Patent Publication Number: US-10319304-B2

Title: OLED pixel driving circuit and pixel driving method

Description:
FIELD OF THE INVENTION 
     The present invention relates to a display technology field, and more particularly to an OLED pixel driving circuit and a pixel driving method. 
     BACKGROUND OF THE INVENTION 
     The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device. 
     The OLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the OLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional OLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current. 
     As shown in  FIG. 1 , the traditional 2T1C pixel driving circuit used for OLED comprises: a first thin film transistor T 10 , a second thin film transistor T 20  and a capacitor C 10 . The first thin film transistor T 10  is a N type thin film transistor, which is used to be a switch thin film transistor; the second thin film transistor T 20  is a P type thin film transistor, which is used to be a driving thin film transistor; the capacitor C 10  is a storage capacitor. Specifically, a gate of the first thin film transistor T 10  receives a scan signal Scan, and a source receives a data signal Data, and a drain is electrically coupled to the gate of the second thin film transistor T 20  and one end of the capacitor C 10 ; a source of the second thin film transistor T 20  receives a power source voltage VDD, and a drain is electrically coupled to an anode of the organic light emitting diode D 10 ; a cathode of the organic light emitting diode D 10  receives a common ground voltage VSS; one end of the capacitor C 10  is electrically coupled to a gate of the second thin film transistor T 20 , and the other end is electrically coupled to a source of the second thin film transistor T 20 . As the OLED displays, the scan signal Scan controls the first thin film transistor T 10  to be activated, and the data signal Data enters the gate of the second thin film transistor T 20  and the capacitor C 10  via the first thin film transistor T 10 . Then, the first thin film transistor T 10  is deactivated. With the storage function of the capacitor C 10 , the gate voltage of the second thin film transistor T 20  can remain to hold the data signal voltage to make the second thin film transistor T 20  to be in the conducted state to drive the current to enter the organic light emitting diode D 10  via the second thin film transistor T 20  and to drive the organic light emitting diode D 10  to emit light. 
     The formula of the current flowing through the thin film transistor and the organic light emitting diode according to calculation is:
 
