Abstract:
A control circuit for use in an active matrix organic light emitting diode (AMOLED) panel comprising a memory unit and a voltage control unit is provided. The operating time of the AMOLED panel is counted and saved by the memory unit. According to the memory unit&#39;s information, the voltage control unit decides a common voltage. The purpose of the present invention is to reduce the common voltage when the turn-on time of the AMOLED panel is increased so that the increase in the voltage difference between the two terminals of an organic light emitting diode (OLED) of the AMOLED panel may be compensated. Thus, the present invention may provide a stable driving current for the OLED and a stable picture definition for the AMOLED panel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 95138018, filed Oct. 16, 2006. All disclosure of the Taiwan application is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an active matrix organic light emitting diode (AMOLED) panel, and more particularly, to an AMOLED panel with an adjustable common voltage. 
     2. Description of Related Art 
     The current generation is frequently proclaimed as the 3C era: the Computer, the Communication and the Consumer electronics era. In our daily life, we encounter all kinds of information products such as mobile phones, personal digital assistants (PDAs), global positioning satellite (GPS) systems, digital cameras and displays. However, most information equipment uses a flat panel display as the main communication medium. 
     With the proliferation of information equipment, a variety of flat panel displays, for example, liquid crystal display, plasma display and organic light emitting diode (OLED) panel, are available for selection. The OLED panel not only has the advantages of a higher brightness level, lower power consumption, higher contrast, rapid response and lower driving voltage, but also has the capability to be miniaturized according to the current trend of communication equipment. Therefore, a large number of OLED panel products are developed in recent years. 
     According to the driving method of the organic light emitting diode (OLED), OLED panels can be classified into passive matrix organic light emitting diode (PMOLED) panels and active matrix organic light emitting diode (AMOLED) panels. Since AMOLED panels can be applied to a large size panel, it has more development potential. In general, the pixel unit of an AMOLED panel at least comprises a transistor and an organic light emitting diode. The transistor has a first terminal connected to a operating voltage and a second terminal connected to the anode of the organic light emitting diode. The cathode of the organic light emitting diode is coupled to a common voltage. In addition, after the AMOLED panel is being operated for a period of time, the voltage difference across the terminals of the organic light emitting diode (OLED) of the AMOLED panel will increase along with the operating time. However, the voltage difference between the first and second terminals of the transistor together with the voltage difference across the terminals of the OLED are a constant value equivalent to the voltage difference between the operating voltage and the common voltage. Hence, if the voltage difference across the OLED is allowed to continuously increase, the voltage difference between the first and second terminal of the transistor will decrease continuously. When the voltage difference changes too much, the transistor can no longer provide a stable driving current to drive the OLED. Ultimately, the picture quality of the AMOLED panel is also adversely affected. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is to provide a flat panel display and an active matrix organic light emitting diode (AMOLED) panel such that the driving current of the AMOLED panel can be adjusted according to the operating time of the AMOLED panel. 
     In addition, the present invention is to provide a method of controlling an AMOLED panel such that the common voltage is adjusted according to the operating time of the AMOLED panel. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a control circuit for an AMOLED panel with a common voltage. The control circuit of the AMOLED panel includes a memory unit and a voltage control unit. The memory unit counts and saves the operating time when the AMOLED panel is turned on, and the voltage control unit adjusts the common voltage according to the operating time of the AMOLED panel. 
     The present invention also provides a flat panel display comprising an AMOLED panel, a timing controller, a power supply and a control circuit. The AMOLED panel has a plurality of pixel units. The timing controller drives the AMOLED panel and the power supply provides an operating voltage and a common voltage to the AMOLED panel. In addition, the control circuit is electrically coupled to the timing controller for detecting the operating time of the AMOLED panel. According to the operating time of the AMOLED panel, the control circuit in the present invention controls the power supply to adjust the common voltage under the condition of a constant operating voltage. 
     In addition, the present invention provides a method for controlling an AMOLED panel. First, an operating voltage and a common voltage are produced for the AMOLED panel. Then, the using time of the AMOLED panel is detected. Finally, according to the operating time of the AMOLED panel, the common voltage is adjusted under the condition of a constant operating voltage. 
     Furthermore, the memory unit can be used to count the turn-on frequency of the AMOLED panel and save the value. The common voltage can be adjusted through the voltage control unit according to the turn-on frequency of the AMOLED panel. 
     Because the control circuit provided by the present invention is capable of detecting either the operating time of the AMOLED panel or counting the turn-on frequency of the AMOLED panel and using either the using time or turn-on frequency to control the power supply and adjust the common voltage under the condition of a fixed operating voltage, the present invention can provide a stable driving current for the organic light emitting diode and an AMOLED panel with a more stable picture quality. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a circuit diagram of an active organic light emitting display according to a preferred embodiment of the present invention. 
