JP2004102040A - Driving device for luminous element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving device for a luminous element, in which a deterioration in luminance due to aging is suppressed in a simple structure. <P>SOLUTION: The device is equipped with a time totaling means for summing up the driving time of the luminous element, a duty ratio setting means for setting a duty ratio corresponding to the cumulative driving time summed up by the time totaling means, and a driving means which generates, with a prescribed recurring period, a driving pulse signal having a pulse width of the duty ratio set by the duty ratio setting means and which impresses such driving pulse signal to the luminous element. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a driving device for a self-luminous element such as an organic electroluminescence element.
[0002]
[Prior art]
As an image display device used for a mobile terminal such as a mobile phone, a thin display panel is required. A liquid crystal display panel is usually used for a conventional thin display panel. However, a display panel formed by arranging a plurality of organic electroluminescent elements (organic EL elements) in a matrix is not only thin but also lightweight. Therefore, it is promising as an image display device for a portable terminal.
[0003]
There are a constant current driving method and a constant voltage driving method for driving the organic EL element. The organic EL element emits light at a luminance corresponding to the supplied current level. In the driving device of the constant current driving system, the current level supplied to the organic EL element is controlled to a constant current, and in the driving device of the constant voltage driving system, the voltage applied to the organic EL element is controlled to a constant voltage. .
[0004]
[Problems to be solved by the invention]
The constant voltage driving method has a simpler circuit configuration, but has the following problems. When the impedance of the organic EL element rises due to the change over time of the EL element itself, there is a problem that the current flowing through the EL element decreases and the luminance decreases. In particular, there is a problem that the change in impedance in the initial stage of the organic EL element is large, and the luminance is significantly reduced in a short time with respect to the initial value. Further, in order to keep the luminance constant, it is necessary to detect a decrease in the luminance of the organic EL element or a decrease in the current value, and to change the power supply voltage accordingly, but there is a disadvantage that the circuit configuration becomes complicated.
[0005]
The problems to be solved by the present invention include the above-mentioned problems as an example. It is an object of the present invention to provide a driving device for a self-luminous element which can suppress a decrease in light emission luminance due to aging with a simple configuration. It is an object of the invention.
[0006]
[Means for Solving the Problems]
The driving device of the present invention is a driving device that drives a self-luminous element to emit light, and includes a time accumulating unit that accumulates a driving time of the self-luminous element, and a duty corresponding to the total driving time that is accumulated by the time accumulating unit. A duty ratio setting means for setting a ratio, a driving means for generating a driving pulse signal having a pulse width of the duty ratio set by the duty ratio setting means at a predetermined repetition cycle, and applying the driving pulse signal to the self-light emitting element. , Is provided.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a light emission driving device of an organic EL device (organic electroluminescence device) according to the present invention. As shown in FIG. 1, the light emission driving device is a device for driving the organic EL element 1 to emit light, and includes a voltage source 2, a switch 3, a counter 4, a memory circuit 5, and a PWM (pulse width modulation) circuit 6. ing.
[0008]
Voltage source 2 outputs a predetermined DC voltage. The predetermined DC voltage is a voltage higher than the light emission threshold voltage of the organic EL element 1. The positive output terminal of the voltage source 2 is connected to the switch 3, and the negative output terminal is grounded. The switch 3 is an on / off switch that is operated or controlled to be turned on when the organic EL element 1 is driven to emit light. One end of the switch 3 is connected to the positive output terminal of the voltage source 2, and the other end is connected to the PWM circuit 6.
[0009]
The counter 4 is a time counter that accumulates a period during which the switch 3 is turned on. The period during which the switch 3 is turned on is a period during which the organic EL element 1 is driven to emit light. The counter 4 outputs a signal indicating the total time of the light emission driving period, and the signal is supplied to the memory circuit 5. The memory circuit 5 internally holds a data table. The data table shows the relationship between the cumulative time and the duty ratio. FIG. 2 shows the relationship between the cumulative time held as a data table in the memory circuit 5 and the duty ratio. The memory circuit 5 reads the duty ratio corresponding to the cumulative time indicated by the output signal of the counter 4 using the data table, and outputs the data to the PWM circuit 6 as data.
[0010]
The output voltage of the voltage source 2 is supplied from the switch 3 to the PWM circuit 6 when the switch 3 is on. The PWM circuit 6 generates a drive pulse signal at a duty ratio supplied from the memory circuit 5. The anode of the organic EL element 1 is connected to the drive pulse signal output terminal of the PWM circuit 6, and the cathode is grounded.
In the light emission driving device having such a configuration, when the switch 3 is turned on, the output voltage of the voltage source 2 is supplied to the PWM circuit 6. The PWM circuit 6 generates a drive pulse signal at a predetermined repetition cycle. The driving pulse signal is applied to the organic EL element 1 in the forward direction, so that the organic EL element 1 emits light. Since the light emission of the organic EL element 1 is performed at a predetermined repetition cycle, the light emission driving is intermittent. However, the predetermined repetition cycle is a cycle at which human eyes can recognize continuous light emission.
