JPS62510B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Summary> The present invention relates to a circuit that reduces the withstand voltage requirements of an electrode selection circuit that drives a thin film EL element with memory.

In order to drive the matrix thin film EL element with memory, the data electrode selection circuit is composed of N-type transistors, and the scan electrode selection circuit is composed of P-type transistors. N-type transistors can be made to have a high breakdown voltage, as proposed by the applicant, and the driver circuit, including the gate selection circuit, can be fabricated on one semiconductor substrate and integrated into an IC. However, it is theoretically difficult to manufacture P-type transistors with high breakdown voltages, and four or more elements per package have not been manufactured. In this way, high integration of P-type transistors is impossible, and along with logic circuits such as gate selection circuits,
There is no hope of converting it to IC. Also, this transistor is expensive to manufacture.

The present invention provides a circuit and a driving method for lowering the required withstand voltage of a P-type transistor that drives a scan electrode.

<Prior invention> The structure and characteristics of the thin-film EL matrix element with memory have been explained in Japanese Patent Application No. 83767/1983 filed by the present applicant entitled "Drive circuit for large capacitance display element" and others, but they will be briefly explained once again. .

In a thin film EL display device, transparent electrodes are arranged in stripes on a glass substrate, and on top of this, for example, Y ₂ O ₃ , Si ₂ N ₄ ,
A dielectric material such as TiO ₂ or Al ₂ O ₃ is added, and on top of this a fluorescent layer such as ZnS doped with Mn (yellow light emitting), and then Y ₂ O ₃ , Si ₂ N ₄ , TiO ₂ , Al ₂ O ₃ and other dielectric materials are deposited to a thickness of 500 to 10,000 Å using thin film techniques such as vapor deposition and sputtering.
It has a heavy insulation type three-layer structure, and striped electrodes are arranged thereon in a direction perpendicular to the transparent electrode to form a matrix electrode. A three-layer thin film with such a structure
In an EL display device, when one of the first electrode group and one of the second electrode group is selected and an appropriate alternating voltage is applied, a small area sandwiched between the two electrodes will emit light. do. This corresponds to one picture element on the screen. By combining these, characters, symbols, patterns, etc. are displayed.

ELs with this type of structure have superior characteristics compared to conventional distributed EL elements in terms of brightness, lifespan, and stability, but this EL has a new feature called hysteresis between the applied voltage and luminance. Show characteristics. When first applying a pulse of voltage amplitude V ₁ , the brightness is at a low level brightness B ₁ . Here, when the sustaining voltage V ₁ is the light emission threshold voltage Vth, V ₁ >Vth. The brightness B ₁ is maintained by continuous application of the maintenance voltage V ₁ . Next, when the write voltage V ₂ is applied, the brightness increases all at once to the high level brightness B ₃ , and even if the voltage returns to the maintenance voltage V ₁ again within a certain period of time, the brightness remains higher than the previous brightness B ₁ . I settled on B ₂ . The brightness B ₂ is maintained by continuous application of the maintenance voltage V ₁ . In this state, when the erase voltage V ₃ is applied next, the brightness level decreases rapidly, and when it is returned to the sustain voltage V ₁ again, it settles to the previous low level of brightness B ₁ . This hysteresis phenomenon can take any small loop depending on the amplitude, pulse width (not shown), and pulse frequency of the write voltage. That is, it is also possible to display halftones.

The reason for this is that once a write voltage or erase voltage is applied, each picture element continues to emit light without losing the gray level given to it by the sustain pulse.
This is a major feature of BL display devices that other display devices do not have. The above voltages vary greatly depending on the physical conditions of composition and film thickness, manufacturing conditions, and applied waveform, but in a prototype example, Vth = 200V, V ₁ = 210V, V ₂ = 210~
280V, V ₃ = 190V.

The drive circuit for this thin film EL panel was developed by the present inventors in Japanese Patent Application No. 52-126948 entitled "Drive Circuit for Thin Film EL Element".
and Japanese Patent Application No. 52-130529 ``Driving circuit for thin film EL element'', this is shown in FIG. 1 as a prior invention and will be described below.

Reference numeral 10 denotes the thin film EL element, in which column (X) electrodes X ₁ to Xm are made of transparent electrodes 11 and row (Y) electrodes Y ₁ to Yn are made of aluminum electrodes 12.
Only shown.

