# Patent

## Title
Display apparatus and display-apparatus driving method

## Abstract

Disclosed herein is a driving method for driving a display apparatus, the display apparatus including: N×M light emitting units; M scan lines; N data lines; a driving circuit provided for each of the light emitting units to serve as a circuit having a signal writing transistor, a device driving transistor, a capacitor and a first switch circuit; and a light emitting device.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the present invention relates to a display apparatus and a driving method for driving the display apparatus. More particularly, the present invention relates to a display apparatus employing light emitting units, which each have a light emitting device and a driving circuit for driving the light emitting device, and relates to a driving method for driving the display apparatus.
2. Description of the Related Art
As already known in general, there is a light emitting unit having a light emitting device and a driving circuit for driving the light emitting device. A typical example of the light emitting device is an organic EL (Electro Luminescence) light emitting device. In addition, a display apparatus employing the light emitting units is also already commonly known. The luminance of light emitted by the light emitting unit is determined by the magnitude of the driving current. A typical example of such a display apparatus is an organic EL display apparatus which employs organic EL light emitting devices. In addition, in the same way as a liquid-crystal display apparatus, the display apparatus employing the light emitting units adopts one of commonly known driving methods such as a simple matrix method and an active matrix method. In comparison with the simple matrix method, the active matrix method has a demerit that the active matrix method entails a complicated configuration of the driving circuit. However, the active matrix method offers a variety of merits such as a capability of increasing the luminance of light emitted by the light emitting device.
As already known, there are a variety of active-matrix driving circuits which each employ transistors and a capacitor. Such a driving circuit serves as a circuit for driving the light emitting device included in the same light emitting unit as the driving circuit. For example, Japanese Patent Laid-open No. 2005-31630 discloses an organic EL display apparatus employing light emitting units, which each have an organic EL light emitting device and a driving circuit for driving the organic EL light emitting device, and discloses a driving method for driving the organic EL display apparatus. The driving circuit employs six transistors and one capacitor. In the following description, the driving circuit employing six transistors and one capacitor is referred to as a 6Tr/1C driving circuit. FIG. 15 is a diagram showing an equivalent circuit of the 6Tr/1C driving circuit included in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix in which N×M light emitting units employed in a display apparatus are laid out. It is to be noted that the light emitting units are sequentially scanned by a scan circuit 101 in row units on a row-after-row basis.
The 6Tr/1C driving circuit employs a signal writing transistor TRW, a device driving transistor TRD and a capacitor C1 in addition to a first transistor TR1, a second transistor TR2, a third transistor TR3 and a fourth transistor TR4.
A specific one of the source and drain areas of the signal writing transistor TRW is connected to a data line DTLn whereas the gate electrode of the signal writing transistor TRW is connected to a scan line SCLm. A specific one of the source and drain areas of the device driving transistor TRD is connected to the other one of the source and drain areas of the signal writing transistor TRW through a first node ND1. A specific one of the terminals of the capacitor C1 is connected to a first power-supply line PS1 to which a reference voltage is applied. In the typical light emitting unit shown in the diagram of FIG. 15, the reference voltage is a reference voltage VCC to be described later. The other one of the terminals of the capacitor C1 is connected to the gate electrode of the device driving transistor TRD through a second node ND2. The scan line SCLm is connected to the scan circuit 101 whereas the data line DTLn is connected to a signal outputting circuit 102.
A specific one of the source and drain areas of the first transistor TR1 is connected to the second node ND2 whereas the other one of the source and drain areas of the first transistor TR1 is connected to the other one of the source and drain areas of the device driving transistor TRD. The first transistor TR1 serves as a first switch circuit connected between the second node ND2 and the other one of the source and drain areas of the device driving transistor TRD.
A specific one of the source and drain areas of the second transistor TR2 is connected to a third power-supply line PS3 to which a predetermined initialization voltage VIni for initializing an electric potential appearing on the second node ND2 is applied. The initialization voltage VIni is typically −4 volts. The other one of the source and drain areas of the second transistor TR2 is connected to the second node ND2. The second transistor TR2 serves as a second switch circuit connected between the second node ND2 and the third power-supply line PS3 to which the predetermined initialization voltage VIni is applied.
