JP2001083924A - Drive circuit and drive method of current control type light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a drive circuit for uniformly gradation-controlling an EL that is a current driving element according to an external input signal by containing, in a buffer, a circuit for compensating the offset voltage that is the difference between the input terminal voltage of a buffer element and the voltage outputted from the output terminal of the buffer element. SOLUTION: Since the charging voltage of a load is VIN-Vt when no offset canceller is attached, the dispersion of threshold voltage Vt appears as an output deviation. However, when an offset canceller is attached, the charging voltage of the load is equal to the input voltage VIN and basically never influenced by the dispersion of the threshold voltage Vt. A circuit for compensating the offset voltage resulted from the dispersion of the threshold voltage Vt is built in a transistor for controlling the luminance of a light emitting element. Therefore, this drive circuit for light emitting element capable of providing a satisfactory image characteristic with a relatively small number of transistors can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving device for a light emitting element used for a display, and more particularly, to a current flowing through the element such as an organic and inorganic EL (electroluminescence) or an LED (light emitting diode). The present invention relates to a configuration and a driving method of a drive circuit of a current control type light-emitting element controlled by the device.

[0002]

2. Description of the Related Art Displays which combine light emitting elements such as organic and inorganic ELs or LEDs in an array and display characters using a dot matrix are widely used in televisions, portable terminals and the like.

[0003] In particular, these displays using self-luminous elements, unlike displays using liquid crystal, have features such as not requiring a backlight for illumination and having a wide viewing angle, and have attracted attention. I have.

[0004] Above all, an active matrix type display which performs static driving by combining a transistor or the like and these light emitting elements is higher in brightness, higher contrast and higher definition than a simple matrix driving display which performs dynamic driving. Etc., and has been attracting attention in recent years.

[0005] As a conventional example of this type of display, FIG. 7 shows a light emission quoted from the announcement of the Society for Information Display's 1997 Autumn Conference Proceedings "Asiadisplay '97", pp. 216-219 (Seiko Epson). 1 shows a light emitting element driving circuit of an active matrix display using EL as an element.

The principle of light emission in this drive circuit will be described with reference to FIG. When the scanning line 72 connected to the gate of the switching transistor 71 is selected and activated, the transistor 71 is turned on, and a signal is transmitted from the data line 73 connected to the transistor 71 to the capacitor 7.
4 is written. The capacitor 74 determines the gate-source voltage of the current control transistor 75.

When the scanning line 72 is not selected and the transistor 71 is turned off, the voltage between both ends of the capacitor 74 is held until the scanning line 72 is selected in the next cycle.

According to the voltage between both ends of the capacitor 74,
Power supply electrode 76 → Drain-source of transistor 75 →
A current flows along the path from the EL element 77 to the common electrode 78, and this current causes the EL element 77 to emit light.

In general, in order to display moving images on a computer terminal, a personal computer monitor, a television, or the like, it is desirable to be able to perform gradation display in which the luminance of each pixel changes.

In order to perform gradation display in the drive circuit of FIG. 7, it is necessary to apply a voltage near the threshold value between the gate and source electrodes of the transistor 75.

However, if the gate voltage-source current characteristics of the transistors have variations as shown in FIG. 8, for example, when a gate voltage VA is applied to the gate electrode of the transistor 75 in FIG. Since IA (intersection between the curve indicated by the solid line and VA) and IB (intersection between the curve indicated by the dashed line and VA), the current flowing through the EL element 77 also changes, and the area that should have the same luminance should be the same. Are different from each other, which causes image quality deterioration such as uneven brightness.

[0012] In a thin film transistor using polysilicon, the variation in the threshold value is generally larger than that in a transistor using crystalline silicon, and the value is estimated to be about ± 0.1 V.

If the threshold voltage varies by ± 0.1 V, the current flowing through the transistor 75 is 2 V
The current value fluctuates by about 5% when operating in the linear region, and by about 10% when operating in the saturation region. FIG. 9 shows current-luminance characteristics of the EL element. Since the current-luminance characteristic is a linear characteristic in the region A where the gradation is displayed, the variation of the current value directly appears as the variation of the luminance characteristic.

To solve this problem, Japanese Patent Laid-Open No. 2-14 / 1990
JP-A-8687 proposes an EL display device that performs gradation display without being affected by variations near the threshold value of the element.

With reference to FIG.
The circuit proposed in Japanese Patent Publication No. 7 will be described. FIG.
3 shows a circuit section corresponding to the current control circuit 79 in a dotted line, and shows an example of a case where 16 gradation display is performed. The number of data lines has been increased to four in order to perform gradation control.

