Pixel circuit and light emitting display comprising the same

This application claims the priority of Korean Patent Application No. 2004-80621, filed on Oct. 8, 2004, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.

The present invention relates to a pixel circuit and a light emitting display comprising the same, and more particularly, to a pixel circuit and a light emitting display comprising the same, in which a threshold voltage is compensated, thereby improving the uniformity of brightness.

Recently, various flat panel displays have been developed, to substitute cathode ray tube (CRT) displays because the CRT displays are relatively heavy and bulky. Among the flat panel displays, a light emitting display (LED) is notable because it has high emission efficiency, high brightness, wide view angle, and fast response time.

The light emitting display comprises a plurality of light emitting devices, wherein each light emitting device has a structure in which an emission layer is placed between a cathode electrode and an anode electrode. Here, an electron and a hole are injected into the emission layer and recombined to create an exciton. Light is emitted when the exciton falls to a lower energy level.

Such a light emitting display is classified into an inorganic light emitting display comprising an inorganic emission layer, and an organic light emitting display comprising an organic emission layer.

FIG. 1 is a circuit diagram of a pixel provided in a conventional light emitting display. Referring to FIG. 1 , the pixel comprises an organic light emitting device OLED, a driving transistor M 2 , a capacitor Cst, a switching transistor M 1 . Further, the pixel is connected to a scan line Sn, a data line Dm, a pixel power line Vdd, and a second power supply line Vss. The second power supply line Vss is a voltage lower that the first voltage supply, for example, a ground voltage. Here, the scan line Sn is arranged in a row direction, and the data line Dm and the pixel power line Vdd are arranged in a column direction. For reference, n is an arbitrary integer between 1 and N, and m is an arbitrary integer between 1 and M.

The switching transistor M 1 comprises a source electrode connected to the data line Dm, a drain electrode connected to a first node A, and a gate electrode connected to the scan line Sn.

The driving transistor M 2 comprises a source electrode connected to the pixel power line Vdd, a drain electrode connected to the organic light emitting device OLED, and a gate electrode connected to the first node A. Here, the driving transistor M 2 supplies current to the organic light emitting device OLED in response to a signal inputted to its gate electrode, thereby allowing the organic light emitting device to emit light. Further, the intensity of the current flowing in the driving transistor M 2 is controlled by a data signal transmitted through the data line Dm and switching transistor M 1 .

The capacitor Cst comprises a first electrode connected to the source electrode of the driving transistor M 2 , and a second electrode connected to the first node A. Here, the capacitor Cst maintains voltage applied between the source and gate electrodes of the driving transistor M 2 in response to the data signal, for a predetermined period.

With this configuration, when the switching transistor M 1 is turned on in response to the scan signal transmitted to the gate electrode of the switching transistor M 1 , the capacitor Cst is charged with a voltage corresponding to the data signal, and then the voltage charged in the capacitor Cst is applied to the gate electrode of the driving transistor M 2 . Hence, the current flows in the driving transistor M 2 , thereby allowing the organic light emitting device OLED to emit light.

At this time, the current supplied from the driving transistor M 2 to the organic light emitting device OLED is calculated by the following equation.

I OLED = β 2 ⁢ ( Vgs - Vth ) 2 = β 2 ⁢ ( Vdd - Vdata - Vth ) 2 [ Equation ⁢ ⁢ 1 ]

where I OLED is a current flowing in the organic light emitting device OLED; Vgs is a voltage applied between the source and gate electrodes of the driving transistor M 2 ; Vth is a threshold voltage of the driving transistor M 2 , Vdata is a voltage corresponding to the data signal; and β is a gain factor of the driving transistor M 2 .

Referring to the equation 1, the current I OLED flowing in the organic light emitting device OLED varies depending on the threshold voltage of the driving transistor M 2 .

However, when the conventional light emitting display is fabricated, deviation arises in the threshold voltage of the driving transistor M 2 . Thus, the deviation in the threshold voltage of the driving transistor M 2 causes in consistencies in the current flowing in the organic light emitting device OLED to be not uniform, thereby deteriorating the uniformity of the brightness of the display device.

