JP2000105574A - Current control type light emission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current control type light emission device which can favorably control the brightness of a display using current control type light emission elements without dispersion of brightness. SOLUTION: This current control type light emission device controls the light emission brightness of a display 2 composed of picture elements constituted of the current control type light emission elements 1 by varying the current value of a voltage/current exchange circuit 4 during the selection period of the picture elements. One picture element is constituted of one light emission element 1, the voltage/current exchange circuit 4 is provided by one as for all the picture elements on the display 2, and by changing output of the voltage/ current exchange circuit 4 to the light emission element of each picture element, the picture element is selected, and when it is not selected, it is applied on offset voltage (Vs) lower than light emission beginning voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current control type light emitting device for controlling the brightness of a display using a current control type light emitting element.

[0002]

2. Description of the Related Art FIG. 6 is a diagram showing an example of the configuration of a display using a conventional organic electroluminescent device (hereinafter referred to as an organic EL device). In the figure, 101 is an organic EL
The element 102 is a display panel, and the number of pixels is m
× n organic EL elements 101. A line control circuit 103 sequentially selects each of the lines x1 to xm and applies a high-level voltage to the selected line.
Reference numeral 104 denotes a voltage-current conversion circuit which samples and holds a luminance signal of each pixel, converts the luminance signal into a current, and outputs the converted signal to each organic EL element 101. Reference numeral 105 denotes a voltage-current conversion circuit assembly, and one voltage-current conversion circuit 104 corresponds to m vertical pixels, and n voltage-current conversion circuits 104 are arranged in the horizontal n-pixel direction.

FIG. 7 is a diagram showing a detailed configuration of a voltage / current conversion circuit arranged at the left end in the voltage / current conversion circuit assembly shown in FIG. In the figure, reference numeral 111 denotes a sample and hold circuit which samples and holds a luminance signal input as an analog voltage at a predetermined timing. 112 is an operational amplifier, 113 is a transistor, 11
Reference numeral 4 denotes a resistor which converts the sampled and held luminance signal into a current and outputs the current.

Next, the operation of brightness control in a display using a conventional organic EL element will be described with reference to FIGS. 6 and 7. FIG. Here, the voltage-to-current conversion circuit of FIG. 7 is shown as one connected to y1 in the voltage-to-current conversion circuit assembly 105 shown in FIG. 6, but y2 to yn
Have the same configuration. That is, the same voltage-to-current conversion circuits 104 as the voltage-to-current conversion circuits shown in FIG. 7 are arranged in the voltage-to-current conversion circuit assembly 105 for the number of pixels (n) in the horizontal direction.

First, a luminance signal input to the voltage-current conversion circuit 104 as an analog voltage is sampled and held at a predetermined timing by a sample-and-hold circuit 111.
Operational amplifier 112, transistor 113, and resistor 11
The current is converted to a current in step 4, and currents corresponding to the luminance signals are output from y1 to yn.

Next, the first line at the top of the panel (x1) is selected by the line control circuit 103, and a high-level voltage is applied to x1. While the high-level voltage is applied to x1, the current output from x1 flows through the organic EL element of each pixel on the first line, flows into the voltage-current conversion circuit 104 via y1 to yn. Thus, the organic EL elements on the first line emit light at a luminance corresponding to the luminance signal.

Next, the first line control circuit 103
The line is not selected, the second line is selected, and a high level voltage is applied to x2. Then, the same brightness control as that of the first line is performed. Further, similar luminance control is performed for the m-th
One image is created by repeating the process up to the line.

[0008]

However, in the above-described conventional luminance control, each voltage-current conversion circuit has a different reference resistance (Rref), and the individual resistance values vary, so that each of the organic EL elements is varied. The current of the element varies, causing variations in light emission luminance. That is,
Since it is impossible to make all the reference resistors (Rref) completely uniform, there has been a problem that the variations appear in the form of uneven brightness.

