Patent Publication Number: US-2006007078-A1

Title: Active matrix organic light emitting diode (AMOLED) display panel and a driving circuit thereof

Description:
BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      This invention relates to an active matrix organic light emitting diode (AMOLED) display panel and a driving circuit thereof, and more particularly to a current-driven AMOLED display panel and a driving circuit thereof.  
      (2) Description of the Related Art  
      With the progress in the fabrication technology of organic light emitting diodes (OELDs), an OLED display with a plurality of OLEDs arranged in matrix for illumination has become a popular choice among all the flat panel displays. Based on the difference in driving methods, the OLED displays in present can be sorted into simple matrix system type and active matrix system type, and the latter is a better choice for large size displays and high resolution usage.  
       FIG. 1  shows an equivalent circuit diagram of a pixel driving unit in a traditional voltage-driven active matrix organic light emitting diode (AMOLED) display. The pixel driving unit 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 (not shown in this figure) for receiving a driving voltage signal Vdata. A gate electrode of the transistor T 1  is connected to a scan line (not shown in this figure) for receiving a scanning voltage signal Scan. 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 Vdd. 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 another potential Vss. Two opposing ends of the capacitor C are connected to the gate electrode of the transistor T 2  and provided with the potential Vdd respectively.  
      As the scanning voltage signal Scan is at a high level state for switching on the transistor T 1 , the driving voltage signal Vdata on the data line is applied to the gate electrode of the transistor T 2  and also the capacitor C. As the scanning voltage level Scan is at a low level state for switching off the transistor T 1 , the capacitor C is floated to store a potential Vcs identical to a difference between the voltage levels of Vdata and Vdd. In this situation, it is understood that the gate to source voltage Vgs of the transistor T 2  equals to a difference between the voltage levels of Vdd and Vdata. A difference between the gate to source voltage Vgs and the threshold voltage Vt of the transistor T 2  further determines the current I passing through the OLED for illuminating.  
       FIG. 2  shows an equivalent circuit diagram of a pixel driving unit in a traditional current-driven AMOLED display. As shown, the pixel driving unit includes an OLED, a transistor T 1 , a transistor T 2 , a transistor T 3 , a transistor T 4 , and a capacitor C. A source electrode of the transistor T 1  is connected to a data line (not shown in this figure) for receiving a driving current signal Idata. A gate electrode of the transistor T 1  is connected to a scan line (not shown in this figure) for receiving a scanning voltage signal Scan. A drain electrode of the transistor T 1  is connected to a source electrode of the transistor T 2 . A gate electrode of the transistor T 2  is connected to a gate electrode of the transistor T 4 . A drain electrode of the transistor T 2  is connected to an anode of the OLED and also a source electrode of the transistor T 4 . A source electrode of the transistor T 3  is connected to the data line for receiving the driving current signal Idata. A gate electrode of the transistor T 3  is connected to the scan line for receiving the scanning voltage signal Scan. A drain electrode of the transistor T 3  is connected to the gate electrode of the transistor T 2  and also the gate electrode of the transistor T 4 . A source electrode of the transistor T 4  is connected to the anode of the OLED. A drain electrode of the transistor T 4  is provided with a potential Vdd. The cathode of the OLED is provided with another potential Vss. Two opposing ends of the capacitor C are connected to the gate electrode of the transistor T 4  and the anode of the OLED respectively.  
      As the scanning voltage signal Scan is at a high level state for switching on the transistors T 1  and T 3 , the driving current signal Idata is applied to the transistor T 2  and the capacitor C and generates a corresponding potential Vcs stored in the capacitor C. It is noted that as the scanning voltage from the scan line is at a low level state for switching off the transistors T 1  and T 3 , two corresponding mirror circuits with respect to the capacitor C and the OLED are created. The transistors T 2  and T 4  are located in the two corresponding mirror circuits respectively. As the two transistors T 2  and T 4  are set with identical electronic properties, the potential Vcs stored in the capacitor C may generate a current I identical to the driving current signal Idata in value passing through the transistor T 4  and determine the illumination of the OLED.  
      In the voltage-driven pixel driving unit shown in  FIG. 1 , the value of the threshold voltage Vt of the transistor T 2  may be significantly increased due to the accumulation of charges inside the transistor T 2  during operation. Since the value of current passing through the OLED is very much influenced by the value of threshold voltage Vt in the transistor T 2 . A decreasing of current passing through the OLED and a worse brightness is unpreventable.  