 I   OLED   =K× ( V   gs   −V   th ) 2  
 
     wherein I OLED  represents a current flowing through the driving thin film transistor and the organic light emitting diode, and K is an intrinsic conductive factor of the driving thin film transistor, and V gs  represents a voltage difference between the gate and the source of the driving thin film transistor, and V th  represents a threshold voltage of the driving thin film transistor. Accordingly, the value of the I OLED  is relevant with the threshold voltage V th  of the driving thin film transistor. 
     The structure of the foregoing traditional OLED pixel driving circuit is simpler and does not possess compensation function, and thus lots of defects exist, wherein the more obvious one is: due to the nonconsistency in the manufacture process of the thin film transistor, the threshold voltages of the driving thin film transistors of all pixels in the OLED display panel are not consistent; and the long period of working time will age the material of the driving thin film transistors to lead to the drifts of the threshold voltages of the driving thin film transistors and the to cause the phenomenon of the nonuniform OLED panel display. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide an OLED pixel driving circuit, which can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
     Another objective of the present invention is to provide an OLED pixel driving method, which can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
     For realizing the aforesaid objectives, the present invention first provides an OLED pixel driving circuit, comprising a first N type thin film transistor, a second N type thin film transistor, a third N type thin film transistor, a fourth N type thin film transistor, a fifth P type thin film transistor, a sixth N type thin film transistor, a seventh P type thin film transistor, a first capacitor, a second capacitor and an organic light emitting diode; the seventh P type thin film transistor serving as a driving thin film transistor of the organic light emitting diode; 
     a gate of the first N type thin film transistor receiving a third scan signal, and a source receiving a data signal, and a drain being electrically coupled to a first node; 
     a gate of the second N type thin film transistor receiving a second scan signal, and a source being electrically coupled to the first node, and a drain being electrically coupled to a second node; 
     a gate of the third N type thin film transistor receiving a light emitting control signal, and a source receiving a power source high voltage, and a drain being electrically coupled to a third node; 
     a gate of the fourth N type thin film transistor receiving the second scan signal, and a source being electrically coupled to the third node, and a drain being electrically coupled to a fifth node; 
     a gate of the fifth P type thin film transistor receiving a light emitting control signal, and a source being electrically coupled to a fourth node, and a drain receiving a common ground voltage; 
     a gate of the sixth N type thin film transistor receiving a first scan signal, and a source being electrically coupled to the fifth node, and a drain time-share receiving a power source low voltage or a luminous brightness adjustment voltage; 
     a gate of the seventh P type thin film transistor being electrically coupled to the first node, and a source being electrically coupled to the third node, and a drain being electrically coupled to the second node; 
     an anode of the organic light emitting diode being electrically coupled to the second node, and a cathode receiving the common ground voltage; 
     one end of the first capacitor being electrically coupled to the first node, and the other end being electrically coupled to the fourth node; 
     one end of the second capacitor being electrically coupled to the fourth node, and the other end being electrically coupled to the fifth node. 
     The first scan signal, the second scan signal, the third scan signal, the light emitting control signal, and the data signal are combined with one another to correspond to a reset stage, a threshold voltage detection stage, a program stage and a light emitting stage one after another. 
     in the reset stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the third scan signal is a low voltage level, and the light emitting control signal is a low voltage level, and the data signal is a low voltage level; the drain of the sixth N type thin film transistor receives the power source low voltage; 
     in the threshold voltage detection stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the light emitting control signal is a low voltage level, and the data signal is a low voltage level; 
     in the program stage, the first scan signal is a low voltage level, and the second scan signal is a low voltage level, and the third scan signal is a high voltage level, and the light emitting control signal is a low voltage level, and the data signal is a high voltage level; 
     in the light emitting stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the third scan signal is a low voltage level, and the light emitting control signal is a high voltage level, and the data signal is a low voltage level; the drain of the sixth N type thin film transistor receives the luminous brightness adjustment voltage; 
     The power source low voltage is higher than a sum of a threshold voltage of the seventh P type thin film transistor and a threshold voltage of the organic light emitting diode. 
     The common ground voltage is not higher than the luminous brightness adjustment voltage, and the luminous brightness adjustment voltage is lower than a sum of the power source high voltage and the threshold voltage of the organic light emitting diode minus a voltage value of a high voltage level provided by the data signal. 
     All of the first scan signal, the second scan signal, the third scan signal, the light emitting control signal and the data signal are generated by an external sequence controller. 
     all of the first N type thin film transistor, the second N type thin film transistor, the third N type thin film transistor, the fourth N type thin film transistor, the fifth P type thin film transistor, the sixth N type thin film transistor, the seventh P type thin film transistor are low temperature poly-silicon thin film transistors, oxide semiconductor thin film transistors or amorphous silicon thin film transistors. 
     The present invention further provides an OLED pixel driving method, comprising steps of: 
     step 1, providing an OLED pixel driving circuit; 
     the OLED pixel driving circuit comprising a first N type thin film transistor, a second N type thin film transistor, a third N type thin film transistor, a fourth N type thin film transistor, a fifth P type thin film transistor, a sixth N type thin film transistor, a seventh P type thin film transistor, a first capacitor, a second capacitor and an organic light emitting diode; the seventh P type thin film transistor serving as a driving thin film transistor of the organic light emitting diode; 
     a gate of the first N type thin film transistor receiving a third scan signal, and a source receiving a data signal, and a drain being electrically coupled to a first node; 
     a gate of the second N type thin film transistor receiving a second scan signal, and a source being electrically coupled to the first node, and a drain being electrically coupled to a second node; 
     a gate of the third N type thin film transistor receiving a light emitting control signal, and a source receiving a power source high voltage, and a drain being electrically coupled to a third node; 
     a gate of the fourth N type thin film transistor receiving the second scan signal, and a source being electrically coupled to the third node, and a drain being electrically coupled to a fifth node; 
     a gate of the fifth P type thin film transistor receiving a light emitting control signal, and a source being electrically coupled to a fourth node, and a drain receiving a common ground voltage; 
     a gate of the sixth N type thin film transistor receiving a first scan signal, and a source being electrically coupled to the fifth node, and a drain time-share receiving a power source low voltage or a luminous brightness adjustment voltage; 
     a gate of the seventh P type thin film transistor being electrically coupled to the first node, and a source being electrically coupled to the third node, and a drain being electrically coupled to the second node; 
     an anode of the organic light emitting diode being electrically coupled to the second node, and a cathode receiving the common ground voltage; 
     one end of the first capacitor being electrically coupled to the first node, and the other end being electrically coupled to the fourth node; 
     one end of the second capacitor being electrically coupled to the fourth node, and the other end being electrically coupled to the fifth node; 
     step 2, entering a reset stage; 
     the first scan signal providing a high voltage level, and the second scan signal providing a low voltage level, and the third scan signal providing a low voltage level, and the light emitting control signal providing a low voltage level, and the data signal providing a low voltage level; the drain of the sixth N type thin film transistor receiving the power source low voltage; 
     the fifth P type thin film transistor and the sixth N type thin film transistor being on, and the other thin film transistors being off, and the power source low voltage charging the second capacitor to perform initializing assignment to the second capacitor, and resetting a voltage difference of two ends of the second capacitor to be VDDL-VSS, wherein VDDL represents the power source low voltage, and VSS represents the common ground voltage; 
     step 3, entering a threshold voltage detection stage; 
     the first scan signal being changed to be a low voltage level, and the second scan signal being changed to be a high voltage level, and the third scan signal being kept to be a low voltage level, and the light emitting control signal being kept to be a low voltage level, and the data signal being kept to be a low voltage level; 
     all of the second N type thin film transistor, the fourth N type thin film transistor, the fifth P type thin film transistor and the seventh P type thin film transistor being on, and all of the first N type thin film transistor, the third N type thin film transistor and the sixth N type thin film transistor being off, and the second capacitor being discharged to the seventh P type thin film transistor until an energy storage voltage of the second capacitor is V th +V th   _ OLED, wherein V th  is a threshold voltage of the seventh P type thin film transistor, and V th   _   OLED  is a threshold voltage of the organic light emitting diode; 
     step 4, entering a program stage; 
     the first scan signal being kept to be a low voltage level, and the second scan signal being changed to be a low voltage level, and the third scan signal being changed to be a high voltage level, and the light emitting diode control signal being kept to be a low voltage level, and the data signal being changed to be a high voltage level; 
     the first N type thin film transistor and the fifth P type thin film transistor being on, and the other thin film transistors being off, and the data signal charging the first capacitor until an energy storage voltage of the first capacitor and a voltage level of the first node are V Data , and V Data  is a voltage value of a high voltage level provided by the data signal; 
     step 5, entering a light emitting stage; 
     the first scan signal being changed to be a high voltage level, and the second scan signal being kept to be a low voltage level, and the third scan signal being changed to be a low voltage level, and the light emitting control signal being changed to be a high voltage level, and the data signal being changed to be a low voltage level; the drain of the sixth N type thin film transistor receiving the luminous brightness adjustment voltage; 
     all of the third N type thin film transistor, the sixth N type thin film transistor and the fifth P type thin film transistor being on, and all of the first N type thin film transistor, the second N type thin film transistor, the fourth N type thin film transistor and the fifth P type thin film transistor being off, and the luminous brightness adjustment voltage being sent to the fifth node, and the voltage level of the first node being changed to be:
 
