Patent Publication Number: US-11037510-B2

Title: Pixel driving system for AMOLED display device and driving method

Description:
FIELD OF THE INVENTION 
     The present invention relates to a display technology field, and more particularly to a pixel driving system for AMOLED display device and driving method. 
     BACKGROUND OF THE INVENTION 
     The Organic Light Emitting Display (OLED) display device has advantages of self-luminous, low driving voltage, high luminous efficiency, short response time, high definition and contrast ratio, near 180° viewing angle, wide temperature range, and flexible display, and large-area full-color display such that the OLED has been recognized by the industry as the most promising display device. 
     The OLED display device can be divided into two types: a passive matrix OLED (PMOLED) and an active matrix OLED (AMOLED), that is, directly addressing and thin-film transistor (TFT) addressing. Wherein, the AMOLED has pixels arranged in an array, belongs to an active display type, has high luminous efficiency, and is generally used as a high-definition large-sized display device. 
     The AMOLED is a current driving device. When a current flows through the organic light-emitting diode, the organic light-emitting diode emits light, and the brightness of the light is determined by the current flowing through the organic light emitting diode itself. Most existing integrated circuit (IC) only transmits voltage signals, so the pixel driving circuit of AMOLED needs to complete the task of converting a voltage signal into a current signal. The conventional AMOLED pixel driving circuit is usually 2T1C, that is, a structure in which two thin-film transistors are added with a capacitor to convert a voltage into a current. 
     As shown in  FIG. 1 , a conventional 2T1C pixel driving circuit for an AMOLED includes a first thin-film transistor T 10 , a second thin-film transistor T 20 , a capacitor C 10 , and an organic light-emitting diode D 10 . The first thin-film transistor T 10  is a switching thin-film transistor. The second thin-film transistor T 20  is a driving thin-film transistor, and the capacitor C 10  is a storage capacitor. Specifically, the gate of the first thin-film transistor T 10  is connected to the scanning signal Gate, the source is connected to the data signal Data, and the drain is electrically connected to the gate of the second thin-film transistor T 20  and one end of the capacitor C 10 . 
     The source of the second thin-film transistor T 20  is connected to the power supply positive voltage OVDD, and the drain is electrically connected to the anode of the organic light-emitting diode D 10 . The cathode of the organic light-omitting diode D 10  is connected to a power supply negative voltage OVSS. One end of the capacitor C 10  is electrically connected to the drain of the first thin-film transistor T 10 , and the other end is electrically connected to the source of the second thin-film transistor T 20 . When displayed, the scanning signal Gate controls the first thin-film transistor T 10  to be turned on, the data signal Data passes through the first thin-film transistor T 10  to enter the gate of the second thin-film transistor T 20  and the capacitor C 10 , and then the first thin-film transistor T 10  is turned off due to the capacitance C 10 . For storage, the gate voltage of the second thin-film transistor T 20  can continue to maintain the data signal voltage, so that the second thin-film transistor T 20  is in an on state, and the driving current enters the organic light-emitting diode D 10  through the second thin-film transistor T 20  to drive the organic light-emitting diode D 10  to emit a light. 
     When the pixel driving circuit shown in  FIG. 1  is in operation, the organic light-emitting diode D 10  is in a DC bias state for a long time, and the internal ion polarization thereof forms a built-in electric field, so that the threshold voltage of the organic light-omitting diode D 10  is continuously increased, so that the threshold voltage of the organic light-emitting diode D 10  is continuously increased. The emission brightness is gradually reduced, and the long-time illuminating also shortens the life of the OLED D 10 , and the aging of the OLED in different sub-pixels may cause uneven display of the screen, affecting the display effect. 
     In the prior art, the brightness of the picture is often adjusted by detecting and adjusting an average picture level (APL) of the OLED display to improve the display effect. The common method for adjusting the APL is to adjust the brightness of the grayscale level of the input data signal, adjust the power supply positive voltage or the power supply negative voltage. However, the algorithm for adjusting the APL and adjusting the brightness of the picture by adjusting the data signal is complicated, and the hardware resources occupied by adjusting the power supply positive voltage or the power supply negative voltage to adjust the APL and adjust the brightness of the picture are more. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to provide a pixel driving system for an AMOLED display device capable of adjusting a driving current flowing through an organic light-emitting diode to adjust the brightness of the entire display picture. 
     Another object of the present invention is to provide a driving method for an AMOLED display device capable of adjusting a driving current flowing through an organic light-emitting diode to adjust the brightness of the entire display picture. 
     In order to achieve the above purpose, the present invention provides a pixel driving system for AMOLED display device, comprising a sub-pixel driving circuit and a node voltage generating module electrically connected to the sub-pixel driving circuit; wherein the sub-pixel driving circuit includes a first TFT, a second TFT, a third TFT, a capacitor, and an organic light-emitting diode; a gate of the first TFT is connected to a scanning signal, a source of the first TFT is connected to a data signal voltage, and a drain of the first TFT is electrically connected to a first node; a gate of the second TFT is electrically connected to the first node, a drain of the second TFT is connected to a power supply positive voltage, and a source of the second TFT is electrically connected to a second node; a gate of the third TFT is connected to the scanning signal, a source of the third TFT is electrically connected to the node voltage generating module, and a drain of the third TFT is electrically connected to a second node; two ends of the capacitor are respectively connected to the first node and the second node; an anode of the organic light-emitting diode is electrically connected to the second node, and a cathode of the organic light-emitting diode is connected to a power supply negative voltage; wherein the first TFT is one of an N-type TFT and a P-type TFT, and the third TFT is the same one of the N-type TFT and the P-type TFT as the first TFT; and wherein the node voltage generating module is inputted with the a red-green-blue display data for processing the red-green-blue display data, obtaining an APL value of a current frame of the AMOLED display device, and according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, the node voltage generating module generates a corresponding node voltage and outputting to the second node. 
     Wherein the node voltage calculation formula is: 
     
