Light emitting display, display panel, and driving method thereof

This application claims priority to and the benefit of Korea Patent Application No. 2003-20432 filed on Apr. 1, 2003 in the Korean Intellectual Property Office, the content of which is incorporated herein by reference.

(a) Field of the Invention

The present invention relates to a light emitting display, a display panel, and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display.

(b) Description of the Related Art

In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1 , the organic emitting cell includes an anode of indium tin oxide (ITO), an organic thin film, and a cathode layer of metal. The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies, and it further includes an electron injecting layer (EIL) and a hole injecting layer (HIL).

Methods for driving the organic emitting cells include the passive matrix method, and the active matrix method using thin film transistors (TFTs) or metal oxide semiconductor field effect transistors (MOSFETs). The passive matrix method forms cathodes and anodes to cross with each other, and selectively drives lines. The active matrix method connects a TFT and a capacitor with each ITO pixel electrode to thereby maintain a predetermined voltage according to capacitance. The active matrix method is classified as a voltage programming method or a current programming method according to signal forms supplied for maintaining a voltage at a capacitor.

Referring to FIGS. 2 and 3 , conventional organic EL displays of the voltage programming and current programming methods will be described.

FIG. 2 shows a conventional voltage programming type pixel circuit for driving an organic EL element, representing one of N×M pixels. Referring to FIG. 2 , transistor M 1 is coupled to an organic EL element (referred to as an OLED hereinafter) to thus supply current for light emission. The current of transistor M 1 is controlled by a data voltage applied through switching transistor M 2 . In this instance, capacitor C 1 for maintaining the applied voltage for a predetermined period is coupled between a source and a gate of transistor M 1 . Scan line S n is coupled to a gate of transistor M 2 , and data line Dm is coupled to a source thereof.

As to an operation of the above-configured pixel, when transistor M 2 is turned on according to a select signal applied to the gate of switching transistor M 2 , a data voltage from data line Dm is applied to the gate of transistor M 1 . Accordingly, current I OLED flows to transistor M 2 in correspondence to a voltage V GS charged between the gate and the source by capacitor C 1 , and the OLED emits light in correspondence to current I OLED .

In this instance, the current that flows to the OLED is given in Equation 1.

I OLED = β 2 ⁢ ( V GS - V TH ) 2 = β 2 ⁢ ( V DD - V DATA -  V TH  ) 2 Equation ⁢   ⁢ 1

where I OLED is the current flowing to the OLED, V GS is a voltage between the source and the gate of transistor M 1 , V TH is a threshold voltage at transistor M 1 , and β is a constant.

As given in Equation 1, the current corresponding to the applied data voltage is supplied to the OLED, and the OLED gives light in correspondence to the supplied current, according to the pixel circuit of FIG. 2 . In this instance, the applied data voltage has multi-stage values within a predetermined range so as to represent gray.

However, the conventional pixel circuit following the voltage programming method has a problem in that it is difficult to obtain high gray because of deviation of a threshold voltage V TH of a TFT and deviations of electron mobility caused by non-uniformity of an assembly process. For example, in the case of driving a TFT of a pixel through 3 volts (3V), voltages are to be supplied to the gate of the TFT for each interval of 12 mV (=3V/256) so as to represent 8-bit (256) grays, and if the threshold voltage of the TFT caused by the non-uniformity of the assembly process deviates, it is difficult to represent high gray. Also, since the value β in Equation 1 changes because of the deviation of the mobility, it becomes even more difficult to represent the high gray.

On assuming that the current source for supplying the current to the pixel circuit is uniform over the whole panel, the pixel circuit of the current programming method can achieve uniform display features even though a driving transistor in each pixel has non-uniform voltage-current characteristics.

FIG. 3 shows a pixel circuit of a conventional current programming method for driving the OLED, representing one of N×M pixels. Referring to FIG. 3 , transistor M 1 is coupled to the OLED to supply the current for light emission, and the current of transistor M 1 is controlled by the data current applied through transistor M 2 .

