode and an anode. In such an organic EL device, when a predetermined voltage is applied between the anode and the cathode, electrons generated from the cathode move to the light emitting layer through the electron injection layer and the electron transport layer, and holes generated from the anode pass through the hole injection layer and the hole transport layer. Move toward the light emitting layer. Accordingly, the light emitting layer emits light by recombination of electrons and holes supplied from the electron transporting layer and the hole transporting layer.

As shown in FIG. 1, an active matrix EL display device using such an organic EL element includes pixel cells arranged in regions defined by intersections of a gate line GL and a data line DL, and including EL cells OLED. An EL panel 20 including a 28, a gate driver 22 for driving the gate lines GL of the EL panel 20, and data lines DL for the EL panel 20; A data driver 24 and a gamma voltage generator 26 for supplying a plurality of gamma voltages VH to VL to the data driver 24 are provided.

The gate driver 22 sequentially drives the gate lines GL by supplying scan pulses to the gate lines GL.

The gamma voltage generator 26 uses different n-level gamma voltages between the high gray gamma voltage VL and the low gray gamma voltage VH by using n resistors connected in series between a supply voltage source and a base voltage source (not shown). (VH to VL) are generated and supplied to the data driver 24.

The data driver 24 converts the digital data signal input from the outside into an analog data signal using the gamma voltages VH to VL from the gamma voltage generator 26. The data driver 24 supplies the analog data signal to the data lines DL every time a scan pulse is supplied.

Each of the pixel cells 28 receives a data signal from the data line DL when a scan pulse is supplied to the gate line GL, and generates light corresponding to the data signal.

To this end, each of the pixels 28 includes an EL cell OLED connected between the supply voltage source VDD and the base voltage source VSS as shown in FIG. 2, and a cell driver for driving the EL cell OLED. 30 is provided.

The cell driver 30 includes a switching thin film transistor T1 in which a gate terminal is connected to the gate line GL, a source terminal is connected to the data line DL, and a drain terminal is connected to the first node N1, and the first The driving thin film transistor T2 in which the gate terminal is connected to the node N1, the drain terminal is connected to the supply voltage source VDD, and the source terminal is connected to the anode terminal of the EL cell EL, and the supply voltage source VDD. And a storage capacitor Cst connected between the first node N1 and the first node N1.

The switching thin film transistor T1 is turned on when a scan pulse is supplied to the gate line GL, and supplies a data signal supplied to the data line DL to the first node N1. The data signal supplied to the first node N1 is charged to the storage capacitor Cst and supplied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 controls the amount of light emitted from the EL cell OLED by controlling the amount of current Id supplied from the supply voltage source VDD via the EL cell OLED in response to the data signal supplied to the gate terminal. Done. In addition, even when the switching thin film transistor T1 is turned off, the driving thin film transistor T2 is kept on by the data signal charged in the storage capacitor Cst, until the data signal of the next frame is supplied. The amount of current Id supplied from the supply voltage source VDD may be controlled via the cell OLED.

On the other hand, each of the switch thin film transistor T1 and the driving thin film transistor T2 of the cell driver 30 uses an amorphous silicon layer as a semiconductor layer. At this time, the amorphous silicon layer has a disadvantage of low mobility. Therefore, researches on polysilicon thin film transistors using a polysilicon layer having high mobility as a semiconductor layer have recently been conducted. Since such polysilicon thin film transistors can integrate a driving drive integrated circuit together on a substrate, they have high integration and price competitiveness. There is an excellent advantage. However, since the deformation temperature of the glass is as low as 600 ° C, the crystal growth technique using a high temperature of 600 ° C or higher for forming the polysilicon layer cannot be used. For this reason, the excimer laser which forms a polysilicon layer at low temperature (100-300 degreeC), and heat-melts an amorphous silicon layer by the pulse irradiation by the excimer laser of wavelength 308nm, and crystallizes in the cooling process, Annealing (Excimer Laser Annealing: hereinafter referred to as ELA) is generally used. By using this ELA, a polysilicon layer can be formed without causing thermal damage to the glass substrate.

However, the excimer laser is unstable in light output and fluctuates in the range of ± 10% of the output intensity. For this reason, in ELA, there exists a problem that the crystal grain size in a polysilicon layer is irregular and its reproducibility is bad. In addition, since the excimer laser has a low pulse driving repetition frequency of 300 Hz, it is difficult to form continuous grain boundaries in ELA, and high carrier mobility is not obtained. Also, the excimer laser has a problem in that large areas cannot be annealed at high speed. .

The size, size uniformity, number and position, direction, etc. of the grains of the semiconductor layer formed by the ELA process may be determined by the threshold voltage (Vth), the threshold slope, the charge carrier mobility, and the leakage current. (let current), and device stability (device stability) and the like has a fatal effect directly or indirectly on the characteristics of the thin film transistor. Accordingly, the characteristics of the thin film transistor formed on the EL panel 20 by the ELA process are not stable because the optical output of the excimer laser irradiated in the form of a line beam is unstable and the output intensity varies in a range of ± 10%. It is changed in units of lines corresponding to the irradiation direction.

