JP2007232902A - Driver of organic el display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To allow luminance of light emission of an organic EL element to be made lower, even when the potential of a signal electrode is set to a high value, before the selection of scan electrodes when reducing the value of the constant current, caused to flow to the organic EL element to perform dimming display. <P>SOLUTION: When dimming display is performed, the potential of a scan electrode to be selected in a selection period, after a precharge period, is set to a non-selection potential V<SB>CH</SB>, and a potential of each signal electrode is set to a potential V<SB>PRE</SB>higher than the potential V<SB>CH</SB>, in the precharge period. In the subsequent offset period, the potential of the scan electrode to be selected in the selection period following the offset period is set to a potential V<SB>MID</SB>, and a constant current is caused to flow to the organic EL element, from the signal electrode of a column where a pixel which will emit light in the selection period following the offset period exists. The potential V<SB>MID</SB>is determined so as to satisfy V<SB>DIM</SB>-V<SB>p</SB><V<SB>MID</SB>≤V<SB>DIM</SB>, where V<SB>DIM</SB>is the potential of the signal electrode converged by causing the constant current to flow, and V<SB>p</SB>is the light emission start voltage. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a drive device for an organic EL (Electroluminescence) display device.

  An organic EL element has an organic thin film between an anode and a cathode. Organic EL elements have characteristics similar to semiconductor light emitting diodes. That is, even when a voltage is applied between the electrodes so that the cathode has a higher potential than the anode, almost no current flows through the organic thin film, and the organic thin film does not emit light. Conversely, when a voltage equal to or higher than a predetermined voltage (light emission start voltage) is applied between the two electrodes so that the anode has a higher potential than the cathode, a current flows through the organic thin film, and the organic thin film emits light. An organic EL display device using this light emission is known.

  When a voltage is applied between both electrodes so that the cathode has a higher potential than the anode, almost no current flows through the organic thin film, but the capacitance of the organic EL element is charged by a minute current. .

  In an organic EL display device, for example, a plurality of scanning electrodes and a plurality of signal electrodes are arranged so as to be orthogonal to each other with an organic thin film interposed therebetween. In such an organic EL display device, the intersection of each scanning electrode and each signal electrode emits light as a pixel. Further, the pixels are arranged in a matrix. In this case, the organic EL display device is driven by line sequential driving. That is, each scan electrode is sequentially selected, and a constant current is supplied to the scan electrode selected from the signal electrode in accordance with the display data of the selected row.

  When the organic EL display device is driven by line sequential driving, a reverse bias voltage is applied to the organic EL elements present in the same column as the pixels that are not allowed to emit light and present in unselected rows. The charge is accumulated. When an organic EL element in which charge is accumulated by applying a reverse bias voltage is caused to emit light, the charge must first be discharged, which delays the start of light emission of the organic EL element. In view of this, a driving method for discharging charges accumulated by applying a reverse bias voltage has been proposed (for example, Patent Document 1). In the driving method described in Patent Document 1, when a constant current is applied to an organic EL element, a predetermined forward bias voltage is directly applied to the organic EL element, and charging to the organic EL element is performed by applying a forward bias voltage. I do. Then, when the voltage is not selected, the discharge of the accumulated charge is accelerated by applying a reverse bias voltage, and the delay of the light emission start of the organic EL element can be prevented.

  Note that the reverse bias means that the level relationship between the potential of the signal electrode and the potential of the scanning electrode is opposite to that when the pixel emits light. Further, the forward bias means that the potential of the signal electrode is higher than the potential of the scanning electrode.

  In addition, in a display device or the like of an in-vehicle navigation system, switching is performed so that an image is displayed with high luminance when the surrounding is bright and an image is displayed with low luminance when the surrounding is dark. In such switching, display with high luminance is referred to as normal display. A display with low luminance is referred to as a dimming display.

  When switching to dimming display, the constant current value that flows through the organic EL element may be lowered. By reducing the constant current value flowing through the organic EL element, the light emission luminance of the organic EL element is reduced, and dimming display is realized. A method for realizing such a dimming display is described in Patent Document 2, for example.

Japanese Patent Laid-Open No. 11-45071 (paragraphs 0021-0023, FIG. 1) JP 2005-181967 (paragraphs 0010, 0021-0130)

  When dimming display is performed by lowering the constant current value flowing through the organic EL element, the potential of each signal electrode is scanned while the scan electrode is set to the non-selected potential before selecting one scan electrode. Setting the potential higher than that of the electrode accelerates the discharge of the accumulated charge when a reverse bias voltage is applied.

