JPS6015277B2 - Drive circuit for thin film EL element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a drive circuit that enables optical erasing of a thin film EL matrix element with a memory having various functions of electrical writing, electrical erasing, electrical logic, and optical writing. It is something.

The structure and characteristics of the thin-film EL matrix element with memory are described in Japanese Patent Application No. 83767/1983 filed by the present applicant entitled "Drive Circuit for Large Capacitance Display Element" (see Japanese Patent Application Publication No. 52-6412) and others. , I will briefly explain it again.

In a thin film EL display device, transparent electrodes are arranged in stripes on a glass substrate, and on this, for example, Y203, Si3N4, Ti0
2. Add a mysterious electric material such as Mt. 203, and add, for example, Mn on top.
A dielectric layer such as ZnS (yellow light emitting) doped with ZnS (yellow light emitting) is formed on top of the dielectric material such as Y, Si3N, TiQ, A03, etc. by thin film technology such as vapor deposition or sputtering.
It was deposited to a thickness of 10,000 Å to form a double-insulated three-layer structure, and a cotton-like electrode was placed thereon in a direction perpendicular to the transparent electrode to form a matrix electrode. 3 of such structure
In a layered thin film L display device, when one of the first electrode group and one of the second electrode group is selected and an appropriate alternating current voltage is applied, the two poles intersect and are sandwiched. A small area emits light. This corresponds to one picture element on the screen. By combining these, characters, symbol patterns, etc. are displayed.
ELs with this type of structure have superior characteristics compared to conventional dispersion type EL elements in terms of brightness, lifespan, and stability.
This EL newly exhibits hysteresis characteristics between applied voltage and luminance. When a pulse of voltage amplitude V, is initially applied, the brightness is at a low level brightness B,. Here, the maintenance voltage V. is V,>Vh, where Vth is the luminous threshold voltage.
When the sustaining voltage V, is continuously applied, the brightness B, is maintained. Next, when the pledge voltage V2 is applied, the brightness increases all at once to a high level of brightness &, and even if the voltage returns to the maintenance voltage V again within a certain period of time, the brightness remains the same as the previous level B, a higher brightness. It settles on the strings. The brightness B2 is maintained by continuously applying the sustaining voltage V. In this state, when the erase voltage V3 is applied next, the brightness level decreases rapidly, and when it is returned to the sustaining voltage V, it settles down to the previous low level of brightness B. 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. A major feature of this EL display device, which other display devices do not have, is that once a write voltage or erase voltage is applied, each picture element continues to emit light without losing the gradation given to it by the sustain pulse. It is.
The above pressures 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, =210V, V2=210~280V
, V3=190y. The present invention relates to a driving circuit for the above three-layer thin film EL display device, and a circuit according to an embodiment of the present invention is shown in FIG. 1, and will be described below. Reference numeral 10 denotes the thin film EL element, and here only column (X) electrodes X, -Xm made of transparent electrodes 11 and row (Y) electrodes Y, -Yn made of aluminum electrodes 12 are shown.

The thin film EL element has a reference electrode r used at the time of display and driving at the right end of the x electrode. 201 Positive sustaining voltage Vs to Y electrode
, is supplied from power line A, and transistor 2
1 and 22, and are connected to each electrode Y, to Yn via a diode.

A switching circuit connects the three electrodes X to ground, and a transistor 31 is connected to each electrode X, -Xm between the electrodes X, -Xm and the ground.

A sustain signal SUS4 and a read signal READ applied to line B are connected to the base of this transistor through a diode 3.
A write signal WRITE, an erase signal ERASE and a sustain signal SUS4, which are applied via lines C and C, are applied via an analog switch 33 operated by a horizontal binary address signal. These transistors 31, . . . act as switching elements for applying sustaining pulses to the entire surface of the thin film EL element, and also act as switching elements for selecting electrodes for writing, erasing, and logical logic. 40' is a circuit that supplies a positive sustaining voltage Vs from line D to all X electrodes,
Consisting of transistors 41 and 42 operated by a sustain signal SUS3 applied to line E, each electrode X, ~Xm
and are connected via diodes 43, . . . .