First, when transistors M 2 and M 3 are turned on because of the select signal from scan line S n , transistor M 1 becomes diode-connected, and the voltage matched with data current I DATA from data line Dm is stored in capacitor C 1 . Next, the select signal from scan line S n becomes high-level to turn on transistor M 4 . Then, the power is supplied from power supply voltage VDD, and the current matched with the voltage stored in capacitor C 1 flows to the OLED to emit light. In this instance, the current flowing to the OLED is as follows.

I OLED = β 2 ⁢ ( V GS - V TH ) 2 = I DATA Equation ⁢   ⁢ 2

where V GS is a voltage between the source and the gate of transistor M 1 , V TH is a threshold voltage at transistor M 1 , and β is a constant.

As given in Equation 2, since current I OLED flowing to the OLED is the same as data current I DATA in the conventional current pixel circuit, uniform characteristics can be obtained when the programming current source is set to be uniform over the whole panel. However, since current I OLED flowing to the OLED is a fine current, control over the pixel circuit by fine current I DATA problematically requires much time to charge the data line. For example, assuming that the load capacitance of the data line is 30 pF, it requires several milliseconds of time to charge the load of the data line with the data current of several tens to hundreds of nA. This causes a problem that the charging time is not sufficient in consideration of the line time of several tens of microseconds.

In accordance with the present invention a light emitting display is provided for compensating for the threshold voltage of transistors or for electron mobility, and sufficiently charging the data line.

In one aspect of the present invention, a light emitting display is provided that includes a display panel on which a plurality of data lines for transmitting the data current that displays video signals, a plurality of scan lines for transmitting a select signal, and a plurality of pixel circuits formed at a plurality of pixels defined by the data lines and the scan lines are formed. The pixel circuit includes: a light emitting element for emitting light corresponding to the applied current; a first transistor, having first and second main electrodes and a control electrode, for supplying a driving current for the light emitting element; a first switch for diode-connecting the first transistor in response to a first control signal; a first storage unit for storing a first voltage corresponding to a threshold voltage of the first transistor in response to a second control signal; a second switch for transmitting a data signal from the data line in response to the select signal from the scan line; a second storage unit for storing a second voltage corresponding to the data current from the first switch; and a third switch for transmitting the driving current from the first transistor to the light emitting element in response to a third control signal. A third voltage determined by coupling of the first and second storage units respectively storing the first and second voltages is applied to the first transistor to supply the driving current to the light emitting element. The second control signal is enabled, the select signal is enabled, and the third control signal is then enabled in order. The pixel circuit further includes a fourth switch turned on in response to the second control signal, and coupled to a control electrode of the first transistor. The second storage unit is formed by a first capacitor coupled between the control electrode and the first main electrode of the first transistor. The first storage unit is formed by parallel coupling of a second capacitor coupled between the first main electrode of the first transistor and a second end of the fourth switch, and the first capacitor. The second control signal is the select signal from the scan line, and the fourth switch responds in the disable interval of the select signal. The first control signal includes a select signal from the previous scan line and a select signal from the current scan line. The first switch includes a second transistor for diode-connecting the first transistor in response to the select signal from the previous scan line, and a third transistor for diode-connecting the first transistor in response to the select signal from the current scan line. The second control signal includes a select signal from the previous scan line, and the third control signal. The pixel circuit further includes a fifth switch coupled in parallel to the fourth switch. The fourth and fifth transistors are respectively turned on in response to the select signal from the previous scan line and the third control signal.

In another aspect of the present invention, a display panel of a light emitting display, on which a plurality of data lines for transmitting the data current that displays video signals, a plurality of scan lines for transmitting a select signal, and a plurality of pixel circuits formed at a plurality of pixels defined by the data lines and the scan lines are formed. The pixel circuit includes: a first transistor having a first main electrode coupled to a first power supplying a first voltage; a first switch coupled between a second main electrode of the first transistor and the data line, and being controlled by a first select signal from the scan line; a second switch controlled by a first control signal to diode-connect the first transistor; a third switch having a first end coupled to a control electrode of the first transistor, and being controlled by a second control signal; a fourth switch having a first end coupled to a second main electrode of the first transistor, and being controlled by a third control signal; a light emitting element, coupled between a second end of the fourth switch and a second power supplying a second voltage, for emitting light corresponding to the applied current; a first storage unit coupled between the control electrode and the first main electrode of the first transistor when the third switch is turned on; and a second storage unit coupled between the control electrode and the first main electrode of the first transistor when the third switch is turned off.