      In the current-driven pixel driving unit shown in  FIG. 2 , the value of current passing through the OLED is determined by the driving current signal Idata and is irrelevant to the variation of the threshold voltages of the transistors T 2  and T 4  so as to prevent a decreasing of current passing through the OLED. However, since the current-driven pixel driving unit needs four transistors T 1 , T 2 , T 3 , and T 4  to show the above mentioned characteristic, an increasing in fabrication cost and a worse transparency is unpreventable.  
      Accordingly, how to prevent the increasing of threshold voltage of the transistors in the traditional voltage-driven pixel driving unit to maintain the brightness of OLEDs, and how to reduce the number of the required electronic elements, such as transistors, in the tradition current-driven pixel driving unit to improve the transparency, are two important issues for the development of OLED display industry.  
     SUMMARY OF THE INVENTION  
      It is a main object of the present invention to improve the transparency of the pixel driving unit in the traditional current-driven active matrix organic light emitting diode (AMOLED) display.  
      It is another object of the present invention to maintain a brightness of the organic light emitting diode (OLED) in the pixel driving unit of the traditional voltage-driven AMOLED display.  
      An AMOLED display with a plurality of pixel driving units of voltage-driven design but applied with a driving current is provided in the present invention. The pixel driving unit having at least a displaying OLED and a driving transistor is connected to a reference unit in parallel. The reference unit has a reference OLED and a reference transistor corresponding to the displaying OLED and the driving transistor in the pixel driving unit respectively for defining a specific relationship between the values of the driving current passing through the pixel driving unit and a reference current passing through the reference unit. 
    
    
     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  shows an equivalent circuit diagram of a pixel driving unit in a traditional voltage-driven AMOLED display;  
       FIG. 2  shows an equivalent circuit diagram of a pixel driving unit in a traditional current-driven AMOLED display;  
       FIG. 3  shows a block diagram depicting a preferred embodiment of the driving circuit of an AMOLED display in accordance with the present invention; and  
       FIG. 4  shows an equivalent circuit diagram depicting the pixel driving unit connected to the reference unit in parallel through the data line shown in  FIG. 3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 3  shows a block diagram depicting a preferred embodiment of the driving circuit  100  of an active matrix organic light emitting diode (AMOLED) display in accordance with the present invention. The driving circuit  100  includes a data driver  120 , a scan driver  140 , a power supplier  150 , a plurality of pixel driving units  160 , and a plurality of reference units  180 . The pixel driving units  160  are arranged on the display in matrix. The data driver  120  is connected to the pixel driving units  160  and the reference units  180  through a plurality of data lines  122 . The scan driver  140  is connected to the pixel driving units  160  through a plurality of scan lines  142 . The power supplier  150  is utilized to apply power to the organic light emitting diodes (not shown in this figure) of each pixel driving unit  160 . As shown, each row of the pixel driving units  160  is coupled to a corresponded reference unit  180  through the data line  122 . The reference unit  180  is arrange at the lower edge of the row of pixel driving units  160  and shielded to prevent some unwanted influence for normal displaying. By contrast with the traditional current-driven pixel driving unit of  FIG. 2 , the reference unit  180  is utilized to play a role as one of the mirror circuits and the required elements of the pixel driving unit  160  in the present invention can be reduced.  
       FIG. 4  shows an equivalent circuit diagram depicting the pixel driving unit  160  connected to the reference unit  180  in parallel through the data line  122  shown in  FIG. 3 . As shown, the pixel driving unit  160  includes a switch transistor T 1 , a driving transistor T 2 , a capacitor C, and a displaying organic light emitting diode (OLED). The switch transistor T 1  has a source electrode connected to the respected data line  122  and a gate electrode connected to the respected scan line  142 . The driving transistor T 2  has a gate electrode connected to a drain electrode of the switch transistor T 1 , and has a drain electrode connected to a power supplier (not shown in this figure) through a power line  152  for receiving a first potential Vdd. The capacitor C has a first end connected to the drain electrode of the driving transistor T 2  and a second end opposing to the first end connected to both the source electrode of the switch transistor T 1  and the gate electrode of the driving transistor T 2 . The displaying OLED has an anode connected to the source electrode of the driving transistor T 2  and a cathode provided with a second potential Vss, which may be corresponding to a grounded potential.  
      The reference unit  180  includes a reference transistor Tm corresponding to the driving transistor T 2  of the pixel driving unit  160  and a reference organic light emitting diode OLEDm corresponding to the displaying organic light emitting diode OLED of the pixel driving unit  160 . The reference transistor Tm has a gate electrode and a drain electrode both connected to the respected data line  122 . In addition, the reference organic light emitting diode OLEDm has an anode connected to a source electrode of the reference transistor Tm and a cathode provided with the second potential Vss.  
