KR100581804B1 - Pixel circuit and driving method thereof and organic light emitting display using the same - Google Patents

Abstract

The present invention relates to a pixel circuit capable of supplying a uniform current to the light emitting element without being substantially affected by the voltage drop of the power supply voltage supplied to the pixel circuit, a driving method thereof, and an organic light emitting display device employing the pixel circuit. . The pixel circuit according to the present invention includes a first transistor, a second transistor sequentially transmitting an initialization voltage and a data voltage in response to the first scan signal, and a power supply voltage on a power line in response to the second scan signal. A third transistor configured to transfer the gate of the first transistor; a first capacitor configured to store a first voltage corresponding to a voltage obtained by subtracting the initialization voltage; and a first voltage corresponding to a voltage difference between the power supply voltage and the initialization voltage, And a second capacitor configured to transfer the data voltage to the gate of the first transistor, wherein the first transistor supplies current on the power line to the light emitting device according to the first voltage applied to the gate.

OLED display, pixel circuit, voltage drop, uniform pixel current

Description

Pixel circuit and driving method according to the invention and organic light emitting display using the same}             

1 is a conceptual diagram of a general organic light emitting device.

2 is an equivalent circuit diagram of a pixel circuit according to an exemplary embodiment of the present invention.

FIG. 3 is a driving waveform diagram of the pixel circuit shown in FIG. 2.

4 is an equivalent circuit diagram of a modified example of a pixel circuit according to an exemplary embodiment of the present invention.

FIG. 5 is a driving waveform diagram of the pixel circuit shown in FIG. 4.

6 is a configuration diagram of a light emitting display device employing a pixel circuit according to an exemplary embodiment of the present invention.

FIG. 7 is a driving waveform diagram of the light emitting display device shown in FIG. 6.

8 is a graph illustrating a pixel current according to a voltage drop of a light emitting display device employing a pixel circuit according to an exemplary embodiment of the present invention compared with a conventional light emitting display device.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pixel circuit of a light emitting display device, and more particularly, to a pixel circuit capable of supplying a uniform current to a light emitting device without being substantially influenced by a voltage drop of a power supply voltage, and a driving method thereof and employing the same. The present invention relates to a light emitting display device.

In general, a light emitting display device refers to a device that displays an image using a self-luminous element. Such light emitting display devices include an inorganic light emitting display device using an inorganic material for a light emitting layer and an organic light emitting display device using an organic material for a light emitting layer.

The organic light emitting diode display is a display device using a phenomenon in which electrons and holes injected through a cathode and an anode are recombined into an organic thin film to form excitons, and light of a specific wavelength is generated from the formed excitons. The organic thin film has a multilayer structure including a hole transport layer, an emitting layer, and an electron transport layer in order to improve luminous efficiency. In addition, the organic thin film may include an electron injection layer, a hole injection layer, or the like in order to improve the injection efficiency of electrons or holes and to balance electrons and holes.

The driving method of the organic light emitting diode display includes a passive matrix method and an active matrix method. In the passive matrix method, the anode and the cathode are formed to be orthogonal and the lines are sequentially selected and driven. The organic light emitting display device using the passive matrix method is easy to implement because its structure is simple. However, the large screen consumes a large amount of current and shortens the driving time of each light emitting device. The active matrix method is a method of controlling the amount of current flowing through each light emitting device by using an active device. As an active element, a thin film transistor (hereinafter referred to as TFT) is mainly used. The active matrix method is somewhat complicated but has the advantage of low current consumption and long emission time.

1 is an equivalent circuit diagram of a general pixel circuit of a conventional organic light emitting display device. In FIG. 1, the pixel circuit is a voltage programming pixel circuit and includes an nth scan line Sn and an mth data line Dm, and two power supply voltages VDD and VSS in an organic light emitting diode display including N * M pixels. ) May be employed as a pixel circuit connected to

As shown in FIG. 1, a conventional pixel circuit controls an organic light emitting diode (OLED). To this end, the conventional pixel circuit includes a first transistor M1, a second transistor M2, and a storage capacitor Cst.