In still another aspect of the present invention, a method is provided for driving a light emitting display including a pixel circuit including a switch for transmitting a data current from a data line in response to a select signal from a scan line, a transistor including first and second main electrodes and a control electrode for outputting the driving current in response to the data current, and a light emitting element for emitting light corresponding to the driving current from the transistor. A first voltage corresponding to a threshold voltage of the transistor is stored in a first storage unit formed between the control electrode and the first main electrode of the transistor. A second voltage corresponding to the data current from the switch is stored in a second storage unit formed between the control electrode and the first main electrode of the transistor. The first and second storage units are coupled to establish the voltage between the control electrode and the first main electrode of the transistor as a third voltage. The driving current is transmitted from the transistor to the light emitting display, wherein the driving current from the transistor is determined corresponding to the third voltage.

In still yet another aspect of the present invention, a method is provided for driving a light emitting display including a pixel circuit including a switch for transmitting a data current from a data line in response to a select signal from a scan line, a transistor including first and second main electrodes and a control electrode for outputting the driving current in response to the data current, and a light emitting element for emitting light corresponding to the driving current from the transistor. The transistor is diode-connected in response to a first control signal. A first storage unit is coupled between the control electrode and the first main electrode of the transistor in response to a first level of a second control signal to store a first voltage corresponding to a threshold voltage of the transistor in the first storage unit. The transistor is diode-connected by the first control signal. A second storage unit is coupled between the control electrode and the first main electrode of the transistor in response to a second level of the second control signal. A second voltage corresponding to the data current is stored in the second storage unit in response to the first select signal. The first and second storage units are coupled in response to the first level of the second control signal to establish the voltage between the control electrode and the first main electrode of the transistor as a third voltage. A driving current is provided corresponding to the third voltage to the transistor. The driving current is provided to the light emitting element in response to a third control signal.

In a still further another aspect of the present invention, in a method for transmitting a data current showing video signals to a transistor in response to a first select signal to drive a light emitting element, a method for driving a light emitting display is provided. First and second control signals are established respectively applied to first and second switches as an enable level to store a first voltage corresponding to a threshold voltage of the transistor. A third control signal is established applied to a third switch as a disable level to electrically intercept the transistor and the light emitting element. The first select signal is established as a disable level to intercept the data current. The first select signal is established as an enable level to supply the data current. The first and second control signals are respectively established as enable and disable levels to store a second voltage corresponding to the data current. The first select signal is established as a disable level to intercept the data current. The first and second control signals are respectively established as disable and enable levels to apply a third voltage to a main electrode and a gate electrode of the transistor. The third control signal is established as an enable level to transmit the current from the transistor to the light emitting element, wherein the third voltage is determined by the first and second voltages.

FIG. 1 shows a concept diagram of an OLED.

FIG. 2 shows an equivalent circuit of a conventional pixel circuit following the voltage programming method.

FIG. 3 shows an equivalent circuit of a conventional pixel circuit following the current programming method.

FIG. 4 shows a brief plane diagram of an organic EL display according to an embodiment of the present invention,

FIGS. 5 , 7 , 9 , 11 , 13 , 14 , and 15 respectively show an equivalent circuit of a pixel circuit according to first through seventh embodiments of the present invention.

FIGS. 6 , 8 , 10 , 12 , and 16 respectively show a driving waveform for driving the pixel circuit of FIGS. 5 , 7 , 9 , 11 , and 15 .

An organic EL display, a corresponding pixel circuit, and a driving method thereof will be described in detail with reference to drawings.

First, referring to FIG. 4 , the organic EL display will be described. FIG. 4 shows a brief ground plan of the OLED.

As shown, the organic EL display includes organic EL display panel 10 , scan driver 20 , and data driver 30 .

