KR20070081950A - Organic light emitting diode driving apparatus and method thereof - Google Patents

Abstract

An apparatus and a method for driving an OLED are provided to maintain constant brightness of the OLED by driving a driving transistor using a capacitor which is charged with a data voltage corresponding to a threshold voltage of the transistor. An apparatus for driving an OLED(Organic Light Emitting Diode) includes an OLED element, a driving transistor(T1), a charge capacitor(C1), an initialization switch(T6), a short switch(T3), and a data switch(T5). The OLED element is connected a common electrode. The driving transistor supplies a driving source voltage to the OLED element according to a gate voltage. The charge capacitor is connected between a gate electrode of the transistor and a data line, and determines a driving degree of the driving transistor using a charged voltage. The initialization switch is parallel-connected to the charge capacitor and selectively shuts off the charge capacitor. The short switch selectively disconnects between drain and gate electrodes of the driving transistor. The data switch is formed between the data line and the charge capacitor and selectively connects the data line and the charge capacitor.

Description

ORGANIC LIGHT EMITTING DIODE DRIVING APPARATUS AND METHOD THEREOF

1 is a cross-sectional view showing the structure of a conventional simple EL element.

2 is a cross-sectional view showing the structure of a conventional active drive OLED display panel.

3 is a circuit diagram showing a structure of a conventional OLED driver.

4 is a circuit diagram showing a structure of an embodiment of the present invention.

5 is an operational waveform diagram of an embodiment of the present invention.

*** Explanation of symbols for main parts of drawing ***

10: substrate 20: drive element

T1-T5: Transistor C1: Charge Capacitor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic light emitting diode driving apparatus and method, and more particularly, to an organic light emitting diode driving apparatus and method for preventing luminance unevenness between pixels due to characteristic variations between driving elements in an organic light emitting diode driving circuit.

Recently, various flat panel displays have been developed to reduce weight and volume, which are disadvantages of cathode ray tubes (CRTs). Such a flat panel display panel is based on a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic electroluminescence (EL). Organic light emitting diode (OLED) displays; and the like.

Among them, the OLED injects electrons and holes into the light emitting layer from the electron injection electrode (cathode electrode) and the hole injection electrode (anode electrode), respectively, so that the excitons, in which the injected electrons and holes combine, fall from the excited state to the ground state. When the device emits light.

Due to this principle, unlike LCD, which is conventionally used as a thin display device, it does not need a separate light source, and thus has the advantage of reducing the volume and weight of the device. Since the reaction speed is 1000 times faster than LCD, afterimage display is performed. As a result, it is emerging as a next-generation display device.

According to the size of these OLEDs, various driving methods are used. Typically, medium- and large-sized OLED displays have an active matrix driving method (AM method), and active matrix driving methods and passive (Passive) methods in small OLED displays. ) Matrix driving method (PM method) is used in combination. In the AM type OLED display, a switching element formed of a thin film transistor is installed in each OLED element (pixel), and the switching element is responsible for current control, thereby emitting a specific pixel until the next control point without a separate control signal. That's the way. On the other hand, the PM type display is a method of operating the entire panel area by using an afterimage by sequentially driving the OLED by a general matrix configuration.

FIG. 1 is a cross-sectional view illustrating a basic structure of an organic light emitting element (hereinafter referred to as an organic EL), which is the basis of the OLED. The first electrode 11 and the organic EL on the substrate 10 as shown in FIG. The layer 12 and the second electrode 13 are sequentially formed, and a getter 14 for adsorbing moisture and impurity gas is attached to the protective cap 16 and then the sealant 15 is used for the protection. The cap 16 encapsulates to protect the OLED device.

FIG. 2 illustrates a typical AM OLED structure in which the basic EL device shown in FIG. 1 is configured together with a driving unit. As shown, the device layer 20 is formed on the substrate 10, and the flat layer 25 is formed. After forming the pixel electrode 30, the organic light emitting layer 40, the common electrode 50 in order to form an OLED structure, while protecting the device on the top of the protective cap (or substrate) 70 and sealing adhesion A protective film and sealant 60 is applied.

FIG. 3 is a circuit diagram illustrating the basic structure of the AM OLED illustrated in FIG. 2, and as shown in FIG. 3, two pixels Ma and Mb and one capacitor C are included in one pixel. In practice, the plurality of transistors may be formed in parallel and then connected in parallel. However, the current-driven structure of the basic circuit configuration may control driving of each pixel using one or more transistors (switching transistor, Ma) as shown in FIG. In addition, the voltage applied using the storage capacitor C is maintained for a predetermined time to continuously provide a constant current to the OLED device 100 to which the current driving transistor Mb is connected to maintain light emission until the next device selection.

