Patent Application
20060262050
This application is based on Japanese Patent Application Nos. 2005-131261, 2005-131262 and 2005-131263, the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to an electroluminescent display device, each pixel of which is provided with an electroluminescent device, and a data line drive circuit that provides the electroluminescent device with a drive current.
2. Description of the Related Art
Organic EL display devices using organic electroluminescent devices (hereafter referred to as organic EL devices) have been developed in recent years as display devices to replace CRT and LCD. An emphasis is laid on development of an active drive type organic EL display panel, each pixel of which is provided with a pixel selection TFT (Thin Film Transistor) and a driver TFT that drives the organic EL device.
On the other hand, an electronic view finder (hereafter referred to as EVF) using the organic EL display device has been also developed. The EVF is attached as a finder to a body of a camera such as a digital camera. It enables magnifying an image of an object displayed on the organic EL display panel by five to ten times with an optical lens.
Further description on the technologies mentioned above is provided in Japanese Patent Application Publication No. 2002-175035.
However, the EVF using the active drive type organic EL display panel has a problem that it gives a grainy look to a light-emitting portion on the display panel, since the pixel selection TFTs, driver TFTs, gate signal lines and drain signal lines included in the active drive type organic EL display panel reduce an aperture ratio, a ratio of an area of open portions (light-emitting portions) to an area of the pixels on the display panel, and make non-open portions at borders between the pixels visible as a grating pattern when the display panel is magnified with the optical lens.
It has another problem that even slight variations in threshold voltages of the driver TFTs in the pixels cause variations in electric currents flowing through the EL devices and cause variations in brightness of the light emitted from the pixels to give a coarse look to the display, since a very little current flowing through the EL device disposed in each of the very small pixels of the EVF can be easily affected by the variations in the threshold voltages.
Therefore, it is conceived to improve the aperture ratio of the pixels using a passive drive type organic EL display device that has no TFTs in its pixels. In this case, the organic EL devices are provided with drive currents corresponding to display data from a data line drive circuit through data lines.
However, there is a problem that variations in threshold voltages of driver transistors integrated in the active drive type display device cause variations in the drive currents and resultant variations in the brightness on the display panel.
This invention provides an electroluminescent display device having a plurality of data lines, a plurality of cathode lines isolated from each other and disposed to intersect the data lines, a plurality of electroluminescent devices each disposed around an intersection of each of the data lines and each of the cathode lines, a horizontal shift register that sequentially samples externally provided display signals, a data line drive circuit that reads-in and retains the display signals sampled by the horizontal shift register and collectively provides the plurality of data lines with drive currents corresponding to the display signals for a predetermined period and a vertical shift register that sequentially selects a cathode line out of the plurality of cathode lines and set an electric potential of the selected cathode line to form a current path through which a current driving the electroluminescent device flows, wherein the plurality of electroluminescent devices, the horizontal shift register, the data line drive circuit and the vertical shift register are formed on a single substrate.
This invention also provides an electroluminescent display device having a plurality of data lines, a plurality of gate lines disposed to intersect the plurality of data lines, a plurality of pixels each disposed around an intersection of each of the data lines and each of the gate lines and including a pixel selection transistor with a gate connected to one of the gate lines and a drain connected to one of the data lines and an electroluminescent device connected to the pixel selection transistor, a horizontal shift register that sequentially samples externally provided display signals, a data line drive circuit that reads-in and retains the display signals sampled by the horizontal shift register and collectively provides the plurality of data lines with drive currents corresponding to the display signals for a predetermined period and a vertical shift register that provides the plurality of gate lines with vertical scanning signals, wherein the plurality of pixels, the horizontal shift register, the data line drive circuit and the vertical shift register are formed on a single substrate.
This invention further offers a data line drive circuit that provides an electroluminescent device with a drive current, the data line drive circuit having a first data line drive circuit that outputs a first drive current to a data line during a first period and a second drive circuit that outputs a second drive current to the data line during a second period, the first and the second currents corresponding to a display data and being compensated for a threshold voltage of a driver transistor.
Next, an organic EL display device according to a first embodiment of this invention will be described hereafter, referring to the drawings.
First, a structure of a pixel region of the organic EL display device will be described. A plurality of data lines DL1-DL6 extends in a vertical direction (top to bottom direction in
An organic EL device 30R that emits red light is disposed around each of four intersections of the first column data line DL1 and the cathode lines CL1-CL4. Anodes of the organic EL devices 30R that emit red light are connected to the data line DL1, while each of their cathodes is connected to corresponding each of the cathode lines CL1-CL4. Similarly, an organic EL device 30G that emits green light is disposed around each of four intersections of the second column data line DL2 and the cathode lines CL1-CL4. Anodes of the organic EL devices 30G that emit green light are connected to the data line DL2, while each of their cathodes is connected to corresponding each of the cathode lines CL1-CL4.
