Abstract:
A driving circuit for an electro-luminescence (EL) cell includes an EL cell, and a supply circuit selectively applying current to the EL cell based on a pixel signal from a data line. A control circuit controls current flow from the supplying circuit to the EL cell such that an amount of current for discriminating between gray scale levels is approximately tens of micro-amps.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an electro-luminescence display (ELD), and more particularly to a driving circuit for driving electro-luminescence cells arranged on an electro-luminescence panel in a matrix type.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, an electro-luminescence (EL) panel converts an electrical signal into light energy to thereby display a picture corresponding to video signals (or image signals). Such an EL panel includes EL cells arranged at intersections between gate lines and data lines. Each of the EL cells responds to a pixel signal from the data line to generate a light corresponding to a magnitude of the pixel signal.  
           [0005]    In order to stably apply a pixel signal to each EL cell, the EL panel has cell-driving circuits scanned sequentially line-by-line. Each of the EL cell-driving circuits responds to a control signal at the gate line to sample a pixel signal at the data line and then holds the sampled pixel signal during the next frame interval, to thereby stably apply the pixel signal to the EL cell.  
           [0006]    As shown in FIG. 1, a conventional EL cell-driving circuit for carrying out such sampling and holding operations of a pixel signal includes a first PMOS thin film transistor (TFT) M 1  connected between an EL cell ELC and a first node N 1 . A gate of the first PMOS TFT M 1 , is connected to a second node N 2 , and the EL cell ELC is also connected to ground. A second PMOS TFT M 2  is connected between the second node N 2  and the EL cell ELC, and is connected at its gate to a gate line CL. A capacitor C 1  is connected between the first and second nodes N 1  and N 2 .  
           [0007]    The capacitor C 1  charges a voltage of a pixel signal when the pixel signal is applied from a data line DL and applies the charged pixel voltage to gate electrodes of the first PMOS TFT M 1 . The first PMOS TFT M 1  is turned on by the pixel voltage charged in the capacitor C 1 , thereby allowing a supply voltage VDD applied, via the first node N 1 , from a voltage supply line VDDL to be supplied to the EL cell ELC.  
           [0008]    At this time, the first PMOS TFT M 1  varies its channel width depending on a voltage level of the pixel signal to control a current amount applied to the EL cell ELC. Then, the EL cell ELC generates a light corresponding to the current amount applied from the first PMOS TFT M 1 . The second PMOS TFT M 2  responds to a gate signal GLS, as shown in FIG. 2, applied from the gate line GL to selectively connect the second node N 2  to the EL cell ELC.  
           [0009]    More specifically, the second PMOS TFT M 2  connects the second node N 2  to the EL cell ELC at a time interval when the gate signal GLS is enabled at a low logic, thereby allowing the pixel signal to be charged in the capacitor C 1 . In other words, the second PMOS TFT M 2  forms a current path of the capacitor Cl at a time interval when the gate signal GLS at the gate line GL is enabled. The capacitor C 1  charges the pixel signal in the enabling interval of the gate signal GLS, thereby allowing the gate electrode of the first PMOS TFT M 1  to have a lower voltage than the drain electrode by a voltage level of the charged pixel signal. Accordingly, a channel width of the first PMOS TFT M 1  is controlled in accordance with a voltage level of the pixel signal to determine a current amount flowing from the first node N 1  into the EL cell ELC.  
           [0010]    The conventional EL cell driving circuit further includes a third PMOS TFT M 3 , connected between the data line DL and the first node N 1 , responding to a gate signal at the gate line GL, and a fourth PMOS TFT M 4 , connected between the voltage supply line VDDL and the first node N 1 , responding to an inverted gate signal /GLS from a gate bar line /GL.  
           [0011]    The third PMOS TFT M 3  is turned on at a time interval when a low logic of gate signal is applied from the gate line GL, thereby connecting the capacitor C 1 , coupled to the first node N 1  and the source electrode of the first PMOS TFT M 1 , to the data line DL. In other words, the third PMOS TFT M 3  responds to a low logic of gate signal GLS to send a pixel signal at the data line DL to the first node N 1 .  
