JP2010185953A - Driving method and driving circuit of organic el active matrix - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To sequentially update a threshold voltage, even when a low response speed amorphous Si-TFT is used for a driving TFT. <P>SOLUTION: In one horizontal period of a display device, the following processing is performed. A pulse voltage Vpre higher than the threshold voltage of the driving TFT is applied, a storage capacitor is discharged, and the gate voltage Vth' of the driving TFT is read during the electric discharge. The read voltage is corrected according to a prescribed relation between the threshold voltage Vth of the driving TFT when the electric discharge is completed and the gate voltage of the driving TFT during the electric discharge, to update the threshold voltage Vth of the driving TFT. Data Vdata are written in a pixel by using the updated threshold voltage Vth. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a driving method and a driving circuit of a display device. In particular, the present invention relates to a driving method and a driving circuit for a self-luminous display panel (display device) such as an EL display panel (display device) using an organic or inorganic electroluminescence (EL) element.

  In an active matrix image display device using an organic electroluminescence (EL) material or an inorganic EL material as an electro-optic conversion substance, light emission luminance changes according to a current written to a pixel. The EL display panel is a self-luminous type having a light emitting element in each pixel. The EL display panel has advantages such as higher image visibility, higher light emission efficiency, no backlight, and faster response speed than the liquid crystal display panel.

  FIG. 1 shows an equivalent circuit of one pixel of an already known active matrix type organic EL display panel. The pixel circuit 10 includes an organic electroluminescence (EL) element 11 which is a light emitting element, driving and switching TFTs (Thin Film Transistors) 12 and 13, and a storage capacitor C14.

  A driver circuit (not shown) for driving the pixel circuit 10 outputs a video (gradation) signal indicated by the strength of voltage, and its configuration is similar to that of a driver circuit for driving a liquid crystal display panel. A voltage signal as a video (gradation) signal is applied to the source signal line 15 from the driver circuit. The applied voltage signal is applied to the pixel circuit 10 and held in the storage capacitor C14.

  When the current Id corresponding to the voltage held in the storage capacitor C14 flows to the driving TFT 12, the organic EL element 11 emits light with a predetermined luminance corresponding to the video (gradation) signal.

  The organic EL display panel is composed of low-temperature polysilicon or amorphous TFT. However, when the characteristics of these TFTs vary in the organic EL element, display (brightness) unevenness occurs. In particular, in the case of an amorphous TFT, the threshold voltage Vth changes with time as the energization time increases as shown in FIG. For this reason, when a constant gate voltage Vg is applied to the driving TFT 12, the drive current Id gradually decreases, causing a problem that the light emission luminance decreases.

  In order to solve this problem, Patent Document 1 discloses that the pixel voltage is expressed by adding the display voltage Vdisp obtained by adding the gradation voltage Vdata for display after temporarily storing the threshold voltage Vth that changes with time by AD conversion or the like. Is disclosed.

  The above method will be described with reference to the timing diagram of FIG. 3 and the operation configuration diagrams of FIGS.

  First, referring to FIG. 4, the pixel circuit 10 has at least two switching TFTs 13 and 15 and one driving TFT 12 so that the threshold voltage Vth can be read.

  On the other hand, the source driver 20 includes a voltage generation source 21 that generates a gradation voltage Vdata, a voltage source 23 that supplies a display voltage, a precharge voltage source 24, an AD converter 25 that reads a threshold voltage Vth and converts it into a digital value, A storage circuit 26 that stores the threshold voltage Vth converted into a digital value, an adder 22 that digitally adds the threshold voltage Vth and the gradation voltage Vdata, and a control circuit 27 and SW1 to SW3 that control these in time series Has as a basic configuration.

  The operation of the above configuration will be described for each of steps ST1, ST2, ST3, and ST4 in FIG.