In FIG. 10, reference numerals 94 to 97 denote transistors for driving light emitting elements, 98 denotes a current mirror circuit, and 99 denotes a current mirror circuit.
Is a light emitting element, and 100 is a resistance component of each source terminal of the transistor and a common electrode to which the light emitting element is connected. The drain electrodes of the transistors 94 to 97 are commonly connected and connected to the input terminal of the current mirror circuit 98.

In FIG. 10, a combination of signal voltages corresponding to gradations from 4-bit inputs is applied to transistors 94-9.
7 is applied as the gate voltage. Then, the same current value as the sum of the currents flowing through the ON transistors among the transistors 94 to 97 is supplied to the light emitting element 99 from the output terminal of the current mirror circuit 98, and the light emitting element 99 emits light according to the current value. I do.

For example, if the values obtained by taking the logarithms of the current values when the transistors 94 to 97 are turned on are respectively doubled (ie, I2 is twice I1 and I3 is twice I2 (= I
(2 times 1), I4 is 2 times I3 (= 23 times I1), and 16 gradations can be displayed by the combination of turning on the transistors 94 to 97. Note that I1
I4 represent source currents when the transistors 94-97 are on.

At this time, if the transistor is used at a voltage in a region where the current corresponding to the gate voltage VB shown in FIG. 8 is saturated, even if the characteristics of the transistor in the vicinity of the threshold value vary, the influence is affected. There is no luminance variation. However, when the number of gradations increases, the number of current mirror circuits increases, and the number of signal lines increases depending on bits, which complicates the driving circuit.

[0020]

As described above, in an active matrix type EL light emitting device, a current mirror circuit or a low current circuit, a current controlling transistor, and the like have hitherto been used to realize a gray scale display. It must be provided within one pixel. When considering production, etc.,
Providing a plurality of transistors in a pixel is expected to decrease the yield due to an increase in the failure probability of the transistor. In order to secure a high yield, a gray scale display is performed with a small number of transistors in one pixel. It is necessary to realize gray scale display with a limited number of transistors. In addition, as the number of transistors increases, the area of an effective portion related to light emission of the EL element decreases. In order to solve such a problem, it is necessary to propose a simple circuit configuration capable of correcting the variation in the threshold voltage of the current driving TFT in the driving circuit of FIG.

[0021]

SUMMARY OF THE INVENTION The present invention proposes a driving circuit for controlling the EL, which is the above-mentioned current driving element, without unevenness in accordance with the level of an external input signal. The specific circuit configuration is as follows.

A scanning line for selecting a pixel composed of a light emitting element whose luminance changes according to a current flowing through the element, and a data line for supplying a voltage for driving the pixel are arranged in a matrix on a substrate. A voltage for controlling the luminance of a light emitting element is supplied from a data line to an intersection of the scanning line and the data line, and a voltage of the data line is changed by a scanning signal given by the scanning line. A first thin film transistor for switching, an output terminal connected to the light emitting element, an output terminal of the thin film transistor for switching and an input terminal connected to the buffer element, an input terminal voltage of the buffer element, and the buffer element A circuit that compensates for the offset voltage, which is the difference between the voltages output from the output terminals of the It proposes a driving circuit of the box-type current-controlled light-emitting elements.

Generally, the buffer element is connected to the light emitting element by a source follower, and a compensation capacitor for canceling a variation in threshold voltage of a current control thin film transistor using the source follower, and a compensation capacitor for the compensation capacitor. It is considered that the configuration in which the switching circuit for storing the threshold voltage is incorporated is a simple configuration.

In this configuration, three basic configurations can be proposed.

(1-1) When the buffer circuit comprises an n-channel type thin film transistor which is connected to the input terminal of the light emitting element by a source follower,
(1-2) When the buffer circuit is composed of a p-channel type thin film transistor connected to the input terminal of the light emitting element and a source follower, (1-3) the source follower is an n-channel transistor and a p-channel transistor This is the case when the transistors have a push-pull connection configuration. As a proposal other than the source follower configuration as the buffer circuit, there is a proposal that “the buffer element is configured by a differential amplifier, a compensation capacitor for canceling an output offset of the differential amplifier, and a threshold voltage for storing a threshold voltage in the compensation capacitor. Circuit in which a switching circuit is incorporated ".

The above circuit configuration is proposed.

A timing chart applied to each node in each of the above structures and the operation of each transistor associated therewith will be described.