       
         
           
             Vcm 
             = 
             
               { 
               
                 
                   
                     
                       
                         V 
                         ⁢ 
                         min 
                       
                       , 
                     
                   
                   
                     
                       APL 
                       &lt; 
                       APL_L 
                     
                   
                 
                 
                   
                     
                       
                         
                           
                             V 
                             ⁢ 
                             max 
                           
                           × 
                           
                             ( 
                             
                               APL 
                               - 
                               APL_L 
                             
                             ) 
                           
                         
                         
                           ( 
                           
                             APL_H 
                             - 
                             APL_L 
                           
                           ) 
                         
                       
                       , 
                     
                   
                   
                     
                       APL_L 
                       ≤ 
                       APL 
                       ≤ 
                       APL_H 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         ⁢ 
                         max 
                       
                       , 
                     
                   
                   
                     
                       APL 
                       &gt; 
                       APL_H 
                     
                   
                 
               
             
           
         
       
     
     wherein, Vcm is a node voltage, Vmin is a preset first voltage, Vmax is a preset second voltage, APL is the APL value of the current frame of the AMOLED display device, and APL_L is a preset first APL threshold, APL_H is a preset second APL threshold, the first voltage is less than the second voltage, and the first APL threshold is less than the second APL threshold. 
     Wherein the first voltage is 0V. 
     Wherein a difference value between the second voltage and the power supply negative voltage is less than a threshold voltage of the organic light-emitting diode. 
     Wherein the node voltage generating module includes a brightness feature value acquiring unit, a de-gamma unit, an APL acquiring unit, and a node voltage acquiring unit that are electrically connected in sequence; the brightness feature value obtaining unit is inputted with the red-green-blue display data for calculating an original brightness feature value of each pixel of the current frame of the AMOLED display device according to the red-green-blue display data and the preset brightness feature value calculation formula, and transmitting to the de-gamma unit; the de-gamma unit is configured to calculate a brightness feature value of each pixel of the current frame of the AMOLED display device after de-gamma according to the original brightness feature value of each pixel of the current frame of the AMOLED display device and a preset de-gamma formula, and transmitting to the APL acquiring unit; the APL acquiring unit is configured to calculate the APL value of the current frame of the AMOLED display according to the brightness feature value of each pixel of the current frame of the AMOLED display device after de-gamma and a preset average image voltage level calculation formula, and transmitting to the node voltage acquiring unit; and the node voltage acquiring unit is configured to calculate the node voltage according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, and transmitting to the source of the third TFT. 
     Wherein the brightness feature value calculation formula is: Y=0.299R+0.589G+0.114B; wherein, Y is the original brightness feature value of the pixel, R is the grayscale value of a red subpixel of the pixel in the red-green-blue display data, and G is the grayscale value of a green subpixel of the pixel in the red-green-blue display data, and B is the grayscale value of a blue sub-pixel of the pixel in the red-green-blue display data. 
     Wherein the de-gamma formula is: Y′=(Y/255) 2.2 ×255; wherein Y′ is the brightness feature value of the pixel after de-gamma. 
     Wherein the average image voltage level calculation formula is: APL=AverageY′/255×100; wherein, AverageY′ is an average value of the brightness feature values of the current frame of the AMOLED display device after de-gamma. 
     Wherein the first TFT and the third TFT are both N-type TFTs. 