Further, the pixel power line Vdd connected to each pixel and supplying pixel power is connected to a first power line (not shown) and supplies the pixel power. In this case, voltage drop arises in the first power supplied from the pixel power line Vdd to the first power line. As the length of the first power line increases, the pixel power line Vdd connected thereto increases in number, thereby causing the voltage drop to get larger.

Particularly, for a large screen of the flat panel display, the voltage drop in the first power line increases further.

Accordingly, it is an aspect of the present invention to provide a pixel circuit and a light emitting display comprising the same, in which current flows in a driving transistor regardless of a threshold voltage of the driving transistor and pixel power. This way, the variations of the threshold voltage is compensated, so that the amount of current flowing in the light emitting device does not vary with voltage drop in first voltage used for the pixel power and the decrease in the pixel power, thereby improving the uniformity of brightness.

In one embodiment, the present invention is a pixel circuit comprising: a light emitting device; a driving transistor to receive a first voltage and supply a current corresponding to the voltage applied to a gate electrode thereof to the light emitting device; a first switching device to supply a data signal in response to a first scan signal; a second switching device to supply a second voltage to the gate electrode of the driving transistor in response to the first scan signal; a capacitor to store a voltage corresponding to the data signal and the second voltage according to operations of the first and second switching devices; a third switching device to apply voltage corresponding to the voltage stored in the capacitor to the gate electrode of the driving transistor in response to a second scan signal; and a fourth switching device to transmit the first voltage to the driving transistor in response to a third scan signal.

In one embodiment, the present invention is a pixel circuit comprising: a light emitting device; a driving transistor to supply a driving current corresponding to a voltage applied to a gate electrode thereof to the light emitting device; a capacitor to store a predetermined voltage corresponding to a data signal and a second voltage applied to the gate electrode of the driving transistor; a first switch to selectively supply the data signal to the capacitor; a second switch to supply either a voltage stored in the capacitor or the second voltage to the gate electrode of the driving transistor; and a third switch to selectively supply a first voltage to the driving transistor.

In one embodiment, the present invention is a pixel circuit comprising: a light emitting device; a capacitor comprising a first terminal connected to a first node, and a second terminal connected to a third node; a first switching transistor comprising source and drain electrodes connected to a data line and the first node, respetively, and a gate electrode connected to a first scan line; a second switching transistor comprising source and drain electrodes connected to second power and a second node, respetively, and a gate electrode connected to the first scan line; a third switching transistor comprising source and drain electrodes connected to the first node and the second node, respetively, and a gate electrode connected to a second scan line; a driving transistor comprising source and drain electrodes connected to the third node and the light emitting device, respetively, and a gate electrode connected to the second node; and a fourth switching transistor comprising source and drain electrodes connected to first power and the driving transistor, respetively, and selectively supplying the first power to the driving transistor.

In one embodiment, the present invention is a light emitting display comprising: a plurality of scan lines; a plurality of data lines; and a plurality of pixel circuits, wherein each pixel circuit comprising: a light emitting device; a driving transistor to receive a first voltage and supply a current corresponding to voltage applied to a gate electrode thereof to the light emitting device; a first switching device to supply a data signal in response to a first scan signal; a second switching device to supply a second voltage to the gate electrode of the driving transistor in response to the first scan signal; a capacitor to store voltage corresponding to the data signal and the second power according to operations of the first and second switching devices; a third switching device to apply voltage corresponding to the voltage stored in the capacitor to the gate electrode of the driving transistor in response to a second scan signal; and a fourth switching device to transmit the first voltage to the driving transistor in response to a third scan signal.

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of some embodiments of the invention, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a circuit diagram of a pixel provided in a conventional light emitting display;

FIG. 2 illustrates configuration of a light emitting display according to an embodiment of the present invention;

FIG. 3 is a circuit diagram of a pixel according to a first embodiment of the present invention;

FIG. 4 is a circuit diagram of a pixel according to a second embodiment of the present invention;

FIG. 5 shows timing between signals for driving the pixels shown in FIGS. 3 and 4 ;

FIG. 6 is a circuit diagram for compensating for variations in the threshold voltage of the pixels shown in FIGS. 3 and 4 ;

FIG. 7 is a circuit diagram formed when a driving voltage is applied to the pixels shown in FIGS. 3 and 4 ;

FIG. 8 is a circuit diagram of a pixel comprising NMOS transistors according to an embodiment of the present invention; and

FIG. 9 shows timing of signals for driving the pixel shown in FIG. 8 .