Organic EL display panel 10 includes a plurality of data lines D 1 through D m in the row direction, a plurality of scan lines S 1 through S n , E 1 through E n , X 1 through X n , and Y 1 through Y n , and a plurality of pixel circuits 11 . Data lines D 1 through D m transmit data signals that represent video signals to pixel circuit 11 , and scan lines S 1 through S n transmit select signals to pixel circuit 11 . Pixel circuit 11 is formed at a pixel region defined by two adjacent data lines D 1 through D m and two adjacent scan lines S 1 through S n . Also, scan lines E 1 through E n transmit emit signals for controlling emission of pixel circuits 11 , and scan lines X 1 through X n and Y 1 through Y n respectively transmit control signals for controlling operation of pixel circuits 11 .

Scan driver 20 sequentially applies respective select signals and emit signals to scan lines S 1 through S n and E 1 through E n , and control signals to scan lines X 1 through X n and Y 1 through Y n . Data driver 30 applies the data current that represents video signals to data lines D 1 through D m .

Scan driver 20 and/or data driver 30 can be coupled to display panel 10 , or can be installed, in a chip format, in a tape carrier package (TCP) coupled to display panel 10 . The same can be attached to display panel 10 , and installed, in a chip format, on a flexible printed circuit (FPC) or a film coupled to display panel 10 , which is referred to as a chip on flexible (CoF) board, or chip on film method. Differing from this, scan driver 20 and/or data driver 30 can be installed on the glass substrate of the display panel, and further, the same can be substituted for the driving circuit formed in the same layers of the scan lines, the data lines, and TFTs on the glass substrate, or directly installed on the glass substrate, which is referred to as a chip on glass (CoG) method.

Referring to FIGS. 5 and 6 , pixel circuit 11 of the organic EL display according to the first embodiment of the present invention will now be described. FIG. 5 shows an equivalent circuit diagram of the pixel circuit according to the first embodiment, and FIG. 6 shows a driving waveform diagram for driving the pixel circuit of FIG. 5 . In this instance, for ease of description, FIG. 5 shows a pixel circuit coupled to an m-th data line D m and an n-th scan line S n .

As shown in FIG. 5 , pixel circuit 11 includes an OLED, PMOS transistors M 1 through M 5 , and capacitors C 1 and C 2 . The transistor is preferably a thin film transistor having a gate electrode, a drain electrode, and a source electrode formed on the glass substrate as a control electrode and two main electrodes.

Transistor M 1 has a source coupled to power supply voltage VDD, and a gate coupled to transistor M 5 , and transistor M 3 is coupled between the gate and a drain of transistor M 1 . Transistor M 1 outputs current I OLED corresponding to a voltage V GS at the gate and the source thereof. Transistor M 3 diode-connects transistor M 1 in response to a control signal CS 1 n from scan line X n . Capacitor C 1 is coupled between power supply voltage VDD and the gate of transistor M 1 , and capacitor C 2 is coupled between power supply voltage VDD and a first end of transistor M 5 . Capacitors C 1 and C 2 operate as storage elements for storing the voltage between the gate and the source of the transistor. A second end of transistor M 5 is coupled to the gate of transistor M 1 , and transistor M 5 couples capacitors C 1 and C 2 in response to a control signal CS 2 n from scan line Y n .

Transistor M 2 transmits data current I DATA from transistor M 1 to data line D m in response to a select signal SE n from scan line S n . Transistor M 4 coupled between the drain of transistor M 1 and the OLED, transmits current I OLED of transistor M 1 to the OLED in response to an emit signal EM n of scan line E n . The OLED is coupled between transistor M 4 and the reference voltage, and emits light corresponding to applied current I OLED .

Referring to FIG. 6 , an operation of the pixel circuit according to the first embodiment of the present invention will now be described in detail.

As shown, in interval T 1 , transistor M 5 is turned on because of low-level control signal CS 2 n , and capacitors C 1 and C 2 are coupled in parallel between the gate and the source of transistor M 1 . Transistor M 3 is turned on because of low-level control signal CS 1 n , transistor M 1 is diode-connected, and the threshold voltage V TH of transistor M 1 is stored in capacitors C 1 and C 2 coupled in parallel because of diode-connected transistor M 1 . Transistor M 4 is turned off because of high-level emit signal EM n , and the current to the OLED is intercepted. That is, in interval T 1 , the threshold voltage V TH of transistor M 1 is sampled to capacitors C 1 and C 2 .

