Cathode potential controller, self light emission display device, electronic apparatus, and cathode potential controlling method

A cathode potential controller for controlling a common cathode potential applied to a self light emission type display panel in which an emission state of each of pixels is driven and controlled in accordance with an active matrix drive system, the cathode potential controller including: a self light emitting element; a constant current source; an electrode-to-electrode voltage measuring portion; a cathode potential determining portion; and a cathode potential applying portion.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2007-144186 filed in the Japan Patent Office on May 30, 2007, the entire contents of which being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for correcting a fluctuation of a driving current due to the temperature characteristics of each of self light emitting elements constituting pixels of a self light emission display panel, respectively. In particular, the invention relates to a cathode potential controller and a cathode potential controlling method each of which is capable of correcting an influence which temperature characteristics of a self light emitting element exerts on a bootstrap operation of a drive transistor by variably controlling a cathode potential of the self light emitting element, a self light emission display device, and an electronic apparatus.

2. Description of the Related Art

At present, the various kinds of flat panel display devices are put to practical use. An organic Electro Luminescence (EL) display panel in which organic EL elements are disposed in a matrix within a display region is known as one of them. The organic EL display panel is not only readily thinned because of its lightness, but also is excellent in the moving image display characteristics because of its high response speed.

However, the following problem is pointed out. That is to say, when a driving current changes depending on an environmental temperature or a temperature change following exothermic heat of the organic EL display panel itself, an emission luminance changes in terms of the characteristics common to the organic EL display panels in each of which the emission luminance changes depending on the magnitude of the driving current.

Actually, the current vs. voltage characteristics of the organic EL element have the temperature characteristics. Therefore, even when a drive transistor is driven with the same voltage, the magnitude of the driving current fluctuates depending on the temperature. Thus, the technique for reducing the luminance change due to the temperature dependency characteristics is desired to be developed.

SUMMARY OF THE INVENTION

The technique for variably controlling a power source voltage, on a high potential side, which is applied to a pixel portion (corresponding to an effective display region described in this specification) in accordance with a voltage developed at an anode electrode of a monitoring element when a constant current is caused to flow through the monitoring element is disclosed in Japanese Patent Laid-Open No. 2006-11388 (hereinafter referred to as Patent Document 1).

That is to say, the technique for variably controlling a potential difference between the (variably controlled) high potential side power source and the (fixed) low potential side power source is disclosed in Patent Document 1. However, with this correcting technique disclosed therein, such an influence that the luminance change is caused due to the fluctuation, of a driving voltage (a gate to source voltage Vgs) of a drive transistor, following a bootstrap operation is not taken into consideration at all.

In the light of the foregoing, it is therefore desire to provide a cathode potential controller and a cathode potential controlling method each of which is capable of correcting an influence which temperature characteristics of a self light emitting element exerts on a bootstrap operation of a drive transistor by variably controlling a cathode potential of the self light emitting element, a self light emission display device, and an electronic apparatus.

In addition, it is also desire to provide correcting techniques for the case where a self light emitting element for voltage measurement is used, and the case where a self light emitting element for display and measurement, respectively.

In order to attain the desire described above, according to an embodiment of the present invention, there is provided a cathode potential controller for controlling a common cathode potential applied to a self light emission type display panel in which an emission state of each of pixels is driven and controlled in accordance with an active matrix drive system, the cathode potential controller including:

(a) a self light emitting element for voltage measurement disposed outside an effective display region;

(b) a constant current source for supplying a constant current to the self light emitting element for voltage measurement;

(c) an electrode-to-electrode voltage measuring portion for measuring a potential developed at an anode electrode of the self light emitting element for voltage measurement, and measuring an electrode to electrode voltage of the self light emitting element for voltage measurement;

(d) a cathode potential determining portion for determining a cathode potential value by using a difference value between a measured value of the electrode to electrode voltage of the self light emitting element for voltage measurement, and a reference voltage value as a correction value; and

(e) a cathode potential applying portion for applying a cathode potential corresponding to the determined cathode potential value to a common cathode electrode of the self light emission type display panel.