     The present invention also provides a driving method for an AMOLED display device, which is applied to the pixel driving system of the AMOLED display device as claimed in claim  1 , comprising steps of: step S 1 , entering a data signal voltage storage stage, wherein a scanning signal controls a first TFT and a third TFT to be turned on, and a data signal voltage is written into a first node; a node voltage generating module obtains an APL value of a current frame of an AMOLED display device by processing a red-green-blue display data; the node voltage generating module generates a corresponding node voltage according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, and outputting to a second node; step S 2 , entering the light-emitting display stage, wherein the scanning signal controls the first TFT and the third TFT to be turned off, due to the coupling effect of a capacitor, a voltage difference between the first node and the second node remains unchanged, a power supply positive voltage charges the second node, and the organic light-emitting diode emits a light. 
     The beneficial effects of the invention: the AMOLED display device of the present invention provides a pixel driving system for AMOLED display device, comprising a sub-pixel driving circuit and a node voltage generating module electrically connected to the sub-pixel driving circuit, the node voltage generating module is inputted with the a red-green-blue display data RGB, and electrically connected to the source of the driving thin-film transistor in the sub-pixel driving circuit, capable of processing the red-green-blue display data, obtaining the APL value of the current frame of the AMOLED display, and according to the APL value of the current frame of the AMOLED display and the preset node voltage calculation formula to generate a corresponding node voltage and outputs the voltage to the source of the driving thin-film transistor, and adjusts the gate-to-source voltage of the driving thin-film transistor by using the APL value, thereby adjusting the driving current flowing through the light-emitting diode in order to adjust the overall display brightness of the AMOLED display. The driving method of the AMOLED display of the present invention can adjust the driving current flowing through the organic light-emitting diode to adjust the brightness of the entire display screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the features and technical aspects of the present invention, reference should be made to the accompanying drawings. The drawings are provided for purposes of illustration and description only and are not intended for limiting. 
       In the drawings, 
         FIG. 1  is a conventional 2T1C pixel driving circuit for AMOLED. 
         FIG. 2  is a schematic structural diagram of a pixel driving system of an AMOLED display device of the present invention. 
         FIG. 3  is a schematic structural diagram of a node voltage generating module of a pixel driving system of an AMOLED display device of the present invention; 
         FIG. 4  is a timing chart showing the operation of the pixel driving system of the AMOLED display device of the present invention, 
         FIG. 5  is a flow chart of a driving method for an AMOLED display of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to further clarify the technical means and effects of the present invention, the following detailed description will be made in conjunction with the preferred embodiments of the invention and the accompanying drawings. 
     Referring to  FIG. 2  to  FIG. 4 , the present invention provides a pixel driving system for AMOLED display device, comprising a sub-pixel driving circuit  10  and a node voltage generating module  20  electrically connected to the sub-pixel driving circuit  10 . 
     The sub-pixel driving circuit  10  includes a first TFT T 1 , a second TFT T 2 , a third TFT T 3 , a capacitor C 1 , and an organic light-emitting diode D 1 . A gate of the first TFT T 1  is connected to a scanning signal Scan, a source of the first TFT T 1  is connected to a data signal voltage Vdata, and a drain of the first TFT T 1  is electrically connected to a first node A. A gate of the second TFT T 2  is electrically connected to the first node A, a drain of the second TFT T 2  is connected to a power supply positive voltage OVDD, and a source of the second TFT T 2  is electrically connected to a second node B. A gate of the third TFT T 3  is connected to the scanning signal Scan, a source of the third TFT T 3  is electrically connected to the node voltage generating module  20 , and a drain of the third TFT T 3  is electrically connected to a second node B. Two ends of the capacitor C 1  are respectively connected to the first node A and the second node B. An anode of the organic light-emitting diode D 1  is electrically connected to the second node B, and a cathode of the organic light-emitting diode D 1  is connected to a power supply negative voltage OVSS. The first TFT T 1  is one of an N-type TFT and a P-type TFT, and the third TFT T 3  is the same one of the N-type TFT and the P-type TFT as the first TFT T 1 . 
     The node voltage generating module  20  is inputted with the a red-green-blue display data RGB for processing the red-green-blue display data RGB, obtaining an APL value of a current frame of the AMOLED display device, and according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, the node voltage generating module  20  generates a corresponding node voltage Vcm and outputting to the second node B. 
     Specifically, the node voltage calculation formula is: 
     