FIG. 2 illustrates a configuration of a light emitting display according to an embodiment of the present invention. Referring to FIG. 2 , the light emitting display comprises a pixel portion 100 , a data driver 200 , and a scan driver 300 . The pixel portion 100 comprises a plurality of pixels 110 including N×M organic light emitting devices; N first scan lines S 1 . 1 , S 1 . 2 , . . . , S 1 .N−1, S 1 .N arranged in a row direction; N second scan lines S 2 . 1 , S 2 . 2 , . . . , S 2 .N−1, S 2 .N arranged in the row direction; N third scan lines S 3 . 1 , S 3 . 2 , . . . , S 3 .N−1, S 3 .N arranged in the row direction; M data lines D 1 , D 2 , . . . DM−1, DM arranged in a column direction; M pixel power lines Vdd to supply pixel power; and M compensation power lines Vinit to supply compensation power. Here, each pixel power line Vdd and each compensation power line Vinit are connected to a first power line 130 and a second power line 120 .

Further, a data signal is transmitted from any of the data lines D 1 , D 2 , . . . DM−1, DM to a pixel 110 in response to a first scan signal and a second scan signal transmitted through any of the first scan lines S 1 . 1 , S 1 . 2 , . . . , S 1 .N−1, S 1 .N, and any of the second scan lines S 2 . 1 , S 2 . 2 , . . . , S 2 .N−1, S 2 .N to generate a driving current corresponding to the data signal. Also, the driving current is supplied to a corresponding organic light emitting device OLED in response to a third scan signal transmitted through one of the third scan lines S 3 . 1 , S 3 . 2 , . . . , S 3 .N−1, S 3 .N, thereby displaying an image.

The data driver 200 is connected to the data lines D 1 , D 2 , . . . DM−1, DM and supplies the data signal to the pixels 110 . The scan driver 300 is provided on a side of the pixel portion 100 , and connected to the first scan lines S 1 . 1 , S 1 . 2 , . . . , S 1 .N−1, S 1 .N, the second scan lines S 2 . 1 , S 2 . 2 , . . . , S 2 .N−1, S 2 .N, and the third scan lines S 3 . 1 , S 3 . 2 , . . . , S 3 .N−1, S 3 .N. The scan driver 300 supplies the first, second and third scan signals to the pixel portion 100 , and selects the rows of the pixel portion 100 in sequence. Then, the data driver 200 supplies the data signal to the selected row, thereby allowing a pixel 110 to emit light based on the data signal.

FIG. 3 is a circuit diagram of a pixel according to a first embodiment of the present invention. As shown in FIG. 3 , the pixel comprises an emission part 111 , a storage part 112 , a driving device 113 , a first switching part 114 , a second switching part 115 , and a third switching part 116 .

The driving device 113 comprises source, gate and drain electrodes, and determines the intensity of current inputted to the emission part 111 on the basis of voltage stored in the storage part 112 , thereby controlling the brightness of the emission part 111 .

The first switching part 114 receives the data signal and selectively transmits it to the storage part 112 . The second switching part 115 selectively transmits either the voltage stored in the storage part 112 or the compensation voltage applied through the compensation power line Vinit to a gate electrode of the driving device 113 , based on scan signals S 1 .n and S 2 .n.

The storage part 112 stores a predetermined voltage and supplies the stored voltage to the gate electrode of the driving device 113 . Further, the storage part 112 stores voltage obtained by subtracting voltage applied to a source electrode of the driving device 113 from the voltage corresponding to the data signal received through the first switching part 114 . Here, the voltage applied to the source electrode of the driving device 113 is higher than the compensation voltage by the absolute value of the threshold voltage of the driving device 113 .