20 is a circuit that supplies a positive sustaining voltage Vs ₁ to the Y electrode from the power supply line A, and is composed of transistors 21 and 22 operated by the sustaining signal T ₁ , and is connected to each electrode.
_Y1 to Yn are the diodes 23 connected to each electrode,
Connect via 23, …….

Reference numeral 30 denotes a circuit that connects all the X electrodes to ground during sustain drive, and is comprised of a transistor 31 that operates in response to a sustain signal _T4 , and is connected to each electrode _X1 to Xm via diodes 32, 32... Ru.

40 is a circuit that supplies a positive sustaining voltage Vs ₁ from line B to all X electrodes, and transistor 4 is operated by a sustaining signal T ₃ applied to line C.
1 and 42, and are connected to each of the electrodes X ₁ to Xm via diodes 43, .

50 is a circuit that connects all the Y electrodes Y ₁ to Yn to the ground, and each electrode is connected to a transistor 5 operated by a sustain signal T ₂ via a diode 51, .
Connected to 2.

60 is a switching circuit for selecting Y electrodes Y ₁ to Yn, and high voltage P-type switching transistors 61, ...... and diodes 62 are connected between each electrode and line D of a power supply 63 that supplies voltages Vw, Ve, and Vr. ,…
. The decoder is configured to directly drive the base of the transistor 61 with a high voltage transistor, or it is configured to level shift the binary address signal using an opto-isolator or the like and drive the base of the transistor 61 with an output of about 5 volts. A write voltage, an erase voltage, and a read voltage are selectively outputted to the power supply line D according to the operation mode of the thin film EL element, and the transistor 61
The above voltage is applied to one of the selected Y electrodes through one of the Y electrodes.

Reference numeral 70 denotes a switching circuit for guiding the X electrodes to the ground, and high voltage N-type transistors 71, . . . are connected to each electrode X ₁ -Xm between the electrodes X ₁ -Xm and the ground. A write signal WRITE and an erase signal ERASE are applied to the base of this transistor via an analog switch (not shown) operated by a horizontal binary address signal. These transistors 71, . . . act as switching elements for selecting electrodes during writing, erasing, and reading. This drive circuit operates as follows. Second
The time chart is shown in the figure.

Γ Maintain Drive At the first timing, the signal T ₁ is applied to the circuit 20 and the signal T ₄ is applied to the circuit 30. Therefore, the sustaining voltage Vs ₁ is the transistor 22→
It is applied via the diode 23, . . . → Y electrode → X electrode → diode 32, …… transistor 31.

At the second timing, the signal T ₂ is applied to the circuit 50, and the diode 44→diode 43,...
…→X electrode→Y electrode→diode 51,……
→The electric charge remaining in the thin film EL element is discharged to the circuit of the transistor 52. This is to prevent breakdown of the thin film EL element due to residual charges.

At the third timing, the signal T ₂ is applied to the circuit 50 and the signal T ₃ is applied to the circuit 40. Therefore, the sustaining voltage Vs ₁ changes from transistor 42 to diode 4.
3,…………→X electrode→Y electrode→diode 5
1, . . . → is applied via the transistor 52. At this time, the sustaining voltage is applied to the thin film EL element in the opposite direction.

At the fourth timing, the signal T ₄ is applied to the circuit 30, and the diode 24→diode 23,...
…→Y electrode→X electrode→diode 32,……
The residual charge is discharged in the circuit →transistor 31→.

The above four timings are sequentially repeated to perform maintenance drive.

ΓWrite, erase, read drive The power supply 63 is set to the write voltage according to the drive mode of the thin film EL element, for example, write, erase, read drive.
Vw, erase voltage Ve, and read voltage Vr are output to line D.

Then, the transistors 61 and 71 of the X electrode and Y electrode connected to the picture element desired to be written, erased, or read are selectively turned on by the electrode selection signal. The electrode selection signal is applied after the fourth timing of sustain driving ends and before the first timing starts. Therefore, the write voltage Vw, erase voltage Ve or read voltage Vr is changed from line D to transistor 6.
1→diode 62→Y electrode→X electrode→transistor 71. Driving at this time is performed by a point sequential method or a line sequential method.

