JP2010008941A - Pixel circuit and display device - Google Patents

Abstract

A light-emitting diode is driven by using a normally-on driving transistor without complicating a peripheral circuit.
A pixel circuit includes an OLED having an anode connected to a predetermined power supply Vdd, a high luminance drive transistor and a low luminance drive transistor having a normally-on characteristic, and one end having a high luminance drive transistor. And a capacitor 38 connected to the gate of the low-brightness drive transistor 34B and having the other end grounded, and connected between the drains of the high-brightness drive transistor 34A and the low-brightness drive transistor 34B and the cathode of the OLED 30. The high luminance switch 32A and the low luminance switch 32B, one end of which is connected to the data line Data for outputting image data, and the other end is one end of the capacitor 38, the high luminance driving transistor 34A and the low luminance driving transistor 34B. The capacitor 38 connected to the gate of It includes the use switch 36, a.
[Selection] Figure 2

Description

  The present invention relates to a pixel circuit and a display device, and more particularly to a pixel circuit and a display device using a light emitting diode.

  In recent years, display devices using organic EL (electroluminescence) elements (hereinafter referred to as OLEDs) as display elements have attracted attention, and various technologies have been proposed for circuits that drive OLEDs ( For example, see Patent Documents 1 to 3). In order to drive this OLED, for example, as shown in FIG. 8, a drive transistor composed of at least two TFTs (Thin Film Transistors) is required. In the pixel circuit 100 illustrated in FIG. 8, when a scan signal is output to the scan line 102, the switching transistor 104 is turned on, and a data signal (image signal) is output to the data line 106, a storage capacitor element is output according to the data signal. 108 is charged. When the switching transistor 104 is turned off, a voltage corresponding to the data signal charged in the storage capacitor element 108 is applied to the gate of the driving transistor 110. As a result, the OLED 112 emits light with a luminance corresponding to the image.

  Unlike the liquid crystal, the OLED 112 is a current driving element, and the luminance thereof is controlled by the gate-source voltage Vgs of the driving transistor. That is, the OLED 112 emits light with a luminance corresponding to the drain current flowing according to the gate-source voltage. For this reason, when the drive characteristic indicating the relationship between the gate-source voltage of the drive transistor 110 and the drain current is shifted (hereinafter, characteristic shift), the luminance of the display panel is changed. Therefore, as the characteristics required for the driving transistor of each pixel, there is little characteristic change such as high reliability of each driving transistor, that is, characteristic shift due to energization or change in charge mobility. There are two points, that is, there is no luminance unevenness and there is in-plane uniformity, that is, there is little variation in characteristics of each driving transistor.

  When an inorganic oxide TFT composed of IGZO (In—Ga—Zn—O) or the like is used in an OLED pixel circuit, among the characteristics required for the driving transistor, the in-plane uniformity is excellent. (Proceedings of 2007 International TFT Conference, Rome, Italy 7.2).

In addition, an inorganic oxide TFT tends to have a normally-on characteristic, that is, a characteristic in which a drain current flows even when the gate-source voltage is zero. With regard to the above reliability, a TFT exhibiting a normally-on characteristic has a characteristic shift. It shows a small tendency and is suitable for a drive transistor.
JP 2003-150104 A JP 2000-347624 A JP 2004-317777 A

  However, when a TFT exhibiting normally-on characteristics is used in an OLED pixel circuit as described in Patent Documents 1 to 3, leakage current flows, the OLED emits light erroneously, and the image quality deteriorates. For this reason, there has been a problem that the peripheral circuit becomes complicated, for example, a negative power source is required to make the ground line of the data driver for controlling the data line a potential lower than 0V. In addition, when a TFT having normally-on characteristics is used, there is a problem that gradation control becomes difficult due to the characteristic shift.

  The present invention has been made to solve the above-described problems, and a pixel circuit and a display device for driving a light-emitting diode as a display element using a normally-on driving transistor without complicating a peripheral circuit. The purpose is to provide.

  In order to solve the above problem, a pixel circuit according to a first aspect of the present invention includes a light emitting diode having an anode connected to a predetermined power source, a plurality of drive transistors having normally-on characteristics and different drive characteristics, and one end of the pixel circuit. A capacitive element connected to the gates of the plurality of driving transistors and having the other end grounded; a plurality of switching switches each connected between the drains of the plurality of driving transistors and the cathode of the light emitting diode; and one end And a charge switch for the capacitive element, wherein the capacitive element is connected to a data line to which image data is output, and the other end is connected to one end of the capacitive element and the gates of the plurality of driving transistors. To do.