The first transistor M1 is connected between the first power supply voltage VDD and the organic light emitting diode OLED to control a current flowing in the organic light emitting diode OLED. The second transistor M2 transfers the data voltage applied to the data line Dm to the gate of the first transistor M1 in response to a scan signal applied from the scan line Sn. The storage capacitor Cst is connected between the source and the gate of the first transistor M1 and charges the data voltage to maintain a predetermined period.

Specifically, when the second transistor M2 is turned on by the scan signal applied to the gate of the second transistor M2, the data voltage applied to the data line Dm is connected to the gate of the first transistor M1. Is applied to the first electrode of the storage capacitor Cst. In this case, the storage capacitor Cst stores a voltage corresponding to the data voltage, more specifically, a voltage corresponding to the voltage difference between the first power supply voltage VDD and the data voltage. Thereafter, a substantially uniform current flows through the first transistor M1 for a predetermined time in response to the voltage stored in the storage capacitor Cst and is provided to the organic light emitting diode OLED. The organic light emitting diode OLED emits light in proportion to a current flowing within a predetermined range.

At this time, the current flowing through the OLED is represented by Equation 1 below.

Here, I _OLED is a current flowing through the OLED, V _GS is a voltage between the gate and the source of the first transistor M1, V _TH is a threshold voltage of the first transistor M1, and V _DD is a power supply of the pixel. The voltage, V _DATA is the data voltage, and β is the constant value.

As shown in Equation 1, in the pixel circuit shown in FIG. 1, a current corresponding to an applied data voltage is supplied to the organic light emitting element OELD, and the organic light emitting element emits light corresponding to the supplied current. At this time, the applied data voltage has a multi-level value in a predetermined range in order to express the gray scale.

However, in the above-described conventional voltage programming pixel circuit, since the current flowing through the organic light emitting element OLED is affected by the power supply voltage VDD, a voltage drop occurs on the power supply line supplying the power supply voltage VDD. When the power supply voltages VDD applied to the plurality of pixel circuits are not the same, a desired amount of current does not flow to the organic light emitting diode OLED, resulting in a problem of deterioration in image quality. This problem becomes more problematic as the area of the organic light emitting diode display increases and the luminance increases, so that the voltage drop increases in the power line that transmits the power voltage VDD.

Accordingly, the present invention was derived to solve the above-described conventional problems, and an object of the present invention is to influence the voltage drop even when a voltage drop occurs on a power supply that transfers a power supply voltage to a pixel circuit controlling a light emitting device. The present invention provides a pixel circuit and a driving method thereof of a light emitting display device capable of supplying a uniform current to a light emitting device without receiving the same.

Another object of the present invention is to provide a light emitting display device suitable for large area and high brightness using the pixel circuit described above.

In order to achieve the above object, according to an aspect of the present invention, a first transistor, a second transistor that sequentially transfers an initialization voltage and a data voltage in response to the first scan signal, and a response to the second scan signal. A third transistor that transfers the power supply voltage on the power supply line to the gate of the first transistor, a first capacitor that stores a first voltage corresponding to a voltage obtained by subtracting the initialization voltage from the data voltage, and a voltage difference between the power supply voltage and the initialization voltage. And a second capacitor configured to store a first voltage corresponding to the first voltage and to transfer a data voltage to the gate of the first transistor, wherein the current is supplied to the light emitting device in response to the first voltage applied to the gate. A pixel circuit of a light emitting display device to be supplied is provided.

According to another aspect of the present invention, a first transistor having a first electrode connected to a power line, and a second electrode connected to a light emitting element, a first electrode connected to a data line, and a first scan line A second transistor having a gate, a first electrode connected to a power line, a second electrode connected to a gate of the first transistor, a third transistor having a gate connected to a second scan line, and connected to a power line A first capacitor having a first electrode and a second electrode connected to the gate of the first transistor, a first electrode connected to the gate of the first transistor, and a second electrode connected to the second electrode of the second transistor A pixel circuit of a light emitting display device including a second capacitor provided with is provided.