In interval T 2 , control signal CS 2 n becomes high level to turn off transistor M 5 , and select signal SE n becomes low level to turn on transistor M 2 . Capacitor C 2 is floated while charged with voltage, because of turned-off transistor M 5 . Data current I DATA from transistor M 1 flows to data line D m because of turned-on transistor M 2 . Accordingly, the gate-source voltage V GS (T 2 ) at transistor M 1 is determined corresponding to data current I DATA , and the gate-source voltage V GS (T 2 ) is stored in capacitor C 1 . Since data current I DATA flows from transistor M 1 , data current I DATA can be expressed as Equation 3, and the gate-source voltage V GS (T 2 ) in interval T 2 is given as Equation 4 derived from Equation 3. That is, the gate-source voltage corresponding to data current I DATA is programmed to capacitor C 1 of the pixel circuit in interval T 2 .

I DATA = β 2 ⁢ (  V GS ⁡ ( T2 )  -  V TH  ) 2 Equation ⁢   ⁢ 3

 V GS ⁡ ( T2 )  = 2 ⁢ I DATA β +  V TH  Equation ⁢   ⁢ 4

where β is a constant.

Next, in interval T 3 , transistors M 3 and M 2 are turned off in response to high-level control signal CS 1 n and select signal SE n , and transistors M 5 and M 4 are turned on because of low-level control signal CS 2 n and emit signal EM n . When transistor M 5 is turned on, the gate-source voltage V GS (T 3 ) at transistor M 1 in interval T 3 becomes Equation 5 because of coupling of capacitors C 1 and C 2 .

 V GS ⁡ ( T3 )  =  V TH  + C 1 C 1 + C 2 ⁢ (  V GS ⁡ ( T2 )  -  V TH  ) Equation ⁢   ⁢ 5

where C 1 and C 2 are respectively the capacitance of capacitors C 1 and C 2 .

Therefore, current I OLED flowing to transistor M 1 becomes as Equation 6, and current I OLED is supplied to the OLED because of turned-on transistor M 4 , to thereby emit light. That is, in interval T 3 , the voltage is provided and the OLED emits light because of coupling of capacitors C 1 and C 2 .

I OLED = β 2 ⁢ { C 1 C 1 + C 2 ⁢ (  V GS ⁡ ( T2 )  -  V TH  ) } 2 = ( C 1 C 1 + C 2 ) 2 ⁢ I DATA Equation ⁢   ⁢ 6

As expressed in Equation 6, since current I OLED supplied to the OLED is determined with no relation to the threshold voltage V TH of transistor M 1 or the mobility, the deviation of the threshold voltage or the deviation of the mobility can be corrected. Also, current I OLED supplied to the OLED is C 1 /(C 1 +C 2 ) squared times smaller than the data current I DATA . For example, if C 2 is M times greater than C 1 (C 2 =M×C 1 ), the fine current flowing to the OLED can be controlled by data current I DATA which is (M+1) 2 times greater than current I OLED , thereby enabling representation of high gray. Further, since the large data current I DATA is supplied to data lines D 1 through D m , charging time for the data lines can be sufficiently obtained.

In the first embodiment, PMOS transistors are used for transistors M 1 through M 5 . However, NMOS transistors can also be implemented, which will now be described referring to FIGS. 7 and 8 .

FIG. 7 shows an equivalent circuit diagram of the pixel circuit according to a second embodiment of the present invention, and FIG. 8 shows a driving waveform diagram for driving the pixel circuit of FIG. 7 .

The pixel circuit of FIG. 7 includes NMOS transistors M 1 through M 5 , and their coupling structure is symmetric with the pixel circuit of FIG. 5 . In detail, transistor M 1 has a source coupled to the reference voltage, a gate coupled to transistor M 5 , and transistor M 3 is coupled between the gate and a drain of transistor M 1 . Capacitor C 1 is coupled between the reference voltage and the gate of transistor M 1 , and capacitor C 2 is coupled between the reference voltage and a first end of transistor M 5 . A second end of transistor M 5 is coupled to the gate of transistor M 1 , and control signals CS 1 n and CS 2 n from scan lines X n and Y n are respectively applied to the gates of transistors M 3 and M 5 . Transistor M 2 transmits data current I DATA from data line D m to transistor M 1 in response to select signal SE n from scan line S n . Transistor M 4 is coupled between the drain of transistor M 1 and the OLED, and emit signal EM n from scan line E n is applied to the gate of transistor M 4 . The OLED is coupled between transistor M 4 and power supply voltage VDD.

