JP2002278497A - Display panel and driving method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce power consumption of a display panel using current control type light emitting elements, such as organic EL elements. SOLUTION: A switch is inserted between a light emitting element and a current supply means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving device for a light emitting device used in a display, and more particularly to a current control device in which light emission luminance is controlled by a current flowing through the device, such as an organic or inorganic electroluminescent device or a light emitting diode. The present invention relates to a driving circuit for a light emitting device.

[0002]

2. Description of the Related Art Displays that combine current-controlled light-emitting elements such as organic and inorganic electroluminescence (EL) elements or light-emitting diodes (LEDs) in an array and display characters in a dot matrix are televisions and portable terminals. Widely used for etc.

[0003] In particular, these displays using self-luminous elements, unlike displays using liquid crystal, have features such as not requiring a backlight for illumination and having a wide viewing angle, and have attracted attention. I have.

[0004] Above all, an active matrix type display which performs static driving by combining a transistor or the like and these light emitting elements is higher in brightness, higher contrast and higher definition than a simple matrix driving display which performs dynamic driving. Etc., and has been attracting attention in recent years.

[0005] As a conventional example of this type of display, FIG. 9 shows Society for Information.
216-219 of "Eurodisplay '90", a preprint of the 1990 Fall Conference published by nDisplay
1 shows a light emitting element driving circuit of an active matrix type display using an EL element as a light emitting element, which is cited from the publication of the page.

Referring to FIG. 9, in this driving circuit, when scanning line 31 connected to the gate electrode of transistor 35 is selected and activated, transistor 35 is turned on and connected to transistor 35. A signal is written from the signal line 32 to the capacitor 38. Capacitor 38
Determines the gate-source voltage of the transistor 41.

When the scanning line 31 is not selected and the transistor 35 is turned off, the voltage between both ends of the capacitor 38 is held until the scanning line 31 is selected in the next cycle.

According to the voltage between both ends of the capacitor 38,
A current flows along a path of the power supply line 39 → the light emitting element 40 → the drain-source of the transistor 41 → the common electrode 42, and the light emitting element 40 emits light by this current.

[0009]

Generally, in order to display moving images on a computer terminal, a monitor of a personal computer, a television, or the like, it is desirable that the light emission amount of each pixel is changed to perform gradation display.

In order to give gradation to an image, in a conventional example,
There are an analog gradation method, an area gradation method, and a time gradation method.

In the analog gray scale method, it is necessary to control the gate potential of the transistor 41 for supplying a current to the light emitting element in accordance with the video signal, that is, to control the conductance of the transistor 41. In this case, it is necessary to change the video signal according to the luminance-voltage characteristics of the organic EL element.

In general, the voltage-current characteristics of an organic EL element exhibit nonlinear diode characteristics, and the voltage-luminance characteristics also exhibit diode characteristics. Therefore, it is necessary to perform gamma correction on the video signal voltage, which complicates the system.

[0013] In addition, even if the video signal voltages input to all the pixels are uniform, display irregularities may occur due to variations in the characteristics of the transistors provided in each pixel on the display panel. .

In order to perform analog gray scale display in the drive circuit of FIG. 9, it is necessary to apply a voltage near the threshold voltage (V _th ) between the gate and source electrodes of the transistor 41.

However, for example, if the gate voltage and source current characteristics of the transistors A and B have variations as shown in FIG. 10 (in FIG. 10, the solid line indicates the characteristics of the transistor A, and the dotted line indicates the characteristics of the transistor B). and are), the case of applying the gate voltage V _G to the gate electrode of the corresponding transistor for example to the transistor 41 in FIG. 9,
Since the current flowing through the respective transistors different as I _A (curve and intersection between V _G shown by the solid line) and I _B (the intersection between the curve shown by a broken line and V _G), also vary the current flowing through the light emitting element 40 However, the luminances of the regions that should have the same luminance are different from each other, and therefore, for example, image quality deterioration such as uneven luminance occurs.