 Va=Vr+V   Data   −V   th   −V   th   _   OLED  
 
     wherein V a  represents a voltage level of the first node, and Vr represents the luminous brightness adjustment voltage; 
     the organic light emitting diode emits light, and a current flowing through the organic light emitting diode is irrelevant with the threshold voltage of the seventh P type thin film transistor. 
     The power source low voltage is higher than a sum of a threshold voltage of the seventh P type thin film transistor and a threshold voltage of the organic light emitting diode. 
     The common ground voltage is not higher than the luminous brightness adjustment voltage, and the luminous brightness adjustment voltage is lower than a sum of the power source high voltage and the threshold voltage of the organic light emitting diode minus a voltage value of a high voltage level provided by the data signal. 
     All of the first scan signal, the second scan signal, the third scan signal, the light emitting control signal and the data signal are generated by an external sequence controller; 
     all of the first N type thin film transistor, the second N type thin film transistor, the third N type thin film transistor, the fourth N type thin film transistor, the fifth P type thin film transistor, the sixth N type thin film transistor, the seventh P type thin film transistor are low temperature poly-silicon thin film transistors, oxide semiconductor thin film transistors or amorphous silicon thin film transistors. 
     The present invention further provides an OLED pixel driving circuit, comprising a first N type thin film transistor, a second N type thin film transistor, a third N type thin film transistor, a fourth N type thin film transistor, a fifth P type thin film transistor, a sixth N type thin film transistor, a seventh P type thin film transistor, a first capacitor, a second capacitor and an organic light emitting diode; the seventh P type thin film transistor serving as a driving thin film transistor of the organic light emitting diode; 
     a gate of the first N type thin film transistor receiving a third scan signal, and a source receiving a data signal, and a drain being electrically coupled to a first node; 
     a gate of the second N type thin film transistor receiving a second scan signal, and a source being electrically coupled to the first node, and a drain being electrically coupled to a second node; 
     a gate of the third N type thin film transistor receiving a light emitting control signal, and a source receiving a power source high voltage, and a drain being electrically coupled to a third node; 
     a gate of the fourth N type thin film transistor receiving the second scan signal, and a source being electrically coupled to the third node, and a drain being electrically coupled to a fifth node; 
     a gate of the fifth P type thin film transistor receiving a light emitting control signal, and a source being electrically coupled to a fourth node, and a drain receiving a common ground voltage; 
     a gate of the sixth N type thin film transistor receiving a first scan signal, and a source being electrically coupled to the fifth node, and a drain time-share receiving a power source low voltage or a luminous brightness adjustment voltage; 
     a gate of the seventh P type thin film transistor being electrically coupled to the first node, and a source being electrically coupled to the third node, and a drain being electrically coupled to the second node; 
     an anode of the organic light emitting diode being electrically coupled to the second node, and a cathode receiving the common ground voltage; 
     one end of the first capacitor being electrically coupled to the first node, and the other end being electrically coupled to the fourth node; 
     one end of the second capacitor being electrically coupled to the fourth node, and the other end being electrically coupled to the fifth node; 
     wherein the first scan signal, the second scan signal, the third scan signal, the light emitting control signal, and the data signal are combined with one another to correspond to a reset stage, a threshold voltage detection stage, a program stage and a light emitting stage one after another; 
     in the reset stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the third scan signal is a low voltage level, and the light emitting control signal is a low voltage level, and the data signal is a low voltage level; the drain of the sixth N type thin film transistor receives the power source low voltage; 
     in the threshold voltage detection stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the light emitting control signal is a low voltage level, and the data signal is a low voltage level; 
     in the program stage, the first scan signal is a low voltage level, and the second scan signal is a low voltage level, and the third scan signal is a high voltage level, and the light emitting control signal is a low voltage level, and the data signal is a high voltage level; 
     in the light emitting stage, the first scan signal is a high voltage level, and the second scan signal is a low voltage level, and the third scan signal is a low voltage level, and the light emitting control signal is a high voltage level, and the data signal is a low voltage level; the drain of the sixth N type thin film transistor receives the luminous brightness adjustment voltage; 
     wherein all of the first scan signal, the second scan signal, the third scan signal, the light emitting control signal and the data signal are generated by an external sequence controller. 
     The benefits of the present invention are: the present invention provides an OLED pixel driving circuit and a pixel driving method, which utilizes the pixel driving circuit of 7T2C structure, and the first scan signal, the second scan signal, the third scan signal, the light emitting control signal and the data signal are combined with one another to correspond to a reset stage, a threshold voltage detection stage, a program stage and a light emitting stage one after another and can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the characteristics and technical aspect of the invention, please refer to the following detailed description of the present invention is concerned with the diagrams, however, provide reference to the accompanying drawings and description only and is not intended to be limiting of the invention. 
