JPS6133197B2 - - Google Patents

Description

[Detailed Description of the Invention] <Summary> The present invention provides a three-layer thin film EL having hysteresis memory characteristics in the relationship between applied voltage and luminance.
Regarding a memory reading device for a display device, the present invention provides a circuit that can perform reading, writing, and erasing with only one system of switches, thereby simplifying the circuit.

<Description of prior art> A three-layer structure thin film EL display device has transparent electrodes arranged in stripes on a glass substrate, and on this, for example,
A dielectric material such as Y ₂ O ₃ , Si ₃ N ₄ , TiO ₂ , Al ₂ O ₃ , etc. is further layered on top of the dielectric material, for example, a fluorescent layer such as Mn-doped ZnS (yellow light emitting) is further added on top of the dielectric material such as Y ₂ O 3 , Si 3 N 4 , TiO 2 , Al 2 O 3 , etc. ₃ , _{Si3N4 ,}
A dielectric material such as TiO ₂ or Al ₂ O ₃ is deposited to a thickness of 500 to 10,000 Å using thin film techniques such as evaporation or sputtering to form a double-insulated three-layer structure, and the above-mentioned transparent electrode is placed on top of this. Striped electrodes are arranged in orthogonal directions 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, symbol patterns, etc. are displayed.

ELs with this type of structure have superior characteristics compared to conventional distributed EL elements in terms of brightness, lifespan, and stability, but this EL has a new feature called hysteresis between the applied voltage and luminance. Show characteristics. To explain this, when a pulse with the darkest initial voltage amplitude V ₁ is applied, the brightness is at the level of brightness B ₁ . Here the maintenance voltage
When V ₁ is the emission threshold voltage Vth, V ₁ <Vth.
The brightness B ₁ is maintained by continuous application of the maintenance voltage V ₁ . Then when the write voltage V ₂ is applied, the brightness is B ₃
After that, even if the voltage returns to the maintenance voltage _V1 within a certain period of time, the brightness will settle down to a brightness _B2 which is higher than the previous brightness _B1 . The brightness B ₂ is maintained by continuous application of the maintenance voltage V ₁ . In this state, when the erase voltage V ₃ is applied next, the brightness level decreases rapidly, and when it is returned to the sustain voltage V ₁ again, the brightness level settles to B ₁ . This hysteresis phenomenon can take any small loop depending on the amplitude, pulse width (not shown), and pulse frequency of the write voltage. That is, it is also possible to display halftones. In this way, once a write voltage or erase voltage is applied, each pixel continues to emit light without losing the gradation given by the sustain pulse.
This is a major feature of this EL 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 =
210-280V, _V3 = 190V.

This three-layer thin film EL display device can not only perform writing, erasing, and memory using the applied voltage, pulse width, and pulse frequency as described above, but also can read out the memory state electrically.

Thin-film EL devices sandwich the fluorescent layer between double insulating layers, so they can be thought of as capacitive devices, and a displacement current flows when a sustaining voltage is applied, but when the device is memorizing the light-emitting state, By applying the sustaining voltage, a current corresponding to the magnitude of the luminance of the emitted light flows superimposed on the displacement current. This current is called a polarization current. Actually, even in the erased state, there is a slight rise in the background, and a correspondingly small amount of polarization current flows. For example, when a sustaining voltage pulse as shown in FIG. 1a is applied to an EL element, the current flowing through the element exhibits a displacement current waveform as shown by the solid line 11 in FIG. 1b in the erased state. However, in the light emitting state, as shown by a broken line 12, a waveform obtained by superimposing a polarization current is shown.

In order to determine the polarization current, the inventors of the present invention used Japanese Patent Application No. 101718/1983 entitled "Method for Detecting Memory Contents of Light Emitting Elements" (see Japanese Patent Application Laid-open No. 28446/1983).
A specific point in time when the current flowing through the EL element is present (for example, if there is a polarization current, the point at which it reaches its peak)
proposed a method and a circuit for detecting whether a light emitting state or an erasing state is present based on whether or not the displacement current exceeds a certain predetermined level including a noise margin.

However, since this method separates the displacement current and polarization current at a specific level, careful consideration is not only required in setting the specific level, but also because the magnitude of the displacement current is similar to the magnitude of the polarization current. As the value increases compared to , the S/N deteriorates accordingly.

In addition, a current similar to the above displacement current is connected to a capacitor,
It is conceivable to eliminate the erase current by configuring an erase equivalent circuit using analog elements such as resistors, or by storing the erase waveform itself in a ROM or the like. However, in this method, when the EL element is a single unit, there is only one waveform at the time of erasing, and the above idea can be applied, but in the case of a matrix element, there is the following problem.