Looking at the driving thereof, when the device is selected, the switching transistor Ma is turned on by the selection signal, so that the data voltage is applied to the gate A of the driving transistor Mb and through the driving transistor Mb. The driving voltage VDD causes a current to flow through the OLED element 100 to emit light. In this case, the storage capacitor C is charged with the applied data voltage to continuously apply a constant voltage to the gate of the corresponding driving transistor.

However, in the OLED display panel, since the driving transistor has a structure formed directly on the substrate as shown in FIG. 2, the threshold voltages of the individual transistors and the electric field mobility of each element are not uniform and have a predetermined deviation. Since a problem arises in that luminance uniformity is poor for each pixel.

The object of the present invention is to propose a method for reducing the luminance variation between pixels by overcoming the characteristic variation of each driving element as described above, and to drive the transistor for driving the OLED with a charging capacitor charged with a data voltage considering the threshold voltage of the transistor. The organic light emitting diode driving apparatus is to provide an organic light emitting diode driving apparatus capable of maintaining the OLED luminance uniformly regardless of the characteristics of each driving element of each pixel.

Another object of the present invention is to unstable by applying a gate voltage considering the characteristics of the transistor to the driving transistor and finally determining the path connection between the driving transistor and the OLED element using a predetermined control signal and transistor. The present invention provides an organic light emitting diode driving apparatus capable of preventing incorrect setting of light emission and gate voltage.

Another object of the embodiment of the present invention is to use a reference voltage provided separately from the driving voltage of the OLED element in order to continuously maintain the charge value of the charging capacitor when driving the device, thereby using a uniform reference voltage regardless of the driving of the OLED element An organic light emitting diode driving apparatus is provided to reduce luminance variations.

Another object of the embodiment of the present invention is to discharge the charging capacitor for a unit scan time, and then driving the driving transistor to the diode state and connected to one side of the charging capacitor, the other side of the charging capacitor is connected to the data line to the driving transistor in the charging capacitor The present invention provides a method of driving an organic light emitting diode which can reduce a luminance deviation between pixels by charging a data voltage considering a threshold voltage of the LED and continuously providing the data voltage to a gate of a driving transistor to emit an OLED device.

Another object of the present invention is to cut off the connection between the driving element and the OLED and the connection between the charging capacitor and the charging voltage during the process of charging the data capacitor considering the threshold voltage of the driving transistor in the charging capacitor, The present invention provides an organic light emitting diode driving method capable of ensuring stable driving of a driving transistor by restoring the disconnected connection when the charging is completed, thereby driving the OLED device.

In order to achieve the above object, the organic light emitting diode driving apparatus according to an embodiment of the present invention, the organic light emitting device of which one side is connected to the common electrode; A driving transistor for supplying driving power to the other electrode of the organic light emitting element according to a gate voltage; A charging capacitor connected between the gate electrode of the driving transistor, an external power supply, and a data line to determine a driving degree of the driving transistor based on a charged voltage; An initialization switch connected in parallel with the charging capacitor to selectively short the charging capacitor; A short switch for selectively shorting between the drain electrode and the gate electrode of the driving transistor; And a data switch located between the data line and the charging capacitor to selectively connect them.

An organic light emitting diode driving apparatus according to another embodiment of the present invention includes an organic light emitting diode having a driving transistor, a charging capacitor driving the driving transistor at a charging voltage, and a switching transistor selectively connecting the charging capacitor and a data line. A drive device comprising: an initialization switch for selectively shorting said charging capacitor; A short switch for selectively shorting a gate electrode and a drain electrode of the driving transistor; A reference voltage switch for selectively providing a reference voltage to one electrode of the charging capacitor to maintain a voltage charged in the charging capacitor; And a light emitting switch for selectively providing a driving current provided by the driving transistor to the organic light emitting diode.

In another embodiment, an organic light emitting diode driving method includes: shorting a gate electrode and a drain electrode of a driving transistor providing a driving voltage to an organic light emitting diode, and the other side of the charging capacitor having one side connected to the gate electrode of the driving transistor; Connecting the data line to an electrode to charge the charging capacitor; Disconnecting the charging capacitor and the data line and releasing a short circuit between the gate electrode and the drain electrode of the driving transistor.

Embodiments of the present invention as described above will be described in detail with reference to the accompanying drawings.

4 illustrates a cell structure of an organic light emitting diode panel according to an exemplary embodiment of the present invention, wherein a plurality of signal lines for active driving, driving circuits and power sources connected thereto to emit light of the organic light emitting diode are shown in FIG. Shown in circuit diagram form.

