Abstract:
A light emitting circuit and a display device which supply a forward driving current to an organic electroluminescence element in response to generation of a light emit command to cause to light-emit the organic electroluminescence element, and supply a charging current to the organic electroluminescence element after the generation of the light emit command to charge the capacity component of the organic electroluminescence element.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a light emitting circuit for an organic electroluminescence element and a display device.  
           [0003]    2. Description of the Related Background Art  
           [0004]    An electroluminescence element (hereinafter referred to as ‘EL element’), which is a capacitive light emitting element, can be electrically expressed as an equivalent circuit, as shown in FIG. 1. As can be understood from FIG. 1, an element can be substituted by a constitution of a capacity component C and a component E having a diode characteristic coupled in parallel to the capacity component. Therefore, an EL element can be considered to be a capacitive light emitting element. When a light emitting driving DC voltage is applied between the electrodes of the EL element, electric charge is accumulated in the capacity component C. When the voltage across the electrodes exceeds the barrier voltage or the light emission threshold voltage peculiar to the EL element, electric current starts flowing from the electrode (the anode side of the diode component E) to an organic functional layer forming a light emitting layer. As a result, the EL element emits light at light intensity proportional to the current.  
           [0005]    The voltage V-current I-luminance L characteristic of the EL element is, as shown in FIG. 2, similar to the characteristic of a diode in that the current I is very small at a voltage lower than the light emission threshold voltage Vth and increases at a voltage higher than the light emission threshold voltage Vth. Further, the current I and the luminance L are nearly proportional to each other. The EL element shows light emitting luminance proportional to the current I which flows in accordance with a driving voltage V when the driving voltage applied to the EL element exceeds the light emission threshold voltage Vth, and shows no light emitting luminance when the driving voltage V applied to the EL element is equal to or lower than the light emission threshold voltage Vth.  
           [0006]    A display panel on which a plurality of EL elements are mounted in a matrix shape is already known. A display device that actively drives a display panel with EL elements is mounted with light emitting circuits configured as illustrated in FIG. 3 for respective pixels.  
           [0007]    As shown in FIG. 3, the light emitting circuit for a single pixel includes two FETs (Field Effect Transistor)  1  and  2  and a capacitor  3  so as to drive an EL element  5 . The gate G of the FET  1  is connected to a scan line Ai supplied with a scan signal, and the source S of the FET  1  is connected to a data line Bj supplied with a data signal. The Drain D of the FET  1  is connected to the gate G of the FET  2  and one terminal of the capacitor  3 . The source S of the FET  2  is connected to a common power supply line  6  as well as the other terminal of the capacitor  3 . The drain D of the FET  2  is connected to the anode of the EL element  5 . The cathode of the EL element  5  is grounded. The earth, to which the power supply line  6  and the cathode of the EL element  5  are connected, is connected to a power source (not shown).  
           [0008]    Although the El element  5  is shown in FIG. 3 by a symbol for a diode, it can more exactly be shown in an equivalent circuit as shown in FIG. 1. The same thing can be said for a later-stated EL element  25 .  
           [0009]    With regard to the operation of the light emitting circuit, first, a scan signal is supplied to the gate G of the FET  1  through the scan line Ai, and then, the FET  1  turns on to pass a current from the source S to the drain D. The current is flowed in accordance with the voltage of a data signal supplied to the source S. The capacitor  3  is charged during the period of the ON-state of the FET  1 , and the charged voltage is supplied to the gate G of the FET  2  to turn the FET  2  into the ON-state (active or saturation state). In the ON-state of the FET  2 , a driving current flows from the power supply line  6  through the source S and the drain D of the FET  2  and to the EL element  5  to make the EL element  5  emit light. When the supply of the scan signal to the gate G of the FET  1  disappears, the FET  1  turns into an OPEN-state. Then, the voltage of the gate G of the FET  2  is maintained by the accumulated charge in the capacitor  3 , whereby the driving current is maintained until a next scanning and the light emission of the EL element  5  is maintained as well.  