Preferably, in the pixel circuit of the light emitting display device, during the first period, the second transistor is turned on in response to the first scan signal at the enable level, and the third transistor is turned on in response to the second scan signal at the enable level. The second voltage is stored in the second capacitor. Next, during the second period, the third transistor is turned off in response to the second scan signal having the disable level, and the first voltage is stored in the first capacitor. Next, during the third period, the second transistor is turned off in response to the first scan signal having the disable level, and the first transistor supplies a current to the light emitting device in response to the first voltage.

According to still another aspect of the present invention, there is provided a pixel circuit according to any one of claims 1 to 4, and a plurality of pixels each having an organic light emitting element controlled by the pixel circuit, and a plurality of pixels with an initialization voltage. And a plurality of data lines for transmitting a data voltage, and a plurality of scan lines for transmitting a scan signal to the plurality of pixels.

According to still another aspect of the present invention, in a method of driving a pixel circuit of a light emitting display device including first to third transistors and first and second capacitors, during a on period of the second and third transistors, During the step of storing the second voltage corresponding to the voltage difference between the initialization voltage transferred through the transistor and the power supply voltage transferred through the third transistor in the second capacitor, during the on period of the second transistor and the off period of the third transistor, A data voltage transferred through the second transistor is transferred to a gate of the first transistor through a second capacitor, and a first voltage corresponding to a voltage obtained by subtracting an initialization voltage from the data voltage is stored in the first capacitor; and During the off periods of the second and third transistors, the electric power on the power supply line which transfers the power supply voltage according to the first voltage applied to the gate of the first transistor. A method of driving a pixel circuit of a light emitting display device comprising the step of supplying a type to a light emitting element via a first transistor is provided.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, when a part is connected to another part, it includes not only the case where it is directly connected but also the case where it is electrically connected with another element between them. In addition, in the following description, a transistor includes a source, a drain, and a gate, or a first electrode representing a source or a drain, a second electrode representing a drain or a source, and a gate. It explains. In the drawings, parts irrelevant to the present invention are omitted for clarity, and like reference numerals designate like parts throughout the specification.

2 is an equivalent circuit diagram of a pixel circuit according to an exemplary embodiment of the present invention. In FIG. 2, the pixel circuit is formed of a PMOS circuit in which the transistor is composed of only PMOS transistors. Therefore, in the present embodiment, the first electrode of the transistor becomes a source and the second electrode becomes a drain.

Referring to FIG. 2, the pixel circuit controls the organic light emitting diode OLED by supplying a current to the organic light emitting diode OLED displaying colors of red, green, blue, and white. To this end, the pixel circuit includes a first transistor M1, a second transistor M2, and a data holding unit 210. In other words, the pixel circuit is connected to the first power supply voltage VDD and the second power supply voltage VSS by a scan signal and a data signal applied to the scan lines Sn1 and Sn2 and the data line Dm, respectively. Control the device OLED. In the present embodiment, the second power supply voltage VSS becomes a common voltage commonly connected to the ground voltage or the reference voltage.

In detail, the first transistor M1 includes a first electrode connected to the first power supply voltage VDD, a second electrode connected to the anode of the OLED, and a gate connected to the data holding part 210. It is provided. The first transistor M1 provides a predetermined current to the organic light emitting diode OLED according to the gate voltage maintained by the data holding unit 210.

The second transistor M2 includes a first electrode connected to the data line Dm, a second electrode connected to the data holding part 210, and a gate connected to the first scan line Sn1. The second transistor M2 transmits a data signal applied to the data line Dm to the data holding part 210 in response to the first scan signal having the enable level transmitted through the first scan line Sn1. Here, since the second transistor M2 is a PMOS transistor, the first scan signal of the enable level becomes a low level signal. In addition, in the present invention, the data signal transmitted to the data holding unit includes an initialization voltage and a data voltage.

The data holding unit 210 maintains the gate voltage of the first transistor at a voltage level corresponding to the data voltage without being substantially affected by the voltage drop of the first power supply voltage VDD. To this end, the data holding part 210 includes a third transistor M3, a first capacitor C1, and a second capacitor C2.