  According to the present invention, a plurality of drive transistors having normally-on characteristics with little characteristic shift and different drive characteristics, and a plurality of switching transistors respectively connected between the drains of the plurality of drive transistors and the cathode of the light emitting diode. And a switch. In addition, as described in Claim 2, the said light emitting diode can be set as the structure which is an organic EL element.

  In this manner, since the normally-on driving transistor is used, the characteristic shift is small, and erroneous light emission of the light-emitting diode can be prevented, and deterioration in image quality can be prevented. In addition, since a desired gradation region can be shared by a plurality of driving transistors having different driving characteristics, a decrease in the number of gradations can be prevented and gradation control can be facilitated.

  According to a third aspect of the present invention, there is provided a plurality of pixel circuits according to the first or second aspect, and the charging switch in a data output period in which image data is output to the data line in a predetermined frame period. A switch control unit for turning on, and a switch control for switching which turns on one of the plurality of switching switches according to the image data during a driving transistor driving period after the end of the data output period in the frame period Means.

  According to the present invention, a plurality of drive transistors having normally-on characteristics with little characteristic shift and different drive characteristics, and a plurality of switching transistors respectively connected between the drains of the plurality of drive transistors and the cathode of the light emitting diode. And a pixel circuit having a configuration including a switch, so that erroneous light emission of the light emitting diode can be prevented, image quality can be prevented from being lowered, and gradation can be prevented from being lowered. Control can be facilitated.

  According to a fourth aspect of the present invention, the frame period is divided according to the number of the plurality of driving transistors, and the charging switch control means sets the charging switch in the data output period of each divided period. The changeover switch control means may turn on the changeover switch corresponding to the drive transistor drive period after the end of the data output period of each divided period.

  According to the present invention, there is an effect that a light emitting diode as a display element can be driven by using a normally on driving transistor without complicating a peripheral circuit.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In FIG. 1, the schematic block diagram of the display apparatus 10 which concerns on this embodiment was shown. The display device 10 includes a display panel 12 using an OLED, a first scanning line driving circuit 14, a second scanning line driving circuit 16, a charging line driving circuit 18, and a data line driving circuit 20. Yes.

The display panel 12 includes pixel circuits 22 _{11 to} 22 _NM for N rows and M columns. The first scanning line driving circuit 14 and the pixel circuits 22 _{11 to} 22 _{1M in the} first row are connected by the first scanning line Ascan 1, and the first scanning line driving circuit 14 and the pixel circuit in the second row are connected. 22 _{21 to} 22 _2M are connected by a first scanning line Ascan 2. Similarly, the first scanning line driving circuit 14 and the pixel circuits 22 _{N 1 to} 22 _{NM in the} N-th row are connected to the first scanning line Asc ₂ . They are connected by a scanning line AscanN.

The second scanning line driving circuit 16 and the pixel circuits 22 _{11 to} 22 _{1M in the} first row are connected by the second scanning line Bscan1, and the second scanning line driving circuit 16 and the pixel circuit in the second row are connected. 22 _{21 to} 22 _2M are connected by a second scanning line Bscan 2. Similarly, the second scanning line driving circuit 16 and the pixel circuits 22 _{N 1 to} 22 _{NM in the} N-th row are connected to the second scanning line Bscan 2. They are connected by a scanning line BscanN.

The charging line driving circuit 18 and the pixel circuits 22 _{11 to} 22 _{1M in the} first row are connected by the charging line Vscan1, and the charging line driving circuit 18 and the pixel circuits 22 _{21 to} 22 _{2M in the} second row are charged. Similarly, the charging line driving circuit 18 and the Nth pixel circuits 22 _{N1 to} 22 _NM are connected by the charging line VscanN.

The data line driving circuit 20 and the pixel circuits 22 _{11 to} 22 _{N1 in the} first column are connected by the data line Data1, and the data line driving circuit 20 and the pixel circuits 22 _{12 to} 22 _{N2 in the} second column are data Similarly, the data line driving circuit 20 and the M-th column pixel circuits 22 _{1M to} 22 _NM are connected by the data line DataM.

