JPS62511B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Summary> The present invention relates to a circuit that performs halftone writing in a thin film EL element with memory by changing the writing voltage.

In order to drive the matrix thin film EL element with memory, its drive system has a data electrode selection circuit of N
The scanning electrode selection circuit is composed of a P-type transistor. N-type transistors can be made to withstand high voltage as previously filed by the applicant of the present application, and the driver circuit including the gate selection circuit can be fabricated on one semiconductor substrate and integrated into an IC. However, in principle, it is difficult to manufacture P-type transistors with high voltage resistance, and as evidenced by the fact that no more than 4 elements per package have been manufactured, it is difficult to achieve high integration of P-type transistors. Although this is close to possible, there is very little hope that it will be integrated into an IC together with logic circuits such as gate selection circuits, and this transistor also has many problems, including high manufacturing costs.

The present invention proposes a technique for performing halftone writing using a circuit that lowers the required withstand voltage of a P-type transistor that drives a scan electrode.

<Prior Application Invention> The structure and characteristics of the thin film EL matrix element with memory are described in Japanese Patent Application No. 50-83767 "Drive circuit for large capacitance display element" filed by the applicant of the present invention and others.

That is, in a thin film EL display device, transparent electrodes are arranged in stripes on a glass substrate, and on this, for example, Y ₂ O ₃ ,
A dielectric material such as Si ₃ N ₄ , TiO ₂ , Al ₂ O ₃ , etc., a fluorescent layer such as ZnS or ZnSe doped with Mn on top of this, and then Y ₂ O ₃ , Si ₃ N ₄ , etc. _{TiO2 , Al2O3}
A dielectric material such as the above is deposited to a thickness of 500 to 10,000 Å using a thin film technique such as evaporation or sputtering, and a double-insulated three-layer structure is formed. Striped electrodes are arranged to form a matrix electrode. In a three-layer thin film EL display device having such a structure, when one of the first electrode group and one of the second electrode group is selected and an appropriate AC voltage is applied, the two electrodes intersect and are sandwiched. The small area covered by the light emits light. 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 in terms of brightness, lifespan, and stability compared to technology's distributed EL elements, but this EL has new characteristics such as changes in applied voltage and luminance. It exhibits hysteresis characteristics between the two. That is, when a pulse with voltage amplitude V ₁ is applied as a sustaining voltage, the brightness is at a low level of brightness B ₁ .
Here, the sustaining voltage V ₁ is defined as the emission threshold voltage Vth.
V ₁ > Vth is set. The brightness is maintained at B ₁ during the period of continuous application of the maintenance voltage V ₁ . Next, when the write voltage V ₂ (V ₂ > V ₁ ) is applied, the brightness increases all at once to the high level brightness B ₃ , and thereafter, even if the voltage returns to the maintenance voltage V ₁ again, the brightness remains the same as before. The brightness settles to _B2 , which is greater than _B1 . The brightness is maintained at _B2 by continuous application of the sustaining voltage _V1 . In this state, when the erase voltage V ₃ (V ₃ <V ₁ ) is applied next, the brightness level decreases rapidly, and when it is returned to the sustaining 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 or pulse frequency of the write voltage. That is, it is also possible to display halftones.

In this way, once a write voltage or an erase voltage is applied, each picture element continues to emit light without losing the gradation given by the sustain pulse.
This is a major feature of the display device 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~
The values are 280V and 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 leads 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, . . . .

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, 43, . . . .

50 is a circuit that leads all the Y electrodes Y ₁ to Yn to ground, and each electrode is connected to a transistor 52 operated by a sustain signal T ₂ via diodes 51, 51, . . . .

60 is a switching circuit for selecting Y electrodes Y ₁ to Yn, and between each electrode and line D of a power supply 63 that supplies voltages Vw, Ve, and Vr, high voltage P-type switching transistors 61, . . . and diodes 62, ……
... are connected, and the transistor 61 is selectively operated by a decoder (not shown) operated in response to a vertical binary address signal. The decoder is directly connected to the transistor 6 by the high voltage transistor.
The base of the transistor 61 is driven by an output of about 5 volts by level shifting the binary address signal using an opto-isolator or the like. A write voltage, an erase voltage, and a read voltage are selectively output to the power supply line D according to the operation mode of the thin film EL element, and the various voltages are applied to one of the selected Y electrodes through one of the transistors 61. Apply.