The third transistor M3 includes a first electrode connected to the first power supply voltage VDD, a second electrode connected to the gate of the first transistor M1, and a gate connected to the second scan line Sn2. . The third transistor M3 is connected to the gate of the first transistor M1 in response to the second scan signal having the enable level applied to the second scan line Sn2, that is, the first power voltage VDD at the node A. ).

The first capacitor C1 includes a first electrode connected to the first power supply voltage VDD and a second electrode connected to the gate of the first transistor M1. The first capacitor C1 stores a first voltage applied between the first power voltage VDD and the node A. Here, the first voltage is a voltage corresponding to a voltage obtained by subtracting the initialization voltage from the data voltage. In other words, the first voltage is divided by the first and second capacitors from the data voltage minus the initialization voltage to become a divided voltage applied to the first capacitor. At this time, the initialization voltage in this embodiment becomes a voltage higher than the data voltage and lower than the power supply voltage.

The second capacitor C2 includes a first electrode connected to the gate of the first transistor M1, and a second electrode connected to the second electrode of the second transistor M2. The second capacitor C2 stores the voltage between the node A and the node (node B) to which the drain of the second transistor M2 is connected. In addition, the second capacitor C2 transfers the data voltage applied after the initialization voltage to the gate of the first transistor.

By the above-described configuration, the pixel circuit according to the present invention can supply a uniform current to the light emitting element substantially without being affected by the voltage drop of the power supply voltage.

Next, the driving of the pixel circuit of the present invention shown in FIG. 2 will be described with reference to FIG. 3 is a driving waveform diagram of the pixel circuit of FIG. 2.

Referring to FIGS. 2 and 3, the pixel circuit may include a first period in which the data holding unit 210 is initialized with an initialization voltage transferred through the data line, and a data holding unit initialized with the data voltage transferred through the data line ( A second period for programming 210 and a third period for providing a current to the organic light emitting diode OLED by operating the first transistor M1 by the programmed voltage are driven.

Specifically, in the first period T1, the first scan signal having the enable level is applied to the gate of the second transistor M2, and the second scan signal having the enable level is applied to the gate of the third transistor M3. In this case, the initialization voltage is applied to the node B through the second transistor M2, and the first power supply voltage VDD is applied to the node A through the third transistor M3. The second capacitor C2 is connected between the node A and the node B to store a voltage applied between both ends. The voltage stored in the second capacitor C2 is referred to as a second voltage.

Next, in the second period T2, when the second scan signal having the disable level is applied to the gate of the third transistor M3, and the voltage applied to the data line Dm is changed from the initialization voltage to the data voltage, The voltage of the node B becomes the data voltage, and the amount of change of the voltage of the node B is transmitted to the node A to which the gate of the first transistor is connected through the second capacitor.

At this time, the change amount of the voltage of the node B is as shown in Equation 2 below.

Here, ΔV _DATA is the amount of change in the voltage of the node B, V _DATA2 is the data voltage, and V _DATA1 is the initialization voltage.

Therefore, the voltage of the node A becomes a voltage obtained by subtracting the voltage distributed by the first capacitor and the second capacitor from the power supply voltage applied to the node A in the first period from the change amount of the voltage minus the initialization voltage.

At this time, the voltage applied to the node A is as shown in Equation 3 below.

Where V _A is the voltage of node A, V _DD is the power supply voltage, ΔV _DATA is the voltage minus the initialization voltage, C1 is the capacity of the first capacitor C1, and C2 is the capacity of the second capacitor C2. Indicates.

Therefore, the first capacitor C1 stores a predetermined voltage corresponding to the voltage of the first power supply voltage VDD and the node A. FIG. Here, the voltage stored in the first capacitor C1 is referred to as a first voltage.

At this time, the voltage stored in the first capacitor C1 becomes as shown in Equation 4 below.

Here, V _C1 represents a voltage stored in the first capacitor C1.

