Patent Publication Number: US-7911459-B2

Title: Pixel circuit

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to a pixel circuit, and more particularly relates to an AMOLED compensation pixel circuit with improved IR drop. 
     2. Description of Related Art 
       FIG. 1  shows an organic light emitting diode pixel circuit of the prior art. The pixel circuit is a voltage type pixel circuit. The pixel circuit has a light emitting diode  110 , a driving transistor  130 , a capacitor  150 , and a first switch  170 . The light emitting diode  110  has a first end  112  receiving a first supply voltage  120 . The driving transistor  130  has a source  132  and a drain  136  respectively receiving a second supply voltage  140  and coupled to a second end  116  of the light emitting diode  110  through the first switch  170 . The capacitor  150  has a first end  151  coupled to a gate  134  of the driving transistor  130  and a second end  152  receiving the second supply voltage  140 . The first switch  170  is controlled by a first scan signal (SN 1 ) to couple the drain  136  of the driving transistor  130  to the second end  116  of the light emitting diode  110 . 
     The pixel circuit has a second switch  180  controlled by a second scan signal (SN 2 ) to couple a data line  185  to the pixel circuit through a transistor  187 . 
     The transistor  190  is controlled by the first scan signal from the neighbor data line (SN 1 - 1 ). The transistors  187  and  190  are arranged to compensate the driving voltage when the pixel circuit operates. 
     The drawback of the conventional pixel circuit is that it has an IR drop issue. Especially when the panel display gets bigger, the IR drop issue gets worse. 
     SUMMARY 
     According to one embodiment of the present invention, the pixel circuit has a light emitting diode, a driving transistor, a capacitor, and a first switch. The light emitting diode had a first end to receive a first supply voltage. The driving transistor has a source and drain respectively receiving a second supply voltage and coupled to a second end of the light emitting diode. The capacitor has a first end coupled to a gate of the driving transistor and a second end receiving a reference voltage. The first switch is controlled by a first scan signal to couple the source of the driving transistor to the second end of the capacitor. The pixel circuit operates in a pre-charge period, a programming period, and an emission period sequentially, and the first scan signal is asserted to turn on the first switch during the pre-charge and emission periods. 
     According to another embodiment of the present invention, the display panel has several pixel circuits coupled to a first scan line and a second scan line. The pixel circuits are respectively coupled to several data lines. Each pixel circuit has a light emitting diode, a driving transistor, a capacitor, and a first switch. The light emitting diode has a first end to receive a first supply voltage. The driving transistor has a source and drain respectively receiving a second supply voltage and coupled to a second end of the light emitting diode. The capacitor has a first end coupled to a gate of the driving transistor and a second end receiving a reference voltage. The first switch is controlled by a first scan signal to couple the source of the driving transistor to the second end of the capacitor. The pixel circuit operates in a pre-charge period, a programming period, and an emission period sequentially, and the first scan signal is asserted to turn on the first switch during the pre-charge and emission periods. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows an organic light emitting diode pixel circuit of the prior art; 
         FIG. 2  shows an organic light emitting diode pixel circuit according to an embodiment of the invention; 
         FIG. 3  shows an organic light emitting diode pixel circuit according to another embodiment of the invention; 
         FIG. 4  shows an organic light emitting diode pixel circuit according to another embodiment of the invention; and 
         FIG. 5  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 4 . 
     
    
    