       In drawings, 
         FIG. 1  is a circuit diagram of a pixel driving circuit of traditional 2T1C structure; 
         FIG. 2  is a circuit diagram of an OLED pixel driving circuit according to the present invention; 
         FIG. 3  is a sequence diagram of an OLED pixel driving circuit according to present invention; 
         FIG. 4  is a circuit diagram of an OLED pixel driving circuit in a reset stage, and also a circuit diagram of step 2 in the OLED pixel driving method according to the present invention; 
         FIG. 5  is a circuit diagram of an OLED pixel driving circuit in a threshold voltage detection stage, and also a circuit diagram of step 3 in the OLED pixel driving method according to the present invention; 
         FIG. 6  is a circuit diagram of an OLED pixel driving circuit in a program stage, and also a circuit diagram of step 4 in the OLED pixel driving method according to the present invention; 
         FIG. 7  is a circuit diagram of an OLED pixel driving circuit in a light emitting stage, and also a circuit diagram of step 5 in the OLED pixel driving method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For better explaining the technical solution and the effect of the present invention, the present invention will be further described in detail with the accompanying drawings and the specific embodiments. 
     Please refer to  FIG. 2  and  FIG. 3 . The present invention first provides an OLED pixel driving circuit, which utilizes the 7T2C structure and comprises a first N type thin film transistor T 1 , a second N type thin film transistor T 2 , a third N type thin film transistor T 3 , a fourth N type thin film transistor T 4 , a fifth P type thin film transistor T 5 , a sixth N type thin film transistor T 6 , a seventh P type thin film transistor T 7 , a first capacitor C 1 , a second capacitor C 2  and an organic light emitting diode D 1 . 
     A gate of the first N type thin film transistor T 1  receives a third scan signal Scan 3 , and a source receives a data signal Data, and a drain is electrically coupled to a first node a; a gate of the second N type thin film transistor T 2  receives a second scan signal Scan 2 , and a source is electrically coupled to the first node a, and a drain is electrically coupled to a second node b; a gate of the third N type thin film transistor T 3  receives a light emitting control signal EM, and a source receives a power source high voltage VDDH, and a drain is electrically coupled to a third node c; a gate of the fourth N type thin film transistor T 4  receives the second scan signal Scan 2 , and a source is electrically coupled to the third node c, and a drain is electrically coupled to a fifth node e; a gate of the fifth P type thin film transistor T 5  receives a light emitting control signal EM, and a source is electrically coupled to a fourth node d, and a drain receives a common ground voltage VSS; a gate of the sixth N type thin film transistor T 6  receives a first scan signal Scan 1 , and a source is electrically coupled to the fifth node e, and a drain time-share receives a power source low voltage VDDL or a luminous brightness adjustment voltage Vr; the seventh P type thin film transistor T 7  is a driving thin film transistor which directly drives the organic light emitting diode D 1 , of which a gate is electrically coupled to the first node a, and a source is electrically coupled to the third node c, and a drain is electrically coupled to the second node b; an anode of the organic light emitting diode D 1  is electrically coupled to the second node b, and a cathode receives the common ground voltage VSS; one end of the first capacitor C 1  is electrically coupled to the first node a, and the other end is electrically coupled to the fourth node d; one end of the second capacitor C 2  is electrically coupled to the fourth node d, and the other end is electrically coupled to the fifth node e. 
     Specifically, all of the first N type thin film transistor T 1 , the second N type thin film transistor T 2 , the third N type thin film transistor T 3 , the fourth N type thin film transistor T 4 , the fifth P type thin film transistor T 5 , the sixth N type thin film transistor T 6  and the seventh P type thin film transistor T 7  are low temperature poly-silicon thin film transistors, oxide semiconductor thin film transistors or amorphous silicon thin film transistors; all of the first scan signal Scan 1 , the second scan signal Scan 2 , the third scan signal Scan 3 , the light emitting control signal EM and the data signal Data are generated by an external sequence controller. 
     The first scan signal Scan 1 , the second scan signal Scan 2 , the third scan signal Scan 3 , the light emitting control signal EM, and the data signal Data are combined with one another to correspond to a reset stage S 1 , a threshold voltage detection stage S 2 , a program stage S 3  and a light emitting stage S 4  one after another. 
     Furthermore, combining  FIG. 3  and  FIG. 4 , in the reset stage S 1 : 
     the first scan signal Scan 1  is a high voltage level, and the second scan signal Scan 2  is a low voltage level, and the third scan signal Scan 3  is a low voltage level, and the light emitting control signal EM is a low voltage level, and the data signal Data is a low voltage level; the drain of the sixth N type thin film transistor T 6  receives the power source low voltage VDDL. 
     The fifth P type thin film transistor T 5  and the sixth N type thin film transistor T 6  is on, and the other thin film transistors is off, and the power source low voltage VDDL charges the second capacitor C 2  via the sixth N type thin film transistor T 6  which is on, and a voltage difference of two ends of the second capacitor C 2  after charging is accomplished, i.e. a voltage difference V ed  between the fifth node e and the fourth node d is:
 