Next, in the third period T3, the first transistor M1 provides a predetermined current to the organic light emitting diode OLED corresponding to the first voltage applied to the gate. In more detail, the first capacitor C1 is connected between the source and the gate of the first transistor M1. Therefore, the voltage (first voltage) stored in the first capacitor C1 is applied to the gate of the first transistor M1 as the gate voltage. In this case, when the first voltage is higher than the threshold voltage of the first transistor M1, the first transistor M1 provides a current having a predetermined level corresponding to the first voltage to the organic light emitting diode OLED. As such, in the third section, the organic light emitting diode OLED emits light at a predetermined level corresponding to the current provided through the first transistor M1.

At this time, the current flowing through the OLED is represented by Equation 5 below.

Where I _OLED is the current flowing through the OLED, V _GS is the gate-to-source voltage of the first transistor M1, V _TH is the threshold voltage of the first transistor M1, and ΔV _DATA is the data voltage. Voltage minus the initialization voltage, C1 represents the capacity of the first capacitor, and C2 represents the capacity of the second capacitor.

As can be seen from Equation 5, the pixel circuit according to the present invention provides a substantially uniform current to the organic light emitting device without being substantially affected by the voltage drop of the power supply voltage VDD. Therefore, according to the present invention, it is possible to provide a new pixel circuit which is not affected by the voltage drop of the power supply voltage of the pixel circuit.

Meanwhile, in FIG. 3, after the second scan signal applied to the second scan line Sn2 is changed from the low level to the high level at the end of the first period, the data signal applied to the data line Dm is initialized to the high level. The change from the voltage to the low level data voltage is an allowable width so that the initialization voltage can be properly transferred during the first period. Similarly, after the first scan signal applied to the first scan line Sn1 is changed from the low level to the high level at the end of the second period, the data signal applied to the data line Dm is changed from the low level data signal. The change to the high level initialization voltage is a margin for properly transferring the data voltage during the second period. In addition, if the second scan signal applied to the second scan line Sn2 in the first section can store the second voltage in the second capacitor, the first scan signal applied to the first scan line Sn1. It is not applied at the same time as, and may be applied with a predetermined margin before or after the first scan signal is applied.

4 is an equivalent circuit diagram of a modified example of a pixel circuit according to an exemplary embodiment of the present invention. In Fig. 4, the pixel circuit is formed of an NMOS circuit in which the transistor is composed only of NMOS transistors. Therefore, in the present embodiment, the first electrode of the transistor represents a drain, and the second electrode represents a source. FIG. 5 is a driving waveform diagram of the pixel circuit of FIG. 4.

Referring to FIG. 4, the pixel circuit supplies a predetermined current to the organic light emitting diode OLED to control the organic light emitting diode OLED displaying a color such as red, green, blue, or white. In other words, the pixel circuit is connected to the first power supply voltage VDD and the second power supply voltage VSS by a scan signal and a data signal applied to the scan lines Sn1 and Sn2 and the data line Dm, respectively. Control the device OLED. To this end, the pixel circuit includes a first transistor M1, a second transistor M2, and a data holding unit 410. In particular, in the pixel circuit of the present exemplary embodiment, the data holding part 410 maintains the gate voltage of the first transistor at a predetermined data voltage level without being affected by the voltage drop of the second power supply voltage VSS. The second power supply voltage VSS is independently applied to each pixel circuit in order to selectively control each organic light emitting element.

In the present embodiment, the pixel circuit is substantially the same as the pixel circuit described above with reference to FIG. 2 except that the transistor is formed of an NMOS transistor. Therefore, detailed description of the pixel circuit of this embodiment is omitted to avoid duplication.

Then, the pixel circuit of this embodiment is driven as shown in FIG. This driving method is described above except that the first scan signal is applied at a high level in the first and second sections, the second scan signal is applied at a high level in the first sections, and is applied at a low level in other sections. It is substantially the same as the driving method of the pixel circuit described with reference to FIG. Therefore, the detailed description of the driving method of the pixel circuit of this embodiment is omitted to avoid duplication.

However, in order to explain that the pixel circuit according to the present embodiment can supply a substantially uniform current to the light emitting device without being affected by the voltage drop of the power supply voltage, the voltage of the node A, the voltage of the node A, and the second The voltages stored in the first capacitor C1 having the power supply voltage VSS as the voltage at both ends thereof are represented by Equations 6 and 7 below.