Since the pixel circuit of FIG. 7 includes NMOS transistors, the driving waveform for driving the pixel circuit of FIG. 7 has an inverse form of the driving waveform of FIG. 6 , as shown in FIG. 8 . Since the detailed operation of the pixel circuit according to the second embodiment of the present invention can be easily obtained from the description of the first embodiment and FIGS. 7 and 8 , no further detailed description will be provided.

According to the first and second embodiments, since transistors M 1 through M 5 are the same type transistors, a process for forming TFTs on the glass substrate of display panel 10 can be easily executed.

Transistors M 1 through M 5 are PMOS or NMOS types in the first and second embodiments, but without being restricted to this, they can be realized using combination of PMOS and NMOS transistors, or other switches having similar functions.

Two control signals CS 1 n and CS 2 n are used to control the pixel circuit in the first and second embodiments, and in addition, the pixel circuit can be controlled using a single control signal, which will now be described with reference to FIGS. 9 through 12 .

FIG. 9 shows an equivalent circuit diagram of the pixel circuit according to a third embodiment of the present invention, and FIG. 10 shows a driving waveform diagram for driving the pixel circuit of FIG. 9 .

As shown in FIG. 9 , the pixel circuit has the same configuration as the first embodiment except for transistors M 2 and M 5 . Transistor M 2 includes an NMOS transistor, and gates of transistors M 2 and M 5 are coupled in common to scan line S n . That is, transistor M 5 is driven by select signal SE n from scan line S n .

Referring to FIG. 10 , in interval T 1 , transistors M 3 and M 5 are turned on because of low-level control signal CS 1 n and select signal SE n . Transistor M 1 is diode-connected because of turned-on transistor M 3 , and the threshold voltage V TH at transistor M 1 is stored in capacitors C 1 and C 2 . Also, transistor M 4 is turned off because of high-level emit signal EM n , and the current flow to the OLED is intercepted.

In interval T 2 , select signal SE n becomes high level to turn transistor M 2 on and transistor M 5 off. Then, the voltage V GS (T 2 ) expressed in Equation 4 is charged in capacitor C 1 . In this instance, since the voltage charged in capacitor C 2 can be changed when transistor M 2 is turned on because of select signal SE n , in order to prevent this, transistor M 3 is turned off before transistor M 2 is turned on, and again, transistor M 3 is turned on after transistor M 2 is turned on. That is, control signal CS 1 n is inverted to high level for a short time before select signal SE n becomes high level.

Since other operations in the third embodiment of the present invention are matched with those of the first embodiment, no further corresponding description will be provided. According to the third embodiment, scan lines Y 1 through Y n for supplying control signal CS 2 n can be removed, thereby increasing the aperture ratio of the pixels.

In the third embodiment, transistors M 1 and M 3 through M 5 are realized with PMOS transistors, and transistor M 2 with an NMOS transistor, and further, the opposite realization of the transistors are possible, which will be described with reference to FIGS. 11 and 12 .

FIG. 11 shows an equivalent circuit diagram of the pixel circuit according to a fourth embodiment of the present invention, and FIG. 12 shows a driving waveform diagram for driving the pixel circuit of FIG. 11 .

As shown in FIG. 11 , the pixel circuit realizes transistor M 2 with a PMOS transistor, and transistors M 1 and M 3 through M 5 with NMOS transistors, and their coupling structure is symmetric with that of the pixel circuit of FIG. 9 . Also, as shown in FIG. 12 , the driving waveform for driving the pixel circuit of FIG. 11 has an inverse form of that of FIG. 10 . Since the coupling structure and the operation of the pixel circuit according to the fourth embodiment can be easily obtained from the description of the third embodiment, no detailed description will be provided.

