KR102486082B1 - Electroluminescence display and pixel circuit thereof - Google Patents

Abstract

The present invention relates to an electroluminescent display device and a pixel circuit thereof, and includes a driving transistor for driving a light emitting element and a current control transistor for adjusting a current supplied to the driving transistor when a pixel driving voltage changes. The light emitting element operates within a linear region of the driving transistor.

Description

Electroluminescent display device and its pixel circuit

[0001] The present invention relates to an electroluminescent display device having driving elements for driving light emitting elements of sub-pixels and a pixel circuit thereof.

The electroluminescent display device is divided into an inorganic light emitting display device and an organic light emitting display device according to the material of the light emitting layer. Since an active matrix type organic light emitting display device reproduces an input image by using an organic light emitting diode (hereinafter referred to as "OLED") that emits light by itself in each sub-pixel, response speed It is fast and has high luminous efficiency, luminance, and viewing angle. In addition, since the organic light emitting display device can express a black gradation as complete black, it can reproduce an image with a superior level in contrast ratio and color gamut.

The pixels of the organic light emitting display device include an OLED and a driving element that drives the OLED by supplying a current to the OLED according to a voltage between a gate and a source. An OLED of an organic light emitting display device includes an anode, a cathode, and an organic compound layer formed between the electrodes. The organic compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer, EIL). When current flows through the OLED, holes passing through the hole transport layer (HTL) and electrons passing through the electron transport layer (ETL) move to the light emitting layer (EML) to form excitons, and as a result, the light emitting layer (EML) emits visible light. .

As shown in FIGS. 1 and 2 , each of the sub-pixels of the organic light emitting display device includes a driving element DT for driving an OLED. The driving element DT may be implemented as a transistor having a metal oxide semiconductor field effect transistor (MOSFET) structure. The current of the OLED operates in a saturation region (SR) on the current-voltage characteristic curve (hereinafter referred to as “TR IV curve”, 11) of the driving element DT. The operating point of the OLED is determined by the point where the current-voltage characteristic curve of the OLED (hereinafter “OLED IV curve”, 12) and the TR IV curve 11 meet within the saturation region (SR) of the TR IV curve 11 . In FIG. 2 , the horizontal axis is the drain-to-source voltage (VDS) of the driving element (DT), and the vertical axis is the current of the OLED. In FIG. 2 , reference numeral “LR” denotes a linear region of the TR IV curve 11 .

Pixel power sources EVDD and EVSS are supplied to sub-pixels of the organic light emitting display. The pixel power sources EVDD and EVSS include a pixel driving voltage EVDD and a pixel base voltage EVSS. Voltages of these pixel power supplies (EVDD, EVSS) may vary according to pixel positions on the screen due to wiring resistance. Hereinafter, the pixel driving voltage EVDD is referred to as “EVDD” and the pixel base voltage EVSS is referred to as “EVSS”. EVDD may drop due to the wiring resistance of the EVDD, and EVSS may rise due to the EVSS wiring resistance. When the operating point of the OLED is determined in the saturation region SR of the TR IV curve 11, the OLED current can be maintained constant even if EVDD and EVSS fluctuations occur due to the EVDD drop and EVSS rising.

The electrical characteristics of the driving element should be uniform among all pixels, but there may be differences between the pixels due to process variation and element characteristic variation, and may change according to the lapse of display driving time. Such changes in electrical characteristics of the driving element may cause an afterimage problem on the screen of the organic light emitting display device.

An internal compensation circuit or an external compensation circuit may be applied to the organic light emitting diode display to compensate for variations in electrical characteristics of the driving element. An internal compensation circuit is built into each pixel to sample the gate-source voltage (Vgs) of the driving device, which varies according to the electrical characteristics of the driving device, and compensates for the data voltage by the gate-source voltage. The external compensation circuit senses the voltage of the pixel, which varies according to the electrical characteristics of the driving element, and modulates data of an input image in an external circuit based on the sensed voltage, thereby compensating for deviations in electrical characteristics of the driving element between pixels.

In order for the operating point of the OLLED to be determined on the saturation region SR of the TR IV curve 11, it is difficult to reduce power consumption because a voltage difference between EVDD and EVSS (EVDD-EVSS) of 15 V or more is required. When the OLED operates in the saturation region (SR), the operating point of the OLED is moved to the linear region (LD) when EVDD is reduced to reduce power consumption. In this case, a current deviation of the OLED occurs due to variations in EVDD, EVSS, and OLED device characteristics, and the current deviation causes a difference in luminance between pixels, resulting in a stain on the screen and deterioration of image quality.

Accordingly, the present invention provides an electroluminescent display capable of reducing power consumption without deteriorating image quality and a pixel circuit thereof.

