# Patent

## Title
Apparatus and method for driving multi-color light emitting display panel

## Abstract

A driving apparatus for a multi-color light-emitting display panel including drive lines and scanning lines intersecting with each other, and capacitive light-emitting elements which have polarities connected to the scanning lines and the drive lines at the intersections and which are divided into a plurality of types by a color of light emission, the capacitive light-emitting elements of the same type being arranged on each drive line. The drive apparatus comprises a scanning circuit for selectively supplying a first potential and a second potential higher than the first potential to each of the scanning lines, and a drive circuit for selectively supplying a drive current from a current source and a third potential for an offset voltage not higher than a light emission threshold voltage of the element to each of the drive lines, the drive current and the third potential are variable.

## Background
This is a continuation application of application Ser. No. 09/624,194, filed on Jul. 24, 2000, now U.S. Pat. No. 6,707,438, which is incorporated by reference herein in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving apparatus for a multi-color light-emitting display panel that uses capacitive light-emitting elements such as organic electroluminescence elements and a drive method for the same.
2. Description of the Related Art
In recent years, with the trend of increasing the size of display devices, thinner display devices have been required, and a variety of thin display devices have been brought into practical use. An electroluminescence display comprising a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as one of the thin display devices.
As shown in FIG. 1, the organic electroluminescence element comprises at least one layer of an organic functional layer 102, made up of an electron transport layer, a light-emitting layer, a hole transport layer or the like, and a metallic electrode 103, stacked on a transparent substrate 100 of a glass plate or the like on which a transparent electrode 101 is formed. The organic functional layer 102 emits light by applying a positive voltage to the anode of the transparent electrode 101 and a negative voltage to the cathode of the metallic electrode 103, that is, by applying a current across the transparent electrode and the metallic electrode. The electroluminescence display is practicable by using, as the organic functional layer, an organic compound that can be expected to provide good light emission characteristics.
The organic electroluminescence element (hereinafter simply referred to as an EL element) can be represented electrically by an equivalent circuit as shown in FIG. 2. As can be seen from the drawing, the EL element can be replaced by a capacitance component C and a diode characteristic component E that is connected in parallel to the capacitance component. Therefore, the EL element can be considered a capacitive light-emitting element. In the EL element, when a DC light emission drive voltage is applied across the electrodes, electric charge is stored in the capacitance component C. When a barrier voltage or light emission threshold voltage, which corresponds to the element, is exceeded thereafter, a current starts to flow from the electrode (the anode of the diode component E) to the organic functional layer which serves as a light-emitting layer to allow the EL element to emit light at an intensity in proportion to the current.
As shown in FIG. 3, the characteristic of voltage V−current I−luminosity L of such an EL element is very similar to that of a diode, where the current I is extremely small at voltages not larger than the light emission threshold voltage Vth and suddenly increases at voltages equal to or larger than the light emission threshold voltage Vth. In addition, the current I is generally proportional to the luminosity L. In the EL element, when a drive voltage larger than the light emission threshold voltage Vth is applied to the EL element, the element emits light at luminosity proportional to the current corresponding to the drive voltage. On the other hand, when the drive voltage applied thereto is equal to or smaller than the light emission threshold voltage Vth, no drive current flows and the luminosity of light emission remains zero.
As a method for driving a light-emitting display panel that employs such EL elements, known is a simple matrix drive method. FIG. 4 shows the configuration of an example of a drive apparatus that uses the simple matrix drive method for a multi-color light-emitting display panel. In the light-emitting display panel, n cathode lines (metallic electrodes) B1, . . . , Bn are provided in the horizontal direction and 3m anode lines (transparent electrodes) A1R, A1G, A1B, . . . , AmR, AmG, AmB are provided in the vertical direction. EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n are formed at the respective intersections (a total of n×3m). The EL elements E1R, 1, . . . , EmR, n emit red light; the EL elements E1G, 1, EmG, n emit green light; and EL elements E1B, 1, . . . , EmB, n emit blue light. Three EL elements (for example, E1R, 1, E1G, 1, E1B, 1) of each of three primary colors of red, green, and blue, consecutive in each of the cathode lines, form one pixel. The EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n are arranged in the shape of lattice with one end thereof (the anode line side of the diode component E in the aforementioned equivalent circuit) connected to the anode lines and the other end thereof (the cathode side of the diode component E in the aforementioned equivalent circuit) connected to the cathode lines, corresponding to the intersections of the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB, which are directed along the vertical direction, and the cathode lines B1, . . . , Bn, which are directed along the horizontal direction. The cathode lines are connected to a cathode line scanning circuit 1, while the anode lines are connected to an anode line drive circuit 2 and an anode line reset circuit 3.
The cathode line scanning circuit 1 has scanning switches 51, . . . , 5n, corresponding to the cathode lines B1, . . . , Bn, for determining individually the electric potential of each of the cathode lines, each relaying and supplying either one of a positive potential Vcc which serves as a reverse bias voltage or the ground potential (0V) to a corresponding cathode line.
The anode line drive circuit 2 has current sources 21R, 21G, 21B, . . . , 2mR, 2mG, 2mB, (for example, constant current sources), corresponding to the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB, for supplying drive currents to individual EL elements through the respective anode lines, and drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB. The anode line drive circuit 2 performs on/off control to allow the drive switches to supply currents to individual anode lines. Voltage sources such as constant voltage sources can be used as the drive sources. However, current sources (current circuits that are controlled to provide a desired amount of supply current) are generally used due to a fact that the aforementioned current—luminosity characteristic is stable against a variation in temperature, whereas the voltage—luminosity characteristic is unstable against a variation in temperature. The amount of supply current of the current sources 21R, 21G, 21B, . . . , 2mR, 2mG, 2mB is an amount of current required for the EL elements to sustain a state of emitting light at desired instantaneous luminosity (hereinafter, this state is referred to as the steady light emitting state). In addition, the aforementioned capacitance component C of the EL element is charged with electric charge corresponding to the amount of supply current when the EL element is under a light-emitting state. Accordingly, the voltage across the EL element becomes a specified value Ve (hereinafter, this is referred to as the light emission regulating voltage).
The anode line reset circuit 3 has shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB, which are provided for each of the anode lines. The shunt switches are selected to set the cathode lines to the ground potential.
The cathode line scanning circuit 1, the anode line drive circuit 2, and the anode line reset circuit 3 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1, the anode line drive circuit 2, and the anode line reset circuit 3 to display, in accordance with image data supplied from an image data generation system (not shown), the image to be served by the image data. The light emission control circuit 4 generates a scanning line select control signal for the cathode line scanning circuit 1 to select one cathode line from the cathode lines B1, . . . , Bn, corresponding to a horizontal scanning period of the image data, so that the selected cathode line is set to the ground potential and the remaining cathode lines are supplied with the positive potential Vcc. The positive potential Vcc is applied to EL elements by constant voltage sources connected to the cathode lines to prevent the EL elements, which are connected to the intersections of the driven anode lines and the cathode lines which are not selected for scanning, from producing cross-talk light emission. The positive potential is set such that Vcc=Ve. As the scanning switches 51, . . . , 5n are sequentially switched to the ground potential in each horizontal scanning period, a cathode line set at the ground potential functions as a scanning line which enables the EL elements connected thereto to emit light.
The anode line drive circuit 2 performs light emission control for the scanning line. The light emission control circuit 4 generates a drive control signal (a drive pulse) that shows which EL elements connected to the scanning line is allowed to emit light, the timing, and the duration of time for the light emission, in accordance with pixel color information shown by image data, and supplies the signal to the anode line drive circuit 2. In accordance with the drive control signal, the anode line drive circuit 2 turns on some of drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB to supply drive currents to the corresponding EL elements through the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB. This allows the EL elements to which drive currents are supplied to emit light in accordance with the pixel color information. Any color can be obtained depending on the light emission luminosity of each of the EL elements in a pixel or depending on the duration of time for light emission within a light emission period.
The reset operation of the anode line reset circuit 3 is carried out in accordance with the reset signal from the light emission control circuit 4. The anode line reset circuit 3 turns on some of the shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB, corresponding to the anode lines, shown by the reset control signal, to be reset, and turns off the remaining shunt switches.
Japanese Patent Laid-Open Publication No.Hei 9-232074, applied by the same applicant as the present applicant, discloses a drive method for performing a reset operation (hereinafter referred to as the reset drive method) in which electric charge stored in each EL element, disposed in a lattice shape, immediately before a scanning line is changed in a simple matrix display panel. This reset drive method is to accelerate the rise time of light emission in EL elements when a scanning line is changed. The reset drive method for a simple matrix display panel will be explained with reference to FIGS. 4 to 6.