In general, the operating point Q of the driving thin film transistor T2 is present in the saturation region as shown in the characteristic graph of the transistor illustrated in FIG. 3. This is because a stable current Id can be supplied to the EL cell OLED even when the voltage Vds between the drain terminal and the source terminal of the driving thin film transistor T2 changes. At this time, the amount of change of the current Id flowing in the driving thin film transistor T2 in the saturation region is larger than the unsaturated region with respect to the deviation of the threshold voltage Vth of each of the driving thin film transistors T2. Accordingly, when the threshold voltage Vth is large, as described above with respect to the voltage Vgs between the same gate terminal and the source terminal of each of the driving thin film transistors T2, the current flows to the driving thin film transistor T2. The change of the current Id becomes large.

Therefore, in the conventional EL display device, the gray scale is represented by the change of the data voltage. Therefore, when the threshold voltage Vth of the driving thin film transistor T2 is not uniform for each line of the EL panel 20, the EL is applied to the same data voltage. Since the amount of current flowing through the cell OLED cannot be precisely controlled (actually, the amount of current is decreased), there is a problem that the desired image is not displayed due to uneven brightness.

Accordingly, an object of the present invention is to provide an electro-luminescence display device and a driving method thereof in which a thin film transistor for driving an electro-luminescence cell is operated in an unsaturation region to prevent a deterioration in image quality by compensating a threshold voltage. It is.

In addition, another object of the present invention is to operate a thin film transistor for driving an electro-luminescence cell in an unsaturated region to compensate for a threshold voltage to prevent image degradation and to adjust luminance according to two modes. An electroluminescent display and its driving method are provided.

In order to achieve the above object, an electroluminescent display device according to an embodiment of the present invention is connected between a first supply voltage source and a base voltage source and emits light by electric current supplied from the first supply voltage source. And a driving thin film transistor formed at each intersection of the sense cell and the gate lines and the data lines and connected between the first supply voltage source and the electro-luminescence cell to control a current flowing in the pixel cell. A pulse supply coupled to a cell driver, between the electro-luminescence cell and the base voltage source, to supply a pulse width modulation signal to the electro-luminescence cell for operating the driving thin film transistor in an unsaturated region; It is characterized by comprising a part.

The electro-luminescence display further comprises a data driver for supplying an on-off signal for driving the driving thin film transistor to the data line, and a gate driver for supplying a scan pulse to the gate line. It features.

In the electro-luminescence display, the cell driver is connected to the gate line, the data line, and the driving thin film transistor, and in response to the scan pulse, an on / off signal on the data line is applied to the gate terminal of the driving thin film transistor. And a storage capacitor connected between the switch thin film transistor to be supplied and a gate terminal of the driving thin film transistor and the first supply voltage source.

In the electroluminescent display, the data driver includes first and second resistors connected in series between a second supply voltage source and the base voltage source, and a first switch element connected between the second resistor and the base voltage source. Characterized in having a.

In the electro-luminescence display, the data driver is in a high state or a low state by a voltage difference between the first supply voltage source and the voltage on the node between the first and second resistors according to the switching of the first switch element. The on-off signal is supplied to the data line.

In the electro-luminescence display, the gate terminal of the first switch element has a duty cycle corresponding to the number of bits of digital data while a scan pulse is supplied to the gate line, and in step N (where N is a natural number). The modulated data signal to be divided is supplied.

The modulated data signal of each of the N steps in the electro-luminescence display device has a read section having a first voltage level and a write section having a second voltage level different from the first voltage level.

In the electro-luminescence display, the pulse supply unit supplies the pulse width modulated signal, which is synchronized with the modulated data signal and has the same duty cycle, divided into the N stages, to the cathode terminal of the electro-luminescence cell. Characterized in that.

In the electro-luminescence display, the pulse width modulated signal of each of the N stages has a lead period equal to the voltage level from the first supply voltage source, and a voltage between the lead level and the base voltage from the base voltage source. And a light section having a level.

In the electro-luminescence display, the driving thin film transistor has a voltage difference between the drain and the source due to the voltage supplied to the write period of the pulse width modulation signal of each of the N stages with respect to the fixed gate-source voltage. It operates in the non-saturated region.

In the electro-luminescence display, the electro-luminescence cell emits light by the current caused by the voltage level of the write period of each of the N-stage pulse width modulation signals and the voltage difference between the first supply voltage source, The gray level corresponding to the N bits is expressed by the sum of the emission times of each of the N stages.

In the electro-luminescence display, the data driver comprises a third resistor connected between the node between the first and second resistors and the second supply voltage source, and between the third resistor and the second supply voltage source. And a second switch element for connecting the third resistor to the first resistor in parallel in response to a mode selection signal connected and supplied from the outside.

