Patent Application
20110187888
1. Field of the Invention
The present invention relates to a drive circuit and a drive method of a light emitting display apparatus using light emitting display devices, especially using organic electro-luminescence (hereinafter briefly referred to as EL) devices.
2. Related Background Art
In recent years, the development of displays using self light emitting devices, such as an inorganic EL device, an organic EL device, and a light emitting diode, has actively performed as the next generation large-screen thin-shaped display apparatus followed by a liquid crystal display apparatus and a plasma display apparatus.
In particular, because the organic EL device can be formed by depositing layers on a glass substrate and a flexible substrate such as a film, the full-scale practical realization of the organic EL device has been expected from the recent improvement of the luminous efficiency and the reliability thereof.
The drive of the organic EL device is principally performed by an active matrix drive system, which has been proven in liquid crystal displays and is realized by thin film transistors (hereinafter referred to as TFTs) using poly-silicon, an amorphous semiconductor, or the like.
Among them, the amorphous semiconductor can be formed on a film substrate by a low temperature process, and thereby has a technological advantage in the use of large-size and thin-shaped displays.
The active-matrix (hereinafter referred to as AM) type organic EL display apparatus adjusts the brightness thereof and performs gradation display by controlling the voltages and the currents that are supplied to the organic EL devices according to voltage signals or current signals that are supplied on drive transistors (see
However, there is the following actual problem in controlling the brightness of the organic EL devices by means of the drive transistors.
That is, the electric characteristics of the drive transistors are sometimes dispersed in respective pixels owing to the variation of manufacturing processes. Moreover, the electric characteristics of the drive transistors sometimes change dependently on environments and electrification times. When the electric characteristics change, the changes of threshold values are especially remarkable.
Even if the same data signal is applied, the currents flowing through the drive transistors are consequently different from one another. The changes of the currents cause production of the differences of brightness of the light emitting devices at each pixel and each time, and therefore bring about display unevenness extending over the whole display screen.
On the other hand, it has been proposed to cancel the dispersion of the electric characteristics of the drive transistors by operating the voltages supplied to light emitting devices to have the same electric potential as the data signals by negative-feedback loops using differential amplifiers (see, for example, U.S. Pat. No. 6,809,706). In this case, the control of the brightness of the light emitting devices is performed based on voltages.
However, if the brightness of organic EL devices is controlled based on voltages, the gradation control of the devices is more complicated because the brightness-voltage characteristics of the devices do not have linearity.
Moreover, because the brightness-voltage characteristics change with time, the method of such a control is also needed to be changed according to the aged deterioration of the characteristics.
From the above reasons, it is desirable to control the brightness not using voltages but using currents.
Accordingly, it is an object of the present invention to provide a drive circuit realizing good image quality even if the threshold values of drive transistors show variation, or aged deterioration when stable currents are supplied to light emitting devices in a light emitting display apparatus. That is, according to a scope of the present invention, a drive circuit and a drive method of a light emitting display apparatus are both capable of realizing good image quality by correcting the threshold values of drive transistors when the light emitting operations of light emitting devices are performed at desired brightness gradation.
The inventors have made the present invention with great efforts to solve the above-described problem.
A drive circuit of the present invention for a light emitting display apparatus including a pixel circuit having a light emitting device for emitting a light having brightness determined based on a supplied current and a drive transistor for supplying the current to the light emitting device, comprises: a threshold value correction circuit for converting a second signal including a threshold voltage of the drive transistor and a data voltage, the second voltage being output from the drive transistor when a first signal including the data voltage is input into a control electrode of the drive transistor, into a third signal including the threshold voltage of an inverted polarity and the data voltage or a voltage corresponding to the data voltage, and outputting the third voltage to the pixel circuit, wherein the pixel circuit includes a switch for supplying the third signal to the control electrode of the drive transistor.
Moreover, a drive circuit of the present invention for a light emitting display apparatus including a pixel circuit having a light emitting device for emitting a light having brightness determined based on a supplied current, a drive transistor to supply the current to the light emitting device, a first to a fifth switches, and a capacitor, and a threshold value correction circuit for correcting a threshold value of the drive transistor of the pixel circuit, wherein the threshold value correction circuit includes a first and a second operational amplifiers, and a first and a second resistor devices so that the first operational amplifier has an output terminal connected to an inversion input (also called negative input) terminal of the second operational amplifier through the first resistor device, a non-inversion input (also called positive input) terminal connected to a source terminal of the drive transistor through the first switch, and an inversion input terminal connected to the output terminal of the first operational amplifier, wherein the second operational amplifier has an output terminal connected to an inversion input terminal thereof through the second resistor device, wherein the capacitor is connected to the output terminal of the second operational amplifier through the second switch, wherein the third switch is connected between the source terminal of the drive transistor and the ground, and wherein one of terminals of each of the fourth switch and the fifth switch is connected to a gate terminal of the drive transistor, the other terminal of the fourth switch is connected to a connection point between the capacitor and the second switch, and the other terminal of the fifth switch is connected to a signal line from a data driver.