<Problems with the prior invention> In the above circuit, the write voltage Vw, the erase voltage
Since Ve and read voltage Vr are applied when no sustain voltage is applied, that is, when the voltage is 0V, the withstand voltage of transistors 61, 71, 71, etc. needs to be higher than the write voltage Vw, for example 250 volts or higher. shall be. This situation is caused by diodes 23, 32, 43, 5
This also applies to 1, 24, and 44, and the same amount of withstand voltage is required. transistors 61, 71,
Since it is necessary to prepare the same number of diodes 23, 32, 43, and 51 as the number of electrodes of the thin film EL element, each of these elements cannot be miniaturized unless they are integrated into ICs. By the way, the N-type transistor 71 can be integrated into an IC including a logic circuit, but the P-type transistor 61 is not only difficult to manufacture with high voltage resistance, but also almost impossible to integrate. It is possible.

<Description of the Present Invention> In view of the above points, the present invention provides a circuit and a driving method that particularly reduce the required breakdown voltage of the P-type transistor 61.

The basic circuit of the present invention is shown in FIG. 3, and its operation will be explained with reference to the time chart of FIG. 4.

In FIG. 3, circuit parts that are the same as those in FIG. 1 are given the same reference numerals, and explanations thereof will be omitted. However, although the transistors 61 and 71 are bipolar transistors in FIG. 1, they are MOS transistors in FIG. 3, so the symbols are changed and the symbols are 61' and 71'. Further, the power supply 81 is a withstand voltage reduction voltage generating section, and in this embodiment is a sustaining voltage source. In FIG. 3, the power supply 63' generates the write auxiliary voltage Vwe,
This voltage has a relationship of Vwe=Vw−Vp between the write voltage Vw and the withstand voltage reduction voltage Vp. Power supply 63'
is connected between the source common line of the transistor 61' and the anode common line of the diode 23.

FIG. 5 shows an equivalent circuit that is a simplified version of FIG. 3.
In FIG. 5, only one picture element EL of the thin film EL element is shown, and its X and Y electrodes are shown by only one. Also, selection transistors 61', 71', diode 2
Only one number 3, 32, 43, and 51 is also represented.

In the circuit of the present invention, sustain driving is performed in the same manner as in the circuit of FIG. 1, so a description thereof will be omitted.

The timing of selecting and driving the electrodes such as writing, erasing, and reading is different from that in Figure 1, and is
It works as shown in the figure. Here, write drive will be explained as an example.

The write drive consists of three stages.

Signals T ₁ and T ₄ are applied to circuits 20 and 30 to apply a sustaining voltage Vs ₁ to all pixels of the thin film EL device and to
Apply until the voltage across the EL element reaches the sustaining voltage Vs ₁ .

Then the signal T ₁ continues to be applied and the signal T ₄ goes to zero. Then, in order to select the X electrode and Y electrode containing the picture element desired for writing, electrode selection signals Tv and Th
is added to the transistors 61' and 71'. Therefore, the write picture element has a sustain voltage Vs ₁ and a write auxiliary voltage Vwe.
are applied in a superimposed manner, and the written picture element emits light.

Finally, the signals T ₁ , Tv, and Th are set to 0, and the signals T ₂ ,
A discharge circuit is formed by applying _T4 , and the voltage across the thin film EL element is set to zero.

As is clear from the equivalent circuit of FIG. 5, the present invention connects a series circuit of a write power source 63', a transistor 61', and a diode 62 in parallel across the diode 23, and as is clear from FIG. After applying the sustaining voltage Vs ₁ to all pixels, write auxiliary voltage
The feature is that Vwe is applied.

Therefore, according to the present invention, the breakdown voltage is reduced for the following reason.

After applying the sustaining voltage Vs ₁ to all the pixels of the thin film EL element, when applying the write auxiliary voltage Vwe, first turn on transistors 22 and 31 and apply the sustaining voltage to all the pixels, then the thin film EL element Since there is an insulating layer sandwiching the fluorescent layer between the electrodes, it can be equivalently thought of as a capacitor, so even if the transistor 31 is turned off after applying the sustaining voltage, the voltage across the thin film EL element maintains the sustaining voltage. ing. Therefore, when the write auxiliary voltage Vwe is applied, the on-breakdown voltage of the transistors 61' and 71' becomes the write auxiliary voltage Vwe.