In the electro-luminescence display, the data driver is configured such that when the second switch element is turned off by the mode selection signal, the voltage on the node between the first and second resistors is changed according to the switching of the first switch element. When the on-off signal having a high state or a low state having a first level is supplied to the data line by the voltage difference between the first supply voltage source, and the second switch element is turned on by the mode selection signal. A low state having a high state or a second level due to the voltage difference between the first supply voltage source and the voltage on the node between the parallel resistance of the first and second resistors and the second resistor according to the switching of the first switch element. The on-off signal is supplied to the data line.

In the electro-luminescent display, the driving thin film transistor has different voltages between first and second gate-sources according to the on-off signal in a low state having the first and second levels. do.

In the electro-luminescent display, the driving thin film transistor controls the magnitude of the current flowing through the electro-luminescence cell to two levels according to the voltage between the first and second gate-sources. .

A method of driving an electroluminescent display device according to an embodiment of the present invention includes an electroluminescent cell connected between a first supply voltage source and a base voltage source and emitting light by a current supplied from the first supply voltage source; And a cell driver formed at each intersection of gate lines and data lines and including a driving thin film transistor connected between the first supply voltage source and the electro-luminescence cell to control a current flowing in the pixel cell. In the electro-luminescence display having a light emitting device, the driving thin film transistor is operated in an unsaturated region by using a pulse supply connected between the electro-luminescence cell and the base voltage source. And supplying a pulse width modulated signal to make the pulse width modulated signal.

The driving method may further include generating an on-off signal for driving the driving thin film transistor and supplying a scan pulse to the gate line.

In the driving method, the generating of the on-off signal has a duty cycle corresponding to the number of bits of digital data while a scan pulse is supplied to the gate line, and the modulation data signal is divided into N steps (where N is a natural number). And generating the on-off signals in a high state and a low state by using the modulated data signal.

In the driving method, each of the modulated data signals of step N has a read section having a first voltage level and a write section having a second voltage level different from the first voltage level.

In the driving method, the pulse width modulated signal is synchronized with the modulated data signal, has the same duty cycle, is divided into the N steps, and is supplied to the cathode terminal of the electro-luminescence cell.

In the driving method, the pulse width modulation signal of each of the N steps has a read period equal to a voltage level from the first supply voltage source, and a write interval having a level between the voltage level of the read period and the base voltage from the base voltage source. It is characterized by having a.

In the driving method, the driving thin film transistor is formed in the unsaturated region by a voltage difference between the drain and the source due to the voltage supplied to the write period of the pulse width modulation signal of each of the N steps with respect to the fixed gate-source voltage. It is characterized in that the operation.

In the driving method, the electro-luminescence cell emits light by the current caused by a voltage difference between the voltage level of each write period of each of the N-stage pulse width modulation signals and the voltage of the first supply voltage source, and emits light of each of the N-steps. The gray level corresponding to the N bits is expressed by the sum of time.

The generating of the on-off signal in the driving method may include generating the on-off signal in a high state or a low state having a first level by a mode selection signal, and in a high state or a second by the mode selection signal. Generating the on-off signal in a low state having a level.

In the driving method, the driving thin film transistor has different voltages between first and second gate-sources according to the on / off signal having a low state having the first and second levels.

In the driving method, the driving thin film transistor controls the magnitude of the current flowing through the electro-luminescence cell to two levels according to the voltage between the first and second gate-sources.

Other objects and features of the present invention in addition to the above object will be apparent from the description of the embodiments with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described with reference to FIGS. 4 to 10.

4 and 5, an electro-luminescence display device according to a first embodiment of the present invention is an intersection of a gate line GL and a data line DL. An EL panel 120 having pixel cells 128 arranged in a defined region and including an EL cell OLED and a driving thin film transistor T2 for driving the EL cell OLED, and an EL panel The gate driver 122 driving the gate lines GL of the 120 and the on / off signal Vdata for driving the pixel cells 128 of the EL panel 120 are connected to the data lines DL. A pulse supply unit 140 for supplying a pulse width modulation signal Vs to the data driver 124 for supplying the data driver 124 and the cathode electrode of the EL cell OLED so that the driving thin film transistor T2 operates in an unsaturated region. Equipped.

The gate driver 122 sequentially drives the gate lines GL by supplying scan pulses to the gate lines GL.

Each of the pixel cells 128 receives an on-off signal Vdata from the data line DL when a scan pulse is supplied to the gate line GL, and receives a pulse width modulation signal supplied from the pulse supply unit 140. It generates light corresponding to Vs).

To this end, each of the pixels 128 includes an EL cell OLED connected between the first supply voltage source VDD1 and the pulse supply unit 140 and an EL cell OLED as shown in FIG. 5. The cell driver 130 is provided.

