KR101072757B1 - Driving Circuit of Passive Matrix Organic Electroluminescent Display Device - Google Patents

Abstract

If the gray scale level of each data channel in the common cathode line is different from each other, the precharge level is varied according to each gray level, so that the precharge voltage and the driving voltage applied to the anode electrode of the organic EL element are changed. A driving circuit of a passive matrix organic EL display device for reducing the occurrence of luminance deviation due to a change in voltage difference. The present invention provides a driving constant current source for supplying a driving current to an organic EL element during a driving period, a precharge constant current source for supplying a precharge current to the organic EL element during a precharge period, and a bias voltage for controlling current values of the driving constant current source. The organic EL element according to the driving period of the driving current control unit for applying a precharge current control unit for applying a bias voltage for controlling the current value of the precharge constant current source and the precharge constant current source, the driving constant current source and the ground terminal And a switching controller for applying a control signal for connecting to the anode electrode of the precharge current controller, wherein the precharge current controller changes the precharge current value by adjusting the bias voltage according to the change of the data gray level. According to the present invention, not only the luminance error according to the data amount between scan lines can be reduced, but also the power consumption can be reduced.

Organic EL, OLED, luminance error, precharge current, charging current

Description

Driving Circuit of Passive Matrix Organic Electroluminescent Display Device

1 is a block diagram illustrating a typical passive matrix organic EL display device.

2 is a representative waveform diagram for driving a passive matrix organic EL display device.

3 is a view showing a driving circuit of a conventional organic EL display device in which a driving circuit and an organic EL element are connected.

4 is a diagram showing a driving circuit of a passive matrix organic EL display device according to a first embodiment of the present invention.

5 is a view showing a driving circuit of a passive matrix organic EL display device according to a second embodiment of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a passive matrix organic electroluminescent display (hereinafter, referred to as 'EL'). Specifically, each gray level is different when gray levels are different between data channels in a common cathode line. A passive matrix organic EL display device for reducing the occurrence of a luminance error caused by a variation in the voltage difference between the precharge voltage and the driving voltage applied to the anode electrode of the organic EL element by varying the precharge level according to the It relates to a driving circuit.

In recent years, remarkable developments have been made in the field of flat panel displays. As flat panel displays, LCD (Plasma Display Panel), FED (Field Emission Display) and EL (Electro-luminescent) are typical.

Currently, LCD is widely used as a video display device of a TV, a computer, or a mobile communication terminal. The LCD is not only heavy because it requires a backlight, but also has a disadvantage of being thick and slow in response.

Therefore, the organic EL display device (or OLED) is attracting attention as a next-generation video display device replacing the LCD.

The organic EL element (OLED) includes an extremely thin organic layer of 0.1 μm or less, and when current flows through the organic layer, electrons and holes are recombined near the interface between the electron transport layer and the hole transport layer. Light emission, which has an extremely fast response time of several hundreds of microseconds or less.

As such, the organic EL element OLED has a two-pole structure of an anode electrode (anode) and a cathode electrode (cathode), and the current is driven due to the difference in voltage-current characteristics of the individual organic EL elements OLED forming the panel. Will be

The display means using the organic EL element (OLED), that is, the driving method for the organic EL display device is classified into a passive matrix method and an active matrix method, which are orthogonal to the anode and the cathode. The active matrix method is a driving method in which a thin film transistor TFT and a capacitor Cst are integrated in each pixel to maintain a voltage by a capacitor capacity. to be.

Fig. 1 is a block diagram showing a typical passive matrix organic EL display device, and Fig. 2 is a representative waveform diagram for driving a passive matrix organic EL display device.

Referring to FIG. 1, the passive matrix organic EL display device is composed of a display panel 10, a data driver 20, and a scan driver 30.

In the display panel 10, a plurality of common anode lines D1 -Dm and a plurality of common cathode lines S1 -Sn are arranged in a lattice shape, and each pixel includes an R / G / B pixel at each intersection of the lattice. Organic electroluminescent element (EL11-ELnm) is arrange | positioned. Each organic EL element is composed of an anode electrode, an organic film layer, and a cathode electrode, and a parasitic capacitor C is formed between the anode electrode and the cathode electrode. In addition, the data driver 20 is connected to the common anode lines D1 -Dm, and the scan driver 30 is connected to the common cathode lines S1 -Sn.