           [0010]    The light emitting luminance of the EL element  5  is controlled to obtain a display gradation in accordance with image data. Here, either a driving current modulation system, which controls luminance by the level of the driving current for each frame, or a frame modulation system, which controls the driving period in one frame with a constant current driving level, can be applied for luminance control. As shown in FIG. 4, the driving current modulation system uses the FET  1  in an active state in accordance with image data to vary the driving current, whereby luminance is varied for each frame. On the other hand, the frame modulation system, as shown in FIG. 5, divides one frame into a plurality of sub frames of SF 1 , SF 2 , SF 3 , . . . and uses the FET  1  in a saturation state only for the sub frame periods selected in accordance with image data to supply the driving current with a constant level, whereby light is emitted or not in the unit of sub frame.  
           [0011]    However, since an EL element has a capacity component as stated above, when the flow of a driving current into the EL element is started, the forward voltage of the EL element gradually increases by charge accumulated in its capacity component. It may take time for the forward voltage to exceed the light emission threshold voltage. Particularly in gradation driving of a driving current modulation system, the driving current is controlled in accordance with the display gradation of a pixel. If the driving current level is low as shown in FIG. 6A, the forward voltage of the EL element gradually increases to exceed the light emission threshold voltage Vth. Accordingly, as shown in FIG. 6B, the EL element emits light only for the last short period of one frame. The light emitting luminance gradually increases and is not constant, and thus it is not allowed to obtain desired luminance.  
           [0012]    In addition, even when flowing the driving current with a constant level at the start of supplying the driving current to the EL element, the time required for exceeding the light emission threshold voltage varies in accordance with the quantity of accumulated charge left in the capacity component of the EL element then. Particularly in a frame modulation system, if a large quantity of accumulated charge is left in the capacity component of the EL element, as shown in FIG. 7A, at the time of starting the supply of the driving current, the forward voltage of the EL element is present with a level in accordance with the quantity of accumulated charge, and the voltage starts increasing from this voltage level. Accordingly, it takes a short time for the voltage to exceed the light emission threshold voltage Vth, and the EL element, as shown in FIG. 7B, starts emitting light in a comparatively short time after the start of the supply of the driving current. On the other hand, if a quantity of accumulated charge is hardly left in the capacity component in the EL element, as shown in FIG. 8A, at the time of starting the supply of the driving current, the forward voltage level of the EL element in accordance with the quantity of accumulated charge is nearly zero volts, and the voltage starts increasing from this low voltage level of nearly zero volts. Accordingly, it takes a long time for the voltage to exceed the light emission threshold voltage Vth. The EL element, as shown in FIG. 8B, starts emitting light at a late time after the start of the supply of the driving current. As a result, even when supplying the driving current to the EL element with the same level for the same period of time, the light emitting luminance varies in accordance with the quantity of accumulated charge left in the capacity component of the EL element at the start of supplying the driving current to the EL element. Thus it is not allowed to obtain desired luminance.  
         SUMMARY OF THE INVENTION  
         [0013]    An object of the present invention is to provide an active driving light emitting circuit, which can provide desired luminance whatever the quantity of accumulated charge of an EL element at the start of supply of a driving current to the EL element is, and also to provide a display device using the circuit.  
           [0014]    A light emitting circuit according to the present invention makes an organic electroluminescence element emit light by supplying, in response to generation of a light emit command, a forward driving current to the organic electroluminescence element, the light emitting circuit comprising a charging current supply device which supplies a charging current to the organic electroluminescence element so as to charge a capacity component of the organic electroluminescence element after the generation of the light emit command.  
