Abstract:
A driver circuit for an EL element is proposed. The driver circuit for the EL element includes a first TFT, a second TFT, a third TFT, a storage capacitor, and an EL element. The EL element includes an anode connected to a first supply voltage and a cathode connected to a source of the first TFT. A source of the second TFT is connected to a drain of the first TFT. A source of the third TFT is connected to a drain of the second TFT. A drain of the third TFT is connected to the ground. Brightness of the EL element can be prevented from being lowered due to EL element degradation by adopting the method of such connections.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to display technology, and more particularly, to a driver circuit for an electroluminescent (EL) element. 
     2. Description of the Prior Art 
     Electroluminescent displays (ELDs) grab people&#39;s attention because the ELDs have many advantages such as wide viewing angles, quick response, excellent color gamut, etc. So the ELDs are gradually accepted and used in products, especially in cellphones, media players, small introductory TV sets. 
     The ELDs should be equipped with a driver circuit for driving an EL element to illuminate. Please refer to  FIG. 1  showing a schematic diagram of a driver circuit for an EL element in the conventional technology. The conventional driver circuit for the EL element includes a switch thin-film transistor (TFT) T 1 ′, a driver TFT T 2 ′, a storage capacitor C′, and an EL element D′. The driver circuit for the EL element is realized in that an electric signal is charged and stored in the storage capacitor C′ at the sampling phase. The electric signal is in a relation with the brightness of the EL element D′. The electric signal stored in the storage capacitor C′ ensures that the EL element D′ keeps shining at the holding phase. However, the conventional EL element D′ may have a degradation in operation, resulting in variations of the resistance of the EL element D′. The brightness of the EL element D′ is thus greatly affected by the degradation of the EL element D′. As a result, the ELDs do not have good display effects. 
     Therefore, a driver circuit for an EL element has to be provided for solving the above-mentioned problem. 
     SUMMARY OF THE INVENTION 
     A driver circuit for an EL element is proposed in the present invention for avoiding the negative effects on the brightness of the EL element due to EL element degradation. 
     According to the present invention, a driver circuit for an electroluminescent (EL) element comprises a first thin-film transistor (TFT), a second TFT, a third TFT, a fourth TFT, a fifth TFT, a sixth TFT, a storage capacitor, and an EL element. The second TFT comprises a source connected to a drain of the first TFT. The third TFT comprises a source connected to a drain of the second TFT and a drain connected to the ground. The storage capacitor comprises a first terminal connected to a gate of the second TFT and a second terminal connected to the drain of the second TFT. The fourth TFT comprises a source connected to a second supply voltage and a drain connected to the source of the second TFT. The fifth TFT comprises a source connected to the drain of the fourth TFT and a drain connected to the gate of the second TFT. The sixth TFT comprises a source connected to the drain of the second TFT and a drain connected to a data signal. The EL element comprises an anode connected to a first supply voltage and a cathode connected to a source of the first TFT. A duty cycle of the driver circuit for the EL element comprises a sampling phase and a holding phase, the first TFT, the second TFT, and the third TFT are conducted at the holding phase, an impedance of the first TFT, the second TFT, and the third TFT in cascode is greater than an impedance of the EL element. The fourth transistor, the fifth transistor, and the sixth transistor are cut-off in the holding phase. The second transistor, the fourth transistor, the fifth transistor, and the sixth transistor are conducted at the sampling phase, while the first transistor and the third transistor are cut-off at the sampling phase. The storage capacitor is used for storing an electric signal across the gate of the second TFT and the drain of the second TFT at the sampling stage and for holding a voltage which is between the gate of the second TFT and the drain of the second TFT and varies with the electric signal at the holding phase for controlling brightness of the EL element. 
     In one aspect of the present invention, gates of the fourth TFT, the fifth TFT, and the sixth TFT are all connected to a first gate signal, gates of the first TFT and the third TFT are both connected to a second gate signal, which is the reversed-phase first gate signal, and the first gate signal and the second gate signal are used for controlling the fourth TFT, the fifth TFT, and the sixth TFT to be cut-off at the holding phase and for controlling the first TFT, the second TFT, and the third TFT to be conducted at the holding phase. 