  Since each pixel circuit has the same configuration, the pixel circuit is simply referred to as a pixel circuit 22 when the pixel circuit is not particularly limited. Further, the data line, the first scanning line, the second scanning line, and the charging line are also simply referred to as Data, Asscan, Bscan, and Vscan, respectively, when not particularly limited.

  FIG. 2 shows a circuit diagram of the pixel circuit 22. As shown in the figure, the pixel circuit 22 includes an OLED 30, a high luminance switch 32A, a low luminance switch 32B, a high luminance driving transistor 34A, a low luminance driving transistor 34B, a charging switch 36, and a capacitor 38. It consists of

  The anode of the OLED 30 is connected to a predetermined power supply Vdd, and the cathode is connected to one end of the high luminance switch 32A and the low luminance switch 32B.

  The other end of the high brightness switch 32A is connected to the drain of the high brightness drive transistor 34A, and the other end of the low brightness switch 32B is connected to the drain of the low brightness drive transistor 34B. The sources of the high luminance drive transistor 34A and the low luminance drive transistor 34B are grounded.

  One end of a capacitor 38 is connected to the gates of the high luminance drive transistor 34A and the low luminance drive transistor 34B. The other end of the capacitor 38 is grounded. The capacitor 38 has a function of holding the gate-source voltage of the high luminance drive transistor 34A and the low luminance drive transistor 34B.

  One end of the charging switch 36 is connected to the data line Data, and the other end is connected to one end of the capacitor 38, and the gates of the high luminance driving transistor 34A and the low luminance driving transistor 34B.

  A voltage corresponding to the image data (luminance value) of the pixel to be displayed by the pixel circuit 22 is output to the data line Data by the voltage supply circuit 20 </ b> A of the data line driving circuit 20. As shown in FIG. 2, the voltage supply circuit 20 </ b> A includes a voltage source 40 that outputs a voltage according to image data, and a switch 42 that is turned on in a predetermined period for causing the OLED 30 to emit light.

  The high-luminance drive transistor 34A and the low-luminance drive transistor 34B are composed of inorganic oxide TFTs having normally-on characteristics, that is, characteristics in which a drain current flows even when the gate-source voltage is 0V.

  FIG. 3 is a drive characteristic chart showing the relationship between the gate-source voltage and the drain current as a characteristic of the high brightness drive transistor 34A and the low brightness drive transistor 34B. In the figure, a drive characteristic 44A indicated by a solid line indicates the characteristic of the high-luminance drive transistor 34A, and a drive characteristic 44B indicated by an alternate long and short dash line indicates the characteristic of the low-luminance drive transistor 34B.

Here, assuming that the driving current when the OLED 30 emits light with the maximum luminance is 1 μA (1 × 10 ⁻⁶ A), when the OLED 30 is driven only by a normally on driving transistor such as the driving characteristic 44A, The drain current ID is 1 × 10 ⁻⁸ A when the source-to-source voltage Vgs = 0 V, and the ON / OFF ratio is 100: 1. For this reason, in a current drive type light emitting element such as the OLED 30, not only the contrast is lowered, but also the number of gradations is limited.

  On the other hand, in order to reduce the drain current ID at the gate-source voltage Vgs = 0V, when the OLED 30 is driven only by the drive transistor having the normally-on characteristic such as the drive characteristic 44B, it is necessary for emitting light with high luminance. The voltage becomes very high and power consumption increases.

  Therefore, in the present embodiment, as shown in FIG. 3, in the high gradation region 46A, the OLED 30 is driven by the high luminance drive transistor 34A, and in the low gradation region 46B, the OLED 30 is driven by the low luminance drive transistor 34B. To drive. As a result, it is possible to prevent a reduction in contrast and the number of gradations, and to suppress an increase in power consumption.

  When the gate width of the driving transistor is W and the gate length is L, by adjusting the W / L ratio, the high luminance driving transistor 34A and the driving characteristic 44B like the driving characteristic 44A in FIG. A low-luminance drive transistor 34B can be manufactured. That is, when W / L is increased, a transistor in which a large drain current flows with respect to a predetermined gate-source voltage can be manufactured. When W / L is decreased, a drain current is increased with respect to a predetermined gate-source voltage. A small number of transistors can be manufactured.

  The TFTs used as the high-luminance drive transistor 34A and the low-luminance drive transistor 34B can be manufactured by, for example, a method described in “APPLIED PHYSICS LETTERS 92, 133503 2008” as non-patent literature.