The cell driver 130 includes a switching thin film transistor T1 in which a gate terminal is connected to the gate line GL, a source terminal is connected to the data line DL, and a drain terminal is connected to the first node N1, A driving thin film transistor T2 having a gate terminal connected to the node N1, a drain terminal connected to the first supply voltage source VDD1, and a source terminal connected to the anode terminal of the EL cell EL, and the first supply voltage source ( A storage capacitor Cst is connected between VDD1 and the first node N1.

The switching thin film transistor T1 is turned on when a scan pulse is supplied to the gate line GL, and supplies the on-off signal Vdata supplied to the data line DL to the first node N1. The on-off signal Vdata supplied to the first node N1 is charged to the storage capacitor Cst and supplied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 is turned on / off by the on-off signal Vdata supplied to the gate terminal, and the current amount Id supplied from the first supply voltage source VDD1 via the EL cell OLED. To control. In addition, even when the switching thin film transistor T1 is turned off, the driving thin film transistor T2 is maintained in the on state by the on / off signal Vdata charged in the storage capacitor Cst.

The EL cell OLED has a voltage difference between the pulse width modulation signal Vs and the first supply voltage source VDD1 supplied from the pulse supply unit 140 to its cathode while the driving thin film transistor T2 is turned on. The current corresponding to is supplied from the first supply voltage source VDD1 to emit light for a time corresponding to the pulse width modulation signal Vs.

The data driver 124 includes a data modulation circuit (not shown) for modulating to have a duty cycle of n steps (where n is a natural number) corresponding to the number of bits of digital data input from the outside, and the second supply voltage source VDD2. ) And first and second resistors R1 and R2 connected in series between the base voltage source VSS and the switch element SW connected between the second resistor R2 and the base voltage source VSS. At this time, the second supply voltage source VDD2 has a level smaller than the voltage level of the first supply voltage source VDD1.

The data modulation circuit modulates digital data input from the outside to have a duty cycle of n steps corresponding to the number of bits, and supplies the digital data to the gate terminal of the switch element SW. At this time, when the digital data from the outside is 4 bits, the modulated data signal data has a digital value (0 to 0) corresponding to 4 bits while the scan pulse is supplied to the gate line GL as shown in FIG. According to 15, it is dividedly supplied to have a duty cycle of four steps (8, 4, 2, 1). At this time, each step of the modulation data signal data is divided into a read section for turning off the switch element SW and a write section for turning on the switch element SW. Accordingly, 16 grays are represented by the sum of grays represented by the four steps (8, 4, 2, 1) of the modulated data signal data. That is, among the four steps (8, 4, 2, 1), one step expresses eight gradations, the second step four gradations, the third step two gradations, and the fourth step expresses one gradation.

The node between the first and second resistors R1 and R2 is connected to the data line DL. The switch element SW selectively connects the second resistor R2 to the ground voltage source VSS in accordance with the modulation data signal data supplied from the data modulation circuit.

The data driver 124 turns off the switch element SW by a read interval of the modulated data signal data supplied to the switch element SW, and passes the second supply voltage source VDD2 via the first resistor R1. The on-off signal Vdata of the high state HIGH, ie, the high state HIGH, is supplied to the data line DL. On the other hand, the data driver 124 turns on the switch element SW by the write section of the modulated data signal data supplied to the switch element SW to connect the second resistor R2 to the base voltage source VSS. do. Thus, the on-off signal Vdata of the low state LOW is supplied to the data line DL connected to the node between the first and second resistors R1 and R2. That is, when the scan pulse is supplied to the gate line GL, the gate terminal of the driving thin film transistor T2 is the second resistor R2 of the switching thin film transistor T1, the data line DL, and the data driver 124. And between the first resistor R1 and the second resistor R2 when the switch element SW of the data driver 124 is turned on because it is connected to the base voltage source VSS via the switch element SW. The on-off signal Vdata in the low state LOW is supplied to the gate terminal of the driving thin film transistor T2 by the voltage difference between the voltage on the node and the first supply voltage source VDD1.

The pulse supply unit 140 is connected between the cathode electrode of the EL cell OLED and the ground voltage source VSS. The pulse supply unit 140 synchronizes with each step of the modulated data signal data supplied to the switch element SW of the data driver 124 and transmits a pulse width modulated signal Vs having the same duty cycle to the EL cell. It is supplied to the cathode electrode of (OLED).