  First, in the first step ST1, as shown in FIG. 4, the switching TFT 13 is turned on to read the threshold voltage Vth, and a voltage Vpre sufficiently higher than the threshold voltage Vth is applied to the pixel circuit 10 in a pulse manner. The storage capacitor C14 is charged. At this time, the parasitic capacitors Cs1 and Cs2 are also charged.

  In the second step ST2, as shown in FIG. 5, when the switching TFT 15 is turned on in addition to the switching TFT 13 to discharge the storage capacitor C14 and the parasitic capacitors Cs1 and Cs2 gradually, the gate voltage Vg gradually decreases. When the gate voltage Vg decreases and becomes equal to the threshold voltage Vth of the driving TFT 12, the discharge is stopped. That is, the threshold voltage Vth of the driving TFT 12 is automatically stored in the storage capacitor C14. Note that the driving TFT 12 can store the threshold voltage Vth as described above because the resistance between the drain and the source where the gate voltage Vg is equal to or lower than the threshold voltage is high (in an off state).

  In the third step ST3, as shown in FIG. 6, the SW3 is turned on and the threshold voltage Vth is read by the AD converter 25.

  In the fourth step ST4, as shown in FIG. 7, the threshold voltage Vth and the gradation voltage Vdata read as described above are added by the adder 22, and the added voltage Vdisp is added to the pixel in the third step ST3 of the next cycle. By supplying the voltage to the circuit 10, the fluctuation of the threshold voltage Vth is compensated.

  When the above steps ST1 to ST4 are executed for each row of the matrix display panel, the change with time of the threshold voltage Vth of the driving TFT 12 in all the pixel circuits is compensated.

JP 2006-284716 A

  However, in the conventional method described above, since the mobility is low particularly when the driving TFT is an amorphous Si-TFT, it takes time to read the threshold voltage Vth due to the influence of the output capacitance Cs1 and the wiring capacitance Cs2 of the driving circuit IC. Take it. In addition, when the display panel resolution is improved, the TFT size becomes smaller, and this reading time becomes longer. FIG. 3 shows that the ideal broken line discharge voltage Vth actually becomes the solid line discharge voltage Vth ′ under the influence of the capacitance described above.

  The reading time of the threshold voltage Vth is determined by the on-resistance of the switching TFTs 13 and 15 in the pixel circuit 10, the storage capacitor C14, the wiring capacitor Cs2, and the output capacitor Cs1 of the source driver 20. Assuming that each on-resistance is several MΩ and each capacitance is around 50 pF, the time constant is about 100 us. Therefore, it takes three times as much time until the discharge is completed, that is, until the correct threshold voltage Vth is stored. .

  Therefore, when the display panel is, for example, QVGA (240 × 320) and has a frame frequency of 60 Hz, the time during which one row can be processed for sequential updating must be shorter than about 52 us in one horizontal period. For this reason, the threshold voltage Vth of the driving TFT of the display panel cannot be read and stored within one horizontal period, and there is an important problem that the threshold voltage Vth cannot be updated sequentially.

  An object of the present invention is to provide a driving method and a driving circuit for an organic EL display device capable of sequentially updating the threshold voltage Vth even when an amorphous Si-TFT having a low response speed is used as a driving TFT. is there.

  In order to achieve the above object, a driving method of an organic EL display device provided by the present invention includes an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT. In a driving method of a display device configured by arranging a plurality of pixels arranged in a matrix, a pulse voltage higher than the threshold voltage of the TFT is applied to the TFT gate within one horizontal period of the display device. Applying a precharge step, discharging the capacitor charged in the precharge step, reading a gate voltage of the TFT during the discharge, and reading the gate voltage when the discharge is finished By correcting according to a predetermined relationship between the threshold voltage of the TFT and the gate voltage of the TFT during the discharge, Updating step of updating the threshold voltage of the FT, and, and executes a writing step of writing data to the pixels using a threshold voltage the update.