A circuit that is not easily affected by variations in transistor characteristics as described above has also been proposed. IDRC
(International Display Re
search Conference) 2000, Di
guest P.G. 358 to 361 propose an embodiment in which a current mirror circuit is applied to a pixel circuit for driving a light emitting element (FIG. 11).

The current mirror circuit can supply the supplied current to the load irrespective of the threshold voltage of the transistor, so that the current, ie, the load, that is, the light-emitting element is fixed irrespective of the variation of the transistor. A current can be supplied.

On the other hand, documents AM-LCD2000, AM3
For -1, the area gradation method is proposed.

In this method, one pixel is divided into a plurality of pixels, each pixel is turned on / off, and a gray scale is output based on the total area of the turned-on pixels.

However, in this method, it is difficult to increase the aperture ratio, so that the driving current density to the light emitting element must be increased, which may cause problems such as an increase in driving voltage and a reduction in the life of the element.

The time gray scale method is a method in which the gray scale is controlled by the light emission time of the light emitting element in order to solve the above-mentioned problem. For example, SID2000 DIGEST
36.1 (pp. 912-915). However, to reduce transistor variations,
It is necessary to operate a transistor for driving a light emitting element at a constant current in a linear region, which causes a problem such as an increase in power supply voltage and power consumption.

In this method, as described in the above report, the light emission time is adjusted by selecting a plurality of light emission periods. For example, when an attempt is made to display 8 bits (256 gradations), the light emission time has a time ratio of 1: 2: 4:
Eight subfield periods of 8: 16: 32: 64: 128 will be selected. Immediately before each subfield period, there is an addressing period of all pixels each time to select light emission / non-light emission in the subfield. This addressing period is basically non-display. Therefore, the effective light emitting period in one field is equal to the effective light emitting period = (one field period)-(one screen) when N bit gradation display is to be performed. (Addressing period × N), and the light emission amount decreases. Therefore, it is necessary to increase the light emission amount per subfield to compensate for the light emission amount in the entire field. For this purpose, it is necessary to increase the emission luminance of each light emitting element, which leads to a reduction in the life of the light emitting element. Further, in a normal liquid crystal display (LCD), it is necessary to address only one addressing per field by the number of gradation bits, so that a higher-speed addressing circuit is required.

The present invention has been made in view of the above problems, and has as its object to provide a display panel which is superior to the conventional display panel.

[0024]

According to a first aspect of the present invention, a scanning line and a signal line are formed in a matrix on a substrate, and the scanning line and the signal line are connected to each other. In an active matrix display panel having a pixel circuit having at least one current control type light emitting element whose luminance changes according to an input current near an intersection, the pixel circuit supplies a current to the current control type light emitting element And a switch means connected between the current supply means and the current control type light emitting element.

According to a second aspect of the present invention, in the display panel according to the first aspect, the current supply is further connected to the scanning line and the signal line, and the current is supplied by a signal from the scanning line and the signal line. Preferably, the apparatus further comprises a current control means for switching between supply and non-supply of current from the means to the switch means.

The third aspect of the present invention is the first or second aspect.
The display panel according to any one of the first to third aspects, wherein the switch means preferably includes at least one transistor.

According to a fourth aspect of the present invention, in the display panel of the third aspect, the transistor preferably includes a thin film transistor.

[0028] The invention according to claim 5 is the invention according to claims 1 to 4.
In the display panel according to any one of the above, it is preferable that the display panel further includes drive control means for performing two-input multiplex drive control, and the switch means constitutes a two-input multiplexer together with the drive control means. It is included as an embodiment.

According to a sixth aspect of the present invention, there is provided a method of driving a display panel according to any one of the first to fifth aspects, wherein the opening and closing time of the switch means is reduced. The light emission amount of the current control type light emitting element is adjusted by controlling.