     According to the present invention, a driver circuit for an electroluminescent (EL) element comprises a first thin-film transistor (TFT), a second TFT, a third TFT, a storage capacitor, and an EL element. The second TFT comprises a source connected to a drain of the first TFT. The third TFT comprises a source connected to a drain of the second TFT and a drain connected to the ground. The storage capacitor comprises a first terminal connected to a gate of the second TFT and a second terminal connected to the drain of the second TFT. The EL element comprises an anode connected to a first supply voltage and a cathode connected to a source of the first TFT. A duty cycle of the driver circuit for the EL element comprises a sampling phase and a holding phase. The first TFT, the second TFT, and the third TFT are conducted at the holding phase. The storage capacitor is used for storing an electric signal between the gate of the second TFT and the drain of the second TFT at the sampling stage and for holding a voltage which is between the gate of the second TFT and the drain of the second TFT and varies with the electric signal at the holding phase for controlling brightness of the EL element. 
     In one aspect of the present invention, the driver circuit for the EL element further comprises a fourth TFT, a fifth transistor, and a sixth transistor. The fourth TFT comprises a source connected to a second supply voltage and a drain connected to the source of the second TFT. The fifth TFT comprises a source connected to the drain of the fourth TFT and a drain connected to the gate of the second TFT. The sixth TFT comprises a source connected to the drain of the second TFT and a drain connected to a data signal. The second TFT, the fourth TFT, the fifth TFT, and the sixth TFT are conducted at the sampling phase, and the first TFT and the third TFT are cut-off at the holding phase. 
     In another aspect of the present invention, gates of the fourth TFT, the fifth TFT, and the sixth TFT are all connected to a first gate signal. Gates of the first TFT and the third TFT are both connected to a second gate signal, which is the reversed-phase first gate signal, and the first gate signal and the second gate signal are used for controlling the fourth TFT, the fifth TFT, and the sixth TFT to be cut-off at the holding phase and for controlling the first TFT, the second TFT, and the third TFT to be conducted at the holding phase. 
     In another aspect of the present invention, an voltage level of the first gate signal at the sampling phase is reverse to an voltage level of the first gate signal at the holding phase. The first gate signal and the second gate signal are also used for controlling the first TFT and the third TFT to be cut-off at the sampling phase and for controlling the second TFT, the fourth TFT, the fifth TFT, and the sixth TFT to be conducted at the sampling phase. 
     In another aspect of the present invention, the first TFT, the second TFT the third TFT, the fourth TFT, the fifth TFT, and the sixth TFT are all N-type IGZO (indium gallium zinc oxide) TFTs. The first gate signal is at a high voltage level at the sampling phase and at a low voltage level at the holding phase. 
     In another aspect of the present invention, duration at the holding phase is longer than duration at the sampling phase when the driver circuit for the EL element is during a duty cycle. 
     In another aspect of the present invention, the EL element is an organic EL element. 
     In another aspect of the present invention, the first TFT, the second TFT the third TFT, the fourth TFT, the fifth TFT and the sixth TFT are all N-type IGZO TFTs. 
     In still another aspect of the present invention, the sixth TFT is a current source at the sampling phase equivalently. 
     In yet another aspect of the present invention, a current flowing through the current source is determined by both of the data signal and the first gate signal. 
     Differing from the conventional technology, a plurality of TFTs are cascode-connected for forming a driver circuit for the EL element working at the passage at the holding phase in the present invention. The driver circuit for the EL element can avoid the brightness of the EL element from being affected due to the EL element degradation. An in-cell current source of a pixel is equipped for compensating uniformity and reliability of a threshold voltage and electron mobility of the TFTs and an IR drop of the de supply voltage at lower costs. These are the merits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a conventional driver circuit for an EL element. 
         FIG. 2  shows a schematic diagram of a driver circuit for an EL element according to a preferred embodiment of the present invention. 
         FIG. 3  shows an equivalent circuit diagram (Norton equivalent) of the driver circuit for the EL element at the holding phase according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is illustrated in detail in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings. 
     Please refer to  FIG. 2  showing a schematic diagram of a driver circuit for an EL element according to a preferred embodiment of the present invention. The driver circuit for the EL element comprises a first TFT T 1 , a second TFT T 2 , a third TFT T 3 , a fourth TFT T 4 , a fifth TFT T 5 , a sixth TFT T 6 , an EL element D, and a storage capacitor C. The first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  are all N-type IGZO (indium gallium zinc oxide) TFTs in the present invention. In other embodiments, the first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  are another kind of TFT. The EL element D is preferably an OLED (organic light-emitting diode) in the present invention. In other embodiments, the EL element D is another kind of element. 