Specifically, the voltage level supplied to the cathode electrode of the EL cell OLED in the lead section of the pulse width modulation signal Vs has the same voltage level as the first supply voltage source VDD1, and the EL cell OLED in the write section. The voltage level supplied to the cathode electrode of) has a voltage level between the first supply voltage source VDD1 and the base voltage source VSS. Accordingly, the voltage level between the first supply voltage source VDD1 and the base voltage source VSS supplied to the write section of the pulse width modulation signal Vs is controlled by the data driver 124 to gate the driving thin film transistor T2. As the voltage Vds of the driving thin film transistor T2 is reduced while the voltage Vgs of the terminal and the source terminal is fixed, the driving thin film transistor T2 of the driving thin film transistor T2 is reduced as shown in FIG. 7. The operating point Q is present in the unsaturated region. Therefore, the EL display device and the driving method thereof according to the first embodiment of the present invention are applied to the fixed Vgs supplied from the data driver 124 because the operating point Q of the driving thin film transistor T2 is in the unsaturated region. On the other hand, the amount of change in the current Id flowing in the driving thin film transistor T2 due to the variation in the threshold voltage Vth can be made smaller than before. As a result, the EL display device and the driving method thereof according to the first embodiment of the present invention can compensate for the deviation of the threshold voltage Vth of the driving thin film transistor T2 to prevent deterioration in image quality.

At the same time, the EL cell OLED is connected to the first supply voltage source by the voltage from the first supply voltage source VDD1 supplied through the driving thin film transistor T2 and the voltage difference DT from the pulse supply unit 140. The light is emitted by receiving a current from VDD1). Therefore, the EL cell OLED is supplied step by step from the pulse supply unit 140 so as to be synchronized with the on / off signal Vdata supplied step by step from the data driver 124 during the period in which the scan pulse is supplied to the gate line GL. The gray level corresponding to the number of bits of the digital data is expressed by the sum of the light emission times of n steps by the pulse width modulated signal Vs.

According to the EL display device and the driving method thereof according to the first embodiment of the present invention, as shown in FIG. 8, digital data supplied from the outside is 4 bits, and one EL cell (OLED) is used by using the 4-bit digital data. In the following example, 12 gradations are expressed as follows.

While the scan pulse SP is supplied to the gate line GL, the data driver 124 may perform the modulation data signal 8 of the first stage and the first stage having a duty cycle corresponding to the digital data 1000 of eight. Subsequently, the second stage modulated data signal 4 having a duty cycle corresponding to four digital data 0100 is sequentially supplied to the switch element SW. Accordingly, the switching element SW sequentially switches the on / off signal Vdata in response to each of the first and second stage modulation data signals 8 and 4 supplied sequentially from the data driver 124. It is supplied to the gate terminal of the driving thin film transistor T2 via T1 and is synchronized with each of the modulation data signals 8 and 4 in the first and second stages from the pulse supply unit 140, and also has the same duty cycle. The pulse width modulated signals Vs of the first and second stages are supplied stepwise to the cathode electrode of the EL cell OLED.

As a result, the driving thin film transistor T2 is turned on by the on-off signal Vdata sequentially supplied by the first and second stages, and thus the first supply voltage source VDD1 via the EL cell OLED. The amount of current Id supplied from) is controlled. At this time, the EL cell OLED emits light during the duty cycle of each of the pulse width modulated signals Vs of the first and second stages supplied to its cathode electrode.

Therefore, according to the EL display device and the driving method thereof according to the first embodiment of the present invention, the EL cell OLED is subjected to the first and second steps while the scan pulse SP is supplied to the gate line GL. By emitting light, eight gray scales of the light emission time of the first stage and four gray scales of the light emission time of the second stage are added to represent 12 gray scales.

9, the EL display device according to the second embodiment of the present invention is the same as the EL display device according to the first embodiment of the present invention except for the data driver 224. FIG. Accordingly, in the EL display device according to the second embodiment of the present invention, the description of the components except for the data driver 224 will be replaced with the description of the EL display device according to the first embodiment of the present invention.

The EL display device according to the second embodiment of the present invention can adjust the luminance of the EL panel 120 in two ways according to the mode selection signal MD. In this case, the mode selection signal MD becomes the high state HIGH in the bright mode, and the mode selection signal MD becomes the low state HIGH in the dark mode.

To this end, the data driver 224 of the EL display device according to the second embodiment of the present invention modulates digital data input from the outside to have a duty cycle of n steps (where n is a natural number) corresponding to the number of bits. A data modulation circuit (not shown), first and second resistors R1 and R2 connected in series between the second supply voltage source VDD2 and the base voltage source VSS, and the second resistor R2 and the base voltage source ( The first switch element SW1 connected between VSS, the second switch element SW2 connected between the node between the first resistor R1 and the second resistor R2 and the second supply voltage source VDD2. And a third resistor R3 connected between the node between the first resistor R1 and the second resistor R2 and the second switch element SW2.

The data modulation circuit modulates digital data input from the outside to have a duty cycle of n steps corresponding to the number of bits, and supplies the digital data to the gate terminal of the switch element SW. At this time, when the digital data from the outside is 4 bits, the modulated data signal data has a digital value (0 to 0) corresponding to 4 bits while the scan pulse is supplied to the gate line GL as shown in FIG. According to 15, it is dividedly supplied to have a duty cycle of four steps (8, 4, 2, 1). At this time, each step of the modulation data signal data is divided into a read section for turning off the switch element SW and a write section for turning on the switch element SW. Accordingly, 16 grays are represented by the sum of grays represented by the four steps (8, 4, 2, 1) of the modulated data signal data. That is, among the four steps (8, 4, 2, 1), one step expresses eight gradations, the second step four gradations, the third step two gradations, and the fourth step expresses one gradation.