An electroluminescent display device of the present invention includes a display panel on which a plurality of pixels are disposed, to which data lines and scan lines cross each other and to which a pixel driving voltage and a pixel base voltage are supplied; a data driver supplying data voltages to the data lines; and a scan driver supplying scan signals to the scan lines.
A pixel circuit of each of the pixels includes a light emitting element; a driving element including a gate electrode, a first electrode, and a second electrode to drive the light emitting element; a first switch element connected to a gate electrode to which a first scan signal is applied, a first electrode connected to a data line to which the data voltage is applied, and a gate electrode of the driving element; a second switch element including a gate electrode to which a second scan signal is applied, a first electrode to which a second reference voltage is applied, and a second electrode; a third switch element including a gate electrode connected to the second electrode of the second switch element, a first electrode to which the pixel driving voltage is applied, and a second electrode connected to the first electrode of the driving element; a gate electrode to which the second scan signal is applied, a first electrode connected to the second electrode of the driving element and the anode of the light emitting element, and a fourth switch element to which a first reference voltage lower than the second reference voltage is applied; and a first capacitor connected between the gate electrode of the driving element and the second electrode of the driving element. and a second capacitor connected between the gate electrode of the third switch element and the second electrode of the driving element. The light emitting element operates within a linear region of the driving transistor.

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According to the present invention, power consumption can be reduced without deterioration in image quality by adding a current control transistor between the driving transistor and the EVDD line, which adjusts the current applied to the drain of the driving element according to the change in EVDD.

1 is a circuit diagram showing a driving element and an OLED between EVDD and EVSS in one sub-pixel of an organic light emitting display device.
2 is a graph showing an operating point of an OLED on an IV curve of a transistor used as a driving element of a pixel and an IV curve of an OLED.
3 is a graph showing a relationship between EVDD-EVSS and an IV curve of an OLED IV curve in an electroluminescent display device according to an embodiment of the present invention.
4 is a block diagram showing an electroluminescent display device according to an exemplary embodiment of the present invention.
5 is a circuit diagram illustrating a pixel circuit according to an exemplary embodiment of the present invention.
FIG. 6 is a waveform diagram illustrating pixel data and a scan signal supplied to the pixel circuit shown in FIG. 5 .
7A and 7B are diagrams illustrating an initialization section operation of the pixel circuit shown in FIG. 5 .
8A and 8B are diagrams illustrating an operation of a sampling period of the pixel circuit shown in FIG. 5 .
9A and 9B are diagrams illustrating an operation of a data write section of the pixel circuit shown in FIG. 5 .
10 is a diagram showing simulation results in which a drain voltage of a driving device varies according to a change in EVDD.
11 is a circuit diagram showing a pixel circuit selected as a comparative example in simulation.
FIG. 12 is a waveform diagram illustrating signals applied to the pixel circuit shown in FIG. 11 .
13 is a diagram showing simulation results showing OLED current according to EVDD change in a pixel circuit of the present invention and a comparative example.
14 is a diagram showing simulation results showing a rate of change of OLED current according to a change in EVDD in a pixel circuit of the present invention and a comparative example.
15 is a diagram showing simulation results showing OLED current according to EVSS in a pixel circuit of the present invention and a comparative example.
16 is a diagram showing simulation results showing a rate of change of OLED current according to a change in EVSS in a pixel circuit of the present invention and a comparative example.
17 is a diagram showing simulation results showing a change in OLED current according to an OLED driving voltage in a pixel circuit of the present invention and a comparative example.
18 is a diagram showing simulation results showing an OLED current change rate according to an OLED driving voltage in a pixel circuit of the present invention and a comparative example.
19 is a diagram showing a compensation error rate according to a change in threshold voltage of a driving element in a pixel circuit of the present invention and a comparative example.

Advantages and features of the present invention, and methods of achieving them, will become clear with reference to the detailed description of the following embodiments taken in conjunction with the accompanying drawings. The present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, only the embodiments will make the disclosure of the present invention complete, and those of ordinary skill in the art to which the present invention belongs It is provided to fully inform the scope of the invention, the invention is defined only by the scope of the claims.

Since the shape, size, ratio, angle, number, etc. disclosed in the drawings for explaining the embodiments of the present invention are exemplary, the present invention is not limited to those shown in the drawings. Like reference numbers designate substantially like elements throughout the specification. In addition, in describing the present invention, if it is determined that a detailed description of related known technologies may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted.

When "comprises", "includes", "has", "consists of", etc. mentioned in this specification is used, other parts may be added unless '~ only' is used. When a component is expressed in the singular, it may be interpreted in the plural unless specifically stated otherwise.