 V   ed   =VDDL−VSS  
 
     to accomplish the reset and initializing assignment to the second capacitor C 2 . 
     Significantly, the power source low voltage VDDL is higher than a sum of a threshold voltage of the seventh P type thin film transistor T 7  and a threshold voltage of the organic light emitting diode D 1 , i.e.:
 
 VDDL&gt;V   th   +V   th   _   OLED  
 
     wherein Vth represents a threshold voltage of the seventh P type thin film transistor T 7 , and V th   _   OLED  is a threshold voltage of the organic light emitting diode D 1 . 
     Combining  FIG. 3  and  FIG. 5 , in the threshold voltage detection stage S 2 : 
     in the threshold voltage detection stage S 2 , the first scan signal Scan 1  is a low voltage level, and the second scan signal Scan 2  is a high voltage level, and the third scan signal Scan 3  is a low voltage level, and the light emitting control signal EM is a low voltage level, and the data signal Data is a low voltage level. 
     All of the second N type thin film transistor T 2 , the fourth N type thin film transistor T 4 , the fifth P type thin film transistor T 5  and the seventh P type thin film transistor T 7  are on, and all of the first N type thin film transistor T 1 , the third N type thin film transistor T 3  and the sixth N type thin film transistor T 6  are off, and the fifth node e is coupled with the third node c, i.e. the source of the seventh P type thin film transistor T 7  via the fourth N type thin film transistor T 4  which is on, and the first node a, i.e. the gate of the seventh P type thin film transistor T 7  is coupled to the second node b, i.e. the anode of the organic light emitting diode D 1  via the second N type thin film transistor T 2  which is on, and the second capacitor C 2  is discharged to the seventh P type thin film transistor T 7  until a voltage difference V ed  between the fifth node e and the fourth node d is:
 
 V   ed   =V   th   +V   th   _   OLED  
 
     then, an energy storage voltage of the second capacitor C 2  is V th +V th   _   OLED . 
     Combining  FIG. 3  and  FIG. 6 , in the program stage S 3 : 
     the first scan signal Scan 1  is a low voltage level, and the second scan signal Scan 2  is a low voltage level, and the third scan signal Scan 3  is a high voltage level, and the light emitting control signal EM is a low voltage level, and the data signal Data is a high voltage level. 
     The first N type thin film transistor T 1  and the fifth P type thin film transistor T 5  are on, and the other thin film transistors are off, and the data signal charges the first capacitor C 1  via the first N type thin film transistor T 1  until an energy storage voltage of the first capacitor C 1  and a voltage level of the first node a are a voltage value V Data  of a high voltage level provided by the data signal Data. 
     In the program stage S 3 , the second capacitor C 2  is in a floating state. 
     Combining  FIG. 3  and  FIG. 7 , in the light emitting stage S 4 : 
     the first scan signal Scan 1  is a high voltage level, and the second scan signal Scan 2  is a low voltage level, and the third scan signal Scan 3  is a low voltage level, and the light emitting control signal EM is a high voltage level, and the data signal Data is a low voltage level; the drain of the sixth N type thin film transistor T 6  receives the luminous brightness adjustment voltage Vr. 
     All of the third N type thin film transistor T 3 , the sixth N type thin film transistor T 6  and the seventh P type thin film transistor T 7  are on, and all of the first N type thin film transistor T 1 , the second N type thin film transistor T 2 , the fourth N type thin film transistor T 4  and the fifth P type thin film transistor T 5  are off, and the luminous brightness adjustment voltage Vr is sent to the fifth node e via the sixth N type thin film transistor T 6 , and the voltage level Va of the first node a, i.e. a gate voltage level of the seventh P type thin film transistor T 7  is changed to be:
 
