Patent Publication Number: US-7218296-B2

Title: Active matrix organic electroluminescence light emitting diode driving circuit

Description:
FIELD OF THE INVENTION 
   This invention relates to a driving circuit of active matrix organic electroluminescence light emitting diode display. More particularly, the invention is directed to a driving device that improves the non-uniform phenomena on an active matrix organic light-emitting diode display panel. 
   BACKGROUND OF THE INVENTION 
   An OLED Display can be classified according to its driving method, passive-matrix (PMOLED) and active-matrix (AMOLED). AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals that can control OLED levels of brightness. 
   The manufacturing procedure of PMOLED is simpler in comparison and is less costly of the two; however, it is limited in its size (&lt;5 inches) because of its driving mode and a lower resolution display application. In order to produce an OLED display with higher resolution and larger size, utilizing active-matrix driving is necessary. The so-called AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals, so that pixels can maintain their brightness after line scanning; on the other hand, pixels of passive matrix driving only light up when the scan line selects them. Therefore, with active matrix driving, the brightness of OLED is not necessarily ultra-bright, resulting in longer lifetime, higher efficiency and higher resolution. Naturally, TFT-OLED with active matrix driving is suitable for display application of higher resolution and excellent picture due to the unique qualities of OLED. 
   LTPS (Low Temperature Poly-Silicon) and a-Si (amorphous Silicon) are both technologies of TFT integrating on glass substrate. The obvious differences are electric characteristics and complexity of process. Although LTPS-TFT possesses higher carrier mobility and higher mobility means more current can be supplied, the process is much more complex. However, the process of a-Si TFT is simpler and more mature, except for low carrier mobility. Therefore, a-Si process has better competitive advantages in cost. 
   As mobility of LTPS-TFT is up to 100˜200 cm 2  /V-sec currently, TFT-OLED driving IC and data IC can be LTPS processed; however, due to limitations of LTPS processing capability, properties of each TFT element vary. The most pressing problem of AMOLED is how to reduce the impact of uneven LTPS-TFT characteristics. Such an issue requires an immediate solution for follow-up development and applications since images with erroneous gray scales show up on OLED panels and seriously damage image uniformity. 
   U.S. Pat. No. 6,229,506 discloses an Active Matrix Light Emitting Diode Pixel Structure And Concomitant Method. A 4T2C (4 TFTs and 2 capacitors) pixel circuit is proposed as shown in  FIG. 1 . An Auto-Zero mechanism is applied to compensate for threshold voltage differences of TFT elements to improve the uniformity of images. Driving sequences of control signals include Auto-Zero Phase  210 , Load Data Phase  220  and Illuminate Phase  230 . Refer to  FIG. 2  for the sequences of control signals. 
   Transistors T 3  and T 4  are off and transistor T 2  is on prior to Auto-Zero Phase  210 . The current passing through OLED  160  at this moment is current of the previous frame and controlled by Vsg of transistor T 1  (voltage difference between source and gate; i.e., voltage difference of both ends of Cs). 
   After entering the Auto-Zero Phase  210 , transistor T 4  is on and then transistor T 3  is on, too so that drain and gate of transistor T 1  can connect as a diode. As transistor T 2  is off, gate voltage of transistor T 1  will increase, which equals to Vdd minus threshold voltage (Vth) of transistor T 1 . That is to say, the voltage difference stored at both ends of capacitor Cs is the threshold voltage of transistor T 1 . After placing transistor T 3  off, threshold voltage (Vth) of transistor T 1  can be stored into capacitor Cs and Auto-Zero Phase  210  is completed. 
   On Load Data Phase  220 , when the voltage difference of data line  110  is ΔV, it couples to the gate of transistor T 1  through transistor T 4  and capacitor Cc. Thus, voltage difference stored at both ends of capacitor Cs will be ΔV×[Cc/(Cc+Cs)] adding Vth that is stored in capacitor Cs previously. That is, Vsg of transistor T 1  includes Vth of transistor T 1 , which makes output current of transistor T 1  relate to voltage change (ΔV) of data line  110  and capacity of capacitors Cc and Cs, instead of being affected by Vth of transistor T 1  in every pixel. 