First, the operation of the source follower will be described.
Here, a case where the current control transistor is an n-channel transistor will be described.

(1) When the input voltage Vin is applied to the gate voltage, the transistor turns on and a large drain current flows at first to charge the load. However, when the load voltage increases, the source voltage of the transistor increases. Since the gate-source voltage gradually decreases, the drain current decreases. When the load is charged until the gate-source voltage reaches the threshold voltage Vt, the transistor turns off and charging stops.

(2) When the input voltage Vin is applied to the gate of the source follower, the load is charged up to Vin-Vt, and when the threshold voltage Vt varies, the output voltage appears as an output deviation as it is.

In the current polysilicon, the variation of the threshold voltage is in the range of about ± 0.5 V. Therefore, the variation of the threshold voltage appears as the variation of the luminance as it is. Next, the operation of the source follower with an offset canceller will be described with reference to FIG. The operation is roughly divided into three steps.

<First Step> Switches S1, S2, S
3 is turned on, the threshold detection voltage Vof is applied to the gate, and the source is set to the ground, so that the source follower transistor is turned on. At the same time, by turning on the switch S5, the load is discharged. The reason for resetting the load is that the load can be charged in the case of the Nch source follower, but the charge stored in the load cannot be discharged.

<Second Step> By turning off the switch S3, the current flowing through the transistor is reduced to zero. As a result, the source voltage of the transistor increases until the gate-source voltage becomes equal to the threshold voltage Vt. As a result, the threshold detection capacitor holds a value equal to the threshold.

<Third Step> When the switches S1 and S2 are turned off and the switch S0 is turned on, the input voltage V is applied to the gate of the transistor through the threshold detection capacitor.
in + threshold voltage Vt is applied. Therefore, the source voltage of the transistor becomes Vin, which is a value obtained by subtracting the threshold voltage from the gate voltage, and the load is charged to Vin by turning off the switch S5 and turning on the switch S4.

According to the above operation, when the offset canceller is not provided, the load charging voltage is Vin-Vt. Therefore, when the threshold voltage Vt varies, the output appears as an output deviation. The charging voltage of the load becomes equal to the input voltage Vin, and is basically not affected by the variation of the threshold voltage Vt.

FIG. 2 is a simulation result of a source follower with an offset canceller. The simulation conditions are as follows: (1) Threshold voltage variation is assumed to be ± 0.5 V (2) Threshold detection capacitance is 0.5 pF, threshold detection voltage Vof = 7.5 V (3) Input voltage = 8 0.0 V (4) One horizontal time = 30 μsec.

In the first step, the transistor of the source follower is turned on, but the output voltage (source voltage) does not become completely zero due to the ON resistance of the switch S3. In the second step, it can be seen that the output voltage varies due to the influence of the threshold variation. The difference between the threshold detection voltage Vof and the output voltage is equal to the threshold voltage Vt, and this value is stored in the threshold detection capacitor. In the third step, a value obtained by adding the threshold voltage Vt to the input voltage Vin is applied to the gate, so that the output voltage is substantially equal to the input voltage Vin regardless of the threshold voltage. The offset canceling ability is shown in the enlarged view in the lower part of FIG.
The output deviation is ± 10 for a threshold variation of 0.5V.
mV.

The above is the case where the current control transistor is an n-channel transistor, but it is also conceivable that the transistor is a p-channel transistor.

FIG. 4 shows this circuit configuration. The p-channel transistor has lower current driving capability than the n-channel transistor, but is more stable than the n-channel transistor in terms of transistor reliability. Basically, a similar offset canceling operation is possible in the case of the p-channel type, but since the p-channel source follower can only discharge the load, a precharge circuit for charging the load to the power supply voltage in advance is required.

In addition, a circuit in which an n-channel and a p-channel are push-pull connected can be proposed. Since the push-pull can charge and discharge the load, no charging circuit is required. FIG. 5 shows a configuration using a push-pull circuit.

Next, it is possible to use a differential amplifier circuit as the buffer configuration. Since the differential amplifier circuit receives a negative feedback as compared with the source follower circuit, the difference between Vin and Vout can be detected regardless of the cause of all offsets including the offset caused by threshold voltage variation. .

The operation of the differential amplifier circuit with an offset canceller will be described with reference to FIG.