A specific one of the source and drain areas of the third transistor TR3 is connected to the first power-supply line PS1 to which the predetermined reference voltage VCC of typically 10 volts is applied. The other one of the source and drain areas of the third transistor TR3 is connected to the first node ND1. The third transistor TR3 serves as a third switch circuit connected between the first node ND1 and the first power-supply line PS1 to which the predetermined reference voltage VCC is applied.
A specific one of the source and drain areas of the fourth transistor TR4 is connected to the other one of the source and drain areas of the device driving transistor TRD whereas the other one of the source and drain areas of the fourth transistor TR4 is connected to a specific one of the terminals of a light emitting device ELP. The specific one of the terminals of the light emitting device ELP is the anode electrode of the light emitting device ELP. The fourth transistor TR4 serves as a fourth switch circuit connected between the other one of the source and drain areas of the device driving transistor TRD and the specific terminal of the light emitting device ELP.
The gate electrodes of the signal writing transistor TRW and the first transistor TR1 are connected to the scan line SCLm whereas the gate electrode of the second transistor TR2 is connected to a scan line SCLm−1 provided for a matrix row right above a matrix row associated with the scan line SCLm. The gate electrodes of the third transistor TR3 and the fourth transistor TR4 are connected to a third/fourth-transistor control line CLm.
Each of the transistors is a TFT (Thin Film Transistor) of a p-channel type. The light emitting device ELP is provided typically on an inter-layer insulation layer which is created to cover the driving circuit. The anode electrode of the light emitting device ELP is connected to the other one of the source and drain areas of the fourth transistor TR4 whereas the cathode electrode of the light emitting device ELP is connected to a second power-supply line PS2 for supplying a cathode voltage VCat of typically −10 volts to the cathode electrode. Reference notation CEL denotes the parasitic capacitance of the light emitting device ELP.
It is impossible to prevent the threshold voltage of a TFT from varying to a certain degree from transistor to transistor. Variations of the threshold voltage of the device driving transistor TRD cause variations of the magnitude of a driving current flowing through the light emitting device ELP. If the magnitude of the driving current flowing through the light emitting device ELP varies from a light emitting unit to another, the uniformity of the luminance of the display apparatus deteriorates. It is thus necessary to prevent the magnitude of the driving current flowing through the light emitting device ELP from being affected by variations of the threshold voltage of the device driving transistor TRD. As will be described later, the light emitting device ELP is driven in such a way that the luminance of light emitted by the light emitting device ELP is not affected by variations of the threshold voltage of the device driving transistor TRD.
By referring to diagrams of FIGS. 16A and 16B, the following description explains a driving method for driving an light emitting device ELP employed in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column of a two-dimensional matrix in which N×M light emitting units employed in a display apparatus are laid out. FIG. 16A is a model timing diagram showing timing charts of signals appearing on the scan line SCLm−1, the scan line SCLm and the third/fourth-transistor control line CLm. On the other hand, FIG. 16B and FIGS. 16C and 16D are model circuit diagrams showing the turned-on and turned-off states of the transistors employed in the driving circuit. For the sake of convenience, in the following description, the scan period in which the scan line SCLm−1 is scanned is referred to as the (m−1)th horizontal scan period whereas the scan period in which the scan line SCLm is scanned is referred to as the mth horizontal scan period.
As shown in the timing diagram of FIG. 16A, during the (m−1)th horizontal scan period, a second-node electric-potential initialization process is carried out. The second-node electric-potential initialization process is explained in detail by referring to the circuit diagram of FIG. 16B as follows. At the beginning of the (m−1)th horizontal scan period, an electric potential appearing on the scan line SCLm−1 is changed from a high level to a low level but an electric potential appearing on the third/fourth-transistor control line CLm is conversely changed from a low level to a high level. It is to be noted that, at that time, an electric potential appearing on the scan line SCLm is sustained at a high level. Thus, during the (m−1)th horizontal scan period, each of the signal writing transistor TRW, the first transistor TR1, the third transistor TR3 and the fourth transistor TR4 is put in a turned-off state whereas the second transistor TR2 is put in a turned-on state.