According to another embodiment of the present invention, there is provided a cathode potential controller for controlling a common cathode potential applied to a self light emission type display panel in which an emission state of each of pixels is driven and controlled in accordance with an active matrix drive system, the cathode potential controller including:

(a) a constant current source for voltage measurement disposed outside an effective display region for supplying a constant current to a self light emitting element for display and measurement constituting a specific pixel, the self light emitting element for display and measurement being disposed inside the effective display region;

(b) an electrode-to-electrode voltage measuring portion for measuring a potential developed at an anode electrode of the self light emitting element for display and measurement constituting the specific pixel in a phase of measuring an electrode to electrode voltage of the self light emitting element for display and measurement, and measuring the electrode to electrode voltage of the self light emitting element for display and measurement;

(c) a cathode potential determining portion for determining a cathode potential value by using a difference value between a measured value of the electrode to electrode voltage of the self light emitting element for display and measurement, and a reference voltage value as a correction value; and

(d) a cathode potential applying portion for applying a cathode potential corresponding to the determined cathode potential value to a common cathode electrode of the self light emission type display panel.

According to the present embodiment, the cathode potential value of the organic EL element for display and measurement is controlled in accordance with the difference value between the measured value of the electrode to electrode voltage of the self light emitting element for display and measurement, and the reference voltage value (the electrode to electrode voltage of the self light emitting element for display and measurement at a room temperature).

For example, when a temperature is lower than the room temperature, the electrode to electrode voltage of the self light emitting element for display and measurement moves to lower voltages with respect to the reference voltage value. In this case, therefore, the control is carried out such that the cathode potential value moves to higher voltages by the difference value.

On the other hand, for example, when the temperature is higher than the room temperature, the electrode to electrode voltage of the self light emitting element for display and measurement moves to higher voltages with respect to the reference voltage value. In this case, therefore, the control is carried out such that the cathode potential value is reduced by the difference value.

As a result, even when the temperature changes, the driving voltage for the drive transistor after completion of the bootstrap operation is controlled so as to become the same state as that at the room temperature. That is to say, the control can be carried out such that the temperature change in current vs. voltage characteristics of the self light emitting element does not appear in the form of a change in driving current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description will be given with respect to the case where the present invention is applied to cathode potential control for an active matrix drive type organic EL display panel.

It is noted that the well-known or known technique in this technical field is applied to any of portions which are especially illustrated or described in this specification.

(A) Principles of Generation of Temperature Characteristics of Driving Current

(a) Principles of Fluctuation of Driving Current Due to Temperature Characteristics of Organic EL Element

Firstly, a mechanism in which a driving current for a drive transistor fluctuates due to the temperature characteristics of an organic EL element will now be described by giving a current control type organic EL display panel as an example.

FIG. 1shows the temperature characteristics which current vs. voltage characteristics of an organic EL element generally have. As shown inFIG. 1, when a constant current is caused to flow through the organic EL element, an electrode to electrode voltage Velof the organic EL element falls with a rise in a temperature.

Hereinafter, a bootstrap operation of a drive transistor shown inFIG. 3will be described with reference to a circuit diagram of a pixel circuit shown inFIG. 2. By the way,FIG. 2shows the case where a pixel circuit2is composed of two N-channel thin film transistors T1and T2.

Of the two N-channel thin film transistors T1and T2, the N-channel thin film transistor T1is a transistor for controlling the operation for writing pixel data to a storage capacitor C. On the other hand, the N-channel thin film transistor T2is a transistor for supplying a driving current Idhaving a magnitude corresponding to a voltage Vgsheld in the storage capacitor C to the organic EL element. That N-channel thin film transistor T2corresponds to a drive transistor as an object of the description given herein.

The operation of the pixel circuit makes progress as follows. Firstly, the N-channel thin film transistor T1is controlled so as to become an ON state. As a result, the pixel circuit is connected to a signal line Vsig. At this time, the charges corresponding to a signal potential Vdataapplied to the signal line Vsigare accumulated in the storage capacitor C. It is noted that in a phase of writing the signal potential Vdata, a power source voltage VDD is controlled so as to become a grounding potential.

When the operation for writing the signal potential Vdatais completed, the N-channel thin film transistor T1is controlled so as to be turned OFF, and at the same time, the power source voltage VDD is controlled so as to become a driving voltage (positive power source voltage). A driving current corresponding to a gate to source voltage Vgs(a voltage held in the storage capacitor C) in a moment when the N-channel thin film transistor T1is controlled so as to be turned OFF starts to be caused to flow through the drive transistor T2along with that control operation for the power source voltage VDD.

At this time, a voltage (electrode to electrode voltage) Velcorresponding to a magnitude of the driving current is developed across the electrodes of the organic EL element. The magnitude of the electrode to electrode voltage Velfluctuates depending on the temperature characteristics, though. A rise amount when a source potential Vschanges to Vs′ owing to the electrode to electrode voltage Velis expressed by Vanode. At the same time, the gate potential Vgfor the drive transistor T2rises to Vg′.