In the above circuit configuration, the switching transistor T5, the driving transistor T1 and the charging capacitor C1, which are generally used in the current driven OLED driving circuit, perform the essential operations, but additionally provided a short circuit. A switch T3, a reference power switch T4, an initialization switch T6, a light emitting switch T2, and a signal (line) for controlling them are added to the charging voltage considering the threshold voltage of the driving transistor T1. The capacitor C1 is charged to prevent the luminance unevenness caused by the threshold voltage variation of the driving transistor T1 for each cell.

That is, the threshold voltage of the driving transistor T1 driving the driving transistor T1 means a minimum gate voltage capable of driving the driving transistor T1, and the driving transistor T1 is set by the voltage exceeding the threshold voltage. Current path capacitance between the drain and source

Therefore, the same voltage data voltage is provided to the charging capacitor C1. When the threshold voltages of the driving transistors T1 are different, the size of the OLED driving current flowing through each driving transistor T1 is also different. Differences occur in the emission luminance of the OLED.

The basic operation of the present embodiment is, first, when a voltage charged in the charging capacitor C1 to drive the driving transistor T1 is provided from the data line, the reference voltage is not the fixed power supply voltage, but the driving transistor T1. It is to be the threshold voltage value of.

Therefore, when the short switch T3 is connected in parallel between the gate and the drain of the driving transistor T1, and the charging capacitor C1 receives the data voltage from the data line by the operation of the switching transistor T5, the driving is performed. By shorting the gate electrode and the drain electrode of the transistor T1 so that the driving transistor T1 becomes a diode, the threshold voltage of the diode can be regarded as a reference voltage. That is, at this time, the voltage charged in the charging capacitor C1 becomes the data voltage of the data line based on the threshold voltage of the driving transistor T1. In other words, if the data voltages are the same regardless of the threshold voltages of the individual driving transistors T1, the currents flowing through the driving transistors T1 are the same and the luminance of the connected OLEDs is the same.

As described above, additional switches are further configured to more fully and effectively implement the configuration of charging the charging capacitor C1 with the charging voltage considering the threshold voltage of the driving transistor T1.

First, an initialization switch T6 in parallel to the charging capacitor C1 so as to discharge the electric charge previously charged for driving before charging the charging capacitor C1 to effectively charge the newly provided data voltage. Configure Through this, the charging capacitor C1 may be initialized (short circuit due to the initialization switch T6 on) to be discharged at the inlet of the charging section, and then driven to perform the remaining charging.

In addition, since the charges charged in the charging capacitor C1 may flow directly to the OLED through the short-circuit switch T3 connected to each other, such as the path P1 shown during the charging process, it is difficult to smoothly charge the OLED. After the light emitting switch T2 is connected in series between the OLED, the driving transistor T1, and the shorting switch T3 so as to be separated, the light emitting switch T2 is turned off while the charging capacitor C1 is being charged. Be sure to

Through the above configurations, the charging capacitor C1 may be effectively charged with the data voltage considering the threshold voltage of the driving transistor T1. Subsequently, when light emission is actually performed, the light emission switch T2 is turned on, and the short circuit switch T3 and the switching transistor T5 are turned off to be charged in the charging capacitor C1. The voltage continuously provides a constant data voltage to the driving transistor T1, and thus a constant driving current is provided to the OLED through the driving transistor T1, so that the OLED can continuously emit light uniformly.

However, an external power source is required in order for the charging capacitor C1 to maintain a uniform charging voltage continuously during the OLED emission time. For this purpose, a switching transistor T5 of both electrodes of the charging capacitor C1 is basically required. Electrode connected to the OLED driving power source (VDD-EL). However, in the present embodiment, since the OLED driving power supply (VDD-EL) does not always provide a stable voltage due to the change of the OLED driving time and the OLED driving environment (number of driven OLEDs), a separate reference voltage source (constant voltage) Configured to use the reference voltage Vref provided from the circle). Since the reference voltage Vref is also preferably not connected while the charging capacitor C1 is being charged, the reference voltage is connected to one electrode of the charging capacitor C1 (the electrode connected to the switching transistor T5) only at the time of actual light emission. A reference voltage switch T4 is added to provide.

Each switch is preferably a MOS transistor as shown, and uses three separate signals except for the OLED driving power supply (VDD-EL), the OLED cavity electrode (GND-EL), the reference voltage (Vref), and the data line. By controlling the on / off of the configuration switch in each driving step, it is possible to effectively manage the charging process of the charging capacitor and the light emitting process of the OLED.