Moreover, a drive circuit of the present invention for a light emitting display apparatus including a pixel circuit having a light emitting device for emitting a light having brightness determined based on a supplied current, a drive transistor for supplying the current to the light emitting device, a first to a fourth switches, and a capacitor, and a threshold value correction circuit for correcting a threshold value of the drive transistor of the pixel circuit, wherein the threshold value correction circuit includes a first and a second operational amplifiers, and a first and a second resistor devices, wherein the first operational amplifier has an output terminal connected to an inversion input terminal of the second operational amplifier through the first resistor device, a non-inversion input terminal connected to a source terminal of the drive transistor through the first switch, and an inversion input terminal connected to the output terminal of the first operational amplifier, wherein the second operational amplifier has an output terminal connected to the capacitor through the second switch, wherein the third switch is connected between the source terminal of the drive transistor and the ground, and wherein a connection point between the capacity device and the second switch is connected to a gate terminal of the drive transistor and is connected to a signal line from a data driver through the fourth switch.
A drive method of the present invention for a light emitting display apparatus including a pixel circuit having a light emitting device for emitting a light having brightness determined based on a supplied current and a drive transistor for supplying the current to the light emitting device, comprises: a first period in which a first signal including a data voltage is input into a control electrode of the drive transistor and a second signal, output from the drive transistor and including a threshold voltage of the drive transistor and the data voltage, is converted into a third signal including the threshold voltage of an inverted polarity and the data voltage or a voltage corresponding to the data voltage, to be stored in a capacitor; and a second period in which the third signal stored in the capacitor is supplied to the control electrode of the drive transistor to make the light emitting device emit a light.
A camera of the present invention is the one using any one of the above-mentioned drive circuits of the present invention for the light emitting display apparatus.
According to the present invention, it can be attained to stably provide a high quality light emitting display apparatus that does not cause a problem of display unevenness of image quality and the like even if variation of threshold values of drive transistors is caused owing to the factors of manufacturing processes, environmental conditions, and electrification times.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Next, the exemplary embodiments of the present invention will be described with reference to the attached drawings.
The drive transistors that can be used for the present invention are those having variation of threshold values among the parameters pertaining to electric optical characteristics, or having shift of threshold values as changes of electric optical characteristics owing to electric stresses.
The present invention corrects the variation of the threshold voltages of the transistors for driving to obtain the stable emission intensity of light emitting devices with the suppressed influences of the threshold voltages of the transistors for driving. As the light emitting devices, the light emitting devices emitting lights having the brightness determined by supplied currents, for example, AM type organic EL devices can be used.
In the following, the configuration and the operation of an exemplary embodiment of a drive circuit of the present invention will be described with reference to the drawings. As illustrated in
If the gate-to-source voltage is denoted by Vgs here, then the drain current Ids of the drive transistor is expressed by Ids∝(Vgs−Vth)2. Because there is a relation Vgs=Vdata+Vth, the drain current Ids can be expressed as Ids∝(Vdata)2.
Additional explanation of inversion of the polarity (positive and negative) of the above-described threshold voltage Vth will be given. The expression “inverting the polarity of the Vth” is used in the specification of the present application to mean that the polarity of the Vth included in the above-described equation Vdata−Vth is inverted. That is, let the Vth the polarity of which is inverted be Vth′, then this readily provides Vth′=−Vth. Therefore, a signal resulted in after the polarity inversion is expressed by Vdata−(Vth′)=Vdata−(−Vth)=Vdata+Vth.
Consequently, the current to be supplied to the light emitting device by the drive transistor during the light emitting device driving period (light emitting period) has a value independent of the threshold value Vth of the drive transistor, and the current scarcely receives the influences of the value of the threshold voltage Vth and the changes of the value.