   Lastly, when Illuminate Phase  230  begins, transistor T 4  is off and transistor T 2  is on. Output current of transistor T 1  at the present frame will flow through OLED  160  to illuminate. 
   Though this 4T2C pixel circuit may compensate for the threshold voltage (Vth) differences of transistor elements in each pixel and improve integral uniformity of images; however, other control lines like Auto-Zero Line  130  and Illuminate Line  140  are required in addition to data line  110 , scan Line  120  and supply line (Vdd)  150 . Capacitor Cs has to record all threshold voltages and part of the data voltages loaded. Besides, a capacitance coupling approach is used to load data, which not only makes the driving method more complicated, but also increases manufacturing costs when a non-standard data driving IC is required. 
   To solve the same problem, Philips also published a thesis with the subject of ┌ A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays┘. One 4T2C pixel circuit is presented in the thesis as  FIG. 3  shows. It skillfully changes the location of connecting two capacitors in the pixel circuit of the U.S. Pat. No. 6,229,506 ( FIG. 1 ) to solve the defects of complexity and impracticability. However, control lines like Auto-Zero Line  330  and Illuminate Line  340  are also required in addition to data line  310 , scan line  320  and supply line (Vdd)  350 , just like those in U.S. Pat. No. 6,229,506. 
   The sequences of driving control signals are the same as those in the U.S. Pat. No. 6,229,506 since they consist of Auto-Zero Phase, Load Data Phase and Illuminate Phase. 
   On Auto-Zero Phase, Transistor T 34  is off and then transistor T 33  is on so that drain and gate of transistor T 31  can be connected as a diode. As transistor T 32  is off, gate voltage of transistor T 31  will increase, which equals to Vdd minus threshold voltage (Vth) of transistor T 31 . That is to say, the sum voltage stored at capacitors C 1  and C 2  is the threshold voltage (Vth) of transistor T 31 . After placing transistor T 33  off, Auto-Zero Phase is completed. 
   Data voltage is conducted through connection of transistor T 34 . Data voltage is stored in capacitor C 1  and a certain proportion of Vth previously stored at both ends of capacitor C 2  is still maintained, which equals to [C 1 /(C 1 +C 2 )]×Vth. Thus, the sum of capacitors C 1  and C 2  is (Vdd−Vdata+[C 1 /(C 1 +C 2 )]×Vth); i.e., Vsg of transistor T 31  contains part of Vth of transistor T 31 , which may not only reduce the correlation between the output current and threshold voltage of transistor T 31 , but also compensate for part of the threshold voltage (Vth) difference resulting from processing factors. 
   The threshold voltage of transistor T 31  in the thesis is memorized by two capacitors (C 1  &amp; C 2 ). Part of the threshold voltage data stored in one of the capacitors will get lost while loading data voltage. Therefore, this approach can only make up for part of the threshold voltage difference resulting from processing. 
   SUMMARY OF THE INVENTION 
   Hence, a voltage type of AMOLED driving circuit that can compensate for TFT threshold voltage variations is presented in this invention so as to improve image defects resulting from uneven characteristics of TFT. 
   To achieve the objective above, a driving device of each pixel presented in this invention includes 3 TFTs and 2 capacitors, which are 1 scan reset TFT, 1 detect TFT, 1 driving TFT, 2 capacitors (Cd &amp; Ct) and 1 organic electro-luminescence element. The gate of scan reset TFT is controlled by the scan line of the row where the pixel is located. Detect TFT is controlled by one threshold-lock line. Capacitor Cd is used to store data voltage (Vdata) of image signals and capacitor Ct is used to store threshold voltage (Vth) of driving TFT. Therefore, the sum voltage stored at capacitors Cd and Ct will force driving TFT to output a corresponding current to the organic electroluminescence element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG.1  is a schematic pixel circuit diagram of U.S. Pat. No. 6,229,506. 