After the write voltage Vw is applied to the write picture element, when the S point shown in FIG. 5 becomes 0 potential,
The write voltage Vw is applied to the series circuit of the write power supply 63', the transistor 61', and the diode 62, but at this time, the diode 62 is in the opposite direction to this voltage, so the diode 62 is turned off. This prevents a voltage from being applied to transistor 61'. Therefore, in this case, if the withstand voltage of the diode 62 is sufficient, the withstand voltage of the transistor 61' does not require a high voltage.

For the above reasons, the absolute withstand voltage of the transistor 61' is higher than the write auxiliary voltage Vwe, and the transistor 7
The on-breakdown voltage of the transistor 71' is higher than the write auxiliary voltage Vwe, and the off-breakdown voltage of the transistor 71' is higher than the sustain voltage _Vs1 . As an example, the sustaining voltage Vs ₁ is 210 volts,
The write auxiliary voltage Vwe is 40 volts.

Thus, according to the present invention, the transistor 6
The required withstand voltage of the P-type MOS transistor that constitutes 1' is as low as about 40 volts, so the manufacturing method has been established, and it can be obtained at low cost (for example, 1/8 of a discrete transistor), and gate selection is also required. Both circuits can be integrated on the same substrate. The transistor 71' of the present invention is
It is composed of a DSA MOS transistor, but when used with a capacitor load, this transistor has a high off-breakdown voltage but a low on-breakdown voltage, and is easily damaged if it is operated with the on-breakdown voltage equal to the off-breakdown voltage. However, the present invention does not have the above problem because the on-breakdown voltage is lower than the off-breakdown voltage.

Further, in the present invention, when performing write driving,
After the sustain voltage is applied to all picture elements, the auxiliary voltage necessary for writing is superimposed on this sustain voltage and applied only to the write picture elements, so that at the same time as the write voltage is applied to the write picture elements and writing is performed, A sustaining voltage is applied to the other picture elements so that they can be sustainably driven.

This write drive is performed in accordance with the application timing of the sustain voltage.

Next, erasure or read driving is performed between the fourth timing and the first timing of the sustain pulse, and is performed in the same manner as the write driving described above. However, the voltage initially applied is not the sustain voltage Vs ₁ , but a voltage lower than the erase or read voltage by the write auxiliary voltage Vwe.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of the invention of the prior application, Fig. 2 is a time chart of Fig. 1, Fig. 3 is a circuit diagram of an embodiment of the invention, and Fig. 4 is a circuit diagram of an embodiment of the invention of Fig. A time chart for explaining the operation, and FIG. 5 shows an equivalent circuit diagram of the circuit in FIG. 3. 10: Thin film EL element, 20: Sustaining voltage application circuit, 30: X electrode earthing circuit, 40: Sustaining voltage application circuit, 50: Y electrode earthing circuit, 60: Y
Electrode selection circuit, 63': Write voltage source, 70: X
Electrode selection circuit, 81: Withstand voltage reduction voltage source.

Claims (1)

[Claims] 1. In a drive circuit for a thin film EL element in which a thin film EL layer sandwiched by a dielectric thin film is interposed between mutually orthogonal matrix electrodes, a first voltage source and a first switching element are provided. and when the first switching element is turned on, the thin film
The above first electrode is applied from one electrode to the other electrode of the EL element.
a circuit for applying a voltage from a voltage source, a second voltage source and a second switching element between the first switching element and the one electrode, and after applying the voltage from the first voltage source; By continuing the on state of the first switching element and selectively turning on the second switching element,
a circuit that superimposes and adds the voltage of the first voltage source and the voltage of the second voltage source to apply an operating voltage of the thin film EL element from one electrode to the other electrode; A drive circuit for a thin-film EL element that is characterized by reducing the withstand voltage of the element. 2. The thin film EL element exhibits a hysteresis phenomenon in applied voltage and luminance characteristics, the voltage of the first power supply is a sustaining voltage, and the superimposed operating voltage is a write voltage to the selected picture element. A thin film according to claim 1, characterized in that
EL element drive circuit. 3. A backflow prevention diode is connected between the first voltage source, the first switching element, and the one electrode,
The thin film according to claim 1, characterized in that a series circuit of the second voltage source and a second switching element is connected in parallel with the diode.
EL element drive circuit.