Where V _A is the voltage of node A, ΔV _DATA is the voltage minus the initialization voltage, C1 is the capacity of the first capacitor C1, C2 is the capacity of the second capacitor C2, and V _SS is the second voltage. The power supply voltage VSS is shown.

Where V _C1 is the voltage stored in the first capacitor C1, ΔV _DATA is the voltage obtained by subtracting the initialization voltage from the data voltage, C1 is the capacity of the first capacitor C1, C2 is the capacity of the second capacitor C2, And V _SS represents the second power supply voltage VSS.

As shown in Equation 7, since the voltage stored in the first capacitor C1 is not affected by the second power supply voltage VSS, the first transistor M1 is connected between the gate and the source. ) May supply a current to the light emitting device without being affected by the second power supply voltage VSS. In other words, the pixel circuit of the present embodiment provides the organic light emitting device with a uniform current without being affected by the voltage drop of the second power supply voltage VSS as in Equation (5). Therefore, according to the present invention, there is provided a new pixel circuit capable of supplying a uniform current to the organic light emitting element without being affected by the voltage drop of the power supply voltage of the pixel circuit.

6 is a configuration diagram of a light emitting display device employing a pixel circuit according to an exemplary embodiment of the present invention. 7 is a driving waveform diagram of the light emitting display device of FIG. 6. In Fig. 7, the initialization voltage and the data voltage applied to each data line are shown superimposed as if they are applied to one data line D.

6 and 7, the light emitting display device basically includes an image display unit 630 for displaying an image. The light emitting display device includes a scan line and a data line which respectively transmit a scan signal and a data signal to the image display unit 630. In addition, the light emitting display device includes a scan driver 610 that supplies a scan signal on a scan line, and a data driver 620 that supplies a data signal on a data line.

Specifically, the image display unit 630 includes a plurality of scan lines S11, S12, S21, S22, ... Sn1, Sn2 extending in the row direction, a plurality of data lines D1, D2, ... Dm extending in the column direction, And a plurality of pixels 640 each connected to a scan line, a data line, and a power line for transmitting a power supply voltage.

The pixel 640 includes an organic light emitting element and a pixel circuit for controlling the organic light emitting element. The pixel circuit of FIG. 2 according to an embodiment of the present invention is employed as the pixel circuit included in the pixel 640, and the pixel circuit is driven according to the driving method illustrated in FIG. 3.

The above-described scan lines S11, S12, S21, S22, ... Sn1, Sn2 transfer scan signals for operating the pixel circuit to the pixel circuit. In this case, two scan signals are applied to each pixel circuit in pairs. For example, as shown in FIG. 7, the first and second scan signals by the first and second scan lines S11 and S12 are respectively applied to the pixel circuits of the first row, and the pixel circuits of the second row are next. The other first and second scan lines S21 and S22 are applied to the pixel circuits, and then the other first and second scan lines Sn1 and Sn2 are sequentially applied to the pixel circuits of the n-th row.

The data lines D1, D2, ... Dm sequentially transfer the initialization voltage for initializing the data retention portion and the data voltage corresponding to the image signal to each pixel circuit. For example, in FIG. 7, the initialization voltage and the data voltage transmitted through each data line D1, D2,... Dm are shown superimposed on the data line D, where the high level signal represents the initialization voltage. The low level signal represents a data voltage transmitted to each pixel circuit in correspondence with each scan signal.

In the light emitting display device according to the present exemplary embodiment, a voltage corresponding to an image signal is transferred to a gate of a driving transistor of each pixel circuit by using a voltage difference between an initialization voltage and a data voltage.