     The EL element DI comprises an anode connected to a first supply voltage V 1  and a cathode connected to a source of the first TFT T 1 . A source of the second TFT T 2  is connected to a drain of the first TFT T 1 . A source of the third TFT  13  is connected to a drain of the second TFT T 2 . A drain of the third TFT  13  is connected to the ground. The storage capacitor C comprises a first terminal and a second terminal. The first terminal is connected to a gate of the second TFT T 2 , and the second terminal is connected to the drain of the second TFT T 2 . A source of the fourth TFT T 4  is connected to a second supply voltage V 2 . The source of the second TFT T 2  is connected to a drain of the fourth TFT T 4 . A source of the sixth TFT′ T 6  is connected to the drain of the second TFT T 2 . A drain of the sixth TFT T 6  is connected to a data signal Vdata. A source of the fifth TFT T 5  is connected to the drain of the fourth TFT T 4 . The gate of the second TFT T 2  is connected to a drain of the fifth TFT T 5 . The gates of the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  are all connected to a first gate signal Vsel. The gates of the first TFT T 1  and the third TFT T 3  are both connected to a second gate signal Vsel′, which is the reversed-phase first gate signal Vsel. 
     The duty cycle of the driver circuit for the EL element comprises a sampling phase and a holding phase. When the driver circuit for the EL element is during a duty cycle, the duration at the holding phase is longer than the duration at the sampling phase. The storage capacitor C is used for storing the electric signal between the gate of the second TFT T 2  and the drain of the second TFT T 2  at the sampling phase and for holding the voltage which is between the gate of the second TFT T 2  and the drain of the second TFT T 2  and varies with the electric signal at the holding phase. In this way, the brightness of the EL element D can be controlled. The voltage level of the first gate signal Vsel at the sampling phase is reverse to that of the first gate signal Vsel at the holding phase. The first gate signal Vsel is at a high voltage level at the sampling phase and at a low voltage level at the holding phase in the present embodiment. The first gate signal Vsel can be at a low voltage level at the sampling phase while can be at a high voltage level at the holding phase correspondingly in another embodiment. 
     Below in conjunction with the accompanying drawings, the working principle of the driver circuit for the EL element will be described in further detail. 
     When the first gate signal Vsel is at the high voltage level, the second gate signal Vsel′ is at the low voltage level. At this time, the first TFT T 1  and the third TFT  13  are cut-off, and the second TFT T 2 , the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  are conducted. The driver circuit for the EL element works at the sampling phase. The second TFT T 2 , the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  form a passage between the second supply voltage V 2  and the data signal Vdata. The storage capacitor C is connected to the gate of the second TFT T 2  and to the drain of the second TFT T 2  in parallel. The storage capacitor C is between the gate of the second TFT T 2  and the drain of the second TFT T 2 . The storage capacitor C records the electric signal between the gate of the second TFT T 2  and the drain of the second TFT T 2 . In other words, the storage capacitor C records variations of voltage between the gate of the second TFT T 2  and the drain of the second TFT T 2 . When the first gate signal Vsel is at the low voltage level, the second gate signal Vsel′ is at the high voltage level. At this time, the fourth TFT T 4 , the fifth TFT T 5 , and the sixth TFT T 6  are cut-off at the holding phase, and the first TFT T 1 , the second TFT T 2 , and the third TFT T 3  are conducted at the holding phase. The driver circuit for the EL element works at the holding phase. The first TFT T 1 , the second TFT T 2 , the third TFT T 3  form a passage between the first supply voltage V 1  and the ground GND. The storage capacitor C is connected to the gate of the second TFT T 2  and to the drain of the second TFT T 2  in parallel. The storage capacitor C is between the gate of the second TFT T 2  and the drain of the second TFT T 2 . The voltage between the gate of the second TFT T 2  and the drain of the second TFT T 2  varies with the electric signal. In this ways, the brightness of the EL element D is properly controlled. 
     Please refer to  FIG. 3  showing an equivalent circuit diagram of the driver circuit for the EL element at the holding phase according to the embodiment of the present invention. The passage of the driver circuit for the EL element at the holding phase is equivalently the circuit diagram as shown in  FIG. 2  according to Norton&#39;s theorem where Rout represents an output equivalent impedance after the first TFT T 1 , the second TFT T 2 , and the third TFT T 3  are cascode-connected. A current source Is and the admittance Rout are connected in parallel. A load port is connected to the EL element D. The current I of the current source Is is the current of the load port with short circuit current. From the structure of the circuit, the EL, element D has the current Id as: 
     