       
         
           
             Vcm 
             = 
             
               { 
               
                 
                   
                     
                       
                         V 
                         ⁢ 
                         min 
                       
                       , 
                     
                   
                   
                     
                       APL 
                       &lt; 
                       APL_L 
                     
                   
                 
                 
                   
                     
                       
                         
                           
                             V 
                             ⁢ 
                             max 
                           
                           × 
                           
                             ( 
                             
                               APL 
                               - 
                               APL_L 
                             
                             ) 
                           
                         
                         
                           ( 
                           
                             APL_H 
                             - 
                             APL_L 
                           
                           ) 
                         
                       
                       , 
                     
                   
                   
                     
                       APL_L 
                       ≤ 
                       APL 
                       ≤ 
                       APL_H 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         ⁢ 
                         max 
                       
                       , 
                     
                   
                   
                     
                       APL 
                       &gt; 
                       APL_H 
                     
                   
                 
               
             
           
         
       
     
     Wherein, Vcm is a node voltage, Vmin is a preset first voltage, Vmax is a preset second voltage, APL is the APL value of the current frame of the AMOLED display device, and APL_L is a preset first APL threshold, APL_H is a preset second APL threshold, the first voltage is less than the second voltage, and the first APL threshold is less than the second APL threshold. 
     Specifically, referring to  FIG. 3 , the node voltage generating module  20  includes a brightness feature value acquiring unit  21 , a de-gamma unit  22 , an APL acquiring unit  23 , and a node voltage acquiring unit  24  that are electrically connected in sequence. The brightness feature value obtaining unit  21  is inputted with the red-green-blue display data RGB for calculating an original brightness feature value of each pixel of the current frame of the AMOLED display device according to the red-green-blue display data RGB and the preset brightness feature value calculation formula, and transmitting to the de-gamma unit  22 . The de-gamma unit  22  is configured to calculate a brightness feature value of each pixel of the current frame of the AMOLED display device after de-gamma according to the original brightness feature value of each pixel of the current frame of the AMOLED display device and a preset de-gamma formula, and transmitting to the APL acquiring unit  23 . 
     The APL acquiring unit  23  is configured to calculate the APL value of the current frame of the AMOLED display according to the brightness feature value of each pixel of the current frame of the AMOLED display device after de-gamma and a preset average image voltage level calculation formula, and transmitting to the node voltage acquiring unit  24 . The node voltage acquiring unit  24  is configured to calculate the node voltage Vcm according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, and transmitting to the source of the third TFT T 3 . 
     Further, the brightness feature value calculation formula Y=0.299R+0.589G+0.114B; 
     Wherein, Y is the original brightness feature value of the pixel, R is the grayscale value of a red subpixel of the pixel in the red-green-blue display data RGB, and G is the grayscale value of a green subpixel of the pixel in the red-green-blue display data RGB, and B is the grayscale value of a blue sub-pixel of the pixel in the red-green-blue display data RGB. 
     The de-gamma formula is: Y′=(Y/255) 2.2 ×255; 
     Wherein Y′ is the brightness feature value of the pixel after de-gamma. 
     The average image voltage level calculation formula is: APL=AverageY′/255×100; 
     Wherein, AverageY′ is an average value of the brightness feature values of the current frame of the AMOLED display device after de-gamma. 
     Specifically, in the embodiment shown in AG.  2 , the first TFT T 1  and the third TFT T 3  are both N-type TFTs. 
     Preferably, the first voltage is 0V. 
     Preferably, a difference value between the second voltage and the power supply negative voltage OVSS is less than a threshold voltage of the organic light-emitting diode D 1 . 
     Referring to the embodiment shown in  FIG. 2  to  FIG. 4 , the operation process of the pixel driving system of the AMOLED display device of the present invention is as follows: 
     First, entering a data signal voltage storage stage  1 . The scanning signal Scan is at a high voltage level, and controlling the first TFT T 1  and the third TFT T 3  to be turned on, and the data signal voltage Vdata is written into the first node A. At the same time, the node voltage generating module  20  obtains the APL value of the current frame of the AMOLED display by using the internal brightness feature value acquiring unit  21 , the de-gamma unit  22 , and the APL acquiring unit  23  to process the red-green-blue display data RGB. Besides, the node voltage generating module  20  generates a corresponding node voltage Vcm by using the internal node voltage obtaining unit  24  and according to the APL value of the current frame of the AMOLED display device and the preset node voltage calculation formula, and outputting to the second node B. At this time, the first node A, that is, the voltage value Va at one end of the capacitor C 1  is Vdata, the second node B, that is, the voltage value Vb at the other end of the capacitor C 1  is Vcm, and the gate-to-source voltage of the second TFT T 2  is Vdata-Vcm. 
     After that, entering the light-emitting display stage  2 . The scanning signal Scan is at a low voltage level, and the first TFT T 1  and the third TFT T 3  are controlled to be turned off. Due to the coupling effect of the capacitor C 1 , the voltage difference between the first node A and the second node B remains unchanged, and is still Vdata-Vcm. The power supply positive voltage OVDD charges the second node B, so that the voltage Vb of the second node B and the voltage Va of the first node A continuously increase. When the voltage Vb of the second node B rises to a voltage value such that a difference between the voltage Vb and the power source negative voltage OVSS is greater than the threshold voltage of the organic light-emitting diode D 1 , the organic light-emitting diode D 1  emits light. According to a formula for the current flowing through an organic light-emitting diode in the prior art:
 