As described above, in the conventional light emitting display device, the plurality of pixel circuits in the image display unit 630 are configured to be connected to power lines that supply power voltages VDD / VSS, respectively. Therefore, in the image display unit 630, a voltage drop occurs due to a parasitic resistance, a parasitic capacitor component, or the like present in the power supply line for supplying the power supply voltages VDD / VSS. However, the light emitting display device according to the present embodiment employs a pixel circuit which is not affected by the voltage drop of the power supply voltage, so that a current flowing through the organic light emitting diode OLED is applied to the power supply line supplying the power supply voltage VDD / VSS. It is not affected by the voltage drop of. Therefore, the light emitting display device according to the present embodiment can supply a very uniform current to the light emitting device as compared with the related art, thereby improving image quality and increasing luminance uniformity.

Meanwhile, the above-described scan driver 610 and / or data driver 620 may be electrically connected to an image display unit 630 such as a display panel, and on the other hand, the tape may be adhered to and electrically connected to the image display unit 630. It may be mounted in the form of a chip or the like in a carrier carrier package (TCP). On the other hand, the scan driver 610 and / or the data driver 620 may be bonded to the image display unit 630 and electrically connected to a flexible printed circuit (FPC) or film. It may be mounted in the form of. This structure is commonly referred to as a chip on flexible board, or chip on film (COF) method. On the other hand, the scan driver 610 and / or the data driver 620 may be directly mounted on the glass substrate of the image display unit 630 and formed of the same layers as the scan line, the data line, and the thin film transistor on the glass substrate. It can be mounted in a drive circuit.

8 is a graph illustrating a pixel current according to a voltage drop of a light emitting display device employing a pixel circuit according to an exemplary embodiment of the present invention compared with a conventional light emitting display device. In FIG. 8, (a) shows a pixel current according to a voltage drop of a conventional light emitting display device, and (b) shows a pixel current according to a voltage drop of a light emitting display device according to the present invention.

Referring to FIG. 8, in the conventional light emitting display device as shown by the curve (a), a large voltage drop (IR-drop) is generated on a power supply line (VDD line) connected to a pixel circuit while moving from left to right in the drawing. Accordingly, it can be seen that the current (EL current) flowing through the light emitting element of each pixel circuit is greatly changed.

Meanwhile, in the light emitting display device according to the present invention as shown by the curve (b) of FIG. 8, a voltage drop (IR−) similar to the curve (a) in the power supply line (VDD line) connected to the pixel circuit while moving from left to right in the drawing. Although drop is generated, it can be seen that the current EL current flowing through the light emitting element of each pixel circuit is almost constant.

As described above, in the light emitting display device according to the present invention, as shown in Equation 5, the voltage difference between the data voltage and the initialization voltage, the capacitance of the first capacitor, the capacitance of the second capacitor, and the first transistor (driving transistor). By using the pixel circuit having the threshold voltage as a factor, even when a voltage drop occurs in the power supply voltage as in the curve (a) shown in FIG. 8, the voltage drop of the power supply voltage is not affected. Substantially uniform current can be supplied to each light emitting element. With such a configuration, the present invention provides a light emitting display device in which image quality is improved and luminance uniformity is improved.

On the other hand, in the above description, the pixel circuit of the present invention is formed of a PMOS circuit composed only of P-type transistors or an NMOS circuit composed only of N-type transistors. However, the present invention can form the pixel circuit not only in such a configuration but also in a CMOS transistor structure in which a PMOS transistor and an NMOS transistor are formed together in one pixel circuit. For example, in the pixel circuit of the PMOS circuit described above, the second transistor may be formed as an NMOS transistor, and in the pixel circuit of the NMOS circuit described above, the second transistor may be formed as a PMOS transistor.

In addition, the MOS transistor in the pixel circuit of the present invention is mentioned as an example. Therefore, the pixel circuit of the present invention can be formed of other kinds of transistors in addition to the MOS transistors. For example, a first electrode, a second electrode, and a third electrode are provided, and the amount of current flowing from the second electrode to the third electrode can be controlled by a voltage applied between the first electrode and the second electrode. It can be implemented as an active element.

In addition, the above-described second and third transistors M2 and M3 are elements for switching electrodes on both sides in response to a scan signal, and may be implemented using various switching elements capable of performing the same function. .

As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various deformation | transformation is performed by a person with ordinary skill in the art within the technical idea of this invention. It is possible.