Similarly, an organic EL device 30B that emits blue light is disposed around each of four intersections of the third column data line DL3 and the cathode lines CL1-CL4. Anodes of the organic EL devices 30B that emit blue light are connected to the data line DL3, while each of their cathodes is connected to corresponding each of the cathode lines CL1-CL4. Pixels in a fourth column and beyond are structured in a way to repeat the structure described above. Inorganic EL devices may be used instead of the organic EL devices 30R, 30G and 30B.
Next, structures of a horizontal shift register 10 and a data line drive circuit DLD will be described hereafter. The horizontal shift register 10 and the data line drive circuit DLD are formed on the glass substrate 51. The horizontal shift register 10 has a plurality of horizontal shift register units HSR1, HSR2, and so on and a plurality of sampling transistors ST11, ST12, and so on. The sampling transistors ST11, ST12, and so on are thin film transistors.
Each of the plurality of horizontal shift register units HSR1, HSR2, and so on sequentially outputs each of horizontal scanning pulses SPH1, SPH2, and so on, respectively, by shifting a horizontal start pulse STH in synchronization with a horizontal clock CKH, as shown in
Six of the sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16 are disposed corresponding to the horizontal shift register unit HSR1 in a first stage. The horizontal scanning pulse SPH1 is inputted in common to gates of the six sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16. Similarly, six of the sampling transistors ST21, ST22, ST23, ST24, ST25 and ST26 are disposed corresponding to the horizontal shift register unit HSR2 in a second stage. The horizontal scanning pulse SPH2 is inputted in common to gates of the six sampling transistors ST21, ST22, ST23, ST24, ST25 and ST26.
Regarding the six sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16, sources of a first couple of the sampling transistors ST11 and ST12 are connected to a first display signal line LR that supplies a red display signal Sig(R), sources of a second couple of the sampling transistors ST13 and ST14 are connected to a second display signal line LG that supplies a green display signal Sig(G) and sources of a third couple of the sampling transistors ST15 and ST16 are connected to a third display signal line LB that supplies a blue display signal Sig(B).
The data line drive circuit DLD has a first data line drive circuit DLD1 and a second data line drive circuit DLD2 corresponding to each of the data lines DL1-DL6. For example, the first data line drive circuit DLD1 corresponding to the data line DL1 reads-in and retains the red display signal Sig(R) through the sampling transistor ST11, and provides the data line DL1 with a drive current corresponding to the red display signal Sig(R). The first data line drive circuit DLD1 has a threshold voltage compensation circuit that compensates for a threshold voltage of a driver transistor, as will be described later. The variations in the brightness on the display panel due to the variations in the threshold voltages of the driver transistors can be suppressed because the drive current independent of the threshold voltage of the driver transistor is obtained with the threshold voltage compensation circuit.
The second data line drive circuit DLD2 operates similarly. While the first data line drive circuit DLD1 is controlled by a vertical clock CKV that has a period of two horizontal periods, the second data line drive circuit is controlled by a reverse vertical clock *CKV that is a reverse of the vertical clock CKV. As a result, the first data line drive circuit DLD1 and the second data line drive circuit DLD2 output the drive current to the data line DL1 alternately for every other one horizontal period (1H period). One horizontal period means a time required to scan one horizontal line (the cathode line CL1, for example).
Another first data line drive circuit DLD1 and another second data line drive circuit DLD2 corresponding to each of the other data lines DL2-DL6 are structured similarly.
Next, a structure of a vertical shift register 20 will be described hereafter. The vertical shift register 20 has a plurality of vertical shift register units VSR1, VSR2, and so on, connected in series and switching devices SW1, SW2, SW3 and SW4 disposed on the glass substrate 51. The vertical shift register 20 is formed of thin film transistors. The switching devices SW1, SW2, SW3 and SW4 may be formed of inverters using thin film transistors.
Each of the plurality of vertical shift register units VSR1, VSR2, and so on sequentially outputs each of vertical scanning pulses SPV1, SPV2, and so on, respectively, by shifting a vertical start pulse STV in synchronization with the vertical clock CKV and the reverse vertical clock *CVK. Each of the switching devices SW1, SW2, SW3 and SW4 switches according to each of the vertical scanning pulses SPV1, SPV2, SPV3 and SPV4, respectively, and sets an electric potential of each of the cathode lines CL1-CL4 at a ground electric potential GND or at a power supply electric potential Vcc. That is, each of the switching devices SW1, SW2, SW3 and SW4 sets the electric potential of each of the cathode lines CL1-CL4 at the ground electric potential GND to form current paths for the organic EL devices in the pixels only when each of the vertical scanning pulses SPV1, SPV2, SPV3 and SPV4 is at a high level, respectively.
