JPS624717B2 - - 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 improving driving speed and executing 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 layer of dielectric material such as the above is deposited to a thickness of 500 to 10,000 A using a thin film technique such as vapor deposition or sputtering, and a double-insulated three-layer structure is formed. A striped back electrode is arranged on the surface of the substrate 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 combining these, characters, symbols, patterns, etc. are displayed.

EL elements with this structure have superior characteristics compared to conventional distributed EL elements in terms of brightness, lifespan, and stability, but this EL element also has new characteristics that improve the applied voltage and light emission. 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 sustaining voltage V ₁ is set to be V ₁ >V _th , where V _th is the light emission threshold voltage. 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. B ₁
The brightness settles to _B2 , which is greater. 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, 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 EL 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, V _th = 200 V, V ₁ = 210 V, V ₂ = 210 ~
The values are 280V and V ₃ = 190V.

This thin film EL pulse drive circuit 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,
Connect via 23, .

Reference numeral 30 denotes a circuit that leads all the X electrodes to ground during sustain drive, and is composed of a transistor 31 that operates in response to a sustain signal _T4 , and 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 a high voltage P-type switching transistor 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.

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 of the electrodes X ₁ to X _n between the electrodes X ₁ to _{X n} 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 pinary address signal. These transistors 71, . . . act as switching elements for selecting electrodes during writing, erasing, and reading.

The operation of this drive circuit will be explained with reference to the time chart shown in 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 display 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 24 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 → discharges the residual charge in the circuit.

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 X electrode and the X electrode transistors 61 and 71 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 0 V, so the withstand voltage of the transistors 61, 71, 71, . requires a write voltage Vw _or higher, 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 points, in particular, 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. 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, a power supply 63' generates a write auxiliary voltage V _we , which has a relationship between the write voltage V _w and the sustain voltage V _s1 as V _we =V _w −V _s1 . power supply 6
3' 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 voltage across the thin film EL device reaches the sustain voltage V _s1 .

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 sustain voltage V _s1 and the write auxiliary voltage V _we are applied in a superimposed manner to the write picture element, and the write picture element emits light.

Finally, the signals T ₁ , T _v , T _h are set to 0 and the symbols 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 withstand voltage of the transistor 61' is higher than the write auxiliary voltage _Vwe , 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 sustain voltage V _s1 is about 210 volts, and the write auxiliary voltage V _we is about 35 volts.

The above circuit configuration is based on a point sequential method in which one point is written in one write operation.

<Description of the present invention> The present invention uses the above circuit configuration to ground the selected X electrode, and when applying write, erase, and read voltages to the selected Y electrode, the voltages are applied to the non-selected Y electrodes as well. , by applying 1/2 voltage value of each voltage above, Y
It is an object of the present invention to provide a drive system that can reduce the withstand voltage of an electrode selection switch transistor by approximately half, eliminate the influence of wrap-around capacitance, enable line sequential scanning, and perform halftone writing and erasing.

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 time chart for explaining the operation of the embodiment shown in FIG.

The same reference numerals in Figure 6 refer to Figures 3 and 5, respectively.
Shows the same content as the figure. 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.

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 ground point D of the emitter follower 94 and the diode 66 becomes zero.

The diode 101 is connected to the diode 24, the diode 102 is connected to the diode 66, the transistor 103 is connected to the transistor 94, and the transistor 1
04 acts in the same way as the transistor 92, the voltage source 105 acts in the same way as the voltage source 63, and the resistor 106 acts in the same way as the resistance 93 during the write operation. Since the resistor 106 and the resistor 93 generate the same write auxiliary voltage,
It is necessary to set the resistance values to equal values.

107 is a switching transistor that is turned on during writing and increases the potential at point F to V _s + (voltage charged in capacitor 110);
8 is a diode that turns off when the potential at point F rises and exceeds V _s and disconnects V _s from point F; 109 turns on when the capacitor 110 is charged with the writing auxiliary voltage; a transistor that forms one path for grounding one end of the capacitor 110 and charging the capacitor 110 to a write auxiliary voltage;
10 is a capacitor for holding a write auxiliary voltage, and 111 is a diode that is turned on only during writing and disconnects the capacitor 110 from point F at other times.

