JP2003150106A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem wherein a residual image phenomenon is some times seen when optical elements for which high luminance data are set, are rewritten to low luminance data. SOLUTION: When a scanning line SL is made high for writing luminance data, a transistor Tr1 is made into ON state; a transistor Tr3 is made into OFF state; data voltage is supplied from a data line DL and a holding capacitor SC and a gate electrode of a transistor Tr2 are set to a data potential corresponding to the luminance data. Here, an organic light emitting diode OLED is shut off from a power supply line Vdd by the transistor Tr3, therefore, the potential across both ends of the organic light emitting diode OLED is decreased to a value determined by the shut off period, the time constant of the organic light emitting diode, and the potential difference immediately before the shut-off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device, and more particularly to a technique for improving the display quality of an active matrix display device.

[0002]

2. Description of the Related Art The spread of notebook personal computers and portable terminals has been rapidly increasing. Currently, liquid crystal displays are mainly used for these display devices.
Organic E is expected as a next-generation flat display panel
It is an L (Electro Luminescence) display. The active matrix drive system is central to the display method of these displays. A display using this method is called an active matrix type display, and a large number of pixels are arranged vertically and horizontally to show a matrix shape, and a switching element is arranged in each pixel. Video data is sequentially written for each pixel by the switching element.

Research and development of organic EL displays are in their infancy, and various pixel circuits have been proposed. As an example of such a circuit, a pixel circuit disclosed in JP-A-11-219146 will be briefly described with reference to FIG.

This circuit includes two transistors Tr11 and Tr12 which are n-channel transistors, an organic light emitting diode OLED which is an optical element, and a storage capacitor SC1.
1, a scanning line SL, a power supply line Vdd, and a data line DL for inputting brightness data.

The operation of this circuit is as follows:
In order to write the brightness data of the LED, the scanning line SL becomes high, Tr11 is turned on, and the brightness data input to the data line DL is set in the transistor Tr12 and the storage capacitor SC11. When the scanning line SL becomes low at the timing of light emission, the transistor Tr11 is turned off, the gate voltage of the transistor Tr12 is maintained, and light is emitted with the set brightness data.

[0006]

When the brightness data of the optical element is large, even if an attempt is made to set a small brightness data by rewriting the brightness data, charges corresponding to the previous large brightness data remain without being removed from the optical element. In some cases, the afterimage phenomenon cannot be seen because the luminance data cannot be set accurately. In particular, the image becomes very difficult to see when displaying a moving image.

The present invention has been made in view of these circumstances, and an object thereof is to propose a new circuit for reducing the above-mentioned afterimage phenomenon.

[0008]

One aspect of the present invention relates to a display device. This device is a display device in which brightness data to be set for an optical element is held in the form of a control voltage, and a writing period for changing and setting the brightness data again and charge generated at both ends of the optical element. And an initialization period for discharging. The write period and the initialization period may be generated at the same time by sharing a signal for activating them. Generally, as these signals, a scanning signal input to a scanning line as an update signal of brightness data can be assumed. Further, the writing period and the initializing period may be generated at the same time, and the path for supplying the current to the optical element may be cut off during those periods. Further, the initialization period may be set arbitrarily by dividing the signal for activating the writing period and the signal for activating the initialization period.

Assuming an active matrix type display device, one pixel is generally composed of an optical element, a drive circuit, a data line, a scanning line and a power supply line. Also,
A path connected in series with the drive circuit, the optical element, and the ground potential is formed from the power supply line, and a current having a desired value is supplied to the optical element. Here, a switching element is provided between the drive circuit and the power supply line or between the drive circuit and the optical element, and the switching element is turned off during the writing period in which the luminance data is changed and set again. The discharge of electric charges is promoted, and the optical element is initialized. Further, the switching element, the optical element, and the drive circuit may be connected in series in this order from the power supply line.

Here, an organic light emitting diode can be assumed as the optical element, but the invention is not limited to this. As a switching element, a MOS (Metal Oxide Semiconductor) transistor or a thin film transistor (TFT: Thin Film Transistor) is used.
r) can be assumed, but the intention is not limited to this. Also,
The "luminance data" is data relating to the luminance information set in the driving transistor and is distinguished from the light intensity emitted by the optical element.

As another form, a switching element is provided between the drive circuit and the power supply line, and a switching element is provided from the anode of the optical element to the ground potential to form a bypass. By turning on this switching element during the writing period when the brightness data is changed and set again,
Initialization of the optical element is performed by extracting the electric charge of the anode of the optical element to the ground potential and setting the potential of the anode of the optical element to the same potential as the ground potential.