Therefore, instead of having a constant current source transistor for the number of pixels, a single voltage / current conversion circuit is used, and a pixel is selected while switching the output to each organic EL element in the line. Can be considered. However, according to this, when the number of pixels in one line is n, the allocation time of one pixel is one in the conventional driving method.
It becomes 1 / n compared to the pixel allocation time.
Performing the luminance control by the duty control during the period requires a very high-speed switching, and thus the realization becomes severe.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and provides a current control type light emitting device capable of favorably controlling the brightness of a display using a current control type light emitting element without variation in brightness. The purpose is to do.

[0011]

In order to solve the above-mentioned problems, a current-controlled light-emitting device according to the present invention (claim 1) is designed to reduce the light emission luminance of a display comprising pixels constituted by current-controlled light-emitting elements. In a current control type light-emitting device that controls by changing the current value of a constant current source during a selection period of the pixel, one pixel is constituted by one light-emitting element, and the constant current source includes all of the elements on the display. One pixel is provided, and the output of the constant current source is switched to the light emitting element of each pixel, thereby selecting the pixel and applying an offset voltage lower than the light emission start voltage when not selected. .

According to a second aspect of the present invention, in the current control type light emitting device according to the first aspect, one pixel includes three types of red, green and blue light emitting elements. The constant current source was configured such that each of the three types of red, green and blue light emitting elements was provided one by one.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. Embodiment 1 FIG. FIG. 1 is a block diagram showing a configuration example of a current-controlled light emitting device for a monochrome display according to a first embodiment of the present invention. In the figure, reference numeral 1 denotes a light-emitting element. One end of each light-emitting element is connected to one of x1 to xm, and the other end is connected to one of y1 to yn. Reference numeral 2 denotes a display panel, which is composed of a light emitting element 1 having m × n pixels. A line control circuit 3 sequentially selects each of the lines x1 to xm and applies a high-level voltage to the selected line. Reference numeral 4 denotes a voltage-current conversion circuit that outputs a luminance signal of each pixel to each light emitting element 1. Reference numeral 5 denotes a y-direction address control circuit which, when applied to the line selected by the line control circuit 3, selects each pixel in the selected line. More specifically, when the selected line is applied, the y-direction address control circuit 5 sequentially selects each of the switches swy1 to swyn, and switches the selected switch to the voltage / current conversion circuit 4 Connected, and switches that are not selected at that time are connected to the voltage source Vs to sequentially select each pixel in the selected line.

Therefore, each light emitting element 1 in the display panel passes through one of the switches swy1 to swyn controlled by the y-direction address control circuit 5 and receives one of the voltage-current conversion circuit 4 and the voltage source Vs. Will be connected. Note that the switches swy1 to swy
A state in which yn is connected to the voltage-current conversion circuit 4 side is called an ON state, and a state in which yn is connected to the voltage source Vs side is called an OFF state.

FIG. 2 is a diagram showing a detailed configuration of the voltage-current conversion circuit shown in FIG. Here, it is assumed that the input luminance signal has been digitized by an AD converter in advance. The voltage-current conversion circuit in the figure is the same as the configuration of a general-purpose 6-bit current addition type DA converter, and the luminance signal is 1-bit D1 (Least Significant Bit, LS).
B) to 6-bit D6 (Most Significant Bit, MS)
In the 6-bit gradation up to B), the output current of the constant current source weighted corresponding to each bit is switched according to the input signal and output from the output terminal Vy.

FIG. 3 is a diagram showing a schematic relationship between a voltage and an element current when the first line x1 is active and the swy1 is on in the current control type light emitting device for monochrome display of FIG.

In the figure, Vg is a voltage applied to x1 by the line control circuit when active. Vs and Vd are the potentials of y1 when swy1 is on and off, respectively. Therefore, the voltage applied to the selected light emitting element is Vg-Vd, and Vg-Vs is applied to the non-selected light emitting element. Note that Vs
Needs to be set to a voltage that does not emit light even when Vg-Vd is applied to the light emitting element. Reference numeral 11 denotes a light emitting element characteristic curve, which indicates a relationship between a voltage applied to the light emitting element and a current flowing through the light emitting element (element current).
Reference numeral 12 denotes a constant current source characteristic curve, which shows the relationship between the voltage and the current of the voltage-current conversion circuit 4. 13 is an operating point,
The intersection of the light emitting element characteristic curve 11 and the constant current source characteristic curve 12 is the operating point of the light emitting element. The operating point 13 changes as shown in the figure according to the magnitude of the luminance signal because the voltage-current conversion circuit 4 is an aggregate of a plurality of constant current sources.

