Abstract:
A pixel circuit for active matrix OLED and driving method is proposed in this invention, which includes five transistors and one capacitance, it&#39;s mainly use a first-transistor connected to a control line to let a second transistor connected to the former scan line off when writing a low voltage in, so to avoid large current generation and IR-drop, finally the illumination will be more uniform than prior art.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a pixel circuit for active matrix OLED and driving method which provide the illumination uniformity.  
           [0003]    2. Description of the Related Art  
           [0004]    Organic light emitting devices is a new light emitting technology, its principle is a sandwich structure that organic film to place in between two electrode layers. The light need transparent to device, so one of electrodes needs to use ITO electrode. When drive a forward bias to device between anode and cathode, the electron and hole that generated by anode and cathode will empty into light emitting material and then emit light by radiation and re-combine method.  
           [0005]    The major application of Organic Light Emitting Devices is display, the pixel circuit is similar with the circuit of TFT LCD, they are all matrix arrays. An illustrated view showing a pixel circuit of Organic Light Emitting Devices of the prior art is shown as FIG. 1. After scan light  12  turn on the transistor  100 , data line  10  provides a voltage and stores to capacitance  102 . It&#39;s equal to voltage of transistor V GS , and transistor  101  convert voltage to current and current through transistor  101  by power line  11  and then transmit to Organic Light Emitting Diode. The current formula is  
       I   =       1   2              k        (       V     G                 S       -     V   t       )       2     .                             
 
           [0006]    The problem which pixel circuit of prior art is the threshold voltage of TFT has big variation. It causes big variation of current I, and different current of OLED in pixel circuit. Finally, the uniformity of illumination isn&#39;t well.  
           [0007]    From FIG. 2 is an illustrated view showing a local pixel circuit layout on display panel of the prior art. If the voltage VDD of signal line  21  is 12V, then maintain wholly white frame need 8V that data line  22  writing a voltage. When the first scan line S N−1  scan and turn on, writing 8V to point A. Thus, the voltage on capacitance  23  is 4V and current generated by transistor M 1  under V GS  transmit to OLED  24 , transmitting from transistor M 1  to OLED through signal line  21 . When the first scan line S N−1  cut-off and the second scan line S N  turn on, the data writing 8V to point B and transistor M 2  generate current through signal line  21 , but point C is even lower than 12V because parasitic resistance of signal line  21  has IR-drop. It causes the voltage of capacitance  25  on pixel circuit P 2  is not equal to voltage of capacitance  23  on pixel circuit P 1 , and the frame from top to bottom generates non-uniformity when writing the same data. This kind of phenomenon which parasitic resistance of signal line  21  to descend the voltage VDD is called IR-drop.  
           [0008]    Refer to FIG. 3 is an illustrated view showing a pixel circuit of OLED of the another prior art. This circuit uses four Thin-film Transistors (TFT)  30 , 31 , 32 , 33  and two capacitance  36 , 37 , wherein the value of capacitance for capacitance  36  is C 1  and the value of capacitance for capacitance  37  is C 2 . Four transistors include drive transistor  30  which convert voltage to current and three transistors  31 , 32 , 33  which to do turn on or cut-off. Driving has two statement, one is AutoZero statement that using transistor  31 , 32  short, transistor  33  open and data line  34  transmits a VDD data, transistor  30  forms a connection of diode because transistor  32  short and point A stores the threshold voltage V t1  of transistor  30 . Another statement is writing statement that transistor  32  cut-off, data line  34  transmit a correct data and using capacitance couple principle, voltage of point A stores the value of  
           Δ                 V   ×       c   1         c   1     +     c   2           +     V   t1       ,                         
 
           [0009]    ΔV is the voltage volume of couple. When transistor  33  turns on, the voltage of point A lets transistor  30  generate current, the current formula is  
         I   =       1   2            k        (       V     G                 S       -     V   t       )       2         ,                         
 
