Patent Publication Number: US-7903059-B2

Title: Lighting emitting display, pixel circuit and driving method thereof

Description:
This application claims the benefit of Taiwan application Serial No. 95108476, filed Mar. 13, 2006, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a display, a pixel circuit and a driving method thereof, and more particularly to a lighting emitting display, a pixel circuit of a light emitting diode (LED) and a driving method thereof. 
     2. Description of the Related Art 
     An organic light emitting display has no limitation to the viewing angle, has the low power consumption, may be easily manufactured and has the high response speed, so the organic light emitting display has become the next generation of the display technology. In the organic light emitting display, an organic film is evaporated between a transparent anode and a metal cathode, and electrons and holes are introduced between the transparent anode and the metal cathode to combine together between the organic films so that the energy can be converted into visible light. In addition, different organic materials may be used to output the light with different colors so that the requirement of the full-color display may be satisfied. 
     In a pixel circuit of the organic light emitting display, the luminance outputted from the pixel may be different from an expected one due to the threshold voltage variation of each MOS transistor and the influence of the mobility shift. Thus, it is very important to compensate the pixel structure or the driving method in the above-mentioned state.  FIG. 1  (PriorArt) shows a conventional pixel circuit  100  capable of compensating a mobility shift. Referring to  FIG. 1 , the pixel circuit  100  includes transistors K 1  to K 5 , a capacitor Cst and an organic light-emitting diode O 1 . In the pixel circuit  100 , the transistors K 2  and K 4  form a current mirror to control the current flowing through the organic light-emitting diode O 1 . However, the dismatch between the transistors K 2  and K 4  may make the behavior of the pixel be worse than the expected behavior. 
       FIG. 2  (Prior Art) shows another conventional pixel circuit  200  capable of compensating the mobility shift. Referring to  FIG. 2 , the pixel circuit  200  includes transistors T 1  to T 7 , a capacitor C and an organic light-emitting diode OLED. Compared with the pixel circuit of  FIG. 1 , the problem of dismatch between the transistors K 2  and K 4  can be overcome, but the number of transistors in the pixel circuit  200  is greater than that in the pixel circuit  100 . So, the aperture ratio is influenced and the cost is increased. 
     SUMMARY OF THE INVENTION 
     The invention is directed to an organic light emitting display, a pixel circuit and a driving method thereof capable of preventing the problems of the increase of the cost and the decrease of the aperture ratio, which are caused by the circuit dismatch and the too many transistors. 
     According to a first aspect of the present invention, a pixel circuit of a light emitting diode (LED) is provided. The pixel circuit includes a driving transistor, a capacitor and a light emitting diode (LED). The capacitor has one end coupled to receive a first supply voltage and the other end coupled to a gate of the driving transistor. The LED has a cathode coupled to receive a second supply voltage. The gate and the drain of the driving transistor are coupled to an anode of the LED, and the source of the driving transistor is coupled to a charging voltage during a pre-charge period. The gate and the drain of the driving transistor are commonly coupled to an anode of the light emitting diode, and the source of the driving transistor is coupled to receive a charging voltage during a pre-charge period. The source of the driving transistor is coupled to receive a data signal and the drain and gate of the driving transistor are coupled to each other during a programming period. The source of the driving transistor is coupled to receive the first supply voltage and the drain of the driving transistor is coupled to the anode of the light emitting diode during a display period. 
     The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (Prior Art) shows a conventional pixel circuit capable of compensating a mobility shift. 