In interpreting the components, even if there is no separate explicit description, it is interpreted as including the error range.

In the case of a description of a positional relationship, for example, when a positional relationship between two components is described as 'on ~', 'on top of ~', 'on the bottom of ~', 'next to', etc., ' One or more other components may be interposed between those components where 'immediately' or 'directly' is not used.

Although first, second, etc. may be used to distinguish the components, the function or structure of these components is not limited to the ordinal number or component name attached to the front of the component.

The following embodiments may be partially or wholly combined or combined with each other, and technically various interlocking and driving operations are possible. Each of the embodiments may be implemented independently of each other or together in an association relationship.

In the electroluminescent display device of the present invention, the pixel circuit includes a driving element and a switch element. The driving element and the switch element may be implemented with one or more of n-channel transistors (NMOS) and p-channel transistors (PMOS). The transistor may be implemented as an oxide transistor having an oxide semiconductor pattern or an LTPS transistor having a low temperature poly-silicon (LTPS) semiconductor pattern. A transistor is a three-electrode device including a gate, a source, and a drain. The transistor may be implemented as a TFT (Thin Film Transistor) on the display panel. The source is an electrode that supplies a carrier to the transistor. Within a transistor, carriers start flowing from the source. The drain is an electrode through which carriers exit from the TFT. The flow of carriers in a transistor flows from the source to the drain. In the case of an n-channel transistor (NMOS), since carriers are electrons, the source voltage has a voltage lower than the drain voltage so that electrons can flow from the source to the drain. In an n-channel transistor (NMOS), the direction of current flows from the drain to the source. In the case of a p-channel transistor (PMOS), since the carrier is a hole, the source voltage is higher than the drain voltage so that holes can flow from the source to the drain. In a p-channel transistor (PMOS), current flows from the source to the drain because holes flow from the source to the drain. Accordingly, it should be noted that the source and drain of the transistor are not fixed because the source and drain may change depending on the applied voltage. In the following description, the source and drain of the transistor will be referred to as first and second electrodes.

A gate signal of a TFT used as a switch element swings between a gate on voltage and a gate off voltage. The gate-on voltage is set to a voltage higher than the threshold voltage of the TFT, and the gate-off voltage is set to a voltage lower than the threshold voltage of the TFT. A TFT is turned on in response to a gate-on voltage, while it is turned off in response to a gate-off voltage. In the case of NMOS, the gate-on voltage may be a gate high voltage (VGH), and the gate-off voltage may be a gate low voltage (VGL). In the case of PMOS, the gate-on voltage may be the gate low voltage (VGL), and the gate-off voltage may be the gate high voltage (VGH).

Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following embodiments, the electroluminescent display device will be mainly described as an organic light emitting display device including an organic light emitting material, but is not limited thereto.

3 is a graph showing a relationship between EVDD-EVSS and an IV curve of an OLED IV curve 13 in an electroluminescent display device according to an embodiment of the present invention.

Referring to FIG. 3 , the present invention lowers the EVDD-EVSS to 1/2 the level of the prior art so that the OLED can operate in the linear region LR where the OLED IV curve 13 meets. Since the EVDD-EVSS is reduced, the OLED IV curve 13 meets the TR IV curve 11 in the linear region LR of the TR IV curve 11. Therefore, the operating point of the OLED is in the linear region LR of the TR IV curve 11.

In the present invention, a current control transistor (or a third switch element, S3) and a switch element as shown in FIG. (S2) is added. In the present invention, EVDD-EVSS can be lowered to 10V or less based on a mobile device such as a smart phone.

4 is a block diagram showing an electroluminescent display device according to an exemplary embodiment of the present invention.

Referring to FIG. 4 , an electroluminescent display device according to an embodiment of the present invention includes a display panel 100, display panel driving circuits 110 and 120 that write data of an input image into pixels of the display panel 100, and a timing controller 130 for controlling the display panel driving circuits 110 and 120 .

The screen of the display panel 100 includes a pixel array AA displaying an input image. The pixel array AA includes a plurality of data lines 102, a plurality of scan lines 104 crossing the data lines 102, and pixels arranged in a matrix form. The pixel array AA further includes power lines 1031 , 1032 , 105 , and 106 as shown in FIG. 5 .

Each of the pixels may be divided into a red sub-pixel, a green sub-pixel, and a blue sub-pixel for color implementation. Each of the pixels may further include a white sub-pixel. Each of the subpixels 101 is implemented as a pixel circuit having a current deviation compensation function when the OLED operates in the linear region LR of the TR IV curve 11 as shown in FIG. 5 .