  On the other hand, the high-brightness switch 32A and the low-brightness switch 32B are normally-off type in this embodiment, that is, a TFT having a drive characteristic as shown in FIG. 4 in which no drain current flows when the gate-source voltage is 0V. Composed. This is to prevent the OLED 30 from emitting light erroneously due to the drain current flowing when the gate-source voltage is 0V. Such a TFT can be manufactured, for example, by a method described in “APPLIED PHYSICS LETTERS 91, 0991910 2007” as a non-patent document. Note that a characteristic shift may occur when the high luminance switch 32A and the low luminance switch 32B are normally-off type TFTs. However, the high luminance switch 32A and the low luminance switch 32B are simply used as switches. Therefore, even if a characteristic shift occurs, there is no particular problem if it is controlled within a range in which it can be appropriately turned on / off in consideration of the margin.

  Next, a specific operation of the pixel circuit 22 will be described.

  FIG. 5 shows a timing chart of signals output to the first scanning line Ascan, the second scanning line Bscan, the charging line Vscan, and the data line Data.

  In FIG. 5, the period from t1 to t5 is a period of one frame. In this embodiment, one frame is divided into a driving period of the high luminance driving transistor 34A and a driving period of the low luminance driving transistor 34B. Driving transistors are driven separately. Therefore, the light emission luminance of the OLED 30 is the sum of the light emission during the driving period of the high luminance driving transistor 34A and the light emission during the driving period of the low luminance driving transistor 34B. Therefore, in the voltage supply circuit 20A, the high-luminance driving current and the low-luminance driving transistor 34B are driven so as to drive the high-luminance driving transistor 34A so that the OLED 30 finally emits light with luminance corresponding to the image data. In order to achieve this, it is divided and supplied to a low-luminance drive current. Note that the ratio of the driving period of the high luminance driving transistor 34A and the driving period of the low luminance driving transistor 34B is not particularly limited as long as the OLED 30 emits light satisfactorily.

  First, the switch 42 of the voltage supply circuit 20A of the data line driving circuit 20 is turned on in the data output period (high luminance TFT writing period) of t1-t2. As a result, a high-luminance driving current determined according to the image data is output to the data line Data. In synchronization with this, the charging line Vscan is set to the high level by the charging line driving circuit 18. Accordingly, the charging switch 36 is turned on, and the high-luminance driving current output to the data line Data flows through the path R1 shown in FIG. 6A, so that the capacitor 38 is charged.

  When the capacitor 38 is charged and the high-brightness TFT writing period ends, the switch 42 of the voltage supply circuit 20A of the data line drive circuit 20 is switched during the drive transistor drive period (high-brightness TFT energization period) t2-t3. At the same time, the charging line driving circuit 18 sets the charging line Vscan to the low level, and the charging switch 36 is turned off. In synchronization with this, the first scanning line Asscan is set to the high level by the first scanning line driving circuit 14. As a result, the high brightness switch 32A is turned on, and the drive voltage is applied between the gate and the source of the high brightness drive transistor 34A by the charged capacitor 38, and therefore, as shown in FIG. A current flows through the path R2, and the OLED 30 emits light. The OLED 30 emits light with a luminance corresponding to the gate-source voltage of the high luminance driving transistor 34A, that is, a luminance corresponding to the drain current of the high luminance driving transistor 34A.

  When the high luminance TFT energization period ends, the first scanning line drive circuit 14 sets the first scanning line Asscan to the low level.

  Then, the switch 42 of the voltage supply circuit 20A of the data line driving circuit 20 is turned on again during the period from t3 to t4 (low luminance TFT writing period). As a result, a low-luminance drive current determined according to the image data is output to the data line Data. In synchronism with this, the charging line driving circuit 18 sets the charging line Vscan to the high level again. Accordingly, the charging switch 36 is turned on again, and the low-luminance driving current output to the data line Data flows through the path R3 illustrated in FIG. 7A, so that the capacitor 38 is charged.