         FIG. 2  (Prior Art) shows another conventional pixel circuit capable of compensating a mobility shift. 
         FIG. 3  shows a pixel circuit of an organic light-emitting diode according to a first embodiment of the invention. 
         FIG. 4  shows waveforms of driving signals in one example of the pixel circuit according to the first embodiment of the invention. 
         FIG. 5  shows waveforms of driving signals in another example of the pixel circuit according to the first embodiment of the invention. 
         FIG. 6  shows a pixel circuit of an organic light-emitting diode according to a second embodiment of the invention. 
         FIG. 7  shows waveforms of driving signals in the pixel circuit according to the second embodiment of the invention. 
         FIG. 8  shows a pixel circuit of an organic light-emitting diode according to a third embodiment of the invention. 
         FIG. 9  shows waveforms of driving signals in one example of the pixel circuit according to the third embodiment of the invention. 
         FIG. 10  shows waveforms of driving signals in another example of the pixel circuit according to the third embodiment of the invention. 
         FIG. 11  shows a pixel circuit of an organic light-emitting diode according to a fourth embodiment of the invention. 
         FIG. 12  shows waveforms of driving signals in the pixel circuit according to the fourth embodiment of the invention. 
         FIG. 13  is a block diagram showing an organic light emitting display according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows a pixel circuit  300  of an organic light-emitting diode according to a first embodiment of the invention. Referring to  FIG. 3 , the pixel circuit  300  includes a driving transistor, for example PMOS transistor MP, a capacitor C 1 , an organic light-emitting diode O 3  and switches M 1  to M 4 . The capacitor C 1  is coupled between a first supply voltage VDD and a gate of the PMOS transistor MP. A cathode of the organic light-emitting diode O 3  is coupled to a second supply voltage VSS. 
     The switch M 1  is coupled between a source of the PMOS transistor MP and the first supply voltage VDD. The switch M 2  is coupled between the source of the PMOS transistor MP and a data signal. The switch M 3  is coupled between the gate and a drain of the PMOS transistor MP. The switch M 4  is coupled between the drain of the PMOS transistor MP and an anode of the organic light-emitting diode O 3 . 
       FIG. 4  shows waveforms of driving signals in one example of the pixel circuit according to the first embodiment of the invention. As shown in  FIGS. 3 and 4 , the switches M 1  to M 4  in this embodiment are PMOS transistors. The gate of the switch M 1  receives a signal SCAN 1  and is controlled to being turned on or off according to the signal SCAN 1 . The gate of the switch M 2  receives a signal SCAN 1 B and is controlled to being turned on or off according to the signal SCAN 1 B. The gate of the switch M 3  receives a signal SCAN 2  and is controlled to being turned on or off according to the signal SCAN 2 . The gate of the switch M 4  receives the signal SCAN 1  and is controlled to being turned on or off according to the signal SCAN 1 . The second supply voltage VSS remains on a constant voltage, such as the ground voltage GND. 
     During a pre-charge period Precharge, the switch M 1  is turned on, the switch M 2  is turned off, the switch M 3  is turned on and the switch M 4  is turned on so that the gate and the drain of the PMOS transistor MP are coupled to the anode of the organic light-emitting diode O 3 , and the source of the PMOS transistor MP is coupled to a charging voltage, which is provided by the first supply voltage VDD in this embodiment. At this moment, the voltage at a node A is VAini. 
     During a programming period Programming, the switch M 1  is turned off, the switch M 2  is turned on, the switch M 3  is turned on and the switch M 4  is turned off, so that the source of the PMOS transistor MP is coupled to a data signal VDATA, and the drain and the gate of the PMOS transistor MP are coupled to the node A to generate a node voltage VA. The data signal VDATA, the voltage VAini and the threshold voltage Vth of the PMOS transistor MP have the following relationship:
 