An operation in which each of the source potential Vsand the gate potential Vgchanges along with the supply of the driving current is called a bootstrap operation. As a result, a value of the driving current for the drive transistor T2changes to a value corresponding to the gate to source voltage Vgs′ after completion of that change.

It is noted that a relationship expressed by the following Expression (1) is recognized between the gate to source voltage Vgs′ after completion of the bootstrap operation and the gate to source voltage Vgsbefore completion of the bootstrap operation:
Vgs′=Vgs−(1−Gb)×Vanode(1)

Where a value of Gbis a bootstrap gain which is equal to or smaller than 1.0.

FIG. 4shows a temperature change in the bootstrap operation of the organic EL element. In the figure, an operation at a room temperature is indicated by a fine broken line, and an operation at a high temperature is indicated by a heavy solid line.

The electrode to electrode voltage Velof the organic EL element changes to decrease with the rise in the driving temperature. Along with this change, Vanoderegulating a rise amount of source potential Vsfollowing the bootstrap operation further falls than that at the room temperature.

This means that a term of (1−Gb)×Vanodein Expression (1) decreases. As a result, the gate to source voltage Vgs′ increases. When the gate to source voltage Vgs′ becomes larger than that at the room temperature, an amount of driving current naturally further increases than that at the room temperature.

On the other hand, when the driving temperature is lower than at the room temperature, the electrode to electrode voltage Velof the organic EL element increases. Also, Vanodegiving a rise amount of source potential Vsfollowing the bootstrap operation becomes larger than that at the room temperature.

As a result, the term of (1−Gb)×Vanodein Expression (1) increases, the gate to source voltage Vgs′ after completion of the bootstrap operation decreases, which results in that the driving current decreases.

The foregoing is the reason that the temperature characteristics appear in the driving current after completion of the bootstrap operation.

(b) Principles of Fluctuation of Driving Current Due to Temperature Characteristics of Drive Transistor

FIG. 5shows temperature characteristics which the current vs. voltage characteristics of the drive transistor generally have.

As shown inFIG. 5, a mobility of the drive transistor T2increases with a rise in the driving temperature. Also, when the same gate to source voltage Vgsis applied to the drive transistor T2, a current which is caused to flow through the drive transistor T2further increases at the high temperature than at the low temperature. Contrary, the current decreases at the low temperature.

As has been described so far, in the current control type organic EL display panel, the driving current and the emission luminance fluctuate due to the temperature fluctuation caused by the exothermic heat or the like of the organic EL display panel itself following the environmental temperature and the light emission.

(B) Principles of Correcting Fluctuation of Driving Current

For the purpose of correcting the fluctuation of the driving current due to the temperature characteristics of the organic EL element, it is necessary to hold the gate to source voltage Vgs′ after completion of the bootstrap operation at a constant value irrespective of the temperature change.

FIG. 6shows the control principles for correcting the gate to source voltage Vgs′ at the high temperature to the same value as that at the room temperature.

As shown inFIG. 6, the inventors of the present embodiment makes a device in such a way that a cathode potential Vcathodeof the organic EL element is caused to rise from the grounding potential GND, which results in that an anode potential Vanodeof the organic EL element is controlled so as to become the same voltage value as that at the room temperature.

Performing this control operation results in that a value of the anode potential Vanoderegulating a rise amount of source potential Vsbecomes identical to the value of the anode potential Vanodeat the room temperature. As a result, the gate to source voltage Vgs′ is controlled so as to become the same state as that at the room temperature. In the manner as described above, the fluctuation of the driving current due to the temperature characteristics of the organic EL element is properly corrected.

By the way, for realization of this correcting operation, it is necessary to perform the following operation. That is to say, a change in electrode to electrode voltage Velof the organic EL element following the fluctuation of the driving temperature is measured. Also, a difference value between that electrode to electrode voltage Velof the organic EL element thus changed and the electrode to electrode voltage Velof the organic EL element at the room temperature is fed back to the cathode potential of the organic EL element.

However, there is a problem in supplying the driving current from the drive transistor T2for the purpose of measuring the electrode to electrode voltage Velof the organic EL element. The reason for this is because as described above, the drive transistor T2has the temperature characteristics (refer toFIG. 5), and thus the driving current fluctuates depending on the driving temperatures.

In view of the foregoing, the inventors of the present embodiment proposes the technique with which a constant current source (a current source capable of causing a constant current to flow irrespective of the temperature) which has no temperature characteristics unlike the case of the drive transistor T2is specially prepared, and a constant current is caused to flow through the organic EL element from the constant current source, thereby measuring the electrode to electrode voltage of the organic EL element.