An interlayer insulation film 54 is formed on the gate electrode 45. In a region where the shift register unit VSR1 is formed, an aluminum electrode 47 is formed on the interlayer insulation film 54, making a contact with an underlying chromium electrode 46.
In the pixel region, the data line DL1 made of aluminum is formed on the interlayer insulation film 54. A protection film 55 and a first planarizing insulation film 56 are formed on the aluminum electrode 47 and the data line DL1. An anode 58 made of ITO (Indium Tin Oxide) is formed on the first planarizing insulation film 56 in the pixel region. An organic EL layer 60 is formed on the anode 58, and a second planarizing insulation film 59 is formed to cover a part of the organic EL layer 60. The cathode line CL1 is formed on the organic EL layer 60. The cathode line CL1 extends over to the region where the vertical shift register unit VSR1 is formed, and is connected to the aluminum electrode 47 through a contact hole.
Next, operation of the organic EL display device described above will be explained hereinafter, referring to a timing chart shown in
During a second 1H period, the plurality of first data line drive circuits DLD1 outputs drive currents compensated for the threshold voltage to the data lines DL 1-DL6 all together. Only the cathode line CL 1 drops to the ground electric potential GND during the second 1H period. As a result, the drive currents flow through the organic EL devices 30R, 30G and 30B in the first row connected to the cathode line CL1, and each of the organic EL devices 30R, 30G and 30B emits light to a brightness corresponding to each of the drive currents, respectively. That is, regarding the organic EL device 30R, for example, the drive current supplied to the data line DL1 flows into the cathode line CL1 through the organic EL device 30R.
On the other hand, in the second 1H period, during which the plurality of first data line drive circuits DLD1 outputs the drive currents, the display signals Sig(R), Sig(G) and Sig(B) sampled by the plurality of sampling transistors ST12, ST14, ST16, and so on are sequentially read into the plurality of second data line drive circuits DLD2 and compensated for the threshold voltages of the driver transistors while they are retained.
During a third 1H period, the plurality of second data line driver circuits DLD2 outputs drive currents compensated for the threshold voltages to the data lines DL1-DL6 all together. Only the cathode line CL2 drops to the ground electric potential GND during the third 1H period, and the drive currents flow through the organic EL devices 30R, 30G and 30B in the second row connected to the cathode line CL2, and each of the organic EL devices 30R, 30G and 30B emits light to a brightness corresponding to each of the drive currents, respectively.
On the other hand, in the third 1H period, during which the plurality of second data line drive circuits DLD2 outputs the drive currents, the display signals Sig(R), Sig(G) and Sig(B) sampled by the plurality of sampling transistors ST11, ST13, ST15, and so on are sequentially read into the plurality of first data line drive circuits DLD1 and compensated for the threshold voltages of the driver transistors while they are retained.
A frame of image is displayed by repeating the operation described above over one frame period. The organic EL display device according to the first embodiment is a passive drive type that does not have a TFT in the pixel as described above. As a result, the aperture ratio of the pixel is improved and the coarse look on the display is reduced to realize a high quality display. And because various kinds of drive circuits such as the horizontal shift register 10, the data line drive circuit DLD and the vertical shift register 20 are formed on the single glass substrate 51 together with the pixel region in which the organic EL devices 30R, 30G and 30B are formed, there is no need to attach an external driver IC, making it possible to reduce the cost.
Also, because the first embodiment adopts a line-sequential drive method by which the data line drive circuit DLD provides the data lines DL1-DL6 with the drive currents all together for one horizontal period, a light-emitting period of the organic EL devices 30R, 30G and 30B can be made longer compared with an ordinary passive drive type display device, making it possible to realize a relatively bright display panel.
Next, detailed structures and operation of the first data line drive circuit DLD1 and the second data line drive circuit DLD2 will be described, referring to
The first thin film transistor T1 is for reading-in of the display signal, with its source connected to the sampling transistor, and a first control signal GS1 is applied to its gate. The first thin film transistor T1 is turned on when the first control signal GS1 is at a high level to read-in the display signal Sig(R), for example, and applies the display signal Sig(R) to a first terminal P1 of the coupling capacitor Cs, which is connected to a drain of the first thin film transistor T1. A second terminal P2 of the coupling capacitor Cs, which is facing to the first terminal P1, is connected to a gate of the second thin film transistor T2. The second thin film transistor T2 is a driver transistor, and the power supply electric potential PVdd is applied to its source.
And the first control signal GS1 is applied to a gate of the third thin film transistor T3 that is connected between the gate and the drain of the second thin film transistor T2. The third thin film transistor T3 is turned on to short-circuit the gate and the drain of the second thin film transistor when the first control signal GS1 is at the high level.
