JP2004302288A - Display element and display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce variation in luminance of a digital drive type display element. <P>SOLUTION: In a pixel circuit 100 constituting a digital drive type organic EL display device, an organic light emitting device OLED which is supplied with a current to emit light and two driving transistors Tr2a and Tr2b controlling the supply of the current to the organic light emitting device OLED are connected in series. The driving transistor Tr2b functions as a current adjusting element for adjusting the current supplied to the organic light emitting device OLED. As the current flowing to the organic light emitting device OLED decreases owing to deterioration of the organic light emitting device OLED and a fall of temperature, the driving transistor Tr2b increases the current. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a display element and a display device, and more particularly to a digitally driven display element and a display device.
[0002]
[Prior art]
2. Description of the Related Art Organic electroluminescent display devices (hereinafter, also referred to as “organic EL display devices” or “organic EL panels”) have attracted attention as new flat display devices. It is expected that organic EL display devices will soon sweep the widespread use of liquid crystal display devices, and there is fierce competition for development toward practical use.
[0003]
The driving method of the organic EL display device is roughly classified into two types, an analog driving method and a digital driving method. The analog driving method is a method in which a current having a magnitude corresponding to a data voltage is supplied to each organic EL element, and the organic EL elements are turned on with a luminance corresponding to the data voltage. Various digital driving methods have been proposed. For example, in the time gray scale method, a pulse current having a duty ratio corresponding to a data voltage is supplied to each organic EL element, and a period of time corresponding to the data voltage is lit. This is a method of expressing multiple gradations.
[0004]
Among the time gray scale methods, in the subfield drive method, one field (frame) period, which is a display cycle of one screen, is divided into a plurality of subfield (frame) periods, and lighting on / off in each subfield period is performed. By controlling, the organic EL element is turned on for a period corresponding to the data voltage. At this time, a current of the same magnitude is supplied to the organic light emitting element, and the organic light emitting element emits light with the same luminance, but a gradation is expressed by the length of the lighting time. The light emission period of each subfield has a length of 2 n (n = 0, 1, 2,..., N−1), for example, 1, 2, 4, 8, 16, 32, 64 , 128, 256 gradations are represented by ON / OFF of the light emission period set to the length.
[0005]
In the organic EL panel adopting the time gray scale method as described above, it is preferable to operate a driving transistor for driving the organic light emitting element at a constant current in a linear region in order to reduce the influence of variations in the transistors.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-open No. Hei 10-310173
[Problems to be solved by the invention]
However, in the case of operating the driving transistor in the linear region in the digital driving system, there is a problem that the fluctuation of the current due to temperature change and deterioration with time is larger than in the analog driving system in which the driving transistor operates in the saturation region. When the fluctuation of the current supplied to the organic light emitting element is large, the luminance of the organic light emitting element varies, and the display quality may be deteriorated.
[0008]
The present invention has been made in view of such a situation, and an object of the present invention is to provide a technique for reducing variation in luminance in a digitally driven display element. Another object of the present invention is to provide a technique for reducing a decrease in luminance due to deterioration with time of a digitally driven display element. Still another object of the present invention is to provide a technique for suppressing the occurrence of thermal runaway in a digitally driven display device.
[0009]
[Means for Solving the Problems]
One embodiment of the present invention relates to a display element. This display element is a digitally driven display element, a light emitting element that emits light when a current is supplied thereto, and a driving transistor that operates in a linear region and controls the supply of current to the light emitting element. A current adjusting element connected in series with the light emitting element and the driving transistor for adjusting a current flowing through the light emitting element.
[0010]
By providing the current adjusting element, a change in current caused by a change in temperature or deterioration of the light-emitting element over time can be reduced, and a change in luminance can be reduced.
[0011]
The current adjusting element may be a transistor. When the voltage between the two electrodes of the light-emitting element increases due to a change in temperature or a change with time, the transistor may be connected so that the operating point shifts in a direction to reduce the increase in the voltage. The same signal as the signal input to the gate electrode of the driving transistor may be input to the gate electrode of the transistor. Thus, current adjustment can be realized with a simple configuration. A control signal for variably controlling a current flowing through the light-emitting element may be input to a gate electrode of the transistor. Thereby, the current can be adjusted more effectively.
