JPS6055835B2 - Driving method of EL display device - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to thin film electroluminescence (hereinafter referred to as EL).
The present invention relates to a method of driving a display device having a history characteristic between brightness and applied voltage.

First, the characteristics of thin film EL will be explained.

As shown in FIG. 1, a transparent electrode 2 is placed on a glass substrate 1.
are arranged in parallel.

For example, Y on top of this. A dielectric material 3 such as Oa, a phosphor layer 4 such as Mn-doped ZnS, and the same dielectric material 3' as above are sequentially laminated by vapor deposition to form a three-layer structure, and a transparent electrode 2 is formed on top of the dielectric material 3. Electrodes 5 are arranged in parallel so that they are orthogonal to each other. With this structure, when an appropriate AC voltage is applied to one of the first electrode group 2 and one of the second electrode group 5, the two electrodes are crossed and sandwiched. Only a small area of the screen will emit light, and this corresponds to one picture element on the screen. FIG. 2 shows, as an example, the relationship between a voltage waveform a applied to a picture element and a light emission waveform. Furthermore, although the EL with the structure shown in Figure 1 has superior characteristics in terms of brightness, lifespan, and stability compared to conventional distributed WL elements, the individual picture elements have a new The hysteresis phenomenon shown in FIG. 3 is shown between the voltage and the applied voltage. In other words, if the amplitude of the positive and negative pulse train is slowly increased, EL will become the light emission threshold voltage Vth
to start emitting light. As the voltage is further increased, the brightness increases along the curve a in Figure 3, with an amplitude of Vm and a brightness of Bm.
reach. If the voltage is lowered from this point, the brightness will reach the 3rd level.
As shown in the figure, for the same voltage, the brightness value decreases while taking a brightness value larger than a. Therefore, the voltage Vs with the large amplitude of the luminance difference shown in FIG. 3a is selected as the light emission sustaining voltage. First, it is assumed that a pulse train of amplitude Vs is applied as shown in FIG. 4A, and the brightness is at a brightness level Bsa on the curve a in FIG. 3.
Then, at a certain point J, after applying a write pulse with an amplitude Vw larger than Vs and a width tw as shown in FIG. Depending on the amplitude (VW) and width (tw) of the write pulse, the brightness level after writing may be the third level.
For example, between Bsal■ on diagram a and Bw on diagram b,
,,Bw. It can be taken arbitrarily, such as... In other words, it is possible to write in halftones. To erase the EL after writing, the amplitude V is smaller than the amplitude Vs as shown in Fig. 4C.
One or more erasing pulses with a width TE of E are applied. Furthermore, the sustain pulse train is then applied again as shown in FIG. As will be described in detail later, depending on the erase pulse amplitude ■E and the width stop size, Bsa. The brightness levels BE1, BE2, . . . are between 15Bsb.
By the way, the dependence of the erasing pulse amplitude (E) on the erasing effect is as shown in FIG.

In other words, assuming that the brightness level after writing is equal to Bw, if the amplitude of the erase pulse ■E is made smaller than the amplitude of the sustain pulse ■S, the erase effect will increase (the brightness level after adding the erase pulse (becomes smaller), but as the amplitude (■E) is further reduced, the erasing effect becomes smaller after a certain point (the luminance level after adding the erasing pulse becomes larger).
That is, as shown in FIG. 5, there is an optimum erase voltage EOpt. The figure also shows that the larger the number n of erasing pulses, the greater the erasing effect. The details mentioned above are described in Japanese Patent Application No. 75663/1989 filed by the same applicant. By the way, the dependence of the width T of the erase pulse on the erase effect is as shown in FIG.
It was newly determined that it would be pt3. Therefore, the method according to the present invention is applicable to display devices that can be switched between high speed and low speed depending on the application.For example, when the write/erase cycle is high, such as for moving images, the frequency becomes high, and the erase pulse width is inevitably limited, making it difficult to use a narrower one. becomes preferable. In such cases, the amplitude of the erase pulse should be made closer to the amplitude of the sustain pulse (
Make it bigger. ) is better. On the other hand, when the write/erase cycle is slow, such as when displaying Roman letters or Arabic numeral symbols manually entered by a human on a typewriter of a computer terminal, the frequency can be lowered, and therefore the time width of the erase pulse can be increased accordingly. Therefore, the optimum erasing effect can be obtained by making the amplitude of the erasing pulse smaller.
The erasing pulse width D can be easily varied by varying the resistor or capacitor that determines the time constant of a so-called monostable circuit. For example, if two resistors are provided and one of the resistors is selectively connected as the display device is switched between high speed and low speed ζ, the desired erasing pulse width can be set depending on the resistance value. I can do it. Further, a clock is appropriately input to the trigger terminal of the monostable circuit, and the application of the erase pulse is synchronized. By changing the amplitude and width of the erase pulse in this manner, the contrast ratio of the screen can be changed.

The system according to the present invention is useful because it not only allows high-speed screen switching, but also adjusts visual perception to be optimal depending on the surrounding conditions of the display device and the content being displayed.

[Brief explanation of the drawing]

Fig. 1 shows an example of an EL display element, a is a partially cutaway perspective view, b is a side sectional view, Fig. 2 is a time chart showing the operation of the EL display element, and Fig. 3 is this EL display. A characteristic diagram showing the operation of the element, Figure 4 is an example of a driving wave.
The charts, FIGS. 5 and 6, are characteristic diagrams showing the relationship between erase voltage and brightness. A...maintain pulse, mouth...write pulse, c...
Erase pulse.

Claims (1)

[Claims] 1 EL with history characteristics between applied voltage and luminance
In a method of driving a display element by applying a sustain drive signal, a write signal, and an erase signal, when the application cycle of the write signal and erase signal is fast, the signal width of the erase signal is narrowed and the amplitude is increased. When the application cycle of the signal and the erasing signal is slow, the erasing signal is applied by widening the signal width of the erasing signal and decreasing the amplitude to obtain the lowest erasing luminance state. Driving method.