The operation that is shown in FIGS. 4 to 6 and described below includes an example in which the cathode line B1 is scanned to allow the EL elements E1R, 1, E1G, 1, E1B, 1 to emit light and thereafter, the scan is transferred to the cathode line B2 to emit the EL elements E2R, 2, E2G, 2, E2B, 2. In addition, for the sake of clarity in explanation, EL elements that are emitting light are indicated by diode symbols, whereas those that are not emitting light are indicated by capacitor symbols. Moreover, the positive potential Vcc applied to the cathode lines B1, . . . , Bn is made equal to the light emission regulating voltage Ve of an EL element.
Referring to FIG. 4, first, only scanning switch 51 is switched over to the ground potential of 0 (V) and the cathode line B1 is scanned. The positive potential Vcc is applied to the remaining cathode lines B2, . . . , Bn by means of the scanning switches 52, . . . , 5n. At the same time, the current sources 21R, 21G, 21B are connected to the anode lines A1R, A1G, A1B by means of the drive switches 61R, 61G, 61B. In addition, the remaining anode lines A2R, A2G, A2B, . . . , AmR, AmG, AmB are switched over to the ground potential of 0 (v) by means of the shunt switches 72R, 72G, 72B, . . . , 7mR, 7mG, 7mB. Thus, in the case of FIG. 4, a voltage is applied to only the EL elements E1R, 1, E1G, 1, E1B, 1 in the forward direction, where drive currents flow in from the current sources 21R, 21G, 21B, as indicated by arrows, to allow only the EL elements E1R, 1, E1G, 1, E1B, 1, to emit light. In this state, the EL elements E2R, 2, E2G, 2, E2B, 2, . . . , EmR, n, EmG, n, EmB , n, which emit no light and are indicated by hatching, are charged in the direction of the polarity shown in the figure.
The following reset control is carried out immediately before the scan is transferred from the light-emitting state of FIG. 4 to the state where the subsequent EL elements E2R, 2, E2G, 2, E2B, 2 emit light. That is, as shown in FIG. 5, all the drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB are opened, and all the scanning switches 51, . . . , 5n and all the shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB are switched over to the ground potential of 0 (V). The anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB and the cathode lines B1, . . . , Bn are set to the ground potential of 0 (V), thus all being reset. By resetting all, all the anode lines and the cathode lines are made equal to the same potential of 0 (V), so that electric charge stored in each of the EL elements is discharged and thus the charged electric charge in all EL elements disappear instantly.
After the charged electric charge in all of the EL elements is zero, only the scanning switch 52 corresponding to the cathode line B2 is then switched over to the 0 (V) side to carry out scanning over the cathode line B2 as shown in FIG. 6. At the same time, the drive switches 62R, 62G, 62B are closed to connect the current sources 22R, 22G, 22B to the corresponding anode lines A2R, A2G, A2B; and the shunt switches 71R, 71G, 71B, 73R, 73G, 73B, . . . , 7mR, 7mG, 7mB are turned on to give 0 (V) to the anode lines A1R, A1G, A1B, A3R, A3G, A3B, AmR, AmG, AmB. Accordingly, in the case of FIG. 6, a voltage is applied to only the EL elements E2R, 2, E2G, 2, E2B, 2 in the forward direction, where drive currents flow in from the current sources 22R, 22G, 22B to allow only the EL elements E2R, 2, E2G, 2, E2B, 2 to emit light.
The light emission control of the aforementioned reset drive method is to repeat the scan mode or a period for making any one of the cathode lines B1, , Bn active and the subsequent reset mode. The scan mode and the reset mode are carried out for every one horizontal scanning period (1H) of image data. Suppose that the state of FIG. 4 is directly transferred to that of FIG. 6 without carrying out the reset control. For example, the drive currents supplied from the current sources 22R, 22G, 22B, . . . , 2mR, 2mG, 2mB not only flow into the EL elements E2R, 2, E2G, 2, E2B, 2 but also are dissipated to cancel out the electric charge stored in the reverse direction (shown in FIG. 4) in the EL elements E2R, 3, . . . , E2R, n, E2G, 3, . . . , E2G, n, E2B, 3, . . . , E2B, n. Consequently, it will take time to make the EL elements E2R, 2, E2G, 2, E2B, 2 emit light (to make the voltage across the EL elements E2R, 2, E2G, 2, E2B, 2 equal to the light emission regulating voltage Ve).
However, carrying out the aforementioned reset control allows the potential of the anode lines A2R, A2G, A2B to become generally Vcc at the instant of changing the scan to the cathode line B2. Then, charge currents flow into the EL elements E2R, 2, E2G 2, E2B, 2, which should be subsequently allowed to emit light, not only from the current sources 22R, 22G, 22B but also through a plurality of routes from the constant voltage sources connected to the cathode lines B1, B3, . . . , Bn as shown in FIG. 6. These charge currents charge parasitic capacitances (the aforementioned capacitive components C) to allow the EL elements E2R, 2, E2G, 2, E2B, 2 to reach the light emission regulating voltage Ve and to be transferred to the state of light emission. After that, since within a scanning period of the cathode line B2, as described above, the amount of current supplied from each of the current sources is restricted to an amount of current just enough for each of the EL elements to sustain the state of light emission at a light emission regulating voltage Ve, the currents supplied from the respective current sources 22R, 22G, 22B flow into only the EL elements E2R, 2, E2G, 2, E2B, 2, all being dissipated for light emission, and the state of light emission shown in FIG. 6 is sustained.
As described above, according to the reset drive method, all the cathode lines and anode lines are once connected to either the ground potential of 0 (V) or the same potential of positive potential Vcc to reset the EL elements before the process proceeds to the light emission control of a subsequent scanning line. Accordingly, when the scan is changed over to the subsequent scanning line, it is possible to charge the EL elements quickly up to the light emission regulating voltage Ve and thus to provide EL elements which should emit light on the changed scanning line with a quick increase of light emission.
However, EL elements for red, green, and blue colors have element structures and materials, different from each other, so that the EL elements have the characteristics of voltage V−luminosity L, which are different from each other. Therefore, when all the EL elements constituting one pixel emit light to display white color, voltages applied to the both ends of each of the EL elements are different from each other. Thus, it is generally true that each of EL elements for red, green, and blue colors has a different light emission regulating voltage Ve. Therefore, when the reverse bias voltage Vcc is applied to each of the EL elements for red, green, and blue colors by the reset control as described above, and the cathode line for a subsequent scan is selected after the reset control, a difference in time will be produced until voltages across the EL elements, which should be allowed to emit light, on the cathode line selected reaches the light emission regulating voltage Ve of each of the red, green, and blue colors. Thus, since light emission at the light emission regulating voltage Ve did not take place at the same time, a problem of producing a difference in color was present.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus and method for driving a multi-color light-emitting display panel, which can improve the rise characteristic of light emission of each of the capacitive light-emitting elements that have colors of light emission different from each other.
A drive apparatus for a multi-color light-emitting display panel according to the present invention, the multi-color light-emitting display panel including a plurality of drive lines and a plurality of scanning lines intersecting with each other, and a plurality of capacitive light-emitting elements having polarities connected to the scanning lines and the drive lines at a plurality of intersections of the drive lines and the scanning lines, and being divided into a plurality of types by a color of light emission, the capacitive light-emitting elements of the same color type being arranged on each of the plurality of drive lines, comprises scanning means for selectively supplying a first potential and a second potential higher than the first potential to each of the plurality of scanning lines; and drive means for selectively supplying a drive current from a current source and a third potential for an offset voltage, equal to or less than a light emission threshold voltage of the element to each of the plurality of drive lines, wherein the drive current and the third potential are variable.
According to the driving apparatus of the present invention, the drive current and the third potential are made variable. Variations in voltages across respective capacitive light-emitting elements for emitting light of colors different from each other can be thereby made equal to each other, the variations being produced by the time the voltages reach each desired voltage during a scanning period. Thus, the rise characteristic of each of the capacitive light-emitting elements that emit light of colors different from each other can be improved.
Furthermore, a method for driving a multi-color light-emitting display panel according to the present invention, multi-color light-emitting display panel including a plurality of drive lines and a plurality of scanning lines intersecting with each other, and a plurality of capacitive light-emitting elements having polarities connected to the scanning lines and the drive lines at a plurality of intersections of the drive lines and the scanning lines, and being divided into a plurality of types by a color of light emission, the capacitive light-emitting elements of the same color type being arranged on each of the plurality of drive lines, comprises the steps of repeatedly setting a scanning period to select one scanning line of the plurality of scanning lines and a subsequent reset period in accordance with a scan timing of input image data, designating at least one drive line of the plurality of drive lines corresponding to at least one capacitive light-emitting element which should be emitted light on the one scanning line during the scanning period in accordance with the input image data, supplying, during the scanning period, the first potential to the one scanning line and the second potential to scanning lines other than the one scanning line, and supplying, during the reset period, the first potential to all scanning lines, and supplying, during the scanning period, the drive current to the at least one drive line to apply a positive voltage equal to or greater than the light emission threshold voltage to the at least one capacitive light-emitting element in the forward direction, and supplying, during the reset period, the third potential to at least one subsequent drive line to be designated during a next scanning period to apply an offset voltage equal to or less than the light emission threshold voltage to at least one capacitive light-emitting element which should be allowed to emit light during the next scanning period, the drive current and the third potential being variable for each type of the capacitive light-emitting elements.