  However, after that, when the selection period starts, the pixel emits light with a higher brightness than the desired brightness, and the light emission brightness during the dimming display becomes higher than the desired brightness. Hereinafter, this reason will be described with reference to FIG. In the following description, the constant current that flows through the light emitting pixel during normal display is referred to as a first constant current. In addition, a constant current flowing through the light emitting pixel during dimming display is referred to as a second constant current.

FIG. 5 shows the potential change of each electrode when the potential of each signal electrode is set higher than that of the scan electrode while the scan electrode is set to the potential at the time of non-selection before selecting one scan electrode. It is explanatory drawing which shows. Further, FIG. 5 shows a potential change in the dimming display. The potential _VCH shown in FIG. 5 is a potential set to the scan electrode of the non-selected row (hereinafter referred to as a non-selection potential). The potential V _SS shown in FIG. 5 is a potential (hereinafter referred to as a selection potential) set to the scan electrode of the selected row. The potential V _CH is higher than the potential V _SS . V _DIM shown in FIG. 5 is a signal electrode potential when a second constant current is passed through the pixel. In this example, it is assumed that the potential of the signal electrode in the column where the pixel that does not emit light in the selected row exists is set to _VSS . V _PRE shown in FIG. 5 is a potential set in each signal electrode before the start of each selection period, and is higher than the non-selection potential V _CH . The potential V _DIM is lower than the potential V _PRE .

A period in which the potential of each signal electrode is set higher than the potential of the scan electrode before the start of the selection period of a scan electrode is referred to as a precharge period. The precharge period, the scanning electrodes selected in the selection period immediately after is set to non-selection period potential V _CH, the potential of each signal electrode is set to V _PRE. Then, the charge accumulated by applying the reverse bias voltage is discharged within the precharge period.

When the selection period is started after the precharge period, a second constant current is supplied from the signal electrode in the column where the pixels to be lit are present during the selection period. At this time, it is preferable that the potential of the signal electrode immediately changes from the potential V _PRE to the potential V _DIM as indicated by a broken line in FIG. However, in practice, as indicated by the solid line in FIG. 5, it takes time until the potential of the signal electrode converges from V _PRE to V _DIM .

When dimming display is performed at a desired low luminance, it is ideal that the integral value of the current in the selection period is “the product of the current value of the second constant current and the selection period”. However, since it takes time until the potential of the signal electrode converges from V _PRE to V _DIM , the integrated value of the current within the selection period becomes larger than an ideal value. The integral value of the current within the selection period is larger than the ideal value by the integral value of the current corresponding to the integral value of the voltage indicated by the hatching in FIG. This means that more current than the desired current flows through the organic EL element at the start of the selection period. Therefore, when the selection period starts, the pixel emits light with a luminance higher than the desired luminance, and the light emission luminance during dimming display becomes higher than the desired luminance.

FIG. 6 is an explanatory diagram showing the relationship between the current flowing through the organic EL element and the light emission luminance of the organic EL element. The solid line shown in FIG. 6 shows the relationship between the constant current supplied from the constant current circuit connected to the signal electrode and the light emission luminance. As shown by the solid line in FIG. 6, the constant current flowing from the constant current circuit and the light emission luminance are substantially proportional. However, as described with reference to FIG. 5, it takes time for the potential of the signal electrode to converge from V _PRE to V _DIM, and during that period, in addition to the constant current flowing from the constant current circuit, A current due to a potential difference between the signal electrode and the scan electrode is also generated. The broken line shown in FIG. 6 shows the relationship between the luminance and the current added with the current other than the constant current flowing from the constant current circuit. At the time of dimming display, the second constant current having a low current value is supplied to drive down the luminance, but a current different from the constant current supplied from the constant current circuit is also generated. As indicated by the broken line in FIG. 6, the luminance does not decrease.

  In view of this, the present invention reduces the light emission luminance of the organic EL element even when the potential of the signal electrode is set high before selecting the scanning electrode when performing dimming display by reducing the constant current value passed through the organic EL element. An object of the present invention is to provide a drive device for an organic EL display device that can achieve low luminance.