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. The timing signals are shown in FIG.

(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 61 → 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 auxiliary voltage, leakage etc. can be ignored and it can be used as is. Hold voltage.

(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.

In FIG. 7, A is a charge signal appearing at the base terminal of the transistor 109, which is transmitted from the transistor 103 to the diode 10.
This is a signal for configuring a write auxiliary voltage charging section along the path 2→capacitor 110→transistor 109. Further, B is a voltage waveform across the capacitors 64 and 110.

In the above circuit configuration, transistors 92,1
04 is the same current source, that is, the output current of the DA converter 91, and it is necessary to divide the current into two through transistors 92 and 104, so V _BE , hfe
It is preferable to use a pair type with a matching number. If V _BE and hfe differ, the current distribution will be uneven depending on the degree, and therefore the capacitors 64,
This causes an imbalance in the charge and pressure of 110,
It has a negative effect on non-selected picture elements.

When configuring an electrode designation logic circuit that determines which transistor to turn on in the transistor group 61', it is advantageous to use CMOS in a floating state in terms of space and power consumption. It is necessary to insert a fixed voltage source (about 10V). In this case, unless the voltage of the voltage source 105 is set to the sum of the voltage of the application source 63 and the fixed voltage, the relationship of V _t =1/2V _w will not be satisfied, resulting in voltage imbalance. But transistor 9
In both cases, when a constant base current is applied, the collector current remains almost constant even if the collector voltage changes, so no problem occurs. The withstand voltage of transistor 61' is 1/ of the maximum write auxiliary voltage to the EL element.
2, and around 18V is sufficient.

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.

That is, based on the change in halftone pulse width, the pulse width that provides the lowest brightness even at about 8 levels is about several microseconds, and due to the presence of a large transparent electrode resistance,
Even if the voltage is almost completely applied to the electrode lead-out portion, there is a risk that the voltage will not be applied to the tip portion, resulting in increased 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 hfo (gm in MOS) of the electrode selection transistor have a large effect on the pulse width, resulting in considerable variation in the output pulse. 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 maintenance voltage is applied to all pixels, and the charging voltage of the capacitor for writing is applied only to the writing pixels.
At the same time that a write voltage is applied to the write picture element to perform writing, a sustain voltage is applied to the other picture elements so that they can be sustain-driven.

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

By writing to the X electrodes at the same writing level on the selected Y electrode at once, one line can be written only once (set gradation - 1) times, no matter how many Since writing of a halftone image can be completed, the present invention is very effective for displaying still images or slow-moving moving images. (Writing to the lowest level is unnecessary.) 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 _ws .

[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 schematic circuit diagram of FIG. 3. FIG. 6 is a basic circuit diagram showing one embodiment of the present invention, and FIG. 7 is a time chart for explaining the operation of FIG. 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 driving circuit for a thin-film EL element having an EL layer interposed therein and exhibiting 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, a first capacitor and a second switching element between the first switching element and the sustaining voltage source; a circuit that maintains the first switching element in an on state after application of a sustaining voltage, and superimposes the charged voltage of the first capacitor on the sustaining voltage by turning on the second switching element; between the switching element and one electrode of the matrix electrode, and includes a second capacitor and a third switching element, and the third switching element is turned on in synchronization with the turning-on operation of the second switching element.
a circuit for selectively turning on the switching element of the second capacitor to superimpose the charging voltage of the second capacitor on a superimposed voltage of the sustaining voltage and the charging voltage of the first capacitor; and a circuit for converting each charging voltage value of the capacitor and the second capacitor and making each charging voltage the same voltage value, and passing the selection electrode from the one electrode to the other electrode to perform the maintenance. A write voltage in which the voltage and the charging voltage of the first and second capacitors are superimposed is applied, and a voltage in which the sustaining voltage and the charging voltage of the first capacitor are superimposed is applied through the non-selected electrode, respectively. A drive circuit for a thin film EL element, which is characterized by executing writing.