This Application claims priority of Taiwan Patent Application No. 096132437, filed on Aug. 31, 2007, the entirety of which is incorporated by reference herein.
1. Field of the Invention
The invention relates to a display device, and more particularly to a display device for obtaining the threshold voltage of a transistor.
2. Description of the Related Art
Because cathode ray tubes (CRTs) are inexpensive and provide high definition, they are utilized extensively in televisions and computers. With technological development, new flat-panel displays are continually being developed. When a larger display panel is required, the weight of the flat-panel display does not substantially change when compared to CRT displays. Generally, flat-panel displays comprises liquid crystal displays (LCD), plasma display panels (PDP), field emission displays (FED), and electroluminescent (EL) displays.
Electroluminescence (EL) display devices include organic light emitting diode (OLED) displays and polymeric light emitting diode (PLED) displays. In accordance with associated driving methods, an OLED can be an active matrix type or a positive matrix type. An active matrix OLED (AM-OLED) display typically is thin and exhibits lightweight characteristics, spontaneous luminescence with high luminance efficiency and low driving voltage. Additionally, an AM-OLED display provides the perceived advantages of increased viewing angle, high contrast, high-response speed, full color and flexibility.
An AM-OLED display is driven by electric current. Specifically, each of the pixel units of an AM-OLED display includes a driving transistor and an OLED. The driving transistor provides a driving current such that the OLED can be lit. The brightness of the OLED is determined by the driving current. Due to manufacturing procedures, different driving transistors comprise different threshold voltages. Thus, conventional OLEDs generate abnormal brightness.
Display devices are provided. An exemplary embodiment of a display device comprises a pixel unit, a selection unit, and a control unit. The pixel unit comprises a driving transistor and a capacitor. The driving transistor comprises a gate and a source. The capacitor is coupled between the gate and the source. The selection unit selectively transmits a first voltage or a second voltage to the driving transistor. The control unit controls the selection unit and receives the voltage of the source.
Electronic systems are also provided. An exemplary embodiment of an electronic system comprises a display device and a transformation device. The display device displays an image according to a power signal. The transformation device transforms an external power into the power signal. The display device comprises a pixel unit, a selection unit, and a control unit. The pixel unit comprises a driving transistor and a capacitor. The driving transistor comprises a gate and a source. The capacitor is coupled between the gate and the source. The selection unit selectively transmits a first voltage or a second voltage to the driving transistor. The control unit controls the selection unit and receives the voltage of the source.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The invention can be more fully understood by referring to the following detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
In one embodiment, when the transformation device 110 does not receive the external power SAC, the display device 130 displays an image according to the power signal SDC2. When the transformation device 110 receives the external power SAC, the display device 130 displays an image according to the power signal SDC1.
The pixel unit P11 comprises a driving transistor 241 and a capacitor 242. The capacitor 242 is coupled between the gate and the source of the driving transistor 241. In this embodiment, the pixel unit P11 further comprises a switching transistor 243 and a lighting element 244. The switching transistor 243 transmits a signal to the gate of the driving transistor 241 according to a scan signal provided by a scan line S1 wherein the signal is originated from a data line D1. The lighting element 244 is lit according to a data signal originated from the data line D1. In this embodiment, the lighting element 244 is an organic light emitting diode (OLED).
The selection units 210 and 220 selectively transmit voltage Vref or voltage PVDD to the driving transistors of the corresponding pixel units P11˜P22. Since each of selection units is used to control the pixel units of the same data line, the amount of selection units is determined by the amount of data lines. To simplify the description, two selection units are shown in
The control unit 230 controls the selection units 210 and 220 and receives the source voltages of the driving transistors of the pixel units P11˜P22. In this embodiment, the control unit 230 comprises a gate driver 231 and a source driver 232. In addition to transmitting scan signals provided by the scan lines S1 and S2 to the pixel units P11˜P22, the gate driver 231 also provides switching signals SSW1˜SSW3 and pre-charge signals SPre-charge1 and SPre-charge2. Similarly, in addition to transmitting data signals provided by the data lines D1 and D2 to the pixel units P11˜P22, the source driver 232 also receives the source voltages of the driving transistors of the pixel units P11˜P22. The source driver 232 further provides data signals according to the source voltages of the driving transistors of the pixel units P11˜P22.