Specially preparing the constant current source in such a manner makes it possible to separate the temperature characteristics of the driving transistor T2from the measured value of the electrode to electrode voltage of the organic EL element. As a result, there is ensured the correcting operation in which only the temperature characteristics of the organic EL element are reflected.

In Embodiment 1 of the present invention, a detailed description will be given hereinafter with respect to the case where the electrode to electrode voltage (the voltage developed across the anode electrode and the cathode electrode) Velof the organic EL element is measured by using one of the pixels (a pixel for display and measurement) disposed within an effective display region constituting an organic EL panel, and thus the cathode potential supplied to the organic EL panel is controlled.

(C-1) Examples of Disposition of Pixel for Display and Measurement

FIGS. 7A and 7Bshow examples of disposition of the pixel (the pixel for display and measurement) which is used not only for normal picture display, but also for measurement. Each of the pixels7for display and measurement shown inFIGS. 7A and 7B, respectively, is disposed on an organic EL panel3constituting an organic EL panel module1. It is noted that in this case, each of the pixels7for display and measurement shown inFIGS. 7A and 7B, respectively, is disposed within an effective display region5constituting the organic EL panel3.

FIG. 7Ashows an example in which the pixel7for display and measurement is disposed in a lower right-hand corner of the effective display region5constituting the organic EL panel3. Also,FIG. 7Bshows an example in which the pixel7for display and measurement is disposed in an upper right-hand corner of the effective display region5constituting the organic EL panel3.

It is noted that the number of pixels7for display and measurement, and the positional disposition of the pixels7for display and measurement are arbitrarily set, respectively. However, the pixels7for display and measurement are preferably dispersively disposed within the effective display region5from a viewpoint of an influence exerted on the displayed image quality, and panel design. More preferably, the pixels7for display and measurement are dispersively disposed in a peripheral portion of a screen. Dispersively disposing a plurality of pixels7for display and measurement within the effective display region5results in that even when there is a temperature dispersion within the screen, an influence thereof can be removed by averaging the measured values.

A pixel configuration of the pixel7for display and measurement is assumed to be the same as that of any other pixel within the effective display region5except that an extension wiring for measurement of the anode potential of the organic EL element is additionally formed. Therefore, the pixel7for display and measurement is formed in exactly the same processes as those for any other pixel within the effective display region5.

(C-2) Entire Configuration

FIG. 8shows a main constituent portion of the organic EL panel module1. The organic EL panel module1shown inFIG. 8includes an organic EL panel3, a data line driver11, a scanning line driver13, and a cathode potential controlling portion15as main constituent elements.

In the case of Embodiment 1, the organic EL panel3is one for color display, and thus the pixels9are disposed in a matrix in accordance with the arrangement of emission colors and in correspondence to the panel resolution. However, when the organic EL element having a structure obtained by laminating organic emitting layers for emitting respective lights having a plurality of colors one upon another constitutes the pixels9, one pixel corresponds to a plurality of emission colors.

It is noted that one of the pixels9corresponds to the pixel7for display and measurement with which the anode potential of the organic EL element is measured. In the case of Embodiment 1, it is assumed that only one pixel7for display and measurement is disposed in the lower right-hand corner of the effective display region5.

The data line driver11is a circuit device for successively applying pixel data (having respective signal voltages Vdata) to data lines DL, respectively. The pixel data stated herein is one in image positions corresponding to the pixels9and the pixel7for display and measurement which constitute the effective display region5.

The scanning line driver13is a circuit driver for giving writing timings for the signal voltages Vdata. Of course, the scanning line driver13drives and controls a scanning line WL as well to which the pixel7for display and measurement is connected. It is noted that the scanning lines WL becoming destinations to which the writing timings are given, respectively, are controlled so as to be successively switched in units of horizontal scanning time periods.

The cathode potential controlling portion15is a processing device for switching-controlling the supply of a current used for the measurement to the pixel7for display and measurement provided for measurement of the anode potential, and controlling the cathode potential common to all the pixels in accordance with the anode potential generated in the phase of supplying the current used for the measurement.

FIG. 9shows an internal configuration of the cathode potential controlling portion15. It is noted that a pixel structure of the pixel7for display and measurement is identical to that of each of the general pixels constituting the effective display region5. In this connection, in the phase of the mounting the cathode potential controlling portion15, the transistors which are used for correction of a threshold value and mobility correction for the drive transistor T2, respectively, and other elements are connected to the cathode potential controlling portion15in some cases.