[0012]
Another embodiment of the present invention relates to a display device. This display device is characterized in that one of the display elements described above is arranged in a matrix.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a configuration of an organic EL display device 10 according to the embodiment. The organic EL display device 10 includes an organic EL panel 5 in which a plurality of pixels are arranged in a matrix, a scan driver 3 for supplying a scan signal to the organic EL panel 5, and a data driver 4 for supplying a luminance data signal to the organic EL panel 5. , An image signal processing circuit 6 for processing an image signal, and a timing signal generating circuit 7 for generating a timing signal for controlling display timing.
[0014]
The video signal processing circuit 6 performs processing necessary for display on the input video signal, outputs video signals of three primary colors of R, G, and B to the data driver 4, and outputs a horizontal synchronization signal H _Sync and a vertical synchronization signal H _Sync. The signal V _Sync is output to the timing signal generation circuit 7. The timing signal generation circuit 7 generates a timing signal for controlling display timing based on the horizontal synchronization signal H _Sync and the vertical synchronization signal V _Sync , and supplies the timing signal to the scanning driver 3 and the data driver 4.
[0015]
FIG. 2 shows a configuration of a pixel circuit 100 for one pixel of the organic EL panel 5. The pixel circuit 100 includes an organic light emitting element OLED, a switching transistor Tr1 for controlling timing of writing data to the organic light emitting element OLED, driving transistors Tr2a and Tr2b for controlling energization to the organic light emitting element OLED, and a storage capacitor C And a pulse width modulation (PWM) circuit 50 for modulating the luminance data voltage to a pulse width, a scanning line SCAN for transmitting a scanning signal, a data line DATA for transmitting luminance data, and a current for the organic light emitting element OLED. And a power supply line VDD for supplying. The power supply line VDD supplies a current for causing the organic light emitting element OLED to emit light. The data line DATA passes a signal of luminance data for controlling the luminance of each of the organic light emitting elements OLED. The scanning line SCAN passes a scanning signal for controlling the emission timing of each of the organic light emitting elements OLED.
[0016]
The pixel circuit 100 of the present embodiment includes two driving transistors Tr2a and Tr2b connected in series. At this time, the one driving transistor Tr2b also functions as a current adjusting element for adjusting the current flowing between the source and the drain of the driving transistors Tr2a and Tr2b and the organic light emitting element OLED, as described later. Actually, since the current adjusting action is realized by the cooperation of the driving transistors Tr2a and Tr2b, both may be called current adjusting elements. By providing the current adjusting element, it is possible to reduce a change in luminance due to the influence of the temperature or the aging of the organic light emitting element OLED.
[0017]
The switching transistor Tr1 has a gate electrode connected to the scanning line SCAN, a source electrode connected to the data line DATA, and a drain electrode connected to the PWM circuit 50. The switching transistor Tr1 may have any of a single-gate structure, a double-gate structure, and a multi-gate structure having three or more gate electrodes. Further, the switching transistor Tr1 may be an n-channel transistor or a p-channel transistor. The source electrode and the drain electrode may be reversed.
[0018]
The driving transistor Tr2a has a source electrode connected to the power supply line VDD, a drain electrode connected to the source electrode of the driving transistor Tr2b, and a gate electrode connected to the PWM circuit 50. The driving transistor Tr2b has a source electrode connected to the drain electrode of the driving transistor Tr2a, a drain electrode connected to the anode of the organic light emitting element OLED, and a gate electrode connected to the PWM circuit 50. In this example, two driving transistors Tr2 are provided in series, but three or more driving transistors Tr2 may be provided.
[0019]
The organic light emitting element OLED has an anode connected to the drain electrode of the driving transistor Tr2b, and a cathode grounded. One end of the storage capacitor C is connected to the drain electrode of the switching transistor Tr1, and the other end is connected to a wiring (not shown). The other end of the storage capacitor C may be connected to the power supply line VDD. The PWM circuit 50 is provided between the drain electrode of the switching transistor Tr1 and the gate electrodes of the driving transistors Tr2a and Tr2b.