The data driver 20 includes a plurality of driving circuits 21 to apply a precharge current Iprec, a driving current Idrv, and a ground voltage GND for each common anode line D1 -Dm. Accordingly, under the control of a driving controller (not shown), each driving circuit 21 selectively selects each common anode line D1-Dm to the precharge current stage Iprec, the driving constant current stage, and the ground terminal GND. Connect with

The scan driver 30 includes a plurality of scan output units 31 that apply a high voltage VCC or a ground voltage GND for each common cathode line S1 -Sn. Accordingly, according to the control of the driving controller (not shown), each scan output unit 31 selectively selects each common cathode line S1-Sn to the high voltage terminal VCC and the ground terminal GND in a predetermined pattern. Connect.

In the above configuration, when the plurality of scan output units 31 are sequentially turned on / off and select the common cathode line from the first column S1 to the nth column Sn, the driving is performed. The circuit unit 21 is connected to the common anode line D1-Dm toward the driving constant current source Idrv in accordance with the gray scale of the pixel, that is, the organic EL element EL11-ELnm, in synchronization with the corresponding pixel. One screen frame is formed by applying a current to the EL element.

Here, a method of expressing a gray scale of a pixel includes a pulse width modulation (PWM) method for adjusting the time of the applied current and a pulse amplitude modulation (PAM) method for adjusting the magnitude of the applied current.

On the other hand, as described above, since the organic EL element ELnm is formed of an organic thin film, parasitic capacitor C is present at both ends of the anode and the cathode of the diode EL, which is low due to the parasitic capacitor C. Since there is a problem in that gray scale processing cannot be performed, the parasitic capacitor C is precharged by applying a voltage such that the diode EL can be turned on before applying current to the common anode lines D1 -Dm. Let's go. To this end, the driving circuit unit 21 is provided with a precharge current stage I _PRE for outputting a current of a predetermined level.

Therefore, the passive matrix organic EL display device precharges the parasitic capacitor C of the organic EL element EL11-ELnm by connecting all common anode lines D1-Dm to the precharge current stage I _PRE for a predetermined time. According to the pre-charge period and the control of the external control signal PWM, the driving circuit unit 21 connects each common anode line D1-Dm to the pixel gray level of the organic EL element 12 connected thereto. According to the driving period (Driving Period) for emitting an organic EL element by connecting to the driving constant current source (Idrv) for a predetermined PWM time, and then again driving circuit unit 21 under the control of the external control signal (Reset) the common anode By connecting the lines (D1-Dm) to the ground terminal (GND) is driven by the discharge period (Discharge Period) for discharging the voltage charged in the parasitic capacitor (C).

Referring to FIG. 2, a driving waveform diagram of a conventional organic EL display device is shown. 'Sn-1' is a waveform of an n-1th common cathode line which is not selected during driving, and 'Sn' is n selected during driving. The waveform of the first common cathode line is shown, and 'Vop (n)' is a waveform diagram showing the voltage application form of the common anode line. In the conventional driving waveform, the charge is charged by applying a positive potential Vop_prec of the charging voltage level in the charging section in the waveform of 'Vop (n)', and the driving section corresponds to the gradation level of each organic EL element. The potential Vop_drv is applied, and the charged charge is discharged by connecting the ground GND in the discharge period.

3 shows a driving circuit of the conventional organic EL display device in which the driving circuit 21 and the organic EL element are connected. During the charging period, the precharging current Iprec is charged with a constant current regardless of the gray level between each channel. Therefore, the voltage Vop of the anode electrode of the organic EL element at the end of the charging is changed to the charging voltage Vop_prec.

Next, when the driving period starts, the driving current Idrv flowing in the driving current source of each driving circuit unit 21 varies depending on the gray level of each channel. At this time, the voltage Vop of the anode of the organic EL element is changed to the driving voltage Vop_drv.

Therefore, the voltage difference Vop of the voltage Vop of the anode electrode of the organic EL element at the time of switching from the end of the charging period to the driving period is expressed by Equation 1 below.

OpVop = Vop_prec-Vop_drv

Here, Vop_prec is the voltage of the anode electrode of the organic EL element when the precharge current is applied, and Vop_drv is the voltage of the anode electrode of the organic El element when the driving current is applied.

As shown in Equation 1, when precharging with a constant charging current as in the conventional driving method, Vop_prec always has a constant voltage for every common cathode line S1 -Sn. However, Vop_drv has a different value for each common cathode line D1-Dm according to the gradation level. That is, as the amount of data applied to the organic EL element is different for each common cathode line S1 -Sn, the voltage difference (Vop) of the anode electrode is different for each common cathode line S1 -Sn. .