<First Step> The two switches A are turned on to form a buffer circuit in which a detection capacitor is connected between the inverting input terminal and the non-inverting input terminal of the differential amplifier. It is assumed that the input voltage Vin is applied to the non-inverting input terminal of the differential amplifier, and the output voltage is Vout = Vin + ΔV. Since the buffer circuit is configured, the voltage of the inverting input terminal is also Vi.
n + ΔV. Therefore, the detection capacitor has an input voltage Vin of the non-inverting input terminal and an output voltage Vin + of the inverting input terminal.
ΔV, which is the difference between ΔV, is detected and held.

<Second Step> Since the switch A is turned off and the switch B is turned on, the output voltage is fed back to the inverting input terminal through the detection capacitor. Since the input voltage Vin is applied to the non-inverting input terminal as in the first step, the voltage at the inverting input terminal must be Vin + ΔV in order to maintain the same state in the internal circuit of the differential amplifier. Since a detection capacitor having a potential difference of ΔV is connected between the output terminal and the inverting input terminal, in order for the voltage of the inverting input terminal to be Vin + ΔV, the output voltage must be V
out = Vin.

By adding an offset canceller to the differential amplifier circuit by the above operation, it is possible to suppress the output deviation ΔV caused by various factors. The output voltage is always equal to the input voltage Vin.

FIG. 11 shows a simulation result of an operational amplifier with an offset canceller. The simulation conditions are as follows: (1) The push-pull type buffer section of the differential amplifier circuit (2) Output deviation detection capacitance = 1.0 pF (3) Input voltage = 7.0 V (4) Load resistance = 1.0 kΩ, load Capacitance = 20 pF (5) Assuming that 1 horizontal time = 30 μsec, the result of ± 0.5 V variation in threshold voltage is shown in FIG.
It is shown in FIG.

In the first step, the output voltage greatly varies with respect to the input voltage due to the variation in the threshold value. The difference is detected and corrected by the output deviation detecting capacitor, and the second step is the threshold. The output voltage is almost equal to the input voltage without being affected by the value variation. From the enlarged view at the bottom of FIG. 12, the offset canceling capability shows that the output deviation ±
It can be suppressed up to 5 mV.

[0048]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a circuit diagram of a first embodiment of the present invention, wherein a charge injection type organic thin film EL element (hereinafter abbreviated as "organic thin film EL element") is used as a light emitting element. It is when it was.

In FIG. 1, reference numeral 15 denotes an organic thin-film EL element which is a light-emitting element; 12, a resistance element for controlling a current flowing through the organic thin-film EL element 15; 14, a capacitor which keeps current flowing through the organic EL element; , A scanning line for supplying a scanning signal for selecting the switching transistor 13, and a scanning line 16 is turned on to supply a charge to the capacitor 14 via the selected switching transistor 13. A data line, 18 is a power supply electrode for supplying a current to the organic thin-film EL element 14, and 19 is a common electrode for determining the operating point of the transistor based on a potential difference between the data line 17.

The principle of light emission by the above driving circuit is as described above.

Polysilicon is melt-recrystallized by depositing amorphous silicon by a vapor phase growth method and performing laser annealing to form polysilicon. Phosphorus ions are implanted into the polysilicon by an ion doping method to form source and drain electrode portions of the transistor. The resistance element 12 is formed by the same process as that for forming the source and drain regions of the transistor. Example 1
In this example, the current control transistor is an n-channel transistor. Therefore, since a load cannot be discharged as a switching circuit, a circuit for resetting the charge of the load is provided.

(Embodiment 2) FIG. 4 shows a circuit configuration in Embodiment 2. In this embodiment, the current control transistor is a p-channel type. The light emission principle of this drive circuit is basically the same as that of the first embodiment. However, since the p-channel transistor can only discharge to the load, a precharge circuit for charging the load to the power supply voltage in advance is loaded.

(Embodiment 3) In Embodiment 3, the configuration of the source follower is an n-channel, p-channel push-pull configuration. The circuit configuration is the same as the circuit shown in FIG. In the push-pull configuration, both charging and discharging can be performed with respect to the load, but the power consumption is relatively large because the circuit scale is large and a steady current flows.

(Embodiment 4) In Embodiment 4, the buffer is configured using a differential amplifier. The circuit configuration is the same as the circuit shown in FIG. When a configuration using a differential amplifier is used, the circuit configuration is slightly more complicated than the above-described source follower configuration, but the level of the offset voltage that can be canceled is higher because the circuit itself has a feedback action. .