           [0010]    the V t  in formula will be eliminated. The current has relationship with voltage on data line  34  and no relationship with the threshold voltage V t  of transistor. It can overcome the threshold voltage has variation induced current and illumination also has variation in former prior art. Due to this circuit need four transistors and two capacitance and need two statements, so also need two complex control signals.  
           [0011]    Refer to FIG. 4 is an illustrated view showing a pixel circuit  4  of OLED of the another prior art. This pixel circuit  4  uses four Thin-film Transistors (TFT)  41 , 42 , 43 , 44  and one capacitance  45 , wherein the function of transistor  41  is a switch, transistor  42  convert voltage to current and provide Organic light emitting diode (OLED)  46 , and the function of transistor  43 , 44  is compensating threshold voltage (V t ) of transistor  42 . Thus, scan signal SN turn on transistor  41 , data line  47  provide a lowest voltage, and then transistor  44  will turn on and decrease voltage of B point to turn on transistor  43 , data line  47  provide higher voltage V DATA . Due to low voltage of B point will turn on transistor  43 , thus, providing the current of OLED  46 , the formula is  
                 I                 d     =     k        (       V     G                 S       -     V   t       )         ,                k   =       1   2          μ   ·     C   OX              W   L     .                 (   1   )                               
  V   G 42   =V   B   =V   A   −V   t43   (2)  
             Id==k ( V   DD −( V   A   −V   t43 )− V   t42 ) 2   (3)  
           [0012]    In formula (3), V t43 =V t42  because the difference is close between transistor  42  and transistor  43 , and process variation small. It replaces to formula (2) is Id=k(V DD −V A ) 2 , V A =V DATA , it shows no relationship with current and threshold voltage V th  of transistor.  
           [0013]    In formula (3), V G42  is a voltage of gate of transistor  42 ; V t43  is a threshold voltage of transistor  43 ; V t42  is a threshold voltage of transistor  42 ; V DD  is a voltage transmitted by signal line  48 .  
           [0014]    From the result of formula mention above, this circuit  4  can overcome threshold voltage variation of transistor on display induced illumination non-uniformity and layout area is smaller. But before writing a real data, it need provide a low voltage and then transistor  42  provide a high current to OLED  46 , the illumination of display will brighter first and recover to normal status. It causes shorten the life-time of OLED and worse image quality, and operation complex because it need to provide a low voltage before writing correct data in data driving circuit.  
           [0015]    To resolve problems mentioned above that threshold voltage and IR-drop induced illumination non-uniformity of OLED. In this invention propose a pixel circuit for active matrix OLED and driving method and achieve the purpose of the illumination uniformity in display.  
         SUMMARY OF THE INVENTION  
         [0016]    A pixel circuit for active matrix OLED and driving method is proposed in this invention, it use a first-transistor connect to a control line to let a second transistor which connect to the former scan line cut-off when writing a low voltage in, so to avoid large current generation and IR-drop.  
           [0017]    To achieve the purpose mentioned above, a pixel circuit for active matrix OLED in this invention includes the first transistor which received control signal output by signal line and then cut-off; the second transistor which received scan signal output by former scan line and provide a low voltage; the third transistor which received scan signal output by corresponding scan line and then turn on it; the fourth transistor which received data voltage output by signal line and convert to current output to organic light emitting diode; the fifth transistor to compensate threshold voltage of the fourth transistor.  
           [0018]    According to pixel circuit mentioned above, a circuit driving method for active matrix OLED in this invention includes:  
           [0019]    Input a control signal to Kth parallel signal and cut-off the first transistor controlled by Kth and (K−1)th control line; Input a scan signal to turn on the second transistor controlled by (K−1)th parallel scan line and writing a low voltage to compensate threshold voltage; Input next scan signal to turn on the third transistor controlled by Kth parallel line and writing data in pixel circuit of Kth parallel line; Finally, to finish the scan control flow of pixel circuit of Kth parallel line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0021]    [0021]FIG. 1 is an illustrated view showing a pixel circuit of Organic Light Emitting Devices of the prior art;  
         [0022]    [0022]FIG. 2 is an illustrated view showing a local pixel circuit layout on display panel of the prior art;  
         [0023]    [0023]FIG. 3 is an illustrated view showing a pixel circuit of Organic Light Emitting Devices of the another prior art;  
         [0024]    [0024]FIG. 4 is an illustrated view showing a pixel circuit of Organic Light Emitting Devices of the another prior art;  
         [0025]    [0025]FIG. 5 is an illustrated view showing a pixel circuit in accordance to an embodiment of the present invention;  
         [0026]    [0026]FIG. 6 is an illustrated view showing a wave of control signal in accordance to an embodiment of the present invention;  
         [0027]    [0027]FIG. 7 is an illustrated view showing a scan control flow of pixel circuit in accordance to another embodiment of the present invention;  
         [0028]    [0028]FIG. 8 is an illustrated view showing a circuit layout which can resolve IR-drop of signal line in accordance to another embodiment of the present invention; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0030]    Refer to FIG. 5 is an illustrated view showing a pixel circuit  5  in accordance to an embodiment of the present invention, wherein includes a data line  50 , a former scan line  51 , a scan line  52 , a signal line  53 , the first transistor  54 , the second transistor  55 , the third transistor  56 , the fourth transistor  57 , the fifth transistor  58  and a storage capacitance  59 .  
         [0031]    The function of the first transistor is a switch which received control signal SB K  output by control line  61  to cut-off the first transistor  54 ; the second transistor  55  which received scan signal S K− 1 output by former scan line  51  and provide a low voltage to saturate the fifth transistor  58 . The gate  550  of the second transistor  55  connect to former (K−1)th scan line  51  and drain  55  connect to a low voltage signal (GND); the third transistor  56  which received scan signal S K  output by Kth scan line  52  and then turn on the third transistor  56  and write a data to D point, that is means store to capacitance; the fourth transistor  57  which received data voltage (V DATA ) of storage capacitance and convert to current output to organic light emitting diode  60 ; the fifth transistor  58  which setting between the third  56  and the fourth transistor  57  to compensate threshold voltage of the fourth transistor  57 .  
         [0032]    Actual circuit driving status refers to FIG. 6. The first, control line  61  output a control signal SB K  to the first transistor  54  and cut-off it, and former scan line  51  is also output a scan signal to the second transistor  55 . This signal S K− 1 is a low voltage, so reduce the voltage of D point to turn on the fifth transistor  58  and form diode connection method. The difference of voltage of point C and point D is a threshold voltage (V t58 ) and then this Kth scan line  52  output control signal S K  to turn on the third transistor  56 , a data line  50  written voltage V DATA  to the third transistor  56  and the fourth transistor  57  store to storage capacitance  59 . At this moment, the first transistor  54  is still cut-off, and after the third transistor  56  cut-off by control signal S K , the first transistor  54  will turn on and generate current. The voltage of point C is V C =V DATA′  the gate voltage of the fourth voltage  57  (V G57 ) is equal to the voltage of point C (V C ) minus the threshold voltage on the fifth transistor  58  (V t58 ); the formula is  
         