Touch sensors may be disposed on the display panel 100 . A touch input may be sensed using separate touch sensors or sensed through pixels. Touch sensors are implemented as on-cell type or add-on type touch sensors disposed on the screen of a display panel or embedded in a pixel array. can

The display panel driving circuits 110 and 120 include a data driving unit 110 and a scan driving unit 120 . A demultiplexer (not shown) may be disposed between the data driver 110 and the data lines 102 . The demultiplexer is disposed between the data driver 110 and the data lines 102 to distribute the data voltage output from the data driver 110 to the data lines 102 . Since one channel of the data driver 110 is connected to a plurality of data lines by the demultiplexer, the number of data lines 102 may be reduced.

The display panel driving circuits 110 and 120 write pixel data of an input image into pixels of the display panel 100 pixel by pixel under the control of the timing controller 130 to display the input image on the screen. The display panel driving circuits 110 and 120 may further include a touch sensor driving unit for driving the touch sensors. The touch sensor driver is omitted in FIG. 1 . In a mobile device or a wearable device, the data driver 110, the timing controller 130, and a power supply unit (not shown) may be integrated into one integrated circuit. The power supply unit generates power necessary for driving the pixels, the display panel driving circuits 110 and 120, and the display panel 100.

The data driver 110 converts the pixel data of the input image received from the timing controller 130 into a gamma compensation voltage for each frame period by using a Digital to Analog converter (DAC). to generate data voltage (Vdata). The data voltage Vdata is supplied to the pixels through the data line 102 .

The scan driver 120 may be implemented as a gate in panel (GIP) circuit formed directly on a bezel area of the display panel 100 together with a transistor array in an active area. The scan driver 120 outputs scan signals to the scan lines 104 under the control of the timing controller 130 . The scan driver 120 may sequentially supply the scan signals to the scan lines 104 by shifting scan signals using a shift register. The scan signal is divided into a first scan signal SCAN1 and a second scan signal SCAN2 as shown in FIGS. 5 and 6 . The scan signal swings between a gate on voltage (VGH) and a gate low voltage (VGL).

The timing controller 130 receives input image data received from a host system (not shown) and a timing signal synchronized therewith. The timing controller 130 transmits pixel data of an input image to the data driver 110 . The timing controller 130 controls operation timings of the data driver 110 and the scan driver 120 based on the timing signal received from the host system.

The host system may be any one of a television (TV) system, a set top box, a navigation system, a personal computer (PC), a home theater system, a mobile device, and a wearable device.

5 is a circuit diagram illustrating a pixel circuit according to an exemplary embodiment of the present invention. 6 is a waveform diagram illustrating pixel data and a scan signal supplied to a pixel circuit.

5 and 6, the pixel circuit includes an OLED, a driving element DT, a first switch element S1, a second switch element S2, a third switch element S3, and a fourth switch element S4. ), and capacitors Cst and Cb. The switch elements S1 to S4 are turned on according to the gate-on voltage VGH.

The transistors T1 and T2 may be implemented as n-channel MOSFET structured thin film transistors (hereinafter referred to as "TFTs").

In this pixel circuit, the third switch element S3, the driving element DT, and the OLED are connected in series between the EVDD line 105 and the EVSS line 106.

Power such as EVDD, EVSS, Vref1, and Vref2 is applied to the pixel circuit. EVDD is set to a DC voltage less than 1/2 of the conventional one so that the operating point of the OLED exists in the leading region (LR) of the TR I-V curve. For large display devices such as TV products, EVDD can be as low as 15V to 17V. In the case of a small display device such as a mobile device, EVDD can be reduced to a level of 8V to 10V. The first reference voltage (Vref1) is set to a voltage lower than the data voltage (Vdata) and set to a DC voltage that initializes the source voltage (Vs) of the driving element (DT) and the anode voltage of the OLED. The second reference voltage Vref2 is set to a DC voltage appropriately adjusting the current flowing through the third switch element S3. The second reference voltage Vref2 is higher than the first reference voltage Vref1. Vref1 is a voltage lower than the lowest gradation voltage of Vdata. Vref1 can be set to -2V when Vdata has a voltage range of 0V to 6V. Vref2 can be set to a voltage of 1V to 2V. The scan signals SCAN1 and SCAN2 are generated as pulses swinging between a gate-on voltage VGH and a gate-off voltage VGL. VGH is 16V and VGL can be -6V. These voltages do not limit the present invention. The voltage applied to the pixel circuit may vary depending on the panel characteristics and driving method of the display panel 100 .

An OLED is a light emitting device including an organic compound layer formed between an anode and a cathode. The organic compound layer may include, but is not limited to, a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL). The OLED includes an anode connected to the driving element DT through the fourth node n4 and a cathode connected to the VSS line 105 to which EVSS is applied. The OLED emits light by a current determined according to a gate-to-source voltage (Vgs) of the driving element (DT).