When an image is displayed on the display panel, the voltage difference Vop for each common cathode line S1 -Sn is directly connected to the luminance of each line. The luminance error occurs due to the above phenomenon, which is directly related to the image quality in the display method of the passive matrix organic EL display device, and this phenomenon becomes more serious as the resolution of the display increases.

An object of the present invention for solving the above problems is to vary the precharge current according to the gradation level of the driving current to switch organic pre-charge regardless of the amount of data applied to each common cathode line when switching from the precharge period to the driving period. The voltage difference (Vop) of the anode electrode of the device is intended to have the same value.

That is, the same voltage difference (VoV) is achieved regardless of the number of pixels emitted from each common cathode line S 1 -Sn or the amount of driving current applied to implement a gray scale required.

The driving circuit of the organic EL display device according to the present invention for achieving the above object is driven in the precharge section, the driving section, and the discharge section to drive the organic EL display device of the organic EL display device of the passive matrix organic EL display device displaying a predetermined image. A furnace, comprising: a driving constant current source for supplying a driving current to an organic EL element during a driving period; A precharge constant current source supplying a precharge current to the organic EL element during a precharge period; A ground terminal for discharging a charge of the organic EL device during a discharge period; A driving current controller for applying a bias voltage for controlling a current value of the driving constant current source; A precharge current controller for applying a bias voltage for controlling a current value of the precharge constant current source; And a switching controller configured to apply a control signal for connecting the precharge constant current source, the driving constant current source, and the ground terminal to the anode electrode of the organic EL element according to a driving period, wherein the precharge current controller is configured to control the data gray level. The precharge current value is changed by adjusting the bias voltage according to the change, and the precharge current value is adjusted by receiving information about the change amount of the driving voltage of the anode electrode of the organic EL element from the switching controller. .

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Example  One

4 is a diagram showing a driving circuit of an organic EL display device of a passive matrix organic EL display device according to a first embodiment of the present invention.

Referring to FIG. 4, the driving circuit of the organic EL display device of the passive matrix organic EL display device according to the first embodiment of the present invention is a pulse width modulation (PWM) driving method, and the driving current Idrv is converted into an organic EL element. The gray scale is represented by adjusting the time to be applied.

The driving circuit of the organic EL display device according to the first embodiment includes a driving constant current source 100, a pre-charge constant current source 120, a ground terminal GND, a driving current controller 130, and a precharge current. The controller 140, the precharge switch 160, the driving switch 170, the discharge switch 180, and the switching (PWM) controller 190 are configured.

The driving constant current source 100 is connected to the power supply voltage VCC, and applies a driving current Idrv for emitting the organic EL element EL11 at a predetermined luminance during the driving period.

The precharge constant current source 120 is connected to the power supply voltage VCC and applies a precharge current Iprec for charging the capacitor of the organic EL element EL11 during the precharge period.

The ground terminal GND discharges the charge of the charged capacitor through the ground GND during the discharge period.

The driving current controller 130 controls the amount of current of the driving constant current source 100. In the PWM driving method, since the driving current controller 130 always applies the same bias voltage to the driving constant current source 100, the driving current Idrv has the same value.

The precharge current controller 140 controls the amount of current of the precharge constant current source 120. That is, the precharge current control unit 140 does not always apply the same bias voltage to the precharge current control unit 140 as in the related art, but according to the first embodiment of the present invention, the precharge current control unit 140 changes according to the gray level of each data channel. ), Different bias voltages are applied to the precharge constant current source 120 to apply different precharge currents Iprec according to the gray level.

Here, the method for determining the precharge constant current Iprec according to the gradation level, for example, reads data of one cathode line from a memory in which data applied to the common anode lines D1 -Dm is stored and transfers it to the driving current controller. In this case, the precharge bias level is determined by calculating the total amount of data transferred, or the precharge bias level is calculated by calculating the total amount of data stored in the resist before the data of the cathode line is output from the driving current controller. May be used.

The precharge current controller 140 implements a digital / analog converter (DAC) for precharge bias and transfers the precharge level determined according to the gradation level to the digital / analog converter (DAC) to precharge the constant current source 120. The precharge current applied to the organic EL device may be controlled by controlling the bias of the bias or by transferring the precharge level determined according to the gradation level to the precharge switching controller to control the precharge current Iprec. Vop_prec) can be adjusted.