All five Y electrodes lead ~Yn to the ground,
Each electrode is connected via a diode 51, . . . to a transistor 52 operated by a sustain signal SUS2.

60 is a switching circuit for selecting Y electrodes &, ~Yn, and between each electrode and the voltage Vwer of power supply line F, high voltage switching transistors 61, . . . and diodes 62, . . . are connected, and the transistor 61 is controlled by a decoder 63 operated by a vertical binary address signal.

The decoder 63 is configured to directly drive the base of the transistor 61 with a high voltage transistor, or 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. be done. The power supply line F selectively outputs a write voltage, an erase voltage, and a read voltage in combination with the operation mode W of the thin film EL element by a circuit shown in FIG. The above voltage is applied to one of the two. 70 is a selection logic switching circuit which connects the collectors of the transistors 71, . . . to the X electrodes and selects the X electrode.

The emitters of the transistors 71, . . . are all connected to the output current detection resistor R. Transistor 71,... ... is selectively turned on/off of one transistor by an analog multiplexer 72 which receives a binary address signal and whose output is combined with its base. The RCA CD-4051 is suitable for this analog multiplexer. The analog multiplexer is composed of MOS transistors, and since MOB transistors are voltage-controlled elements, the input gate capacitance is as small as several PF or less, so no current flows after a small transient current flows. Moreover, since both ends of the output current detection resistor R are clamped by diodes connected in antiparallel, the potential drop at both ends does not exceed 0.7V. Therefore, the address input of the analog multiplexer is only 0.7 V lower than the output of the binary address circuit, and at this level there is no problem with the operation of the decoder in the analog multiplexer. In particular, if C-MOS is used, almost no current is required for logic operation, and only the base current of the selection transistor flows, resulting in very little power consumption. 80
is a logic circuit, and the transistor 81 outputs the signals SUS4 and READ obtained via the open collector buffer 82 when all the X electrodes are grounded during the start and sustain drive.
This transistor is turned on by the resistor R2 and generates a reference current in the resistor R2 to remove the erase current at the time of readout.

At this time, in order to prevent the base current of the transistor 81 from flowing to the current detection resistor R2, a constant voltage is always supplied to the capacitor C while the transistor 81 is off, and only when the transistor 81 is on, the capacitor C is connected to the resistor R2. The transistor 81 is turned on for a time corresponding to a time constant (=C, R3) formed together with R3, and the base current flows between the capacitor C and the base emitter of the resistor R3. The amplifier 83 receives the read current obtained through the resistor R, and the reference current obtained through the resistor R2, and removes the in-phase signal component, that is, the displacement current in the output current. The output of this amplifier 83 is connected to a bell detection and determination circuit to detect the light emitting state (written state and erased state) of the selected picture element of the thin film EL element. FIG. 2 shows a power supply circuit that supplies a sustain voltage, a write voltage, an erase voltage, and a read voltage in the circuit of FIG.
A sustaining voltage Vs, is supplied to.

The transistor 02 is constantly supplied with base current by a resistor 4 inserted between the collector and base of this transistor, and is always turned on. Also, transistor 10
A parallel circuit of a resistor R5 and a capacitor C2, and a full erase transistor 103 are inserted between the base of the transistor 2 and the ground.
The entire erase signal ALERA is applied to the base of the transistor 103.
When SE is applied, transistor 103 is turned on and the base of transistor 102 is grounded. Therefore, the voltage of the power supply lines A, 0 becomes below the sustain voltage, that is, below the erase voltage or the erase voltage. However, transistor 1
Since capacitor C2 and resistor R5 are connected to the base of transistor 102, the voltage drop between the collector and emitter of transistor 102 decreases according to this time constant (approximately several hundred skin seconds), and gradually recovers to the maintenance voltage Vs. . Since the power supply lines A and D combine the voltages to the line A of the circuit 20 and the line D of the circuit 40, the switching operations of the circuits 20, 30, 40, and 50 connect all of the thin film EL elements to the X electrode or the Y electrode. A sustain voltage Vs or an erase voltage is applied to the thin film EL element, and the thin film EL element alternately performs sustain drive and full erase drive. Transistor 104 is set by resistors R6 and R7 to provide an erase voltage on line 105.