According to such a driving method of the present invention, a subsequent drive line corresponding to capacitive light-emitting elements which should be allowed to emit light during the subsequent scanning period is designated during a reset period in accordance with the input image data. In addition, the third potential is supplied to the subsequent drive line and thereby an offset voltage not greater than the light emission threshold voltage is applied to the capacitive light-emitting elements. Moreover, the drive current and the third potential are made variable depending on the type of the capacitive light-emitting elements. Consequently, variations in voltages across respective capacitive light-emitting elements for emitting light of colors different from each other can be thereby made equal to each other, the variations being produced by the time the voltages reach each desired voltage during a scanning period. Thus, the rise characteristic of each of capacitive light-emitting elements that emit light of colors different from each other can be improved.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view showing an organic electroluminescence element;
FIG. 2 is a view showing an equivalent circuit of an organic electroluminescence element;
FIG. 3 is a schematic plot showing the drive voltage current light-emitting luminosity characteristic of an organic electroluminescence element;
FIG. 4 is an explanatory block diagram showing the light emission control operation of a prior art drive apparatus;
FIG. 5 is an explanatory block diagram showing the light emission control operation of the prior art drive apparatus;
FIG. 6 is an explanatory block diagram showing the light emission control operation of the prior art drive apparatus;
FIG. 7 is a schematic block diagram showing the configuration of a display apparatus to which the present invention is applied;
FIG. 8 is a view showing the specific configuration of the cathode line scanning circuits, the anode line drive circuits, and the light-emitting display panel of the apparatus of FIG. 7;
FIG. 9 is a flow diagram showing a light emission control routine;
FIG. 10 is a flow diagram showing a hue control routine;
FIG. 11 is an explanatory block diagram showing the light emission control operation during a scanning period;
FIG. 12 is an explanatory block diagram showing the light emission control operation during a reset period;
FIG. 13 is an explanatory block diagram showing the light emission control operation during the subsequent scanning period;
FIG. 14 shows variations in voltage across an EL element due to hue control;
FIG. 15 is a block diagram showing part of another display device to which the present invention is applied;
FIG. 16 is a flow diagram showing a hue control routine;
FIG. 17 is a view showing a data table;
FIGS. 18A–18C show the relationship between the offset voltage and reverse bias voltage of each of three primary colors;
FIG. 19 is a view showing the relationship between the offset voltage and reverse bias voltage when the luminosity of red is extremely decreased;
FIG. 20 is a flow diagram showing an initialization routine;
FIG. 21 is a flow diagram showing a brightness control routine;
FIG. 22 is a flow diagram showing a hue control routine;
FIG. 23 is a plot showing the voltage V−current I characteristic of an EL element when the total time of light emission is short; and
FIG. 24 is a plot showing the voltage V−current I characteristic of an EL element when the total time of light emission is long.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of the present invention will be explained below with reference to the drawings.
FIG. 7 shows a schematic configuration of a display device in which the present invention is applied to a multi-color light-emitting display panel employing EL elements as capacitive light-emitting elements. This display device has a capacitive light-emitting display panel 11, light emission control circuit 12, cathode line scanning circuit 13, and anode line drive circuit 14.
As shown in FIG. 8, the light-emitting display panel 11 is constructed in the same manner as those shown in FIGS. 4 to 6. That is, the light-emitting display panel 11 has a plurality of EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n disposed at a plurality of intersections in a matrix of drive lines or anode lines A1R, A1G, A1, . . . , AmR, AmG, AmB and scanning lines or cathode lines B1-Bn. The EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n are coupled to the anode lines and cathode lines at each of the plurality of intersections of the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB and the cathode lines B1, . . . , Bn. The EL elements E1R, 1, . . . , EmR, n emit red light, the EL elements E1G, 1, . . . , EmG, n emit green light, and the EL elements E1B, 1, . . . , EmB, n emit blue light. Three EL elements of red (R), green (G), and blue (B) (for example, E1R, 1, E1G, 1, and E1B, 1) form one pixel at each of the cathode lines.
The cathode line scanning circuit 13 is coupled to the cathode lines B1, . . . , Bn of the display panel 11, while the anode line drive circuit 14 is coupled to the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB. The cathode line scanning circuit 13 has scanning switches 151, . . . , 15n which are provided corresponding to the respective cathode lines B1, . . . , Bn. Each of the scanning switches 151, . . . , 15n, supplies either the ground potential or bias potential Vcc to a corresponding cathode line. The scanning switches 151, . . . , 15n are controlled by the light emission control circuit 12 so as to be switched over to the ground potential in sequence at every horizontal scanning period. Accordingly, the cathode lines B1, . . . , Bn which are set to the ground potential are to function as the scanning lines enabling the elements connected to the cathode lines to emit light.
The anode line drive circuit 14 has drive switches 161R, 161G, 161B, . . . , 16mR, 16mG, 16mB, variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB, and variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB, which are provided corresponding to the respective anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB. Each of the drive switches 161R, . . . , 16mR supplies one of the current from the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB, the potential from the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB, and the ground potential to the corresponding anode line. The variable voltage sources 181R, . . . , 18mR output offset voltage VR; the variable voltage sources 181G, . . . , 18mG output offset voltage VG; and the variable voltage sources 181B, . . . , 18mB output offset voltage VB. The light emission control circuit 12 controls the current value of each of the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB and the voltage value of each of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB.
In accordance with pixel color information provided by image data, the light emission control circuit 12 generates a drive control signal (drive pulse) that shows which EL element connected to a scanning line is to be allowed to emit light, the timing for the light emission, and the duration of the light emission. Then, the light emission control circuit 12 supplies the drive control signal to the anode line drive circuit 14. In response to the drive control signal, the anode line drive circuit 14 switches those drive switches corresponding to the light emission among the drive switches 161R, 161G, 161B, . . . , 16mR, 16mG, 16mB to the current source side. Then, the anode line drive circuit 14 supplies drive currents IR, IG, IB to corresponding elements in response to the pixel information through corresponding anode lines (current addressing drive lines) among the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB. In addition, the anode line drive circuit 14 supplies the ground potential to other anode lines via drive switches.
The light emission control circuit 12 is connected with a data input portion 19 and a memory 20. The data input portion 19 is adapted to be operable to control the luminosity of red, green, and blue colors of the light-emitting display panel 11. The data input portion 19 outputs, to the light emission control circuit 12, information regarding hue according to the user-actuated position of a control lever (not shown) corresponding to each of the red, green, and blue colors, that is, luminosity data of each of the red, green, and blue colors. The memory 20 stores beforehand control data such as data tables, which are described later.
The light emission control operation of the light-emitting display panel 11 by means of the light emission control circuit 12 is explained with reference to the flow diagram of FIG. 9.
The light emission control circuit 12 carries out a light emission control routine every one horizontal scanning period of pixel data supplied. In the light emission control routine, first, pixel data within one horizontal scanning period is captured (step S1). Then, in accordance with the pixel information provided by the captured pixel data within one horizontal scanning period, a scan select control signal and a drive control signal is generated (step S2).
The scan select control signal is supplied to the cathode line scanning circuit 13. In order to set one of the cathode lines B1, . . . , Bn, which corresponds to the current horizontal scanning period shown by the scan select control signal, to the ground potential, the cathode line scanning circuit 13 switches to the ground side the scanning switch corresponding to the one cathode line (one scanning switch 15S among the switches 151, . . . , 15n, where S is one of 1 to n). In order to apply positive potential Vcc to other cathode lines as the reverse bias potential, scanning switches (all the scanning switches of switches 151, . . . , 15n except the one scanning switch 15i) are switched over to the ground side.