The driving element DT controls the current of the OLED according to the gate-source voltage Vgs to drive the OLED. The driving element DT has a gate connected to the first node n1, a first electrode (or drain) to which EVDD is applied through the second node n2 and the third switch element S3, and a fourth node n4. ) through a second electrode (or source) connected to the anode of the OLED.

The first capacitor Cst is connected between the gate and the source of the driving element DT to store the gate-source voltage Vgs of the driving element DT.

The first switch element S1 is turned on according to the first scan signal SCAN1 and supplies the data voltage Vdata to the gate of the driving element DT connected to the first node n1. . The first switch element S1 includes a gate connected to the first gate line 1041 to which the first scan signal SCAN1 is applied, a first electrode connected to the data line 102, and a first node connected to the first node n1. Contains 2 electrodes.

The second switch element S2 is a switch element that is turned on according to the second scan signal SCAN2 and applies Vref2 to the gate of the third switch element S3 connected to the third node n3. The second switch element S2 includes a gate connected to the second gate line 1042 to which the second scan signal SCAN2 is applied, a first electrode connected to the second REF line 1032 to which Vref2 is applied, and a third node. and a second electrode connected to (n3).

The third switch element S3 is a current control transistor that adjusts the current flowing to the driving element DT according to Vref2. The third switch element S3 includes a gate connected to the third node n3, a first electrode connected to the EVDD line 105, and a second electrode connected to the second node n2.

The second capacitor (Cb) is connected between the third node (n3) and the fourth node to provide a difference voltage between the gate voltage (Ve) of the third switch element (S3) and the source voltage (Vs) of the driving element (DT). charge

The fourth switch element S4 is turned on according to the second scan signal SCAN2 and supplies Vref1 to the fourth node n4 to initialize the source voltage Vs of the driving element DT and the anode voltage of the OLED. do. The fourth switch element S4 has a gate connected to the second gate line 1042 to which the second scan signal SCAN2 is applied, a first electrode connected to the fourth node n4, and a first REF line 1031. It includes a second electrode connected to it.

The pixel circuit is driven in an initialization period Tini, a sampling period Tsam, a data writing period Twr, and an emission period Tem in every frame period. The first scan signal SCAN1 defines an initialization period Tini, a sampling period Tsam, and a data writing period Twr. The first scan signal SCAN1 is generated as a pulse of the gate-on voltage VGH during the initialization period Tini, the sampling period Tsam, and the data writing period Twr in one frame period, and the gate is turned off during the emission period Tem. Hold the voltage (VGL). The second scan signal SCAN2 defines an initialization period Tini. The second scan signal SCAN2 is generated as a pulse of the gate-on voltage VGH during the initialization period Tini in one frame period, and is gated during the sampling period Tsam, the data writing period Twr, and the emission period Tem. Maintain off voltage (VGL). The pixel data DATA is converted into a data voltage Vdata by the data driver 110 and supplied to the pixel circuit during the data writing period Twr.

7A and 7B are diagrams illustrating an initialization section operation of the pixel circuit shown in FIG. 5 .

Referring to FIGS. 7A and 7B , the first and second scan signals SCAN1 and SCAN2 are simultaneously inverted to the gate-on voltage VGH during the initialization period Tini. During the initialization period Tini, there is no pixel data DATA of the input image. The voltage Vin applied to the gate of the driving element DT through the data line 102 in the initialization period Tini is an initialization voltage for sensing the threshold voltage Vth of the driving element DT. Vin must be set to Vin > Vref1 +Vth so that the driving element DT can be turned on. The threshold voltage Vth of the driving element DT can be sensed only when Vin is greater than Vref1 +Vth.

During the initialization period Tini, the switch elements S1, S2, S3, and S4 and the driving element DT are turned on. In the initialization period Tini, the gate voltage Ve of the third switch element S3 is initialized to Ve=Vref2. During the initialization period Tini, the gate voltage Vg of the driving element DT is initialized to Vin. In the initialization period Tini, the source voltage Vs of the driving element DT is initialized to Vs = Vref1. In the initialization period Tini, the drain voltage Vd of the driving element DT is initialized to a value higher than Vref1 (Vref+A) by rising Vref1. A is the drain-to-drain voltage of the driving element DT, that is, A=Vds=Vd-Vs. A is proportional to EVDD. A increases as EVDD increases or as EVDD Drop decreases.

When the voltage of EVDD drops, the value of A decreases at Vd=Vref1+A. In the initialization period Tini, the voltage between the gate and source of the third switch element S3 (Vgs = Ve-Vd) increases, and in the light emission period Tem, the current of the OLED increases and is reduced by EVSS rising. OLED current is compensated.