Aspect 1 of the present invention is a drive device for an organic EL display device in which an organic thin film is disposed between a plurality of scan electrodes and a plurality of signal electrodes, and includes a scan electrode driver for selecting a scan electrode, and a signal electrode A signal electrode driver for passing a constant current through the organic thin film;
A control unit that instructs the signal electrode driver to display a first display mode or a second display mode that displays an image at a lower luminance than the first display mode, and the control unit instructs the second display mode. The first period, the second period following the first period, and the selection period, which is the period following the second period and selecting the scan electrode, with respect to the scan electrode driver. Instruct the signal electrode driver to start the first period and start the second period, respectively, and when the scan electrode driver is instructed to start the first period, The potential of the scan electrode selected in the subsequent selection period is set to the non-selection potential, and when the start of the second period is instructed, the potential of the scan electrode selected in the selection period after the second period is not selected Decrease from the time potential, the start of the selection period Sometimes, the potential of the scanning electrode to be selected is set to the potential at the time of selection, and the signal electrode driver causes the first constant current to flow when the first display mode is instructed, and the second display mode is instructed. When a second constant current having a current value lower than that of the first constant current is supplied, and the start of the first period is instructed, the potential of each signal electrode is set to a predetermined potential higher than the non-selection potential. When the start of the second period is instructed, the driving of the organic EL display device is characterized in that a second constant current is supplied to a signal electrode in which a pixel that emits light in a selection period after the second period exists. Providing equipment.

  According to Aspect 2 of the present invention, in Embodiment 1, when the scan electrode driver is instructed to start the second period, the potential of the scan electrode selected in the selection period after the second period is equal to the second constant current. Of the organic EL display device that lowers the potential of the scan electrode so that the difference between the potential of the scan electrode and the potential of the signal electrode is less than the light emission start voltage of the organic thin film. A drive device is provided.

  Aspect 3 of the present invention is the aspect 1 or aspect 2, in which when the start of the second period is instructed by the scan electrode driver, the potential of the scan electrode selected in the selection period after the second period is not selected. Provided is a drive device for an organic EL display device in which the time potential is lowered stepwise.

  According to the present invention, when performing display in the second display mode (when performing dimming display), even if the potential of the signal electrode is set high before selecting the scan electrode, the emission luminance of the organic EL element is reduced. can do.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of the present invention. The organic EL display device (hereinafter referred to as display panel) 10 includes a plurality of scanning electrodes (not shown) and a plurality of signal electrodes (not shown). Each scanning electrode and each signal electrode are arranged so as to be orthogonal to each other with the organic thin film interposed therebetween. The intersection between each scanning electrode and each signal electrode is a pixel. In each pixel, when an electric current flows from the signal electrode side to the scanning electrode side, the organic thin film of the pixel emits light. In the present embodiment, the signal electrode is an anode and the scanning electrode is a cathode. The display panel 10 performs both normal display (first display mode) and dimming display (second display mode) according to the driving device of the present invention.

  The drive device that drives the display panel 10 includes a controller (control unit) 1, a display RAM 5, a decoder 6, a scan electrode driver 2, a signal electrode driver 4, and a power supply circuit 8. The scan electrode driver 2 includes a scan control circuit 3.

  The display RAM 5 is a storage device that stores image data of an image to be displayed on the display panel 10. The decoder 6 decodes the image data stored in the display RAM 5 and outputs the decoded image data to the signal electrode driver 4.

  The controller 1 controls the scanning electrode driver 2 and the signal electrode driver 4 to display an image on the display panel 10. In addition, the controller 1 causes the display RAM 5 to output image data to the decoder 6. Alternatively, the controller 1 may cause the decoder 6 to read the image data stored in the display RAM 5.

  The controller 1 instructs the scan control circuit 3 to start scanning from the first row (FLM: first line marker), a control signal that instructs the start of the precharge period (first period), and an offset period. A control signal instructing the start of the (second period) and a control signal instructing the start of the selection period are output. However, the controller 1 outputs a control signal instructing the start of the offset period only during dimming display, and does not output a control signal instructing the start of the offset period during normal display. That is, the controller 1 controls the scan control circuit 3 to instruct the start of the precharge period in addition to the FLM during normal display (when the signal electrode driver 4 is instructed to perform normal display). A signal and a control signal instructing the start of the selection period are output. In addition, the controller 1 controls the scan control circuit 3 to start the precharge period in addition to the FLM during dimming display (when the signal electrode driver 4 is instructed to perform dimming display). A signal, a control signal for instructing the start of the offset period, and a control signal for instructing the start of the selection period are output. As will be described later, the signal electrode driver 4 changes the constant current value to be output depending on whether normal display or dimming display is instructed.

  The precharge period is a period in which the potential of each signal electrode is set to a potential higher than the potential of the scan electrode before the start of a certain scan electrode selection period. Within the precharge period, the charge accumulated by applying a reverse bias voltage is discharged.

  The offset period is a period in which the potential of the signal electrode set in the precharge period is converged to a potential when a constant current is passed from the signal electrode to the organic thin film. The offset period is provided only during dimming display, and is not provided during normal display.