 I=K ( Vgs−Vth ) 2 ;
 
     Wherein, I is the current flowing through the organic light-emitting diode D 1 , K is the structural parameter of the driving thin-film transistor, that is, the second TFT T 2 , and Vgs is the gate-to-source voltage of the driving thin-film transistor, that is, the second TFT T 2 , and Vth is the threshold voltage of the second TFT T 2 . Due to the storage function of the capacitor C 1 , the gate-to-source voltage of the second TFT T 2  is always Vdata-Vcm in the light-emitting display stage  2 , and thus the current I flowing through the organic light-omitting diode D 1  at this time is I=K(Vgs−Vth) 2 =K(Vdata−Vcm−Vth) 2 . 
     It should be noted that, in the present invention, the node voltage generating module  20  is used to obtain the APL value of the current frame of the AMOLED display device, and the APL value of the current frame of the AMOLED display is substituted into the calculation formula according to the preset node voltage. And substituting the APL value of the current frame into the preset node voltage calculation formula, calculating the corresponding node voltage Vcm and inputting the second node B. Through the change of the APL value, the change of the node voltage Vcm can be controlled correspondingly so that the value of the gate-to-source voltage of the driving thin-film transistor, that is, the second TFT T 2  can be controlled. The current value flowing through the second TFT T 2  and the organic light-emitting diode D 1  is further controlled to achieve the purpose of controlling the display brightness of the AMOLED display. 
     Specifically, when the APL value is greater than the preset second APL threshold, it indicates that the APL value of the current frame of the AMOLED display is too large, and the brightness of the display panel of the AMOLED display requires to be adjusted in a maximum degree. At this time, the node voltage generating module  20  generates a second voltage having a larger voltage value as the node voltage Vcm and outputs it to the second node B of the sub-pixel driving circuit  10  such that the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is small. Therefore, in the light-emitting display stage  2 , the current value flowing through the organic light-emitting diode becomes smaller, so that the display brightness of the AMOLED display becomes lower. When the APL value is smaller than the preset first APL threshold, 
     It indicates that the APL value of the current frame of the AMOLED display device is small, and there is no need to adjust the picture brightness of the AMOLED display device. Therefore, the node voltage generating module  20  generates the first voltage with a small voltage value or even 0 as the node voltage Vcm, and outputting to the second node of the sub-pixel driving  10  such that the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is close to the data signal voltage Vdata, so that in the light-emitting display stage  2 , the current value flowing through the organic light-emitting diode can maximally close to a driving current value corresponding to the data signal voltage Vdata; When the APL value is greater than or equal to the preset first APL threshold and less than or equal to the preset second APL threshold, it indicates that the picture brightness of the AMOLED display device should be adjusted in a certain degree. At this time, the node voltage generating module  20  generates a voltage value that is between the first voltage and the second voltage and positively correlated with the APL value as a node voltage Vcm, and outputting the node voltage Vcm to the second node B of the sub-pixel driving circuit  10  such that when the APL value is larger, the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is smaller, when the current value flowing through the organic light-emitting diode is smaller in the light-emitting display stage  2 , the brightness of the organic light-emitting diode D 1  is lower. 