A second control signal CS1 is applied to a gate of the fourth thin film transistor T4, a reference electric potential Vref is applied to its source and its drain is connected to the first terminal P1 of the coupling capacitor Cs. The fourth thin film transistor T4 is turned on to set the first terminal P1 of the coupling capacitor Cs at the reference electric potential Vref when the second control signal CS1 is at the high level.
The fifth thin film transistor T5 and the sixth thin film transistor T6 are connected in series between the second thin film transistor T2 and the ground. A third control signal ES1 is applied to a gate of the fifth thin film transistor T5, and a reverse signal *CS1 that is a reverse of the second control signal CS1 is applied to a gate of the sixth thin film transistor T6.
The fifth thin film transistor T5 is connected to the data line DLi through the seventh thin film transistor T7 that is to control outputting of the drive current. An output of the first NAND circuit ND1 is applied to a gate of the seventh thin film transistor T7. The vertical clock CKV and an output enable signal ENB are inputted to the first NAND circuit ND1. The output enable signal ENB is a signal to prevent overlapping between the output signal of the first NAND circuit ND1 and an output signal of a second NAND circuit ND2 in the second data line drive circuit DLD2, which will be described later. And the organic EL device 30R, for example, is connected with the data line DLi, as described above.
Like the first data line drive circuit DLD1, the second data line drive circuit DLD2 is made of a first through seventh thin film transistors T1-T7, a coupling capacitor Cs and the second NAND circuit ND2, as shown in
Operation of the first data line drive circuit DLD1 and the second data line drive circuit DLD2 will be explained, referring to
Detailed explanation on the operation of the first data line drive circuit DLD1 is as follows.
First, when the first control signal GS1 turns to the high level, the first thin film transistor T1 is turned on to apply the display signal Sig(R), for example, from the sampling transistor to the first terminal P1 of the coupling capacitor Cs through the first thin film transistor T1. Also, the third thin film transistor T3 is turned on to short-circuit the gate and the drain of the second thin film transistor T2. Next, when the third control signal ES1 turns to the high level, electric charges at the gate of the second thin film transistor T2 are discharged to the ground through the fifth thin film transistor T5 and the sixth thin film transistor T6.
After that, the fifth thin film transistor T5 is turned off when the third control signal ES1 turns to a low level. Since the gate and the drain of the second thin film transistor T2 become electrically floating as a result, an electric potential at the gate and the drain becomes PVdd−Vtp. Vtp is an absolute value of the threshold voltage of the second thin film transistor T2. Then, the first thin film transistor T1 and the third thin film transistor T3 are turned off when the first control signal GS1 turns to the low level.
After that, when the second control signal CS1 turns to the high level in the second 1H period, the fourth thin film transistor T4 is turned on to set an electric potential at the first terminal P1 of the coupling capacitor Cs at the reference electric potential Vref. And the sixth thin film transistor T6 is turned off.
Since the electric potential at the first terminal P1 of the coupling capacitor Cs changes from Vsig to Vref when the fourth thin film transistor T4 is turned on, an electric potential at the second terminal P2 of the coupling capacitor Cs, that is the electric potential Vg at the gate of the second thin film transistor T2, changes from PVdd−Vtp to PVdd−Vtp+Vref−Vsig consequently.
After that, the fifth thin film transistor T5 is turned on when the third control signal ES1 turns to the high level again and the seventh thin film transistor T7 is turned on when an output of the first NAND circuit ND1 rises to a high level. Thus the second thin film transistor T2 is connected to the data line DLi through the fifth thin film transistor T5 and the seventh thin film transistor T7.
The drive current I flowing through the second thin film transistor T2 is represented by the following equation:
I=1/2·β·(Vgs+Vtp)2
where β is a constant.
Since,
Vgs=Vg−PVdd=−Vtp+Vref−Vsig
It follows:
I=1/2·β·(Vref−Vsig)2
That is, the drive current I is made to be independent of the threshold voltage Vtp of the second thin film transistor T2. The drive current I is supplied to the organic EL device 30R through the data line DLi to make a display corresponding to the display signal Vsig(R).
Next, an organic EL display device according to a second embodiment of this invention will be described hereafter, referring to the drawings.
First, a structure of a pixel region of the organic EL display device will be described. A plurality of data lines DL1-DL6 extends in a vertical direction (top to bottom direction in
A pixel selection transistor GT and an organic EL device 30R that emits red light are disposed around each of four intersections of the first column data line DL1 and the gate lines GL1-GL4. A drain of the pixel selection transistor GT is connected to the data line DL1 and its source is connected to an anode of the organic EL device 30R that emits red light. And a cathode of the organic EL device 30R is connected to a common cathode layer CL formed all over the pixel region.
