JPH0638360B2 - Driving method for thin film EL display device - Google Patents

Description

The present invention relates to an AC drive type capacitive flat matrix display panel, that is, a drive circuit of a thin film EL (electroluminescence) display device.

<Outline of the Invention> In the present invention, an EL layer is provided between a scanning side electrode and a data side electrode arranged in a direction intersecting with each other, and the data side electrode is divided into an odd number side and an even number side. In the driving method of the thin film EL display device, wherein the writing voltage or OV is applied to the scanning side electrode and the modulation voltage or OV is applied to the data side electrode, the data side electrode is modulated. A voltage is applied, and a writing voltage is applied to the scanning-side electrode after a period until the modulation current due to the application of the modulation voltage finishes flowing in the EL layer, thereby reducing the current flowing in the EL layer during writing driving. However, it is intended to reduce the brightness unevenness that occurs in each horizontal picture element due to the voltage drop of the data side electrode resistance.

<Prior Art> For example, a double-insulation (or three-layer structure) thin film EL element is configured as follows.

As shown in FIG. 5, a strip-shaped transparent electrode 2 made of In ₂ O ₃ is provided in parallel on a glass substrate 1 and, for example, Y ₂ O ₃ , Si
₃ N ₄ , Al ₂ O _{3 and} other dielectric substances 3, Mn and other activator-doped ZnS EL layer 4, and Y ₂ O ₃ and Si ₃ N as described above
Dielectric materials 3'such as ₄ , TiO ₂ and Al ₂ O ₃ are sequentially laminated to a film thickness of 500 to 10000Å using thin film technology such as vapor deposition and sputtering to form a three-layer structure, on which the transparent electrode is formed. A strip-shaped back electrode 5 made of Al ₂ O ₃ is provided in parallel in a direction orthogonal to ₂ .

Since the thin film EL element has the EL material 4 held between the electrodes by the dielectric materials 3 and 3 ', it can be regarded as a capacitive element in terms of an equivalent circuit. Further, as is clear from the voltage-luminance characteristics shown in FIG. 6, this thin film EL element is driven by applying a relatively high voltage of about 200V. This thin film EL element has the characteristics that it emits light with high brightness due to an AC electric field and has a long life.

Conventionally, for the display device using such a thin film EL element, the present applicant has a transistor for applying a loadable voltage to the data side electrode as a drive circuit of the scanning side electrode for the purpose of reducing modulation power consumption. And a transistor for applying a positive voltage to the scanning side driver IC, and a transistor for charging the EL layer with a modulation voltage and a transistor for discharging the EL layer as a driving circuit for the data side electrode, and a current direction opposite to each transistor. A data side driver IC with a diode connected in the direction is provided, and modulation driving is performed using a transistor that charges and discharges according to display data on the data side, while field inversion driving is performed using an Nch transistor and a Pch transistor on the scanning side. Do not reverse the polarity of the write waveform applied to the picture element for each scanning line. By performing et line sequential driving, the driving time of one horizontal period is short, moreover proposed a driving apparatus good AC pulse symmetry is excellent can reliability applied to the EL layer.

<Problems to be Solved by the Invention> However, in the case of realizing the above drive device,
Since the data side electrode is made of a transparent electrode such as In ₂ O _3, it has a relatively large resistance compared to the aluminum of the scanning side electrode, and the data side electrode is divided into an odd number and an even number and alternately taken out from both ends. In the case of the thin film EL display device described above, at both end portions of the thin film EL element, picture elements closer and farther than the output of the data side driver IC, that is, data side driver IC
From the output to the picture element, the picture element having a small electrode resistance and the picture element having a large electrode resistance are arranged in the lateral direction. When a writing voltage is applied to one horizontal line from the scanning side in this configuration, the actual voltage applied to each pixel is the voltage drop due to the In ₂ O ₃ resistance even if the output of the data side driver IC is the same potential. Makes a difference. The potential difference between the picture elements directly affects the brightness unevenness and deteriorates the display quality.

