Abstract:
A pixel driving circuit for use in an active matrix organic light-emitting diode with threshold voltage compensation includes a transistor, a first capacitor and a second capacitor. The organic light-emitting diode is in communication with the transistor. The first capacitor has a first and a second ends, wherein the first end is coupled to a gate electrode of the transistor. The second capacitor has a third and a fourth ends coupled to the second end of the first capacitor and a ground voltage, respectively. A threshold voltage of the transistor is stored in the first capacitor in a first state, a driving voltage received from a data line is stored in the second capacitor in a second state, and the gate electrode of the transistor is biased with a specified voltage applied to the first and the second capacitors interconnected in series in a third state. A current passing through the organic light-emitting diode is controlled accordingly.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pixel driving circuit, and more particularly to a pixel driving circuit for use in an active matrix organic light-emitting diode with threshold voltage compensation. The present invention also relates to a pixel driving method of such circuit. 
     BACKGROUND OF THE INVENTION 
     Since the typical liquid crystal display (LCD) needs backlight to operate and is complicated in fabrication, alternative displays are further developed. Recently, a display by means of organic light-emitting diodes (OLEDs) has been developed due to its self-light-emitting and easily manufactured features. In addition, the OLED panel has advantages of wide viewing angles, low cost, reduced thickness and flexible operational temperature. The OLEDs can be used as pixel units of an active matrix electron luminescent display, and thus the OLED panel is expected to substitute for the LCD in the near future. 
       FIG. 1  illustrates a conventional driving circuit for driving an OLED pixel. The pixel unit comprises an organic light-emitting diode OLED, two transistors M 1 ˜M 2  and a capacitor Cs (so-called as 2T1C). The gate electrode of the transistor M 1  is coupled to a scan line  10 , and the other two electrodes of the transistor M 1  are coupled to a data line  20  and the gate electrode of the transistor M 2 , respectively. The source and drain electrodes of the transistor M 2  are coupled to a source voltage Vdd and the P electrode of the organic light-emitting diode OLED. The N electrode of the organic light-emitting diode OLED is coupled to a ground voltage GND. The capacitor Cs is coupled between the source electrode and gate electrode of the transistor M 2 . 
     During operation of the scan line  10 , the transistor M 1  is switched on. Meanwhile, via the data line  20 , a driving voltage is inputted and stored in the capacitor Cs. The driving voltage can also bias the transistor M 2  to result in a constant current Id passing through the organic light-emitting diode OLED. The organic light-emitting diode OLED emits light accordingly. 
     For a purpose of forming the active matrix and its peripheral circuit on the same substrate, a so-called low-temperature polysilicon thin film transistor (LTPS-TFT) technology was developed with improved electrical properties of TFTs and other benefits. However, since the threshold voltage and mobility of such LTPS-TFT vary with manufacturing processes to a certain extent, some problems may occur. For example, under a constant voltage applied to the capacitor Cs, the resulting intensity of current passing through the organic light-emitting diode OLED may be different for the LTPS-TFT manufactured by different processes. The light intensity emitted by the OLED cannot be well expected. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pixel driving circuit and a pixel driving method for use in an active matrix organic light-emitting diode, in which the current passing through the organic light-emitting diode is precisely controlled with threshold voltage compensation. 
     In accordance with a first aspect of the present invention, there is provided a pixel driving circuit for use in an active matrix organic light-emitting diode with threshold voltage compensation. The pixel driving circuit comprises a transistor, a first capacitor and a second capacitor. The organic light-emitting diode is in communication with the transistor. The first capacitor has a first and a second ends, wherein the first end is coupled to a gate electrode of the transistor. The second capacitor has a third and a fourth ends coupled to the second end of the first capacitor and a ground voltage, respectively. The first capacitor stores therein a threshold voltage of the transistor in a first state, the second capacitor stores therein a driving voltage received from a data line in a second state, and the first and the second capacitors interconnected in series and having a specified voltage applied thereto bias the gate electrode of the transistor in a third state. A current passing through the organic light-emitting diode is controlled accordingly. 
     In one embodiment, the pixel driving circuit further comprises a first switch for controlling the driving voltage received from the data line to be stored in the second capacitor in the second state. 
     In one embodiment, when the pixel driving circuit is in the first state, a source voltage coupled to the source electrode of the transistor is inputted into the second end of the first capacitor via the data line such that the threshold voltage is stored into the first capacitor. 
     In one embodiment, the first, the second and the third states are a compensation, a data write-in and an emission states, respectively. 
     In accordance with a second aspect of the present invention, there is provided a method for driving a pixel of an active matrix organic light-emitting diode. Firstly, a threshold voltage is recorded in a first state. Then, a driving voltage is recorded in a second state. Afterward, a gate electrode of a transistor is biased with a summation voltage of the threshold voltage and the driving voltage to control a current passing through the organic light-emitting diode in a third state. 
     In one embodiment, the threshold voltage is a threshold voltage of the transistor of the pixel. 
     In one embodiment, the threshold voltage is recorded into a first capacitor of the pixel. 
