Abstract:
A pixel driving circuit of an active matrix organic light emitted diode display is provided with an input first scanning voltage signal and an input displaying voltage signal. The pixel driving circuit comprises a driving thin film transistor (TFT), an organic light emitted diode (OLED), and a capacitor. The capacitor has a first end connected to a gate electrode of the driving TFT to store a potential respect to the displaying voltage signal and having the driving TFT generate a steady current flowing through the OLED. The capacitor has a second end provided with a second scanning voltage signal, which has a level range larger than that of the displaying voltage signal, partially overlapping with the first scanning voltage signal so as to generate a negative bias in the driving TFT.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to an active matrix organic light emitting diode (AMOLED) display, a pixel driving circuit, and a driving method thereof, and more particularly to a voltage-driven AMOLED display. 
     (2) Description of the Related Art 
     With the progress in the fabrication technology of organic light emitting diodes (OELDs), an organic light emitting display with a plurality of OLEDs arranged in matrix has become a popular choice among all the flat panel displays. Based on different driving methods, the organic light emitting display can be sorted into simple matrix system type and active matrix system type. In addition, the active matrix system type is more suitable for large size displays and high resolution usage. 
       FIG. 1  shows a circuit diagram of a pixel driving circuit in a traditional voltage-driven active matrix organic light emitting display. The pixel driving circuit includes an OLED, a transistor T 1 , a transistor T 2 , and a capacitor C. A source electrode of the transistor T 1  is connected to a data line D 1  for receiving a driving voltage signal V D . A gate electrode of the transistor T 1  is connected to a scan line S 1 . A source electrode of the transistor T 2  is connected to an anode of the OLED. A drain electrode of the transistor T 2  is provided with a potential V dd . A gate electrode of the transistor T 2  is connected to a drain electrode of the transistor T 1 . A cathode of the OLED is provided with a different potential V ss . Both ends of the capacitor C are connected to the gate electrode of the transistor T 2  and provided with the potential V dd  respectively. 
     As to generate a steady current I passing through the OLED to maintain the brightness, a scanning voltage V S  is firstly applied through the scan line S 1  to turn on the transistor T 1 . Then, a driving voltage signal V D  on the data line D 1  is able to apply to the gate electrode of the transistor T 2  and create a potential V cs  stored in the capacitor C. It is understood that the potential V cs  equals to a difference of the voltage levels of V dd  and the driving voltage signal V D . Therefore, the gate to source voltage Vgs (not shown) of the transistor T 2  is determined. Since a difference between the gate to source voltage Vgs and the threshold voltage Vt of the transistor T 2  determines the value of current I, the brightness of the OLED may be decided by setting the value of the driving voltage signal V D . 
     Although the usage of amorphous silicon thin film transistor (a-Si TFT) can reduce the cost of an organic light emitting display, most of the thin film transistors (TFT) applied for driving OLEDs nowadays are made of low temperature poly-silicon (LTPS) technology due to a major consideration of a shifting threshold voltage Vt of an a-Si TFT during operation. That is, even the gate to source voltage Vgs of the transistor remains constant, the value of the current passing through the OLED may be reduced due to the increasing threshold voltage Vt, and a decreasing brightness of the OLED is predictable. 
     The total variation of the threshold voltage Vt of the a-Si TFT is equal to a sum of the variations under positive bias and negative bias, which is disclosed in “Threshold Voltage Variation of Amorphous Silicon Thin-Film Transistor During Pulse Operation” of Japanese Journal of Applied Physics Vol. 30, December, 1991, pp. 3719-3723. Furthermore, the variation of the threshold voltage under positive bias is positive, and the variation of the threshold voltage under negative bias is negative, which is disclosed in “Electrical Instability of Hydrogenated Amorphous Silicon Thin-Film Transistors for Active-Matrix Liquid-Crystal Displays” of Japanese Journal of Applied Physics Vol. 37, September, 1998, pp. 4704-4710. 
     As mentioned above, the problem of increasing threshold voltage of the a-Si TFT can be effectively resolved by having the a-Si TFT properly supplied with negative bias. Therefore, modifying the pixel driving circuit by providing the TFT with negative bias is quite helpful for the application of a-Si TFT for driving OLEDs. 
     SUMMARY OF THE INVENTION 
     It is a main object of the present invention to generate a negative bias to a driving thin film transistor (TFT) in a pixel driving circuit so as for driving an OLED to overcome the problem of increasing threshold voltage. 
     A pixel driving circuit of a voltage-driven active matrix organic light emitting display is provided in the present invention. The pixel driving circuit is applied with a first scanning voltage signal and a displaying voltage signal, and it comprises a driving transistor, an organic light emitting diode (OLED) connected to the driving transistor, and a capacitor having a first end connected to a gate electrode of the driving transistor to store a potential respect to the displaying voltage signal so as to generate a steady current passing through the OLED. A second end of the capacitor is provided with a second scanning voltage signal, which partially overlaps with the first scanning voltage signal and has a level range larger than that of the displaying voltage signal so as to generate a negative bias in the driving transistor. 