  The selection period is a period for selecting individual scan electrodes.

  During normal display, an offset period is not provided, a precharge period is provided immediately before the selection period, and the precharge period and the selection period are alternately provided. In the dimming display, a precharge period is first provided before the start of the selection period of each scan electrode, followed by an offset period, and then the selection period is started. Therefore, at the time of dimming display, a precharge period, an offset period, and a selection period are repeatedly provided.

The non-selection time potential (potential of setting the scanning electrodes of non-selected rows) denoted V _CH. Further, representing the selected period potential (a potential set to the scanning electrodes in a selected row) and V _SS. The potential V _CH is higher than the potential V _SS . The potential _VSS is, for example, a ground potential.

In addition, a potential set to each signal electrode in the precharge period is represented as _VPRE . Potential _{V PRE} is a voltage higher than the potential _{V CH.} However, V _PRE is determined so that the relationship of V _PRE −V _CH <V _p is satisfied when the light emission start voltage is V _p . In addition, the potential of the signal electrode when a second constant current (a constant current that flows to the pixel that emits light during dimming display) is supplied to the pixel that emits light from the signal electrode is represented as V _DIM . Further, in this embodiment mode, description is made on the assumption that the potential of the signal electrode in the column where the pixel that does not emit light in the selected row exists is set to _VSS .

During the offset period, the scan electrode selected in the selection period after the offset period is set to a predetermined potential V _MID . Note that the potential V _MID is a potential that satisfies V _DIM −V _p <V _MID ≦ V _DIM . Also, V _SS <V _MID .

  The scan electrode driver 2 starts scanning from the first row when FLM is input from the controller 1. That is, the operation of sequentially selecting scan electrodes from the first scan electrode is started.

  The scan electrode driver 2 scans the scan electrodes by sequentially selecting individual scan electrodes one by one in accordance with the controller 1. The scan control circuit 3 provided in the scan electrode driver 2 sets the potential of each scan electrode.

The scan electrode driver 2 includes a switch (not shown) corresponding to each scan electrode. One switch, the scanning electrodes corresponding to the switch, the output terminal of the voltage V _CH, the output terminal of the voltage V _MID, or is connected to the output terminal of the voltage V _SS.

When a control signal instructing the start of the precharge period (hereinafter referred to as _SPRE ) is input from the controller 1, the scan control circuit 3 sets the potentials of all the scan electrodes to the non-selection potential _VCH . . That is, switching each switch so that all scanning electrodes are connected to the output terminal of the voltage V _CH. Therefore, during the precharge period, the potential of the scan electrode selected in the selection period after the precharge period is _VCH .

The scan control circuit 3, a control signal instructing the start of the offset period (hereinafter, referred to as S _OS.) When is input from the controller 1, the potential of the scan electrodes selected in the selection period after the offset period _Is set to _VMID , and the potentials of the other scan electrodes are maintained at _VCH . That is, the switch of the scan electrode is switched so that the scan electrode selected in the selection period is connected to the output terminal of the voltage _VMID , and the other switches are not changed. Therefore, during the offset period, the potential of the scan electrode selected in the selection period after the offset period becomes _VMID .

The scan control circuit 3, a control signal instructing the start of the selection period (hereinafter, referred to as LP.) Is the input from the controller, to set the potential of the scan electrodes be selected during the selection period to V _SS , to maintain the potential of the other scanning electrodes remain V _CH. That is, the scanning electrodes selected in the selection period switches the switch of the scanning electrodes to be connected to the output terminal of the voltage V _SS, do not change with respect to other switches. Thus, during the selection period, the potential of the scan electrodes being selected becomes V _SS.

Further, the controller 1 outputs S _PRE (a control signal for instructing the start of a precharge period) and LP (a signal for instructing the start of a selection period) to the signal electrode driver 4 during normal display, and dimming At the time of display, S _PRE and S _OS (control signal for instructing the start of the offset period) are output to the signal electrode driver 4.

Signal electrode driver 4 according to the controller 1, to set the potential of the signal electrodes to V _PRE or V _SS, or a constant current is supplied from the signal electrode to the organic thin film.

FIG. 2 is an explanatory diagram showing a configuration example of the signal electrode driver 4. In the configuration illustrated in FIG. 2, the signal electrode driver 4, in correspondence with the signal electrodes, provided with a switch 21, a constant current circuit 22, the precharge voltage output unit 23, and an output terminal 24 of the voltage V _SS Yes.