According to the present invention, it is possible to provide a pixel circuit for supplying a uniform current to the light emitting element without being substantially affected by the voltage drop of the power supply voltage.

Further, by employing the pixel circuit according to the present invention, it is possible to provide a light emitting display device suitable for large area and high brightness.

Claims (15)

A first transistor; A second transistor sequentially transmitting an initialization voltage and a data voltage in response to the first scan signal; A third transistor transferring a power supply voltage on a power supply line to a gate of the first transistor in response to a second scan signal; A first capacitor configured to store a first voltage corresponding to a voltage obtained by subtracting the initialization voltage from the data voltage; And A second capacitor configured to store a first voltage corresponding to a voltage difference between the power supply voltage and the initialization voltage, and to transfer the data voltage to a gate of the first transistor, And the first transistor supplies a current on the power supply line to a light emitting device in response to the first voltage applied to a gate. The method of claim 1, And wherein the first voltage is a voltage distributed by the first capacitor and the second capacitor. A first transistor having a first electrode connected to a power line and a second electrode connected to a light emitting element; A second transistor having a first electrode connected to the data line and a gate connected to the first scan line; A third transistor having a first electrode connected to the power line, a second electrode connected to a gate of the first transistor, and a gate connected to a second scan line; A first capacitor having a first electrode connected to the power line, and a second electrode connected to the gate of the first transistor; And And a second capacitor having a first electrode connected to the gate of the first transistor and a second electrode connected to a second electrode of the second transistor. The method of claim 3, During the first period, the second transistor is turned on in response to the first scan signal at the enable level, the third transistor is turned on in response to the second scan signal at the enable level, and the second capacitor is turned on. 2 voltages are stored, During the second period, the third transistor is turned off in response to the second scan signal at the disable level, and the first voltage is stored in the first capacitor. During the third period, the second transistor is turned off in response to the first scan signal at the disable level, and the pixel of the light emitting display device in which the first transistor supplies current to the light emitting element in response to the first voltage. Circuit. The method according to any one of claims 1 to 4, And the first to third transistors are formed of transistors having channels of the same type as each other. The method of claim 5, The first to third transistors are formed of transistors having a P-type channel, wherein the first electrode is a source electrode, and the second electrode is a drain electrode. The method of claim 6, And the initialization voltage is higher than the data voltage and lower than the power voltage. The method of claim 5, The first to third transistors are formed of transistors having an N-type channel, wherein the first electrode is a drain electrode, and the second electrode is a source electrode. The method of claim 8, And the initialization voltage is higher than the power voltage and lower than the data voltage. The method according to any one of claims 1 to 4, And the second transistor is formed of a transistor having a channel of a different type from the third transistor. A plurality of pixels each having the pixel circuit according to any one of claims 1 to 4 and an organic light emitting element controlled by the pixel circuit; A plurality of data lines transferring the initialization voltage and the data voltage to the plurality of pixels; And And a plurality of scan lines for transmitting the scan signals to the plurality of pixels. The method of claim 11, And a scan driver connected to the plurality of scan lines to supply the scan signal. The method of claim 12, And a data driver connected to the plurality of data lines to supply the initialization signal and the data signal. A driving method of a pixel circuit of a light emitting display device comprising first to third transistors and first and second capacitors, During the on periods of the second and third transistors, a second voltage corresponding to a voltage difference between an initialization voltage transferred through the second transistor and a power supply voltage transferred through the third transistor is stored in the second capacitor. step; During an on period of the second transistor and an off period of the third transistor, a data voltage transferred through the second transistor is transferred to the gate of the first transistor through the second capacitor, and the initialization is performed at the data voltage. Storing a first voltage corresponding to the voltage minus the voltage in the first capacitor; And During the off periods of the second and third transistors, a current on a power supply line which transfers the power supply voltage according to the first voltage applied to the gate of the first transistor is supplied to the light emitting device through the first transistor. A method of driving a pixel circuit of a light emitting display device comprising the step. The method of claim 14, And storing the first voltage in the first capacitor comprises storing the voltage distributed by the second capacitor and the first capacitor in the first capacitor.

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