In the first through fourth embodiments, capacitors C 1 and C 2 are coupled in parallel to power supply voltage VDD, and differing from this, capacitors C 1 and C 2 can be coupled in series to power supply voltage VDD, which will now be described referring to FIGS. 13 and 14 .

FIG. 13 shows an equivalent circuit diagram of the pixel circuit according to a fifth embodiment of the present invention.

As shown, the pixel circuit has the same structure as that of the first embodiment except for the coupling states of capacitors C 1 and C 2 , and transistor M 5 . In detail, capacitors C 1 and C 2 are coupled in series between power supply voltage VDD and transistor M 3 , and transistor M 5 is coupled between the common node of capacitors C 1 and C 2 and the gate of transistor M 1 .

The pixel circuit according to the fifth embodiment is driven with the same driving waveform as that of the first embodiment, which will now be described referring to FIGS. 6 and 13 .

In interval T 1 , transistor M 3 is turned on because of low-level control signal CS 1 n to diode-connect transistor M 1 . The threshold voltage V TH of transistor M 1 is stored in capacitor C 1 because of diode-connected transistor M 1 , and the voltage at capacitor C 2 becomes 0V. Also, transistor M 4 is turned off because of high-level emit signal EM n to intercept the current flow to the OLED.

In interval T 2 , control signal CS 2 n becomes high level to turn off transistor M 5 , and select SE n becomes low level to turn on transistor M 2 . Data current I DATA flows from transistor M 1 to data line D m because of turned-on transistor M 2 , and the gate-source voltage V GS (T 2 ) at transistor M 1 becomes as shown in Equation 4. Hence, the voltage V C1 at capacitor C 1 charging the threshold voltage V TH becomes as shown in Equation 7 because of coupling of capacitors C 1 and C 2 .

V C1 =  V TH  + C 2 C 1 + C 2 ⁢ (  V GS ⁡ ( T2 )  -  V TH  ) Equation ⁢   ⁢ 7

Next, in interval T 3 , transistors M 3 and M 2 are turned off in response to high-level control signal CS 1 n and select signal SE n , and transistors M 5 and M 4 are turned on because of low-level control signal CS 2 n and emit signal EM n . When transistor M 3 is turned off, and transistor M 5 is turned on, the voltage V C1 at capacitor C 1 becomes the gate-source voltage V GS (T 3 ) of transistor M 1 . Therefore, current I OLED flowing from transistor M 1 becomes as shown in Equation 8, and current I OLED is supplied to the OLED according to transistor M 4 thereby emitting light.

I OLED = β 2 ⁢ { C 2 C 1 + C 2 ⁢ (  V GS ⁡ ( T2 )  -  V TH  ) } 2 = ( C 2 C 1 + C 2 ) 2 ⁢ I DATA Equation ⁢   ⁢ 8

In the like manner of the first embodiment, current I OLED supplied to the OLED is determined with no relation to the threshold voltage V TH of transistor M 1 or the mobility. Also, since the fine current flowing to the OLED using data current I DATA that is (C 1 +C 2 )/C 2 squared times current I OLED can be controlled, high gray can be represented. By supplying large data current I DATA to data lines D 1 through D M , sufficient charging time of the data lines can be obtained.

Transistors M 1 through M 5 are realized with PMOS transistors in the fifth embodiment, and they can also be realized with NMOS transistors, which will now be described with reference to FIG. 14 .

FIG. 14 shows an equivalent circuit diagram of the pixel circuit according to a sixth embodiment of the present invention.

As shown, the pixel circuit realizes transistors M 1 through M 5 with NMOS transistors, and their coupling structure is symmetric with that of the pixel circuit of FIG. 13 . The driving waveform for driving the pixel circuit of FIG. 14 has an inverse driving waveform of the pixel circuit of FIG. 14 , and it is the same driving waveform as that of FIG. 8 . Since the coupling structure and the operation of the pixel circuit according to the sixth embodiment can be easily derived from the description of the fifth embodiment, no further detailed description will be provided.

Two or one control signal is used to control the pixel circuit in the first through sixth embodiments, and differing from this, the pixel circuit can be controlled by using a select signal of a previous scan line without using the control signal, which will now be described in detail with reference to FIGS. 15 and 16 .