  The constant current circuit 22 supplies a constant current. The constant current circuit 22 is controlled by the signal electrode driver 4 so that a first constant current flows during normal display and a second constant current flows during dimming display. The current value of the second constant current is lower than the current value of the first constant current.

The precharge voltage output unit 23 converts the voltage (V _SH ) output from the power supply circuit 8 (see FIG. 1) to the signal electrode driver 4 into the voltage V _PRE and outputs the voltage V _PRE . The voltage V _SH and the voltage V _PRE may be equal.

The output terminal 24 of the voltage _{V SS,} a voltage _{V SS} supplied from the power supply circuit 8 (see FIG. 1.).

In Figure 2, switch 21, constant current circuit 22, the precharge voltage output unit 23, and an output terminal 24 of the voltage _{V SS} shown a set, each signal electrode, the switch 21, the constant current circuit 22, a precharge voltage output unit 23, and an output terminal 24 of the voltage _{V SS} is provided.

S _PRE is input from the controller 1 to the signal electrode driver 4 during normal display and during dimming display. When S _PRE is input from the controller 1, the signal electrode driver 4 sets the potentials of all the signal electrodes to V _PRE . That is, the switch 21 of each signal electrode is switched so that all the signal electrodes are connected to the voltage output terminals of the precharge voltage output unit 23. Therefore, the potential of each signal electrode is V _PRE during the precharge period.

LP is input from the controller 1 to the signal electrode driver 4 during normal display. When LP is input from the controller 1, the signal electrode driver 4 is connected to the current output terminal of the constant current circuit 22 in the column where the pixel to be lit during the selection period is present, and does not emit light in the selected row. there so as to connect the signal electrode of the present column to an output terminal 24 of the voltage V _SS, switches the switch 21 of the signal electrodes. During normal display, the signal electrode driver 4 controls the constant current circuit 22 so that the first constant current flows from the constant current circuit 22. Therefore, during the selection period, the first constant current flows from the signal electrode in the column where the pixel to be lit exists to the organic EL element 25, and the organic EL element 25 emits light. The potential of the signal electrodes of the column in the selection period, the pixels do not emit light in the selected row is present, it is set to V _SS, no current flows through the organic EL element 25 from the signal electrode.

S _OS, upon dimming the display, input from the controller 1 to the signal electrode driver 4. When _SOS is input from the controller 1, the signal electrode driver 4 is connected to the current output terminal of the constant current circuit 22 and the signal electrode of the column in which the pixel to be lit during the selection period immediately thereafter is connected to the selected row. the signal electrodes of the columns that do not emit light pixels are present so as to connect the output terminal 24 of the voltage V _SS in, switches the switch 21 of the signal electrodes. Thereafter, the signal electrode driver 4 maintains the state of the switch 21 of each signal electrode until _SPRE is input next time. During dimming display, the signal electrode driver 4 controls the constant current circuit 22 so that the second constant current flows from the constant current circuit 22.

When the signal electrode connected to the voltage output terminal of the precharge voltage output unit 23 is connected to the current output terminal of the constant current circuit 22 at the start of the offset period, the potential of the signal electrode decreases from V _PRE. Converge to V _DIM . The offset period is defined as a period during which the potential of the signal electrode can be converged to V _DIM as it decreases from V _PRE . After the potential of the signal electrode converges from V _PRE to V _DIM during the offset period, the period shifts to the selection period. Even during the selection period, the potential of the signal electrode remains V _DIM .

The signal electrode is connected to a voltage output terminal of the precharge voltage output unit 23, when connected to an output terminal 24 of the voltage V _SS at the start of the offset period, the potential of the signal electrode becomes V _SS. After the transition to the selection period, the potential of the signal electrode remains at V _SS.

  If the signal electrode driver 4 determines, based on the image data output from the decoder 6, the signal electrode of the column where the pixel to be lit during the selection period and the signal electrode of the column where the pixel which is not lit in the selected row are present. Good.

  Further, the controller 1 outputs a control signal for instructing the signal electrode driver 4 to perform normal display or dimming display, and the signal electrode driver 4 outputs the first signal if normal display is instructed. The constant current circuit 22 is controlled so that a constant current flows. If the dimming display is instructed, the constant current circuit 22 is controlled so that the first constant current flows.

  Note that the controller 1 only has to indicate whether normal display or dimming display is performed in accordance with a command from an external device (not shown). For example, when the driving device drives a display panel provided in the in-vehicle navigation system, the controller 1 instructs whether to perform normal display or dimming display according to a command from the MPU of the in-vehicle navigation system. do it.

In the present embodiment, the power supply circuit 8 shown in FIG. 1 outputs voltages V _CH , V _MID , and V _SS to the scan electrode driver 2. The power supply circuit 8 outputs voltages V _SH and V _SS to the signal electrode driver 4. The voltage V _SH is a voltage for operating the constant current circuit 22 shown in FIG. 2 (that is, a voltage for causing the constant current circuit 22 to output a constant current).

  Next, light emission of the organic EL element during normal display will be described. During normal display, the signal electrode driver 4 controls the constant current circuit 22 (see FIG. 2) so that the first constant current flows. Further, during normal display, a precharge period and a selection period are alternately provided.

During the precharge period, the scan control circuit 3 sets the potential of the scan electrode selected in the immediately subsequent selection period to _VCH , and the signal electrode driver 4 sets the potential of each signal electrode to _VPRE . Since V _PRE > V _CH , the forward bias voltage is applied to the pixels in the row selected in the immediately _subsequent selection period, and the charge accumulated when the reverse bias voltage is applied is discharged. Further, since V _PRE is determined so that the relationship of V _PRE −V _CH <V _p is established, each pixel does not emit light during the precharge period.

Subsequently, at the selection period, the scan control circuit 3 sets the potential of the scan electrode to be selected V _SS, sets the potential of the other scanning electrodes to V _CH. Further, the signal electrode driver 4 connects a signal electrode in which a pixel to be lit during the selection period exists to the constant current circuit 22 (see FIG. 2). Then, the potential of the signal electrode decreases from V _PRE and changes to a potential (referred to as V _S ) when a first constant current is passed from the signal electrode to the light emitting pixel. The potential V _S of the signal electrode when the first constant current is passed from the signal electrode to the light emitting pixel is higher than V _DIM . Therefore, the time until the potential of the signal electrode decreases from V _PRE to converge to V _S is short, and the integrated value of the current within the selection period is an ideal value (the current value of the first constant current and the selected value). It is a value close to the product of the period. Accordingly, the pixel can emit light with desired bright brightness.

The signal electrode driver 4 sets the potential of the signal electrodes of the column there is a pixel which does not emit light in the selected row to V _SS. Since the potential of the signal electrode is equal to the potential of the selected row scanning electrode, no current flows from the signal electrode, and the pixels in the column do not emit light.

  Next, the light emission of the organic EL element during dimming display will be described with reference to FIG. As already described, at the time of dimming display, the signal electrode driver 4 controls the constant current circuit 22 (see FIG. 2) so that the second constant current flows. In dimming display, a precharge period, an offset period, and a selection period are repeatedly provided.

FIG. 3 is an explanatory diagram illustrating an example of changes in the potentials of the scan electrode and the signal electrode during dimming display. During the precharge period, the scan control circuit 3 sets the potential of the scan electrode selected in the selection period after the precharge period to _VCH . The signal electrode driver 4 sets the potential of each signal electrode to V _PRE . Since V _PRE > V _CH , the forward bias voltage is applied to the pixels in the row selected in the immediately _subsequent selection period, and the charge accumulated when the reverse bias voltage is applied is discharged. Further, since V _PRE is determined so that the relationship of V _PRE −V _CH <V _p is established, each pixel does not emit light during the precharge period. Note that in FIG. 3, the height of the potential V _PRE is shown high for the sake of convenience in order to easily show the change from the potential V _PRE to the potential V _DIM . This also applies to FIG.

Subsequently, at the offset period, the scan control circuit 3 sets the potential of the scanning electrodes selected in the selection period immediately after the V _MID, maintaining the potential of the other scanning electrodes remain V _CH. Further, the signal electrode driver 4 connects the signal electrodes in the column where the pixels to be lit in the immediately subsequent selection period are connected to the constant current circuit 22 (see FIG. 2). Then, the potential of the signal electrode decreases from V _PRE to V _DIM (see FIG. 3). At this time, if the difference between the potential of the signal electrode and the potential V _{MID of the} scan electrode is equal to or higher than the light emission start voltage V _p , the pixel in the column emits light, but the potential of the signal electrode and the potential V _{MID of the} scan electrode When the difference between the two becomes less than the emission start voltage V _p , no light is emitted. Further, since the relationship of V _DIM −V _p <V _MID ≦ V _DIM is established, after the potential of the signal electrode converges to V _DIM during the offset period, the pixel does not emit light until the selection period starts.

The signal electrode driver 4 sets the potential of the signal electrodes of the column in which the pixel does not emit light in the selected line after the selection period is present to V _SS. Since V _SS <V _MID , the pixels in that column do not emit light during the offset period.