  When the capacitor 38 is charged and the low luminance TFT writing period ends, the switch 42 of the voltage supply circuit 20A of the data line driving circuit 20 is turned off in the period t4-t5 (low luminance TFT energization period), and The charging line Vscan is set to a low level by the charging line driving circuit 18, and the charging switch 36 is turned off. In synchronization with this, the second scanning line Bscan is set to the high level by the second scanning line driving circuit 16. As a result, the low luminance drive switch 32B is turned on, and the drive voltage is applied between the gate and the source of the low luminance drive transistor 34B by the charged capacitor 38, so that FIG. A current flows through the path R4 shown, and the OLED 30 emits light. The OLED 30 emits light with luminance according to the gate-source voltage of the low-luminance drive transistor 34B, that is, luminance according to the drain current of the low-luminance drive transistor 34B.

Such processing is sequentially scanned and executed from the pixel circuits 22 _{11 to} 22 _{1M in the} first row to the pixel circuits 22 _{N1 to} 22 _{NM in the} N row, whereby an image is displayed on the display panel 12.

  As described above, in the present embodiment, the normally on high-luminance drive transistor 34A and the low-luminance drive transistor 34B are connected in parallel, and each drive transistor is driven separately in the high-luminance region and the low-luminance region. Without complicating the peripheral circuit, a reduction in contrast can be suppressed and gradation control can be facilitated.

  In the present embodiment, the configuration in which two drive transistors are connected in parallel to cause the OLED 30 to emit light has been described. However, a configuration in which three or more drive transistors are connected in parallel to cause the OLED 30 to emit light may be used. For example, when the OLED 30 is configured to emit light by connecting three high-luminance driving transistors, medium-luminance driving transistors, and low-luminance driving transistors having different characteristics in parallel, the pixel circuit 22 in FIG. The transistor is connected in parallel to the high-brightness drive transistor 34A and the low-brightness MOS-FET 34B, and the display device 10 of FIG. 1 is further provided with a third scan line drive circuit via the third scan lines Cscan1 to CscanN. What is necessary is just to set it as the structure which controls the drive transistor for medium brightness | luminance for each pixel circuit.

It is a schematic block diagram of a display apparatus. It is a circuit diagram of a pixel circuit. FIG. 6 is a characteristic diagram illustrating a relationship between a gate-source voltage and a drain current of a normally-on high-luminance drive transistor and a low-luminance drive transistor. It is a characteristic view of the switch for charge of normally-off characteristics. 6 is a timing chart of a first scanning line Asscan, a second scanning line Bscan, a charging line Vscan, and a data line Data. (A) is a figure for demonstrating the flow of the electric current at the time of charge, (B) is a figure for demonstrating the flow of the electric current at the time of the drive of the high-intensity drive transistor. (A) is a figure for demonstrating the flow of the electric current at the time of charge, (B) is a figure for demonstrating the flow of the electric current at the time of the drive of the low-intensity drive transistor. It is a circuit diagram of the conventional OLED drive circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Display apparatus 12 Display panel 14 1st scanning line drive circuit 16 2nd scanning line drive circuit 18 Charging line drive circuit 20 Data line drive circuit 20A Current supply circuit 22 Pixel circuit 32A High brightness switch 32B Low brightness switch 34A High Brightness Drive Transistor 34B Low Brightness Drive Transistor 36 Charging Switch 38 Capacitor 40 Voltage Source 42 Switch Asscan First Scan Line Bscan Second Scan Line Vscan Charge Line Data Data Line

Claims (4)

A light emitting diode having an anode connected to a predetermined power source;
A plurality of drive transistors having normally-on characteristics and different drive characteristics;
A capacitive element having one end connected to the gate of each of the plurality of drive transistors and the other end grounded;
A plurality of switching switches respectively connected between drains of the plurality of driving transistors and cathodes of the light emitting diodes;
A switch for charging the capacitive element, one end of which is connected to a data line from which image data is output and the other end is connected to one end of the capacitive element and the gates of the plurality of driving transistors;
A pixel circuit comprising: The pixel circuit according to claim 1, wherein the light emitting diode is an organic EL element. A plurality of pixel circuits according to claim 1 or 2, and
Charging switch control means for turning on the charging switch during a data output period in which image data is output to the data line in a predetermined frame period;
A switch control means for switching for turning on one of the plurality of switches according to the image data in a drive transistor driving period after the end of the data output period in the frame period;
A display device comprising: The frame period is divided according to the number of the plurality of driving transistors,
The charging switch control means turns on the charging switch during the data output period of each divided period,
The display device according to claim 3, wherein the switching switch control unit turns on the switching switch corresponding to a driving transistor driving period after the end of the data output period of each divided period.

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