     In one embodiment, the threshold voltage is recorded into the first capacitor under the condition that the first capacitor has a first end coupled to the gate electrode of the transistor and a second end for inputting therein a source voltage coupled to a source electrode of the transistor. 
     In one embodiment, the driving voltage is recorded into a second capacitor of the pixel. 
     In one embodiment, the driving voltage to be recorded into the second capacitor is received from a data line via a switch of the pixel in the second state. 
     In accordance with a third aspect of the present invention, there is provided a pixel driving circuit for use in an active matrix current-controllable light-emitting device with threshold voltage compensation. The pixel driving circuit comprises a transistor, a current-controllable light-emitting device, a first capacitor and a second capacitor. The current-controllable light-emitting device is in communication with the transistor. The first capacitor has a first and a second ends, wherein the first end is coupled to a gate electrode of the transistor. The second capacitor has a third and a fourth ends coupled to the second end of the first capacitor and a ground voltage, respectively. The first capacitor stores therein a threshold voltage of the transistor in a first state, the second capacitor stores therein a driving voltage received from a data line in a second state, and the first and the second capacitors interconnected in series and having a specified voltage applied thereto bias the gate electrode of the transistor in a third state. A current passing through the current-controllable light-emitting device is controlled accordingly. 
     In one embodiment, the current-controllable light-emitting device is an organic light-emitting diode. 
     The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a conventional pixel driving circuit of an OLED display; 
         FIG. 2  is a circuit diagram illustrating a pixel driving circuit of an OLED display according to an embodiment of the present invention; 
         FIG. 3  is a timing waveform diagram showing the signal variations in different states; 
         FIGS. 4(   a ) and  4 ( b ) are a schematic diagram and a table illustrating operation principle of the pixel driving circuit of  FIG. 2 ; 
         FIG. 5  is a plot illustrating voltage variation of the gate electrode of the transistor M 4  according to various threshold voltages of the transistor M 4  and a constant driving voltage; 
         FIG. 6  is a plot illustrating current variation of the current passing through the transistor M 4  according to various threshold voltages of the transistor M 4  and a constant driving voltage; and 
         FIG. 7  is a circuit diagram illustrating a pixel driving circuit of an OLED display according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to overcome the problem resulting from variation of the threshold voltage in the prior art, the present invention provides a pixel driving circuit for use in an active matrix organic light-emitting diode with threshold voltage compensation. 
       FIG. 2  illustrates a pixel driving circuit for driving an OLED pixel according to a preferred embodiment of the present invention. Each of the pixel units comprises an organic light-emitting diode OLED, four transistors M 1 ˜M 4  and two capacitors C 1 ˜C 2  (also referred to as 4T2C). The gate electrode of the transistor M 1  is coupled to a first scan line  130 , and the other two electrodes of the transistor M 1  are coupled to a data line  150  and a node “a”, respectively. The gate electrode of the transistor M 2  is coupled to a second scan line  135 , and the other two electrodes of the transistor M 2  are coupled to the drain and the gate electrodes of the transistor M 4 , respectively. The source, the gate and the drain electrodes of the transistor M 3  are coupled to the drain electrode of the transistor M 4 , a third scan line  140  and the P electrode of the organic light-emitting diode OLED, respectively. The source electrode of the transistor M 4  is coupled to a source voltage Vdd. The N electrode of the organic light-emitting diode OLED is coupled to a ground voltage GND. The capacitor C 1  is coupled between the gate electrode of the transistor M 4  and the node “a”. The capacitor C 2  is coupled between the node “a” and the ground voltage GND. 
     The pixel driving circuit of  FIG. 2  is operated in alternate reset, compensation, data write-in and emission states, which are controlled by the first scan line  130 , the second scan line  135  and the third scan line  140 .  FIG. 3  is a timing waveform diagram showing the signals passing through the scan lines  130 ,  135  and  140  and the data line  150  in different states. The operation principle of this pixel driving circuit will be illustrated as follows with reference to  FIGS. 2 and 3 . 
     In the reset state, the first scan line  130 , the second scan line  135  and the third scan line  140  are enabled to switch on the transistors M 1 , M 2  and M 3 , and the source voltage Vdd is also transmitted via the data line  150 . At this time, all of the charges stored in the capacitors C 1 , C 2  and the parasitic capacitor of the organic light-emitting diode OLED are cleared. 
     In the compensation state, the first scan line  130  and the second scan line  135  are enabled to switch on the transistors M 1  and M 2 , and the third scan line  140  is disabled such that the transistor M 3  is switched off. At this time, the voltage at the node “a” equals to the source voltage Vdd. Thus, the voltage applied to the capacitor C 1  defines the threshold voltage Vth of the transistor M 4 . 
     In the data write-in state, the first scan line  130  is enabled to switch on the transistor M 1 , and the second scan line  135  and the third scan line  140  are disabled such that the transistors M 2  and M 3  are switched off. A driving voltage Vdrv is transmitted via the data line  150 . Since the voltage at the node “a” equals to the driving voltage Vdrv, the voltage applied to the capacitor C 2  will equal to Vdrv. Thus, the gate voltage V G  of the transistor M 4  is computed as V G =Vdrv−|Vth|. 