Transistors 106 and 107 operated by the erase signal ERASE are inserted in the line 105, and provide an erase voltage to the wire line F of the circuit 60 when the erase signal is applied. When the erase signal ERASE is generated, a horizontal pipe read address signal is applied to the analog switch 33 to select the transistor 31, and a vertical binary address signal is applied to the decoder in order to select the X electrode and Y electrode containing the selected picture element. 63 and selects transistor 61. Transistor 108 provides an output voltage on line 109. Transistor 108 is turned on and off by transistor 110 operated by read signal READ. When the read signal READ is obtained, in order to select the X electrode and Y electrode that include the selected picture element, one of the transistors 61 is turned on and only one of the transistors 31 is turned off, operating as described above. , one of the transistors 71 is turned on. The logic output then appears at resistor R, which detects the read signal via common mode rejection amplifier 83. Transistor 111 is always on, transistor 112 is on except during writing, and charges capacitor C3 to the polarity shown.

When the write signal WRITE is applied, the inverter 11
3, the base potential of the transistor 112 is set to 0 potential to turn it off. On the other hand, the signal WRITE turns on transistors 114, 115, and 116 and raises one side of capacitor C3 to the potential of power supply 101, thus turning off transistor 111 and charging power supply 101 and capacitor C3 to line 117. Supply the total voltage of the voltage. Since the charging voltage of capacitor C3 is set to the difference between the write voltage and the voltage of voltage source 101, line 1
A write voltage is obtained at 17. When a write signal is obtained, in order to select the × electrode and Y electrode that include the selected picture element,
Operating as described above, one of the transistors 61 and one of the transistors 31 are turned on. As described above, the thin film E
The L element is subjected to electrical sustain drive, full erase drive, selective erase drive, selective write drive, and selective output drive by the circuits shown in FIGS. i and 2. Among the above circuits, the present invention particularly provides a circuit for optical erasure driving.

That is, in the circuit of FIG. 1, a diode 201 is inserted between the collector of the transistor 22 of the circuit 20 and the ground, and a diode 202 is inserted between the collector of the transistor 42 of the circuit 40 and the ground.

These diodes create a path from the ground to the thin film EL element and facilitate the photo-erasing operation. Next, the optical erasing operation will be explained with reference to the time chart in FIG. When the voltage across the EL element is 0, pulse a of sustain pulse SUS2 and pulse b of SUS4
, the transistors 31, . . . , between the X electrode and the ground of the thin film EL element 10, and the transistor 52 between the Y electrode and the ground are turned on to short-circuit the X electrode and the Y electrode. .

When the above pulses a and b are applied at the same time, a trigger pulse c of the xenon lamp (not shown) is output,
The thin film EL element is irradiated with light from the xenon lamp. Due to this light irradiation, the polarized charges accumulated at the interface of the EL layer are
It is relaxed and erased by photoconduction in the L layer. The relaxation current due to photo-erasing is generated by the transistors 31, .・・・．・・・
．．・In addition to flowing through the diodes 201 and 2
02, and in the present invention, the polarization relaxation current circuit is connected to transistors 31.02. By forming a circuit that flows from the thin film EL element to the ground by . Polymer is formed to facilitate optical erasure. During this light erasing, the thin film 5L is
Both ends of the element are connected to a diode 201, a transistor 31,...
. . . Since the diode 202 and the transistor 52 are short-circuited, electrical input, erasure, and output cannot be performed. Therefore, these operations must be controlled to be prohibited during optical erasure, but it is not possible to maintain them. The pulse frequency is about 500Hz, whereas the repetition frequency of a normal xenon lamp is limited to about 40Hz, so even when light erasing is performed continuously, there is only one cycle of light erasing, and the next writing etc. As a practical matter, other operations for light erasing will not be delayed by much.

The operation of the thin film EL element other than optical erasing will be explained with reference to the time chart of FIG. 3. In the optical writing drive, when the sustain driving pulse is applied, the xenon trigger pulse d is applied, and optical writing is performed.