FIG. 4 is a diagram schematically showing a change in y1 potential when the current control type light emitting device for a monochrome display is turned off and opened during a non-selection period in FIG. Here, the selection period is set to 1 / n (n is the number of pixels in one line) of the selection period of one pixel in the driving method using the conventional voltage-current conversion circuit 104 (see FIG. 6). For example, if the number of pixels in the horizontal direction is 320 in a general-purpose display at present, it is set to 1/320 of the conventional selection period.

In the figure, reference numeral 14 denotes a potential change in an off state during a non-selection period, and y1 when the switch swy1 is turned on during a selection period and turned off during a non-selection period.
3 schematically shows the change in the potential. That is, this is a case where the brightness control operation is performed in the monochrome display according to the first embodiment. Reference numeral 15 denotes a potential change in the switch open state during the non-selection period. The switch swy1 is turned on during the selection period, like the potential change 14 during the non-selection period, and is also turned off during the non-selection period. In the open state without being connected to the voltage-current conversion circuit 4 or the voltage source Vs.
1 schematically shows a potential change.

Next, a change in the potential y1 of the potential change 15 in the switch open state during the non-selection period will be described. Since the voltage is applied to the light-emitting element during the non-selection period, the voltage applied to the light-emitting element becomes zero V and does not emit light. When the selection period starts, the light-emitting element is turned on, current is drawn into the voltage-current conversion circuit 4, and the potential of y1 drops. Go. However, since the parasitic capacitance is attached to the wiring y1, the potential drop speed of y1 becomes slower as the parasitic capacitance is larger, and in some cases, the operating point voltage Vd corresponding to the luminance signal is not reached within the selection period as shown in the drawing. When the selection period ends and the non-selection period starts again, the potential of y1 rises.

Here, in the potential change 15 in the switch open state in the non-selection period, the change in the potential from zero voltage in the non-selection period to the potential drop in the selection period is determined by the conventional voltage-current conversion circuit 104 (FIG. 6). This corresponds to a potential change in a current-controlled light-emitting device for monochrome display using the above-described method. Therefore, during the conventional selection period, that is, n times the selection period shown in FIG. 4, the potential drops from Vg and reaches Vd sufficiently within the conventional selection period.

On the other hand, when the potential change 14 in the switch-off state, that is, the brightness control operation in the first embodiment, is performed, the potential of y1 is Vs in the off state during the non-selection period, and Does not emit light because the applied voltage is Vg-Vs. When the selection period starts, the non-selection period is turned on similarly to the potential change 15 of the switch open state, the current is drawn into the voltage-current conversion circuit 4, and the potential of y1 becomes , During the non-selection period, at the same speed as the potential change 15 in the switch open state. However, since the drop start voltage is low, the operating point voltage (Vd) corresponding to the luminance signal during the selection period
Can be reached.

Next, the operation of luminance control in the current control type light emitting device for monochrome display according to the first embodiment will be described with reference to FIGS. First, the line control circuit 3 applies an active voltage to the first line x1. Next, the y-direction address control circuit 5 sets the
1 is turned on, and the first pixel on the first line x1 is selected while keeping swy2 to swyn in the off state. That is, as shown in FIG. 1, since the switch swy1 is on and x1 is active, the light emitting element on the intersection of x1 and y1 emits light.

At this time, while the first pixel is selected, that is, during the selection period (1 / n of the conventional selection period) shown in FIG. Draw the corresponding current. As a result, the current flowing through the light emitting element changes, and the potential of y1 sufficiently reaches the operating point voltage (Vd) during the selection period, and the light emission luminance of the first pixel is controlled.