In the conventional driving, the timing of applying the modulation voltage from the data side and the timing of applying the writing voltage from the scanning side were substantially the same. When the modulation voltage is applied, the entire capacitance of the EL element becomes a load, so a considerably larger current flows than when the write voltage, which is a capacitive load for one horizontal line, is applied.
For this reason, when the modulation drive and the write drive are simultaneously performed as in the conventional drive, the current flowing through the data side electrode becomes large, and the potential difference between the picture elements due to the voltage drop of the electrode resistance is large, and thus the uneven brightness is also large. There was a problem called.

Therefore, in the present invention, when the write voltage is applied, the modulation voltage is applied and the potential on the data side is fixed as in the conventional driving, but the timing of applying the modulation voltage is not the same as the application of the write voltage, but earlier than that. By applying the modulation voltage, the modulation current ends in the EL layer, that is, by applying the write voltage from the scanning side after charging is completed, the current flowing in the EL layer at the time of writing is reduced, causing the voltage drop of the data side electrode resistance. The present invention aims to improve display quality by eliminating uneven brightness.

<Means for Solving Problems> In the present invention, an EL layer is provided between a scanning side electrode and a data side electrode arranged in a direction intersecting with each other, and the data side electrode is an odd number side. In the driving method of the thin film EL display device, the writing voltage or OV is applied to the scanning side electrode, and the modulation voltage or OV is applied to the data side electrode. A modulation voltage is applied to the data-side electrode, and a writing voltage is applied to the scanning-side electrode after a period until the modulation current due to the application of the modulation voltage ends in the EL layer.

<Operation> By applying the write voltage after the modulation current has finished flowing through the EL layer, the current flowing through the data side electrode when the write voltage is applied is reduced. When the current decreases, the voltage drop due to the electrode resistance on the data side also decreases, and the voltages applied to the respective picture elements become uniform, so that the uneven brightness is eliminated.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a drive circuit block diagram showing an embodiment of the present invention.

Reference numeral 10 denotes a thin film EL display device having a light emission threshold voltage of 190 V (Vw). In this figure, only the electrodes are shown with the X-direction electrode as the data side electrode and the Y-direction electrode as the scanning side electrode.

Reference numerals 20 and 30 denote scanning-side high breakdown voltage driver ICs (corresponding to a first switching circuit, hereinafter referred to as scanning-side driver ICs) corresponding to the odd lines and the even lines of the Y-direction electrodes of the thin film EL display device 10, respectively. A transistor NTodd for applying a negative voltage and a transistor PTodd for applying a positive voltage to the data side electrode are connected to the odd-numbered line, and a current flows in the opposite direction to each of the transistors NPodd and PTodd. Diodes NDodd and PDodd for are connected. Further, a transistor NTeven for applying a negative voltage and a transistor PTeven for applying a positive voltage to the data side electrode are connected to the even line, and a current flows in the opposite direction to each of the transistors NPeven, PTeven. Diodes NDeven, PD for flowing
even is connected. Reference numerals 21 and 31 are logic circuits such as shift registers in the scan side driver ICs 20 and 30.

Reference numerals 40 and 50 denote data-side high breakdown voltage driver ICs (corresponding to a second switching circuit, hereinafter referred to as data-side driver ICs) corresponding to the X-direction electrodes of the thin film EL display device 10. One side of each line is modulated. Switching element UT such as Pch MOSFET, thyristor, PNP type transistor, etc. connected to a power supply and having a pull-up function
_1- UTi (hereinafter simply referred to as transistor), Nch MOSFET having a pull-down function with one side grounded,
Switching elements DT _{1 to} DTi (hereinafter simply referred to as transistors) such as thyristors and NPN type transistors, and diodes UD to UDi, DD ₁ to allow current to flow in the opposite direction to the respective transistors UT and DT.
DDi, each of which is a logic circuit 41, 5 such as a shift register of the data side driver IC 40, 50.
Controlled by 1.

Reference numeral 100 denotes a circuit (corresponding to a third switching circuit) for switching the pull-down common line potential of the scan side driver ICs 20 and 30. The control signal "NSC" causes a negative write voltage of -160V (-Vw + 1 / 2Vm) and OV.
It is composed of a switch SW1 for switching to.