The drive control signal is supplied to the anode line drive circuit 14. The anode line drive circuit 14 switches a drive switch (one of the drive switches 161R, 161G, 161B, . . . , 16mR, 16mG, 16mB) corresponding to an anode line to a current source side (one of the current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB corresponding thereto), the anode line containing an element of the pixel, which should be driven to emit light, of the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB within the current horizontal scanning period shown by the drive control signal. Other anode lines are switched over to the ground side. For example, when the drive switches 161R, 161G, 161B are switched over to the current sources 171R, 171G, 171B, a drive current IR flows from the current source 171R through the drive switch 161R, the anode line A1R, the element E1R, S, the cathode line BS, the scanning switch 15S to the ground. On the other hand, a drive current IG flows from the current source 171G through the drive switch 161G, the anode line A1G, the element E1G, S, the cathode line BS, the scanning switch 15S to the ground. Moreover, a drive current IB flows from the current source 171B through the drive switch 161B, the anode line A1B, the element E1B, S, the cathode line BS, the scanning switch 15S to the ground. The EL elements E1R, S, E1G, S, E1B, S, to which are the drive currents IR, IG, IB are supplied, emit light according to the corresponding pixel information. The time for light emission in each of the EL elements E1R, S, E1G, S, E1B, S is set individually in accordance with information regarding pixel colors, thereby allowing a pixel comprising the EL elements E1R, S, E1G, S, E1B, S to be displayed in a desired color.
The light emission control circuit 12 determines whether a predetermined time has elapsed after the execution of step S2 (step S3). The predetermined time is set corresponding to a predetermined horizontal scanning period. If the predetermined time has elapsed, the light emission control circuit 12 generates a reset signal (step S4). The reset signal is supplied to the cathode line scanning circuit 13 and the anode line drive circuit 14. The cathode line scanning circuit 13 switches the movable contacts of all the scanning switches 151, . . . , 15n to the stationary contacts on the ground side in response to the reset signal. The reset signal shows the designation of the anode lines (subsequent drive lines) corresponding to the EL elements that should be driven to emit light during the subsequent scanning period. The anode line drive circuit 14 switches the movable contacts of the drive switches, which are coupled to the anode lines corresponding to the EL elements that should be driven to emit light during the subsequent scanning period, to the stationary contacts on the offset voltage side in response to the reset signal. This causes an offset voltage to be applied to the EL elements that should be driven to emit light during the subsequent scanning period. That is, the offset voltage VR is applied to the EL element for emitting red light that should be driven to emit light during the subsequent scanning period; the offset voltage VG is applied to the EL element for emitting green light; and the offset voltage VB is applied to the EL element for emitting blue light. This will cause the capacitive component of each of the EL elements to be charged, which should be driven to emit light during the subsequent scanning period.
After having completed the execution of step S4, the light emission control circuit 12 completes the light emission control routine and will be on standby until the subsequent horizontal scanning period starts. Even during the time until the subsequent horizontal scanning period is started, the reset operation of step S4 is continued. When the subsequent horizontal scanning period starts, the aforementioned operations in step S1 to S4 are repeated.
Next, the hue control operation by means of the light emission control circuit 12 is explained with reference to the flow diagram of FIG. 10.
The light emission control circuit 12 carries out the hue control routine in response to the luminosity data of each of red, green, and blue colors at the time when the user actuates the control lever of the data input portion 19. In the hue control routine, first, the luminosity data of each of red, green, and blue colors, which is outputted from the data input portion 19 is read (step S11). Then, voltages VeR, VeG, VeB, corresponding to the luminosity data of each of red, green, and blue colors and appearing at the time of light emission across each of the red, green, and blue EL elements are set (step S12). The memory 20 stores as a data table in FIG. 17, for example, the characteristic of voltage V−Current I−luminosity L, like the one which is shown in FIG. 3, for every red, green, and blue color. Accordingly, this table can be used to determine the voltages VeR, VeG, VeB, corresponding to the luminosity data of each of red, green, and blue colors and appearing at the time of light emission across each of the red, green, and blue EL elements. The luminosity data is shown in 32 levels of halftone.
After having carried out step S12, the light emission control circuit 12 sets drive currents IR, IG, IB according to the voltages VeR, VeG, VeB, appearing at the time of light emission across the elements (step S13). Moreover, the light emission control circuit 12 sets the offset voltages VR, VG, VB, corresponding to the voltages VeR, VeG, VeB across the elements (step S14). The drive currents IR, IG, IB can be set corresponding to the luminosity data of each of the red, green, and blue colors, using the aforementioned data table of the characteristic of voltage V−Current I−luminosity L of each EL element for emitting red, green, and blue light. Each of the offset voltages is calculated and set, so that the offset voltage VR=VeR−Vcc; the offset voltage VB=VeB−Vcc; and the offset voltage VB=VeB−Vcc. At each of EL elements for emitting red, green, and blue light, it holds true in order to prevent cross-talk light emission that VR<VthR, VG<VthG, and VB<VthB, where the light emission threshold voltages are VthR, VthG, and VthB.
The light emission control circuit 12 controls the variable current sources 17 R1R, 171G, 171B, . . . , 17mR, 17mG, 17mB so as to obtain the drive currents IR, IG, IB which have been set (step S15). In addition, the light emission control circuit 12 controls the output voltage of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB so as to obtain the offset voltages VR, VG, VB, which have been set (step S16). That is, the supply current by means of the variable current sources 171R, . . . , 17mR is made equal to the drive current Ithat has been set in step S13. On the other hand, the supply current by means of the variable current sources 171G, . . . , 17mG is made equal to the drive current IG that has been set in step S13, and the supply current by means of the variable current sources 171B, . . . , 17mB is made equal to the drive current IB that has been set in step S13. The output voltage of the variable voltage sources 181R, . . . , 18mR is made equal to the offset voltage VR that has been set in step S14. On the other hand, the output voltage of the variable voltage sources 181G, . . . , 18mG is made equal to the offset voltage VG that has been set in step S14, and the output voltage of the variable voltage sources 181B, . . . , 18mB is made equal to the offset voltage VB that has been set in step S14.
Each of the EL elements for emitting red, green, and blue light has the characteristic of voltage V−Current I−luminosity L of EL elements, shown in FIG. 3, different from each other. Accordingly, in the aforementioned data table, data such as the voltage across an EL element, a drive current, and an offset voltage, corresponding to the luminosity data of the red, green, and blue colors are determined.
Next, such a case is explained with reference to FIGS. 11 to 13 as the cathode line B1 is scanned by the light emission control operation of the light emission control circuit 12 to allow EL elements E1R, 1, E1G, 1, E1B, 1 of a pixel to emit light and thereafter the scan is transferred to the cathode line B2 to cause EL elements E2R, 2, E2G, 2, E2B, 2 of a pixel to emit light. For the sake of clarity in explanation, like FIGS. 3 and 5, FIGS. 11 to 13 show elements that are emitting light by diode symbols, while elements that are not emitting light are expressed with capacitor symbols.
First, FIG. 11 shows an operating state in which the elements E1R, 1, E1G, 1, E1B, 1 that should emit light are emitting light under the steady light-emitting state within a scanning period during which the cathode line B1 is being selectively scanned with only the scanning switch 151 being switched over to the ground potential side of 0 (V). The positive potential Vcc is applied to other cathode lines B2, . . . , Bn by means of the scanning switches 152, . . . , 15n. At the same time, the anode lines A1R, A1G, A1B are connected with the variable current sources 171R, 171G, 171B by means of the drive switches 161R, 161G, 161B. In addition, other anode lines A2R, A2G, A2B, . . . , AmR, AmG, AmB are switched over to the ground potential side of 0 (V) by means of the drive switches 162R, 162G, 162B, . . . , 16mR, 16mG, 16m,B. Therefore, in the case of FIG. 11, a forward voltage is applied only to the EL elements E1R 1, E1G, 1, E1B, 1, so that the drive currents IR, IG, IB flow in from the variable current sources 171R, 171G, 171B to cause only the EL elements E1R, 1, E1G, 1, E1B, 1 to emit light.
In this state, the voltage Vcc is applied across the terminal of each of the non-light-emitting EL elements E2R, 2, E2G, 2, E2B, 2, . . . , EmR, n, EmG, n, EmB, n, which are shown by being shaded, the capacitive components thereof are to be charged opposite to the forward direction shown in the drawing. Moreover, the anode line A1R to which the EL elements E1R, 2, . . . , E1R, n of the non-light-emitting EL elements E1R, 2, E1G, 2, E1B, 2, . . . , E1R, n, E1G, n, E1B, n are coupled has a voltage VeR, and the voltage Vcc is applied to the cathode lines B2, . . . , Bn of the EL elements E1R, 2, . . . , E1R, n. Therefore, a voltage, VeR−Vcc, is applied to the EL elements E1R, 2, . . . , E1R, n in the forward direction, and the capacitive components thereof are charged. The anode line A1G to which the EL elements E1G, 2, . . . , E1G, n are coupled has a voltage VeG, and the voltage Vcc is applied to the cathode lines B2, . . . , Bn of the EL elements E1G, 2, . . . , E1G, n. Therefore, a voltage, VeG−Vcc=0, is applied to the EL elements E1G, 2, . . . , E1G, n, and the capacitive components thereof are not charged. The anode line A1B to which the EL elements E1B, 2, . . . , E1B, n are coupled has a voltage VeB, and the voltage Vcc is applied to the cathode lines B2, . . . , Bn of the EL elements E1B, 2, . . . , E1B, n. Therefore, a voltage, Vcc−VeG, is applied to the EL elements E1B, 2, . . . , E1B, n in the reverse direction, and the capacitive components thereof are charged.