Subsequently, at the selection period, the scan control circuit 3 sets the potential of the scan electrode to be selected V _SS, sets the potential of the other scanning electrodes to V _CH. Further, the signal electrode driver 4 does not switch the switch of each signal electrode even when the selection period is started. As a result, the second constant current flows from the signal electrode in the column where the pixel to be lit during the selection period to the selected row scanning electrode via the organic EL element, and the organic EL element (pixel) emits light. The potential of the signal electrodes of the column there is a pixel that does not emit light in the selected row, since it is V _SS is equipotential and the potential of the selected row scanning electrodes, the pixel does not emit light.

In the present embodiment, the integrated value of the current within the selection period is an ideal value (product of the current value of the second constant current and the selection period). In the offset period, the pixel emits light when the difference between the signal electrode potential and the scan electrode potential V _MID is equal to or higher than the light emission start voltage V _p, but the difference between the signal electrode potential and the scan electrode potential V _MID. There goes below the emission start voltage V _p, not emit light. Further, after the potential of the signal electrode converges to V _DIM during the offset period, the pixel does not emit light until the selection period starts. Therefore, generation of an undesirable current can be suppressed as compared with the conventional driving method. As a result, even if the potential of the signal electrode is set to V _PRE before selecting the scanning electrode, the light emission luminance of the organic EL element at the time of dimming display can be made lower than before to improve the light emission luminance.

  Next, a modification of the embodiment of the present invention will be described. Also in this modification, the structure of the drive device of the organic EL display device is the same as that described above (see FIG. 1). Further, the operation of the driving device during normal display is the same as the operation during normal display already described.

  The operations of the components other than the scan control circuit 3 during the dimming display are the same as the operations during the dimming display already described.

In the dimming display, the operation of the scan control circuit 3 provided in the scan electrode driver 2 is different from the operation of the scan control circuit 3 in the dimming display already described. In addition to the voltages V _CH , V _MID , and V _SS , the power supply circuit 8 outputs other levels of voltage to the scan electrode driver 2. This voltage referred to as _{V 1.} The voltage V ₁ is a voltage that satisfies V _MID <V ₁ <V _CH . A switch (each switch corresponding to each scan electrode) included in the scan electrode driver 2 includes a scan electrode as an output end of the voltage V _CH, an output end of the voltage V _1, an output end of the voltage V _MID , or the voltage V Connect to the _SS output.

The scan control circuit 3 in the present modification operates in the precharge period and selection period during dimming display in the same manner as the precharge period and selection period during dimming display already described. However, the operation in the offset period is different from the operation already described. When S _OS (control signal instructing the start of the offset period) is input from the controller 1, the scan control circuit 3 sets the potential of the scan electrode selected in the selection period after the offset period during the offset period. _Therefore, the potential of the other scan electrode is maintained at _VCH . That is, the scan control circuit 3 switches the switch of the scan electrodes such that scan electrodes selected by the selection period is connected to the output terminal of the voltage V _1, then further output terminal of the voltage V _MID is the scanning electrodes The switch of the scan electrode is switched so as to be connected to Further, the scan control circuit 3 does not change the switches of the other scan electrodes. Therefore, during the offset period, the potential of the scan electrode selected in the selection period after the offset period decreases from V _CH to V ₁ and further decreases to V _MID . As described above, in this modification, the potential of the scan electrode selected immediately after the offset period is lowered stepwise from V _CH to V _MID .

  FIG. 4 is an explanatory diagram showing an example of changes in the potentials of the scan electrode and the signal electrode during dimming display in the present modification. Since the operation of the signal electrode driver 4 is the same as the operation already described, the potential change of the signal electrode is the same as the potential change of the signal electrode shown in FIG. In addition, since the operation of the scan control circuit 3 in the precharge period and the selection period is the same as that already described, the potential of the scan electrode in this period is the same as the potential of the scan electrode shown in FIG.

As shown in FIG. 4, in the offset period, the scan control circuit 3 _first sets the potential of the scan electrode selected in the immediately subsequent selection period to V1. Then, during the offset period, the potential of the scan electrode is reset from V ₁ to V _MID . Scan control circuitry 3 maintains the potential of the other scanning electrodes remain V _CH. Further, the signal electrode driver 4 connects the signal electrodes in the column where the pixels to be lit in the immediately subsequent selection period are connected to the constant current circuit 22 (see FIG. 2). Then, the potential of the signal electrode decreases from V _PRE to V _DIM (see FIG. 4).