According to a seventh aspect of the present invention, there is provided a method of driving a display panel according to any one of the second to fifth aspects, wherein an opening and closing time of the switch means is reduced. Controlling the light emission amount of the current control type light emitting element, and selecting, by the current control means, light emission or non-light emission in one field of the current control type light emitting element included in each pixel. I do.

The invention according to claim 8 is the driving method according to claim 7, wherein the selection is preferably performed only once per one field.

According to a ninth aspect of the present invention, there is provided a method of driving a display panel according to the fifth aspect, wherein the light emission time of the current control type light emitting element is controlled by multiplex driving of two inputs. Is controlled.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION (Display Panel) In the present invention, a scanning line and a signal line are formed in a matrix on a substrate, and a luminance near an intersection of the scanning line and the signal line is determined in accordance with an input current. An active matrix type display panel having a pixel circuit having at least one current control type light emitting element, wherein the pixel circuit is a means for supplying current to the current control type light emitting element; And a switch unit connected between the supply unit and the current control type light emitting element.

According to such a configuration, by controlling the opening and closing time of the switch means, the light emission amount of the pixel in one field can be adjusted, and as a result, a gradation display can be obtained.

Further, it is connected to a scanning line and a signal line,
In a preferred embodiment, the apparatus further comprises current control means for opening and closing the connection between the current supply means and the switch means in accordance with signals from the scanning lines and the signal lines.

According to such a configuration, the scanning lines, the signal lines, and the current control means can be used for addressing each pixel, and information of light emission and non-light emission in one field, for example, in only a selected pixel is provided. The current can be supplied from the current supply means to the light emitting element based on the writing and the information. At this time, by further controlling the opening / closing time of the switch means, the amount of light emission in one field is adjusted, and as a result, a stable gradation display without crosstalk can be obtained.

In the following specific embodiments, an organic EL element will be described as an example of a light emitting element.

FIG. 1 is a schematic diagram of a circuit configuration of one pixel of a display panel according to the present invention. In FIG. 1, reference numeral 1 denotes current control means;
Is a current supply means for supplying a current for causing the organic EL element to emit light, 3 is a switch means, and 4 is an organic EL element corresponding to the light emitting element. FIG. 1 shows a preferred embodiment including a current control means.

A feature of the present invention is that, as shown in FIG. 1, a switch means 3 is inserted in series between a current supply means 2 and an organic EL element 4.

If the current control means 1 is provided, by using the same for addressing, for example, light emission / non-light emission information is written only in the selected pixel, and the current supply means 2 sends the information from the current supply means 2 based on the information. A current is supplied to the EL element. At this time, by controlling the opening / closing time of the switch means 3 arranged in series, the light emission amount can be adjusted, and as a result, a gradation display can be obtained.

FIG. 2 is a schematic diagram of a circuit configuration of one pixel of a more preferred embodiment of the display panel of the present invention. FIG.
1, the same reference numerals as those in FIG. 1 denote the same members, and reference numeral 5 denotes a drive control unit that performs multiplex drive control of two inputs.

In the embodiment shown in FIG. 2, two-input multiplex drive is used to control the light emission time of the organic EL element 4.

FIG. 3 is a pixel circuit diagram of one embodiment of the display panel of the present invention. 27 is an organic EL element, 10 is a scanning line, 11 is a signal line, 15 is a power supply line, 12 is a control signal line,
20 to 22 are thin film transistors (TFTs), and 14 is a cathode electrode line. Current supply TFT 21 and organic EL
The feature is that another TFT 22 is inserted in series with the element 14.

In this embodiment, the current supply means corresponds to the power supply line 15 and the TFT 21, and the current control means corresponds to the TF
The switch means corresponds to the TFT 22 corresponding to T20. The current supply means may be a combination of a power supply and a switch as in the present embodiment, or may be a power supply itself. Further, the current control means may be a switch as in this embodiment, or a driver for controlling the power supply itself.

FIG. 4 is a pixel circuit diagram of another embodiment of the display panel of the present invention. 4, the same symbols as those in FIG. 3 represent the same members, 3 is a switch means, 13 is a control signal line,
23 and 24 are TFTs.

