JPH11329721A - Method and equipment for driving light-emitting element in multi-gradation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for driving a light-emitting element in multi- gradation enabling control of a sufficiently large number of gradations in an image and also capable of displaying a high definition image even when driving a display consisting of light emitting elements whose thresholds varies largely, and equipment therefor. SOLUTION: This multiple-gradation driving equipment comprises: a temporal modulation means 2 for outputting a temporally modulated signal according to an input signal; an amplitude modulation means 3 for outputting amplitude modulation signal according to the input signal; and a judging means 1 for judging the magnitude of an input signal level for driving a light-emitting element and choosing an output signal from an output of temporal modulation means 2 and that of amplitude modulation means 3 based on this judgement result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an LED and an organic EL.
The present invention relates to a driving device and a driving method for a light-emitting element having a light emission gradation control function of a self-light-emitting element such as an element.

[0002]

2. Description of the Related Art A self-luminous display such as a light emitting diode (LED), a laser diode (LD), and an organic EL element is expected to be used together with a liquid crystal display or a next-generation display. These have been put into practical use as constant-luminance displays, such as segment displays and dot-matrix displays, and image display is possible if emission gradations can be displayed well.

[0003] Since LEDs and organic EL elements can control light emission luminance by power supply, see, for example, Japanese Patent Application Laid-Open No.
As disclosed in Japanese Patent No. 0351, there has been proposed a device in which a gradation signal is a voltage signal applied to an element. Further, as disclosed in Japanese Patent Application Laid-Open No. 6-301355, a so-called PWM that controls the power supply time while keeping the instantaneous power constant.
Attempts have also been made to introduce scheme control.

[0004] As disclosed in Japanese Patent Application Laid-Open No. Hei 5-30351, the method of controlling gradation by voltage amplitude can be applied to an organic EL element including current amplitude modulation in terms of amplitude modulation. is there. However, the voltage,
Alternatively, when the organic EL element is driven by current amplitude modulation, there is a large variation in a threshold value, which is a level at which the organic EL element emits light, which is a problem. That is, there is no problem with a signal level that gives relatively bright light emission, but at a low signal level that gives dark light emission, variation in light emission luminance at the same voltage (current) level is enlarged (visual acuity is increased with respect to dark images. And the image quality is greatly impaired.

On the other hand, in a method of modulating a pulse width as disclosed in Japanese Patent Application Laid-Open No. 6-301355, since a voltage or current duration is controlled, a voltage sufficiently higher than the threshold is applied. And dark current can be obtained with high quality without being affected by variations in the threshold value.

However, the brightness gradation has a minimum pulse width,
Limited by the ratio of the maximum pulse width. For this reason, in the display of a moving image, in the case of sequentially scanning the simplest pixels one by one, even a normal television image has about 0.25 μs per pixel.
Only signal time can be taken. Therefore, it is difficult to drive this time with a signal that is further divided,
In the first place, the light emission speed cannot keep up.

Even when scanning one line at a time, in the case of a television signal, the signal time is about 64 μs,
In the case of the L element, assuming that the minimum response time is about 10 μs, the ratio of the minimum pulse width to the maximum pulse width is only about 6, and
Considering the contrast ratio, it can be seen that it is difficult to increase the number of gradations. Here, as a measure for increasing the number of gradations, there is a method in which a screen is divided and each divided screen is driven simultaneously. However, the display device must draw the drive electrodes in the XY directions, but the number of divisions from which the drive electrodes can be drawn in the XY directions is limited to four or less. Therefore, even if this technique is used, the number of gradations is only 25 at most in the above example.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a multi-gradation driving apparatus and a driving method for a light-emitting element which enable gradation control with a sufficiently large number of gradations.

Another object of the present invention is to provide a multi-gradation driving device and a driving method capable of obtaining a high-quality image even when driving a display having a large variation in light emission threshold.