In these states, the initialization voltage VIni for initializing the second node ND2 is applied to the second node ND2 by way of the second transistor TR2 which has been set in a turned-on state. Thus, during this period, the second-node electric-potential initialization process is carried out.
Then, as shown in the timing diagram of FIG. 16A, during the mth horizontal scan period, the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state so that the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. During this mth horizontal scan period, a threshold-voltage cancelling process is also carried out. To put it concretely, the second node ND2 is electrically connected to the other one of the source and drain areas of the device driving transistor TRD. When the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state, the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. As a result, the electric potential appearing on the second node ND2 rises to a level obtained by subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig.
The processes described above are explained in detail by referring to the diagrams of FIGS. 16A and 16C as follows. At the beginning of the mth horizontal scan period, the electric potential appearing on the scan line SCLm−1 is changed from a low level to a high level but the electric potential appearing on the scan line SCLm is conversely changed from a high level to a low level. It is to be noted that, at that time, the electric potential appearing on the third/fourth-transistor control line CLm is sustained at the high level. Thus, during the mth horizontal scan period, each of the signal writing transistor TRW and the first transistor TR1 is put in a turned-on state whereas each of the second transistor TR2, the third transistor TR3 and the fourth transistor TR4 is conversely put in a turned-off state.
The second node ND2 is electrically connected to the other one of the source and drain areas of the device driving transistor TRD through the first transistor TR1 which has been put in a turned-on state. When the electric potential appearing on the scan line SCLm is changed from a high level to a low level in order to put the signal writing transistor TRW in a turned-on state, the video signal VSig appearing on the data line DTLn is written into the first node ND1 by way of the signal writing transistor TRW. As a result, the electric potential appearing on the second node ND2 rises to a level obtained by subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig.
That is to say, if the electric potential appearing on the second node ND2 connected to the gate electrode of the device driving transistor TRD has been initialized at a level putting the device driving transistor TRD in a turned-on state at the beginning of the mth horizontal scan period by carrying out the second-node electric-potential initialization process during the (m−1)th horizontal scan period, the electric potential appearing on the second node ND2 rises toward the video signal VSig applied to the first node ND1. As the difference in electric potential between the gate electrode and the specific one of the source and drain areas of the device driving transistor TRD attains the threshold voltage Vth of the device driving transistor TRD, however, the device driving transistor TRD is put in a turned-off state in which the electric potential appearing on the second node ND2 is about equal to an electric-potential difference of (VSig−Vth).
Later on, a driving current flows from the first power-supply line PS1 to the light emitting device ELP by way of the device driving transistor TRD, driving the light emitting device ELP to emit light.
The process is explained in detail by referring to the diagrams of FIGS. 16A and 16D as follows. At the beginning of a (m+1)th horizontal scan period not shown, the electric potential appearing on the scan line SCLm is changed from a low level to a high level. Afterwards, the electric potential appearing on the third/fourth-transistor control line CLm is changed conversely from a high level to a low level. It is to be noted that, at that time, the electric potential appearing on the scan line SCLm−1 is sustained at a high level. As a result, each of the third transistor TR3 and the fourth transistor TR4 is put in a turned-on state whereas each of the signal writing transistor TRW, the first transistor TR1 and the second transistor TR2 is conversely put in a turned-off state.
During the (m+1)th horizontal scan period, a driving voltage VCC is applied to the specific one of the source and drain areas of the device driving transistor TRD through the third transistor TR3 which has been put in the turned-on state. The other one of the source and drain areas of the device driving transistor TRD is connected to the specific electrode of the light emitting device ELP by the fourth transistor TR4 which has been put in the turned-on state.
Since the driving current flowing through the light emitting device ELP is a source-to-drain current Ids flowing from the source area of the device driving transistor TRD to the drain area of the same transistor, if the device driving transistor TRD is ideally operating in a saturated region, the driving current can be expressed by Eq. (A) given below. As shown in the circuit diagram of FIG. 16D, the source-to-drain current Ids is flowing to the light emitting device ELP, and the light emitting device ELP is emitting light at a luminance determined by the magnitude of the source-to-drain current Ids.