The precharge switch 160 is turned on during the precharge period to apply the precharge current Iprec output from the precharge constant current source 120 to the organic EL element EL11.

The driving switch 170 is turned on during the driving period to apply the driving current Idrv output from the driving constant current source 100 to the organic EL element EL11.

The discharge switch 180 is turned on during the discharge period to discharge the charge charged in the organic EL element EL11 to the ground terminal GND.

The switching controller 190 applies a control signal to the precharge switch 160, the driving switch 170, and the discharge switch 180 to control an on / off time. In particular, in the PWM driving method, the switching controller 190 controls the on time of the driving switch 170 so that an amount of current corresponding to each gray level is applied to the organic EL element EL11. In addition, the switching controller 190 applies information about the change amount Vop_drv of Vop_drv to the precharge current controller 140 according to the gray level of the organic EL elements connected to one common cathode line. It is possible to generate a precharge current Iprec.

As described above, in the driving circuit of the organic EL display device of the PWM driving method according to the first embodiment of the present invention, the switching (PWM) controller 190 for the precharge current controller 140 to implement gray levels. Information on the amount of change in Vop_drv is received from. Therefore, as shown in Equation 1 mentioned in FIG. 3, the precharge voltage Vop_prec is changed to correspond to the change of the driving voltage Vop_drv of the anode of the organic EL device to be switched from the precharge period to the driving period. Luminance deviation can be reduced by making the voltage difference (Vop) generated at the same time have the same value for each common cathode line.

For example, the driving voltage Vop_drv according to the driving current Idrv value flowing through the anode of the first organic EL element EL11 connected to the first common cathode electrode in the driving section is 3V, and the second organic If the driving voltage Vop_drv according to the driving current Idrv flowing through the anode electrode of the EL element EL12 is 5 V, the precharge voltage according to the precharge current Iprec of the first organic EL element EL11 ( When Vop_prec is 2V, the precharge current controller 140 adjusts the bias voltage so that the precharge voltage Vop_prec according to the precharge current Iprec of the second organic EL element EL12 is 4V. Adjust Iprec). Therefore, when switching from the precharge period to the driving period, the luminance difference can be reduced by making the voltage difference (Vop) of the organic EL elements in the same common cathode line all have the same value as -1V.

Example  2

5 is a diagram showing a driving circuit of an organic EL display device of a passive matrix organic EL display device according to a second embodiment of the present invention.

Referring to FIG. 5, the driving circuit of the organic EL display device of the passive matrix organic EL display device according to the second embodiment of the present invention is a pulse amplitude modulation (PAM) driving method, and has a different size depending on the gradation level in the driving current source. The luminance is expressed by applying the driving current Idrv to the organic EL element.

The driving circuit of the organic EL display device according to the second embodiment includes a driving constant current source 100, a precharge constant current source 120, a ground terminal GND, a drive current controller 130 ′, and a precharge current controller 140. , Precharge switch 160, driving switch 170, discharge switch 180, and switching controller 190 ′. Since the driving circuit of the organic EL display device according to the second embodiment is a PAM driving method, only the driving current control unit 130 'and the switching control unit 190' are different from those in the first embodiment.

The driving constant current source 100 is connected to the power supply voltage VCC, and applies a driving current Idrv for emitting the organic EL element EL11 at a predetermined luminance during the driving period.

The precharge constant current source 120 is connected to the power supply voltage VCC, and applies a precharge current Iprec for charging the capacitor of the organic EL element EL11 during the precharge period.

The ground terminal GND discharges the charge of the charged capacitor through the ground GND during the discharge period.

The driving current controller 130 ′ controls the amount of current output from the driving constant current source 100. That is, in the PAM driving method, since the driving current controller 130 applies a different bias voltage to the driving constant current source 100 according to each gray level, the value of the driving current Idrv is different for each gray level. The driving current controller 130 ′ may adjust the bias using a resistor or may control the digital current using an analog converter (DAC). That is, by adjusting the bias of the driving constant current source 100 by using a resistor or a DAC outside the IC, the driving current Idrv applied to the organic EL element can be controlled.

The precharge current controller 140 receives information about the change amount Vop_drv of Vop_drv according to the gray level of the organic EL elements from the driving current controller 130 ′ and generates a corresponding precharge current Iprec. To be able. That is, the precharge current control unit 140 does not always apply the same bias voltage to the precharge current control unit 140 as in the related art, but according to the second embodiment of the present invention, the precharge current control unit 140 changes according to the gray level of each data channel. ), Different bias voltages are applied to the precharge constant current source 120 to apply different precharge currents Iprec according to the gray level.