[0010]

A self-luminous element such as an LED or an organic EL element can control the light emission luminance to multiple gradations by appropriately controlling the power supplied to the element. Therefore, when the supplied power is small (low luminance), PWM or F
When time modulation such as M is used and supply power is large (high brightness), amplitude modulation is used to give a signal with a short time width even to a dark screen, thereby sufficiently exceeding the threshold value of the element. An amplitude signal can be provided. As a result, the deterioration of the image quality due to the variation in the threshold level of the element is eliminated. In addition, for a bright screen, even if the signal pulse width has an upper limit, the signal amplitude can be increased instead of the pulse width, so that the number of gradations can be sufficiently increased.

That is, the above object is achieved by the following constitution. (1) A time modulation means for outputting a time modulation signal according to an input signal, an amplitude modulation means for outputting an amplitude modulation signal according to an input signal, and judging a level of an input signal level for driving a light emitting element. A multi-gradation driving device for a light-emitting element, comprising: a judging unit that outputs either the time modulation unit or the amplitude modulation unit as an output signal based on the judgment result. (2) The determination means compares a reference value with an input signal level, and when the input signal level is equal to or less than the reference value or less than the reference value, the signal of the time modulation means is set as an output signal, and the input signal level is set to the reference value. The multi-gradation driving device for a light-emitting element according to the above (1), wherein a signal of the amplitude modulation means having a constant output width is used as an output signal when the value exceeds the value or is equal to or more than the reference value. (3) The multi-gradation driving device for a light emitting element according to the above (1) or (2), wherein the amplitude modulation means performs amplitude modulation on an output signal of the time modulation means. (4) The multi-gradation driving device for a light-emitting element according to any one of (1) to (3), wherein the reference value corresponds to a limit output time width of the amplitude modulation unit. (5) A multi-gradation driving device for a light-emitting element according to any one of (1) to (4), which drives the organic EL element. (6) The input signal level for driving the light emitting element is compared with a reference value, and when the input signal level is equal to or less than the reference value or less than the reference value, a time-modulated output is obtained, and the input signal level exceeds the reference value. A multi-gray-scale driving method for a light-emitting element in which an amplitude modulation output is output when the output is equal to or greater than a reference value. (7) The method of (6), wherein the amplitude modulation output is obtained by performing amplitude modulation on the time modulation output. (8) The method of (6) or (7), wherein the reference value corresponds to a limit output time width of the amplitude modulation means. (9) A multi-tone driving method for a light-emitting element according to any one of the above (6) to (8), which drives an organic EL element.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A multi-gradation driving device for a light emitting device according to the present invention, as shown in FIG. 1, for example, a time modulation means 2 for outputting a time modulation signal according to an input signal, and a time modulation means 2 according to the input signal. An amplitude modulating means 3 for outputting an amplitude modulated signal, a judging means 1 for judging the magnitude of the input signal level, and using either the output of the time modulating means 2 or the amplitude modulating means 3 as an output signal according to the judgment result; Having.

As described above, by using either the output of the time modulation means 2 or the output of the amplitude modulation means 3 as an output signal in accordance with the input signal level, the deterioration of the image quality due to the variation in the threshold level of the element is eliminated. , The number of gradations can be made sufficiently large.

The judging means 1 controls, as shown in FIG. 2, for example, the output of either the time modulating means 2 or the amplitude modulating means 3 to be an output signal in accordance with the input signal level.

Here, in FIG. 2, level A is the maximum amplitude level of the time modulation means, and level B is the organic EL.
The threshold level of a light emitting element such as an element. As described above, the time modulation unit 2 is driven at the power level A sufficiently higher than the light emission threshold level B of the light emitting element, so that the light emission quality does not deteriorate due to the variation in the threshold value. The low-level light emission luminance is adjusted by reducing the driving time width.

First, during the time T1 to T3, the input signal level is low, and is controlled by the PWM method (or the FM method) to adjust the supply power. When the input signal level becomes equal to or larger than a predetermined level (or exceeds this level),
As shown in T4 to T6, the system is switched to the amplitude control system, and the supply power is adjusted.

That is, a PWM method in which the instantaneous power and the repetition period are constant and the output time changes until the input signal level reaches a certain value (the level at which the output power becomes a certain value).
Alternatively, the output signal is controlled by the FM method in which the instantaneous power and the output time are constant and the repetition period changes. When the input signal level becomes higher than a certain value (a level at which the output power becomes a certain value), the output signal is preferably controlled by an AM method in which the time width is constant and the amplitude of the output signal (power) changes.