Reference numeral 200 denotes a circuit (corresponding to a fourth switching circuit) for switching the pull-up common line potential of the scanning side driver ICs 20 and 30 and switching to the positive write voltage 220V (Vw + 1 / 2Vm) and OV by the control signal "PSC". It is composed of the switch SW2.

The 300 switches the switch SW4 to "O" by the control signal "T2".
N ", and the capacitor Cm has a 1/2 modulation voltage of 30 V (1
/ 2Vm) and after charging, the switch SW4 is turned "OFF" by the control signal "T2" and the control signals "T1", "T".
By turning on the switches SW3 and W5 by "3", the modulation voltage 60V (V
m) is a circuit (corresponding to the fifth switching circuit) for supplying the switch SW controlled by the control signal "T3".
It is connected to the data side driver IC 40 through 5.
On the other hand, after the thin film EL display device 10 emits light, the control signal "T
It is also a circuit for storing a charge in the capacitor Cm through the diode Dr as a switch by turning on the switch SW4 by "2" to turn on the switch SW4.

Reference numeral 400 is a data inversion control circuit.

Next, the operation of FIG. 1 will be described with reference to the time chart of FIG.

Here, it is assumed that the scan electrode of Y ₁ including the picture element A is selected as the selective scan electrode by the line sequential drive. Further, in this driving device, the polarity of the write voltage applied to the picture element is inverted for each line to be driven, but the pull-down transistor NTn of the scanning side driver ICs 20 and 30 connected to the scanning side selection electrode is driven. Only one line is turned on, and the drive timing of one line for applying a negative write pulse to the picture element on that electrode line is called N drive timing.
On the other hand, the driving timing of one line for turning on only the pull-up transistor PTn of the scanning side driver ICs 20 and 30 connected to the selection electrode and applying a positive write pulse to the picture element on that electrode line. Will be referred to as P drive timing.

Data side drive is basically display data "DATA"
Accordingly, the voltage applied to each data side line is switched between Vm and OV in a cycle of one horizontal period.

Next, the switching timing will be described.

As shown in FIG. 2, the data is latched by the control signal "DLS" after the data transfer of one line, and even if the transistors UT and DT of the data side driver IC 40 are "ON" by the latched data, The modulation voltage Vm is not immediately applied, and the fifth switching circuit 300
The stepwise charging is performed from the potential ½ Vm to the potential Vm, the power consumption during modulation is reduced to 3/4, and the potential 1
At the time of / 2Vm, a part of the electric charge accumulated in the EL layer is charged to the external capacitor Cm through the diode Dr and is reused as a part of the driving energy when the modulation voltage Vm is applied next. Power consumption is reduced.

The operation of such a drive circuit is roughly divided into two types of timings, that is, an NP field and a PN field. By completing the execution of these two fields, all the picture elements of the thin film EL display device emit light. This is to close the AC pulse required for. Further, each field (screen) is composed of two types of timings, N drive and P drive. In the NP field, N drive is performed for odd-numbered select lines on the scanning side and even-numbered select lines are driven. Execute P drive. In the PN field, the reverse driving is performed. Further, the N drive and the P drive are each constituted by a discharge period and a writing period.

Next, each drive period will be described.

(A) NP field 1. Discharge period in N drive (TN ₁ ) Switch SW by control signals "NSC" and "PSC"
1 and SW2 are set to "OFF" and the common line is set to OV.
All transistors N of scan side driver ICs 20 and 30
By turning off T and PT, all electrodes on the scanning side are brought into a floating state. At this time, on the data side, the switches SW3 and SW5 are turned off by the control signals "T1" and "T3", and the switch SW is turned on by the control signal "T2".
When "4" is turned "ON", a part of the electric charge accumulated in the EL layer is charged in the capacitor Cm through the diode Dr and also from the 1/2 Vm power source through the diode Dm.
Then, the electrode potential connected to the selected picture element of the data side electrode becomes 1/2 Vm. After that, the control signal "DLS" is input, and the transistor U of the data side driver IC 40 is
T, "ON" of the DT, the switch SW5 by the control signal "T3" when "OFF" is switched to "OFF", the modulated power supply Vcc ₂ floating. At the same time, when all the scanning side transistors PT and NT are turned "ON", the electric charge of the EL layer is discharged and the scanning side electrode potential becomes OV.