Reference numeral 70 denotes a switching circuit for guiding the X electrodes to the ground, and a high voltage N-type transistor 71, . . . is connected to each electrode X ₁ -Xm between the electrodes X ₁ -Xm and the ground. A write signal WRITE and an erase voltage 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. Figure 2 shows the time chart.

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, . . . →Discharge the charge remaining in the circuit of 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 51,
......→Added via transistor 52.
At this time, the sustaining voltage is applied to the thin film EL element in the opposite direction to that described above.

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, ......→transistor 31→residual charge is discharged in the circuit.

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.

In the above circuit, write voltage Vw, 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, and so on needs to be higher than the write voltage Vw, for example 250 volts or higher .
This is the diode 23, 32, 43, 51, 2
The same applies to 4 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.

In view of the above problems, especially the P-type transistor 61
Figure 3 shows a basic circuit that reduces the required withstand voltage.
The operation will be explained with reference to the time chart shown in FIG.

In FIG. 3, circuit parts that are the same as those in FIG. 1 are given the same reference numerals and their explanations 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 source 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-Vs1 between the write voltage Vw and the sustain voltage Vs1. A 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 a simplified circuit of FIG. 3. Fifth
In the figure, only one pixel EL of a thin film EL element is shown, and its
and only one Y electrode is shown. Also, selection transistors 61', 71', diodes 23, 3
Only one number 2, 43, and 51 is also represented.

In the circuits shown in FIGS. 3 and 5, sustain driving is performed in the same manner as in the circuit shown in 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 circuit of FIG. 5, the above circuit configuration has a series circuit of a write power supply 63', a transistor 61', and a diode 62 connected in parallel across both ends of the diode 23, and as is clear from the circuit of FIG. After applying the sustain voltage Vs ₁ to all pixels, write auxiliary voltage
The feature is that Vwe is applied.

<Description of the Present Invention> The present invention uses the circuit configuration described above to change the write brightness for each selected picture element by changing the voltage of the write auxiliary voltage source 63, thereby making it possible to perform halftone writing. The purpose is to provide a circuit system for thin film EL devices.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 6 is a basic circuit diagram showing one embodiment of the present invention. FIG. 7 is a simplified circuit of FIG. 6.
8 is a time chart for explaining the operation of the embodiment shown in FIG. 6. FIG.

The same reference numerals in Figures 6 and 7 are number 3, respectively.
Figure 5 shows the same content as Figure 5. 64 is a capacitor for holding the write auxiliary voltage, 65 is a transistor for resetting the potential across the capacitor 64 to zero together with the diode 24 before charging the capacitor 64 to the write auxiliary voltage, and 66 is the capacitor 64.
This diode is turned off when the common connection point A between the diodes 23 and 24 rises to a high voltage such as the sustain voltage, and protects the emitter follower 94, etc., which will be described later. 91 is a DA converter that outputs a current corresponding to binary data; 92 is a transistor that receives the output current of the DA converter 91 with low impedance and allows a constant current to flow regardless of the voltage at the output terminal; 93 is a transistor 9
2 and the base terminal of the emitter follower 94, and the voltage at the common connection point C is transferred from the voltage of the write voltage source 63 to the DA converter 9.
a resistor for lowering the output current in accordance with the output current of 1 to generate a voltage corresponding to binary data;
4 is an emitter follower added to sufficiently lower the high impedance at point C.

A diode 95 is interposed on the collector side of the transistor 65 on a path connected to the base terminal of an emitter follower 94 and on a path connected to the diode 66. Therefore transistor 6
5 is on, the base potential of the emitter follower 94 becomes zero, and the potential at the common connection point D between the emitter follower 94 and the diode 66 becomes zero.

Before a write operation is performed, the potentials across the write picture element are set to zero, as described above.

The basic operation for writing is performed as follows.

(1) Before proceeding to the write operation, the transistor 65 is turned on by the DIS signal, and the capacitor 64 is discharged along the path from the diode 24 to the capacitor 64 to the transistor 65, and the potential at both ends is brought to zero (reset). At the same time, the required values are set in the DA converter 91 up to this point. As a result, the voltage at point D = (voltage of write voltage source 63) - resistance value R of resistor 93 x (output current of DA converter 91), resulting in an appropriate write auxiliary voltage.