The cathode potential controlling portion15is composed of a changing-over switch (constituted by an N-channel thin film transistor T3), a constant current source21, an electrode-to-electrode voltage measuring portion23, a cathode potential determining portion25, and a cathode potential applying portion27.

In the case of Embodiment 1, the changing-over switch is constituted by the N-channel thin film transistor T3. That is to say, the N-channel thin film transistor T3operates as a switch. Also, in the case of Embodiment 1, the switching timing for the N-channel thin film transistor T3is switched and controlled in accordance with a control signal supplied from the electrode-to-electrode voltage measuring portion23. Of course, the switching timing can also be given from the outside by using an exclusive line.

Here, when an input image is displayed on the pixel7for display and measurement, the N-channel thin film transistor T3is controlled so as to be turned OFF. On the other hand, when the anode potential of the organic EL element constituting the pixel7for display and measurement is measured, the N-channel thin film transistor T3is controlled so as to be turned ON.

The constant current source21is one which can usually supply a constant current because it has no temperature characteristics. Thus, the known current source can be used as the constant current source21.

The electrode-to-electrode voltage measuring portion23is a circuit device for measuring the anode potential of an organic EL element D constituting the pixel7for display and measurement.

FIG. 10shows an example of an internal configuration of the electrode-to-electrode voltage measuring portion23. The electrode-to-electrode voltage measuring portion23is composed of a voltage follower circuit31for measuring an anode potential Vs, an analog-to-digital conversion circuit (A/D conversion circuit)33and an electrode-to-electrode voltage calculating portion35.

Here, the reason for use of the voltage follower circuit31is because the magnitude of the driving current supplied to the organic EL element D is very minute, that is, on the nanometer order. It is noted that the anode potential Vsmeasured through the voltage follower circuit31has an analog value.

The analog-to-digital conversion circuit33is a circuit device for converting the analog potential Vsmeasured as the analog potential into a digital value.

The electrode-to-electrode voltage calculating portion35is a processing device for calculating a potential difference between the anode potential Vsdeveloped at the anode electrode of the organic EL element D, and the cathode potential value Dcathodedeveloped at the cathode electrode of the organic EL element D. The arithmetic operation processing as described above is executed by executing digital processing.

A measured value DVelof the electrode to electrode voltage Velof the organic EL element D is calculated by executing the arithmetic operation processing as described above. The reason for the execution of the arithmetic operation processing is because the cathode potential Vcathode(p)applied to the cathode electrode of the organic EL element D is variably controlled similarly to the case of other pixels9constituting the effective display region5.

In the case of Embodiment 1, the electrode-to-electrode voltage calculating portion35outputs the switching timing signal for the N-channel thin film transistor T3described above. The reason for this is because the measured value DVelcorresponding to the electrode-to-electrode voltage Velis calculated. The electrode-to-electrode voltage calculating portion35supplies the measured value DVelthus calculated to the cathode potential determining portion25.

The cathode potential determining portion25calculates a difference value between the measured value DVelcalculated in the electrode-to-electrode voltage measuring portion23, and the electrode-to-electrode voltage Velat the room temperature. Also, the cathode potential determining portion25sets the difference voltage thus calculated as a correction value. After that, the cathode potential determining portion25adds or subtracts the correction value to or from a reference voltage value, thereby determining the cathode potential value Vcathodeas a control target value.

The reference voltage value stated herein differs depending on how to give a power source potential on the cathode side as a fixed potential. For example, as shown inFIG. 11, when a reference potential Vcathode(i)in the cathode potential applying portion27is supplied from a negative power source, zero is used as the reference voltage value. Of course, the reference potential Vcathode(i)is set sufficiently lower than a change width of the correction value.

In this case, the cathode potential determining portion25directly outputs the correction value (difference value) as a cathode potential value Dcathode.

As a result, the cathode potential value Dcathodeat the low temperature becomes equal to or smaller than 0 V. The cathode potential value Dcathodeat the room temperature becomes 0 V. Also, the cathode potential value Dcathodeat the high temperature becomes equal to or larger than 0 V.

In addition, for example, when as shown inFIG. 12, the reference potential Vcathode(i)in the cathode potential applying portion27is the grounding potential, an offset potential (>0) is used as the reference voltage value.

In this case, the cathode potential Dcathodeat the low temperature becomes equal to or lower than the offset potential. The cathode potential Dcathodeat the room temperature becomes the offset value. Also, the cathode potential Dcathodeat the high temperature becomes equal to or higher than the offset potential.