Immediately before the scan is transferred from the light-emitting state of FIG. 11 to the state where the subsequent EL elements E2R, 2, E2G, 2, E2B, 2 emit light, a reset period comes during which the aforementioned step S4 performs reset control. That is, as shown in FIG. 12, the drive switches 161R, 161G, 161B, and 163R, 163G, 163B, . . . , 16mR, 16mG, 16mB, other than the drive switches 162R, 162G, 162B corresponding to the EL elements E2R, 2, E2G, 2, E2B, 2, are switched over to the ground potential side. In addition, all scanning switches 151, . . . , 15n are switched over to the ground potential side, and the anode lines A1R, A1G, A1B, A3R, A3G, A3B, . . . , AmR, AmG, AmB and the cathode lines B1, . . . , Bn are once made equal to the ground potential side of 0 (V). This resets the EL elements E1R, 1, . . . , E1R, n, E1G, 1, . . . , E1G, n, E1B, 1, . . . , E1B, n, E3R, 1, E3G, 1, E3B, 1, . . . , EmR, n, EmG, n, EmB, n, and an equal voltage of 0 (V) appears between the anode and cathode of the EL elements. Accordingly, the electric charge that has been charged in each of the EL elements is discharged, and thus the charged electric charge in all the EL elements are discharged instantly, so that no charge is left therein. An equal potential of 0 (V) also appears between the anode and the cathode of the EL elements E1G, 2, . . . , E1G, n, however, no discharge occurs since the EL elements E1G, 2, . . . , E1G, n have not-been charged when the EL elements E1R, 1, E1G, 1, E1B, 1 emit light, and thus no charge is stored.
The reset control causes the drive switches 162R, 162G, 162B to be switched over to the side of the variable voltage sources 182R, 182G, 182B. Accordingly, the positive voltage VR of the variable voltage source 182R is applied to the anode of each of the EL elements E2R, 1, . . . , E2R, n for emitting red light via the drive switch 162R and the anode line A2R. The positive voltage VG of the variable voltage source 182G is applied to the anode of each of the EL elements E2G, 1, . . . , E2G, n for emitting green light via the drive switch 162G and the anode line A2G. Moreover, the positive voltage VB of the variable voltage source 182B is applied to the anode of each of the EL elements E2B, 1, . . . , E2B, n for emitting red light via the drive switch 162B and the anode line A2B. The cathode of each of the EL elements E2R, 1, E2G, 1, E2B, 1, . . . , E2R, n, E2G, n, E2B, n is maintained at the ground potential via the corresponding scanning switches 151, . . . , 15n. Accordingly, the offset voltage VR is applied across the anode and the cathode of the EL elements E2R 1, . . . , E2R, n for emitting red light. In addition, the offset voltage VG is applied to across the anode and the cathode of the EL elements E2G, 1, . . . , E2G, n for emitting green light, while the offset voltage VB is applied to across the anode and the cathode of the EL elements E2B, 1, . . . , E2B, n for emitting blue light. Here, if it holds true for the initial values of the offset voltages VR, VG, VB, that VR>0 (V), VG=0 (V), and VB<0 (V), the capacitive components of the EL elements E2R, 1, . . . , E2R, n for emitting red light are charged in the forward direction, while the capacitive components of the EL elements E2G, 1, . . . , E2G, n for emitting green light are not charged and the capacitive components of the EL elements E2B, 1, . . . , E2B, n for emitting blue light are charged opposite to the forward direction, as shown in FIG. 11.
The stored charge of all the EL elements E1R, 1, E1G, 1, E1B, 1, . . . , E1R, n, E1G, n, E1B, n, E3R, 1, E3G, 1, E3B, 1, . . . , EmR, n, EmG, n, EmB, n, is made zero, and the voltage across each of the EL elements E2R, 1, E2G, 1, E2B, 1, . . . , E2R, n, E2G, n, E2B, n is made equal to the offset voltages VR, VG, VB. Thereafter, the subsequent scanning period comes now. As shown in FIG. 13, only the scanning switch 152 corresponding to the cathode line B2 is switched over to the ground potential side and the cathode line B2 is selectively scanned. At the same time, the drive switches 162R, 162G, 162B are switched over to the variable current source side to allow the variable current sources 172R, 172G, 172B to be connected to the corresponding anode lines A2R, A2G, A2B.
At the moment the scanning switches and the drive switches are switched over, that is, at the moment the scanning switches and the drive switches are switched over as shown in FIG. 13 and the charged state of the parasitic capacitance of each EL element remains as in the state of FIG. 12, the potential of the anode line A2R is generally equal to Vcc+VR (precisely speaking, equal to (n−1)·(Vcc+VR)/n). Accordingly, the voltage across the EL element E2R, 2 that is allowed to emit light is to take instantly on approximately Vcc+VR. Therefore, charge currents quickly charge the EL element E2R, 2 by flowing therein from a plurality of routes such as the route from the scanning switch 151 through the cathode line B1, the EL element E2R, 1, the anode line A2R, and the EL element E2R, 2 to the scanning switch 152; the route from the scanning switch 153 through the cathode line B3, the EL element E2R, 3, the anode line A2R, and the EL element E2R, 2 to the scanning switch 152; . . . ; and the route from the scanning switch 15n through the cathode line Bn, the EL element E2R, n, the anode line A2R, and the EL element E2R, 2 to the scanning switch 152, in addition to the route from the variable current source 172R through the drive switch 162R, the anode line A2R, and the EL element E2R, 2 to the scanning switch 152. Consequently, this causes the EL element E2R, 2 to go into the steady light-emitting state instantly. Thereafter, during the scanning period of B2, the steady light-emitting state is sustained by a drive current flowing in via the route from the variable current source 172R through the drive switch 162R, the anode line A2R, and the EL element E2R, 2 to the scanning switch 152.
Likewise, at the moment the scanning switches and the drive switches are switched over, the voltage across the EL element E2G, 2 starts becoming approximately Vcc (precisely speaking, equal to (n−1)·Vcc/n). Therefore, charge currents drive the EL element E2G, 2 into the steady light-emitting state instantly by flowing therein from a plurality of routes such as the route from the scanning switch 151 through the cathode line B1, the EL element E2G, 1, the anode line A2G, and the EL element E2G, 2 to the scanning switch 152; the route from the scanning switch 153 through the cathode line B3, the EL element E2G, 3, the anode line A2G, and the EL element E2G, 2 to the scanning switch 152; . . . ; and the route from the scanning switch 15n through the cathode line Bn, the EL element E2G, n, the anode line A2G, and the EL element E2G, 2 to the scanning switch 152, in addition to the route from the variable current source 172G through the drive switch 162G, the anode line A2G, and the EL element E2G, 2 to the scanning switch 152. Thereafter, during the scanning period of B2, the steady light-emitting state is sustained by a drive current flowing in via the route from the variable current source 172G through the drive switch 162G, the anode line A2G, and the EL element E2G, 2 to the scanning switch 152.
Moreover, at the moment the scanning switches and the drive switches are switched over, the voltage across the EL element E2B, 2 starts becoming approximately Vcc+VB (precisely speaking, equal to (n−1),(Vcc+VB)/n). Therefore, charge currents drive the EL element E2B, 2 into the steady light-emitting state instantly by flowing therein from a plurality of routes such as the route from the scanning switch 151 through the cathode line B1, the EL element E2B, 1, the anode line A2B, and the EL element E2B, 2 to the scanning switch 152; the route from the scanning switch 153 through the cathode line B3, the EL element E2B, 3, the anode line A2B, and the EL element E2B, 2 to the scanning switch 152; . . . ; and the route from the scanning switch 15n through the cathode line Bn, the EL element E2B, n, the anode line A2B, and the EL element E2B, 2 to the scanning switch 152, in addition to the route from the variable current source 172B through the drive switch 162B, the anode line A2B, and the EL element E2B, 2 to the scanning switch 152. Thereafter, during the scanning period of B2, the steady light-emitting state is sustained by a drive current flowing in via the route from the variable current source 172B through the drive switch 162B, the anode line A2B, and the EL element E2B, 2 to the scanning switch 152.
Each of the light-emitting EL elements E2R, 2, E2G, 2, E2B, 2 reaches the light emission regulating voltage VeR, VeG, VeB substantially at the same time the scan is changed over to come into the steady light-emitting state. Consequently, the pixel made up of the EL elements E2R, 2, E2G, 2, E2B, 2 is to display the desired color without a difference in color.