8A and 8B are diagrams illustrating an operation of a sampling period of the pixel circuit shown in FIG. 5 .

Referring to FIGS. 8A and 8B , the first scan signal SCAN1 is maintained at the gate-on voltage VGH during the sampling period Tsam, while the second scan signal SCAN2 is inverted to the gate-off voltage VGL. do. As a result, the first switch element S1 maintains an on state, while the second and fourth switches S2 and S4 are turned off. During the sampling period Tsam, since there is no pixel data DATA of the input image, the data voltage Vdata is Vin. During the sampling period Tsam, the gate voltage Ve of the third switch element S3 changes to Ve=Vref2+(Vin-Vth-Vref1).

In the sampling period Tsam, current flows through the third switch element S3 and the driving element DT so that the source voltage Vs and the drain voltage Vd of the driving element DT increase. When EVDD drops, since B decreases at the drain voltage Vd=Vref1+A+B of the driving element DT, Vgs of the third switch element S3 increases. B is the variation of Vd in the sampling period (Tsam), that is, B = Vd (after sampling) - Vd (after initialization). B is a change in Vd after sampling compared to Vd initialized in the initialization period Tini. The higher the EVDD or the smaller the EVDD Drop, the larger the Vd after Vth sampling, so B also increases.

In the sampling period Tsam, the gate voltage Ve of the third switch element S3 is Ve=Vref2+(Vin-Vth-Vref1).

During the sampling period Tsam, the gate voltage Vg of the driving element DT is Vg=Vin. In the sampling period Tsam, the drain voltage Vd of the driving element DT is Vd=Vref1+A+B. In the sampling period Tsam, the source voltage Vs of the driving element DT is Vs=Vin-Vth. Here, Vth is the threshold voltage of the driving element DT and Vth=Vgs=Vg-Vs. In the sampling period Tsam, the gate voltage Ve of the third switch element S3 is Ve=Vref2+(Vin-Vth-Vref1).

9A and 9B are diagrams illustrating an operation of a data write section of the pixel circuit shown in FIG. 5 .

Referring to FIGS. 9A and 9B , during the data writing period Twr, the first scan signal SCAN1 is maintained at the gate-on voltage VGH, while the second scan signal SCAN2 is maintained at the gate-off voltage VGL. maintain. As a result, the first switch element S1 maintains an on state, while the second and fourth switches S2 and S4 maintain an off state. During the data writing period Vdata, the data voltage Vdata is generated as a grayscale voltage of the pixel data DATA.

Since the data voltage Vdata is applied to the gate of the driving element DT in the data writing period Twr, the third switch element is coupled to the gate of the driving element DT through the second capacitor Cb. The gate voltage Ve of (S3) rises. In the data writing period Twr, the drain voltage Vd of the driving element DT is Vd = Vref1+A+B+C. Here, C is the variation of Vd in the data writing period Twr, that is, C=Vd (after data writing) - Vd (right after Vth sampling). That is, as Vs rises, Vd also rises, and the amount of change in Vd after writing data compared to Vd immediately after Vth sampling is C. The higher the EVDD or the smaller the EVDD Drop, the higher the Vd after writing data, and thus the higher the C.

When an EVDD drop occurs, Vgs of the third switch element S3 increases because the C value decreases at the drain voltage Vd = Vref1 + A + B + C of the driving element DT. As a result, since the OLED current flowing through the third switch element S3 increases when EVDD drops in the emission period Tem, the reduced OLED current due to EVSS rising is compensated for.

In the data writing period Twr, the gate voltage Vg of the driving element DT is Vg=Vdata. In the data writing period Twr, the drain voltage Vd of the driving element DT is Vd=Vref1+A+B+C. In the data writing period Twr, the source voltage Vs of the driving element DT is Vs = Vin-Vth + C'(Vdata-Vin). Here, C' is a coupling ratio of Vs when data is written and is determined by the capacitance connected to the source of the driving element DT. C' is C'=Cst/(Cst+Coled). Coled is the capacitance between the anode and cathode of the OLED.

In the data writing period Twr, the gate voltage Ve of the third switch element S3 is Ve=Vref2+(Vin-Vth-Vref1)+C'(Vdata-Vin).

During the emission period Tem, the first scan signal SCAN1 is inverted to the gate-off voltage VGL, and the second scan signal SCAN2 is maintained at the gate-off voltage VGL. As a result, the switch elements S1, S2, and S4 are turned off. As a result, the gate of the driving element DT and the gate of the third switch element S3 float, and a current generated according to the gate-source voltage Vgs of the driving element DT flows through the OLED.