The cathode potential applying portion27is a circuit device for generating a common cathode potential Vcathode(p)corresponding to the determined cathode potential value Dcathode, and applying the common cathode potential Vcathode(p)thus generated to a common cathode electrode of the organic EL panel3.

FIG. 13shows an example of an internal configuration of the cathode potential applying portion27. The cathode potential applying portion27shown inFIG. 13is composed of a digital potentiometer41, and a voltage follower circuit (composed of an operational amplifier OP1and a P-channel field effect transistor T11)43.

The digital potentiometer41is a semi-fixed resistor for generating a voltage in the form of steps (for example, 256 steps (8 bits)) corresponding to a bit length of the cathode potential value Dcathodewhich is inputted thereto in the form of a digital value.

The voltage follower circuit43is a circuit device for applying the cathode potential Vcathode(p)identical to the input voltage value to the common cathode electrode in accordance with the feedback control. As a result, the common cathode electrode in the organic EL panel3can be controlled so as to follow the temperature change in the organic EL element D.

As has been described so far, according to Embodiment 1 of the present invention, it is possible to realize the separation of the organic EL element D from the temperature characteristics of the drive transistor T2, and it is possible to readily correct the fluctuation of the driving current owing to the temperature characteristics which the current vs. voltage characteristics of the organic EL element generally have.

In addition, in the case of Embodiment 1 of the present invention, the potential applied to the cathode electrode of the organic EL element D rises with the rise in the temperature. For this reason, the voltage applied to the potential circuit portion can fall by an amount of potential risen.FIG. 14shows this voltage relationship.

It is understood fromFIG. 14that the voltage between the power source voltage VDD and a reference potential Vcathode(i)is fixed, and also the voltage applied to the voltage follower circuit43increases or decreases by a amount of voltage changed applied to the pixel circuit portion.

Therefore, even when this control method is adopted, the power consumption of the entire organic EL panel module can be held unchanged.

If anything, it is also possible to expect an effect of suppressing a rise in the panel temperature because the power consumed in the pixel circuit portion is reduced (that is, an exothermic quantity of pixel circuit portion is reduced) in the phase of the rise in the temperature.

In addition, according to Embodiment 1 of the present invention, for a time period other than the time period for measurement, of the electrode to electrode voltage of the organic EL element, performed along with the temperature fluctuation, the N-channel thin film transistor T3is controlled so as to be turned OFF, so that the pixel7for display and measurement can be used in the normal display operation. Therefore, the circuit configuration can be simplified as compared with the case where the dummy pixel dedicated to the measurement is prepared. As a result, it is possible to avoid the cost-up of the self light emission display device.

In addition, according to Embodiment 1 of the present invention, it is possible to directly add the dispersion as well in the temperature distribution within the surface of the organic EL panel3because the pixels disposed within the effective display region5can be used.

In Embodiment 2 of the present invention, a detailed description will now be given with respect to the case where the electrode to electrode voltage Velof the organic EL element is directly measured by using dummy pixels each having the same configuration as that of each of the pixels disposed within an effective display region, and the cathode potential in the organic EL panel is controlled. However, the actual processing operation in Embodiment 2 is the same as that in Embodiment 1 except that measurement elements are merely exclusively disposed.

(D-1) Examples of Disposition of Pixels for Display and Measurement

FIGS. 15A and 15Bshow respectively examples of disposition of pixels (pixels for display and measurement) which are used not only for the normal picture display, but also for the measurement. The dummy pixels57shown in each ofFIGS. 15A and 15Bare also displayed on an organic EL panel53constituting an organic EL panel module51.

However, each of the dummy pixels57is disposed outside the effective display region55. That is to say, each of the dummy pixels57is disposed in a region (a region which can not be normally seen from a user) which is unrelated to the picture display.

FIG. 15Ashows an example in which the dummy pixels57are disposed on an outer right-hand side of the effective display region55constituting the organic EL panel53. Also,FIG. 15Bshows an example in which the dummy pixels57are disposed on a lower outer side of the effective display region55constituting the organic EL panel53.

It is noted that a pixel configuration of each of the dummy pixels57is the same as that of each of the pixels constituting the effective display region55. Therefore, each of the dummy pixels57is formed in the same processes as those for each of the pixels constituting the effective display region55.

(D-2) Entire Configuration

FIG. 16shows a main constituent portion of the organic EL panel module51. The organic EL panel module51includes the organic EL panel53, the data line driver11, the scanning line driver13, the cathode potential controlling portion15, and a frame average value calculating portion59as the main constituent elements.