FIG. 15 shows an equivalent circuit diagram of the pixel circuit according to a seventh embodiment of the present invention, and FIG. 16 shows a driving waveform diagram for driving the pixel circuit of FIG. 15 .

As shown in FIG. 15 , the pixel circuit has the same structure as that of the first embodiment except for transistors M 3 , M 5 , M 6 , and M 7 . In detail, transistor M 3 diode-connects transistor M 1 in response to select signal SE n−1 from previous scan line S n−1 , and transistor M 7 diode-connects transistor M 1 in response to select signal SE n from current scan line S n . Transistor M 7 is coupled between data line D m and the gate of transistor M 1 in FIG. 15 , and it can also be coupled between the gate and the drain of transistor M 1 . Transistors M 5 and M 6 are coupled in parallel between capacitor C 2 and the gate of transistor M 1 . Transistor M 5 responds to select signal SE n−1 from previous scan line S n−1 , and transistor M 6 responds to emit signal EM n from scan line E n .

Next, the operation of the pixel circuit of FIG. 15 will be described referring to FIG. 16 .

As shown, in interval T 1 , transistors M 3 and M 5 are turned on because of low-level select signal SE n−1 . Capacitors C 1 and C 2 are coupled in parallel between the gate and the source of transistor M 1 because of turned-on transistor M 5 . Transistor M 1 is diode-connected because of turned-on transistor M 3 to store the threshold voltage V TH of transistor M 1 in capacitors C 1 and C 2 coupled in parallel. Transistors M 2 , M 7 , M 4 , and M 6 are turned off because of high-level select signal SE n and emit signal EM n .

In interval T 2 , select signal SE n−1 becomes high level to turn off transistor M 3 , and transistor M 7 is turned on because of low-level select signal SE n to diode-connect transistor M 1 and maintain the diode-connected state of transistor M 1 . Transistor M 5 is turned off because of select signal SE n−1 to have capacitor C 2 be floated while storing the voltage. Transistor M 2 is turned on because of select signal SE n to make data current I DATA from transistor M 1 flow to data line D m . The gate-source voltage V GS (T 2 ) of transistor M 1 is determined corresponding to data current I DATA , and the gate-source voltage V GS (T 2 ) is given as Equation 4 in the same manner of the first embodiment.

Next, in interval T 3 , select signal SE n becomes high level to turn off transistors M 2 and M 7 , and transistors M 4 and M 6 are turned on because of low-level emit signal EM n . When transistor M 6 is turned on, the gate-source voltage V GS (T 3 ) of transistor M 1 is given as Equation 5 because of coupling of capacitors C 1 and C 2 in the like manner of the first embodiment. Therefore, current I OLED shown in Equation 6 is supplied to the OLED because of turned-on transistor M 4 to emit light.

The two control signals CS 1 n and CS 2 n are removed in the seventh embodiment, and differing from this, one of control signals CS 1 n and CS 2 n can be removed. In detail, in the case of additionally using control signal CS 1 n in the seventh embodiment, transistor M 7 is removed from the pixel circuit of FIG. 15 , and transistor M 3 is driven by not select signal SE n−1 but by control signal CS 1 n . In the case of additionally using control signal CS 2 n in the seventh embodiment, transistor M 6 is removed from the pixel circuit of FIG. 15 , and transistor M 5 is not driven by the select signal SE n−1 and emit signal EM n but by control signal CS 2 n . Accordingly, the number of wires increases compared to FIG. 15 , but the number of transistors can be reduced.

In the above, PMOS and/or NMOS transistors are used to realize a pixel circuit in the first through seventh embodiments, and without being restricted to this, the pixel circuit can be realized by PMOS transistors, NMOS transistors, or a combination of PMOS and NMOS transistors, and by other switches having similar functions.

Accordingly, since the current flowing to the OLED can be controlled using the large data current, the data line can be sufficiently charged during a single line time frame. Also, the deviation of the threshold voltage of the transistor or the deviation of the mobility is corrected, and a light emission display with high resolution and a wide screen can be realized.

While this invention has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.