As shown in FIGS. 7A to 9B , the pixel circuit of the present invention operates in the linear region LR of the driving element DT with EVDD-EVSS reduced, and the drain voltage Vd of the driving element DT is at EVDD. Since it is charged proportionally, when EVDD decreases, Vgs = Ve-Vd of the third switch element S3 increases, whereas when EVDD increases, Vgs = Ve-Vd of the third switch element S3 decreases, resulting in OLED current can be compensated.

10 is a diagram showing simulation results in which a drain voltage of a driving device varies according to a change in EVDD.

The source voltage Vs of the driving element DT rises when the threshold voltage Vth of the driving element DT is sampled, and the drain voltage Vd of the driving element DT rises along with the source voltage Vs. do. When an EVDD drop occurs, the B value becomes smaller at Vd=Vref+A+B. At this time, Vgs of the third switch element S3 increases and the current of the OLED increases during the light emission period Tem, so that the current of the OLED that decreases due to the EVSS rising is compensated.

According to the simulation confirming the change in the drain voltage (Vd) of the driving element (DT) when EVDD is changed between 7V and 9.5V, as shown in FIG. reduced to V. As a result, it was confirmed that the decrease in pixel current due to the decrease in EVDD was compensated for by increasing the Vgs of the third switch element S3 and thus increasing the OLED current, that is, the current of the pixel.

The inventors of the present application compared the power consumption, Vth compensation characteristics, etc. when the OLED operates in the linear region LR of the driving element DT with a circuit in which the OLED operates in the saturation region SR of the driving element DT. was carried out.

Pixel circuits applied as comparative examples are shown in FIGS. 11 and 12 . In this pixel circuit, EVDD is 17V and EVSS = 0V. As shown in FIG. 11 , the pixel circuit of the comparative example includes a driving element DT for driving an OLED and a first driving element DT for applying a data voltage Vdata to a gate of the driving element DT according to a first scan signal SCAN1. A switch element S01, a second switch element S02 that applies EVDD to the drain of the driving element DT according to the emission signal EM, and a driving element that applies the reference voltage Vref according to the second scan signal SCAN2. A third switch element S03 applied to the source of (DT), a capacitor Cst connected between the gate and the source of the driving element DT, and a capacitor connected between the gate of the second switch element S02 and the EVDD line ( C0b).

13 is a diagram showing simulation results showing OLED current according to EVDD change in a pixel circuit of the present invention and a comparative example. 14 is a diagram showing simulation results showing a rate of change of OLED current according to a change in EVDD in a pixel circuit of the present invention and a comparative example. The pixel circuit of the present invention applied to this simulation is as described in the embodiment of FIGS. 5 to 9B. As can be seen from the simulation results shown in FIGS. 13 and 14 , when the EVDD is changed between 7V and 9.5V in the pixel circuit of the present invention, the current fluctuation of the OLED is much smaller than that of the comparative example. Therefore, since the pixel circuit of the present invention has almost no current deviation of the OLED even when the EVDD is lowered to 8V or less, power consumption compared to the comparative example can be reduced to 1/2 or less without deterioration in image quality.

15 is a diagram showing simulation results showing OLED current according to EVSS in a pixel circuit of the present invention and a comparative example. 16 is a diagram showing simulation results showing a rate of change of OLED current according to a change in EVSS in a pixel circuit of the present invention and a comparative example. As can be seen from the simulation results shown in FIGS. 15 and 16, when the EVSS is changed between 0V and 2.5V in consideration of the EVSS rising in the pixel circuit of the present invention, the current fluctuation of the OLED is much smaller than that of the comparative example. . Therefore, since the pixel circuit of the present invention has almost no current deviation of the OLED even during EVSS rising, power consumption compared to the comparative example can be reduced to 1/2 or less without deterioration in image quality.

17 is a diagram showing simulation results showing a change in OLED current according to an OLED driving voltage in a pixel circuit of the present invention and a comparative example. 18 is a diagram showing simulation results showing an OLED current change rate according to an OLED driving voltage in a pixel circuit of the present invention and a comparative example. When the driving history of the pixels increases and the OLED deteriorates, the driving voltage OLED_Dvth of the OLED can be increased to prevent the decrease in luminance, so the driving voltage OLED_Dvth increases when the OLED deteriorates. As can be seen from the simulation results shown in FIGS. 17 and 18, when the OLED driving voltage (OLED_Dvth) is changed between 0V and 2.5V in the pixel circuit of the present invention, the current fluctuation of the OLED is much smaller than that of the comparative example. . In the pixel circuit of the present invention, the compensating effect for the increase in OLED driving voltage and the compensating effect for the increase in EVSS are substantially the same.