           [0015]    A display device according to the present invention comprises: a display panel having a plurality of drive lines, a plurality of scan lines intersecting with the plurality of drive lines, and a plurality of sets of an organic electroluminescence element and a light emitting circuit of an active driving method, the plurality of sets being arranged at a plurality of intersecting positions by the drive lines and the scan lines, respectively; and a controller which supplies a scan signal to one scan line of the plurality of scan lines in sequence at a specified timing and supplies a data signal to a data line of the plurality of data lines which is associated to at least an organic electroluminescence element to be driven to emit light on the one scan line, wherein the light emitting circuit includes: a switching element which turns on in response to the scan signal to allow the data signal to pass therethrough; a capacitor which is charged by the data signal supplied through the switching element; an EL driving element which activates in accordance with a charged voltage of the capacitor to supply a driving current to the organic electroluminescence element in the same set; and a charging current supply device which supplies a charging current to the organic electroluminescence element in the same set immediately after the supply of the scan signal to charge a capacity component of the organic electroluminescence element in the same set.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates an equivalent circuit of an EL element;  
         [0017]    [0017]FIG. 2 schematically illustrates the driving voltage-current-light emitting luminance characteristic of the EL element;  
         [0018]    [0018]FIG. 3 is a diagram illustrating a configuration of a conventional light emitting circuit;  
         [0019]    [0019]FIG. 4 is the light emitting luminance characteristic diagram of an EL element by a light emitting circuit to which gradation driving of a driving current modulation system is applied;  
         [0020]    [0020]FIG. 5 is the light emitting luminance characteristic diagram of an EL element by a light emitting circuit to which gradation driving of a frame modulation system is applied;  
         [0021]    [0021]FIG. 6A and FIG. 6B respectively illustrate the forward voltage and luminance of an EL element in the case of gradation driving of a driving current modulation system;  
         [0022]    [0022]FIG. 7A and FIG. 7B respectively illustrate the forward voltage and luminance of an EL element in the case of gradation driving of a frame modulation system;  
         [0023]    [0023]FIG. 8A and FIG. 8B respectively illustrate the forward voltage and luminance of an EL element for gradation driving of a frame modulation system;  
         [0024]    [0024]FIG. 9 is a block diagram illustrating the configuration of a display device according to the present invention;  
         [0025]    [0025]FIG. 10 is a circuit diagram illustrating a configuration of a light emitting circuit in the device in FIG. 9;  
         [0026]    FIGS.  11 A- 11 E are waveform diagrams illustrating the operation of the light emitting circuit in the FIG. 10;  
         [0027]    [0027]FIG. 12 is a circuit diagram illustrating another configuration of the light emitting circuit in the device in FIG. 9;  
         [0028]    [0028]FIG. 13 is a circuit diagram illustrating still another configuration of the light emitting circuit in the device in FIG. 9;  
         [0029]    [0029]FIG. 14 illustrates a voltage superimposing circuit for supplying a data signal to the light emitting circuit in FIG. 13;  
         [0030]    FIGS.  15 A- 15 D are waveform diagrams illustrating the operation of the light emitting circuit in FIG. 13;  
         [0031]    [0031]FIG. 16 illustrates another voltage superimposing circuit for supplying a data signal to the light emitting circuit in FIG. 13;  
         [0032]    [0032]FIG. 17 is a circuit diagram illustrating another configuration of the light emitting circuit in the device in FIG. 9;  
         [0033]    FIGS.  18 A- 18 E are waveform diagrams illustrating the operation of the light emitting circuit in FIG. 16; and  
         [0034]    [0034]FIG. 19 is a circuit diagram illustrating another configuration of the light emitting circuit in the device in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Preferred embodiments of the invention will be described as follows, with reference to the drawings.  
         [0036]    [0036]FIG. 9 illustrates a display device using a matrix display panel according to the present invention. The display device includes a display panel  11 , a scan line driving circuit  12 , a data line driving circuit  13 , a charging control line driving circuit  14 , and a controller  15 .  
         [0037]    The display panel  11  is an active matrix type constituted of m×n pixels, and has EL light emitting circuits  11   1,1 - 11   m,n  for respective pixels as shown in FIG. 9. The EL light emitting circuits  11   1,1 - 11   m,n  all have the same configuration, and are connected to the scan line driving circuit  12  through scan lines A 1 -An, to the data line driving circuit  13  through data lines B 1 -Bm, and to the charging control line driving circuit  14  through charging control lines C 1 -Cn. The controller  15  generates a scan control signal, a data control signal, and a charging control signal in accordance with input image data. The scan control signal indicates a scan line to be selected and is supplied to the scan line driving circuit  12 . The data control signal indicate the data lines corresponding to EL elements to be driven to emit light and is supplied to the data line driving circuit  13 . The charging control signal indicates the charging control lines corresponding to the EL elements to be driven to emit light and is supplied to the charging control line driving circuit  14 .  
         [0038]    The scan line driving circuit  12 , the data line driving circuit  13 , and the charging control line driving circuit  14 , which are not shown specifically, are respectively constituted of a power source and a switch circuit. The scan line driving circuit  12  selects from the scan lines A 1 -An in sequence for each frame in accordance with a scanning control signal, and supply a scan signal to a selected scan line. The data line driving circuit  13  selects from the data lines B 1 -Bm in accordance with a light emitting control signal, and supplies a data signal to a selected data line. The charging control driving circuit  14  selects from the charging control lines C 1 -Cn in accordance with a charging control signal, and supplies a charging signal to a charging control line.  
         [0039]    As described above, since the light emitting circuits  11   1,1 - 11   m,n  all have the same configuration, the configuration of the light emitting circuit  11   1,1  will be explained as follows.  
         [0040]    The light emitting circuit  11   1,1 , as shown in FIG. 10, includes three FETs  21 - 23  and a capacitor  24  to drive an EL element  25 . The gate G of the FET  21  is connected to a scan line A 1  to which a scan signal is supplied, and the source S of the FET  21  is connected to a data line B 1  to which a data signal is supplied. The drain D of the FET  21  is connected to the gate G of the FET  22  and one terminal of the capacitor  24 . The source S of the FET  22  is connected to a common power supply line  26  as well as the other terminal of the capacitor  24 . The drain D of the FET  22  is connected to the anode of the EL element  5  as well as the drain D of the FET  23 , and the cathode of the EL element  25  is grounded. The source S of the FET  23  is connected to a power supply line  27 , and the gate G thereof is connected to a charging control line C 1 . A specified voltage V A  is supplied to the power supply line  26 , and a specified voltage V S  is supplied to the power supply line  27 .  
         [0041]    With regard to the operation of the light emitting circuit  11   1,1  first, a scan signal is supplied to the gate G of the FET  21  through the scan line A 1 , and at the same time, a charging signal is supplied to the gate G of the FET  23  through the charging control line C 1 . The scan signal is a pulse voltage having a waveform as shown in FIG. 11B. The charging signal is a pulse voltage having a waveform as shown in FIG. 11A and has a shorter pulse width than that of a scan signal.  
         [0042]    The FET  21  is turned on by the supply of the scan signal, and flows a current in accordance with the voltage of the data signal supplied to the source S through the data line B 1  from the source S to the drain D. The capacitor  24  is charged, and its voltage is supplied to the gate G of the FET  22  to turn the FET  22  on (saturation or active state). On the other hand, the FET  23  turns on by the supply of the charging signal. Therefore, the FET  22  and the FET  23  almost simultaneously turn on. Consequently, the FET  22  supplies the EL element  25 , from the specified Voltage V A  with a driving current in accordance with the data signal supplied to the gate G, and the specified voltage V S  is applied to the EL element  25  through the source S—drain D of the FET  23 . If the EL element  25  has a small quantity of accumulated charge then, a driving current by the specified voltage V S  flows to the EL element  25  through the source S—drain D of the FET  23 . This driving current flows in order to charge the capacity component of the EL element  25  rapidly. That is, a driving current flows into the EL element  25  as shown in FIG. 11C. As the capacity component of the EL element  25  is charged, the driving current decreases. When the charging signal disappears, the driving current in accordance with the data signal supplied to the gate G of the FET  22  flows into the EL element  25 . This driving current by the specified voltage V A  flows with a constant level.  