Let us look at the process of driving the OLED using the configuration shown.

First, the light switch T2 is turned off and the charge capacitor C1 is turned off to prevent the generation of a current path that may be formed between the OLED and the charging capacitor C1 and the shorting switch T3 before the charging process. The reference voltage switch T4 is turned off to cut off the connection between the reference power supply and the charging capacitor C1 to maintain charge, and the control of the switches T2 and T4 produces a single light emission signal. It is available.

In the charging process, the switching transistor T5 and the short-circuit switch T3 are turned on using a single scan signal to provide a charging voltage to the charging capacitor C1, and are controlled to be turned off when charging is completed. .

The on-off switch T6 is turned on and off for a predetermined time by an initialization signal so that the charged capacitor C1 is discharged and initialized during the charging process, thereby shorting the charge capacitor C1 for a predetermined time so that the charged charge is discharged. .

When charging is completed, the light emitting switch T2 is turned on to connect the driving transistor T1 to the OLED using the light emitting signal, and a reference voltage is provided to provide the reference voltage to the charging capacitor C1. Turn on the voltage switch T4.

Through the above process, charging is performed according to the new data voltage in the non-emission period, and the OLED is continuously emitted by providing the charging voltage according to the data voltage to the gate of the driving element during the emission period.

The driving transistor T1 is effectively charged in the non-light emitting period of the OLED through the configuration and driving method except for all or a part of the above-described configuration, and is charged in the charging capacitor C1 in the light emitting period. Since the driving transistor T1 that provides the current to the OLED can be stably driven with the charging voltage considering the threshold voltage of, the luminance uniformity of the panel is greatly increased, and the display quality is greatly improved.

FIG. 5 is a waveform diagram of control signals illustrated to further explain a method of driving an active OLED cell using the configuration of FIG. 4 described above through time and specific signal operations.

The unit scan time means a non-light emitting period, and the other time becomes a light emitting period.

First, when entering the non-light emitting period to drive the OLED device with new data, the light emission signal is brought into a high potential state to turn off the light switch T2 and the reference voltage switch T4. Through this, the charging capacitor C1 is electrically disconnected from the reference voltage Vref or the OLED.

Then, the scan signal is made low and the switching transistor T5 and the short switch T3 driven by the scan signal are turned on. As a result, the driving transistor T1 diode-connected to one electrode of the charging capacitor is connected, and the data line is connected to the other electrode to perform the charging process.

In addition, the initialization signal is provided to the initialization switch T6 in the form of a pulse to discharge and initialize the existing charging charges of the charging capacitor C1 to the data voltage provided through the data line for the remaining non-emitting period. Recharge C1).

When the charging capacitor C1 is charged during the non-light emitting period and enters the light emitting period, the scan signal is brought into a high potential state, thereby disconnecting the connection between the data line and the charging capacitor C1 and closing the gate of the driving transistor T1. A voltage charged in the charging capacitor C1 by disconnecting the drain connection and connecting the driving transistor T1 to the OLED while providing the reference voltage Vref to the charging capacitor C1 by making the light emission signal low. The OLED emits light with the driving current provided by the driving transistor T1 operating by receiving the gate electrode as a gate electrode. This light emission lasts until the next non-light emitting period.

As described above, the organic light emitting diode driving apparatus and method according to an embodiment of the present invention drives the transistor for driving the OLED with a charging capacitor charged with a data voltage considering the threshold voltage of the transistor to emit the OLED. By doing so, there is an effect of maintaining the OLED brightness uniformly regardless of the characteristics of each pixel driving element.

The organic light emitting diode driving apparatus according to the embodiment of the present invention applies a gate voltage considering the characteristics of the transistor to the driving transistor and finally connects the path between the driving transistor and the OLED element using a predetermined control signal and a transistor. By making the determination, it is possible to prevent unstable light emission and incorrect setting of the gate voltage, thereby improving display quality and driving reliability.

The organic light emitting diode driving apparatus according to the embodiment of the present invention uses a reference voltage provided separately from the driving voltage of the OLED element to continuously maintain the charging value of the charging capacitor when driving the device, thereby providing a power source for driving the OLED element. Even if unstable, the display quality can be improved by maintaining the gate voltage of the driving device by using a stable reference voltage at all times.

In the method of driving an organic light emitting diode according to an exemplary embodiment of the present invention, the connection between the driving element and the OLED and the connection between the charging capacitor and the charging voltage are cut off while the charging capacitor is charged with the data voltage considering the threshold voltage of the driving transistor. In addition, when charging of the charging capacitor is completed, the disconnected connection is restored to drive the OLED device, thereby increasing stability and efficiency of charging and improving light emission uniformity.