      It is noted that the value of the current passing through the reference transistor Tm determines the difference between the gate to source voltage Vgs′ and the threshold voltage Vt′ of the reference transistor Tm. In addition, the voltage level of the data line Vdata equals to the sum of the gate to source voltage Vgs′, the anode to cathode voltage Voled′ of the reference organic light emitting diode OLEDm, and the second potential Vss (Vdata=Vgs′+Voled′+Vss). Therefore, a preset driving current signal I determines the voltage level Vdata on the data line.  
      As a scanning voltage applied through the scan line  142  to the pixel driving unit  160  is at a high level state to switch on the switch transistor T 1 , a voltage difference between the first potential Vdd and the voltage level on the data line  122  Vdata is applied and stored in the capacitor C. In this situation, the gate to source voltage Vgs of the driving transistor T 2  equals to Vdata−Voled−Vss, wherein Voled is the anode to cathode voltage of the displaying OLED.  
      Since the voltage level on the data line  142  Vdata equals to Vgs′+Voled′+Vss, and the gate to source voltage Vgs of the driving transistor T 2  equals to Vdata−Voled−Vss, it is calculated that the gate to source voltage Vgs of the driving transistor T 2  equals to Vgs′−(Voled−Voled′). In addition, the difference between the voltage Vgs and the threshold voltage Vt of the driving transistor T 2  determines the value of the current I′ passing through the displaying OLED for illumination.  
      As mentioned above, the driving circuit in accordance with the present invention has the following advantages.  
      Firstly, it is predictable that the reference organic light emitting diode OLEDm and the displaying organic light emitting diode OLED in the present invention may present similar operation time. Therefore, the increasing events of the anode to cathode voltages of the OLEDm and the OLED are similar. That is, the voltage Voled may be substantially identical to the voltage Voled′ dynamically. Since the gate to source voltage Vgs of the driving transistor T 2  equals to Vgs′−(Voled−Voled′), the gate to source voltage Vgs of the driving transistor T 2  is substantially identical to the gate to source voltage Vgs′ of the reference transistor Tm. The value of the gate to source voltage Vgs′ of the reference transistor Tm is determined by the driving current signal I. The value of the gate to source voltage Vgs of the driving transistor T 2  determines the current I′ passing through the displaying OLED in the pixel driving unit  160  for illumination. Therefore, in the driving circuit in accordance with the present invention, a preset driving current signal I is able to determine the value of the current I′ without a bad influence of the increasing of the anode to cathode voltage of the displaying OLED.  
      Secondly, since the reference transistor Tm and the driving transistor T 2  are predicted to have similar operating time, the threshold voltages Vt and Vt′ of the two transistor T 2  and Tm may show similar increasing events. In addition, because the gate to source voltage Vgs of the driving transistor T 2  is substantially identical to the gate to source voltage Vgs′ of the reference transistor Tm, the difference between the threshold voltage and the gate to source voltage of the driving transistor T 2  and that of the reference transistor Tm may be substantially the same. That is, by setting the driving transistor T 2  and the reference transistor Tm with identical channel width/length (W/L) ratio, the value of the current I′ passing through the displaying OLED may be substantially equal to the value of the driving current signal I and is irrelevant to the increasing of the threshold voltages Vt and Vt′ as the transistors are operating.  
      Thirdly, because the differences between the threshold voltage and the gate to source voltage of the driving transistor T 2  and that of the reference transistor in accordance with the present invention are substantially the same, the differences of the channel W/L ratios of the reference transistor Tm and the driving transistor T 2  is able to decide the relationship between the driving current signal I and the current I′ passing through the displaying OLED. For example, if the channel W/L ratio of the reference transistor Tm is two times larger than the channel W/L ratio of driving transistor T 2 , and the voltage Vgs equals to the voltage Vgs′, the value of the driving current signal I passing through the reference transistor Tm will be twice the value of the current I ‘passing through the driving transistor T 2 .  
      Based on this concept, it is understood that even in a low brightness condition with a small current I′ passing through the driving transistor T 2 , by setting a proper relationship between the channel W/L ratios of the two transistors T 2  and Tm, a greater driving current signal I with respect to the current I′ can be applied on the data line  122  for charging the capacitor C of the pixel driving circuits  160  with an acceptable speed and also guarantees that the capacitor C is charged to the needed potential.  
      Fourthly, by contrast to the traditional current-driven pixel driving unit shown in  FIG. 2 , which needs four transistors for current driving utility, the pixel driving unit  160  in the present invention as shown in  FIG. 3  and  FIG. 4  needs only two transistors T 1  and T 2  for such current driving utility. Therefore, a better transparency and a greater aperture ratio is predictable.  
      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.