Similarly, a pixel selection transistor GT and an organic EL device 30G that emits green light are disposed around each of four intersections of the second column data line DL2 and the gate lines GL1-GL4. A drain of the pixel selection transistor GT is connected to the data line DL2 and its source is connected to an anode of the organic EL device 30G that emits green light. And a cathode of the organic EL device 30G is connected to the common cathode layer CL.
Similarly, a pixel selection transistor GT and an organic EL device 30B that emits blue light are disposed around each of four intersections of the third column data line DL3 and the gate lines GL1-GL4. A drain of the pixel selection transistor GT is connected to the data line DL3 and its source is connected to an anode of the organic EL device 30B that emits blue light. And a cathode of the organic EL device 30B is connected to the common cathode layer CL. Pixels in a fourth column and beyond are structured in a way to repeat the structure described above. Inorganic EL devices may be used instead of the organic EL devices 30R, 30G and 30B.
Next, structures of a horizontal shift register 10 and a data line drive circuit DLD will be described hereafter. The horizontal shift register 10 and the data line drive circuit DLD are formed on the glass substrate 51. The horizontal shift register 10 has a plurality of horizontal shift register units HSR1, HSR2, and so on and a plurality of sampling transistors ST11, ST12, and so on. The sampling transistors ST11, ST12, and so on are thin film transistors.
Each of the plurality of horizontal shift register units HSR1, HSR2, and so on sequentially outputs each of horizontal scanning pulses SPH1, SPH2, and so on, respectively, by shifting a horizontal start pulse STH in synchronization with a horizontal clock CKH, as shown in
Six of the sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16 are disposed corresponding to the horizontal shift register unit HSR1 in a first stage. The horizontal scanning pulse SPH1 is inputted in common to gates of the six sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16. Similarly, six of the sampling transistors ST21, ST22, ST23, ST24, ST25 and ST26 are disposed corresponding to the horizontal shift register unit HSR2 in a second stage. The horizontal scanning pulse SPH2 is inputted in common to gates of the six sampling transistors ST21, ST22, ST23, ST24, ST25 and ST26.
Regarding the six sampling transistors ST11, ST12, ST13, ST14, ST15 and ST16, sources of a first couple of the sampling transistors ST11 and ST12 are connected to a first display signal line LR that supplies a red display signal Sig(R), sources of a second couple of the sampling transistors ST13 and ST14 are connected to a second display signal line LG that supplies a green display signal Sig(G) and sources of a third couple of the sampling transistors ST15 and ST16 are connected to a third display signal line LB that supplies a blue display signal Sig(B).
The data line drive circuit DLD has a first data line drive circuit DLD1 and a second data line drive circuit DLD2 corresponding to each of the data lines DL1-DL6. For example, the first data line drive circuit DLD1 corresponding to the data line DL1 reads-in and retains the red display signal Sig(R) through the sampling transistor ST11, and provides the data line DL1 with a drive current corresponding to the red display signal Sig(R). The first data line drive circuit DLD1 has a threshold voltage compensation circuit that compensates for a threshold voltage of a driver transistor, as will be described later. The variations in the brightness on the display panel due to the variations in the threshold voltages of the driver transistors can be suppressed because the drive current independent of the threshold voltage of the driver transistor is obtained with the threshold voltage compensation circuit.
The second data line drive circuit DLD2 operates similarly. While the first data line drive circuit DLD1 is controlled by a vertical clock CKV that has a period of two horizontal periods, the second data line drive circuit is controlled by a reverse vertical clock *CKV that is a reverse of the vertical clock CKV. As a result, the first data line drive circuit DLD1 and the second data line drive circuit DLD2 output the drive current to the data line DL 1 alternately for every other one horizontal period (1H period). One horizontal period means a time required to scan one horizontal line (the gate line GL1, for example).
Another first data line drive circuit DLD1 and another second data line drive circuit DLD2 corresponding to each of the other data lines DL2-DL6 are structured similarly.
Next, a structure of a vertical shift register 20 will be described hereafter. The vertical shift register 20 has a plurality of vertical shift register units VSR1, VSR2, and so on, connected in series and disposed on the glass substrate 51. The vertical shift register 20 is formed of thin film transistors.
Each of the plurality of vertical shift register units VSR1, VSR2, and so on sequentially outputs each of vertical scanning pulses SPV1, SPV2, and so on to corresponding each of the gate lines GL1, GL2, GL3 and GL4, respectively, by shifting a vertical start pulse STV in synchronization with the vertical clock CKV and the reverse vertical clock *CVK. The pixel selection transistor GT connected to each of the gate lines GL1, GL2, GL3 and GL4 is turned on only for a period during which the corresponding each of the vertical scanning pulse SPV1, SPV2, SPV3 and SPV4 is at a high level.