The third switching part 116 prevents the first power Vdd from being applied to the driving device 113 while the pixel power is selectively applied to the pixel through the pixel power line Vdd and stored in the storage part 112 . Further, the third switching part 116 supplies the first power Vdd to the driving device 113 when the pixel power is completely stored in the storage part 112 .

In other words, the pixel 110 comprises the organic light emitting device OLED and its peripheral circuits including a first switching transistor M 1 , a second switching transistor M 2 , a third switching transistor M 3 , a driving transistor M 4 , a fourth switching device M 5 , and a capacitor Cst. Each of the first through third switching transistors M 1 , M 2 , M 3 , the driving transistors M 4 , and the switching device M 5 comprises a gate electrode, a source electrode, and a drain electrode. Further, the capacitor Cst comprises a first electrode and a second electrode.

The gate electrode of the first switching transistor M 1 is connected to the first scan line S 1 .n, the source electrode is connected to the data line Dm, and the drain electrode is connected a first node A. Here, the first switching transistor M 1 supplies the data signal to the first node A, in response to the first scan signal inputted through the first scan line S 1 .n.

The gate electrode of the second switching transistor M 2 is connected to the first scan line S 1 .n, the source electrode is connected to the compensation power line Vinit, and the drain electrode is connected to a second node B. Here, the second switching transistor M 2 supplies the compensation power from the compensation power line Vinit to the second node B, in response to the first scan signal inputted through the first scan line S 1 .n. Further, the compensation power inputted through the compensation power line Vinit is maintained as a high signal.

The capacitor Cst is connected between the first node A and a third node C, and charged with the voltage difference between the voltage applied to the first node A and the voltage applied to the third node C, thereby supplying the charged voltage to the gate electrode of the driving transistor M 4 for a period corresponding to one frame.

The gate electrode of the third switching transistor M 3 is connected to the second scan line S 2 .n, the source electrode is connected to the first node A, and the drain electrode is connected to the second node B. Here, the third switching transistor M 3 supplies the voltage charged in the capacitor Cst to the gate electrode of the driving transistor M 4 in response to the second scan signal inputted through the second scan signal S 2 .n.

The gate electrode of the driving transistor M 4 is connected to the second node B, the source electrode is connected to the third node C, and the drain electrode is connected to the anode electrode of the organic light emitting device OLED. Here, the driving transistor M 4 controls the current corresponding to the voltage applied to its own gate electrode to flow via its own source and drain electrodes, thereby supplying the current to the organic light emitting device OLED.

The gate electrode of the fourth switching device M 5 is connected to the third scan line S 3 .n, the source electrode is connected to the pixel power line Vdd to supply the pixel power, and the drain electrode is connected to the third node C. Here, the fourth switching device M 5 is switched in response to the third scan signal inputted through the third scan line S 3 .n, and thus selectively supplies the pixel power to the organic light emitting device OLED, thereby controlling the current flowing in the organic light emitting device OLED.

FIG. 4 is a circuit diagram of a pixel according to a second embodiment of the present invention. Referring to FIG. 4 , the pixel comprises an additional fifth switching transistor M 6 connected in parallel to the organic light emitting device OLED, relative to the pixel circuit of the first embodiment.

The fifth switching transistor M 6 comprises a gate electrode connected to a third scan line, a source electrode connected to a cathode electrode of the organic light emitting device OLED, and a drain electrode connected to an anode electrode of the organic light emitting device OLED. Further, the fifth switching transistor M 6 has a reverse polarity relative to the fourth switching transistor M 5 . For example, when the fourth switching device M 5 is of a p-type transistor as shown in FIG. 4 , the fifth switching transistor M 6 is of an n-type transistor. In this case, the fifth switching transistor M 6 is turned off while the fourth switching device M 5 is turned on. On the other hand, the fifth switching transistor M 6 is turned on while the fourth switching device M 5 is turned off.

Therefore, in a case that the organic light emitting device OLED emits light, the fifth switching transistor M 6 is turned off, so that the current flows only in the organic light emitting device OLED. On the other hand, in a case that the organic light emitting device OLED does not emit light (particularly, while the threshold voltage is detected), the fifth switching transistor M 6 is turned on, so that the current flows in the fifth switching transistor M 6 and not in the organic light emitting device OLED, thereby preventing the organic light emitting device OLED from emitting light.