FIG. 16also shows the case where only one dummy pixel57is disposed on a lower right-hand corner of the organic EL panel53. Now, it is known that the electrode to electrode voltage Velfluctuates depending on the degree as well of the progress of the deterioration of the pixel. For this reason, it is preferably from a viewpoint of the measurement precision that the deterioration state of each of the dummy pixels57reflects on the deterioration state of the entire organic EL panel. In view of this respect, in Embodiment 2, the frame average value calculating portion59for calculating a frame average value about input image data Dinis disposed in the organic EL panel module51. Thus, the frame average value calculating portion59supplies the frame average value calculated therein to the dummy pixel57for a time period other than the time period necessary for giving the measurement timing.

Of course, when the dummy pixel57can be regarded as reflecting the deterioration state and the driving temperature of the entire organic EL panel53, the frame average value calculating portion59is not necessarily disposed in the organic EL panel module51. In this case, the light emission of the dummy pixel57must be controlled with specific gradation values for the time period other than the time period necessary for giving the measurement timing.

For example, the driving current may be supplied from the constant current source21to the dummy pixel57. Of course, in this state, it is preferably that the driving current is not continuously supplied from the constant current source21to the dummy pixel57, but the control is carried out so that a given ratio is obtained between the time period for the supply and the supply-stop time period.

In Embodiment 2 as well of the present invention, the same effects as those in Embodiment 1 can be offered except for use of the dummy pixel57.

(E) Other Embodiments

(E-1) Another Circuit Configuration of the Cathode Potential Controlling Portion

In each of Embodiments 1 and 2, the description has been given so far with respect to the case where the changing-over switch (constituted by the N-channel thin film transistor T3) is disposed on the wiring path connecting the constant current source21and the anode electrode of the organic EL element.

However, in the case where it is thought that a resistance component is generated due to the disposition of the changing-over switching, and it exerts an influence on the measurement precision of the anode voltage Vanodeto be measured, it is recommended to adopt the configuration of using no changing-over switching.

(E-2) Correction for Temperature Characteristics which Emission Property has

In each of Embodiments 1 and 2, the description has been given with respect to the case where the cathode potential of the organic EL element is controlled so as to remove the fluctuation of the driving current due to only the temperature characteristics of the organic EL element.

However, even when the fluctuation of the driving current due to the temperature characteristics of the organic EL element is corrected, there is the possibility that the emission luminance fluctuates due to the emission property of the organic EL element for the driving current.

In this case, the correction value (difference value) calculated in the cathode potential determining portion25must be corrected in accordance with the temperature characteristics of the emission property.

(E-3) Adjustment for White Balance

In each of Embodiments 1 and 2, the description has been given with respect to the case where the cathode potential of the organic EL element common to all the pixels is variably controlled in accordance with the measurement results irrespective of the difference among the emission colors.

However, when the cathode electrodes of the organic EL elements are partitively disposed on the organic EL panel so as to correspond to R, G and B, respectively, the electrode to electrode voltages Velof the organic EL elements must be measured individually so as to correspond to R, G and B, and each of the cathode potentials of the organic EL elements must be controlled so that the gate to source voltage Vgsafter completion of the bootstrap operation becomes constant.

In this case, even when the temperature characteristics which the current vs. voltage characteristics of the organic EL element generally have differ among the colors, the white balance can be held by correcting the fluctuation of the driving current.

However, with the method of individually controlling the cathode electrodes of the organic EL elements partitively disposed so as to correspond to R, G and B, respectively, it may be impossible to avoid that the circuit configuration is complicated.

Therefore, when the simplification of the circuit configuration is prioritized, it is preferably that similarly to the case of each of Embodiments 1 and 2 described above, the cathode electrode of the organic EL element common to all the colors is prepared, and the cathode potential of the organic EL element is controlled by using either the average value of the electrode to electrode voltages Velindividually measured so as to correspond to R, G and B, respectively, or any one of these electrode to electrode voltages Velthus measured.

(E-4) Examples of Products

(a) Drive Integrated Circuit

In the explanation stated above, the description has been given so far with respect to the organic EL panel module in which the pixel array portion (organic EL panel) and the drive circuits (such as the data line driver, the scanning line driver, and the cathode potential controlling portion) are formed on one base.

However, the pixel array portion, the drive circuit portion and the like can be individually manufactured, and can be distributed in the form of the independent products, respectively. For example, the drive circuits can be manufactured in the form of the independent drive integrated circuits (ICs), and can be distributed independently of the pixel array portion.