[0056]

As described above, according to the present invention, a circuit for compensating for the offset voltage caused by the variation of the threshold voltage of the transistor and the like of the luminance of the light emitting element is built in, and the number of transistors is relatively small. Thus, it is possible to realize a drive circuit of an active matrix type current control type light emitting element which can obtain good image characteristics.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is a diagram showing a simulation result of a source follower with an offset canceller;

FIG. 3 is an enlarged view of the result of FIG. 2;

FIG. 4 is a diagram showing a circuit configuration of a source follower with an offset canceller when a current control transistor is a p-channel type.

FIG. 5 is a diagram showing a case where a push-pull circuit is configured as a source follower circuit;

FIG. 6 is a diagram showing a differential amplifier circuit with an offset canceller.

FIG. 7 is a diagram showing a driving circuit of a conventional active matrix EL display.

FIG. 8 shows a gate voltage of a polysilicon thin film transistor.
Diagram showing source current characteristics

FIG. 9 shows current-voltage characteristics of an EL element.

FIG. 10 is a diagram showing an EL drive circuit for gradation display corresponding to 4 bits according to a conventional example.

FIG. 11 is a diagram showing a simulation result of a differential amplifier circuit with an offset canceller.

FIG. 12 is an enlarged view of the result of FIG. 11;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 12 Resistance element 13 Switching transistor 14 Capacitor 15 Organic thin film EL element 16 Scanning line 17 Data line 18 Power supply electrode 19 Common electrode

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Okada 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. (reference) 5C080 AA06 AA07 BB05 DD05 DD22 DD25 DD28 EE19 EE29 FF11 JJ03 JJ04 JJ05 KK43

Claims (7)

[Claims] 1. A scanning line for selecting a pixel composed of a light emitting element whose luminance changes according to a current flowing through the element, and a data line for supplying a voltage for driving the pixel are formed in a matrix on a substrate. A voltage for controlling the luminance of the light emitting element is supplied from a data line to an intersection of the scanning line and the data line, and a voltage of the data line is controlled by a scanning signal given by the scanning line. A first thin film transistor for switching a voltage, an output terminal connected to the light emitting element, an output terminal of the switching thin film transistor and an input terminal connected to the buffer circuit, and an input terminal voltage of the buffer circuit. And a circuit for compensating for the offset voltage, which is the difference between the voltage output from the buffer circuit and the output terminal of the buffer circuit. Ibumatorikusu type driving circuit of the current control type light emitting device. 2. A buffer circuit comprising a thin film transistor connected to an input terminal of a light emitting element by a source follower.
The buffer circuit includes a compensation capacitor for canceling a variation in threshold voltage of the current control thin film transistor connected to the source follower, and a switching circuit for storing the threshold voltage in the compensation capacitor. 2. A drive circuit for an active matrix type current control type light emitting device according to claim 1. 3. A buffer circuit comprising an n-channel type thin film transistor connected to an input terminal of a light emitting element and a source follower, and a switching circuit preloads a load before a threshold voltage is written to a compensation capacitor. A method for driving an active matrix type current control type light emitting element, characterized by discharging stored electric charge. 4. A buffer circuit comprising a p-channel type thin film transistor connected to an input terminal of a light emitting element and a source follower, wherein a load is previously set before a threshold voltage is written to a compensation capacitor by a switching circuit. A method for driving an active-matrix current-controlled light-emitting element, comprising precharging to charge to a power supply voltage. 5. The drive circuit for an active matrix type current control type light emitting device according to claim 2, wherein the source follower-connected transistors are n-channel transistors and p-channel transistors are push-pull connected. 6. A buffer circuit comprising a differential amplifier, a compensation capacitor for canceling an output offset of the differential amplifier, and a switching circuit for storing a threshold voltage in the compensation capacitor. Item 2. A drive circuit for an active matrix type current control type light emitting element according to item 1. 7. A scanning line for selecting a pixel formed of a light emitting element whose luminance changes according to a current flowing through the element, and a data line for supplying a voltage for driving the pixel are formed in a matrix on a substrate. A voltage for controlling the luminance of the light emitting element is supplied from a data line to an intersection of the scanning line and the data line, and a voltage of the data line is controlled by a scanning signal given by the scanning line. A thin film transistor for switching a voltage, an output terminal connected to the light emitting element, an output terminal of the thin film transistor for switching and an input terminal connected to the buffer circuit, an input terminal voltage of the buffer circuit, and the buffer A compensation circuit for compensating for an offset voltage which is a difference between voltages output from output terminals of the circuit. Current-controlled light-emitting element characterized in that the compensation circuit is incorporated in the buffer circuit.