An interlayer insulation film 54 is formed on the gate electrode 45. In a region where the shift register unit VSR1 is formed, an aluminum electrode 47 is formed on the interlayer insulation film 54, making a contact with an underlying chromium electrode 46.
In the pixel region, the data line DL1 made of aluminum is formed on the interlayer insulation film 54. A protection film 55 and a first planarizing insulation film 56 are formed on the aluminum electrode 47 and the data line DL1. An anode 58 made of ITO (Indium Tin Oxide) is formed on the first planarizing insulation film 56 in the pixel region. An organic EL layer 60 is formed on the anode 58, and a second planarizing insulation film 59 is formed to cover a part of the organic EL layer 60. The cathode layer CL is formed on the organic EL layer 60. The cathode layer CL extends over the region where the vertical shift register unit VSR1 is formed, and is connected to the aluminum electrode 47 through a contact hole.
Next, operation of the organic EL display device described above will be explained hereinafter, referring to a timing chart shown in
During a second 1H period, the plurality of first data line drive circuits DLD1 outputs drive currents compensated for the threshold voltage to the data lines DL1-DL6 all together. During the second one horizontal period, the vertical scanning pulse SPV1 outputted to the gate line GL1 rises from a ground electric potential GND to a power supply electric potential Vcc. As a result, the pixel selection transistors GT connected to the first row gate line GL1 are turned on to provide the organic EL devices 30R, 30G and 30B connected to the first row gate line GL1 with the drive currents, and each of the organic EL devices 30R, 30G and 30B emits light to a brightness corresponding to each of the drive currents, respectively. That is, regarding the organic EL device 30R, the drive current supplied to the data line DL1 flows into the cathode layer CL through the pixel selection transistor GT and the organic EL device 30R to drive the organic EL device 30R to emit light to the brightness corresponding to the drive current.
On the other hand, in the second 1H period, during which the plurality of first data line drive circuits DLD1 outputs the drive currents, the display signals Sig(R), Sig(G) and Sig(B) sampled by the plurality of sampling transistors ST12, ST14, ST16, and so on are sequentially read into the plurality of second data line drive circuits DLD2 and compensated for the threshold voltages of the driver transistors while they are retained.
During a third 1H period, the plurality of second data line driver circuits DLD2 outputs drive currents compensated for the threshold voltages to the data lines DL1-DL6 all together. During the third 1H period, the vertical scanning pulse SPV2 outputted to the gate line GL2 rises from the ground electric potential GND to the power supply electric potential Vcc. As a result, the pixel selection transistors GT connected to the second row gate line GL2 are turned on to provide the organic EL devices 30R, 30G and 30B connected to the second row gate line GL2 with the drive currents, and each of the organic EL devices 30R, 30G and 30B emits light to a brightness corresponding to each of the drive currents, respectively. That is, regarding the organic EL device 30R, the drive current supplied to the data line DL1 flows into the cathode layer CL through the pixel selection transistor GT and the organic EL device 30R to drive the organic EL device 30R to emit light to the brightness corresponding to the drive current.
On the other hand, in the third 1H period, during which the plurality of second data line drive circuits DLD2 outputs the drive currents, the display signals Sig(R), Sig(G) and Sig(B) sampled by the plurality of sampling transistors ST11, ST13, ST15, and so on are sequentially read into the plurality of first data line drive circuits DLD1 and compensated for the threshold voltages of the driver transistors while they are retained.
A frame of image is displayed by repeating the operation described above over one frame period. Because the organic EL display device according to the second embodiment is a semi-passive drive type that has the electroluminescent device 30R, 30G or 30B and the pixel selection transistor GT in the pixel, the aperture ratio of the pixel is improved and a high quality display is realized by reducing the coarse look on the display. And because various kinds of drive circuits such as the horizontal shift register 10, the data line drive circuit DLD and the vertical shift register 20 are formed on the single glass substrate 51 together with the pixel region in which the organic EL devices 30R, 30G and 30B are formed, there is no need to attach an external driver IC, making it possible to reduce the cost. Also, because the second embodiment adopts a line-sequential drive method by which the data line drive circuit DLD provides the data lines DL1-DL6 with the drive currents all together for one horizontal period, a light-emitting period of the organic EL devices 30R, 30G and 30B can be made longer compared with an ordinary passive drive type display device, making it possible to realize a relatively bright display panel.