Next, when the selection time of the first pixel is over, the y-direction address control circuit 5 turns off the switch swy1, turns on the switch swy2, and keeps the switches swy3 to swyn off, thereby setting the first line x1. Is selected.

Next, the voltage-current conversion circuit 4 draws a current corresponding to the luminance signal from swy2, and the potential of y2 reaches the operating point voltage sufficiently during the selection period, and the light emission luminance of the second pixel is reduced. Controlled. Similarly, the first line x
The third and subsequent pixels of 1 are sequentially selected.

When the selection of each pixel on the first line x1 is completed, the line control circuit 3 sequentially selects all the second to m-th lines (x2 to xm), and the y-direction address control circuit 5 Sequentially selects each pixel on each line. As described above, each pixel on the entire screen of the display panel is selected, and the light emission luminance of each pixel on the entire panel is controlled.

As described above, the current control type light emitting device for monochrome display according to the first embodiment of the present invention switches one voltage / current conversion circuit and applies an offset voltage to the light emitting element during the non-selection period. By selecting each pixel, the light emission luminance of all pixels is controlled. Therefore, the selection period of one pixel is shortened to reach a sufficient operating voltage, and the luminance can be controlled satisfactorily and the current value varies. It is also possible to avoid the occurrence of luminance unevenness due to.

Embodiment 2 FIG. 5 is a block diagram showing a configuration example of a current control type light emitting device for a color display according to the second embodiment of the present invention. Here, in the color display, one pixel in the monochrome display according to the first embodiment is formed of light emitting elements of three colors of red, green and blue (RGB).

In the figure, 21 is a light emitting element for R,
One end of each R light emitting element is any of xA1 to xAm,
The other end is connected to any of yA1 to yAn. Similarly, 31 and 41 are the light emitting element for G and B
Each of the G light emitting elements has one end at xB1 to xB
Bm, the other end is connected to one of yB1 to yBn, and each B light emitting element has one end connected to one of xC1 to xCm and the other end connected to one of yC1 to yCn. Reference numeral 22 denotes a display panel having a pixel number m
It is composed of × n R light emitting elements 21, G light emitting elements 31, and B light emitting elements 41.

FIG. 3 shows a light emitting element for R connected to xA1 and yA1, and a G light emitting element connected to xB1 and yB1.
Light-emitting device and B connected to xC1 and yC1
Only one pixel composed of a light emitting element for use is shown, and other light emitting elements are omitted.

Reference numeral 23 denotes a line control circuit for R, and xA1
To xAm are sequentially selected, and a high-level voltage is applied to the selected line. Similarly, 33 and 4
3 is a line control circuit for G and a line control circuit for B, respectively, and xB1 to xBm and xC1 to x, respectively.
Each line up to Cm is sequentially selected, and a high-level voltage is applied to the selected line.

Reference numeral 24 denotes an R voltage-current conversion circuit, which outputs a red (R) luminance signal in each pixel to each R light emitting element 21. Similarly, reference numerals 34 and 44 denote a G voltage-current conversion circuit and a B voltage-current conversion circuit, respectively, which respectively output green (G) and blue (B) luminance signals in each pixel to the G light emitting elements 31 and B, respectively. Light emitting element 41 for
Output to Note that the R voltage-current conversion circuit 24, the G voltage-current conversion circuit 34, and the B voltage-current conversion circuit 44
Is similar to that shown in FIG.

Reference numeral 25 denotes an R y-direction address control circuit which, when applied to the line selected by the R line control circuit 23, selects the R light-emitting element 21 of each pixel in the selected line. I do. Similarly, 35 and 4
5 is a y-direction address control circuit for G and y for B
Direction address control circuit, and when applied to a line selected by the G line control circuit 33 and the B line control circuit 43, respectively, the G light emitting elements 31 and B of each pixel in the selected line. Light emitting element 41 for
Select

More specifically, the y-direction address control circuits 25, 35, and 45 for R, G, and B respectively operate the switches swyA1 to swyAn, swyB1 to swyA1 when the selected line is applied. The switches SwyBn and SwyC1 to SwyCn are sequentially selected, and the selected switches are connected to the R, G, and B voltage / current conversion circuits 24, 34, and 44, respectively. Is connected to a voltage source to sequentially select the R, G, and B light emitting elements 21, 31, and 41 of each pixel in the selected line.