The node between the first and second resistors R1 and R2 is connected to the data line DL. The third resistor R3 is selectively connected in parallel with the first resistor R1 according to the switching of the second switch element SW2.

The first switch element SW1 selectively connects the second resistor R2 to the ground voltage source VSS in accordance with a modulated data signal data supplied from the data modulation circuit. The second switch element SW2 is switched by the input mode selection signal MD to selectively connect the third resistor R3 to the first resistor R1 in parallel.

The data driver 224 turns off the first switch element SW1 according to a read interval of the modulated data signal data supplied to the first switch element SW1 to turn the second switch via the first resistor R1. The voltage from the supply voltage source VDD2, that is, the on-off signal Vdata in the high state HIGH is supplied to the data line DL.

On the other hand, the data driver 224 is a modulated data signal data supplied to the first switch element SW1 when the mode selection signal MD is turned off due to the high state HIGH. The first switch element SW1 is turned on by the write period of R to connect the second resistor R2 to the ground voltage source VSS. Therefore, the on-off signal Vdata of the low state LOW having the first level is supplied to the data line DL connected to the node between the first and second resistors R1 and R2. That is, when the scan pulse is supplied to the gate line GL, the gate terminal of the driving thin film transistor T2 has the second resistor R2 of the switching thin film transistor T1, the data line DL, and the data driver 224. And the first resistor R1 and the second resistor (w) when the first switch element SW1 of the data driver 224 is turned on because it is connected to the base voltage source VSS via the first switch element SW1. Due to the voltage difference between the node between R2 and the first supply voltage source VDD1, the gate terminal of the driving thin film transistor T2 has a base low voltage, that is, an on-off signal of a low state LOW having a first level ( Vdata) is supplied.

On the other hand, the data driver 224 of the modulated data signal data supplied to the first switch element SW1 when the mode selection signal MD is turned on in the low state LOW is turned on. The first switch element SW1 is turned on by the light section to connect the second resistor R2 to the ground voltage source VSS, and the third resistor R3 is connected to the first resistor by the second switch element SW2. It is connected in parallel with (R1). Accordingly, the on-off signal Vdata of the low state LOW having a second level different from the first level is supplied to the data line DL connected to the node between the first and second resistors R1 and R2. do. That is, when the scan pulse is supplied to the gate line GL, the gate terminal of the driving thin film transistor T2 has the second resistor R2 of the switching thin film transistor T1, the data line DL, and the data driver 224. And the first and third resistors R1 and 3 when the first switch element SW1 of the data driver 224 is turned on because it is connected to the base voltage source VSS via the first switch element SW1. The gate terminal of the driving thin film transistor T2 has a base low voltage, i.e., a second level, due to a voltage difference between the parallel resistance of R1 and the voltage on the node between the second resistor R2 and the first supply voltage source VDD1. The on-off signal Vdata of the low state LOW is supplied.

The EL display device and the driving method thereof according to the second embodiment of the present invention have the first and second levels at the gate terminal of the driving thin film transistor T2 of the pixel cell 128 in response to the mode selection signal MD. The voltage Vgs of the gate terminal and the source terminal of the driving thin film transistor T2 is selectively supplied by supplying the on-off signal Vdata of the low state LOW having a second level (Vgs1, Vgs2). In addition, the EL display device and the driving method thereof according to the second embodiment of the present invention, as described in the first embodiment of the present invention, the duty cycle of n steps according to the digital data is applied to the cathode electrode of the EL cell OLED. The drain terminal of the driving thin film transistor T2 with the gate terminal of the thin film transistor T2 and the voltage Vgs of the source terminal fixed at two levels Vgs1 and Vgs2 by supplying a pulse width modulation signal Vs. And the voltage Vds of the source terminal can be reduced so that the operating points Q1 and Q2 of the driving thin film transistor T2 are present in the unsaturated region as shown in FIG. Accordingly, in the EL display device and the driving method thereof according to the second embodiment of the present invention, since the operating points Q1 and Q2 of the driving thin film transistor T2 are in the unsaturated region, the data driver is driven according to the mode selection signal MD. The amount of change in the current Id flowing in the driving thin film transistor T2 due to the variation in the threshold voltage Vth with respect to the fixed Vgs1 and Vgs2 supplied from 224 can be made smaller than before. As a result, the EL display device and the driving method thereof according to the second embodiment of the present invention can compensate for the deviation of the threshold voltage Vth of the driving thin film transistor T2 to prevent deterioration of image quality.

According to the second embodiment of the present invention, as shown in Fig. 8, an EL display device and a driving method thereof have 4 bits of digital data supplied from the outside and a single EL cell (OLED) using the 4 bits of digital data. In the following example, 12 gradations are expressed as follows.