Ids=k*μ*(Vgs−Vth)2   (A)

In the above equation, reference notation μdenotes the effective mobility of the device driving transistor TRD whereas reference notation L denotes the length of the channel of the device driving transistor TRD. Reference notation W denotes the width of the channel of the device driving transistor TRD. Reference notation Vgs denotes a voltage applied between the source area of the device driving transistor TRD and the gate electrode of the same transistor. Reference notation C0X denotes a quantity expressed by the following expression:

(Specific dielectric constant of the gate insulation layer of the device driving transistor TRD)×(Vacuum dielectric constant)/(Thickness of the gate insulation layer of the device driving transistor TRD)

Reference notation k denotes an expression as follows:

k≡(1⁄2)*(W/L)*C0X 

The voltage Vgs applied between the source area of the device driving transistor TRD and the gate electrode of the same transistor is expressed as follows:

Vgs≈VCC−(VSig−Vth)   (B)

By substituting the expression on the right-hand side of Eq. (B) into the expression on the right-hand side of Eq. (A) to serve as a replacement of the term Vgs included in the expression on the right-hand side of Eq. (A), Eq. (C) can be derived from Eq. (A) as follows:




  
    
      
        
          
            
              
                I
                ds
              
              =
              
              ⁢
              
                k
                *
                μ
                *
                
                  
                    (
                    
                      
                        V
                        CC
                      
                      -
                      
                        (
                        
                          
                            V
                            Sig
                          
                          -
                          
                            V
                            th
                          
                        
                        )
                      
                      -
                      
                        V
                        th
                      
                    
                    )
                  
                  2
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                k
                *
                μ
                *
                
                  
                    (
                    
                      
                        V
                        CC
                      
                      -
                      
                        V
                        Sig
                      
                    
                    )
                  
                  2
                
              
            
          
        
      
    
    
      
        (
        C
        )
      
    
  