The switches swyA1 to swyAn,
swyB1 to swyBn and swyC1 to swyCn
Are connected to the R, G, and B voltage / current conversion circuits 24, 34, and 44, respectively, and are referred to as an ON state, and a state connected to the voltage source is referred to as an OFF state.

Here, in the current control type light emitting device for a color display shown in FIG. 5, the first line xA for R is used.
The schematic relationship between the voltage when 1 is active and the swyA1 is in the ON state and the R element current is the same as in FIG. Also, the first lines xB1 and xC for G and B
The schematic relationship between the voltage when 1 is active and swyB1 and swyC1 are on and the device currents for G and B are the same as in FIG.

Further, in the current control type light emitting device for a color display shown in FIG. 5, the potential change in yA1, yB1 or yC1 when turned off and open during the non-selection period is compared and shown. Is the same as in FIG.

Next, the operation of brightness control in the current control type light emitting device for color display according to the second embodiment will be described with reference to FIGS. First, R
The active line control circuit 23 applies an active voltage to the first R line xA1. At the same time, the G line control circuit 33 and the B line control circuit 43 apply an active voltage to the first G line xB1 and the first B line xC1, respectively.

Next, the y-direction address control circuit for R 25
The first R light emitting element 21 of the first R line xA1 is selected while turning on the swyA1 and keeping the swyA2 to the swyAn in the off state. At the same time, the y-direction address control circuit 35 for G and the y-direction address control circuit 45 for B turn on the swyB1 and the swyC1 respectively, and
With the Cn kept in the off state, the first G light emitting element 21 and the first B line xC1 of the first G line xB1
The first B light emitting element 21 is selected.

At this time, the first R light emitting element 21
Is selected, that is, during the selection period shown in FIG. 4 (1 / n of the conventional selection period), the R voltage / current conversion circuit 24 draws a current corresponding to the luminance signal from the swyA1. As a result, the current flowing through the R light emitting element 21 changes, and the potential of yA1 sufficiently reaches the operating point voltage during the selection period, and the light emission luminance of the first R light emitting element 21 is controlled. At the same time, the luminance of the first light emitting element 31 for G and the light emitting element 41 for B are controlled in the same manner.

Next, when the selection time of the first R light emitting element 21 ends, the R y direction address control circuit 25
Turns off swyA1, turns on swyA2, and turns
The second R light emitting element 21 on the first R line xA1 is selected while A3 to swyAn remain off.
At the same time, in exactly the same way, the second G light emitting element 31 of the first G line xB1 and the second light emitting element 31 of the B first line xC1
The third B light emitting element 41 is selected.

Next, the R voltage / current conversion circuit 24 sets sw
A current corresponding to the luminance signal is drawn from yA2, and the potential of yA2 sufficiently reaches the operating point voltage within the selection period, so that the emission luminance of the second R light emitting element 21 is controlled. At the same time, the luminance of the second light emitting element 31 for G and the light emitting element 41 for B are controlled in the same manner. Similarly, the third and subsequent light emitting elements of the first R line xA1, the first G line xB1, and the first B line xC1 are sequentially selected.

When the selection of each R light emitting element 21 of the first R line xA1 is completed, the R line control circuit 2
3 indicates all the R-th to R-th m-th lines (xA2 to
xAm) are sequentially selected, and the y-direction address control circuit 2 for R
5 sequentially selects each R light emitting element 21 on each R line. Line control circuits 33 and 43 for G and B,
The y-direction address control circuits 35 and 45 for G and B operate in exactly the same manner.

As described above, the selection of the R, G, and B light emitting elements 21, 31, and 41 in each pixel on the entire screen of the color display panel is performed.
The light emission luminance of the three color light emitting elements in each pixel of the entire panel is controlled.