Incidentally, only the thing required for the threshold value correction circuit is to have the function of inverting the polarity (positive or negative) of the threshold voltage Vth, and the threshold value correction circuit is not necessarily required to output the voltage (Vdata+Vth). The threshold value correction circuit may output, for example, a voltage (2V1−Vdata+Vth) as it will be described in a second exemplary embodiment described below. V1 denotes an arbitrary DC voltage, and (2V1−Vdata) indicates a signal corresponding to the data voltage Vdata here. If V1=Vdata, then the output of the threshold value correction circuit is the voltage (Vdata Vth).
In the present exemplary embodiment, the threshold value correction circuit is provided to each pixel circuit including a drive transistor, and the threshold value correction circuit and the pixel circuit may constitute a pixel. However, as illustrated in
Incidentally, in the present specification, a field-effect transistor for driving is described as the drive transistor.
As illustrated in
As illustrated in
In the pixel circuit 11, the cathode of the organic EL device 9 is connected to the drain terminal of the N type drive transistor Tr10. The data voltage Vdata is input into the gate of the drive transistor Tr10, which is the control electrode thereof, through the fifth switch SW5. On the other hand, the output from the threshold value correction circuit 31 is accumulated in the capacitor 19 through the second switch SW2. In a period during which the organic EL device 9 emits a light, the signal from the threshold value correction circuit 31 is output to the gate of the drive transistor Tr10 by turning off the fifth switch SW5 and by turning on the fourth switch SW4.
The output terminal of the operational amplifier AMP7, which is a first operational amplifier, is connected to the inversion input terminal of the operational amplifier AMP8, which is a second operational amplifier, through the resistor device R14, which is a first resistor device. The non-inversion input terminal of the operational amplifier AMP7 is connected to the source terminal of the drive transistor Tr10 through the first switch SW1, and the inversion input terminal of the operational amplifier AMP7 is connected to its own output terminal. The output terminal of the operational amplifier AMPS is connected to its own inversion input terminal through the resistor device R15. The capacitor 19 is connected to the output terminal of the operational amplifier AMP8 through the second switch SW2. The third switch SW3 is connected between the source terminal of the drive transistor Tr10 and the ground. One of the terminals of each of the fourth switch SW4 and the fifth switch SW5 is connected to the gate terminal of the drive transistor Tr10. The other terminal of the fourth switch SW4 is connected to the connection point of the capacitor 19 and the second switch SW2. The other terminal of the fifth switch SW5 is connected to the signal line from the data driver 42. The sixth switch SW6 is connected to both the terminals of the organic EL device 9 in parallel to the organic EL device 9.
The transistors and the capacitor 11 in the pixel circuit 11 are formed by an amorphous semiconductor process, and the current source 12, the resistor devices R14 and R15, and the operational amplifiers AMP7 and AMP8 in the threshold value correction circuit 31 are produced by a crystal Si process. The data driver section is similarly produced by the crystal Si process.
The circuit configuration of the present exemplary embodiment uses the first switch SW1 to the sixth switch SW6, and includes a circuit configuration to realize at least two modes of a “threshold value programming period” and a “light emitting device driving period” at the time of image display. The timing chart of each switch at that time is illustrated in
In
Threshold Value Programming Period
As illustrated in
Light Emitting Device Driving Period
In the following, the operation of the circuit will be described.
In the threshold value programming period, as illustrated in
The drain of the drive transistor Tr10 at this time is connected to the power source, and the source thereof is connected to the current source 12. The drive transistor Tr10 and the current source 12 constitutes a source follower circuit, and the source voltage is input into the inversion input terminal of the operational amplifier AMP8, which is an inversion amplifier having the gain of one time, through the operational amplifier AMP7, which is a voltage follower. The resistor devices R14 and R15 are set to have the same resistance values.
If it is supposed that the output of the source follower is a node 16, and that the output of the voltage follower amplifier is a node 17 in
(Vin+Vout)/2=Vref
Vout=2Vref−Vin.
If Vin=Vdata−Vth and Vref=Vdata, then
Vout=Vdata+Vth (1)
This electric potential is held in the capacitor 19. Moreover, the organic EL device 9 does not emit any lights in this period.
Next, in the light emitting device driving period, as illustrated in
The drain terminal of the drive transistor Tr10 is connected to the cathode of the organic EL device, and the source terminal thereof is connected to the ground.
The drain current Ids of the drive transistor Tr10 can be expressed here as
Ids∝(Vgs−Vth)2
where Vgs denotes the gate-to-source voltage thereof.
Because Vgs=Vdata Vth here owing to the operation in the previous threshold value programming period, the drain current Ids can be expressed as
Ids∝(Vdata)2 (2).