     By using the above mentioned pixel driving circuit, a method for voltage-driving an organic light emitting display is provided in the present invention. Firstly, the pixel driving circuit is provided with the first scanning voltage signal and the second scanning voltage signal. The scanning voltage signals does not have any overlap so as to operate the pixel driving circuit ordinarily. As a negative bias is desired to be provided in the driving transistor, the first scanning voltage signal and the second scanning voltage signal are overlapped to generate a negative voltage level to the first end of the capacitor for generating the negative bias. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
         FIG. 1  is a circuit diagram depicting a traditional pixel driving circuit of a voltage-driven active matrix organic light emitting display; 
         FIG. 2  is a functional block diagram depicting a preferred embodiment of an active matrix organic light emitting display in accordance with the present invention; 
         FIG. 3  is a circuit diagram depicting a pixel driving circuit shown in  FIG. 2 ; 
         FIG. 4  is a timing chart showing the waveforms of the first scanning voltage signal, the second scanning voltage signal, the displaying voltage signal, and the voltage applied on the gate electrode of the driving transistor; 
         FIG. 5  shows a schematic drawing of a driving method by using the driving circuit shown in  FIG. 2 ; 
         FIG. 6A  is a timing chart showing the waveforms of the overlapped first and second scanning voltage signals; and 
         FIG. 6B  is a timing chart showing the waveforms shows the waveforms of the non-overlapped first and second scanning voltage signals. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is a functional block diagram depicting a preferred embodiment of an active matrix organic light emitting display in accordance with the present invention. As shown, the organic light emitting display includes a substrate  100 , a data driver  120  arranged on the substrate  100 , a scan driver  140  on the substrate  100 , a power supplier  160 , a plurality of scan lines  122 , a plurality of data lines  142  perpendicular to the scan lines  122 , and a plurality of pixel driving circuits  180  arranged on the substrate  100  in matrix. The pixel driving circuits  180  of the same column is connected to the data driver  120  through a single data line  122 . The pixel driving circuits  180  of the same row is connected to the scan driver  140  through a single scan line  142 . The power supplier  160  located on the substrate  100  is utilized for applying power to activate the organic light emitting diodes (OLEDs) of each pixel driving circuits  180 . 
       FIG. 3  is a circuit diagram depicting a pixel driving circuit  180  of  FIG. 2 , and two adjacent pixel driving circuits  180  of the same column is shown. As shown in  FIG. 3 , each pixel driving circuit  180  includes a switching transistor T 1 , a driving transistor T 2 , an OLED, and a capacitor C. The switching transistor T 1  has a source electrode connected to a data line D 1  (identical to the data line  122  of  FIG. 2 ) for receiving a displaying voltage signal V D , a gate electrode connected to a first scan line S 1  (identical to the scan line  142  of  FIG. 2 ) for receiving a first scanning voltage signal V S1 , and a drain electrode. The driving transistor T 2  has a gate electrode connected to the drain electrode of the switching transistor T 1 , a drain electrode connected to a power line P for receiving a first voltage level V DD  generally ranging from 0 to 12 volts, and a source electrode. The OLED has an anode connected to the source electrode of the driving transistor T 2  and a cathode provided with a second voltage level V ss  generally ranging from 0 to −12 volts. The capacitor C has a first end e 1  connected to both the drain electrode of the switching transistor T 1  and the gate electrode of the driving transistor T 2 , and a second end e 2  connected to an adjacent scan line S 2  for receiving a second scanning voltage signal V S2 . 
     As the first scanning voltage signal is at a high level state generally ranging from 0 to 15 volts, the switching transistor T 1  is turned on so as to allow the displaying voltage signal V D  generally ranging from 0 to 15 volts passing through the switching transistor T 1  and stored in the capacitor C. As the first scanning voltage signal is at a low level state generally ranging from 0 to −15 volts, the switching transistor T 1  is turned off to stop input displaying voltage signal V D . At the same time, a potential stored in the capacitor C respect to the displaying voltage signal V D  drives the driving transistor T 2  to generate a steady current I passing through the OLED to illuminate. 
     All the level ranges of the first voltage level V dd , the second voltage level V ss , the first scanning voltage signal V S1 , and the second scanning voltage signal V S2  are restricted by the allowable loading of the pixel driving circuit  180  and outputs of the data driver  120 , the scan driver  140 , and the power supplier  160 . 
     It is noted that a scanning timing of the second scanning voltage signal V S2  is right behind that of the first scanning voltage signal V S1 . Therefore, the second scan line S 2  connected to the second end e 2  of the capacitor C can be regarded as a next scan line with respect to the first scan line S 1 . 
     As the first scanning voltage signal V S1  does not overlap the second scanning voltage signal V S2 , the first end e 1  of the capacitor C maintains a voltage level of the displaying voltage signal V D . The voltage level of the gate electrode of the driving transistor T 2  is thus maintained to a value corresponding to the displaying voltage signal V D  to generate a steady current I passing through the OLED normally. 