Next, detailed structures and operation of the first data line drive circuit DLD1 and the second data line drive circuit DLD2 will be described, referring to
The first thin film transistor T1 is for reading-in of the display signal, with its source connected to the sampling transistor, and a first control signal GS1 is applied to its gate. The first thin film transistor T1 is turned on when the first control signal GS1 is at a high level to read-in the display signal Sig(R), for example, and applies the display signal Sig(R) to a first terminal P1 of the coupling capacitor Cs, which is connected to a drain of the first thin film transistor T1. A second terminal P2 of the coupling capacitor Cs, which is facing to the first terminal P1, is connected to a gate of the second thin film transistor T2. The second thin film transistor T2 is a driver transistor, and the power supply electric potential PVdd is applied to its source.
And the first control signal GS1 is applied to a gate of the third thin film transistor T3 that is connected between the gate and the drain of the second thin film transistor T2. The third thin film transistor T3 is turned on to short-circuit the gate and the drain of the second thin film transistor when the first control signal GS1 is at the high level.
A second control signal CS1 is applied to a gate of the fourth thin film transistor T4, a reference electric potential Vref is applied to its source and its drain is connected to the first terminal P1 of the coupling capacitor Cs. The fourth thin film transistor T4 is turned on to set the first terminal P1 of the coupling capacitor Cs at the reference electric potential Vref when the second control signal CS1 is at the high level.
The fifth thin film transistor T5 and the sixth thin film transistor T6 are connected in series between the second thin film transistor T2 and the ground. A third control signal ES1 is applied to a gate of the fifth thin film transistor T5, and a reverse signal *CS1 that is a reverse of the second control signal CS1 is applied to a gate of the sixth thin film transistor T6.
The fifth thin film transistor T5 is connected to the data line DLi through the seventh thin film transistor T7 that is to control outputting of the drive current. An output of the first NAND circuit ND1 is applied to a gate of the seventh thin film transistor T7. The vertical clock CKV and an output enable signal ENB are inputted to the first NAND circuit ND1. The output enable signal ENB is a signal to prevent overlapping between the output signal of the first NAND circuit ND1 and an output signal of a second NAND circuit ND2 in the second data line drive circuit DLD2, which will be described later. And the organic EL device 30R, for example, is connected with the data line DLi, as described above.
Like the first data line drive circuit DLD1, the second data line drive circuit DLD2 is made of a first through seventh thin film transistors T1-T7, a coupling capacitor Cs and the second NAND circuit ND2, as shown in
Operation of the first data line drive circuit DLD1 and the second data line drive circuit DLD2 will be explained, referring to
Detailed explanation on the operation of the first data line drive circuit DLD1 is as follows.
First, when the first control signal GS1 turns to the high level, the first thin film transistor T1 is turned on to apply the display signal Sig(R), for example, from the sampling transistor to the first terminal P1 of the coupling capacitor Cs through the first thin film transistor T1. Also, the third thin film transistor T3 is turned on to short-circuit the gate and the drain of the second thin film transistor T2. Next, when the third control signal ES1 turns to the high level, electric charges at the gate of the second thin film transistor T2 are discharged to the ground through the fifth thin film transistor T5 and the sixth thin film transistor T6.
After that, the fifth thin film transistor T5 is turned off when the third control signal ES1 turns to a low level. Since the gate and the drain of the second thin film transistor T2 become electrically floating as a result, an electric potential at the gate and the drain becomes PVdd−Vtp. Vtp is an absolute value of the threshold voltage of the second thin film transistor T2. Then, the first thin film transistor T1 and the third thin film transistor T3 are turned off when the first control signal GS1 turns to the low level.
After that, when the second control signal CS1 turns to the high level in the second 1H period, the fourth thin film transistor T4 is turned on to set an electric potential at the first terminal P1 of the coupling capacitor Cs at the reference electric potential Vref. And the sixth thin film transistor T6 is turned off.
Since the electric potential at the first terminal P1 of the coupling capacitor Cs changes from Vsig to Vref when the fourth thin film transistor T4 is turned on, an electric potential at the second terminal P2 of the coupling capacitor Cs, that is the electric potential Vg at the gate of the second thin film transistor T2, changes from PVdd−Vtp to PVdd−Vtp+Vref−Vsig consequently.
After that, the fifth thin film transistor T5 is turned on when the third control signal ES1 turns to the high level again and the seventh thin film transistor T7 is turned on when an output of the first NAND circuit ND1 rises to a high level. Thus the second thin film transistor T2 is connected to the data line DLi through the fifth thin film transistor T5 and the seventh thin film transistor T7.
The drive current I flowing through the second thin film transistor T2 is represented by the following equation:
I=1/2·β·(Vgs+Vtp)2
where β is a constant.
Since,
Vgs=Vg−PVdd=−Vtp+Vref−Vsig
It follows:
I=1/2·β·(Vref−Vsig)2
That is, the drive current I is made to be independent of the threshold voltage Vtp of the second thin film transistor T2. The drive current I is supplied to the organic EL device 30R through the data line DLi to make a display corresponding to the display signal Vsig(R).