 Va=Vr+ ( V   Data −( V   th   +V   th   _   OLED ))= Vr+V   Data   −V   th   −V   th   _   OLED  
 
     the power source high voltage level VDDH is sent to the third node c, i.e. the source of the seventh P type thin film transistor T 7  via the third N type thin film transistor T 3  which is on:
 
 Vc=VDDH  
 
     wherein Vc represents a voltage of the third node c, i.e. the source of the seventh P type thin film transistor T 7 . 
     The formula of the current flowing through the P type thin film transistor and the organic light emitting diode according to calculation is: 
     
       
         
           
             
               
                 
                   
                     I 
                     OLED 
                   
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           Vc 
                           - 
                           Va 
                           - 
                           
                             V 
                             th 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           VDDH 
                           - 
                           
                             ( 
                             
                               Vr 
                               + 
                               
                                 V 
                                 Data 
                               
                               - 
                               
                                 V 
                                 th 
                               
                               - 
                               
                                 V 
                                 th_OLED 
                               
                             
                             ) 
                           
                           - 
                           
                             V 
                             th 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           VDDH 
                           - 
                           Vr 
                           - 
                           
                             V 
                             Data 
                           
                           + 
                           
                             V 
                             th_OLED 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
           
         
       
     
     wherein I OLED  represents a current flowing through the driving thin film transistor, i.e. the seventh P type thin film transistor T 7  and the organic light emitting diode D 1 , and K is an intrinsic conductive factor of the driving thin film transistor, i.e. the seventh P type thin film transistor T 7 . 
     The organic light emitting diode D 1  emits light, and the current I OLED  flowing through the organic light emitting diode D 1  is irrelevant with the threshold voltage Vth of the seventh P type thin film transistor T 7 , which can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
     Significantly, the common ground voltage VSS is not higher than the luminous brightness adjustment voltage Vr, and the luminous brightness adjustment voltage Vr is lower than a sum of the power source high voltage VDDH and the threshold voltage V th   _   OLED  of the organic light emitting diode D 1  minus a voltage value V Data  of a high voltage level provided by the data signal Data.
 