         FIG. 2  is a circuit diagram of a pixel unit according to a preferred embodiment of the present invention. 
         FIG. 3  is a flow diagram of a method for controlling an AMOLED panel according to a preferred embodiment of the present invention. 
         FIG. 4  is a diagram of a common voltage curve according to a preferred embodiment of the present invention. 
         FIG. 5  is a diagram of a curve showing the relationship between the voltage difference between the operating voltage and the common voltage and the operating time of the AMOLED panel. 
         FIG. 6  is a flow diagram of a method for controlling an AMOLED panel according to another preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 1  is a circuit diagram of an active organic light emitting display according to a preferred embodiment of the present invention. As shown in  FIG. 1 , the present invention provides a flat panel display  100  including an active matrix organic light emitting diode (AMOLED) panel  102 , a control circuit  105 , a timing controller  106  and a power supply  108 . The power supply  108  provides the power for operating the AMOLED panel  102  and the control circuit  105  controls the power supply  108  for adjusting the power to the AMOLED panel  102  according to the operating condition of the AMOLED panel  102 . 
     The AMOLED panel  102  includes a scan driving circuit  130 , a data driving circuit  132  and a pixel array  140 . The scan driving circuit  130  is electrically coupled to the pixel array  140  through a plurality of scan lines, and the data driving circuit  132  is electrically coupled to the pixel array  140  through a plurality of data lines  136 . 
     The pixel array  140  has a plurality of pixels  140  arranged to form an array, and each pixel  141  is disposed at the junction between each scan line and data line. Thus, the scan driving circuit  130  may generate a scan signal to the pixel array  140  according to the output from the timing controller  106  so that the pixels on each scan line are enabled sequentially. Meanwhile, the data driving circuit  132  generates a data voltage signal to the pixel array  140  according to the output from the timing controller  106  to turn on the enabled pixels  141 . 
     In general, the power supply  108  provides an operating voltage and a common voltage to each pixel  141  (a more detailed description is provided below) of the pixel array  140 . The control circuit  105  may control the power supply  108  to adjust the common voltage provided to the pixel  141  according to the on or off state of each pixel  141 . 
     The control circuit  105  includes a memory unit  120  and a voltage control unit  122 . The memory unit  120  can be used to count and save the operating time of the pixel  141  in the AMOLED panel  102 . Thus, the voltage control unit  122  is able to control the power supply  108  according to the information stored in the memory unit  120  such that the common voltage to the pixel array  140  can be adjusted according to the operating time of the AMOLED panel  102 . 
       FIG. 2  is a circuit diagram of a pixel unit  1141  according to a preferred embodiment of the present invention. The pixel units  141  in  FIG. 1  can be implemented using the pixel units  1141 . As shown in  FIG. 2 , the pixel unit  1141  includes transistors  210 ,  212 , an organic light emitting diode  220  and a storage capacitor  230 . In the present embodiment, the transistors  210  and  212  can be implemented using PMOS transistors. The transistor  212  has a controlling terminal used for receiving a data voltage, a first terminal coupled to an operating voltage V DD  and a second terminal coupled to the anode of the organic light emitting diode  220 . The cathode of the organic light emitting diode  220  is coupled to a common voltage V SS  of the AMOLED panel  210 . Furthermore, the transistor  210  has a controlling terminal coupled to a scan line  134 , a first terminal coupled to a data line  136  and a second terminal electrically coupled to the controlling terminal of the transistor  212 . 
     As shown in  FIGS. 1 and 2 , when the pixel unit  1141  is driven, the timing controller  106  generates a control signal to the scan driving circuit  130  so that the scan driving circuit  130  generates a scan signal and activates the transistor  210  to conduct through the scan line  134 . Meanwhile, the timing controller  106  also generates a control signal so that the data driving circuit  132  generates and transmits a data voltage signal to the pixel unit  1141  through the data line  136  and then transmitted to the controlling terminal of the transistor  212  through the transistor  210 . Thus, the transistor  212  is turned on and a driving current I D  is generated to drive the organic light emitting diode  220 . 
     It can be clearly seen from  FIG. 2  that the voltage difference between the operating voltage VDD and the common voltage VSS is fixed. This voltage difference is equivalent to the voltage difference between the first terminal and the second terminal of the transistor  212  and the voltage difference across the organic light emitting diode  220 . Because the voltage across the terminals of the organic light emitting diode  220  will increase with the total operating time, the voltage difference between the first and second terminal of the transistor  212  reduces continuously. However, when the AMOLED panel  102  is in operation, the transistor  212  operates in the saturated region with the condition V DS ≧V GS −V T . Here, V DS  represents the voltage difference between the first terminal and the second terminal of the transistor  212 , V GS  represents the voltage difference between the controlling terminal and the first terminal of the transistor  212 , and V T  represents the threshold voltage. If the voltage V DS  is continuously reduced, the transistor  212  originally operating in the saturated region may start operating in the linear region. 