While the scan pulse SP is applied to the gate line GL, the data driver 224 may perform the first step of the modulation data signal 8 and the first step of the digital data, having a duty cycle of which the digital data value corresponds to eight. The modulated data signal 4 of the second stage having a duty cycle whose data value corresponds to 4 is sequentially supplied to the first switch element SW1. Accordingly, the first switching element SW1 responds to each of the modulated data signals 8 and 4 of the first and second stages sequentially supplied from the data driver 224 and according to the first mode selection signal MD. And an on-off signal Vdata in a low state LOW having any one of the second levels is sequentially supplied to the gate terminal of the driving thin film transistor T2 via the switching thin film transistor T1 and simultaneously pulsed. The pulse width modulated signal Vs of the first and second stages is synchronized with each of the first and second stages of the modulated data signals 8 and 4 from the supply unit 140 and has the same duty cycle. Is supplied stepwise to the cathode electrode.

Therefore, the driving thin film transistor T2 is driven by the on-off signal Vdata in the low state LOW having any one of the first and second levels sequentially supplied by the first and second steps. By being turned on, the amount of current Id supplied from the first supply voltage source VDD1 is controlled via the EL cell OLED. At this time, the EL cell OLED emits light during the duty cycle of each of the pulse width modulated signals Vs of the first and second stages supplied to its cathode electrode. Therefore, according to the EL display device and the driving method thereof according to the second embodiment of the present invention, the EL cell OLED is subjected to the first and second steps while the scan pulse SP is supplied to the gate line GL. By emitting light, eight gray scales of the light emission time of the first stage and four gray scales of the light emission time of the second stage are added to represent 12 gray scales. At this time, the 12 gradations represented by the EL table market value and the driving method thereof according to the second embodiment of the present invention are represented by bright 12 gradations or dark 12 gradations according to the mode selection signal MD.

As described above, the electro-luminescence display device and the driving method thereof according to the embodiment of the present invention supply and drive the on-off signal of the high or low state to the driving thin film transistor of the pixel cell and simultaneously drive the EL cell. By supplying the pulse width modulation signal to the cathode electrode and expressing the gray scale by the sum of the light emission time by controlling the light emission time of the EL cell, the voltage between the drain and source terminals of the fixed gate thin film transistor is reduced. By making it small, the driving thin film transistor is operated in an unsaturated region. Accordingly, the present invention can prevent the deterioration of image quality due to the variation of the threshold voltage by reducing the variation of the threshold voltage generated between the driving thin film transistors due to the non-uniformity of the excimer laser irradiated when the driving thin film transistors are formed.

In addition, the electro-luminescence display device and its driving method according to an embodiment of the present invention control the current flowing through the EL cell in two stages according to the mode selection signal and at the same time the pulse width modulation signal on the cathode electrode of the EL cell. The gray level is represented by the sum of the luminous times by controlling the luminous time of the EL cell, thereby reducing the voltage between the drain and source terminals with respect to the voltage between the gate and the source of the fixed thin film transistor. It operates in an unsaturated region. Accordingly, the present invention can prevent the deterioration of image quality due to the variation of the threshold voltage by reducing the variation of the threshold voltage generated between the driving thin film transistors due to the non-uniformity of the excimer laser irradiated when the driving thin film transistors are formed. According to the mode selection signal, the overall luminance of the electroluminescent panel can be adjusted in two modes.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (26)