In the case of matrix driving, depending on the driving method, as the number of matrices increases, the displacement current increases significantly due to causes such as rotation. Therefore, it is difficult to detect polarized current with good S/N ratio.

In order to efficiently guide the light emitted from the thin film EL surface to the outside, at least the electrode on the display side is made of a transparent electrode, but the resistance of this electrode is The electrode line width is narrow and the resistance is high, and there is a large difference in resistance value between the vicinity of the lead wire extraction part and the display electrode tip part far from the lead wire extraction part, and the effective applied voltage changes greatly. Therefore, even if the same pulse is applied from the outside, the current waveform changes greatly depending on the readout position, and it becomes complicated to have a corresponding number of equal value circuits for erasing, and to prepare waveforms for erasing. This makes it large and uneconomical.

In view of the above points, the inventors of this case
On April 27th, we filed an application for ``driving device for matrix-type thin film EL display device''. In this method, a reference electrode is prepared in a part of the display device, and the current flowing through this reference electrode is used as the above-mentioned erasing current, thereby canceling out the displacement current and extracting only the polarization current. be.

In addition, in order to eliminate wraparound, the levels of all the electrodes at non-selected points that were left floating are fixed. In other words, it is grounded.

The circuit of the above application will be explained below with reference to FIG. This circuit consists of six major blocks,
The first block is a three-phase resonance sustain drive circuit 10, which has two sustain voltage sources E ₁ and -E 2 and three switches SW ₁ , SW ₂ , and _{SW 3 that} operate in sequence to supply three-phase resonance to the EL display device 50 . voltages E ₁ , O, −E ₂ . The transformer T forms a series resonant circuit together with the capacitive component of the EL display device 50, and supplies voltage with high efficiency.

The second block is a write/read switch circuit 20, which switches the write voltage Vw from the power source Ew to the line Y to be written in the write phase by switching Wsk (k=
1 to m). Further, when reading is desired, the switch circuit applies a read voltage Vr from the read drive circuit Er to the line Y to be read during the read phase via the switch RS.
The read drive circuit Er is a circuit for applying a voltage equal to the sustain voltage Vs.

The third block is a switch group 30 provided on a transparent electrode constituting the horizontal scanning electrode of the EL display element. This switch group 30 is all short-circuited during sustain driving, and only the desired X line is turned on during write and erase driving, and the others are turned off. Conversely, during read drive, only the desired X line is turned off, and the others are turned on and grounded.

The fourth block is write and read separation and sustain amplitude holding circuit 40. This is a diode circuit for separating the write line and non-write line and at the same time for maintaining resonance drive amplitude.

The fifth block is a matrix type three-layer EL display device, and only the electrodes are shown in this figure. The electrode at the far end of the panel on the transparent electrode side is prepared as a reference electrode r.

The sixth block is a switch group 60 for selecting a pixel point to be read out from among the horizontal scanning electrodes, turning on the switch of the electrode including the pixel point to be read out, and turning off the others.

The seventh block is a readout circuit 70, which cancels out the voltages appearing across the resistor R1 commonly connected to the horizontal scanning electrodes ₁ to n and the resistor _R2 connected to the reference electrode r using a common-mode signal removal amplifier 71, and generates only a polarization current. Extract and output.

The specifications of the 6-inch EL panel prototyped by the inventors are as follows.

Line pitch: 2 lines/mm X lines (transparent electrodes) 240 lines Y lines (AI electrodes) 180 lines Display characters: 5 x 7 dots, 64 types of Roman letters, Arabic numerals, and symbols can be displayed and the start and end points can be specified. , _vector type graph _display with _linear interpolation between E ₁ is applied to the capacitive element (in this figure, the entire EL panel is approximately considered as a capacitive element Ct with constant capacitance), and the first holding potential is VS ₁ = E ₁ + η (B ₁ − V _H ) ……………(1) holds. Similarly, at the second timing _φ2 , the second
When the holding switch SW ₂ is closed, the second holding potential becomes −VS ₂ = −E ₂ −η(V ₁ −E ₂ ) ……………(2), and then at the third timing φ ₃ . When the third holding switch SW ₃ is closed, the third holding potential becomes V _H =ηV ₂ (3). In this way, three-phase sustaining drive is realized.

The three-phase sustain drive can reduce the withstand voltage requirements of the switches DS ₁ to n by performing writing at an intermediate hold potential (in this embodiment, the third hold potential V _H ).

Writing is performed by selecting the X and Y sides of the write picture element M (i, j) using the write and read switch circuit 20 and the switch circuit 30, respectively, and applying the write voltage Vw during the intermediate holding potential ( _V H period). and do it.

Erasing is performed in the same manner as writing, except that an erase voltage (not shown) Ve is applied to the selected picture element.

Next, read driving will be explained.