Another aspect of the present invention also relates to a display device. In this device, one pixel includes an optical element, a driving transistor, and a power cutoff transistor, and a power source is supplied while a data update instruction signal for writing the luminance data of the optical element to the driving transistor is activated. By turning off the cutoff transistor, the current supply path to the optical element is cut off, and after the period for writing the brightness data to the driving transistor is completed, the power supply cutoff transistor is turned on to cut off the current supply path. Connect. By blocking the current supply path, the electric charges generated at both ends of the optical element are discharged.

It should be noted that any combination or rearrangement of the above components is also effective as an aspect of the present invention.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the following embodiments, an active matrix type organic EL display is assumed as a display device. In the embodiment, when the brightness data of the optical element is rewritten, the electric charge of the optical element is discharged and the optical element is initialized.

Embodiment 1 FIG. 1 shows a circuit of one pixel of a display device according to an embodiment. This pixel includes an organic light emitting diode OLED which is an optical element, transistors Tr1 and Tr3 which function as switching elements, a transistor Tr2 which is a driving transistor in which luminance data is set, a storage capacitor SC, and a transistor Tr1.
A scanning line SL for turning on / off each Tr3 is provided. The transistor Tr1 is an n-channel transistor, and the transistors Tr2 and Tr3 are p-channel transistors. Further, the data line D to which the brightness data is input
L and a power supply line Vdd for supplying a current to the organic light emitting diode OLED. Scan line SL, data line D
L and the power supply line Vdd are shared with other pixels. In addition, a part of the circuit configured by the transistors Tr1 and Tr2 and the storage capacitor SC is particularly called a drive circuit 10.

The gate electrode of the transistor Tr1 is connected to the scanning line SL, and the drain electrode of the transistor Tr1 is connected to the data line DL. The source electrode of the transistor Tr1 and the gate electrode of the transistor Tr2 are connected to one electrode of the storage capacitor SC. The source electrode of the transistor Tr2 and the other electrode of the storage capacitor SC are the power supply line Vdd
In addition, the drain electrode of the transistor Tr2 is connected to the source electrode of the transistor Tr3. Transistor Tr3
And the anode of the organic light emitting diode OLED are connected at node A. The cathode of the organic light emitting diode OLED is connected to the ground potential. Therefore, the transistor Tr2, the transistor Tr3, and the organic light emitting diode OLED are arranged in this order in the power supply line Vdd.
To a ground potential to form a path (hereinafter also simply referred to as a main path) which is connected in series and causes the optical element to emit light.

The operation of the circuit configured as above will be described. When the scanning line SL becomes high for writing the brightness data, the transistor Tr1 is turned on and the transistor Tr3 is turned off, a data voltage corresponding to the brightness data is supplied from the data line DL, and the storage capacitor SC and the transistor Tr2 are supplied.
Luminance data is set to the gate electrode of. Here, since the organic light emitting diode OLED is cut off from the power supply line Vdd by the transistor Tr3, the potential difference between both ends of the organic light emitting diode OLED depends on the cutoff period, the time constant of the organic light emitting diode OLED, and the previous potential difference. It falls to the voltage decided. The potential of the node A at this time may be at a level that does not hinder the image display.

Next, when the writing of the brightness data is completed, the scanning line SL becomes low, the transistor Tr1 is turned off, and the transistor Tr3 is turned on, a current corresponding to the voltage set in the gate electrode of the transistor Tr2 and the storage capacitor SC. Flow into the organic light emitting diode OLED.

According to the first embodiment described above, when writing the brightness data, the potential difference between both ends of the organic light emitting diode OLED decreases as described above. It is possible to eliminate the afterimage phenomenon seen when setting.

Embodiment 2 FIG. 2 shows a circuit of one pixel of a display device according to Embodiment 2. This pixel includes an organic light emitting diode OLED, which is an optical element, and a driving circuit 10.
And holding capacitors SC1 and SC2, and transistors Tr4 and Tr3 functioning as switching elements. The drive circuit 10 further includes transistors Tr1 and Tr2.

The data line D to which the brightness data is input
L, a power supply line Vdd for supplying a current to the organic light emitting diode OLED, and a scanning line SL to which a signal for updating the brightness data is input. The data line DL, the power supply line Vdd, and the scanning line SL are shared with other pixels.

Also, the transistors Tr1, Tr2, Tr
4 is an n-channel transistor, which is a transistor Tr
3 is a p-channel transistor.