 VAini+|Vth|&lt;VDATA.  
 
     Thus, the data signal VDATA can be inputted into the pixel circuit  300 . In this case, the data signal VDATA, the node voltage VA and the threshold voltage Vth of the PMOS transistor MP have the following relationship:
 
 VA=VDATA−|Vth|.  
 
     During a displaying period Display, the switch M 1  is turned on, the switch M 2  is turned off, the switch M 3  is turned off, the switch M 4  is turned on, the source of the PMOS transistor MP is coupled to the first supply voltage VDD, and the drain of the PMOS transistor MP is coupled to the anode of the organic light-emitting diode O 3 . At this moment, the current Io of the organic light-emitting diode O 3  is as follows:
 
 Io=K (| Vgs|−|Vth |) 2 ,
 
     wherein Vgs is a voltage difference between the gate and the source of the PMOS transistor and K is a constant. Also, substitute “Vgs=VDATA−|Vth|−VDD” into the above-mentioned equation can get:
 
 Io=K ( VDD−VDATA ) 2 .
 
     Thus, the organic light-emitting diode O 3  can emit light based on the node voltage VA, and emit light based on the data signal VDATA. 
       FIG. 5  shows waveforms of driving signals in another example of the pixel circuit according to the first embodiment of the invention. The difference between  FIGS. 5 and 4  is that the second supply voltage VSS does not remain on a constant voltage all the time. In this embodiment, the second supply voltage VSS has a second voltage level, such as the ground voltage GND, during the programming period Programming and the displaying period Display, and is reduced to the first voltage level, such as the voltage VSL, during the pre-charge period Precharge, such that the VAini may become smaller. Then, during the programming period Programming, the voltage level of the data signal VDATA may be configured to be adjustable. The first voltage level is lower than the second voltage level. 
       FIG. 6  shows a pixel circuit  600  of an organic light-emitting diode according to a second embodiment of the invention. As shown in  FIG. 6 , the difference between the pixel circuit  600  and the pixel circuit  300  is that the pixel circuit  600  does not have the switch M 4 , and the drain of the PMOS transistor MP is directly coupled to the anode of the organic light-emitting diode O 3 .  FIG. 7  shows waveforms of driving signals in the pixel circuit according to the second embodiment of the invention. Compared with the driving signals of  FIG. 5 , the second supply voltage VSS is changed to substantially equal to the charging voltage, such as the first supply voltage VDD during the programming period Programming in order to disconnect the drain of the PMOS transistor MP from the anode of the organic light-emitting diode O 3  during the programming period Programming when no switch M 4  is provided. The second supply voltage VSS is the ground voltage GND during the displaying period Display, and the second supply voltage VSS is the voltage VSL lower than the ground voltage GND during the pre-charge period Precharge. 
       FIG. 8  shows a pixel circuit  800  of an organic light-emitting diode according to a third embodiment of the invention. As shown in  FIG. 8 , the difference between the pixel circuit  800  and the pixel circuit of the first embodiment is that the switch M 1  is controlled by the signal SCAN 2 B, and the signal SCAN 2 B and the signal SCAN 2  have opposite phases. Consequently, the switch M 1  is turned off during the pre-charge period Precharge, the switch M 2  is turned on, and the capacitor C 1  is charged by the data signal VDATA (the charging voltage is provided by data signal VDATA) to generate the voltage VAini.  FIG. 9  shows waveforms of driving signals in one example of the pixel circuit according to the third embodiment of the invention. The associated driving methods can be derived by one of ordinary skill in the art, so detailed descriptions thereof will be omitted. 
       FIG. 10  shows waveforms of driving signals in another example of the pixel circuit according to the third embodiment of the invention. The difference between  FIGS. 10 and 9  is that the magnitude of the second supply voltage VSS is not constant. In this embodiment, the second supply voltage VSS has the second voltage level, such as the ground voltage GND, during the programming period Programming and the displaying period Display, and is changed to the first voltage level, such as the voltage VSL, during the pre-charge period Precharge, so that the VAini may become smaller. Thus, the level of the data signal VDATA is configured to be adjustable during the programming period Programming. The first voltage level is lower than the second voltage level. 
       FIG. 11  shows a pixel circuit  110  of an organic light-emitting diode according to a fourth embodiment of the invention. As shown in  FIG. 11 , the difference between the pixel circuit  110  and the pixel circuit  800  is that the pixel circuit  110  does not have the switch M 4  so that the drain of the PMOS transistor MP is directly coupled to the anode of the organic light-emitting diode O 3 .  FIG. 12  shows waveforms of driving signals in the pixel circuit according to the fourth embodiment of the invention. Compared with the driving signals of  FIG. 10 , the magnitude of the second supply voltage VSS is increased to substantially equal to the magnitude of the charging voltage, such as the first supply voltage VDD during the programming period Programming in order to disconnect the drain of the PMOS transistor MP from the anode of the organic light-emitting diode O 3  during the programming period Programming when no switch M 4  is provided. The second supply voltage VSS is changed to the ground voltage GND during the displaying period Display, and the second supply voltage VSS is changed to the voltage VSL which is lower than the ground voltage GND during the pre-charge period Precharge. 
     According to the concept of the invention, all the switches of the pixel circuit may be implemented by NMOS transistors, and controlled by the complementary signals corresponding to the original PMOS transistors without departing from the scope of the invention. Taking the pixel circuit  300  as an example, the switch M 2  may be implemented by a NMOS transistor and is controlled by the signal SCAN 1 . 
       FIG. 13  is a block diagram showing an organic light emitting display  130  according to the invention. Referring to  FIG. 13 , the display  130  includes a scan driver  131 , a data driver  132  and a pixel array  133 . The scan driver  131  provides a scan signal SCAN, such as the signal SCAN 1 , SCAN 1 B, SCAN 2  or SCAN 2 B, to the pixel array  133 . The data driver  132  provides the data signal VDATA to the pixel array  133 , which includes the pixel circuit  300 ,  600 ,  800  or  110 , or the pixel circuit included in the concept of the invention. 
     In the display, the pixel circuit and the driving method thereof according to the embodiments of the invention, the novel architecture is provided. Thus, the luminance will not be influenced by the dismatch between the MOS transistors, and no extra circuit has to be provided to solve the problem of dismatch. Also, the circuit may be designed flexibly and may be adjusted according to the product under the concept of the invention. 
     While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.