           [0012]    As a result, the third PMOS TFT M 3  is turned on during a time interval when a gate signal at the gate line GL remains at a low logic, thereby charging a pixel signal into the capacitor Cl connected between the first and second nodes N 1  and N 2 . The fourth PMOS TFT M 4  is turned on in a time interval when a low logic of inverted gate signal /GLS from the gate bar line /GL is applied to the gate electrode thereof, thereby connecting the first node N 1  to which the capacitor C 1  and the source electrode of the first PMOS TFT M 1  are connected, to the voltage supply line VDDL.  
           [0013]    At a time interval when the fourth PMOS TFT M 4  has been turned on, a supply voltage VDD at the voltage supply line VDDL is applied, via the first node N 1  and the first PMOS TFT M 1 , to the EL cell ELC. Thus, The EL cell ELC generates a light of a quantity according to a voltage level of the pixel signal.  
           [0014]    In the conventional EL cell driving circuit, a maximum current amount (i.e., a current margin of a pixel signal) required for obtaining a maximum brightness is small. For this reason, a current difference between gray scale levels of a video signal is approximately several μA. if a current difference between gray scale levels is set to several μA, a data driver integrated circuit (IC) chip must have the ability to control current at a range of several μA accurately. However, it is very difficult to manufacture a data driver IC chip capable of controlling a current at a range of several μA accurately. As a result, the conventional EL cell driving circuit has problems with driving the conventional EL panel to accurately display a gray scale of a picture.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention provides a driving circuit for an electro-luminescence cell that increases a current difference of a pixel signal for identifying gray scale levels.  
           [0016]    The present invention also provides an electro-luminescence panel that more accurately displays a gray scale of a picture.  
           [0017]    In the electro-luminescence (EL) panel according to the present invention, the driving circuit for an electro-luminescence (EL) cell includes an EL cell; a supply is circuit selectively applying current to the EL cell based on a pixel signal from a data line; and a control circuit controlling current flow from the supplying circuit to the EL cell such that an amount of current for discriminating between gray scale levels is approximately tens of micro-amps.  
           [0018]    According to an embodiment of the present invention, the supplying circuit includes a first transistor connected between the EL cell and a voltage supply line, and the control circuit includes a second transistor connected between the data line and the voltage supply line such that the first and second transistors form a current mirror. In a preferred embodiment, the second transistor has a channel width of 3 to 20 times greater than a channel width of the first transistor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a circuit diagram showing a configuration of a driving circuit for a conventional electro-luminescence cell;  
         [0021]    [0021]FIG. 2 is a waveform diagram of driving signals applied to the gate line and the gate bar line shown in FIG. 1; and  
         [0022]    [0022]FIG. 3 is a circuit diagram showing a configuration of a driving circuit for an electro-luminescence cell according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    Referring to FIG. 3, there is shown a driving circuit for an electro-luminescence (EL) cell according to an embodiment of the present invention.  
         [0024]    The EL cell driving circuit includes an EL cell ELC connected to ground, and a first PMOS TFT MP 1  connected between the EL cell ELC and a voltage supply line VDDL. A second PMOS TFT MP 2  is connected between a first node and the voltage supply line VDDL, and both of the gates of the first and second PMOS TFTs MP 1  and MP 2  are connected to a second node N 2 . The first and second PMOS TFTs MP 1  and MP 2  form a current mirror between the first node N 1  and the voltage supply line VDDL. A capacitor C is connected between the second node N 2  and the voltage supply line VDDL. Third and fourth PMOS TFTs MP 3  and MP 4  are connected in series between a data line DL and the second node N 2 . The gates of the third and fourth PMOS TFTs MP 3  and MP 4  are connected to a gate line GL.  
         [0025]    The operation of the driving circuit in FIG. 3 will be described assuming the third and fourth PMOS TFTs MP 3  and MP 4  are on. Then, the operation of the third and fourth PMOS TFTs MP 3  and MP 4  will be described.  