In order to achieve the above object, another driving method of the organic EL display device provided by the present invention includes an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT. In a driving method of a display device in which a plurality of configured pixels are arranged in a matrix, one vertical synchronization period is set for n or more horizontal periods, and all the pixels of the plurality of pixels are set. Determining the reading order, and
The threshold voltage is read for at least one of the plurality of pixels within the vertical synchronization period, and the threshold voltage of each pixel is read according to the reading order determined for all the pixels.

In order to achieve the above object, a drive circuit for an organic EL display device provided by the present invention includes an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT. In the drive circuit of the display device configured by arranging a plurality of pixels configured in a matrix,
Means for updating the threshold voltage of the TFT within one horizontal period of the display device and writing data to the pixel using the updated threshold voltage, and the means for writing the data comprises the TFT Precharge means for applying a pulse voltage higher than a threshold voltage to the gate of the TFT, means for discharging the capacitor charged by the precharge means, and reading the gate voltage of the TFT during the discharge, the read gate Means for updating the threshold voltage of the TFT by correcting the voltage according to a predetermined relationship between the threshold voltage of the TFT when the discharge ends and the gate voltage of the TFT during the discharge, and the update It is characterized by comprising means for writing data into the pixel using the threshold voltage.

  In order to achieve the above object, another drive circuit of the organic EL display device provided by the present invention includes an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT. In a driving circuit of a display device configured by arranging a plurality of pixels arranged in a matrix, one vertical synchronization period is set for n or more horizontal periods, and all the pixels of the plurality of pixels are set. Means for determining the reading order, and reading the threshold voltage for at least one of the plurality of pixels within the vertical synchronization period, and reading the threshold voltage of each pixel according to the reading order determined for all the pixels Means are provided.

  According to the present invention described above, the threshold voltage is read in a short time during the termination to the threshold voltage, corrected by correlating with the ideal final threshold voltage, and the threshold voltage is updated for each pixel. As a result, the threshold voltage can be updated within one horizontal period.

  Further, according to the present invention, the vertical synchronization period is set for n or more horizontal periods, the threshold voltage of at least one pixel is read during this vertical synchronization period, and the threshold voltages are sequentially read for all the pixels. As a result, the threshold voltage can be sequentially updated.

It is a circuit diagram which shows the equivalent circuit of a general pixel circuit. It is a characteristic view explaining the relationship between the threshold voltage Vth and the drive current Id. It is a timing diagram which shows the conventional Vth reading timing. It is operation | movement explanatory drawing explaining the one operation | movement step in the conventional Vth reading. It is operation | movement explanatory drawing explaining the one operation | movement step in the conventional Vth reading. It is operation | movement explanatory drawing explaining the one operation | movement step in the conventional Vth reading. It is operation | movement explanatory drawing explaining the one operation | movement step in the conventional Vth reading. 1 is a configuration diagram of a first embodiment of a drive circuit of an organic EL display device according to the present invention. FIG. It is a timing diagram which shows the operation timing in 1st Embodiment. It is a figure explaining the threshold value reading in 1st Embodiment of this invention. It is a figure explaining the threshold value reading in 1st Embodiment of this invention. It is a block diagram of 2nd Embodiment of the drive circuit of the organic electroluminescence display which concerns on this invention. It is a timing diagram which shows the operation timing in 2nd Embodiment. It is a figure explaining the reading order in 2nd Embodiment.

[First Embodiment]
FIG. 8 is a configuration diagram of the first embodiment of the drive circuit of the organic EL display device according to the present invention. The drive circuit of the present embodiment has a configuration in which a Vth correction unit 31 is added to the conventional configuration shown in FIG. 4, and the Vth correction unit 31 correlates an ideal Vth and a Vth ′ during discharge as described later. It is provided to take

  FIG. 9 is a timing diagram showing an operation example of the drive circuit shown in FIG. 8, and FIG. 10 is a diagram for explaining threshold value reading in the present embodiment.