     Therefore, the present invention provides a method for controlling an active matrix organic light emitting diode (AMOLED) panel.  FIG. 3  is a flow diagram of a method for controlling an AMOLED panel according to a preferred embodiment of the present invention. As shown in  FIG. 3 , the power supply  108  provides a operating voltage VDD and a common voltage VSS to the AMOLED panel  102  in step S 301  for generating a driving current ID to drive the AMOLED panel  102 . The voltage control unit  122  accesses the memory unit  120  to detect the operating time of the AMOLED panel  102  in step S 303 . According to the result of the detection, the power supply  108  adjusts the common voltage VSS in step S 305 . Thereafter, in step S 308 , the memory unit  120  counts and saves the operating time of the AMOLED panel  102  and then repeats the steps from step S 303 . 
     One skilled in the art would understand that the transistor in the pixel circuit  1141  can be implemented using an NMOS transistor instead of the PMOS transistor. Similarly, its operation can be deduced from the foregoing method. 
       FIG. 4  is a diagram of a common voltage curve according to a preferred embodiment of the present invention. As shown in  FIGS. 2 and 4 , an increase in the operating time of the AMOLED panel  102  will lead to an increase in the voltage difference across the organic light emitting diode  220 . Hence, the present invention provides a mechanism for lowering the common voltage V SS  to compensate for the rise in voltage difference across the organic light emitting diode  220 . When the voltage control unit  122  accesses the memory unit  120  to read out the operating time of the AMOLED panel  102 , the relationship between the common voltage curve  410  and the operating time of the AMOLED panel  102  can be used to find the current cumulative operating time of the AMOLED panel  102  and a corresponding common voltage V SS  value. Therefore, the control circuit  105  can use the power supply  108  to adjust the common voltage V SS . In the present invention, the operating voltage V DD  is fixed because adjusting the operating voltage V DD  may affect the gamma (Γ) value of the panel leading to a change in the brightness of the AMOLED panel  102 . However, if the common voltage V SS  is adjusted, no such problem will occur. In other words, the present invention fixes the operating voltage V DD  and only adjusts the common voltage V SS . 
       FIG. 5  is a diagram of a curve showing the relationship between the voltage difference between the operating voltage V DD  and the common voltage V SS  and the operating time of the AMOLED panel  102 . As shown in  FIG. 5 , when the common voltage V SS  is maintained at a constant value, the voltage difference between the operating voltage V DD  and the common voltage V SS  will remain unchanged by operating time of the AMOLED panel  102 . Hence, the relationship between the voltage difference between the operating voltage V DD  and the common voltage V SS  and the operating time of the AMOLED panel  102  is a straight line  510 . In addition, the present invention provides an AMOLED panel  102  with an adjustable common voltage V SS . When the AMOLED panel  102  is turned on, only the initial value of the common voltage V SS  has to be adjusted to the saturation region of the transistor  212 . Then, as the operating time of the AMOLED panel  102  is increased, the common voltage V SS  is gradually lowered according to the common voltage curve  410 . Therefore, the voltage difference between the operating voltage V DD  and the common voltage V SS  and the operating time of the AMOLED panel  102  is a slope line  512 . In addition, the area enclosed by the straight line  510 , the slope line  512  and the voltage axis can be regarded as the energy saved by the AMOLED panel  102 . 
       FIG. 6  is a flow diagram of a method for controlling an AMOLED panel according to another preferred embodiment of the present invention. As shown in  FIG. 6 , another possible method of operation in the present invention is to use the memory unit  120  to count the turn-on frequency of the AMOLED panel  102  and save the value. First, the power supply  108  provides an operating voltage V DD  and a common voltage V SS  to the AMOLED panel  102  for generating a driving current ID driving the AMOLED panel  102  in step S 601 . Then, the voltage control unit  122  accesses the memory unit  120  to detect the turn-on frequency of the AMOLED panel  102  in step S 603 . Thereafter, according to the result of the detection, the power supply  108  adjusts the common voltage V SS  in step S 605 . Next, in step S 608 , the memory unit  120  counts the turn-on frequency of the AMOLED panel  102  and saves the value, and then repeats the steps from step S 603 . 
     In summary, the present invention provides a control circuit for use in an AMOLED panel, which comprises a memory unit and a voltage control unit. The memory unit is used for counting and saving the operating time or the turn-on frequency of the AMOLED panel. Then, the voltage control unit lowers the common voltage according to the operating time or the turn-on frequency of the AMOLED panel. Consequently, the present invention is able to resolve the problem of an increase in the voltage difference across the terminals of an organic light emitting diode as a result of cumulative usage and provides a stable driving current for the organic light emitting diode and an AMOLED panel with a more stable picture quality. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.