Gate electrodes of the transistors Tr1, Tr4, Tr3 are connected to the scanning line SL. Transistor Tr
The drain electrode of 1 is connected to the data line DL and the transistor Tr.
The source electrode of 1 and the gate electrode of the transistor Tr2 are connected to one electrode of the storage capacitor SC2. The source electrode of the transistor Tr2, the anode of the organic light emitting diode OLED, and the other electrode of the storage capacitor SC2 are connected to the drain electrode of the transistor Tr4. The cathode of the organic light emitting diode OLED and the source electrode of the transistor Tr4 are connected to the ground potential. The drain electrode of the transistor Tr2 and one electrode of the storage capacitor SC1 are connected to the drain electrode of the transistor Tr3. The other electrode of the storage capacitor SC1 is connected to the ground potential.
The source electrode of the transistor Tr3 is connected to the power supply line Vdd.

Therefore, the transistors Tr3 and Tr2 and the organic light emitting diode OLED are connected in series in this order from the power supply line Vdd to the ground potential to form a main path. Further, a bypass including the transistor Tr4 is formed from the anode of the organic light emitting diode OLED.

The operation of this circuit will be described. When the scanning line SL becomes high and the transistor Tr1 is turned on for writing the brightness data, the organic light emitting diode OLED is provided to the gate electrode of the transistor Tr2 and the storage capacitor SC2.
The electric potential corresponding to the luminance data of is set. At the same time, the transistor Tr4 is turned on and the organic light emitting diode OL
The electric charge of the anode of the ED is extracted to the ground potential via the transistor Tr4. At the same time, the transistor T
Since r3 is turned off, generation of a through current from the power supply line Vdd is prevented. Accordingly, the organic light emitting diode OL
The anode of the ED has the same potential as the ground potential.

Then, at the timing of light emission, the scanning line SL becomes low, so that the transistors Tr1 and Tr4 are
Turns off and the transistor Tr3 turns on. As a result, a current corresponding to the brightness data set in the transistor Tr2 is supplied from the power supply line Vdd to the organic light emitting diode OLED.
Flow to.

According to the second embodiment, when the brightness data is written, the anode of the organic light emitting diode OLED becomes the same potential as the ground potential and the brightness data is initialized, so that the large brightness data is rewritten to the small brightness data. It is possible to reduce the afterimage phenomenon that is sometimes seen.

The present invention has been described above based on the embodiments. It should be understood by those skilled in the art that this embodiment is an exemplification, and that various modifications can be made to the combinations of the respective constituent elements and the respective processing processes, and that such modifications are within the scope of the present invention. . An example of such a modification will be given.

In the second embodiment, the escape port for the current flowing through the bypass, that is, the source electrode of the transistor Tr4 is connected to the ground potential, but the present invention is not limited to this. For example, this may be equal to the threshold voltage of the organic light emitting diode OLED. In this way, the organic light emitting diode O
The light emission response of the LED is improved. Alternatively, it may be set to a negative potential. In this case, the charge can be quickly extracted.

In the embodiment, the drive circuit 10 shown in FIGS. 1 and 2 is assumed as the drive circuit, but the present invention is not limited to this. There are many driving circuits.

[0031]

The afterimage phenomenon can be reduced by initializing the brightness data of the optical element.

[Brief description of drawings]

FIG. 1 is a diagram showing a pixel circuit according to a first embodiment.

FIG. 2 is a diagram showing a pixel circuit according to a second exemplary embodiment.

FIG. 3 is a diagram showing a pixel circuit of a conventional technique.

[Explanation of symbols]

10 Driving circuit, OLED Organic light emitting diode,
SC, SC1, SC2 retention capacity, Tr1, Tr2, T
r3, Tr4 transistor, SL scanning line, DL
Data line, Vdd power supply line.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Keiichi Sano             2-5-3 Keihan Hondori, Moriguchi City, Osaka Prefecture             Within Yo Denki Co., Ltd. F term (reference) 5C080 AA06 BB05 DD30 FF11 JJ03

Claims (4)

[Claims] 1. A display device in which luminance data to be set for an optical element is held in the form of a control voltage, and a writing period for rewriting the luminance data and an initial period for discharging charges generated at both ends of the optical element. A display device, characterized in that a display period is provided. 2. The display device according to claim 1, wherein signals for activating the writing period and the initializing period are made common to generate the periods at the same time. 3. The display according to claim 1, wherein the writing period and the initializing period are generated at the same time, and a path for supplying a current to the optical element is cut off during those periods. apparatus. 4. A period in which one pixel includes an optical element, a driving transistor, and a power cutoff transistor, and a data update instruction signal for writing the luminance data of the optical element into the driving transistor is activated. By turning off the power cutoff transistor, the current supply path to the optical element is cut off, and after the writing period of the brightness data to the driving transistor is completed, the power cutoff transistor is turned on to cut off the current. A display device characterized in that the conventional current supply paths are connected.