Here, the method for determining the precharge constant current Iprec according to the gradation level, for example, reads data of one cathode line from a memory in which data applied to the common anode lines D1 -Dm is stored and transfers it to the driving current controller. In this case, the precharge bias level is determined by calculating the total amount of data transferred, or the precharge bias level is calculated by calculating the total amount of data stored in the resist before the data of the cathode line is output from the driving current controller. May be used.

The precharge current controller 140 implements a digital / analog converter (DAC) for precharge bias and transfers the precharge level determined according to the gradation level to the digital / analog converter (DAC) to precharge the constant current source 120. By using a method of adjusting the bias of, the precharge current Iprec applied to the organic EL element can be controlled.

The precharge switch 160 is turned on during the precharge period to apply the precharge current Iprec output from the precharge constant current source 120 to the organic EL element EL11.

The driving switch 170 is turned on during the driving period to apply the driving current Idrv output from the driving constant current source 100 to the organic EL element EL11.

The discharge switch 180 is turned on during the discharge period to discharge the charge charged in the organic EL element EL11 to the ground terminal GND.

The switching controller 190 ′ applies a control signal to the precharge switch 160, the driving switch 170, and the discharge switch 180 to control an on / off time.

As described above, in the driving circuit of the organic EL display device of the PAM driving method according to the second embodiment of the present invention, the precharge current controller 140 is provided from the driving current controller 130 'for implementing gray levels. Information on the amount of change in Vop_drv is received. Therefore, as shown in Equation 1 mentioned in FIG. 3, the precharge voltage Vop_prec is changed to correspond to the change of the driving voltage Vop_drv of the anode of the organic EL device to be switched from the precharge period to the driving period. Luminance deviation can be reduced by making the voltage difference (Vop) generated at the same time have the same value for each common cathode line.

According to the present invention having the above configuration, when the precharge current is varied according to the gradation level of the driving current, and the switching from the precharge period to the driving period, the voltage difference (ㅿ Vop) of the anode electrode of the organic EL element is changed for each common cathode line. By having the same value, the luminance deviation can be reduced.

In addition, by making the precharge current variable, the power consumption is reduced.

Claims (8)

A driving circuit of a passive matrix organic EL display device which is driven in a precharge section, a driving section, and a discharge section to display a predetermined image, A driving constant current source supplying a driving current to the organic EL element during the driving period; A precharge constant current source supplying a precharge current to the organic EL element during a precharge period; A ground terminal for discharging a charge of the organic EL device during a discharge period; A driving current controller for applying a bias voltage for controlling a current value of the driving constant current source; A precharge current controller for applying a bias voltage for controlling a current value of the precharge constant current source; And And a switching controller configured to apply a control signal for connecting the precharge constant current source, the driving constant current source, and the ground terminal to the anode electrode of the organic EL element according to a driving period. The precharge current controller varies the precharge current value by adjusting the bias voltage according to the change of the data gradation level, and receives the precharge current information from the switching controller to receive the change amount of the driving voltage of the anode electrode of the organic EL element. A driving circuit of an organic EL display device, characterized by adjusting a charge current value. The method of claim 1, And the switching controller implements a gray scale level by adjusting a switching time of supplying the driving current to the organic EL element. delete The method of claim 1, The precharge current controller implements a digital / analog converter (DAC) for precharge bias and transfers the precharge level determined according to the change amount of the driving voltage of the anode of the organic EL device to the digital / analog converter (DAC). A drive circuit of an organic EL display device, characterized by adjusting a precharge current value by adjusting a bias of a precharge constant current source. The method of claim 1, And the precharge current controller receives a precharge level determined according to a change amount of a driving voltage of an anode of the organic EL element to adjust a precharge time. The method of claim 1, And the driving current controller controls the magnitude of the driving current supplied from the driving constant current source to the organic EL element to implement a gradation level. The method of claim 6, And the precharge current control unit receives information on the amount of change in the drive voltage of the anode electrode of the organic EL element from the drive current control unit to adjust the precharge current value. The method of claim 7, wherein The precharge current controller implements a digital / analog converter (DAC) for precharge bias and transfers the precharge level determined according to the change amount of the driving voltage of the anode of the organic EL device to the digital / analog converter (DAC). A drive circuit for an organic EL display device, characterized by adjusting a precharge current value by adjusting a bias of a precharge constant current source.

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