       FIG. 2  is a schematic diagram of control signal time sequence of U.S. Pat. No. 6,229,506. 
       FIG. 3  is the pixel circuit in the thesis published by PHILIPS. 
       FIG. 4  is the pixel circuit for this invention. 
       FIG. 5  is the connection and control of a pixel circuit in this invention. 
       FIG. 6  is the sequences of control signals in this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Refer to  FIGS. 4 &amp; 5  for the circuit, connection and control of each pixel in this invention. As the Figures show: the driving circuit of pixel  500  on the display panel  400  composed of one scan line  420  and one data line  410  includes 3 TFTs, 2 capacitors and 1 organic electro-luminescence element connected as follows: 
   Gate of a scan reset TFT  510  connected to one scan line  420  and drain connected to a data line  410 . Two ends of storage capacitor Cd installed between source of the scan reset TFT  510  and supply line Vdd  450 . 
   Source of a driving TFT  530  connected to the supply line Vdd  450 . 
   Gate of a detect TFT  520  connected to a Threshold-Lock  430 , drain connected to gate and source connected to drain of driving TFT  530 . Two ends of compensation capacitor Ct installed between source of the scan reset TFT  510  and drain of detect TFT  520 . 
   Anode of an organic electroluminescence element  540  connected to the drain of driving TFT  530  and cathode connected to a common Line  440 . 
   Refer to  FIG. 5 . As the Figure shows: a joint where a scan line  420  (S 1 , S 2 , S 3  . . . Sn) and a data line  410  (D 1 , D 2 , D 3  . . . Dm) meet is a pixel  500 . Refer to  FIG. 4 , and  FIG. 5 . The gate of scan reset TFT  510  is controlled by scan line  420  of the row where Pixel  500  is located, and the drain is connected to data line  410  of the column where Pixel  500  is situated. Detect TFT  520  is controlled by Threshold-Lock  430 . Capacitor Cd is used to store data voltage (Vdata) of image signals and capacitor Ct is used to store threshold voltage (Vth) of driving TFT  530 . Therefore, the sum of voltage stored in capacitors Cd and Ct will force driving TFT  530  for an output of corresponding current to the organic electroluminescence element  540 . 
   Detect TFT  520  of each Pixel  500  on display panel  400  is controlled by the same Threshold-Lock  430  and source of driving TFT  530  is jointly connected to the same supply line (Vdd)  450 . Cathode of organic electroluminescence element  540  in every Pixel  500  is jointly connected to a common line  440 , which is grounded via an external switch  470  controlled by a display line  460 . 
   Actuation procedures of this invention are described as follows: 
   Refer to  FIG. 6  for the sequences of control signals in this invention. A cycle of driving signals can be divided into three phases. First, Threshold-Lock Phase  610 : 
   Signals of scan line  420  and Threshold-Lock  430  will trigger scan reset TFT  510  and detect TFT  520  in every pixel circuit on. The voltage level of reset data line  410  will be the same as that of supply line (Vdd)  450 . When scan reset TFT  510  is on, capacitor Cd storing voltage of image data will discharge via scan reset TFT  510  and data line  410 . Display signal line  460  controls Switch  470  outside of display panel  400  and makes it off. Thus, an open circuit exists between common line  440  and the grounding end of the system. The current of driving TFT  530  stops flowing through organic electroluminescence element  540 , and diverts to detect TFT  520  that is currently on, which forces driving TFT  530  to detect the threshold voltage. As the current of driving TFT  530  passes by detect TFT  520 , capacitor Ct and scan reset TFT  510 , voltage stored in capacitor Ct becomes smaller and smaller, which makes the current of driving TFT  530  become smaller until no current is left. 