     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. 2  shows an organic light emitting diode pixel circuit according to an embodiment of the invention. The pixel circuit is a voltage type pixel circuit. The pixel circuit has a light emitting diode  210 , a driving transistor  230 , a capacitor  250 , and a first switch  270 . The light emitting diode  210  has a first end  212  receiving a first supply voltage  220 . The driving transistor  230  has a source  232  and a drain  236  respectively receiving a second supply voltage  240  and coupled to a second end  216  of the light emitting diode  210 . The capacitor  250  has a first end  251  coupled to a gate  234  of the driving transistor  230  and a second end  252  receiving a reference voltage  260 . A first scan signal (SN 1 ) is applied to control the first switch  270  whether couples the source  232  of the driving transistor  230  to the second end  252  of the capacitor  250  or not. 
     The pixel circuit has a second switch  280  controlled by a second scan signal (SN 2 ) to couple a data line  285  to the pixel circuit through a transistor  287 . Therefore, when the second scan signal is asserted, the data signals from the data line  285  are transmitted to the pixel circuit. 
     The transistor  290  is controlled by the first scan signal from the neighbor data line (SN 1 - 1 ). The transistors  287  and  290  are arranged to compensate the driving voltage when the pixel circuit operates. 
     Moreover, the first supply voltage  220  at the first end  212  of the light emitting diode  210  is a ground voltage (VSS). The first switch  270  and the second switch  280  can be implemented by transistors. In the embodiment shown in the  FIG. 2 , the switches  270  and  280  are PMOS transistors. If the switches  270  and  280  are configured by NMOS transistors, the control signals have to be inverted. 
       FIG. 3  shows an organic light emitting diode pixel circuit according to another embodiment of the invention. The pixel circuit is a current type pixel circuit. The pixel circuit has a light emitting diode  310 , driving transistors  330   a  and  330   b , a capacitor  350 , and a first switch  370 . The light emitting diode  310  has a first end  312  receiving a first supply voltage  320 . The driving transistor  330   a  has a source  332   a  and a drain  336   a  respectively receiving a second supply voltage  340  and coupled to a second end  316  of the light emitting diode  310 . The capacitor  350  has a first end  351  coupled to a gate  334   a  of the driving transistor  330   a  and a second end  352  receiving a reference voltage  360 . The first switch  370  is controlled by a scan signal (SN) to couple the source  332   a  of the driving transistor  330   a  to the second end  352  of the capacitor  350 . The driving transistor  330   b  has a source  332   b  and a gate  334   b  respectively receiving the reference voltage  360  and coupled to the gate  334   a  of the driving transistor  330   a.    
     The pixel circuit has a second switch  380  controlled by the scan signal to couple a data line  385  to the pixel circuit. Therefore, when the scan signal is asserted, the data signals from the data line  385  are transmitted to the pixel circuit. The transistor  390  is controlled by the scan signal to couple a drain  336   b  and the gate  334   b  of the driving transistor  330   b  together. 
       FIG. 4  shows an organic light emitting diode pixel circuit according to another embodiment of the invention. The display panel  400  has several pixel circuits (such as pixel circuits  400   a  and  400   n ) coupled to a first scan line  402  and a second scan line  404 . The pixel circuits are respectively coupled to several data lines. For example, the pixel circuits  400   a  and  400   n  are respectively coupled to the data lines  485   a  and  485   n . Take pixel circuits  400   a  for example; the pixel circuit  400   a  has a light emitting diode  410   a , a driving transistor  430   a , a capacitor  450   a , and a first switch  470   a . The light emitting diode  410   a  has a first end  412   a  receiving a first supply voltage  420   a . The driving transistor  430   a  has a source  432   a  and drain  436   a  respectively receiving a second supply voltage  440  and coupled to a second end  416   a  of the light emitting diode  410   a . The capacitor  450   a  has a first end  451   a  coupled to a gate  434   a  of the driving transistor  430   a  and a second end  452   a  receiving a reference voltage  460 . A first scan signal (SN 1 ) is applied to control the first switch  470   a  whether couples the source  432   a  of the driving transistor  430   a  to the second end  452   a  of the capacitor  450   a  or not. 
     The pixel circuit  400   a  has a second switch  480   a  controlled by a second scan signal (SN 2 ) to couple a data line  485   a  to the pixel circuit through a transistor  487   a . Therefore, when the second scan signal is asserted, the data signals from the data line  485   a  are transmitted to the pixel circuit. 
     The transistor  490   a  is controlled by the first scan signal from the neighbor data line (SN 1 - 1 ). The transistors  487   a  and  490   a  are arranged to compensate the driving voltage when the pixel circuit operates. 
     Moreover, the first supply voltage  420   a  at the first end  412   a  of the light emitting diode  410   a  is a ground voltage (VSS). The first switch  470   a  and the second switch  480   a  can be implemented by transistors. In this embodiment shown in the  FIG. 4 , the switches  470   a  and  480   a  are PMOS transistors. If the switches  470   a  and  480   a  are configured by NMOS transistors, the control signals have to be inverted. The pixel circuit  400   n  has the corresponding configuration of the pixel circuit  400   a.    
       FIG. 5  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 4 . The pixel circuit operates in a pre-charge period, a programming period, and an emission period sequentially. The second scan signal SN 2  is asserted to turn on the second switch  480   a  during the programming period, and de-asserted to turn off the second switch  480   a  during the pre-charge and emission periods. The first scan signal SN 1  is asserted to turn on the first switch  470   a  during the pre-charge and emission periods, and de-asserted to turn off the first switch  470   a  during the programming period. Namely, the first scan signal (SN 1 ) is an inverted signal of the second scan signal (SN 2 ). 
     In the display panel  400 , the power source terminals of the second supply voltage  440  locate at the left side of the display panel  400 . Therefore, when the distance between the pixel circuit and the left side of the display panel  400  increases, the voltage drop (IR drop) of the second supply voltage  440  increases. Namely, the voltage of the second supply voltage  440  in the pixel circuit  400   n  (VDDN) is lower than that of the pixel circuit  400   a  (VDD 1 ). That is why the ordinary display panel has the IR drop issue. 
     Therefore, when the switch  470   n  is turned on by the first scan signal (SN 1 ) in the pre-charge and emission periods, the reference voltage  460  can prevent the second supply voltage  440  in the pixel circuit  400   n  (VDDN) from falling bellow the reference voltage  460  (V ref ). The IR drop issue is improved thereby. 
     Moreover, when the switch  470   n  is turned off by the first scan signal (SN 1 ) in the programming period, the capacitor  450   n  is isolated from the light emitting diode  410   n , and the data signals from the data line  485   n  are written into the capacitor  450   n  more efficiently. 
     Furthermore, a level of the reference voltage  460  is selected for a specific voltage range of a data signal. Namely, the reference voltage  460  can adjust the required voltages of the data signals written into the capacitors in the programming period. For example, if the voltage difference between two ends  451   n  and  452   n  of the capacitor  450   n  during the programming period is 5 volts, and the reference voltage  460  is 10 volts, therefore the required voltage of the data signal written into the capacitor  450   n  is 5 volts. If the reference voltage  460  is 9 volts, the required voltage of the data signal written into the capacitor  450   n  is just 4 volts. Thus, the low reference voltage  460  enables the pixel circuit to be driven by the drivers with low voltage data signals. The power consumption of the pixel circuit and the cost of the drivers and panels are reduced thereby. 
     By the description above, the embodiments of this invention with the voltage compensation function use the reference voltage cooperated with the switch connected thereof to improve the IR drop issue and reduce the power consumption by adjust the voltage of data signals. 
     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.