       V 
       G57 
       =V 
       D 
       =V 
       C 
       −V 
       t58  
     
         [0033]    the current formula:  
                 I                 d     =     k        (       V     G                 S       -     V   t       )         ,                  k   =       1   2          μ   ·     C   OX            W   L         ;             (   1   )                               
  Id=k ( V   DD −( V   C   −V   t58 )− V   t57 ) 2   (2);  
         [0034]    Due to the fourth and fifth transistor ( 57 , 58 ) is very close in process, so their threshold voltage is equivalent.  
         [0035]    In formula (2)  
           V   t58   =V   t57   (3)  
         so  
           Id=k ( V   DD   −V   C ) 2   ,V   C   =V   DATA   (4)  
         [0036]    It shows no relationship between current and threshold voltage of transistor.  
         [0037]    Wherein V t57  of formula (2) and (3) is threshold voltage of the fourth transistor  57 , V DD  of formula (2) is a voltage that transfer by signal line  53 .  
         [0038]    The function of the first transistor  54  and the third transistor  56  is a switch, and the second transistor  55  provides a low voltage. The fourth transistor  57  converts voltage to current for OLED  60 . The fifth transistor  58  compensates the threshold voltage V th  of the fourth transistor  57 .  
         [0039]    The scan control flow of pixel circuit is shown as FIG. 7. At first, to progress step  70 , input a control signal to Kth parallel signal and cut-off the fifth transistor controlled by Kth control line, this time span of control line is two periods of parallel scan; to progress step  71 , input a scan signal to turn on the fourth transistor controlled by (K−1)th parallel line and writing a low voltage in wherein the time span of turn on scan signal is a parallel scan line period; Next, to progress step  72 , input next scan signal to turn on the third transistor controlled by Kth parallel line and writing data in pixel circuit of Kth parallel line, this time span of turn on scan signal is a parallel scan line period; Final, to progress step  73 , turn on the switch of the fifth transistor that is controlled by Kth control line and then finish the scan control flow of pixel circuit of Kth parallel line.  
         [0040]    Refer to FIG. 8 is an illustrated view showing a circuit layout which can resolve IR-drop of signal line in accordance to another embodiment of the present invention, wherein the layout method of signal line is parallel layout with scan line. A driving method mentioned above is when scan line S N−2  turn on, transistor T 1  and T 2  that controlled by control line S BK  is cut-off, so signal line V dd  has no current; when scan line S N−1  turn on and writing voltage to storage capacitance, transistor T 1  and T 2  are also turn off, and transistor T 3  and T 4  turn off because control line S BK+1  is work. When scan line S N−1  finish working, and data line writing the same voltage to storage capacitance  80  of each pixel, then transistor T 1  and T 2  turn on, the (S N−1 )th OLED  81 , 82  are illuminative. Although signal line has current and IR drop, this IR drop generated suddenly will decrease voltage of storage capacitance because of coupling. For driving transistor T 5 , the value V gs  is the same with value that writing voltage but not yet generates current, so no IR-drop. It causes the different effect at storage voltage of each pixel.  
         [0041]    The detail explanation in this invention is mention above, due to add a first transistor in pixel circuit to be a switch to avoid generating high current on the fourth transistor, contrast non-uniformity and increase OLED life time when writing a low voltage before driving in pixel circuit.  
         [0042]    Due to the first transistor is cut-off when scan line turn on the second and the third transistor and writing voltage data, and signal line has no current and no IR-drop, so it can resolve the illumination non-uniformity induced by IR-drop.