   At last, capacitor Cd won&#39;t store any electric charge (0 voltage on both ends) and voltage difference on both ends of capacitor Ct will equal to threshold voltage (Vth) of driving TFT  530 ; i.e. when capacitor Cd discharges and resets, capacitor Ct will memorize the threshold voltage (Vth) of driving TFT  530  (Refer to  FIG. 4  for Pixel  500  circuit.). In summary, threshold voltage (Vth) of driving TFT  530  in every Pixel  500  circuit will be stored in its own capacitor Ct after Threshold-Lock Phase  610 . 
   Next, signals of scan line  420  and Threshold-Lock  430  will trigger scan reset TFT  510  and detect TFT  520  in every Pixel  500  circuit off for the following write Phase  620 . 
   In write Phase  620 , each scan line  420  (S 1 , S 2  . . . Sn) will send out scan signals in order. When scan signals shift to scan line  420 , all scan reset TFT  510  on the same scan line will be on and detect TFT  520  will be off. Data voltage (Vdata) of data line  410  can be stored into capacitor Cd as scan reset TFT  510  is on; however, threshold voltage (Vth) previously memorized by capacitor Ct will still be retained as detect TFT  520  is off. Thus, voltage difference between two ends of capacitor Cd will be equivalent to supply voltage (Vdd) minus data voltage (Vdata); i.e. voltage at both ends of capacitor Cd is (Vdd−Vdata). Therefore, the sum of voltage stored in capacitors Cd and Ct will equal to (Vdd−Vdata+Vth), which enables Driving TFT  530  to output corresponding current to organic electroluminescence element  540  in the following phase (display phase  630 ). Consequently, the current (I) can be expressed with a formula as follows:
 
 I= (½)×β×( Vsg−Vth ) 2 
 
 I= (½)×β×( Vdd−V data+ Vth−Vth ) 2 
 
 I= (½)×β×( Vdd−V data) 2 
 
   From the above equations (β is the Transconductance Parameter of driving TFT  530 ), the current (I) generated by driving TFT  530  is irrelevant to the threshold voltage (Vth) of its own, but only correlated to write data voltage (Vdata). Thus, threshold voltage differences of TFT resulting from processing factors can be compensated for. 
   When the last scan line  420  (Sn) completes writing data voltage (Vdata), display line  460  controls switch  470  to switch on and common line  440  connects to the grounding end of the system for the third stage of display phase  630 . 
   In display phase  630 , driving TFT  530  in each Pixel  500  circuit will output current (I) relating to the written data voltage (Vdata) and organic electroluminescence element  540 , which produces proper luminance. Output current (I) is not related to the threshold voltage (Vth) of driving TFT  530 . 
   In comparison with the U.S. Pat. No. 6,229,506, only one extra reset is required in this invention before loading data voltage to complete the Threshold-Lock Phase  610  and avoid complexity. 
   To compare the thesis published by PHILIPS with the subject of ┌ A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays┘, the technology of this invention is to record all threshold voltages into one capacitor (capacitor Ct) to offset the effects of threshold voltage differences. 
   Two capacitors (Cd &amp; Ct) are used in this invention to deal with two different things. One capacitor Ct is responsible to record all threshold voltage values (Vth) and the other capacitor Cd is in charge of recording all data voltage values (Vdata). It is different from U.S. Pat. No. 6,229,506 as the capacitor Cs has to record all threshold voltages (Vth) and part of data voltage (Vdata) loaded. It is also different from the thesis released by PHILIPS as capacitors C 1  and C 2  record threshold voltages jointly. Part of the threshold voltage stored in Capacitor C 1  will be lost since capacitor C 2  only records part of it. 
   To conclude, the AMOLED driving circuit of this invention has the following advantages: 
   1. As all threshold voltage values (Vth) can be stored in one capacitor Ct (threshold voltage storage capacitor), the effects of threshold voltage differences can be compensated completely. 
   2. Only one extra reset is required for data voltage (Vdata) loading to prevent complexity. 
   3. The technology of placing transistor switch  470  that controls OLED current outside of pixel  500  increases the aperture ratio for pixel  500 .