     In the above manner, the overall display brightness of the AMOLED display device can be effectively adjusted according to the APL value of the current frame of the AMOLED display device, and the display quality of the AMOLED display device can be improved, and at the same time, comparing to the prior art, no complicated algorithm is required and no need to occupy hardware resources to adjust the picture brightness, which can effectively improve the quality of the product. 
     Referring to  FIG. 5 , and in conjunction with  FIG. 2  to AG.  4 , based on the same inventive concept, the present invention further provides a driving method for an AMOLED display device, which is applied to the pixel driving system of the AMOLED display device described above, and includes the following steps: 
     Step S 1 , entering a data signal voltage storage stage  1 . A scanning signal Scan controls a first TFT T 1  and a third TFT T 3  to be turned on, and a data signal voltage Vdata is written into a first node A. A node voltage generating module  20  obtains an APL value of a current frame of an AMOLED display device by processing a red-green-blue display data RGB. Besides, the node voltage generating module  20  generates a corresponding node voltage Vcm according to the APL value of the current frame of the AMOLED display device and a preset node voltage calculation formula, and outputting to a second node B. 
     Specifically, in the step S 1 , the node voltage generating module  20  obtains the APL value of the current frame of the AMOLED display device by using the internal brightness feature value acquiring unit  21 , the de-gamma unit  22 , and the APL acquiring unit  23  to process the red-green-blue display data RGB. Besides, the node voltage generating module  20  generates a corresponding node voltage Vcm by using the internal node voltage obtaining unit  24  and according to the APL value of the current frame of the AMOLED display device and the preset node voltage calculation formula, and outputting to the second node B. At this time, the first node A, that is, the voltage value Va at one end of the capacitor C 1  is Vdata, the second node B, that is, the voltage value Vb at the other end of the capacitor C 1  is Vcm, and the gate-to-source voltage of the second TFT T 2  is Vdata-Vcm. 
     Specifically, in the embodiment shown in  FIG. 2  to AG.  4 , and in the step S 1 , the scanning signal Scan is at a high voltage level, controlling the first TFT T 1  and the third TFT T 3  to be turned on. 
     Step S 2 , entering the light-emitting display stage  2 . The scanning signal Scan controls the first TFT T 1  and the third TFT T 3  to be turned off. Due to the coupling effect of a capacitor G 1 , a voltage difference between the first node A and the second node B remains unchanged. A power supply positive voltage OVDD charges the second node B, and the organic light-emitting diode D 1  emits a light. 
     Specifically, in the step S 2 , due to the coupling effect of the capacitor C 1 , the voltage difference between the first node A and the second node B remains unchanged, and is still Vdata-Vcm. The power supply positive voltage OVDD charges the second node B, so that the voltage Vb of the second node B and the voltage Va of the first node A continuously increase. When the voltage Vb of the second node B rises to a voltage value such that a difference between the voltage Vb and the power supply negative voltage OVSS is greater than the threshold voltage of the organic light emitting diode D 1 , the organic light-emitting diode D 1  emits light. According to a formula for the current flowing through an organic light-emitting diode in the prior art:
 
 I=K ( Vgs−Vth ) 2 ;
 