As compared with the embodiment shown in FIG.
Between the TFT 21 and the organic EL element 27.
4 is inserted in series.

In the present embodiment, the switch means is provided with this T
FTs 23 and 24 are provided. The control signal lines 12, 13 connected to the respective gate electrodes of the series-connected TFT group (in the dotted line in the figure) are used for two inputs when multiplex driving is performed. By controlling these two input signal pulses, it is possible to control the time during which current flows through the organic EL element 27 (the time during which two TFTs are simultaneously turned on).

In this embodiment, the control signal lines 12,
If 13 is connected to a control signal driver (not shown) for performing multiplex drive control of two inputs, this corresponds to the drive control means of the present invention.

FIG. 5 is a schematic diagram of a display panel according to the present invention, which is configured using the pixels having the circuit configuration of FIG. 5, the same symbols as those in FIG. 2 represent the same members, 6 is a scanning driver,
7, a signal driver; 14, a cathode electrode line; 8, 9 control signal drivers for multiplex driving;
0 is a scanning line, 11 is a signal line, 12 and 13 are control signal lines,
Reference numeral 16 denotes a drive control circuit.

Light emission / non-light emission in one field of the pixel is selected by the scanning driver 6 and the signal driver 7, and the current control means 1 switches between supply and non-supply of the light emission current from the current supply means 2. At this time, the opening and closing time of the switch means 3 is controlled by the output of the drive control circuit 5 based on a combination of signals sent from the control signal drivers 8 and 9 to the control signal lines 12 and 13 arranged in a matrix.

In this embodiment, the drive control circuit 5, the control signal drivers 8, 9 and the control signal lines 12, 13 constitute a two-input multiplexer, and correspond to the drive control means.

As a control signal driver used as a multiplexer, for example, a conventional driver technology for a matrix liquid crystal display panel can be used as it is.

FIG. 6 is a circuit diagram of a part of one embodiment of the display panel of the present invention. In the present embodiment, pixel circuits having the form shown in FIG. 3 are arranged in a matrix, and FIG. 6 shows only 2 × 2 of the matrix circuits. It is not limited.

In the present embodiment, the organic E is also obtained only by turning on / off the transistor corresponding to the TFT 22 shown in FIG.
It is possible to control the light emitting time of the L element,
Are connected to the control signal line 13 as shown in FIG.
, And two-input multiplex drive using the control signal lines 12 and 13 is also possible.

FIG. 7 is a circuit diagram of a part of another embodiment of the display panel of the present invention. In this embodiment, the pixel circuits shown in FIG. 4 are arranged in a matrix, and only 2 × 2 matrix circuits are shown. However, the number of rows is not limited to this. Absent.

In the present embodiment, multiplex driving is performed in which the gate electrodes of two TFTs connected in series to the organic EL element are connected in common by matrix wiring (control signal lines 12 and 13 in FIG. 7) and each receives an input. By doing this, it is possible to control the light emission time (control the time during which the two multiplex driven TFTs are simultaneously turned on).

(Driving Method) The driving method of the present invention is the above-described method of driving a display panel of the present invention, wherein the amount of light emitted from the light emitting element is adjusted by controlling the opening and closing time of the switch means. Things.

According to this method, the light emission amount of the light emitting element can be directly and freely adjusted.

Preferably, light emission or non-light emission in one field of each pixel is selected by the current control means.

According to this method, it is possible to control light emission and no light emission without crosstalk.

More preferably, light emission or non-light emission is performed only once per field.

As a result, in the time gray scale method, the writing of video information to each pixel is performed only once per field, and the control of the light emission time required for gray scale expression can be realized simultaneously for all pixels. As described above, writing (addressing) for the number of gradation bits is not required. This will be described in detail below.

FIG. 8 is a conceptual diagram showing a light emission time ratio of one pixel per one field when gradation display is performed by the time gradation method.