Hereby, the current supplied from the drive transistor Tr10 to the organic EL device 9 takes a value independent of the threshold value Vth of the drive transistor Tr10, and is scarcely influenced by the value of the threshold value Vth and the changes of the value.
Incidentally, although the case in which the drive transistor Tr10 is that of the N type has been described in the aforesaid exemplary embodiment, the similar advantages can be expected even in the case of using the P type drive transistor.
Moreover, although the light emitting display apparatus using the organic EL devices has been described, the present invention can be also applied to a light emitting display apparatus that emits a light based on a supplied current and a current load device using a general current load expressing an arbitrary function base on a supplied current.
The configuration of a second exemplary embodiment of the present invention is illustrated in
In this case, in the threshold value programming period, the organic EL device 9 emits a light based on a supplied current having a parameter of the threshold voltage Vth of the drive transistor Tr10 as expressed by: Ids cc (Vdata−Vth)2. However, because the threshold value programming period is a sufficiently short period as compared with the normal light emitting device driving period, the influences of the brightness in that period can be neglected when the influences are observed in the entire frame period.
The configuration of a third exemplary embodiment of the present invention is illustrated in
In this case, in the threshold value programming period, the node 16 takes substantially the electric potential level of (Vdata−Vth), and consequently, almost the same advantages can be obtained by the similar operation to that of the first exemplary embodiment.
The configuration of a fourth exemplary embodiment of the present invention can be illustrated by
In this case, in the aforesaid threshold value programming period, the output of the operational amplifier AMP8 is
(Vin+Vout)/2=Vref,
Vout=2Vref−Vin
where Vin=Vdata−Vth and Vref=V1 (V1 denotes an arbitrary DC voltage), and consequently is
Vout=2V1−Vdata+Vth (3).
On the other hand, the current flowing through the drive transistor Tr10 during the light emitting device driving period is
Ids∝(Vgs−Vth)2,
Vgs=2V1−Vdata+Vth, and consequently is
Ids∝(2V1−Vdata)2 (4).
Then, the supplied current to the organic EL device 9 is scarcely influenced by the value of the threshold voltage Vth and the changes of the value similarly to that of the first exemplary embodiment.
A fifth exemplary embodiment of a drive circuit of the light emitting display apparatus of the present invention is illustrated in
The circuit configuration of the present exemplary embodiment includes a circuit configuration for realizing at least two modes of the “threshold value programming period” and the “light emitting device driving period” at the time of image display similarly to that of the first exemplary embodiment. The timing chart of each switch at that time is illustrated in
Threshold Value Programming Period
Light Emitting Device Driving Period
As illustrated in
In the following, the operation of the circuit will be described.
In the threshold value programming period, the operation of the fifth exemplary embodiment is similar to that of the first exemplary embodiment. However, as illustrated in
Next, in the light emitting device driving period, as illustrated in
Next, the switch SW2 is turned to be in its off-state, and the charges of the capacitor 20 have moved to the capacitor 19.
The gate terminal of the drive transistor Tr10 is held by the capacitor 19 to be the voltage (Vdata+Vth). At the same time, the drain terminal of the drive transistor Tr10 is connected to the cathode of the organic EL device 9, and the source terminal thereof is connected to the ground.
The drain current Ids of the drive transistor Tr10 may be expressed here similarly in the first exemplary embodiment as
Ids∝(Vdata)2 (2).
Hereby, the current supplied from the drive transistor Tr10 to the organic EL device 9 takes a value independent of the threshold value Vth of the drive transistor Tr10, and is scarcely influenced by the changes of the value of the threshold voltage Vth.
In the present exemplary embodiment, the number of the switches in a pixel can be reduced by one as compared with six of the first exemplary embodiment.
Furthermore, if the sixth switch SW6 is also reduced similarly to the second exemplary embodiment, the present exemplary embodiment can operated with four switches in a pixel.
The AM type organic EL display apparatus of each of the aforesaid exemplary embodiments may configure an information display apparatus. The information display apparatus is used in a cellar phone, a portable computer, a camera such as a still camera and a video camera, and an apparatus realizing their plural functions.
In the following, as a desirable exemplary embodiment of the present invention, a digital camera using the AM type organic EL display apparatus described as the first exemplary embodiment is described.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the present invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims priority from Japanese Patent Application No. 2007-061871 filed on Mar. 12, 2007, which is hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-061871 | Mar 2007 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12028588 | Feb 2008 | US |
| Child | 13086226 | US |