[0011]
The drive pulse signal has a pulse width of a duty ratio output from the memory circuit 5 as data. That is, it has a pulse width of a duty ratio corresponding to the total driving time of the light emission driving period of the organic EL element 1 output from the counter 4 at that time. Therefore, in the initial stage, the accumulated driving time is in a state equal to 0, and the duty ratio of the driving pulse signal is minimized.
[0012]
Since the ON period of the switch 3 is accumulated by the counter 4, the count value of the counter 4, that is, the accumulated driving time increases. As the cumulative driving time increases, the duty ratio read from the data table in the memory circuit 5 increases. Therefore, the pulse width of the driving pulse signal output from the PWM circuit 6 increases as the cumulative driving time increases. The amplitude of the drive pulse signal does not change substantially equal to the output voltage of the voltage source 2 even if the cumulative drive time increases.
[0013]
In the organic EL element 1, even at the same applied voltage, the light emission luminance decreases due to the temporal change due to light emission. According to the present invention, this decrease in light emission luminance is compensated for by increasing the pulse width of the drive pulse signal.
FIGS. 3A to 3C show the relationship between the pulse width of the drive pulse signal and the peak luminance by dividing the degree of the change over time of the organic EL element 1 into three stages: an initial stage, an intermediate stage, and a final stage. . In the initial stage of FIG. 3A, the pulse width of the drive pulse signal is minimum, but its peak luminance is maximum. In the middle stage of FIG. 3B, the pulse width of the drive pulse signal increases as compared with the initial stage, and its peak luminance decreases as compared with the initial stage. In the final stage of FIG. 3C, the pulse width of the drive pulse signal is maximum, but its peak luminance is minimum.
[0014]
FIG. 4 shows a change in luminance level over time in the case of the conventional light emission driving device and a change in luminance level over time in the case of using the above-described embodiment.
Therefore, as can be seen from FIG. 4, even when the peak value of the emission luminance decreases due to the aging of the organic EL element 1 itself, the luminance level during the ON period of the switch 3 can be maintained. By using such a driving device in a display panel, display quality can be ensured for a long period of time.
[0015]
In the light emission driving device shown in FIG. 1, the repetition period for generating the driving pulse signal may not be constant, and may be divided into the driving pulse signal generation period and the blank period. The drive pulse signal generation period is constant, and the blank period is controlled to a period corresponding to the duty ratio. This control is performed in the PWM circuit 6 according to the duty ratio. FIGS. 5A and 5B show the drive pulse signal when the degree of the temporal change of the organic EL element 1 in the drive pulse signal generation period and the blank period is in the initial stage and the intermediate stage. The relationship between the generation period and the blank period and the relationship between the pulse width of the drive pulse signal and the peak luminance are shown. As the cumulative driving time increases, the blank period decreases. Further, during the drive pulse signal generation period, gradation control can be performed by changing the pulse width of the drive pulse signal as shown in FIG.
[0016]
FIG. 6 shows a configuration in which a gradation control function is added to the light emission driving device shown in FIG. In the light emission driving device of FIG. 6, a gradation adjustment circuit 7 for correcting a duty ratio is inserted between the memory circuit 5 and the PWM circuit 6. In the gradation adjustment circuit 7, a gradation control signal indicating a gradation is supplied, and the duty ratio read from the memory circuit 5 is corrected according to the gradation control signal. For example, the duty ratio is corrected to be shorter as the gradation is lower. The PWM circuit 6 is supplied with the corrected duty ratio as data.
[0017]
FIG. 7 shows a display device to which the light emission driving device of the present invention is applied for a display panel. This display device includes a display panel 11, a display control circuit 12, an anode driving circuit 13, and a cathode scanning circuit 14. The display panel 11 includes a plurality of organic EL elements at intersections of a plurality of anode lines A1 to Am (m is a positive integer of 2 or more) and a plurality of cathode lines B1 to Bn (n is a positive integer of 2 or more). It is an arranged matrix display panel.
[0018]
The display control circuit 12 includes a CPU, and performs control for displaying an image corresponding to input image data on the display panel 11 in a line-sequential scanning method. For this control, a scanning command is issued to the cathode scanning circuit 14 in synchronization with a predetermined scanning timing, and at the same time, a driving command is issued to a later-described switch circuit 15 of the anode driving circuit 13. The display control circuit 12 accumulates and counts the driving time of each organic EL element of the display panel 11 according to the driving command, and obtains the average driving time of each of the accumulated driving times. Then, a duty ratio corresponding to the average driving time is output as data using a built-in data table as shown in FIG. The data of the duty ratio is supplied to a PWM circuit 22 of the anode drive circuit 13 described later. The display control circuit 12 may include the above-described counter 4 and the memory circuit 5, and may perform operations performed by the components according to a program.