[0020]
The operation of the above configuration will be described. First, the data driver 4 prepares one line of luminance data and supplies it to each data line DATA. Here, when the scan driver 3 sends a scan signal to the scan line SCAN of the horizontal line where data is to be written, the switching transistor Tr1 of the pixel of that horizontal line is turned on, and the luminance data set on the data line DATA is The value is set to the storage capacitor C and is input to the PWM circuit 50. The PWM circuit 50 modulates the voltage of the input luminance data into a pulse signal having a width corresponding to the voltage. The pulse signal is a signal having a constant amplitude and being active during a period according to the luminance data. The pulse signal output from the PWM circuit 50 is input to the gate electrodes of the driving transistors Tr2a and Tr2b. The driving transistors Tr2a and Tr2b are turned on only during a period in which the pulse signal is active, a constant current is supplied to the organic light emitting element OLED, and the organic light emitting element OLED emits light. The above operation is repeated for horizontal lines, and an image of one frame is displayed.
[0021]
FIG. 3 shows the relationship between the source-drain voltage of the driving transistors Tr2a and Tr2b and the voltage between both electrodes of the organic light emitting device OLED. Since the driving transistors Tr2a and Tr2b and the organic light emitting element OLED are connected in series, the current I flowing is the same unless leakage current is considered. The source of the driving transistor Tr2a - drain voltage _{V DS1,} the source of the driving transistor Tr2b - drain voltage _{V DS2,} when the voltage between both electrodes of the organic light emitting device OLED and _{V OLED,} the following equation is established.
VDD = V _DS1 + V _DS2 + _VOLED
[0022]
FIG. 4 is a diagram for explaining how the current adjustment element reduces the current fluctuation when the current flowing through the organic light emitting element OLED fluctuates due to a temperature change or a temporal change. At a certain time point t1, the characteristic curve of the organic light emitting element OLED is indicated by 200a, the characteristic curve of the driving transistor Tr2a is indicated by 210, and the characteristic curve of the driving transistor Tr2b is indicated by 220a. At this time, the operating point of the driving transistor Tr2b is 230a, and a current of the current value I1 flows through the organic light emitting element OLED and the driving transistors Tr2a and Tr2b.
[0023]
It is known that, in the organic light emitting element OLED, it becomes difficult for a current to flow as the light emitting time elapses, and the luminance decreases. That is, the characteristic curve of the organic light emitting element OLED shifts rightward in FIG. In addition, the characteristic curve of the organic light emitting device OLED shifts to the right due to the temperature decrease. When the luminance and the current decrease due to the deterioration over time, the temperature of the display panel decreases, and the characteristic curve shifts in the direction of further decreasing the current.
[0024]
Here, when the characteristic curve of the organic light emitting element OLED at a certain time point t2 shifts to 200b due to a change with time or a decrease in temperature, the operating point moves to 230b, and the current value decreases to I2. At this time, since the operating point of the driving transistor Tr2a also moves to the right, the potential of the source electrode of the driving transistor Tr2b increases, and the gate-source voltage V _{GS of} the driving transistor Tr2b increases. Then, the characteristic curve of Tr2b changes to 220b, the operating point becomes 230c, and the current value increases to I3. When only one driving transistor is provided, the current decreases to I2. However, when two driving transistors are connected in series, the current becomes I3, and the current decreases due to a decrease in temperature or a change with time. Can be reduced.
[0025]
Conversely, when the temperature rises, the characteristic curve of the organic light emitting element OLED shifts to the left, so that the operating point shifts to the upper left and the current increases. Then, there is a possibility that the temperature of the panel will further rise, causing thermal runaway. Also in this case, according to the pixel circuit 100 of the present embodiment, the current adjustment function works to shift the operating point in the direction of decreasing the current value, thereby suppressing thermal runaway. Specifically, when the operating point 230a shifts in the upper left direction with the rise in temperature, the gate-source voltage V _GS of the driving transistor Tr2b decreases, and the characteristic curve of the driving transistor Tr2b shifts downward. I do. As a result, the operating point shifts in the lower left direction, so that an increase in current can be reduced.
[0026]
By providing a plurality of driving transistors, the slope of the linear region of the transistor connected to the organic light emitting element OLED becomes gentle, and there is also an effect that the transistor is less susceptible to temperature change and aging.
[0027]
FIG. 5 shows another example of the pixel circuit 100. In the example of FIG. 5, a control signal for variably controlling a current, which is generated by a current control unit (not shown), is input to a gate electrode of the driving transistor Tr2a functioning as a current adjusting element. Thus, the current flowing through the organic light emitting element OLED can be actively controlled, so that more effective current adjustment can be realized. The current adjusting operation of the pixel circuit 100 will be described with reference to FIG. By adjusting the gate voltage of the driving transistor Tr2a, the characteristic curve 210 shifts up and down. In addition, since the gate-source voltage V _{GS of} the driving transistor Tr2b increases and decreases, the characteristic curve 220a shifts up and down, and the operating point 230a moves.
[0028]
FIG. 6 shows another example of the pixel circuit 100. In the example of FIG. 6, a control signal for variably controlling the current, which is generated by a current control unit (not shown), is input to the gate electrode of the driving transistor Tr2b functioning as a current adjusting element. Thus, the current flowing through the organic light emitting element OLED can be actively controlled, so that more effective current adjustment can be realized. The current adjusting operation of the pixel circuit 100 will be described with reference to FIG. By adjusting the gate voltage of the driving transistor Tr2b, the characteristic curve 220a shifts up and down, and the operating point 230a moves.
[0029]
As described above, according to the technology of the present embodiment, it is possible to minimize a change in luminance of a display element due to a change in temperature or a change with time. Thus, burn-in of the display device can be suppressed, and the life can be extended.
[0030]
The present invention has been described based on the embodiments. This embodiment is an exemplification, and it is understood by those skilled in the art that various modifications can be made to the combination of each component and each processing process, and that such modifications are also within the scope of the present invention. is there.
[0031]
In the embodiment, the example in which the PWM circuit 50 generates a pulse signal having a pulse width corresponding to the luminance data to digitally drive the organic light emitting element OLED has been described. Is also good. Further, the organic light emitting device has been described as an example, but the present invention is applicable to other current driven light emitting devices.
[0032]
【The invention's effect】
According to the present invention, it is possible to provide a technique for reducing a variation in luminance in a digitally driven display element.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an organic EL display device according to an embodiment.
FIG. 2 is a diagram illustrating a configuration of a pixel circuit for one pixel of the organic EL panel.
FIG. 3 is a diagram illustrating a relationship between a source-drain voltage of driving transistors Tr2a and Tr2b and a voltage between both electrodes of an organic light emitting element OLED.
FIG. 4 is a diagram for explaining how the current adjustment element reduces the current fluctuation when the current flowing through the organic light emitting element OLED fluctuates due to a temperature change or a temporal change.
FIG. 5 is a diagram illustrating another example of a pixel circuit.
FIG. 6 is a diagram illustrating another example of the pixel circuit.
[Explanation of symbols]
2 ... organic EL display unit, 3 ... scan driver, 4 ... data driver, 5 ... organic EL panel, 6 ... video signal processing circuit, 7 ... timing signal generation circuit, 10 ... Organic EL display device, 50 ... PWM circuit, 100 ... pixel circuit, C ... storage capacitor, DATA ... data line, OLED ... organic light emitting element, SCAN ... scanning line , Tr1 switching transistor, Tr2 driving transistor, Tr2a driving transistor, Tr2b driving transistor, VDD power supply line.

Claims (5)

A digitally driven display element,
A light-emitting element that emits light when supplied with an electric current,
Controlling the supply of current to the light emitting element, a driving transistor operated in a linear region,
A current adjusting element connected in series with the light emitting element and the driving transistor to adjust a current flowing through the light emitting element;
A display element comprising: The display device according to claim 1, wherein the current adjustment element is a transistor. The display device according to claim 2, wherein the same signal as the signal input to the gate electrode of the driving transistor is input to the gate electrode of the transistor. The display element according to claim 2, wherein a control signal for variably controlling a current flowing through the light emitting element is input to a gate electrode of the transistor. A display device, comprising the display elements according to claim 1 arranged in a matrix.

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