  In the example of FIG. 9, the Vth reading period for reading the threshold voltage Vth and the Vdisp writing period for writing the voltage Vdisp for display are included in one horizontal period Th. Therefore, as shown by the solid line in FIG. 10, the initial Vth ′ read at time tf within the Vth reading period becomes a voltage during discharge, and is higher than the ideal final Vth at time ts outside the Vth reading period. Become. When Vth changes with time from the initial stage, the threshold voltage Vth ′ read at time tf is further increased as indicated by a broken line.

  Here, the process of the present embodiment for obtaining an ideal final Vth that is not affected by various capacities or the like from the threshold voltage Vth ′ read during the discharge will be described.

The voltage Vth ′ during discharge can be expressed by the following equation.
Vth ′ = (Vpre−Vth) exp (−t / RC) + Vth

  In the above equation, Vpre is a preset precharge voltage, Vth is a threshold voltage, R is the on-resistance of the TFT 12, and C is the sum of the storage capacitor C14 and the parasitic capacitors Cs1 and Cs2 of the pixel circuit 10. t is a time for reading, and is represented by an elapsed time from the time when the gate signal 2 becomes high level and the discharge is started.

Based on the above equation, the threshold voltage Vth ′ read during the discharge decreases with the decay rate gradually decreasing according to the time constant RC. Therefore, as shown in FIG. (Determined by the R component and the C component), it can be considered that the following equation holds.
Vth '= K (Vpre-Vth) + Vth
When the above equation is transformed, it can be expressed as the following equation.

  That is, if the attenuation coefficient K determined by the known R component and C component is present, the threshold voltage Vth obtained by correcting Vth ′ according to the above equation is calculated from the voltage Vth ′ read by the AD converter 25 during the discharge. Can do.

  Accordingly, if the AD converter 25 reads Vth ′ after the lapse of the predetermined time t after applying the precharge voltage and corrects it as described above, it does not wait until the final value is settled and does not wait until the final value is reached. It is possible to know the ideal threshold voltage Vth from which the influence is eliminated within the period.

  The threshold voltage Vth ′ corrected as described above is added to the gradation voltage Vdata by the adder 22 and supplied to the gradation voltage source 23.

  As described above, according to the present embodiment, the threshold voltage in the middle of the discharge is read early and corrected by taking the correlation with the final value from which the influence has been eliminated so that it can be processed within one horizontal period. Each time the panel display voltage is changed, Vth can be updated sequentially.

[Second Embodiment]
FIG. 12 is a configuration diagram of a drive circuit of the organic EL display device according to the present invention. The drive circuit of the present embodiment has a configuration in which a vertical synchronization SW control unit 32 and a sequential circuit 33 that control the opening and closing of the SW circuit in synchronization with a vertical signal are added to the conventional configuration shown in FIG. The vertical synchronization SW control unit 32 controls SW1 and SW2.

  FIG. 13 is a timing chart showing an operation example of the drive circuit shown in FIG. 12, and FIG. 14 is a diagram for explaining threshold value reading in this embodiment.

  The sequential circuit 33 controls the vertical synchronization SW control unit 32 in the source driver 20 and the gate driver 40, and for each vertical synchronization period, for example, as shown in FIG. 13, the first pixel in the first row and the two pixels in the first row The order of reading the threshold voltage Vth is determined in the order of the last pixel in the first row, the first pixel in the first row, the first pixel in the second row,..., And the last pixel in the last row (FIG. 14).

  The vertical synchronization SW control 32 controls SW2 and SW3 in synchronization with the sequential circuit 33, applies the precharge voltage Vpre to the gate of the TFT 12 within the vertical synchronization period, and the AD converter 25 reads the threshold voltage Vth. Control the timing as you do.

  In accordance with the operations of the vertical synchronization SW control unit 32 and the sequential circuit 33, the threshold voltage Vth is sequentially read for each of the plurality of pixels arranged in a matrix.

  In the example of FIG. 13, the threshold voltage Vth of one pixel is read during the vertical synchronization period described above.