         [0043]    By supplying the driving current to the EL element  25  in this way, a voltage is applied to the EL element  25 , for example as shown in FIG. 11D, with a constant level, and the light emitting luminance level of the EL element  25  is almost constant, as shown in FIG. 11E, from the start of supplying the driving current to the EL element  25 .  
         [0044]    [0044]FIG. 12 shows another embodiment of the configuration of the light emitting circuit  11   1,1 . The light emitting circuit  11   1,1  in FIG. 12 includes three FETs  21 - 23 , a capacitor  24 , and an EL element  25  as in the circuit of FIG. 10. The light emitting circuit  11   1,1  in FIG. 12 is different from the circuit in FIG. 10 in that, instead of a charging signal, a scan signal is supplied to the gate G of the FET  23  in addition to the gate G of the FET  21  in the circuit  11   1,1  in FIG. 12. Accordingly, a driving current by the specified voltage V S  flows into the EL element  25  through the source S—drain D of the FET  23  during the full period when the scan signal is being supplied, whereby the capacity component of the EL element  25  is rapidly charged. In a display device using the light emitting circuit  11   1,1  in FIG. 12, neither a charging control line driving circuit  14  nor charging control lines C 1 -Cn are needed.  
         [0045]    [0045]FIG. 13 shows still another embodiment of the configuration of the light emitting circuit  11   1,1  The circuit  11   1,1  in FIG. 13 is not provided with the FET  23  that is included in the circuit in FIG. 10, but is provided with the two FETs  21  and  22 , capacitor  24 , and EL element  25  only. In other words, the light emitting circuit  11   1,1  in FIG. 13 has the same configuration as one shown in FIG. 3. Also, in a display device using the light emitting circuit  11   1,1  in FIG. 13, neither a charging control line driving circuit  14  nor charging control lines C 1 -Cn are needed.  
         [0046]    In the data line driving circuit  13  or the controller  15  of the display device, as shown in FIG. 14, a voltage for charging is added to a data signal by a voltage adding circuit  30 . This addition is applied to each data signal line. To each of the light emitting circuits  11   1,1 - 11   m,n  the data signal having a waveform as shown in FIG. 15A is supplied from the data line driving circuit  13 . The signal level is higher by the voltage for charging during a specified period from the start of the supply of the data signal, and becomes a normal level after the specified period. The voltage for charging is set in accordance with the gradation of the pixel corresponding to the image data for each of the light emitting circuits.  
         [0047]    Also, as shown in FIG. 16, the voltage for charging and the data signal may be switched by a changeover switch  40  in accordance with the charging signal.  
         [0048]    With regard to the operation of the light emitting circuit  11   1,1  in FIG. 13, first, the scan signal is supplied to the gate G of the FET  21  through the scan line A 1  as shown in FIG. 15B, and then, the FET  21  turns on in accordance with the scan signal to flow a current from the source S to the drain D in accordance with the voltage of the data signal supplied to the source S through the data line B 1 . The capacitor  24  is charged, and its voltage is supplied to the gate G of the FET  22  to make it an ON-state (saturation or active state). Due to the ON-state of the FET  22 , the driving current in accordance with the data signal supplied to the gate G of the FET  22  flows into the EL element  25 . Since the signal level is higher by the voltage for charging during a specified period from the start of supply of the data signal, the level of the driving current increases, as shown in FIG. 15C, for a specified period, whereby the capacity component of the EL element  25  is rapidly charged. After the specified period, the level of the data signal returns to a normal signal level. Consequently, the resistance between the source S—drain D of the FET  22  increases, whereby the driving current decreases. Therefore, the light emitting luminance of the EL element  25  increases rapidly from the start of supply of the driving current to the EL element  25  as shown in FIG. 15D, and then maintains almost a constant level.  