After writing the video information to the display pixels during the writing (addressing) of the video information to each pixel, the open / close time of the switch is controlled by controlling the open / close timing of the switch. FIG. 8 shows an example in which a 5-bit gradation is displayed as the number of gradations. The display period is divided into five subfields (SF1 to SF5), and gradation display is performed according to the difference in the total light emission time in which period of these subfields light emission. In FIG. 8, each of the subfields SF1 to SF5 is shown in succession. However, in the present invention, the light emission time of the light emitting element 4 only needs to be a predetermined time in the display period. There is no particular limitation on the length of the continuous light emission time or the plurality of light emission times.

According to the present invention, in the gray scale display by the time gray scale method, writing of video information to each pixel only needs to be performed once per field, and writing (addressing) for the number of gray scale bits as in the related art. Is not required.

For example, when considering an 8-bit gray scale for performing full color display, a field frequency is 60 Hz, the number of scanning lines is 240, and the addressing cycle of one scanning line is 250.
Assuming that a frequency of 1 kHz is required, the conventional light emission time (complete white display) = 1 / 60-1 / (25
0 × 10 ³ ) × 8 × 240 = 9.0 msec. Emission time (complete white display) of the present invention = 1 / 60-1 / (2
50 × 10 ³ ) × 240 = 15.7 msec. In the present invention, the emission time can be increased by a factor of 1.7. This means, in other words, that the current flowing through the light emitting element can be reduced by a factor of 1.7 in order to obtain the same light emission amount (in the case of the organic EL element, the current amount and the light emission luminance are almost linear). To change). This is very important for an application that is driven by a battery, such as a display element of a portable device.

More preferably, the light emission time of the current control type light emitting element is controlled by multiplex driving of two inputs.

Thus, a desired display can be performed even on a display panel such as a VGA having a large number of pixels.

Hereinafter, a specific driving method using the display panel of the above embodiment will be described.

First, a specific driving method of the display panel of the embodiment shown in FIG. 6 will be described with reference to FIG.

When the scanning line 10 is selected and the TFT 20 is turned on, a light emission / non-light selection signal of the pixel is transferred from the signal line 11 to the gate capacitance of the TFT 21, and then the TFT 2 is turned on.
It is maintained even if 0 is turned off. Thereby, the addressing of the pixel is performed.

The constant current from the power supply line 15 is supplied via the TFT 21 according to the selection signal. If the light emission selection signal is transferred to the pixel and a pulse signal is supplied from the control signal line 12 to the gate electrode of the TFT 22 in that state, the organic EL element 27 is turned on and off in accordance with the turning on and off of the TFT 22. Off, that is, the organic EL element 27 emits light.
No light emission is performed. By this on / off control, the light emission amount of one pixel can be adjusted to obtain a time gray scale.

As another driving method of the display panel of this embodiment, the cathode electrode line 1 of the organic EL element shown in FIG.
4 is used as the control signal line 13 as shown in FIG.

In this embodiment, the gate electrodes of one TFT connected in series to the organic EL element are commonly connected by one matrix wiring (control signal line 12 in FIG. 6), and the cathode electrode line is connected for each column. By separately forming (the control signal line 13 in FIG. 6), the multiplex driving is performed by inputting each of them to control the emission time (the multiplex-driven TFT is on, and the control signal line is also a cathode electrode line). 13 can be controlled to a low level. In this embodiment, TF is used as a switching means described later.
The number of TFTs is reduced by one as compared with the case of using two T's, which is advantageous for miniaturization of pixels.

Next, a specific driving method of the display panel shown in FIG. 7 will be described with reference to FIG.

In the present embodiment, the addressing is performed as shown in FIG.
OLED is performed in the same manner as the driving method using
The gate electrodes of the two TFTs 23 and 24 connected in series to the element 27 are commonly connected by matrix wiring (the control signal lines 12 and 13 in FIG. 7), and multiplex driving is performed using each of them as an input to thereby achieve a light emission time. (Control of the time during which two multiplex-driven TFTs are simultaneously turned on) can be controlled.