     As the first scanning voltage signal V S1  overlaps the second scanning voltage signal V S2  and both the first scanning voltage signal V S1  and the second scanning voltage signal V S2  are in high level state, the first end e 1  of the capacitor C has the voltage level of the displaying voltage signal V D  and the second end e 2  of the capacitor C has the voltage level of the mentioned high level state. Afterward, as the first scanning voltage signal V S1  is switched to the respected low level state but the second scanning voltage signal V S2  remains in the high level state, the switching transistor T 1  is turned off so as to float the first end e 1  of the capacitor C. A difference between the voltage levels of the displaying voltage signal V D  and the high level state generates a potential stored in the capacitor C. Thereafter, as the second scanning voltage signal V S2  is further switched to the low level state, the voltage level of the second end e 2  of the capacitor C is declined to the voltage level of low level state and leads to a significant decrease of the voltage level of the first end e 1  of the capacitor C. It should be noted that the difference between the voltage levels of the high level state and the low level state is usually much greater than the voltage level of the displaying voltage signal V D . Therefore, the voltage level of the first end e 1  of the capacitor C can be decreased to a negative value. The negative voltage level is also applied on the gate electrode of the driving transistor T 2  to generate a negative bias. 
     For example, as the displaying voltage signal V D  ranges from 0 to 15 volts, the voltage level of the high level state ranges from 0 to 15 volts, and the voltage level of the low level state ranges from 0 to −15 volts, the potential stored in the capacitor C is predictable to have a voltage level ranging from −15 to 15 volts and the difference between the voltage levels of the high and low level states ranging from 0 to 30 volts. It is understood that the difference between the voltage levels of the high and low level states can be greater than the potential stored in the capacitor C, and thus a negative bias can be generated. 
     For a better understanding of the generation of the negative bias, please referring to  FIG.4 , which shows a timing chart depicting the waveforms of the first scanning voltage signal V S1 , the second scanning voltage signal V S2 , the displaying voltage signal V D , voltage level of the gate electrode G of the driving transistor T 2 , and voltage levels of the first end el (denoted El) and the second end e 2  (denoted E 2 ) of the capacitor C, respectively. As shown in  FIG.4 , both the first scanning voltage signal V S1 , and the second scanning voltage signal V S2  are pulse signals. It is assumed that the voltage level of the high level state of both the first scanning voltage signal V S1 , and the second scanning voltage signal V S2  is 10 volts, and the respected voltage level of the low level state is −15 volts. It is also assumed that the voltage level of the displaying voltage signal V D  is 0.5 volt. 
     In the timing chart shown in  FIG. 4 , as both the first scanning voltage signal V S1  and the second scanning voltage signal V S2  are in the high level state of 10 volts, the voltage levels E 1  and E 2  are 0.5 volts and 10 volts respectively. Then, as the first scanning voltage signal V S1  is in the low level state of −15 volts to turn off the switching transistor T 1 , a potential of −9.5 volts, which is also a difference of voltage levels between the first end e 1  and the second end e 2 , is stored in the capacitor C. Thereafter, as the second scanning voltage signal V S2  is at the low level state of −15 volts, the second end e 2  of the capacitor C is forced to shift to a voltage level of −15 volts. Since the difference of voltage levels between the first end e 1  and the second end e 2  is kept constant, the voltage level of the first end e 1  of the capacitor is thus reduced to −24.5 volts and applied to the gate electrode G of the driving transistor T 2  to generate a negative bias. 
     Although the switching transistor T 1  and the driving transistor T 2  in the pixel driving circuit of  FIG. 3  may be polysilicon thin film transistors or amorphous silicon thin film transistors (a-Si TFTs). It is understood that a major objective of the present invention is to generate a negative bias in the driving transistor T 2  to extend the expecting life of the driving transistor. Therefore, the pixel driving circuit of the present invention is particularly suitable for using amorphous silicon thin film transistors as the driving transistor T 2 . Whereas, since the time of applying positive bias on the switching transistor T 1  is much shorter than that on the driving transistor T 2 , the switching transistor T 1  in the pixel circuit  180  of the present invention is not limited to use polysilicon thin film transistors. 
       FIG. 5  shows a schematic drawing of a voltage-driving method by using the driving circuit  100  shown in  FIG. 2 . In the present voltage-driving method, the input scanning voltage signals provided by the scan lines  142  can be sorted into the scanning voltage signals with overlapping V S1 , V S2 , V S3  . . . to generate negative bias in a first cycle, as shown in  FIG. 6A , and the scanning voltage signals without overlapping V S1 ′, V S2 ′, V S3 ′ . . . to operate the pixel driving circuit ordinary to show normal frames in a second cycle, as shown in  FIG. 6B . In  FIG. 5 , the scanning voltage signals without overlapping and the scanning voltage signals with overlapping are alternatively provided to the driving circuit  100 . That is, negative-bias frames formed by applying scanning voltage signals with overlapping are periodically provided to interpose between a predetermined number of the normal frames for adjusting the threshold voltage of the driving transistor in the pixel driving circuits  180 . The predetermined number may be reduced to one to help preventing the increasing of threshold voltage. 
     While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.