         [0049]    [0049]FIG. 17 shows still another embodiment of the configuration of the light emitting circuit  11   1,1 . The light emitting circuit  11   1,1  in FIG. 17 is not provided with the FET  23  that is provided to the circuit in FIG. 10, but provided with the two FETs  21  and  22 , capacitor  24 , and EL element  25 , and still further provided with FETs  31  and  32 . The source S of the FET  31  is connected to the power supply line  26 , and its drain D is connected to the source S of the FET  32 . The drain D of the FET  32  is connected to the anode of the EL element  25  as well as the drain D of the FET  22 . The gate G of the FET  31  is connected to the charging control line C 1 , and the gate G of the FET  32  is connected to the connection line of the gate G of the FET  22 . When the FET, 32  turns into an ON-state, FET  22  also turns into an ON-state. In this case, the circuit is designed such that the current flowing through the FET  32  is  3  times as high as the current flowing through the FET  22  while both FETs are in the On-state.  
         [0050]    With regard to the operation of the light emitting circuit  11   1,1  in FIG. 17, when a scan signal is supplied to the gate G of the FET  21  through the scan line A 1 , a charging signal is simultaneously supplied to the gate G of the FET  32  through the charging control line C 1 . The scan signal is a pulse voltage having a waveform as shown in FIG. 18A. The charging signal is a pulse voltage having a waveform as shown in FIG. 18B, and for example, has a pulse width shorter than the pulse width of the scan signal.  
         [0051]    The FET  21  turns on in response to the supply of the scan signal, and flows a current from the source S to the drain D in accordance with the voltage of the data signal (FIG. 18C) that is supplied to the source S through the data line B 1 . The capacitor  24  is charged, and its voltage is supplied to the respective gates G of the FETs  22  and  32  to turn the FETs  22  and  32  on. On the other hand, the FET  31  turns on by the supply of the charging signal. Accordingly, the FETs  22 ,  31 , and  32  turn on almost at the same time, and then, the driving current through the source S—drain D of the FET  22  and the driving current through the source S—drain D of the FET  31  and through the source S—drain D of the FET  32  flow into the EL element  25 .  
         [0052]    As described above, when the FET  32  is in the ON-state, a current flowing into the FET  32  is  3  times as high as the current flowing into the FET  22  which turns on simultaneously. Accordingly, high current, as shown in FIG. 18D, flows into the EL element  25  for the period when the charging signal is supplied. If Id is defined as the driving current through the source S—drain D of the FET 22 , then the driving current through the source S—drain D of the FET  31  and through the source S—drain D of the FET  32  is added to Id, whereby  4 Id flows into the EL element  25 . As a result, the capacity component of the EL element  25  is rapidly charged by the driving current of  4 Id during the period when this charging signal is supplied.  
         [0053]    When the charging signal disappears, the FET  31  turns off, and the supply of the driving current to the EL element  25  through the source S—drain D of the FET  31  and through the source S—drain D of the FET  32  is stopped. As a result, only the driving current of Id by the FET  22  is supplied to the EL element  25 . Accordingly, the light emitting luminance of the EL element  25 , as shown in FIG. 18E, rapidly increases from the start of the supply of the driving current to the EL element  25 , and then maintains almost a constant luminance level.  
         [0054]    [0054]FIG. 19 illustrates yet another embodiment of the configuration of the light emitting circuit  11   1,1 . The light emitting circuit  11   1,1  in FIG. 19 is a variation of the circuit configuration in FIG. 17, wherein a voltage signal V H  in accordance with the display gradation of each pixel is supplied to the gate G of the FET  32  from the controller  15 . The rest of the configuration is the same as that of the circuit in FIG. 17, and the operation of the circuit  11   1,1  in FIG. 19 is the same as that of the circuit in FIG. 17.  
         [0055]    In the embodiments described above, light emitting circuits for a single pixel are explained. In the case of color display, three light emitting circuits of RGB form one pixel.  
         [0056]    As stated above, according to the present invention, desired luminance can be obtained whatever the quantity of accumulated charge of the EL element is at the start of the supply of the driving current to the EL element.  
         [0057]    This application is based on a Japanese Patent Application No. 2001-372883 which is hereby incorporated by reference.