19 is a diagram showing a compensation error according to a change in threshold voltage of a driving element in a pixel circuit of the present invention and a comparative example. In the pixel circuit of the present invention, when the OLED is driven in the linear region LR of the driving element DT, the change in current according to Vgs of the driving element DT is small. As a result, as can be seen from the simulation results shown in FIG. 19 , the compensation error rate according to the change in threshold voltage (ΔVth) of the driving element in the pixel circuit of the present invention is smaller than that of the comparative example. The following shows the current ILinear of the driving element DT in the linear region LR and the current ISaturation of the driving element DT in the saturation region.

Here, K is a constant value according to physical characteristics such as channel ratio and mobility of the MOSFET.

Through the above description, those skilled in the art will understand that various changes and modifications are possible without departing from the 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 determined by the claims.

11: Current-voltage characteristic curve of driving element (transistor)
12, 13: current-voltage characteristic curve of OLED (light emitting device)
100: display panel 110: data driving unit
120: scan driving unit 130: timing controller
OLED: organic light emitting diode (light emitting element) DT: driving element
S1~S4: switch element Cst, Cb: capacitor

Claims (11)

a display panel on which a plurality of pixels to which data lines and scan lines intersect and to which a pixel driving voltage and a pixel base voltage are supplied are disposed;
a data driver supplying data voltages to the data lines; and
A scan driver supplying scan signals to the scan lines;
Each of the pixels is
light emitting device;
a driving element including a gate electrode, a first electrode, and a second electrode to drive the light emitting element;
a first switch element connected to a gate electrode to which a first scan signal is applied, a first electrode connected to a data line to which the data voltage is applied, and a gate electrode of the driving element;
a second switch element including a gate electrode to which a second scan signal is applied, a first electrode to which a second reference voltage is applied, and a second electrode;
a third switch element including a gate electrode connected to the second electrode of the second switch element, a first electrode to which the pixel driving voltage is applied, and a second electrode connected to the first electrode of the driving element;
a gate electrode to which the second scan signal is applied, a first electrode connected to the second electrode of the driving element and the anode of the light emitting element, and a fourth switch element to which a first reference voltage lower than the second reference voltage is applied;
a first capacitor connected between a gate electrode of the driving element and a second electrode of the driving element; and
and a second capacitor connected between the gate electrode of the third switch element and the second electrode of the driving element. According to claim 1,
The electroluminescent display device of claim 1 , wherein the third switch element, the driving element, and the light emitting element are connected in series between a first power line to which the pixel driving voltage is supplied and a second power line to which the pixel base voltage is supplied. According to claim 1,
a voltage difference between the pixel driving voltage and the pixel base voltage is between 8V and 10V;
The electroluminescent display device of claim 1 , wherein the current-voltage characteristic curve of the light emitting element meets the current-voltage characteristic curve of the driving element in a linear region of the current-voltage characteristic curve of the driving element. delete delete According to claim 1,
The electroluminescence display device of claim 1 , wherein the first reference voltage is a voltage lower than the lowest grayscale voltage of the data voltage. According to claim 1,
The first scan signal is generated as a gate-on voltage pulse during an initialization period, a sampling period, and a data writing period, and maintains a gate-off voltage in an emission period;
The second scan signal is generated as a pulse of the gate-on voltage during the initialization period, and is generated as a pulse of the gate-off voltage during the sampling period, the data write period, and the emission period;
The electroluminescent display device wherein the first to fourth switch elements are turned on according to the gate-on voltage. light emitting device;
a driving element including a gate electrode, a first electrode, and a second electrode to drive the light emitting element;
a first switch element connected to a gate electrode to which a first scan signal is applied, a first electrode connected to a data line to which a data voltage is applied, and a gate electrode of the driving element;
a second switch element including a gate electrode to which a second scan signal is applied, a first electrode to which a second reference voltage is applied, and a second electrode;
a third switch element including a gate electrode connected to the second electrode of the second switch element, a first electrode to which a pixel driving voltage is applied, and a second electrode connected to the first electrode of the driving element;
a gate electrode to which the second scan signal is applied, a first electrode connected to the second electrode of the driving element and the anode of the light emitting element, and a fourth switch element to which a first reference voltage lower than the second reference voltage is applied;
a first capacitor connected between a gate electrode of the driving element and a second electrode of the driving element; and
and a second capacitor connected between the gate electrode of the third switch element and the second electrode of the driving element. According to claim 8,
a voltage difference between the pixel driving voltage and a pixel base voltage lower than the pixel driving voltage is between 8V and 10V;
The pixel circuit of the electroluminescent display device where the current-voltage characteristic curve of the light emitting element meets the current-voltage characteristic curve of the driving element in a linear region of the current-voltage characteristic curve of the driving element. delete delete

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