     In the emission state, the third scan line  140  is enabled to switch on the transistor M 3 , and the first scan line  130  and the second scan line  135  are disabled such that the transistors M 1  and M 2  are switched off. The source voltage Vdd is also transmitted via the data line  150 . At this time, the voltage V GS  crossing the gate and the source electrodes of the transistor M 4  is computed as V GS =V G −V S =(Vdrv−|Vth|)−Vdd, where Vs is the source voltage. Thus, the driving current Id passing through the organic light-emitting diode OLED can be obtained according to the following formula: 
                   Id   =       ⁢       1   /   2     ×   k   ×       (       V   GS     +        Vth          )     2                   =       ⁢       1   /   2     ×   k   ×       (     Vdrv   -        Vth        -   Vdd   +        Vth          )     2                   =       ⁢       1   /   2     ×   k   ×       (     Vdrv   -   Vdd     )     2                   
where k is a device parameter.
 
     As will be understood from the above formula, the current Id passing through the organic light-emitting diode OLED is no longer a function of the threshold voltage of the transistor M 4 . In other words, the current Id is independent of the threshold voltage of the transistor M 4 , and the capacitor C 1  has stored the threshold voltage of the transistor M 4  in the compensation state. Thus, in the emission state, the threshold voltage stored in the capacitor C 1  and the threshold voltage of the transistor M 4  will be offset such that the current Id passing through the organic light-emitting diode OLED is only a function of the driving voltage Vdrv and no longer varies with the threshold voltage. The current Id passing through the organic light-emitting diode OLED can be precisely controlled accordingly. 
     The pixel driving circuit shown in  Fig. 4(   a ) is similar to that of  FIG. 2 , except that the transistors M 1 ˜M 3  are replaced by three alternative switches SW 1 ˜SW 3 . When state changes, these switches SW 1 ˜SW 3  are either switched on or switched off, as is illustrated in a table of  FIG. 4(   b ). In the reset state, the switches SW 1 , SW 2  and SW 3  are all switched on, and thesource voltage Vdd is inputted via the data line. In the compensation state, the switches SW 1  and SW 2  are switched on, but the switch SW 3  is switched off. At this time, the source voltage Vdd is still inputted via the data line. In the data write-in state, the switch SW 1  is switched on, but the switches SW 2  and SW 3  are switched off. At this time, it is the driving voltage Vdrv inputted via the data line. In the emission state, the switches SW 1  and SW 2  are switched off, but the switch SW 3  is switched on. At this time, the source voltage Vdd is inputted via the data line. 
     Please refer to  FIG. 5 , which exemplifies voltage variation of the gate electrode of the transistor M 4  according to various threshold voltages of the transistor M 4  and a constant driving voltage Vdrv. The gate voltages of the transistor M 4  are distinguished after the reset, the compensation, the data write-in and the emission states. That is to say, although the driving voltages Vdrv stored in the capacitor C 2  are identical after the four states, the compensation voltages stored in the capacitor C 1  are different such that the gate voltages of the transistor M 4  are distinguished. 
     Please refer to  FIG. 6 , which illustrates variation of the current passing through the transistor M 4  according to various threshold voltages of the transistor M 4  and a constant driving voltage Vdrv. As shown in  FIG. 6 , the driving currents Id passing through the transistor M 4  are almost identical after the reset, the compensation, the data write-in and the emission states. That is to say, the compensation voltage stored in the capacitor C 1  is offset by the threshold voltage of the transistor M 4 . Meanwhile, the driving currents passing through the transistor M 4  and the organic light-emitting diode OLED are controlled by the driving voltage Vdrv stored in the capacitor C 2 . Since the driving voltage Vdrv is constant, the driving currents passing through the transistor M 4  and the organic light-emitting diode OLED are substantially identical. 
     The transistor M 4  in  FIG. 4(   a ) is implemented by a PMOS transistor. Alternatively, the PMOS transistor M 4  can be replaced by an NMOS transistor M 5 , as is shown in  FIG. 7 . The driving circuit shown in  FIG. 7  is similar to that of  FIG. 4(   a ) except that the two ends of the switch SW 2  are coupled to the drain and the gate electrodes of the transistor M 5 , respectively, the two ends of the switch SW 3  are coupled to the drain electrode of the transistor M 5  and the N electrode of the organic light-emitting diode OLED, the P electrode of the organic light-emitting diode OLED is coupled to a source voltage Vdd, and the source electrode of the transistor M 5  is coupled to a ground voltage GND. The switches SW 1 ˜SW 3  are controlled by the first scan line, the second scan line and the third scan line, respectively. Likewise, after operations in the reset, the compensation, the data write-in and the emission states, the current passing through the organic light-emitting diode OLED is not affected by the threshold voltage of the transistor M 5 . 
     From the above description, it is understood that the pixel driving circuit and the pixel driving method provided by the present invention can effectively compensate the threshold voltage of the transistor M 4  or M 5 . Therefore, the current passing through the organic light-emitting diode OLED will be precisely controlled according to the driving voltage. The present invention is illustrated by referring to an organic light-emitting diode OLED. Nevertheless, the present invention can be applied to any current-controllable light-emitting device. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.