The organic light emitting diode driving method according to an embodiment of the present invention is a path selection process for providing a charging voltage considering the threshold voltage of the driving transistor to the charging capacitor during charging, and an external disturbance path connected to the charging capacitor during the charging capacitor charging process. By performing a path selection process that blocks the signals using a single signal, the number of additional signals required for optimal driving can be minimized.

Claims (18)

An organic light emitting diode having one side connected to the common electrode; A driving transistor for supplying driving power to the other electrode of the organic light emitting element according to a gate voltage; A charging capacitor connected between the gate electrode of the driving transistor, an external power supply, and a data line to determine a driving degree of the driving transistor based on a charged voltage; An initialization switch connected in parallel with the charging capacitor to selectively short the charging capacitor; A short switch for selectively shorting between the drain electrode and the gate electrode of the driving transistor; And a data switch positioned between the data line and the charging capacitor to selectively connect the data line. The organic light emitting diode driving apparatus of claim 1, wherein the external power source connected to the charging capacitor is a driving power source connected to the driving transistor. The power supply of claim 1, wherein the external power source connected to the charging capacitor is a separate reference power source different from the driving power source connected to the driving transistor, and a reference voltage switch for selectively connecting them is connected between the reference power source and the charging capacitor. An organic light emitting diode drive device, characterized in that connected. The organic light emitting diode driving apparatus of claim 1, wherein a light emitting switch selectively connecting the driving transistor and the organic light emitting element is connected in series. The organic light emitting diode driving apparatus according to any one of claims 1 to 4, wherein the switch is a transistor. The organic light emitting diode driving apparatus of claim 3 or 4, wherein the reference voltage switch and the light emitting switch are driven by the same signal. The organic light emitting diode driving apparatus of claim 1, wherein the data switch and the short switch are driven by the same signal. The organic light emitting diode driving apparatus of claim 1, wherein each of the switches is driven by three separate signals besides an organic light emitting diode driving power supply, an organic light emitting diode common electrode, and a data line. An organic light emitting diode driving apparatus comprising a driving transistor, a charging capacitor for driving the driving transistor at a charging voltage, and a switching transistor selectively connecting the charging capacitor and a data line. An initialization switch for selectively shorting the charging capacitor; A short switch for selectively shorting a gate electrode and a drain electrode of the driving transistor; A reference voltage switch for selectively providing a reference voltage to one electrode of the charging capacitor to maintain a voltage charged in the charging capacitor; And a light emitting switch for selectively providing a driving current provided by the driving transistor to the organic light emitting diode. The organic light emitting diode driving apparatus of claim 9, wherein the switching transistor and the short switch are driven in the same manner by a single scan signal, and the reference voltage switch and the light emitting switch are driven in the same manner by a single light emission signal. . 10. The organic light emitting diode driving apparatus according to claim 9, wherein all the transistors and the switches are MOS transistors. Charging the charging capacitor by shorting a gate electrode and a drain electrode of a driving transistor providing a driving voltage to the organic light emitting diode, and connecting a data line to the other electrode of the charging capacitor having one side connected to the gate electrode of the driving transistor; Disconnecting the charging capacitor and the data line and releasing a short circuit between the gate electrode and the drain electrode of the driving transistor. The method of claim 12, wherein the charging of the charging capacitor comprises discharging the charging capacitor for a predetermined time and then charging from the discharged state. The method of claim 12, further comprising: disconnecting the driving transistor from the organic light emitting diode before charging the charging capacitor; Disconnecting the charging of the charging capacitor and the connection of the charging capacitor and the data line, releasing a short circuit between the gate and the drain of the driving transistor, and then restoring the connection of the driving transistor and the organic light emitting diode. An organic light emitting diode driving method characterized in that. The method of claim 12, wherein after the connection of the charging capacitor and the data line is disconnected and the short circuit between the gate and the drain of the driving transistor is released, an external power source for maintaining the charging voltage of the charging capacitor is charged. And providing the other side of the capacitor. The method of claim 15, wherein the external power source is a driving voltage provided to the driving transistor or a separate reference voltage. The organic light emitting diode of claim 12, wherein in the charging of the charging capacitor, the connection switching of shorting the gate and the drain of the driving transistor and connecting the data line to the other side of the charging capacitor uses a single scan signal. Diode drive method. 16. The method of claim 14 or 15, wherein the connection switching to cut off or restore the connection of the driving transistor and the organic light emitting diode and to selectively provide the external power to the other side of the charging capacitor provides a single light emission signal. An organic light emitting diode driving method characterized in that it is used.

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