2. Write period in N drive (TN ₂ ) The data side driver IC 40 continues to drive in the discharge period (TN ₁ ) in the N drive described above, and the modulation power supply Vcc controls the control signal "T 1" by the fifth switching circuit 300.
The switch SW5 is turned "ON" by 3 "to switch the floating state to the potential 1/2 Vm, and then the control signal" T ".
2 ”turns switch SW3“ OFF ”and control signal“ T ”
The switch SW4 is turned "ON" by 1 "to raise it to the potential Vm. After the charging current due to Vm has finished flowing to this modulation voltage, only the driver connected to the selected scan electrode turns on the transistor NTn, and other The transistors NT and PT of the scanning side driver ICs 20 and 30 are all set to "OFF".
The switch SW2 is turned "OFF" by the control signal "PSC" and OV is applied to the pull-up common line of 30. Further, the pull-down common line of all driver ICs 20 and 30 on the scanning side has a negative write voltage −Vw by turning on the switch SW1 by the control signal “NSC”.
+1/2 Vm is applied.

As a result, the electrode potential including the selected picture element on the data side is Vm.
Becomes At the same time, since the negative write voltage -Vw + 1 / 2Vm is applied to the selected scan electrode, Vm-(-Vw + 1 / 2Vm) = Vw + 1 / 2Vm is applied to the selected picture element to emit light. Further, since the data-side electrode potential of the non-selected picture element is OV, and the negative write voltage −Vw + 1 / 2Vm is applied to the selective scan electrode as described above,
OV-(-Vw + 1 / 2Vm) = Vw-1 for non-selected picture elements
/ 2Vm is applied, but it does not emit light because it is less than the light emission threshold voltage Vw.

As for the picture elements on the scanning side non-selection electrode line, since the scanning side electrode is in a floating state, it changes from OV to 60V depending on the ratio of the selection electrode and the non-selection electrode on the data side.

3. Discharge period in P drive (TP ₁ ) Data side driver I according to the inverted data of display data
C40 transistors UT and DT are "ON" and "OFF"
Other than that, the same driving as in the discharge period (TN ₁ ) in the NP field N driving is performed.

4. Write period (TP ₂ ) in P drive The data side driver IC 40 continues to drive in the discharge period (TP ₁ ) in P drive described above, and the fifth switching circuit 300 causes the modulation power supply Vcc ₂ to control signal “T”.
The switch SW5 is turned "ON" by 3 "to switch the floating state to the potential 1/2 Vm, and then the control signal" T ".
2 ”turns switch SW3“ OFF ”and control signal“ T ”
The switch SW4 is turned "ON" by 1 "and pulled up to the potential Vm. After the charging current by the modulation voltage Vm has finished flowing in the EL layer, only the driver connected to the selective scan electrode turns the transistor TPn" ON ", All the transistors UT and PT of the other scanning side driver ICs 20 and 30 are turned off.
The switch SW2 is turned "ON" by the control signal "PSC" and the positive write voltage Vw + 1/2 Vm is applied to the pull-up common lines of 0 and 30. OV is applied to the pull-down common line of the scanning side driver ICs 20 and 30 by turning off the switch SW1 by the control signal "NSC".

As a result, the electrode potential including the selected picture element on the data side is OV.
Becomes At the same time, since the positive write voltage Vw + 1 / 2Vm is applied to the selective operation electrode, (Vw + 1 / 2Vm) -OV = Vw + 1 / 2Vm is applied to the selected picture element, which has a polarity opposite to that of the N drive write voltage. It is applied with and emits light. Further, since the data-side electrode potential of the non-selected picture element is Vm and the positive write voltage Vw + 1 / 2Vm is applied to the selected scan electrode as described above, the non-selected picture element has (Vw + 1 / 2Vm). Although -Vm = Vw-1 / 2 Vm is applied, it does not emit light because it is equal to or lower than the light emission threshold voltage Vw.

(B) PN field 5. Discharge period (TP ₃ ) in P drive The same drive as in the discharge period (TP ₁ ) in NP field P drive is performed.