FIG. 14A shows a rectangular change in voltage across each of the EL elements for emitting light of three primary colors when a reverse bias potential Vcc=20 (V), and offset voltages VR=2 (V), VG=0 (V), VB=−2 (V) are set. In this case, the voltage of each of the EL elements is the current voltage across each of the EL elements at the time of light emission, namely, VeR=22 (V), VeG=20 (V), and VeB=18 (V). Suppose that the user operates the data input portion 19 to increase the luminosity of green by +1 and the luminosity of blue by +2, respectively, under the state of emitting light as such. This operation will cause the offset voltage VR=2 (V) to remain as it is, but the-offset voltages to change into VG=1 (V) and VB=0 (V). Accordingly, a change in voltage across each of the EL elements of the three colors occurs as shown in FIG. 14B at the time of light emission. Thus, since the reverse bias potential Vcc is fixed to 20 (V), the voltage across each of the EL elements becomes VeR=22 (V), VeG=21 (V), and VeB=20 (V) at the time of light emission.
FIG. 15 is a partial view showing another embodiment of a display device of the present invention. This display device comprises the capacitive light-emitting display panel 11, the light emission control circuit 12, the cathode line scanning circuit 13, and the anode line drive circuit 14. Though not shown in FIG. 15, the data input portion 19 and the memory 20 are connected to the light emission control circuit 12 as shown in FIG. 7. The cathode line scanning circuit 13 has the scanning switches 151, . . . , 15n as well as variable voltage sources 211, . . , 21n. The variable voltage sources 211, . . . , 21n generate voltages to obtain the aforementioned reverse bias potential Vcc and the level of the voltage Vcc thereof is controlled by the light emission control circuit 12. The positive terminals of the variable voltage sources 211, . . . , 21n are connected to one side of the stationary contacts of the scanning switches 151, . . . , 15n, while the negative terminals are connected to the ground. Other configuration is the same as those shown in FIGS. 7 and 8.
The light emission control operation of the light-emitting display panel 11 by means of the light emission control circuit 12 shown in FIG. 15 is the same as that shown in the flow diagram of FIG. 9.
When the user actuates the data input portion 19, the light emission control circuit 12 carries out the hue control routine in accordance with the luminosity data of each of the red, green, and blue colors at that time. In this hue control routine, as shown in FIG. 16, the luminosity data of each of the red, green, and blue colors, which are outputted from the data input portion 19, is read (step S21). Then, the drive currents IR, IG, IB, corresponding to each of the red, green, and blue colors, are set by retrieving the data table (step S22). Moreover, the offset voltages VR, VG, VB are set by means of retrieving the data table (step S23). Since the data tables of the drive currents IR, IG, IB and the offset voltages VR, VG, VB, corresponding to the luminosity data of each of the red, green, and blue colors, are formed in the memory 20, these data tables are used to set the drive currents IR, IG, IB and the offset voltages VR, VG, VB. The characteristics of voltage V−Current I−luminosity L of the EL elements shown in FIG. 2 are slightly different from each other in the EL elements for emitting red, green, and blue light. Accordingly, as shown in FIG. 17, in the data tables to be used in steps S22 and S23, drive current data Ir0–Ir31, Ig0–Ig31, Ib0–Ib31 and offset voltage data Vr0–Vr31, Vg0–Vg31, Vb0–Vb31 are determined, which correspond to the luminosity data (luminosity of 32 levels of halftone) of each of the red, green, and blue colors.
The light emission control circuit 12 selects only one common reverse bias voltage Vcc corresponding to each of the offset voltages VR, VG, VB, which have been set (step S24). The light emission control circuit 12 controls the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB so as to obtain the drive currents IR, IG, IB, which have been set (step S25). In addition, the light emission control circuit 12 controls the output voltages of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB so that the output voltages become the offset voltages VR, VG, VB, which have been set (step S26). Moreover, the light emission control circuit 12 controls the output voltages of the variable voltage sources 211, . . . , 21n so that the output voltages become the reverse bias voltage Vcc that has been set (step S27).
The offset voltage VR has a value of VR=VeR−Vcc; the offset voltage VG has a value of VG=VeG−Vcc; and the offset voltage VB has a value of VB=VeB−Vcc. Accordingly, letting the current value of the reverse bias voltage Vcc equal to V1, the reverse bias voltage Vcc is changed to V2 to change the offset voltages VR, VG, VB to within the allowable range of voltages from VLL to VHL with the voltages VeR, VeG, VeB being sustained. For example, where V1=20 (V) and it is set by retrieving the table data in step S23 such that voltage VR=5 (V), VG=1 (V), and VB=0 (V), the offset voltage VR exceeds the aforementioned allowable range of voltage from VLL to VHL, which is, −5 (V) to +3 (V). The offset voltage VR=5 (V) is decreased by 2 (V) to fall within the allowable range of −5 (V) to +3 (V). Thus, it is set such that VR=5 (V)−2 (V)=3 (V), VG=1 (V)−2 (V)=−1 (V), and VB=0 (V)−2 (V)=−2 (V). In addition, the reverse bias voltage Vcc is set to V 2=20 (V)+2 (V)=22 (V) so that it is sustained that voltages VeR=25 (V), VeG=21 (V), and VeB=20 (V).
Each of the offset voltages VR, VG, VB and the reverse bias voltage Vcc can be set in steps S23 and S24 as follows. If the allowable range of offset voltage from VLL to VHL is from −5 (V) to +3 (V), the center voltage thereof is (VL+VHL)/2=1 (V). The average voltage of the offset voltages VR, VG, VB is made equal to the center voltage. That is, the offset voltages VR, VG, VB are set to satisfy that (VR+VG+VB)/3=−1 (V). In step S23, it has been set such that VR=5 (V), VG=1 (V), VB=0 (V), the current average voltage of the offset voltages VR, VG, VB is equal to (VR+VG+VB)/3=2 (V). Therefore, in order to make the average voltage equal to −1 (V), the light emission control circuit 12 decreases the offset voltages VR, VG, VB by 3 (V) to be set such that VR=5 (V)−3 (V)=2 (V), VG=1 (V)−3 (V)=−2 (V), and VB=0 (V)−3 (V)=−3 (V). On the other hand, in step S24, the light emission control circuit 12 can set the reverse bias voltage Vcc such that V2=20 (V)+3 (V)=23 (V).
In the hue control routine of FIG. 16, the drive currents IR, IG, IB and the offset voltages VR, VG, VB are set by means of retrieving the data tables in steps S22 and S23. However, the reverse bias voltage Vcc may also be set by means of retrieving the data tables in step S24. In this case, the voltages VeR, VeG, VeB are determined in accordance with the drive currents IR, IG, IB, and the total voltage of the offset voltages VR, VG, VB and the reverse bias voltage Vcc are made equal to the voltages VeR, VeG, VeB. In the case where VeR=25 (V), VeG=21 (V), and VeB=20 (V), the relationship between the offset voltages VR, VG, VB and the reverse bias voltage Vcc is as shown in FIGS. 18A to 18C for each of the red, green, and blue colors. The voltage that each of the offset voltages VR, VG, VB can provide is to lie within the range of −5 (V) to +3 (V). Within this allowable range of the offset voltages, the bias voltage Vcc becomes a common voltage to the red, green, and blue colors, so that the bias voltage may take any value within the range of 25 (V) to 22 (V) in each of the red, green, and blue colors. Therefore, setting the common reverse bias voltage Vcc to a voltage within the range of 25 (V) to 22 (V) allows each of the offset voltages VR, VG, VB to be set.
For example, in the case where the user operates the data input portion 19 to result in luminosity data for extremely decreasing the luminosity of the red color in step S21, suppose that the relationship between the offset voltage VR and the reverse bias voltage Vcc is shown in FIG. 18A to FIG. 19 in accordance with the luminosity data. If the relationship between the offset voltages VG, VB of the green and blue colors and the reverse bias voltage Vcc remains as shown in FIGS. 18B and 18C, the reverse bias voltage Vcc common to the red, green, and blue colors cannot be obtained. In this case, priority is placed on the green color with high luminosity to allow the reverse bias voltage Vcc to be set using the relationship of FIG. 18B. The reverse bias voltage Vcc=18 (V), which is the lowest in the relationship of FIG. 18B, is selected, and the offset voltages are set such that VR=−5 (V), VG=3 (V), and VB=2 (V).
In the aforementioned embodiment, the data tables are used to retrieve and set the drive currents IR, IG, IB and the offset voltages VR, VG, VB, however, the offset voltages VR, VG, VB may be calculated. Next, the operation is explained in which the offset voltages VR, VG, VB are determined by calculation.