The reference value for switching between the time modulating means and the amplitude modulating means is a power controllable by the time modulating means,
That is, the maximum value of the time width or a value close thereto is preferable. Therefore, it is preferable to detect and switch the input signal level corresponding to this. By supplementing the maximum value of the power that can be controlled by the time modulation means with the amplitude modulation, it is possible to perform gradation control with a larger number of gradations.

The instantaneous power provided by the time modulating means,
Alternatively, the driving voltage is preferably sufficiently higher than the light emission threshold of the light emitting element. Here, when the light emitting element is an organic EL element, the threshold voltage is usually 2 to 20 V, especially 3
About 10V. The threshold voltage of the LED is
Usually, it is about 1.5 to 2V. A sufficiently high value usually means a value that is 1.5 to 2 times or more of these threshold values.

Here, the configuration of FIG. 1 will be described in more detail. FIG. 1 is a block diagram showing the basic configuration of the device of the present invention.

The time modulating means 2 outputs a time modulated signal corresponding to the input signal. As a time-modulated signal, PW in which the instantaneous power and the repetition period are constant and the output time varies
M (pulse width modulation) method, or FM in which the instantaneous power and output time are constant and the repetition cycle changes
(Frequency modulation) system, but the PWM system is more preferable.

The means for performing PWM or FM modulation includes:
A suitable one may be selected from these known circuit systems and used. There are various circuits or processors and control algorithms that perform such operations, and they can be obtained as off-the-shelf devices (ICs).
Note that the time modulation means may apply the time modulation directly to the input signal, may apply modulation to an intermediate signal corresponding to the input signal, or may directly output a PWM or FM signal. It is.

The amplitude modulating means 3 outputs an amplitude modulated signal according to the input signal. Amplitude modulation [AM (amplitude modu
lation)], the amplitude of the output signal (power) changes. As a means for performing modulation, a suitable one may be selected from these known circuit systems and used. There are various circuits or processors and control algorithms that perform such operations, and they can be obtained as off-the-shelf devices (ICs). The time modulating means may apply the time modulation directly to the input signal, may apply the modulation to the intermediate signal corresponding to the input signal, or may directly output the signal in the AM form.

The time width of the output waveform in the amplitude modulation means 3 is preferably the limit output time width of the time modulation means 2. The limit output time width is the maximum output time width that can be output by the time modulating means, in other words, the output time width in which the OFF period cannot be reduced any longer.

The output signals of the time modulating means 2 and the amplitude modulating means 3 have not only a rectangular pulse wave as shown in FIG. 2 but also other levels or analog signals approximated by a set of sine waves. It may be a waveform.

The determining means 1 receives a signal applied to the node N1 and determines the magnitude of the input signal. The determination method is not particularly limited, and may be compared with a predetermined reference value (reference level) to determine whether the signal level is higher or lower, or whether the signal level itself is higher or lower. May be determined. The reference values are as described above.

More specifically, generally, a reference signal is prepared, a comparator (comparator) compares whether or not the amplitude of the input signal is larger than the signal, and uses the output signal. . At this time, when the input signal is a pulse signal or an AC signal, the signal may be input via a filter circuit, a sampling circuit, or the like as a means for extracting a necessary signal component. Further, the output signal of the comparator may be output or held at a certain timing via a gate or a flip-flop (FF).

Further, the input signal may be converted into a digital code by A / D conversion or a coder or the like, and this may be processed and determined using an arithmetic circuit, a processor or the like. In this case, the filter circuit, the sampling circuit, and the like can be replaced with arithmetic processing based on digital theory.

Based on the result of the determination, the determination means 1 converts the output signal supplied to the output node N2 into a time-modulated signal,
Or, switch to either the amplitude modulation signal. In this case, the circuit may be directly switched to either the time modulation means 2 or the amplitude modulation means 3, or the output of either one may be blocked. Further, the same operation may be performed on the input side of the time modulation means 2 or the amplitude modulation means 3. Further, a case where at least one of the time modulating means 2 and the amplitude modulating means 3 is activated is also included.

Further, like the adder, amplitude modulation may be added to the time-modulated signal.