FIG. 5 shows timing of the signals for driving the pixels shown in FIGS. 3 and 4 ; FIG. 6 is a circuit diagram formed when threshold voltage is compensated in the pixels shown in FIGS. 3 and 4 ; and FIG. 7 is a circuit diagram formed when the driving voltage is applied to the pixels shown in FIGS. 3 and 4 . Referring to FIGS. 5 through 7 , operation of the pixel is divided according to a first operation period T 1 and a second operation period T 2 . In the first operation period T 1 , the first scan signal s 1 .n is low, and the second scan signal s 2 .n and the third scan signal s 3 .n are high. In the second operation period T 2 , the first scan signal s 1 .n is high, and the second scan signal s 2 .n and the third scan signal s 3 .n are low.

In the first operation period T 1 , the first and second switching transistors M 1 and M 2 are turned on by the first scan signal s 1 .n, and the third and fourth switching transistors M 3 and M 4 are turned off by the second scan signal s 2 .n and the third scan signal s 3 .n. Hence, the circuit is connected as shown in FIG. 6 .

Referring to FIG. 6 , the data signal is transmitted to the first node A through the first switching transistor M 1 , and the compensation power is supplied to the gate electrode of the driving transistor M 4 through the second switching transistor M 2 . At this time, the first scan signal s 1 .n is changed from a high state to a low state after the second scan signal s 2 .n is changed from a low state to a high state, so that the first and second switching transistors M 1 and M 2 are turned on after the third switching transistor M 3 is turned off. Therefore, the data signal is not distorted by other voltage and is correctly stored in the capacitor, thereby applying a uniform voltage to the gate of the driving transistor M 4 .

Because the applied compensation power is a high signal, the driving transistor M 4 is maintained in the off state, and thus the voltage applied to the source electrode of the driving transistor M 4 is higher than the voltage applied to the gate electrode thereof by the threshold voltage. Therefore, the voltage based on the following equation 2 is applied between the source and gate electrodes of the driving transistor M 4 by the capacitor Cst.

Vcst=Vdata −( Vinit−Vth );  [Equation 2]

where Vcst is a voltage charged in the capacitor; Vdata is a voltage corresponding to the data signal; Vinit is the compensation voltage and Vth is the threshold voltage of the driving transistor M 4 .

In order to correctly operate the driving transistor M 4 , the pixel power voltage should be larger than or equal to the sum of the compensation voltage and the absolute value of the threshold voltage of the driving transistor M 4 .

In the second operation period T 2 , the first scan signal s 1 .n is maintained in the high state, and the second scan signal s 2 .n and the third scan signal s 3 .n are maintained in the low state. The second operation period T 2 is maintained for a period corresponding to one frame. During this time, the first and second switching transistors M 1 and M 2 are turned off by the first scan signal s 1 .n, and the third and fourth switching transistors M 3 and M 5 are turned on by the second scan signal s 2 .n and the third scan signal s 3 .n. Hence, the circuit is connected as shown in FIG. 7 .

Referring to FIG. 7 , the voltage charged in the capacitor Cst is applied to the gate electrode of the driving transistor M 4 , so that the current corresponding to the voltage charged in the capacitor Cst flows in the organic light emitting device OLED through the driving transistor M 4 . At this time, the second scan signal s 2 .n is changed from a high state to a low state after the first scan signal s 1 .n is changed from a low state to a high state, so that the third switching transistor M 3 applies only the voltage charged in the capacitor Cst to the gate electrode of the driving transistor M 4 , thereby applying a uniform voltage to the gate electrode of the driving transistor M 4 .

Therefore, a current based on the following equation 3 flows from the driving transistor M 4 to the organic light emitting device OLED.