As described above, the current control type light emitting device for a color display according to the second embodiment of the present invention is
One voltage-current conversion circuit is switched for each of the light emitting elements for G and B, and an offset voltage is applied to the light emitting elements during the non-selection period. By selecting light emitting elements,
Since the light emission luminance of all pixels is controlled, it is possible to shorten the selection period of one pixel to reach a sufficient operating voltage, to control the luminance satisfactorily, and to avoid the occurrence of luminance unevenness due to current value variation. Can also.

[0047]

As described above, according to the current control type light emitting device of the present invention (claim 1), one voltage / current conversion circuit is switched, and the offset voltage is applied to the light emitting element during the non-selection period. By applying the voltage and selecting each pixel, the light emission luminance of all pixels is controlled. Therefore, the selection period of one pixel is shortened, and the current value of the light emitting element is made to respond within the selection period. It is possible to provide a monochrome display having uniform light emission luminance over the entire screen while being able to control luminance and avoiding the occurrence of luminance unevenness due to current value variation.

Further, according to the current control type light emitting device of the present invention (claim 2), one voltage / current conversion circuit is switched for each of the R, G, and B light emitting elements, so that the In the selection period, an offset voltage is applied to the light emitting elements to select the R, G, and B light emitting elements in each pixel, thereby controlling the light emission luminance of all pixels. By making the pixel selection period shorter, the current value of the light emitting element responds during the selection period, and it is possible to control the luminance satisfactorily and to avoid the occurrence of luminance unevenness due to the variation of the current value, and to achieve uniform light emission over the entire screen. There is an effect that a color display having luminance can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of a current-controlled light-emitting device for a monochrome display according to a first embodiment.

FIG. 2 is a diagram illustrating a detailed configuration of a voltage-current conversion circuit illustrated in FIG. 1;

FIG. 3 is a diagram showing a schematic relationship between voltage and element current in the current control type light emitting device for monochrome display of FIG. 1;

FIG. 4 is a diagram showing a comparison of potential changes of y1 in the current control type light emitting device for monochrome display of FIG. 1;

FIG. 5 is a block diagram illustrating a configuration example of a current control type light emitting device for a color display according to a second embodiment.

FIG. 6 is a diagram showing an example of a configuration of a display using a conventional organic EL element.

FIG. 7 is a diagram illustrating a detailed configuration of a voltage-current conversion circuit in the voltage-current conversion circuit assembly illustrated in FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1,101 Light emitting element 2,22,102 Display panel 3,103 Line control circuit 4 Voltage-current conversion circuit 5 Y direction address control circuit 11 Light emitting element characteristic curve 12 Constant current source characteristic curve 13 Operating point 14 Off during non-selection period State potential change 15 Switch open state potential change during non-selection period 21 R light emitting element 23 R line control circuit 24 R voltage / current conversion circuit 25 R y-direction address control circuit 31 G light emitting element 33 G line Control circuit 34 Voltage-current conversion circuit for G 35 Y-direction address control circuit for G 41 Light-emitting element for B 43 Line control circuit for B 44 Voltage-current conversion circuit for B 45 Y-direction address control circuit for B 104 Conventional voltage-current conversion circuit 105 Voltage-current conversion circuit assembly

Claims (2)

[Claims] 1. A current control type light emitting device for controlling the light emission luminance of a display comprising pixels constituted by current control type light emitting elements by changing a current value of a constant current source within a selection period of the pixel. One pixel is composed of one light emitting element, and one constant current source is provided for every pixel on the display, and the output of the constant current source is switched to the light emitting element of each pixel to select the pixel. And a current control type light emitting device characterized in that the voltage is applied to an offset voltage lower than the light emission start voltage when not selected. 2. The current control type light emitting device according to claim 1, wherein one pixel is composed of three kinds of light emitting elements of red, green and blue, and the constant current source is composed of three kinds of light emitting elements of red, green and blue.
A current-controlled light-emitting device, wherein one light-emitting device is provided.