(b) Display Module

The organic EL panel module according to each of Embodiments 1 and 2 described above can also be distributed in the form of a panel organic EL module having an appearance structure shown inFIG. 17.

An organic EL module61has a structure in which a counter portion63is stuck to a surface of a supporting substrate65.

The counter portion63includes a glass or any other suitable transparent member as a base material. Also, a color filter, a protective film, a light shielding film, and the like are disposed on a surface of the counter portion63.

It is noted that a flexible printed circuit (FPC)67for inputting/outputting a signal or the like the supporting substrate65from the outside, or the like may be provided in the organic EL panel module61.

The organic EL module according to each of Embodiments 1 and 2 described above can also be distributed in the form of a commercial product mounted to an electronic apparatus.

FIG. 18shows an example of a conceptural configuration of the electronic apparatus71. The electronic apparatus71is composed of the organic EL panel module73described above, and a system controlling portion75. The contents of the processing executed in the system controlling portion75differ depending on the commercial product form of the electronic apparatus71.

It is noted that the electronic apparatus71is by no means limited to an apparatus in a specific field as long as it is equipped with a function of displaying an image or a video picture the data on which is generated in the apparatus or is inputted from the outside.

For example, a television receiver is supposed as this sort of electronic apparatus71.FIG. 19shows an appearance example of a television receiver81.

A display screen87composed of a front panel83, a filter glass85, and the like is disposed on the front of a chassis of the television receiver81. In this case, the display screen87corresponds to the organic EL panel module1described in each of Embodiments 1 and 2.

In addition, for example, a digital camera is supposed as this sort of electronic apparatus71.FIGS. 20A and 20Bshow appearance examples of a digital camera91, respectively.FIG. 20Ashows the appearance example on the front side (on a subject side) of the digital camera91, andFIG. 20Bshows the appearance example on a back surface side (on a photographer side).

The digital camera91is composed of a protective cover93, a photographing lens portion95, a display screen97, a control switch99, and a shutter button101. Of these constituent elements, the display screen97corresponds to the organic EL panel module1described in each of Embodiments 1 and 2.

In addition, for example, a video camera is supposed as this sort of electronic apparatus71.FIG. 21shows an appearance example of a video camera111.

The video camera111is composed of a photographing lens115which is provided on the front side of a main body113and which is used to photograph a subject, a start/stop switch117with which the photographing is started/stopped, and a display screen119. Of these constituent elements, the display screen119corresponds to the organic EL module1described in each of Embodiments 1 and 2.

In addition, for example, mobile terminal equipment is supposed as this sort of electronic apparatus71.FIGS. 22A and 22Bshow appearance examples of a mobile phone121as the mobile terminal equipment, respectively. The mobile phone121shown inFIGS. 22A and 22Bis of a folding type.FIG. 22Ashows the appearance example in a state in which a chassis of the mobile phone121is opened, andFIG. 22Bshows the appearance example in a state in which the chassis of the mobile phone121is folded.

The mobile phone121is composed of an upper chassis123, a lower chassis125, a joining portion (a hinge portion in this example)127, a display screen129, an auxiliary display screen131, a picture light133, and a photographing lens135. Of these constituent elements, each of the display screen129and the auxiliary display screen131corresponds to the organic EL panel module1described in each of Embodiments 1 and 2.

In addition, for example, a computer is supposed as this sort of electronic apparatus71.FIG. 23shows an appearance example of a notebook computer141.

The notebook computer141is composed of a lower chassis143, an upper chassis145, a keyboard147and a display screen149. Of these constituent elements, the display screen149corresponds to the organic EL panel module1described in each of Embodiments 1 and 2.

In addition thereto, an audio reproducing apparatus, a game console, an electronic book, an electronic dictionary or the like is supposed as this sort of electronic apparatus71.

(E-5) Examples of Other Display Devices

In each of Embodiments 1 and 2, the description has been given with respect to the case where the common cathode potential of the organic EL element in the organic EL panel module is controlled.

However, the cathode potential controlling function can also be applied to any other self light emission display device. For example, the cathode potential controlling function can also be applied to an inorganic EL display device, a display device having LEDs arranged therein, or any other display device in which light emitting elements each having a diode structure are arranged on a screen.

(E-6) Control Device Configuration

In the above explanation, the description has been given with respect to the case where the cathode potential controlling function is realized in the form of the hardware.

However, a part of the cathode potential controlling function may also be realized in the form of software processing.

Various changes are conceivable for Embodiments 1 and 2 described above without departing from the gist of the invention. In addition, there are conceivable various changes and application examples which are obtained by creation or combination made based on the description in this specification.