An electro-luminescence cell connected between a first supply voltage source and a base voltage source and emitting light by a current supplied from the first supply voltage source; A cell driver formed at each intersection of gate lines and data lines, the cell driver including a driving thin film transistor connected between the first supply voltage source and the electro-luminescence cell to control a current flowing in the pixel cell; And a pulse supply unit connected between the electro-luminescence cell and the base voltage source to supply a pulse width modulation signal to the electro-luminescence cell for operating the driving thin film transistor in an unsaturated region. An electroluminescent display device characterized by the above-mentioned. The method of claim 1, A data driver for supplying an on-off signal to the data line for driving the driving thin film transistor; And a gate driver for supplying a scan pulse to the gate line. The method of claim 2, The cell driver, A switch thin film transistor connected to the gate line, the data line, and the driving thin film transistor and supplying an on / off signal on the data line to a gate terminal of the driving thin film transistor in response to the scan pulse; And a storage capacitor connected between the gate terminal of the driving thin film transistor and the first supply voltage source. The method of claim 2, The data driver, First and second resistors connected in series between a second supply voltage source and said base voltage source; And a first switch element connected between said second resistor and said ground voltage source. The method of claim 4, wherein The data driver transmits the on-off signal to the data line in the high state or the low state by a voltage difference between the first supply voltage source and the voltage on the node between the first and second resistors according to the switching of the first switch element. An electroluminescent display device characterized by being supplied. The method of claim 4, wherein The gate terminal of the first switch element has a duty cycle corresponding to the number of bits of digital data while a scan pulse is supplied to the gate line, and is supplied with a modulated data signal divided into N stages (where N is a natural number). An electroluminescent display device characterized by the above-mentioned. The method of claim 6, And the modulation section of each of the N stages has a read section having a first voltage level and a write section having a second voltage level different from the first voltage level. The method of claim 7, wherein The pulse supply unit, And a pulse width modulation signal which is synchronized with the modulation data signal and has the same duty cycle and is divided into the N stages to a cathode terminal of the electro-luminescence cell. The method of claim 8, The pulse width modulated signal of each of the N stages has a read period having the same read period as the voltage level from the first supply voltage source and a level between the read voltage level and the base voltage from the base voltage source. Electroluminescent display device. The method of claim 9, The driving thin film transistor is operated in the non-saturated region by a voltage difference between a drain and a source due to a voltage supplied to a write period of each pulse width modulation signal of each of the N steps with respect to a fixed gate-source voltage. Electroluminescent display device. The method of claim 9, The electro-luminescence cell emits light by the current caused by the voltage level of each write period of each of the N-stage pulse width modulation signals and the voltage difference between the first supply voltage source and the sum of the emission times of each of the N-stages. And a gray level corresponding to the N bits. The method of claim 4, wherein The data driver, A third resistor connected between the node between the first and second resistors and the second supply voltage source; And a second switch element connected between the third resistor and the second supply voltage source and connected in parallel with the third resistor in response to a mode selection signal supplied from the outside. Electro-luminescence display. The method of claim 12, The data driver, In the case where the second switch element is turned off by the mode selection signal, a high state or a high voltage is caused by a voltage difference between the first supply voltage source and the voltage on the node between the first and second resistors according to the switching of the first switch element. Supplying the on / off signal of a low state having a first level to the data line, When the second switch element is turned on by the mode selection signal, the voltage on the node between the parallel resistance of the first and second resistors and the second resistor and the first supply according to the switching of the first switch element. And the on-off signal of a high state or a low state having a second level is supplied to the data line by a voltage difference between voltage sources. The method of claim 13, And the driving thin film transistor has different voltages between first and second gate-sources according to the on-off signal in a low state having the first and second levels. The method of claim 14, And the driving thin film transistor controls the amount of current flowing through the electro-luminescence cell to two levels according to the voltage between the first and second gate-sources. An electroluminescent cell connected between a first supply voltage source and a base voltage source and emitting light by a current supplied from the first supply voltage source, formed at each intersection of gate lines and data lines, and A driving method of an electro-luminescence display device having a cell driver comprising a driving thin film transistor connected between the electro-luminescence cells and controlling a current flowing in the pixel cell. Supplying a pulse width modulation signal to the electro-luminescence cell to operate the driving thin film transistor in an unsaturated region by using a pulse supply unit connected between the electro-luminescence cell and the base voltage source. Method of driving an electro-luminescence display device comprising a. The method of claim 16, Generating an on / off signal for driving the driving thin film transistor; And supplying a scan pulse to the gate line. The method of claim 17, Generating the on-off signal, Generating a modulated data signal having a duty cycle corresponding to the number of bits of the digital data while a scan pulse is supplied to the gate line and divided into N steps (where N is a natural number); And generating the on-off signals in a high state and a low state by using the modulated data signal. The method of claim 18, And the modulation data signal of each of the N stages has a read section having a first voltage level and a write section having a second voltage level different from that of the first voltage level. The method of claim 19, The pulse width modulated signal is synchronized with the modulated data signal, has the same duty cycle, is divided into the N stages, and is supplied to the cathode terminal of the electro-luminescence cell. Driving method. The method of claim 20, The pulse width modulated signal of each of the N stages has a read period having the same read period as the voltage level from the first supply voltage source and a level between the read voltage level and the base voltage from the base voltage source. A method of driving an electroluminescent display device. The method of claim 21, The driving thin film transistor is operated in the non-saturated region by a voltage difference between a drain and a source due to a voltage supplied to a write period of each pulse width modulation signal of each of the N steps with respect to a fixed gate-source voltage. A method of driving an electroluminescent display device. The method of claim 21, The electro-luminescence cell emits light by the current caused by the voltage level of each write period of each of the N-stage pulse width modulation signals and the voltage difference between the first supply voltage source and the sum of the emission times of each of the N-stages. And a gray level corresponding to the N bits by using the same. The method of claim 17, Generating the on-off signal, Generating the on-off signal in a low state having a high state or a first level by a mode selection signal; And generating the on-off signal having a high state or a low state having a second level by the mode selection signal. The method of claim 24, The driving thin film transistor may have different voltages between first and second gate-sources according to the on-off signal in a low state having the first and second levels. Driving method. The method of claim 25, The driving thin film transistor controls the magnitude of the current flowing in the electro-luminescence cell to two levels according to the voltage between the first and second gate-sources. Way.

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