Note that when performing this optical writing drive, there are no pulses a and b, and a time chart is shown by dotted lines in the portions of pulses a and b. If the xenon lamp irradiates the entire surface of the thin film EL element with light during the optical erasing and optical writing driving described above, simultaneous erasing and writing of the entire surface are performed.

Furthermore, if the xenon lamp partially irradiates light onto the thin film EL element, partial extinction and partial optical writing will occur. 'When performing read drive, logic pulse R
EAD occurs during the absence of sustain pulses and provides an initiation pulse on line F.

At the same time, the decoder 63 operates to turn on the transistor 61 connected to the Y electrode including the selected picture element. Further, the analog multiplexer 33 operates to turn on the transistor 31 connected to the electrode other than the X electrode including the selected picture element,
Further, the analog multiplexer 72 operates to turn on the transistor 71 connected to the X electrode including the selected picture element, and supplies an output to the readout circuit 80 . When the selection pulse is supplied, a pulse voltage e is applied to the selected picture element,
No voltage is applied to non-selected picture elements. When electrically writing or erasing, the pulse WRITE or ER
When ASE occurs, a write pulse voltage g or an erase pulse voltage f is applied to line F by the decoder 63 and analog multiplexer 33 only to the selected picture element.

In FIG. 3, a pulse h is applied to the circuits 30 and 50 after the sustain pulse is applied, and after the write and erase pulses are applied. and to discharge the charge remaining due to the application of these pulses.
This is to prevent breakdown of the thin film EL element.

In addition, there is a small time margin between the sustain, write, erase, optical write, and optical erase operations, but this is because the transistor cannot turn off immediately due to the influence of the base charge accumulated in the base. This is because accumulated charges are released. As described above, the present invention constructs an optical erasing circuit by adding a small number of noisy components to a circuit capable of electrical writing, erasing, maintenance, and optical writing driving to form a simple circuit. Therefore, the functions of the thin film EL element can be increased at low cost.

Note that the diodes 201 and 20 added for the present invention
2 forms an effective circuit during photo-erasing drive, and since this diode is connected with the opposite polarity to the voltage applied during other drives, it will not adversely affect the operation of transistors 22 and 42. do not have. In the circuit of the present invention, it is also possible to insert the diode 210 into the circuit of FIG. 2 to form a path from the ground to the thin film EL element via lines A and D, but in this case, the transistor It becomes necessary to turn on transistors 22 and 42 at the same time, and it is also necessary to prevent a sustaining voltage from being applied to lines A and D during this optical erasing, so the base drive circuit of transistors 22 and 42 becomes complicated, and the first This is not a good idea compared to the embodiment shown in the figure.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of one embodiment of the circuit of the present invention, Fig. 2 is a power supply circuit for applying each required voltage to each part of Fig. 1, and Fig. 3 is a timing diagram for explaining optical erasing drive. It is a chart. 10: Thin film EL element, ×, ~Xm: X electrode, Y, ~Yn
: Y electrode, 20: Sustaining voltage supply circuit, 30: Switching circuit, 40: Sustaining voltage supplying circuit, 50: Earth circuit, 60: Switching circuit, 70: Selective readout switching circuit, 80: Readout circuit, 201, 202: diode. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. In a thin film EL device circuit comprising a matrix type thin film EL device that exhibits a hysteresis phenomenon in applied voltage and luminance characteristics, and a circuit for electrical writing, erasing, and sustain drive, a positive sustain voltage is applied from the Y electrode side. The first Y applying
The electrode side switching circuit and the second grounding circuit for the X electrode.
a third X-electrode side switching circuit that applies a positive sustaining voltage from the X-electrode side;
and a fourth Y electrode side switching circuit that grounds the Y electrode, and a connection point between the first Y electrode side switching circuit and the Y electrode and the third X electrode side switching circuit and the X electrode side switching circuit.
A diode is connected between the connection point of the electrode and the ground in the opposite direction to the polarity of the sustaining voltage, and the second X electrode side switching circuit, the fourth Y electrode side switching circuit, and the above diode are connected during photo-erasure driving. , a drive circuit for a thin film EL element, comprising two circuits for guiding all electrodes of the thin film EL element to ground.