The light emission control circuit 12 carries out the initialization routine for initialization. In the initialization routine, as shown in FIG. 20, a command is generated for supplying drive currents over an entire scanning period (step S31). The user is required to control the data input portion 19 in accordance with the command so that the light-emitting display panel 11 displays white color and the luminosity data of each of the red, green, and blue colors at that time is read from the data input portion 19 (step S32). Then, the drive currents IR, IG, IB are determined in accordance with the read luminosity data (step S33). Then, the voltages VeR, VeG, VeB across the EL elements for emitting red, green, and blue light, corresponding to the drive currents IR, IG, IB, are set (step S34). Since the data tables of the drive currents IR, IG, IB and the voltages VeR, VeG, VeB, corresponding to the luminosity data, are formed in the memory 20 for each of the red, green, and blue colors, the drive currents IR, IG, IB and the voltages VeR, VeG, VeB are set using the data tables.
The light emission control circuit 12 sets the reverse bias voltage Vcc in accordance with the voltages VeR, VeG, VeB, which have been set in step S34 (step S35). In step S35, the voltage levels of the voltages VeR, VeG, VeB are compared with each other, and the second highest voltage is set as the reverse bias voltage Vcc. If the voltages VeR, VeG, VeB have a high and low relationship such that VeR>VeG>VeB, the voltage level of VeG is set as the reverse bias voltage Vcc. In addition, in step S35, the levels of the voltages VeR, VeG, VeB may be compared with each other and an intermediate voltage between the highest and lowest voltages may be set to the reverse bias voltage Vcc. If the voltages VeR, VeG, VeB have a high and low relationship such that VeR>VeG>VeB, the voltage level of (VeR+VeB)/2 is set to the reverse bias voltage Vcc.
After having carried out step S35, the light emission control circuit 12 calculates the offset voltages VR, VG, VB. The offset voltages VR, VG, VB are calculated such that VR=VeR Vcc, VG=VeG−Vcc, and VB=VeB−Vcc. In the case where the former method for setting the reverse bias voltage Vcc in step S35 is used, the offset voltage corresponding to the highest voltage of the voltages VeR, VeG, VeB is positive. The second offset voltage corresponding to the highest voltage is 0 (V), while the offset voltage corresponding to the lowest voltage is negative.
After having carried out step S36, the light emission control circuit 12 writes the drive currents IR, IG, IB, the reverse bias voltage Vcc, and the offset voltages VR, VG, VB into the memory 20 and allows the same to be stored therein (step S37).
In such an initialization operation, if the voltages VeR, VeG, VeB are set, for example, such that VeR=22 (V), VeG=20 (V), and VeB=18 (V), the voltage levels of the voltages VeR, VeG, VeB are compared with each other in step S35 to set the second highest voltage, VeG=20 (V), is set to the reverse bias voltage Vcc. Therefore, in step S36, the offset voltages VR, VG, VB are set such that VR=2 (V), VG=0 (V), and VB=−2 (V).
The allowable range of the offset voltages is set for each of the red, green, and blue colors. For example, the allowable range of red color VLLR to VHLR lies within the range of −5 (V) to 3 (V), the allowable range of green color VLLG to VHLG lies within the range of −5 (V) to 2 (V), and the allowable range of red color VLLB to VHLB lies within the range of −5 (V) to 1 (V).
After having completed the initialization operation, the light emission control circuit 12 allows the user to operate the data input portion 19 to carry out either the brightness control routine or the hue control routine.
When the user actuates the brightness control lever (not shown) of the data input portion 19, the light emission control circuit 12 carries out the brightness control routine in accordance with the luminosity data at that time. The brightness control lever of the data input portion 19 is an actuator for controlling the overall luminosity of the display screen. The user's actuation of the lever causes the luminosity data of each of the red, green, and blue colors, which are outputted from the data input portion 19, to vary by the same luminosity.
In the brightness control routine, as shown in FIG. 21, the light emission control circuit 12 first reads the luminosity data of each of the red, green, and blue colors, which are outputted from the data input portion 19 (step S41). Then, the drive currents IR, IG, IB, corresponding to the luminosity data of each of the red, green, and blue colors, are set by retrieving the data tables (step S42). Moreover, the voltages VeR, VeG, VeB across EL elements for emitting red, green, and blue light, corresponding to the drive currents IR, IG, IB, are set by retrieving the data tables (step S43). The operations of steps S42 and S43 are the same as those of steps S33 and S34.
The light emission control circuit 12 reads the reverse bias voltage Vcc that is stored in the memory 20 (step S44). Then, the light emission control circuit 12 calculates the offset voltages VR, VG, VB, using the voltages VeR, VeG, VeB of step S43 and the reverse bias voltage Vcc that has been read (step S45). That is, the offset voltages are calculated such that VR=VeR−Vcc, VG=VeG−Vcc, and VB=VeB−Vcc.
The light emission control circuit 12 determines whether each of the calculated offset voltages VR, VG, VB lies within a predetermined allowable range (step S46). Since the offset voltages need to be set so as to avoid cross-talk light emission, each of the offset voltages is limited to the red allowable range of VLLR to VHLR, the allowable range of VLLG to VHLG, and the allowable range of VLLB to VHLB. If each of the offset voltages VR, VG, VB lies within the corresponding predetermined allowable ranges of VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB, the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB are controlled so as to obtain the drive currents IR, IG, IB, which have been set (step S47). In addition, the output voltages of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB are controlled so as to be the offset voltages VR, VG, VB, which have been set (step S48).
In step S46, if any one of the offset voltages VR, VG, VB does not lie within the corresponding predetermined allowable ranges of VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB, the reverse bias voltage Vcc and each of the offset voltages VR, VG, VB are reset so that each of the offset voltages VR, VG, VB lies within the corresponding predetermined allowable ranges of VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB (step S49). The reverse bias voltage Vcc in step S49 is reset in the same manner as in the aforementioned step S35, while each of the offset voltages VR, VG, VB is reset in the same manner as in step S36.
After having carried out step S49, the light emission control circuit 12 controls the output voltages of the variable voltage sources 211, . . . , 21n so as to be the reverse bias voltage Vcc which has been set (step S50). The process proceeds to step S47 to allow the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB to be controlled so as to obtain the drive currents IR, IG, IB, which have been set. Thereafter, in step S48, the output voltages of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB are controlled so as to become the offset voltages VR, VG, VB, which have been set.
After having carried out step S48, the light emission control circuit 12 allows the reverse bias voltage Vcc, the offset voltages VR, VG, VB, and the drive currents IR, IG, IB, which have been set, to be stored in the memory 20 (step S51).
If the voltages VeR, VeG, VeB across the EL elements for emitting red, green, and blue light are set, for example, such that VeR=30 (V), VeG=29 (V), and VeB=26 (V) by actuating the brightness control lever of the data input portion 19 in step S43, and differences between each of the voltages VeR, VeG, VeB and the reverse bias voltage Vcc=20 (V) are calculated as the offset voltages VR, VG, VB, then VR=10 (V), VG=9 (V), and VB=6 (V). If the allowable range of red color VLLR to VHLR lies within the range of −5 (V) to 3 (V), the allowable range of green color VLLG to VHLG lies within the range of −5 (V) to 2 (V), and the allowable range of red color VLLB to VHLB lies within the range of −5 (V) to 1 (V) as described above, then all the offset voltages calculated in step S45 lie outside the allowable ranges. Thus, each of the offset voltages VR, VG, VB and the reverse bias voltage Vcc are reset in step S49, and the voltage levels of the voltages VeR, VeG, VeB are compared with each other, so that the second highest voltage VeG=29 (V) is reset to the reverse bias voltage Vcc. Each of the offset voltages VR, VG, VB is reset such that VR=1 (V), VG=0 (V), and VB=−3 (V).
When the user actuates the brightness control lever of the data input portion 19, the light emission control circuit 12 carries out the hue control routine in accordance with the luminosity data at that time. In the hue control routine, as shown in FIG. 22, the light emission control circuit 12 first reads the luminosity data of each of the red, green, and blue colors, which are outputted from the data input portion 19 (step S61). Then, the drive currents IR, IG, IB corresponding to the luminosity data of each of the red, green, and blue colors, are set by retrieving the data tables (step S62). Moreover, the voltages VeR, VeG, VeB across EL elements for emitting red, green, and blue light, corresponding to the drive currents IR, IG, IB, are set by retrieving the data tables (step S63). The operations of steps S62 and S63 are the same as those of steps S33 and S34.
The light emission control circuit 12 reads the reverse bias voltage Vcc that is stored in the memory 20 (step S64). Then, the light emission control circuit 12 calculates the offset voltages VR, VG, VB, using the voltages VeR, VeG, VeB of step S63 and the reverse bias voltage Vcc that has been read (step S65). That is, the offset voltages are calculated such that VR=VeR−Vcc, VG=VeG−Vcc, and VB=VeB−Vcc.