Since the operations of the selection units 210 and 220 are the same, the selection unit 210 is provided as an example. During a first period, the selection unit 210 transmits the voltage PVDD to the gate and the source of the driving transistor 241 of the pixel unit P11. During a second period, the selection unit 210 transmits the voltage Vref to the gate of the driving transistor 241. At this time, the source of the driving transistor 241 is discharged according to the threshold voltage of the driving transistor 241. The source driver 232 obtains the threshold voltage of the driving transistor 241 according to the source voltage of the driving transistor 241 and the voltage Vref.
For example, assuming the threshold voltage of the driving transistor 241 is 1V and the voltage Vref and PVDD are 2V and 5V, respectively, during the first period, the gate voltage and the source of the driving transistor 241 are 5V. Meanwhile, during the second period, the gate voltage of the driving transistor 241 is 2V. Since the threshold voltage of the driving transistor 241 is 1V, the source of the driving transistor 241 is discharged such that the source voltage of the driving transistor 241 is 3V. Thus, the source driver 232 utilizes the source voltage of the driving transistor 241 and the voltage Vref to obtain that the threshold voltage of the driving transistor 241 is 1V.
The source driver 232 obtains the threshold voltage of all driving transistors according to the above method. When the source driver 232 utilizes the threshold voltage of the driving transistor to adjust the data signal transmitted to the pixel units, a phenomenon can be compensated. The phenomenon is caused because the different driving transistors may comprise different threshold voltages.
In this embodiment, a switch 252 is coupled between the data line D1 and the source of the driving transistor 241 and selectively electrically connects the data line D1 and the source of the driving transistor 241 according to the switching signal SSW3. When the switch 252 is turned on or not, the source driver 232 can receive the source voltage of the driving transistor 241. In this embodiment, the switch 252 is turned on during the second period. Additionally, a switch 251 is coupled between the data line D1 and the drain of the switching transistor 243 for transmitting the data signal to the pixel units. In this embodiment, the switch 251 selectively electrically connects the data line D1 and the switching transistor 243.
In this embodiment, the selection unit 210 comprises transistors 211˜213 for selectively providing the voltage Vref or PVDD to the pixel units. As shown in
During the second period T2, the pre-charge signal SPre-charge1 is at the high level and the switching signal SSW1 is at the low level such that the transistors 212 and 211 are turned off. Since the pre-charge signal SPre-charge2 is at the high level, the transistor 213 is turned on. When the switching transistor 243 is turned on, the gate of the driving transistor 241 can receive the voltage Vref. Since the switching signal SSW2 is at the low level and the switching signal SSW3 is at the high level, the switch 251 is still turned off and the switch 252 is turned on. Thus, the source driver 232 can receive the source voltage of the driving transistor 241.
The source driver 232 utilizes the threshold voltages of the driving transistors of the pixel units for actively adjusting the data signal transmitted to each pixel unit. Thus, the phenomenon can be compensated. The phenomenon is caused because the different driving transistors may comprise different threshold voltages.
During the third period T3, the pre-charge signal SPre-charge1 is at the high level and the pre-charge signal SPre-charge2 is at the low level such that the transistors 212 and 213 are turned off. Since the switching signals SSW1 and SSW2 are at the high level and the switching signal SSW3 is at the low level, the switches 211 and 251 are turned on and the switch 252 is turned off.
The source driver 232 adjusts the data signal transmitted to the pixel unit P11 according to the threshold voltage of the driving transistor 241 during the second period T2. Thus, the pixel unit P11 displays the corresponding brightness according to the adjusted data signal during the third period T3 and the fourth period T4.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Number | Date | Country | Kind |
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96132437 A | Aug 2007 | TW | national |
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Number | Date | Country | |
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20090058770 A1 | Mar 2009 | US |