[0077]

As described above, according to the present invention,
When a time gray scale method is employed as a gray scale control method of a current control type light emitting element, a long light emission time can be taken, so that a current flowing through the light emitting element can be reduced. as a result,
The power consumption of the entire display panel can be reduced, and it is possible to sufficiently cope with battery-driven applications such as portable devices.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a circuit configuration of one pixel of a display panel of the present invention.

FIG. 2 is a schematic diagram of a circuit configuration of one pixel of one embodiment of the display panel of the present invention.

FIG. 3 is a pixel circuit diagram of one embodiment of a display panel of the present invention.

FIG. 4 is a pixel circuit diagram of another embodiment of the display panel of the present invention.

FIG. 5 is a schematic diagram of a display panel of the present invention configured using pixels having the circuit configuration of FIG. 2;

FIG. 6 is a circuit diagram of a part of one embodiment of the display panel of the present invention.

FIG. 7 is a circuit diagram of a part of another embodiment of the display panel of the present invention.

FIG. 8 is a conceptual diagram showing a light emission time ratio of one pixel per one field.

FIG. 9 is a circuit diagram illustrating an example of a conventional pixel circuit.

FIG. 10 is a characteristic diagram showing variation in characteristics of two transistors.

FIG. 11 is a circuit diagram showing an example of a conventional pixel circuit using a current mirror circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Current control means 2 Current supply means 3 Switching means 4 Light emitting element 5 Driving control means 6 Scan driver 7 Signal driver 8, 9 Control signal driver 10 Scan line 11 Signal line 12, 13 Control signal line 14 Cathode electrode line 15 Power supply line 16 Drive control circuit 20 to 24 Transistor 27 Organic EL device 30 Constant current circuit 31 Scan line 32 Signal line 33 to 35, 41 Transistor 38 Capacitor 39 Cathode electrode 40 Light emitting element 42 Power supply line 43 Current control circuit

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) H05B 33/14 H05B 33/14 A F term (reference) 3K007 AB02 AB05 AB17 BA06 DA01 DB03 EB00 GA02 GA04 5C080 AA06 AA07 BB05 DD05 EE29 FF11 JJ02 JJ03 JJ04 JJ05

Claims (9)

[Claims] A scanning line and a signal line are formed in a matrix on a substrate, and near a crossing point between the scanning line and the signal line.
In an active matrix display panel having a pixel circuit having at least one current control type light emitting element whose luminance changes according to an input current, the pixel circuit is a unit for supplying a current to the current control type light emitting element. A display panel comprising: a current supply unit; and a switch unit connected between the current supply unit and the current control type light emitting element. 2. The method according to claim 1, further comprising a step of connecting the scanning line and the signal line, and switching between supply and non-supply of a current from the current supply unit to the switch unit according to a signal from the scanning line and the signal line. The display panel according to claim 1, further comprising current control means. 3. The display panel according to claim 1, wherein said switch means comprises at least one transistor. 4. The display panel according to claim 3, wherein the transistor is a thin film transistor. 5. The apparatus according to claim 1, further comprising drive control means for performing two-input multiplex drive control, wherein said switch means constitutes a two-input multiplexer together with said drive control means. The display panel according to any one of the above items. 6. The method of driving a display panel according to claim 1, wherein the amount of light emitted from the current control type light emitting element is controlled by controlling an open / close time of the switch. A driving method characterized by adjusting. 7. The display panel driving method according to claim 2, wherein the amount of light emission of the current control type light emitting element is controlled by controlling an open / close time of the switch means. A driving method, wherein the current control means selects light emission or non-light emission in one field of the current control type light emitting element included in each pixel. 8. The method according to claim 1, wherein the selection is one per one field.
The driving method according to claim 7, wherein the driving is performed only once. 9. The method of driving a display panel according to claim 5, wherein the light emission time of the current control type light emitting element is controlled by two-input multiplex driving.