[0019]
The anode drive circuit 13 is connected to each of the anode lines A1 to Am of the display panel 11, and selectively supplies a drive pulse signal to the anode lines A1 to Am according to a drive command corresponding to m bits from the display control circuit 12. . The cathode scanning circuit 14 is connected to each of the cathode lines B1 to Bn of the display panel 11, selects one of the cathode lines B1 to Bn in a predetermined order in accordance with a scanning command from the display control circuit 12, and selects a predetermined scanning potential (for example, (Ground potential). Among the organic EL elements connected to the cathode line to which a predetermined scanning potential is applied, the organic EL element to which a drive current is supplied via the anode line emits light.
[0020]
The anode drive circuit 13 includes a voltage source 21, a PWM circuit 22, and a switch circuit 23 for generating a drive pulse signal. The voltage source 21 outputs a predetermined DC voltage in the same manner as the voltage source 2 described above. A predetermined DC voltage is supplied from the voltage source 21 to the PWM circuit 22. The PWM circuit 22 generates a drive pulse signal at a duty ratio supplied from the display control circuit 12. The drive pulse signal is generated by the PWM circuit 22 performing pulse width modulation of a predetermined DC voltage from the voltage source 21 in accordance with the duty ratio data.
The switch circuit 23 includes m switches SW1 to SWm. Each of the switches SW1 to SWm is provided between the output of the PWM circuit 22 and the anode lines A1 to Am of the display panel 11. Each of the switches SW1 to SWm is turned on / off according to the above-mentioned drive command.
[0021]
In the display device having such a configuration, each time the display control circuit 12 issues a drive command according to an input image signal, the drive time of the display panel 11 is integrated and counted, and the duty ratio corresponding to the count result is transmitted to the PWM circuit 22 as data. Is output.
Further, one of the switches SW1 to SWm of the switch circuit 23 is turned on in response to the drive command for m bits, and the other switches remain off. That is, the switch of the switch circuit 23 connected to the drive line corresponding to the organic EL element to emit light according to the input image signal is turned on.
[0022]
On the other hand, in response to a scanning command from the display control circuit 12, the scanning circuit 14 applies a scanning potential one scanning line at a time at a predetermined scanning timing in order from the scanning line B1 of the scanning lines B1 to Bn, and this is repeated. The period in which the scanning potential is applied to one scanning line, that is, the scanning period is constant.
The driving pulse signal is applied to the organic EL elements, of the organic EL elements connected to the scanning line to which the scanning potential is applied, for which the switch of the switch circuit 23 connected to the corresponding driving line is turned on. Therefore, the organic EL element emits light. The light emission time is a period determined by a duty ratio with respect to one scanning period.
[0023]
In the above-described embodiment, the display device that performs monochrome display is described. However, in the case of performing color display, three organic EL elements whose emission colors are RGB (red, green, and blue) are provided for one pixel. Further, it is necessary to provide a voltage source and a PWM circuit for each emission color to determine the amplitude of the drive pulse signal.
Further, in each of the above-described embodiments, the case where the organic EL element is used as the self-luminous element has been described. However, the present invention can be applied to a self-luminous element such as an LED other than the organic EL element.
[0024]
As described above, according to the present invention, the time accumulating means for accumulating the driving time of the self-luminous element, the duty ratio setting means for setting the duty ratio corresponding to the accumulated driving time accumulated by the time accumulating means, A driving means for generating a driving pulse signal having a pulse width of a duty ratio set by the setting means at a predetermined repetition cycle and applying the driving pulse signal to the self-luminous element, thereby providing a light emission luminance due to a temporal change. Can be suppressed with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of a light emission driving device to which the present invention is applied.
FIG. 2 is a diagram showing a relationship between a cumulative driving time of a data table and a duty ratio.
FIG. 3 is a diagram showing the operation of the driving device in FIG. 1 in each of three stages of change with time.
FIG. 4 is a diagram showing a luminance change characteristic of FIG. 1;
FIG. 5 is another embodiment of the present invention.
FIG. 6 is a diagram illustrating an operation in a case where a blank period is controlled according to a duty ratio in the driving device of FIG. 1;
FIG. 7 is a diagram showing a display device to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Organic EL element 2, 21 Voltage source 4 Counter 5 Memory circuit 6, 22 PWM circuit 11 Display panel 12 Display control circuit 13 Anode drive circuit 14 Cathode scanning circuit

Claims (5)

A driving device for driving a self-luminous element to emit light,
Time accumulating means for accumulating the driving time of the self-luminous element,
Duty ratio setting means for setting a duty ratio corresponding to the cumulative driving time accumulated by the time accumulating means,
A driving unit that generates a driving pulse signal having a pulse width of a duty ratio set by the duty ratio setting unit at a predetermined repetition period, and applies the driving pulse signal to the self-luminous element. Drive device. 2. The drive device according to claim 1, wherein the duty ratio setting unit increases the duty ratio as the cumulative driving time accumulated by the time accumulating unit increases. 3. 2. The driving device according to claim 1, wherein the driving unit sets the pulse width according to the duty ratio while keeping the predetermined repetition period constant. 2. The driving device according to claim 1, wherein the driving unit sets the predetermined repetition period to a length corresponding to the duty ratio while keeping the pulse width constant. 2. The driving device according to claim 1, wherein the driving unit includes a pulse width modulation circuit.

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