       
         
           
             
               
                 
                   Id 
                   = 
                   
                     I 
                     × 
                     
                       
                         R 
                         out 
                       
                       
                         
                           R 
                           out 
                         
                         + 
                         
                           R 
                           D 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where R D  represents the resistor of the EL element D. 
     Rout represents an equivalent output impedance after the first TFT T 1 , second TFT T 2 , and the third TFT T 3  are cascode-connected, so Rout&gt;&gt;R D . When the EL element D degrades, that is, R D  changes, the current Id of the EL element D nearly does not change as shown in Formula (1), which implies that the EL element D degradation will not cause instability of illumination of the EL element D. 
     Further, referring to  FIG. 2 , the sixth TFT T 6  is a current source at the sampling phase equivalently. The current I 6  flowing through the current source is determined by both of the data signal Vdata and the first gate signal Vsel. Specifically, the current source is an in-cell current source. In other words, the current source is built in the pixel. The working principle of the in-cell current source of the pixel is as follows: because the sixth TFT T 6  reaches saturation at the sampling phase, a formula for the drain current of a field-effect transistor which reaches saturation is: 
     
       
         
           
             
               
                 
                   
                     I 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   = 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     μ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     nCox 
                     ⁢ 
                     
                       W 
                       L 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           Vsel-Vdata-Vth 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where μn represents electron mobility. Cox represents a gate-insulator capacitor per unit area, W/L represents the width-to-length ratio of the sixth TFT T 6 , and Vth represents a threshold voltage of the sixth TFT T 6 . 
     As mentioned previously, μn, Cox, W/L, and Vth maintain unchanged on the whole duty cycle of the driver circuit for the EL element, so the current I 6  flowing through the in-cell current source of the pixel is determined by both of the data signal Vdata and the first gate signal Vsel, as shown in Formula (2). 
     Below in conjunction with the embodiment, the function of the in-cell current source of the pixel will be described in further detail. 
     Take the second TFT T 2  for example. Firstly, the degradation of the first TFT T 1 , the second TFT T 2 , and the third TFT T 3  has to be taken into consideration because the duration at the holding phase is much longer than the duration at the sampling phase. When the second TFT T 2  degrades, the threshold voltage of the second TFT T 2  increases and/or the electron mobility of the second TFT T 2  decreases. The voltage level of the gate of the second TFT  1 T 2  automatically rises at the sampling phase for keeping the current I 2  unchanged. (The current I 6  is determined by the data signal Vdata and the first gate signal Vsel. The current I 2  flowing through the second TFT T 2  is the current I 6 . Thus, the current I 2  will not vary with the degradation of the second TFT T 2 .) The voltage is stored in the storage capacitor C. At the holding phase, the current Id (Id=I 2 ) of the EL element keeps unchanged. Thus, the in-cell current source of the pixel can compensate uniformity and reliability of the threshold voltage and the electron mobility of the second TFT T 2 . 
     The second TFT T 2  is used again for explanation. Secondly, the second supply voltage V 2  lowers when a supply voltage suffers an IR drop, which causes the voltage between the source of the second TFT T 2  and the drain of the second TFT T 2  to be lower. At the sampling phase, the voltage level of the gate of the second TFT T 2  rises automatically due to I 2 =I 6 . The voltage is stored in the storage capacitor C so that the current Id (Id=I 2 ) keeps unchanged at the holding phase. So the in-cell current source of the pixel can compensate an IR drop of the dc supply voltage. 
     Differing from the conventional technology, the plurality of TFTs are cascode-connected for forming the driver circuit for the EL element working at the passage at the holding phase in the present invention. The driver circuit for the EL element can avoid the brightness of the EL element D from being affected due to the EL element degradation. The in-cell current source of the pixel can compensate uniformity and reliability of the threshold voltage and electron mobility of the TFTs and an IR drop of the dc supply voltage. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.