At this time, when the difference between the potential of the scanning electrodes selected immediately after the potential of the signal electrode is the light emission start voltage V _p or more, the pixel in that column are to emit light, selection immediately after the potential of the signal electrodes the difference between the potential of the scan electrodes is less than the emission start voltage V _p, the pixel is not emit light. Further, since the relationship of V _DIM −V _p <V _MID ≦ V _DIM is established, after the potential of the signal electrode converges to V _DIM during the offset period, the pixel does not emit light until the selection period starts.

In this modification, at the start of the offset period, the scan electrode selected immediately after is set to the potential V ₁ that satisfies V _MID <V ₁ <V _CH , and then set to V _MID . Therefore, after the start of the offset period, there occurs a period in which the difference between the potential of the signal electrode that has not decreased so much from V _PRE and the potential V _{1 of the} scan electrode is less than the light emission start voltage V _p , during which the pixel is Does not emit light. Therefore, the period in which the pixels emit light in the offset period is shorter than in the case shown in FIG. Therefore, it is possible to further suppress light emission of an undesired pixel as compared with the case of the above-described embodiment. Further, the point that the integrated value of the current within the selection period becomes an ideal value (the product of the current value of the second constant current and the selection period) is the same as in the above-described embodiment. Therefore, in this modification, the light emission luminance of the organic EL element at the time of dimming display can be further lowered to improve the light emission luminance.

In this modification, the potential of the scan electrode in the offset period is changed to V ₁ and V _MID , but the number of stages of potential change of the scan electrode in the offset period may be further increased. Furthermore, as another modification, the potential of the scan electrode in the offset period may be decreased linearly.

  The present invention is applicable to driving an organic EL display device.

1 is a block diagram illustrating an embodiment of the present invention. Explanatory drawing which shows the structural example of a signal electrode driver. Explanatory drawing which shows the example of the change of the electric potential of a scanning electrode and a signal electrode at the time of a dimming display. Explanatory drawing which shows the example of the change of the electric potential of a scanning electrode and a signal electrode at the time of a dimming display. Explanatory drawing which shows the potential change of each electrode when the potential of each signal electrode is set higher than the scan electrode while the scan electrode is set to the potential at the time of non-selection before selecting one scan electrode. . Explanatory drawing which shows the relationship between the electric current which flows through an organic EL element, and the light-emitting luminance of an organic EL element.

Explanation of symbols

1 controller 2 scan electrode driver 3 scan control circuit 4 signal electrode driver 5 display RAM
6 Decoder 10 Organic EL display device (display panel)

Claims (3)

A driving device for an organic EL display device in which an organic thin film is disposed between a plurality of scanning electrodes and a plurality of signal electrodes,
A scan electrode driver for selecting a scan electrode; and
A signal electrode driver for passing a constant current from the signal electrode to the organic thin film;
A control unit for instructing the signal electrode driver to perform a second display mode for displaying an image with a lower luminance than the first display mode or the first display mode,
The control unit instructs the scan electrode driver to start the first period, start the second period following the first period, and the period following the second period when instructing the second display mode. Each instructing the start of a selection period, which is a period for selecting a scan electrode, and instructing the signal electrode driver to start the first period and the second period, respectively.
Scan electrode driver
When the start of the first period is instructed, the potential of the scan electrode selected in the selection period after the first period is set to the non-selection potential,
When the start of the second period is instructed, the potential of the scan electrode selected in the selection period after the second period is decreased from the non-selection potential,
When the start of the selection period is instructed, the potential of the scan electrode to be selected is set to the potential at the time of selection,
The signal electrode driver
When the first display mode is instructed, a first constant current is passed, and when the second display mode is instructed, a second constant current having a current value lower than the first constant current is passed,
When the start of the first period is instructed, the potential of each signal electrode is set to a predetermined potential higher than the non-selection potential,
When the start of the second period is instructed, a second constant current is supplied to a signal electrode in which a pixel that emits light in a selection period after the second period exists, and the driving device for the organic EL display device . When the scan electrode driver is instructed to start the second period, the potential of the scan electrode selected in the selection period after the second period is equal to or lower than the potential of the signal electrode through which the second constant current is passed. The drive of the organic EL display device according to claim 1, wherein the potential of the scan electrode is lowered so that a difference between the potential of the scan electrode and the potential of the signal electrode is less than a light emission start voltage of the organic thin film. apparatus. The scan electrode driver, when instructed to start the second period, gradually decreases the potential of the scan electrode selected in the selection period after the second period from the non-selection potential. Item 3. A driving device for an organic EL display device according to Item 2.

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