During read driving, the side switch WSi corresponding to the read selection point M(i, j) is selected and closed, and
In the side switch group 30, the switch DSj is turned off and the others are turned on, and in the switch group 60, the switch RSj is turned on and the others are turned off.
This switch state is shown in FIG. Thus, the read pulse voltage Vr is applied to the selection point M. The other electrodes of the horizontal scanning electrodes are switch group 30.
is grounded. A pulse current generated by the read pulse voltage Vr appears in the resistor R ₁ and is supplied to the + terminal of the amplifier 71. At the same time, the reference point P of the reference electrode r
A reference current of (i, j) appears at resistor R ₂ . Since the reference line r is never written to, it always outputs only an erase current, that is, a displacement current. This is supplied to one terminal of the amplifier 71. However, in the amplifier 71, only the displacement current at the readout pixel point is connected to the resistance R ₂
When the read pixel point is in the write state, only the polarization current is output. When the readout pixel points are in the erased state, they are all canceled out, so the output is 0.
becomes.

Since the horizontal scanning electrodes other than the readout selection point M are grounded by the switch group 30, they are fixed at zero potential and do not generate any wrap-around capacitance.
It can be read easily.

In the embodiment shown in FIG. 2, only one reference electrode is prepared, but in a thin film EL display device, the extraction electrodes of the horizontal scanning electrodes are alternately led out to the upper and lower sides due to the electrode density. Two reference electrodes may be prepared so as to match the direction of the extraction electrode, and a current obtained from the reference electrode in the same direction as the extraction electrode may be used.

<Problems to be solved by the present invention> The circuit according to the invention of the earlier application described above has two systems of switches for reading, writing, and erasing, namely, a horizontal scanning electrode switch group 30 and a read pixel point selection switch group 60. We are prepared.

The present invention provides a circuit that can perform reading, writing, and erasing with just one system of switches, and can obtain read data from all horizontal scanning electrodes simultaneously to increase speed.

A problem that arises when a single system of switches is used is readout malfunction due to mutual interference between the readout pixel points and other points during the readout operation.

By the way, the equivalent circuit of a thin film EL front is most simply represented by a capacitor, and this is sufficient for the following discussion. When reading is performed by connecting one system of switches and a current detection resistor R to each electrode, the equivalent circuit at the time of reading is shown in FIG. Here, when a read voltage is applied to the vertical scanning electrode i, interference of signal components from other electrodes as viewed from the horizontal scanning electrode j will be considered.

All the vertical scanning electrodes except the i-th vertical scanning electrode are at the same potential and can be combined into one, so the circuit in FIG. 3 can be rewritten as shown in FIG. 4. Readout selected picture element M(i,
j) The worst condition when reading the contents of point is when all the pixels on the i-th vertical scanning electrode except for the read-selected pixel M(i, j) point have been written. . At this time, (polarization current) + (erasing current) is present at both ends of the current detection resistor R connected to these electrodes.
flows. Since all erasing currents are the same, it is only necessary to consider the polarization current. When a polarization current id(t) flows through each horizontal scanning electrode, a voltage id(t)·R is generated across the resistor R. This can be replaced by a voltage source with a signal source impedance R and a voltage id(t)·R according to Thepnan's theorem. Since we are currently considering signal interference between electrodes, the capacitance (m-1) C that contributes to interference is
is the capacitance C connected to the electrode to which the read voltage is applied.
Since it is sufficiently larger, the circuit shown in FIG. 4 can be written as shown in FIG. And since n≫1 and m≫1,
Capacitance (n-1) (m-1)・C and resistance R/n-1 can be ignored in the circuit compared to capacitance (m-1)・C and resistance R, and in the end, the equivalent circuit can be written as shown in Figure 6.

In other words, the equivalent circuit shows that a current equal to the write current at the point (i, j) is supplied to the erase point from another write point through the capacitor (m-1)·C. Moreover, since the coupling capacitor is (m-1)·C, as the number of picture elements increases, the interference becomes more dense, which is increasingly disadvantageous.
Therefore, the problem cannot be improved simply by changing the value of the resistor R, and it is necessary to detect the current by setting the potential difference between the ground and the electrode to 0.

The reading device of the present invention is based on the above considerations, and will be described below along with examples.

<Preferred Embodiment> FIG. 7 shows a circuit diagram of a reading device according to an embodiment of the present invention.