 VSS≤Vr&lt;VDDH−V   Data   +V   th   _   OLED  
 
     Besides, the above expression formula for calculating I OLED , I OLED =K×(VDDH−Vr−V Data +V th   _   OLED ) 2  contains the item of threshold voltage V th   _   OLED  of the organic light emitting diode D 1 . Since the organic light emitting diode D 1  will age after a long time usage so that the threshold voltage V th   _   OLED  of the organic light emitting diode D 1  rises to result in the decrease of the luminous efficiency. However, it can be told from the expression formula of I OLED  that the rising V th   _   OLED  makes the current I OLED  flowing through the organic light emitting diode D 1  increase, and the increased current can compensate the decreased luminous efficiency for solving the issue of the decreased luminous efficiency. 
     On the basis of the same inventive idea, the present invention further provides an OLED pixel driving method, comprising steps of: 
     step 1, providing an OLED pixel driving circuit. 
     As shown in  FIG. 2 , the OLED pixel driving circuit utilizes the 7T2C structure and comprises a first N type thin film transistor T 1 , a second N type thin film transistor T 2 , a third N type thin film transistor T 3 , a fourth N type thin film transistor T 4 , a fifth P type thin film transistor T 5 , a sixth N type thin film transistor T 6 , a seventh P type thin film transistor T 7 , a first capacitor C 1 , a second capacitor C 2  and an organic light emitting diode D 1 . 
     A gate of the first N type thin film transistor T 1  receives a third scan signal Scan 3 , and a source receives a data signal Data, and a drain is electrically coupled to a first node a; a gate of the second N type thin film transistor T 2  receives a second scan signal Scan 2 , and a source is electrically coupled to the first node a, and a drain is electrically coupled to a second node b; a gate of the third N type thin film transistor T 3  receives a light emitting control signal EM, and a source receives a power source high voltage VDDH, and a drain is electrically coupled to a third node c; a gate of the fourth N type thin film transistor T 4  receives the second scan signal Scan 2 , and a source is electrically coupled to the third node c, and a drain is electrically coupled to a fifth node e; a gate of the fifth P type thin film transistor T 5  receives a light emitting control signal EM, and a source is electrically coupled to a fourth node d, and a drain receives a common ground voltage VSS; a gate of the sixth N type thin film transistor T 6  receives a first scan signal Scan 1 , and a source is electrically coupled to the fifth node e, and a drain time-share receives a power source low voltage VDDL or a luminous brightness adjustment voltage Vr; the seventh P type thin film transistor T 7  is a driving thin film transistor which directly drives the organic light emitting diode D 1 , of which a gate is electrically coupled to the first node a, and a source is electrically coupled to the third node c, and a drain is electrically coupled to the second node b; an anode of the organic light emitting diode D 1  is electrically coupled to the second node b, and a cathode receives the common ground voltage VSS; one end of the first capacitor C 1  is electrically coupled to the first node a, and the other end is electrically coupled to the fourth node d; one end of the second capacitor C 2  is electrically coupled to the fourth node d, and the other end is electrically coupled to the fifth node e. 
     Specifically, all of the first N type thin film transistor T 1 , the second N type thin film transistor T 2 , the third N type thin film transistor T 3 , the fourth N type thin film transistor T 4 , the fifth P type thin film transistor T 5 , the sixth N type thin film transistor T 6  and the seventh P type thin film transistor T 7  are low temperature poly-silicon thin film transistors, oxide semiconductor thin film transistors or amorphous silicon thin film transistors; all of the first scan signal Scan 1 , the second scan signal Scan 2 , the third scan signal Scan 3 , the light emitting control signal EM and the data signal Data are generated by an external sequence controller. 
     step 2, referring to  FIG. 3  and  FIG. 4 , together, entering the reset stage S 1 . 
     The first scan signal Scan 1  provides a high voltage level, and the second scan signal Scan 2  provides a low voltage level, and the third scan signal Scan 3  provides a low voltage level, and the light emitting control signal EM provides a low voltage level, and the data signal Data provides a low voltage level; the drain of the sixth N type thin film transistor T 6  receives the power source low voltage VDDL. 
     The fifth P type thin film transistor T 5  and the sixth N type thin film transistor T 6  is on, and the other thin film transistors is off, and the power source low voltage VDDL charges the second capacitor C 2  via the sixth N type thin film transistor T 6  which is on, and a voltage difference of two ends of the second capacitor C 2  after charging is accomplished, i.e. a voltage difference V ed  between the fifth node e and the fourth node d is:
 
 V   ed   =VDDL−VSS  
 
     to accomplish the reset and initializing assignment to the second capacitor C 2 . 
     Significantly, the power source low voltage VDDL is higher than a sum of a threshold voltage of the seventh P type thin film transistor T 7  and a threshold voltage of the organic light emitting diode D 1 , i.e.:
 
 VDDL&gt;V   th   +V   th   _   OLED  
 
     wherein V th  represents a threshold voltage of the seventh P type thin film transistor T 7 , and V th   _   OLED  is a threshold voltage of the organic light emitting diode D 1 . 
     step 3, Combining  FIG. 3  and  FIG. 5 , entering the threshold voltage detection stage S 2 . 
     The first scan signal Scan 1  is changed to be a low voltage level, and the second scan signal Scan 2  is changed to be a high voltage level, and the third scan signal Scan 3  is kept to be a low voltage level, and the light emitting control signal EM is kept to be a low voltage level, and the data signal Data is kept to be a low voltage level. 
     All of the second N type thin film transistor T 2 , the fourth N type thin film transistor T 4 , the fifth P type thin film transistor T 5  and the seventh P type thin film transistor T 7  are on, and all of the first N type thin film transistor T 1 , the third N type thin film transistor T 3  and the sixth N type thin film transistor T 6  are off, and the fifth node e is coupled with the third node c, i.e. the source of the seventh P type thin film transistor T 7  via the fourth N type thin film transistor T 4  which is on, and the first node a, i.e. the gate of the seventh P type thin film transistor T 7  is coupled to the second node b, i.e. the anode of the organic light emitting diode D 1  via the second N type thin film transistor T 2  which is on, and the second capacitor C 2  is discharged to the seventh P type thin film transistor T 7  until a voltage difference V ed  between the fifth node e and the fourth node d is:
 