More specifically, both outputs may be switched by a contact switch, a semiconductor switch, a data flow direction control mechanism, or the like, which is operated by a control signal of the determination means 1, or the time modulation means 2, Amplitude modulation means 3
Is realized by a processor and software, it may be software for controlling these software. When using a processor,
Any of CISC, RISC, DSP and the like may be used, but RISC and DSP type processors are particularly preferable in terms of operation speed.

The output signal given to node 2 in this way is given to the light emitting element directly or via the buffer means and the amplification means. In this case, if the matrix drive is performed, the control signal or the control device is combined with such a control circuit, and the signal given to each pixel becomes the output signal as described above.

The multi-gradation driving device of the present invention uses an LED, an organic EL element, and the like, and is particularly preferably used for a display that requires high-quality and high-definition image display and requires multi-gradation control. it can.

It should be noted that the above-described driving method of the apparatus is developed into an algorithm, which is stored as a control program on a storage medium, and the storage medium storing the program can be used independently as a control device. .

[0035]

[Embodiment] The size of one pixel is 250 μm × 250 μm
Then, the organic EL display device having 340 × 260 dots of pixels was driven by the device of the present invention. At this time, the threshold voltage of each pixel of the organic EL element had a variation of about 3.1 to 3.6 V.

The drive conditions are such that the drive voltage of the time modulation means is 8 V, which is sufficiently higher than the threshold value, the minimum drive pulse width is 10 μs, and the amplitude level is switched to the input level at which the output time width is 50 μm s. Driven. At this time, the maximum voltage of the amplitude modulation was set to 16V. Under these conditions,
The driving power per element at the minimum luminance is about 240 nW (1 /
The driving power per element at the maximum luminance was about 4800 nW (160 nW in 1/3 sec), the contrast ratio was 2.0, and the number of gradations was 60.

When the driven organic EL display was observed, it was confirmed that high-quality display without variation in light emission was possible even at low luminance.

[0038]

As described above, according to the present invention, it is possible to provide a multi-gradation driving device and a driving method capable of controlling a sufficiently large number of gradations.

Further, it is possible to provide a multi-gradation driving device and a driving method capable of obtaining a high-quality image even when driving a display having a large variation in light emission threshold.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a basic configuration example of a multi-gradation driving device according to the present invention.

FIG. 2 is a pulse waveform diagram exemplarily showing an output waveform of the multi-gradation driving device of the present invention.

[Explanation of symbols]

 1 judgment means 2 time modulation means 3 amplitude modulation means

Claims (9)

[Claims] 1. A time modulation means for outputting a time modulation signal according to an input signal, an amplitude modulation means for outputting an amplitude modulation signal according to an input signal, and a magnitude of an input signal level for driving a light emitting element And a judgment means for setting the output of either the time modulation means or the amplitude modulation means as an output signal based on the judgment result. 2. The method according to claim 1, wherein the determining unit compares a reference value with an input signal level. If the input signal level is equal to or less than the reference value or less than the reference value, the signal of the time modulation unit is set as an output signal. 2. The multi-gradation driving device for a light-emitting element according to claim 1, wherein when is larger than or equal to a reference value, a signal of an amplitude modulation means having a constant output width is used as an output signal. 3. The multi-gradation driving apparatus for a light emitting element according to claim 1, wherein said amplitude modulation means performs amplitude modulation on an output signal of said time modulation means. 4. The multi-gradation driving device for a light-emitting element according to claim 1, wherein the reference value corresponds to a limit output time width of the amplitude modulator. 5. A multi-gradation driving device for a light emitting device according to claim 1, which drives an organic EL device. 6. An input signal level for driving a light emitting element is compared with a reference value, and when the input signal level is equal to or less than the reference value or less than the reference value, a time-modulated output is obtained. A multi-grayscale driving method for a light-emitting element in which amplitude modulation output is output when the value exceeds or exceeds a reference value. 7. The method according to claim 6, wherein the amplitude modulation output is obtained by performing amplitude modulation on a time modulation output. 8. The method according to claim 6, wherein the reference value corresponds to a limit output time width of the amplitude modulation means. 9. The method of driving a light emitting element according to claim 6, wherein the organic EL element is driven.