The electroluminescent display device according to the first embodiment of this invention is of the passive drive type that does not have a TFT in each of the pixels as described above. As a result, a high quality display is realized by reducing the grainy look and the coarse look on the display compared with the conventional active drive type EL display device.
That is, since being of the passive drive type allows practically using drain wirings both for providing image signals and for supplying the drive currents as well as sharing the pixel selection TFTs and the driver TFTs, and eliminates gate lines and capacitor lines to provide storage capacitors to retain the gate electric potentials of the driver TFTs, the aperture ratio can be improved and consequently the visibility of the grating pattern at the borders between the pixels can be reduced to reduce the grainy look on the display panel.
In addition, since the electroluminescent display device according to the first embodiment is of the passive drive type and uses the line-sequential drive, the signals for each row are provided all together and the signals are provided in common to the columns by the switch provided to each column to reduce variations in the brightness among the rows. The variations in the brightness among the columns are reduced by the compensation for the threshold voltages of the driver TFTs in the data line drive circuit that controls the amount of the electric current provided line-sequentially. The effects of suppressing the variations in the brightness among rows and columns result in the suppression of variations in the brightness of all the pixels over the panel to enable reducing the coarse look on the display panel.
Because various kinds of drive circuits such as the horizontal shift register, the data line drive circuit and the vertical shift register are formed on the single glass substrate together with the pixel region in which the electroluminescent devices are formed, there is no need to attach the external driver IC, making it possible to reduce the cost.
Also, because the electroluminescent display device according to the first embodiment adopts the line-sequential drive method used in an LCD and the like, in which the data line drive circuit provides the data lines with the drive currents all together for a predetermined period (one horizontal period, for example), the light-emitting period is made longer and thus a brighter display panel is made available compared with a conventional passive drive type display device that uses a dot-sequential drive method.
The electroluminescent display device according to the first embodiment is suitable for a small display device such as a display device for the EVF, because it is basically of passive drive type and the light-emitting period of the electroluminescent devices is shorter than that of the active drive type.
The electroluminescent display device according to the second embodiment of this invention is of the semi-passive drive type that has an electroluminescent device and a pixel selection transistor in each of the pixels. As a result, a high quality display is realized by reducing the grainy look and the coarse look on the display compared with the conventional active drive type EL display device.
That is, since being of the semi-passive drive type allows practically using the drain wirings both for providing image signals and for supplying the drive currents as well as sharing the driver TFTs, and eliminates capacitor lines to provide storage capacitors to retain the gate electric potentials of the driver TFTs, the aperture ratio can be improved and consequently the visibility of the grating pattern at the borders between the pixels can be reduced to reduce the grainy look on the display panel.
In addition, since the electroluminescent display device according to the second embodiment is of the semi-passive drive type and uses the line-sequential drive, the signals for each row are provided all together and the signals are provided in common to the columns by the switch provided to each column to reduce variations in the brightness among the rows. The variations in the brightness among the columns are reduced by the compensation for the threshold voltages of the driver TFTs in the data line drive circuit that controls the amount of the electric current provided line-sequentially. The effects of suppressing the variations in the brightness among rows and columns result in the suppression of variations in the brightness of all the pixels over the panel to enable reducing the coarse look on the display panel.
Because various kinds of drive circuits such as the horizontal shift register, the data line drive circuit and the vertical shift register are formed on the single glass substrate together with the pixel region in which the electroluminescent devices are formed, there is no need to attach the external driver IC, making it possible to reduce the cost. In addition, there is no need to provide the cathode line isolation material to separate the cathode lines, which is required in the conventional passive drive type display device.
Also, because the electroluminescent display device according to the second embodiment adopts the line-sequential drive method used in an LCD and the like, in which the data line drive circuit provides the data lines with the drive currents all together for a predetermined period (one horizontal period, for example), the light-emitting period is made longer and thus a brighter display panel is made available compared with a conventional passive drive type display device that uses a dot-sequential drive method.
The electroluminescent display device according to the second embodiment of this invention is suitable for a small display device such as a display device for the EVF.
According to the data line drive circuit of the embodiments, the variations in the brightness on the display panel can be reduced to improve the quality of the display because the data line drive circuit outputs drive currents compensated for the threshold voltage of the driver transistors. Also, the light-emitting period of the organic EL devices can be extended by providing the first and second data line drive circuits and by outputting the drive currents alternately from the first and second data line drive circuits.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-131261 | Apr 2005 | JP | national |
| 2005-131262 | Apr 2005 | JP | national |
| 2005-131263 | Apr 2005 | JP | national |