 V   ed   =V   th   +V   th   _   OLED  
 
     then, an energy storage voltage of the second capacitor C 2  is V th +V th   _   OLED . 
     step 4, combining  FIG. 3  and  FIG. 6 , entering the program stage S 3 . 
     The first scan signal Scan 1  is kept to be a low voltage level, and the second scan signal Scan 2  is changed to be a low voltage level, and the third scan signal Scan 3  is changed to be a high voltage level, and the light emitting diode control signal EM is kept to be a low voltage level, and the data signal Data is changed to be a high voltage level. 
     The first N type thin film transistor T 1  and the fifth P type thin film transistor T 5  are on, and the other thin film transistors are off, and the data signal charges the first capacitor C 1  via the first N type thin film transistor T 1  until an energy storage voltage of the first capacitor C 1  and a voltage level of the first node a are a voltage value V Data  of a high voltage level provided by the data signal Data. 
     In the program stage S 3 , the second capacitor C 2  is in a floating state. 
     step 5, combining  FIG. 3  and  FIG. 7 , entering the light emitting stage S 4 . 
     the first scan signal Scan 1  is changed to be a high voltage level, and the second scan signal Scan 2  is kept to be a low voltage level, and the third scan signal Scan 3  is changed to be a low voltage level, and the light emitting control signal EM is changed to be a high voltage level, and the data signal Data is changed to be a low voltage level; the drain of the sixth N type thin film transistor T 6  receives the luminous brightness adjustment voltage Vr. 
     All of the third N type thin film transistor T 3 , the sixth N type thin film transistor T 6  and the seventh P type thin film transistor T 7  are on, and all of the first N type thin film transistor T 1 , the second N type thin film transistor T 2 , the fourth N type thin film transistor T 4  and the fifth P type thin film transistor T 5  are off, and the luminous brightness adjustment voltage Vr is sent to the fifth node e via the sixth N type thin film transistor T 6 , and the voltage level Va of the first node a, i.e. a gate voltage level of the seventh P type thin film transistor T 7  is changed to be:
 
 Va=Vr+ ( V   Data −( V   th   +V   th   _   OLED ))= Vr+V   Data   −V   th   −V   th   _   OLED  
 
     the power source high voltage level VDDH is sent to the third node c, i.e. the source of the seventh P type thin film transistor T 7  via the third N type thin film transistor T 3  which is on:
 
 Vc=VDDH  
 
     wherein Vc represents a voltage of the third node c, i.e. the source of the seventh P type thin film transistor T 7 . 
     The formula of the current flowing through the P type thin film transistor and the organic light emitting diode according to calculation is: 
     
       
         
           
             
               
                 
                   
                     I 
                     OLED 
                   
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           Vc 
                           - 
                           Va 
                           - 
                           
                             V 
                             th 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           VDDH 
                           - 
                           
                             ( 
                             
                               Vr 
                               + 
                               
                                 V 
                                 Data 
                               
                               - 
                               
                                 V 
                                 th 
                               
                               - 
                               
                                 V 
                                 th_OLED 
                               
                             
                             ) 
                           
                           - 
                           
                             V 
                             th 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     K 
                     × 
                     
                       
                         ( 
                         
                           VDDH 
                           - 
                           Vr 
                           - 
                           
                             V 
                             Data 
                           
                           + 
                           
                             V 
                             th_OLED 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
           
         
       
     
     wherein I OLED  represents a current flowing through the driving thin film transistor, i.e. the seventh P type thin film transistor T 7  and the organic light emitting diode D 1 , and K is an intrinsic conductive factor of the driving thin film transistor, i.e. the seventh P type thin film transistor T 7 . 
     The organic light emitting diode D 1  emits light, and the current I OLED  flowing through the organic light emitting diode D 1  is irrelevant with the threshold voltage V th  of the seventh P type thin film transistor T 7 , which can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
     Significantly, the common ground voltage VSS is not higher than the luminous brightness adjustment voltage Vr, and the luminous brightness adjustment voltage Vr is lower than a sum of the power source high voltage VDDH and the threshold voltage V th   _   OLED  of the organic light emitting diode D 1  minus a voltage value V Data  of a high voltage level provided by the data signal Data.
 
 VSS≤Vr&lt;VDDH−V   Data   +V   th   _   OLED  
 
     Besides, the above expression formula for calculating I OLED , I OLED =K×(VDDH−Vr−V Data +V th   _   OLED ) 2  contains the item of threshold voltage V th   _   OLED  of the organic light emitting diode D 1 . Since the organic light emitting diode D 1  will age after a long time usage so that the threshold voltage V th   _   OLED  of the organic light emitting diode D 1  rises to result in the decrease of the luminous efficiency. However, it can be told from the expression formula of I OLED  that the rising V th   _   OLED  makes the current I OLED  flowing through the organic light emitting diode D 1  increase, and the increased current can compensate the decreased luminous efficiency for solving the issue of the decreased luminous efficiency. 
     In conclusion, the OLED pixel driving circuit and the pixel driving method of the present invention utilizes the pixel driving circuit of 7T2C structure, and the first scan signal, the second scan signal, the third scan signal, the light emitting control signal and the data signal are combined with one another to correspond to a reset stage, a threshold voltage detection stage, a program stage and a light emitting stage one after another and can eliminate the influence of the threshold voltage of the driving thin film transistor to the current flowing through the organic light emitting diode to promote the OLED panel display uniformity. 
     Above are only specific embodiments of the present invention, the scope of the present invention is not limited to this, and to any persons who are skilled in the art, change or replacement which is easily derived should be covered by the protected scope of the invention. Thus, the protected scope of the invention should go by the subject claims.