I OLED = β 2 ⁢ ( Vgs - Vth ) 2 = β 2 ⁢ ( Vdata - Vinit ) 2 , [ Equation ⁢ ⁢ 3 ]

where I OLED is a current flowing in the organic light emitting device OLED; Vgs is a voltage applied between the source and gate electrodes of the driving transistor M 4 ; Vdata is a voltage corresponding to the data signal; Vinit is a compensation voltage; and β is a gain factor of the driving transistor M 4 .

Therefore, as shown in the equation 3, the current flowing in the organic light emitting device OLED corresponds only to the data signal voltage and the compensation voltage, regardless of the threshold voltage of the driving transistor M 4 and the pixel power.

At this time, the pixel power allows the current to flow in the light emitting device, so that a voltage drop occurs in the pixel power as the current flows. However, the compensation voltage is connected to the capacitor Cst, so that there is no current flowing to the pixel by the compensation power. Thus, a voltage drop does not occur in the compensation voltage.

Thus, in the pixels shown in FIGS. 3 and 4 , the deviation between the threshold voltages of the driving transistors M 4 is compensated, and the voltage drop in the pixel power is compensated, so that the pixels are suitable for realizing a large sized light emitting display.

FIG. 8 is a circuit diagram of a pixel comprising NMOS transistors according to an embodiment of the present invention. Referring to FIG. 8 , the pixel comprises an organic light emitting device OLED and its peripheral circuits including a first switching transistor M 1 , a second switching transistor M 2 , a third switching transistor M 3 , a driving transistor M 4 , a fourth switching device M 5 , and a capacitor Cst. Each of the first through third switching transistors M 1 , M 2 , M 3 , the driving transistors M 4 , and the switching device M 5 is realized by an NMOS transistor comprising a gate electrode, a source electrode, and a drain electrode. Further, the capacitor Cst comprises a first electrode and a second electrode.

The organic light emitting device OLED is connected to the driving transistor M 4 , and the fourth switching device M 5 is connected between the driving transistor M 4 and a cathode electrode.

FIG. 9 shows timing between signals for driving the pixel shown in FIG. 8 . Referring to FIG. 9 , operation of the pixel is divided according to a first operation period T 1 and a second operation period T 2 . In the first operation period T 1 , the first scan signal s 1 .n is high, and the second scan signal s 2 .n and the third scan signal s 3 .n are low. In the second operation period T 2 , the first scan signal s 1 .n is low, and the second scan signal s 2 .n and the third scan signal s 3 .n are high.

In the first operation period T 1 , the first and second switching transistors M 1 and M 2 are turned on by the first scan signal s 1 .n, and the third and fourth switching transistors M 3 and M 5 are turned off by the second scan signal s 2 .n and the third scan signal s 3 .n. Hence, the compensation voltage is supplied from the compensation power line Vinit to the gate electrode of the driving transistor M 3 , and the capacitor Cst is charged with a voltage based on the equation 2. During this time, the compensation power supplied through the compensation power line Vinit is kept low.

In the second operation period T 2 , the first scan signal s 1 .n is kept low, and the second scan signal s 2 .n and the third scan signal s 3 .n are kept high. The second operation period T 2 is maintained for a period corresponding to one frame. During this time, the first and second switching transistors M 1 and M 2 are kept turned off by the first scan signal s 1 .n, and the third and fourth switching transistors M 3 and M 5 are kept turned on by the second scan signal s 2 .n and the third scan signal s 3 .n. The voltage stored in the capacitor Cst is applied to the organic light emitting device OLED, so that the driving current based on the equation 3 flows therein.

In the foregoing embodiment, the fourth switching device M 5 for controlling the current to flow in the organic light emitting device OLED may be an NMOS transistor when other transistors provided in the pixel are PMOS transistors. Alternately, the fourth switching device M 5 may be a PMOS transistor when other transistors provided in the pixel are NMOS transistors.

As described above, the present invention provides a pixel circuit and a light emitting display, in which current flows in a driving transistor regardless of threshold voltage of the driving transistor and pixel power. Thus, the difference between the threshold voltages is compensated, so that the intensity of current flowing in the light emitting device does not vary due to voltage drop in first power used for the pixel power and a decrease in the pixel power voltage, thereby improving the uniformity of brightness of the light emitting device.

Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.