         [0026]    The capacitor C charges to a difference voltage corresponding to a difference between a voltage of a pixel signal and a supply voltage VDD at the voltage supply line VDDL when the pixel signal is applied from a data line DL, and commonly applies the difference voltage to the gate electrodes of the first and second PMOS TFTs MP 1  and MP 2 . The first PMOS TFT MP 1  is turned on by the difference voltage charged in the capacitor C and applies the supply voltage VDD at the voltage supply line VDDL to the EL cell ELC. At this time, a channel width of the first PMOS TFT MP 1  is varied depending on a voltage level of the pixel signal to control an amount of current applied from the voltage supply line VDDL to the EL cell ELC. Then, the EL cell ELC generates light corresponding to the amount of current applied, via the first PMOS TFT MP 1 , from the voltage supply line VDDL. Meanwhile, the second PMOS TFT MP 2  controls a current amount flowing from the voltage supply line VDDL into the data line DL when a pixel signal is applied from the data line DL, thereby determining an amount of current applied to the EL cell ELC via the first PMOS TFT MP 1 .  
         [0027]    A channel width of the second PMOS TFT MP 2 , which determines a current amount flowing via the first PMOS TFT MP 1  as mentioned above, is formed to be equal to several to tens of times the channel width of the first PMOS TFT MP 1 . For instance, a channel width ratio of the first PMOS TFT MP 1  to the second PMOS TFT MP 2  may be in a range of 1:3 through 1:20. To the contrary, if the channel width ratio is in a range of 3:1 through 10:1, then it has an advantage in power consumption. Making the channel widths of the first and second PMOS TFTs MP 1  and MP 2  different from each other as described above, increases an amount of the current difference in the pixel signal for discriminating gray scale levels to approximately tens of μA. Even though a current amount flowing via the second PMOS TFT MP 2  is varied at a difference of tens of μA by such a pixel signal, a current amount applied, via the first PMOS TFT MP 1  having a channel width as small as several to tens of times the channel width of the second PMOS TFT MP 2 , to the EL cell ELC is varied at a difference of several μA. Accordingly, an EL panel driving IC chip driving the data line DL can be made such that it generates a pixel signal corresponding to a gray scale of video signal or image signal. Furthermore, the EL panel can display a gray scale of a picture with the aid of such a data line driving IC chip.  
         [0028]    The third PMOS TFT MP 3  is turned on at a time interval when a low logic of gate signal is applied from the gate line GL, thereby connecting the drain electrode of the third PMOS TFT MP 3  connected to the first node N 1  to the data line DL. In other words, the third PMOS TFT MP 3  plays a role to send a pixel signal at the data line DL in response to a tow logic of the gate signal. The fourth PMOS TFT MP 4  also is turned on at a time interval when a low logic gate signal is applied from the gate line GL to the gate electrode thereof, thereby connecting the second node N 2  via the first node N 1  to the data line DL. In other words, the third and fourth PMOS TFTs MP 3  and MP 4  are turned on at a time interval when a gate signal at the gate line GL remains at a low logic, thereby charging the pixel signal into the capacitor C connected between the second node N 2  and the voltage supply line VDDL.  
         [0029]    As described above, according to the present invention, a channel width of a PMOS TFT, forming part of a current mirror and responding to a pixel signal, is enlarged by several to tens of times the channel width of the other PMOS TFT in the current mirror that supplies current to the EL cell, thereby increasing an amount of the current difference in the pixel signal for discriminating gray scale levels. Accordingly, the present EL cell driving circuit permits manufacturing a data line driving IC chip suitable for realizing a gray scale of a picture. Also, it permits an EL panel to accurately display a gray scale of a picture.  
         [0030]    Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. For example, the PMOS transistors MP 1  through MP 4  included in the embodiment of the present invention shown in FIG. 3 can be replaced with NMOS transistors, In this case, the gate signal to be applied to the  5  gate line GL has a waveform the same as /GLS of FIG. 2. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents,