JPS62516B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Summary> The present invention relates to a circuit system for performing 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. As previously filed by the applicant of the present application, N-type transistors can be made to withstand high voltage, and a driver circuit including a 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 withstand voltages, and as evidenced by the fact that no more than four 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 has many problems, such as 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. A striped back electrode is placed on the top of the electrode to form a matrix electrode. In a three-layer thin-film EL display device with such a structure, when one of the transparent electrode groups and one of the back electrode groups is selected and an appropriate AC voltage is applied, a small area sandwiched between the two electrodes intersects. Parts glow. This corresponds to one picture element on the screen. By the combination of these,
Characters, symbols, patterns, etc. are displayed.

EL elements with this type of structure have superior characteristics compared to conventional distributed EL elements in terms of brightness, lifespan, and stability, but this EL element has new characteristics such as applied voltage and power generation. Shows hysteresis characteristics during brightness. In other words, when a pulse with voltage amplitude V ₁ is applied as a sustaining voltage, the brightness decreases to the low level brightness B ₁
It is in. Here, the maintenance voltage V ₁ is the emission threshold voltage V _th
Then, V ₁ >V _th is set. Maintaining voltage V ₁
The brightness is maintained at B ₁ during the continuous application period. 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 the previous brightness.
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 returns to the maintenance voltage _V1 again, it settles down to the previous low level of brightness _B1 . This hysteresis phenomenon can take any small loop depending on the amplitude, pulse width, and 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, V _th = 200 V, V ₁ = 210 V, 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 X _n are made of transparent electrodes 11 and row (Y) electrodes Y ₁ to Y _o are made of aluminum electrodes 12.
Only shown.

Reference numeral 20 denotes a circuit that supplies a positive sustaining voltage V _s1 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 .
Y ₁ to _{Y o} are the diodes 23 connected to each electrode,
23, . . .

Reference numeral 30 denotes a circuit that connects all the X electrodes to ground during sustain drive, and is composed of a transistor 31 operated by a sustain signal _T4 , which is connected to each electrode _X1 to _Xn via diodes 32, 32, . . . Ru.

40 is a circuit that supplies a positive sustaining voltage V _s1 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 X _n via diodes 43, 43, .

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

Reference numeral 60 denotes _a switching circuit for selecting Y electrodes Y _{1 to Y o} , and high voltage P-type switching transistors 61, . and diode 62,...
. . . are connected, and the transistor 61 is selectively operated by a decoder (not shown) operated by 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.

70 is a switching circuit that leads the X electrode to the ground, and a high voltage N-type transistor 7 is connected to each electrode _X1 to _Xn .
1, . . . are connected between the electrodes X ₁ to X _n and the ground. The base of this transistor has a write signal
WRITE and ERASE signals are applied via analog switches (not shown) operated by horizontal binary address signals. These transistors 71, . . . act as switching elements for selecting electrodes during writing, erasing, and reading.

The operation of the above drive circuit will be explained with reference to FIG.

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 V _s1 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 V _s1 is 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, ......→Residual charge is discharged in the circuit of transistor 31.

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

Γ Write, erase, read drive The power supply 63 outputs a write voltage V _w , an erase voltage V _e , and a read voltage V _r to the line D according to the drive mode of the thin film EL element, for example, write, erase, or read drive.

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 V _w , erase voltage V _e or read voltage V _r 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, the write voltage V _w , the erase voltage V _e , and the read voltage V _r are applied when the sustain voltage is not applied, that is, when the voltage is OV, so the withstand voltage of the transistors 61, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, ... requires a write voltage Vw _or higher, for example, 250 volts or higher. This is the diode 23, 32, 43, 51,
The same applies to 24 and 44, and the same amount of withstand voltage is required. Transistors 61, 7
1. Diodes 23, 32, 43, 51 are thin films
Since it is necessary to prepare the same number of electrodes as the EL element, each of these elements cannot be miniaturized unless they are integrated into ICs. By the way, N-type transistor 7
1 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.

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 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 a write auxiliary voltage V _we ;
This voltage has a relationship between the write voltage V _w and the sustain voltage V _s1 as V _we =V _w −V _s1 . 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 sustain voltage V _s1 to all pixels of the thin film EL device until the sustain voltage V _s1 is applied across the thin film EL device.

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 T _v and T _h are used.
is added to the transistors 61' and 71'. Therefore, the write picture element has the sustain voltage V _s1 and the write auxiliary voltage V _we
are applied in a superimposed manner, and the written picture element emits light.

Finally, the signals T ₁ , T _v , and T _h 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. The feature is that the write auxiliary voltage V _we is applied after the sustain voltage V _s1 is applied to all picture elements.

Therefore, according to the above circuit configuration, the withstand voltage is reduced for the following reason.

(1) After applying the sustain voltage V _s1 to all picture elements of the thin-film EL element, when applying the write auxiliary voltage V _{we ,} first turn on transistors 22 and 31 and apply the sustain voltage to all picture elements. Since the thin film EL element has an insulating layer interposed between the electrodes to sandwich the fluorescent layer, it can be equivalently thought of as a capacitor.Therefore, even if the transistor 31 is turned off after applying the sustain voltage, the voltage across the thin film EL element remains unchanged. It maintains the maintenance voltage. Therefore, when the write auxiliary voltage V _we is applied, the on-breakdown voltage of the transistors 61' and 71' becomes the write auxiliary voltage V _we .

(2) After the write voltage V _w is applied to the write picture element, when the S point shown in Fig. 5 becomes 0 potential,
A write voltage V _w 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. , which prevents 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 breakdown 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 .

<Description of the present invention> The present invention provides a diode 62 in the above circuit configuration.
The purpose of the present invention is to provide a circuit system for a thin film EL element that is capable of eliminating the need for the writing voltage source 63 and changing the writing brightness for each selected picture element to perform halftone writing. That is. The reason why the diode 62 is not required is because the case (2) mentioned above as the reason why the withstand voltage is reduced does not occur.

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 FIGS. 6 and 7 indicate the same contents as in FIGS. 3 and 5, respectively. diode 62
has been abolished. 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 and the diodes 23, 24. This is a diode that is turned off when the common connection point A with the terminal rises to a high voltage such as a maintenance 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 DA converter 9
A transistor 93 is connected in common to the collector terminal of the transistor 92 and the base terminal of the emitter follower 94, and the common connection portion A resistor 94 is used to reduce the voltage at point C from the voltage of the write voltage source 63 in accordance with the output current of the DA converter 91, and to generate a voltage corresponding to binary data. This is the emitsuta follower added for this purpose.

Diodes 95, 95 are interposed on the collector side of the transistor 65 on a path connected to the base terminal of the emitter follower 94 and on a path connected to the diode 66. Therefore, when the transistor 65 is turned on, the base potential of the emitter follower 94 becomes zero, and the potential at the common ground point D of 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 diode 66 → capacitor 64 → diode 23 → transistor 52, and the voltage across the capacitor 64 is adjusted to an appropriate value. is set to the included 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. Voltage of the included voltage source (~
40v), it is possible to apply voltage 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.

In addition, in the above drive circuit, when the write voltage is applied to the write pixel to drive the write pixel and the write voltage is released, a charge corresponding to the write voltage remains in the write pixel, and this residual voltage is shown in FIG. is applied to point B of The residual voltage is almost the sustaining voltage V
V _s1 +V _we = superimposed of _s1 and write auxiliary voltage V _we
This is the high voltage value of _Vw . In this case, the transistor 22 is kept in a floating state due to the residual voltage by the diode 23, but the transistor 6
A residual voltage will be applied to the drain of 1'. However, since the write auxiliary voltage value V _we of the capacitor 64 that supplies the write auxiliary voltage is applied to the source of the transistor 61', the actual required withstand voltage between the source and gate of the transistor 61' is V _w - It is sufficient that V _we =V _s1 , and therefore, the floating diodes 62, 62 _, . I can do it. That is, the transistor 61' does not need to be floated due to residual voltage, and the number of diodes is greatly reduced.

Note that halftone writing can also be performed by changing the pulse width, but 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 is several microns even at a level level.
Due to the presence of a large transparent electrode resistance, even if the voltage is applied almost completely at the electrode lead-out portion, it 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, when the pulse width is narrow, variations in hfe (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.

The drive circuit has fewer diode arrays than the prior invention, which contributes to cost reduction and also reduces the mounting volume. 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 V _s1 but a voltage lower than the erase or read voltage by the write voltage V _we .

[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'::Writing 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 the second switching element is a thin film that performs halftone writing by applying the residual voltage of the charged charge written to the writing picture element to the drain side when it is off.
EL element drive circuit.