(2) Next, when the transistor 52 is turned on by T ₂ , the voltage at point D is applied to the capacitor 64 via the path of the diode 66 → capacitor 64 → diode 23 → transistor 52, and both ends of the capacitor 64 are properly written. Set to auxiliary voltage. The value of the capacitor 64 is selected to be large (~1μF) so that there is almost no potential drop even when writing to EL, so once it is charged to the write voltage, leakage etc. can be ignored and the voltage remains unchanged. hold.

(3) Subsequently, when a write operation is performed in the same manner as described above, the selected picture element of the EL element is written in accordance with the write voltage.

According to the present invention, the following excellent effects can be obtained.

(1) Since there is a difference of about 15V between the maximum and minimum write level, it is difficult to apply a pulse at the desired level accurately when applying a write pulse from zero. Since the change is 15V with respect to the voltage of the included auxiliary voltage source (~40V), voltage can be applied with high accuracy.

(2) The floating power supply used in the past for writing becomes unnecessary.

(3) Even inexpensive DA converters have a resolution of about 8 bits, so if the number of writing levels is set to 8 (3 bits) in consideration of the non-uniformity of EL luminance, the remaining 5 bits can be used for level correction taking into consideration electrode resistance and the like.
Furthermore, the output voltage can have arbitrary non-linear characteristics with sufficient accuracy using software, resulting in improved image quality.

(4) A halftone writing circuit without an adjustment volume can be realized with a minimum number of parts.

Although halftone writing is also possible by changing the pulse width, the present invention is superior to thin film EL elements for the following reasons.

In other words, if the intermediate tone is determined by changing the pulse width, 8
The pulse width that gives the lowest brightness even at the level of
Due to the presence of a large transparent electrode resistance, even if the voltage is applied almost completely at the electrode lead-out portion, the voltage may not be applied at the tip, resulting in large brightness unevenness. On the other hand, when writing is performed by changing the voltage as in the present invention, a sufficient pulse width can be obtained. Furthermore, if the pulse width is narrow, variations in hf ₀ (gm in MOS) of the electrode selection transistor have a large effect on the pulse width, resulting in considerable variation in the output pulse width. In order to eliminate this problem, it is necessary to select transistors and adjust resistors, etc., which not only takes time and effort, but also leads to a considerable increase in cost, but the present invention is free from such problems. The present invention is applicable not only to a thin film EL element with a memory but also to a case where a basic picture element component includes a capacitor, such as a refresh type thin film EL element.

Further, in the present invention, when performing write driving,
The sustain voltage is applied to all picture elements, and the write voltage is applied only to the write picture element, so at the same time that the write voltage is applied to the write picture element and writing is performed, the sustain voltage is applied to the other picture elements. Can be maintained and 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 voltage Vwe.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of an embodiment of the earlier invention, Figure 2 is a time chart of Figure 1, Figure 3 is a circuit diagram of an improved embodiment of the earlier invention, and Figure 4 is the circuit of Figure 3. FIG. 5 shows a simplified circuit diagram of FIG. 3. FIG. 6 is a basic circuit diagram showing one embodiment of the present invention, FIG. 7 is a simplified circuit diagram of FIG. 6,
FIG. 8 is a time chart for explaining the operation of FIG. 6. 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, 64: Capacitor, 70: X electrode selection circuit, 81: Withstand voltage reduction voltage source, 91: DA converter.

Claims (1)

[Claims] 1. A thin film between mutually orthogonal matrix electrodes.
A drive circuit for a thin film EL element that has an EL layer interposed therein and exhibits a hysteresis phenomenon in applied voltage and luminance characteristics includes a sustaining voltage source and a first switching element, and when the first switching element is turned on, , a circuit for applying a sustaining voltage to the thin film EL layer between the matrix electrodes; and a circuit for applying a sustaining voltage to the thin film EL layer between the first switching element and one of the matrix electrodes, a circuit for applying a sustaining voltage to the thin film EL layer;
After applying the sustaining voltage, the first switching element continues to be on, and the second switching element selectively turns on, thereby superimposing the charging voltage of the capacitor and the sustaining voltage. , a circuit that applies a write voltage from the one electrode to the other electrode, and a circuit that converts the voltage value of the charging voltage of the capacitor in response to a data signal, and executes halftone writing. A thin film characterized by
EL element drive circuit.