  At this time, when the threshold voltage Vth cannot be read within one horizontal period, the vertical synchronization period is set to n horizontal periods. As the value of n, a value that does not hinder display data update (that is, writing of the display voltage Vdisp of each pixel) in a frame period (generally 60 Hz) is selected.

  For example, when the display device has a QVGA resolution, if the display of 320 rows is updated at 60 Hz (16.7 ms), the update time for one row is 52 us. Therefore, if the update time for one line can be reduced to 26us, which is half, one frame can be divided into 640 horizontal lines, which is twice as long as 320, so that n can be allowed up to 320.

  As described above, according to the present embodiment, it is possible to sequentially update the threshold voltage Vth of all the pixels by using the vertical synchronization period set to n or more horizontal periods. Of course, as the number of pixels of the display panel increases, it takes time to update the threshold voltage Vth of all the pixels. However, since the threshold voltage fluctuation of the TFT does not appear immediately, it is a sufficient time.

10 Pixel circuit 12 Driving TFT
13,15 TFT for switching
20 source driver 22 adder 23 gradation voltage source 24 precharge voltage source 25 AD converter 26 storage circuit 27 control circuit 31 Vth correction unit 32 vertical synchronization SW control unit 33 sequential circuit 40 gate driver

Claims (7)

Driving a display device in which a plurality of pixels each having an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT are arranged in a matrix In a method, within one horizontal period of the display device,
A precharge step of applying a pulse voltage higher than a threshold voltage of the TFT to the gate of the TFT;
Discharging the capacitor charged in the precharge step and reading the gate voltage of the TFT during the discharge;
Update to update the threshold voltage of the TFT by correcting the read gate voltage according to a predetermined relationship between the threshold voltage of the TFT when the discharge ends and the gate voltage of the TFT during the discharge Steps and
A driving method comprising: performing a writing step of writing data to the pixel using the updated threshold voltage. The driving method according to claim 1,
In the reading step, the AD converter reads the gate voltage of the TFT during the discharge. The driving method according to claim 1,
In the updating step,
The driving method, wherein the correction is performed using a coefficient that defines the predetermined relationship, and the coefficient is an attenuation coefficient of the gate voltage when the gate voltage of the TFT is lowered by the discharge. The driving method according to claim 3, wherein
The driving method, wherein the attenuation coefficient is determined by a time constant circuit included in the pixel, and the time constant circuit includes the capacitor. Driving a display device in which a plurality of pixels each having an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT are arranged in a matrix In the method
setting one vertical synchronization period for n or more horizontal periods and determining a reading order for all of the plurality of pixels; and
The driving comprising reading a threshold voltage for at least one of the plurality of pixels and reading a threshold voltage of each pixel in accordance with the reading order determined for all the pixels within the vertical synchronization period. Method. Driving of a display device configured by arranging a plurality of pixels arranged in a matrix form having an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT In the circuit
Means for updating the threshold voltage of the TFT within one horizontal period of the display device and writing data to the pixel using the updated threshold voltage;
Means for writing the data;
Precharge means for applying a pulse voltage higher than the threshold voltage of the TFT to the gate of the TFT;
Means for discharging the capacitor charged by the precharge means and reading the gate voltage of the TFT during the discharge;
Means for updating the threshold voltage of the TFT by correcting the read gate voltage according to a predetermined relationship between the threshold voltage of the TFT when the discharge ends and the gate voltage of the TFT during the discharge; as well as,
A drive circuit comprising means for writing data to the pixel using the updated threshold voltage. Driving a display device in which a plurality of pixels each having an organic EL element, an amorphous Si-TFT that drives the organic EL element, and a capacitor connected to the gate of the TFT are arranged in a matrix In the circuit
means for setting one vertical synchronization period for n or more horizontal periods and determining the reading order for all of the plurality of pixels; and
A drive comprising reading means for reading a threshold voltage for at least one of the plurality of pixels and reading a threshold voltage of each pixel in accordance with the reading order determined for all the pixels within the vertical synchronization period. circuit.

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