In this circuit diagram, the resonance sustaining drive circuit 10 for supplying sustaining voltage, write and read separation, and sustaining amplitude holding circuit 40 are omitted to simplify the drawing. Also, the power supply for writing Ew and the power supply for erasing Ee are omitted. In the figure, 80 is a switch group consisting of switches DS ₁ to DSn for selecting picture element points for writing and erasing. Further, 90 is an amplifier A ₁ to 90 which is a feature of the present invention.
An is a current-voltage converter, and one terminal of the switches _DS1 to DSn is -
Connected to the terminal. FIG. 8 shows a circuit diagram of a circuit 91 connected between the - terminal and the output terminal of the amplifier.
The negative terminal of this amplifier can be considered as a virtual zero potential point by connecting a feedback circuit as shown in FIG. At this time, the + terminal of the amplifier is grounded. Therefore, for the electrodes X ₁ to Xn whose writing and erasing points are selected by the switches DS ₁ to DSn, one terminal of the amplifiers A ₁ to An can be viewed as being connected to ground by the virtual 0 potential point. can.

On the other hand, in the amplifiers A ₁ to An, the - terminal is set as a virtual zero potential point as described above, and the current that flows into or out of this - terminal flows to the output terminal through the circuit 91, thereby changing the voltage value. It can be converted to and extracted. Therefore, when reading, all the switch groups 80 are turned on, and the current flowing through the horizontal scanning side electrodes X ₁ to Xn is transferred to the amplifiers A ₁ to An.
Convert to voltage and output. The read current of the reference electrode r is similarly converted into a voltage by the amplifier Ar.

The output of the amplifier group 90 is supplied to the detection circuit 100 and added to the + terminal of the differential amplifier CNi. Also, the output of the reference electrode amplifier Ar is the − of the differential amplifier CNi.
is applied to the terminal, and only the polarized current is output by the differential amplifier. That is, the current obtained in the reference electrode r (since the picture element belonging to the reference electrode is never written, it is an erasing current, that is, a displacement current) is removed.

This differential amplifier is provided for each horizontal scanning side electrode X ₁ to Xn, and each negative terminal has an amplifier Ar.
The outputs of the horizontal scanning electrodes are respectively inputted, and the polarization current of each horizontal scanning electrode is read out and output from the output terminal.

The thus read output is applied to a level correction and read signal determination circuit 110. In this circuit 110, the signal is applied from the amplifier CNi to the + terminal of an analog comparator 111, and the output of the D/A comparator 112 is applied to the - terminal of the analog comparator 111. The output of the analog comparator 111 is applied to an AND gate 113 for extracting a signal only when a read pulse is applied to one terminal. This and gate 1
The output of 13 is applied to input terminal C of flip-flop 114 for holding read data.
The output of the vertical scanning side electrode m is used for a level correction operation by a control microcomputer 115, and this control microcomputer 115 applies data to set a value to the D/A converter 112 via a data line 116.

To explain the operation of the circuit 110, when the sustain pulse Ps is applied and the read pulse Pr is applied, a selected pixel current is obtained at the selected pixel electrode.

The selected pixel current is determined by the analog comparator 111.
, it is compared with the analog output of the D/A converter and outputs "1" and "0". This output is output via AND gate 113. The output of the AND gate 113 causes the flip-flop 11 to
4 and maintain the readout output. Note that this flip-flop is set so that a clear pulse is always applied to the terminal Cr before reading, so the flip-flop is set according to the level of the read current.

<Effects of the Invention> In the present invention, during reading, all the switch groups 80 are turned on at the same time, and read data can be obtained simultaneously from all horizontal scanning electrodes, so that reading can be performed at the highest speed.

[Brief explanation of the drawing]

Fig. 1 is a waveform diagram of displacement current and polarization current, Fig. 2 is a circuit diagram of a readout device according to the invention of the prior application, Fig. 3 is an equivalent circuit diagram of a thin film EL element during readout operation, Fig. 4,
5 and 6 are simplified equivalent circuit diagrams of the circuit in FIG. 3, FIG. 7 is a circuit diagram of a reading device according to an embodiment of the present invention, and FIG. 8 is a part of the circuit in FIG. 7. A circuit diagram is shown. 80 is a switch group, 90 is an amplifier group, and 100 is a detection circuit.

Claims (1)

[Claims] 1. In a thin film EL display device in which a fluorescent layer sandwiched between dielectric layers is interposed between electrodes arranged in a matrix shape, at least one of which is a transparent electrode, and a hysteresis phenomenon appears in the applied voltage and luminance characteristics, vertical scanning is possible. A write circuit, an erase circuit, and a read circuit that supply a write voltage, an erase voltage, and a read voltage through a group of switches are connected to the side electrode, respectively, and a virtual 0 potential point is connected to the horizontal scanning side electrode. The virtual 0 potential point of each amplifier is provided with an amplifier that converts the current input to the virtual 0 potential point into a voltage and outputs the voltage, and the virtual 0 potential point of each amplifier is connected to the horizontal scanning side electrode via one switch group. A drive device for a matrix-type thin film EL display device, characterized in that the input terminals of the potential points are connected, and the output terminals of the respective amplifiers serve as output terminals for the readout output converted to voltage when the readout voltage is supplied. .