The light emission control circuit 12 determines whether each of the calculated offset voltages VR, VG, VB lies within a predetermined allowable range (step S66). Since the offset voltages need to be set so as to avoid cross-talk light emission, each of the offset voltages is limited to the red allowable range of VLLR to VHLR, the allowable range of VLLG to VHLG, and the allowable range of VLLB to VHLB. If each of the offset voltages VR, VG, VB lies within the corresponding predetermined allowable ranges of VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB, it is determined whether the second highest voltage of the voltages VeR, VeG, VeB, which are stored in the memory 20, has changed (step S67). That is, it is determined whether the voltage across an EL element for a color with the second highest voltage of the previous voltages VeR, VeG, VeB, which are stored in the memory 20, has changed into a different voltage due to the current setting of the voltages VeR, VeG, VeB in step S63. If the voltage across an EL element for a color with the second highest voltage of the previous voltages VeR, VeG, VeB has not changed, it is determined whether the voltage across an EL element for the color is currently the second highest voltage (step S68). That is, it is determined whether the second highest voltage of the previous voltages VeR, VeG, VeB and the second highest voltage of the current voltages VeR, VeG, VeB are the voltage across an EL element of the same color.
If the result of the determination in step S68 shows that the voltage across an EL element of a color with the previous second highest voltage has the current second highest voltage, the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB are controlled so as to achieve the drive currents IR, IG, IB, which have been set (step S69). In addition, the output voltages of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB are controlled to achieve the offset voltages VR, VG, VB, which have been set (step S70).
If the determination in step S66 shows that each of the offset voltages VR, VG, VB does not lie within the predetermined allowable range of voltage, if the determination in step S67 shows that the voltage across an EL element for a color with the second highest voltage of the previous voltages VeR, VeG, VeB has changed, or if the determination in step S68 shows that the voltage across an EL element for a color with the second highest voltage of the previous voltages VeR, VeG, VeB is not currently the second highest voltage, the reverse bias voltage Vcc and each of the offset voltages VR, VG, VB are reset so that each of the offset voltages VR, VG, VB becomes a voltage within the corresponding predetermined range of voltage VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB (step S71). The reverse bias voltage Vcc is reset in step S71 in the same manner as in the aforementioned step S35, while each of the offset voltages VR, VG, VB is reset in the same manner as in step S36.
After having carried out step S71, the light emission control circuit 12 controls the output voltages of the variable voltage sources 211, . . . , 21n so as to achieve the reverse bias voltage Vcc which has been set (step S72). Then, in step S69, the light emission control circuit 12 controls the variable current sources 171R, 171G, 171B, . . . , 17mR, 17mG, 17mB so as to achieve the drive currents IR, IG, IB, and thereafter, controls the output voltages of the variable voltage sources 181R, 181G, 181B, . . . , 18mR, 18mG, 18mB so as to achieve the offset voltages VR, VG, VB in step S70.
After having carried out step S70, the light emission control circuit 12 allows the reverse bias voltage Vcc, the offset voltages VR, VG, VB, and the drive currents IR, IG, IB, which have been set, to be stored in the memory 20 (step S73).
If the voltages VeR, VeG, VeB across the EL elements for emitting red, green, and blue light are set, for example, such that VeR=23 (V), VeG=20 (V), and VeB=21 (V) by actuating the brightness control lever of the data input portion 19 in step S63, and differences between each of the voltages VeR, VeG, VeB and the reverse bias voltage Vcc=20 (V) are calculated as the offset voltages VR, VG, VB, then VR=3 (V), VG=0 (V), and VB=1 (V). If the allowable range of red color VLLR to VHLR lies within the range of −5 (V) to 3 (V), the allowable range of green color VLLG to VHLG lies within the range of −5 (V) to 2 (V), and the allowable range of red color VLLB to VHLB lies within the range of −5 (V) to 1 (V) as described above, then all the offset voltages calculated in step S65 lie within the allowable ranges. If the previous voltages VeR, VeG, VeB are such that VeR=22 (V), VeG=20 (V), and VeB=18 (V), the previously second highest voltage is the VeG or the voltage across an EL element for green color. However, the currently second highest voltage is the VeB or the voltage across an EL element for blue color. Thus, each of the offset voltages VR, VG, VB and the reverse bias voltage Vcc are reset in step S61, and the voltage levels of the voltages VeR, VeG, VeB are compared with each other, so that the second highest voltage VeB=21 (V) is reset to the reverse bias voltage Vcc. Each of the offset voltages VR, VG, VB is reset such that VR=2 (V), VG=−1 (V), and VB=0 (V).
The predetermined allowable ranges VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB, of each of the aforementioned offset voltages VR, VG, VB are set as appropriate. The upper limits of the VHLR, VHLG, and VHLB are the light emission threshold voltages VthR, VthG, and VthB. If the offset voltages exceed the light emission threshold voltages, a slight light emission during a reset period or a cross-talk light emission on the EL element that is not scanned may be produced. No limitation is imposed on the lower limits of the VLLR, VLLG, and VLLB in particular. However, in consideration of the power efficiency, the lower limits may be desirably set to within an appropriate range. That is, the parasitic capacitance of an EL element that is located at the intersection of a cathode line that is not scanned and an anode line that is being driven is charged with invalid electric charge, corresponding to the offset voltage, the charge not contributing to light emission. Thus, lower limits are preferably set to within an appropriate range to reduce the amount of electric charge.
If a reverse bias voltage Vcc that satisfies the predetermined allowable ranges VLLR to VHLR, VLLG to VHLG, and VLLB to VHLB, of each of the red, green, and blue colors cannot be set, the reverse bias voltage Vcc is set to a limit value that does not exceed the light emission threshold voltages of EL elements of colors with the maximum voltages VeR, VeG, VeB.
Each of the EL elements of the aforementioned light-emitting display panel deteriorates when allowed to emit light for a long time to cause the V−I characteristic to change. For example, the V−I characteristic is available as shown in FIG. 23 for a total of short time of light emission, however, for a total of long time of light emission, overall current I is reduced for the same value of the voltage V across an EL element as shown in FIG. 24 and thus luminosity L that is proportional to current I is also reduced. Accordingly, it can be thought that the total time of light emission is measured and the V−I characteristic is measured as appropriate in accordance with the time of light emission to compensate for data tables. Currents may be allowed to flow into EL elements at predetermined intervals of current in the measurement, and voltages across the EL elements may be detected to calculate coefficients for compensation.
In the aforementioned embodiment, the voltages VeR, VeG, VeB across the EL elements for emitting red, green, and blue light, corresponding to the drive currents IR, RG, IB, are set by retrieving data tables. However, functional equations showing the characteristic of drive current−voltage across EL element for each of the red, green, and blue colors may be stored to calculate the voltages VeR, VeG, VeB across EL elements using the functional equations.
Furthermore, drive currents are supplied to EL elements that should be allowed to emit light from current sources. However, potentials may be applied to current addressing drive lines from voltage sources so that voltages slightly higher than threshold voltages may be applied to the EL elements.
As described above, according to the present invention, variations in voltages across each of capacitive light-emitting elements for emitting light of colors different from each other can be thereby made equal to each other, the variations being produced by the time the voltages reach each desired voltage during a scanning period. Thus, the rise characteristic of each of capacitive light-emitting elements that emit light of-colors different from each other can be improved.

## Claims

1. An apparatus for driving a multi-color light-emitting display panel including a plurality of drive lines and a plurality of scanning lines intersecting with each other, and a plurality of capacitive light-emitting elements having polarities connected to said scanning lines and said drive lines at a plurality of intersections of said drive lines and said scanning lines, and being divided into three types of red, green and blue by a color of light emission, said capacitive light-emitting elements of the same color type being arranged on each of said plurality of drive lines, and one pixel being formed by three capacitive light-emitting elements of red, green and blue, comprising:
scanning means for selectively applying one of a first potential and a second potential higher than the first potential to each of said scanning lines; and
drive means for supplying drive currents to drive lines which are connected to capacitive light-emitting elements of at least one pixel to be driven to emit light, and for applying a third potential to drive lines other than the drive lines supplied with the drive currents, so as to apply offset voltages, equal to or less than each light emission threshold voltages of said elements of red, green and blue, to capacitive light-emitting elements other than the capacitive light-emitting elements of the at least one pixel,
wherein said drive current and said third potential are variable, so that assuming that voltages across the capacitive light-emitting elements of red, green and blue at the time of light emission are VeR, VeG and VeB, respectively, and the offset voltages of the capacitive light-emitting elements of red, green and blue are VR, VG and VB, respectively, relationships of VeR>VeG>VeB and VR>VG>VB are set.

2. A driving apparatus according to claim 1, wherein said capacitive light-emitting elements are organic electroluminescence elements.

3. A driving apparatus according to claim 1, wherein said drive current and said third potential are different for each color type of the capacitive light-emitting elements arranged on each of said drive lines.