6. Write period in P drive (TP ₄ ) NP except that the selection electrode on the scanning side is selected from the odd number side
The same driving as in the writing period (TP ₂ ) in the field P driving is performed.

7. Discharge period (TN ₃ ) in N drive The same drive as the discharge period (TN ₁ ) in NP field N drive is performed.

8. Write period in N drive (TN ₄ ) NP except that the selection electrode on the scanning side is selected from the even side
The same driving as in the writing period (TN ₂ ) in the field N driving is performed.

By the way, in the conventional driving, the timing of applying the modulation voltage from the data side and the timing of applying the writing voltage from the scanning side were substantially the same. However, in the driving method in this embodiment, the data side is clamped by the modulation voltage when the writing voltage is applied, but the timing of applying the writing voltage from the scanning side is set after the modulation driving, and It is performed after the modulation current has finished flowing to the EL layer (the current stops when the charging current flows for a certain period because it is a capacitive load).

FIG. 3 shows an equivalent circuit of the EL element.

FIG. 4 is a diagram showing the difference in each waveform between the conventional drive (a) and the drive (b) of the present invention. Points A and B correspond to points A and B in FIG. As shown in FIG. 4 (a), in the conventional driving, the data-side electrode X of the same modulation voltage Vm (= 60V) is used.
1 and X2, the modulation voltage Vm and the write voltage (-Vw + 1/2 Vm) are applied at the same timing, so that the current i flowing through the data side electrodes X1 and X2 and the electrode resistance r.
The voltage applied to the picture element at the point B due to the voltage drop due to becomes a waveform in which the rising portion is more blunt than the picture element at the point A. Therefore, the picture element at point B has a darker brightness than the picture element at point A.

On the other hand, as shown in FIG.
Since the modulation current i, which is the same as the current shown in (a), is delayed by the period when the current ends, there is no change in the voltage waveform applied to the picture elements at points A and B. Actually, the current due to the write voltage flows at this time, but the write current in which the capacitance of one horizontal line becomes the load is much smaller than the modulation current in which the total capacitance of the EL element becomes the load, and it is a value that does not substantially affect the brightness. Is.

<Effects of the Invention> As described above, according to the present invention, it is possible to significantly reduce the unevenness in brightness caused by the voltage drop due to the resistance of the data side electrode of the thin film EL element, and to improve the display quality. A driving method of an EL display device can be provided.

[Brief description of drawings]

FIG. 1 is a drive circuit diagram of a thin film EL display device showing an embodiment of the present invention, FIG. 2 is a time chart explaining the operation of FIG. 1, FIG. 3 is an equivalent circuit of an EL element, and FIG. (a) is a waveform diagram showing each waveform of the conventional drive, FIG. 4 (b) is a waveform diagram showing the waveform of the drive circuit according to the present invention, FIG. 5 is a partially cutaway perspective view of the thin film EL display device, and FIG. The figure is a diagram showing the luminance characteristics with respect to the applied voltage of the thin film EL display device. 10 ... Thin-film EL display device, 20, 30 ... Scan side high breakdown voltage driver IC (first switching circuit), 40 ...
Data side high voltage driver IC (second switching circuit), 100 ... Scan side high voltage driver pulldown common line potential switching circuit (third switching circuit), 200
...... Pull-up common line potential switching circuit (fourth switching circuit) of scanning side high voltage driver, 300 ...... Modulation voltage step charging circuit (fifth switching circuit), 400 ...
… Data inversion control circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeyuki Harada 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Co., Ltd. (56)

Claims (1)

[Claims] 1. An EL layer is provided between a scanning side electrode and a data side electrode arranged in a direction intersecting with each other, and the data side electrode is divided into an odd number side and an even number side, and alternately from both ends. In the method of driving a thin film EL display device, wherein a write voltage or OV is applied to the scanning side electrode and a modulation voltage or OV is applied to the data side electrode, the modulation voltage is applied to the data side electrode. A method for driving a thin film EL display device, characterized in that a writing voltage is applied to the scanning-side electrode after a period until the modulation current due to the application of the modulation voltage finishes flowing in the EL layer.