     Wherein, I is the current flowing through the organic light-emitting diode D 1 , K is the structural parameter of the driving thin-film transistor, that is, the second TFT T 2 , and Vgs is the gate-to-source voltage of the driving thin-film transistor, that is, the second TFT T 2 , and Vth is the threshold voltage of the second TFT T 2 . Due to the storage function of the capacitor C 1 , the gate-to-source voltage of the second TFT T 2  is always Vdata−Vcm in the light-emitting display stage  2 , and thus the current I flowing through the organic light-emitting diode D 1  at this time is I=K(Vgs−Vth) 2 =K(Vdata−Vcm−Vth) 2 . 
     Specifically, in the embodiment shown in  FIG. 2  to  FIG. 4 , in the step S 2 , the scanning signal Scan is at a low voltage level, and the first TFT T 1  and the third TFT T 3  are controlled to be turned off. 
     It should be noted that, in the present invention, the node voltage generating module  20  is used to obtain the APL value of the current frame of the AMOLED display, and the APL value of the current frame of the AMOLED display is substituted into the calculation formula according to the preset node voltage. And substituting the APL value of the current frame into the preset node voltage calculation formula, calculating the corresponding node voltage Vcm and inputting the second node B. Through the change of the APL value, the change of the node voltage Vcm can be controlled correspondingly so that the value of the gate-to-source voltage of the driving thin-film transistor, that is, the second TFT T 2  can be controlled. The current value flowing through the second TFT T 2  and the organic light-emitting diode D 1  is further controlled to achieve the purpose of controlling the display brightness of the AMOLED display device. 
     Specifically, when the APL value is greater than the preset second APL threshold, it indicates that the APL value of the current frame of the AMOLED display is too large, and the brightness of the display panel of the AMOLED display requires to be adjusted in a maximum degree. At this time, the node voltage generating module  20  generates a second voltage having a larger voltage value as the node voltage Vcm and outputs it to the second node B of the sub-pixel driving circuit  10  such that the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is small. Therefore, in the light-emitting display stage  2 , the current value flowing through the organic light-emitting diode becomes smaller, so that the display brightness of the AMOLED display device becomes lower. When the APL value is smaller than the preset first APL threshold. It indicates that the APL value of the current frame of the AMOLED display is small, and there is no need to adjust the picture brightness of the AMOLED display. Therefore, the node voltage generating module  20  generates the first voltage with a small voltage value or even 0 as the node voltage Vcm, and outputting to the second node of the sub-pixel driving  10  such that the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is close to the data signal voltage Vdata, so that in the light-emitting display stage  2 , the current value flowing through the organic light-emitting diode can maximally close to a driving current value corresponding to the data signal voltage Vdata; 
     When the APL value is greater than or equal to the preset first APL threshold and less than or equal to the preset second APL threshold, it indicates that the picture brightness of the AMOLED display should be adjusted in a certain degree. At this time, the node voltage generating module  20  generates a voltage value that is between the first voltage and the second voltage and positively correlated with the APL value as a node voltage Vcm, and outputting the node voltage Vcm to the second node B of the sub-pixel driving circuit  10  such that when the APL value is larger, the gate-to-source voltage of the second TFT T 2  stored in the capacitor C 1  is smaller, when the current value flowing through the organic light-emitting diode is smaller in the light-emitting display stage  2 , the brightness of the organic light-emitting diode D 1  is lower. 
     In the above manner, the overall display brightness of the AMOLED display can be effectively adjusted according to the APL value of the current frame of the AMOLED display, and the display quality of the AMOLED display can be improved, and at the same time, comparing to the prior art, no complicated algorithm is required and no need to occupy hardware resources to adjust the picture brightness, which can effectively improve the quality of the product. 
     In summary, the AMOLED display device of the present invention provides a pixel driving system for AMOLED display device, comprising a sub-pixel driving circuit and a node voltage generating module electrically connected to the sub-pixel driving circuit, the node voltage generating module is inputted with the a red-green-blue display data RGB, and electrically connected to the source of the driving thin-film transistor in the sub-pixel driving circuit, capable of processing the red-green-blue display data, obtaining the APL value of the current frame of the AMOLED display, and according to the APL value of the current frame of the AMOLED display and the preset node voltage calculation formula to generate a corresponding node voltage and outputs the voltage to the source of the driving thin-film transistor, and adjusts the gate-to-source voltage of the driving thin-film transistor by using the APL value, thereby adjusting the driving current flowing through the light-emitting diode in order to adjust the overall display brightness of the AMOLED display. The driving method of the AMOLED display of the present invention can adjust the driving current flowing through the organic light-emitting diode to adjust the brightness of the entire display screen. 
     As described above, for those of ordinary skill in the art, various other changes and modifications can be made in accordance with the technical solutions and the technical concept of the present invention, and all such changes and modifications are intended to fall within the scope of the appended claims.