As is obvious from Eq. (C), the source-to-drain current Ids is not dependent on the threshold voltage Vth of the device driving transistor TRD. In other words, it is possible to generate the source-to-drain current Ids in accordance with the video signal VSig as a current flowing to the light emitting device ELP with a magnitude not affected by the threshold voltage Vth of the device driving transistor TRD. In accordance with the driving method described above, variations of the threshold voltage Vth of the device driving transistor TRD from transistor to transistor by no means have an effect on the luminance of light emitted by the light emitting device ELP.
SUMMARY OF THE INVENTION
In order to operate the driving circuit described above, the display apparatus additionally requires a separate power-supply line for supplying the driving voltage VCC, a separate power-supply line for supplying the cathode voltage VCat and a separate power-supply line for supplying the initialization voltage VIni. If the layouts of wires and the driving circuit are to be taken into consideration, however, it is desirable to provide only few power-supply lines.
In order to solve the problems described above, inventors of the present invention have innovated a display apparatus allowing the number of power-supply lines to be reduced and innovated a driving method for driving the display apparatus.
In order to solve the problems described above, there is provided a display apparatus according an embodiment of to the present invention or a display apparatus to which a driving method according to the embodiment of the present invention is applied. The display apparatus employs:
(1): N×M light emitting units laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction;
(2): M scan lines each stretched in the first direction; and
(3): N data lines each stretched in the second direction.
Each of the light emitting units includes:
(4): a driving circuit, which has a signal writing transistor, a device driving transistor, a capacitor and a first switch circuit; and
(5): a light emitting device for emitting light at a luminance according to a driving current output by the device driving transistor.
In each of the light emitting units,
(A-1): a specific one of the source and drain areas of the signal writing transistor is connected to one of the data lines;
(A-2): the gate electrode of the signal writing transistor is connected to one of the scan lines;
(B-1): a specific one of the source and drain areas of the device driving transistor is connected to the other one of the source and drain areas of the signal writing transistor through a first node;
(C-1): a specific one of the terminals of the capacitor is connected to a second power-supply line conveying a reference voltage determined in advance;
(C-2): the other one of the terminals of the capacitor is connected to the gate electrode of the device driving transistor through a second node;
(D-1): a specific one of the terminals of the first switch circuit is connected to the second node;
(D-2): the other one of the terminals of the first switch circuit is connected to the other one of the source and drain areas of the device driving transistor; and
(E): the driving circuit further has a second switch circuit connected between the second node and a data line.
The driving method provided for the display apparatus according to the embodiment of the present invention to serve as a driving method for solving the problems described above has a second-node electric-potential initialization process of applying a predetermined initialization voltage appearing on the data line to the second node by way of the second switch circuit put in a turned-on state and, then, putting the second switch circuit in a turned-off state in order to set an electric potential appearing on the second node at a reference electric potential determined in advance.
The display apparatus according to the embodiment of the present invention is provided with a second switch circuit connected between the second node and the data line. Thus, a predetermined initialization voltage appearing on the data line can be applied to the second node. Since a separate power-supply line for applying the initialization voltage determined in advance to the second node is not required, the number of power-supply lines can be reduced. To put it more concretely, during every scan period, the initialization voltage determined in advance needs to be asserted on the data line to be followed by a video signal asserted on the same data line to serve as a substitute for the initialization voltage. A ratio of a sub-period occupied by the initialization voltage determined in advance to a sub-period occupied by the video signal needs to be properly determined at a stage of designing the display apparatus.
As will be described later, in accordance with a driving method provided by the embodiment of the present invention to serve as a method for driving the display apparatus provided by the embodiment of the present invention, the second switch circuit is put in a turned-on state with a timing adjusted to a period used for asserting the initialization voltage determined in advance on the data line whereas the signal writing transistor is put in a turned-on state with a timing adjusted to a period used for asserting the video signal on the data line. Thus, even if the separate power-supply line for applying the initialization voltage determined in advance to the second node is eliminated, the display apparatus can be driven without causing any problems.
In addition, the driving method provided by the embodiment of the present invention to serve as a method for driving the display apparatus provided by the embodiment of the present invention has a signal writing process of changing an electric potential appearing on the second node toward an electric potential, which is obtained as a result of subtracting the threshold voltage of the device driving transistor from the voltage of a video signal appearing on the data line, by applying the video signal to the first node by way of the signal writing transistor which is put in a turned-on state by a signal appearing on the scan line when the first switch circuit is put in a turned-on state in order to put the second node in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor. In this case, it is possible to provide a desirable configuration in which, prior to the signal writing process, the second-node electric-potential initialization process described above is carried out. On top of that, the driving method also has a light emission process of allowing a driving current, which is generated by the device driving transistor by application of a driving voltage determined in advance to the first node, to flow to the light emitting device in order to drive the light emitting device to emit light. In this case, it is possible to provide a desirable configuration in which the light emission process is carried out after the signal writing process described above.
With the desirable configuration in which the light emission process is carried out after the signal writing process, it is possible to provide a desirable configuration including a second-node electric-potential correction process to be carried out between the signal writing process and the light emission process as a process of changing an electric potential appearing on the second node by applying a voltage having a magnitude determined in advance to the first node for a time period determined in advance with the first switch circuit already put in a turned-on state in order to put the second node in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor. In this case, the driving voltage mentioned above can be applied to the first node to serve as the voltage having a magnitude determined in advance.
The driving method provided for the display apparatus according to the embodiment of the present invention to serve as a driving method including the desirable configurations described above can be configured to make use of a voltage with a fixed magnitude as the initialization voltage. As an alternative, the driving method is configured to make use of a voltage with a magnitude varying in accordance with the video signal as the initialization voltage. By making use of a voltage with a fixed magnitude as the initialization voltage, the driving method offers a merit that the configuration of a circuit for supplying the initialization voltage can be made simple. By making use of a voltage with a magnitude varying in accordance with the video signal as the initialization voltage, on the other hand, the driving method offers a merit that the electric potential appearing on the second node can be changed in a short period of time by execution of the signal writing process toward an electric potential which is obtained as a result of subtracting the threshold voltage of the device driving transistor from the voltage of a video signal appearing on the data line.
In the case of the configuration of making use of a voltage with a magnitude varying in accordance with the video signal as the initialization voltage, the display apparatus is further provided with a voltage conversion circuit having a voltage reduction circuit. In this case, a video signal is supplied to the voltage conversion circuit and, in execution of the second-node electric-potential initialization process, the voltage reduction circuit employed in the voltage conversion circuit applies a voltage obtained as a result of subtracting a voltage having a constant magnitude from the voltage of the video signal to the data line as the initialization voltage.
The voltage conversion circuit and the voltage reduction circuit employed in the voltage conversion circuit are not prescribed in particular. In the case of a configuration in which the input side of the voltage conversion circuit is used for receiving a video signal and the output side of the voltage conversion circuit is connected to the data line, the video signal is supplied to the data line through the voltage reduction circuit in the execution of the second-node electric-potential initialization process. In the execution of the signal writing process, on the other hand, the video signal is supplied to the data line directly. The operation to supply the video signal to the data line through the voltage reduction circuit is properly switched to the operation to supply the video signal to the data line directly and vice versa by making use of a commonly known component such as a transistor. In addition, a circuit having a generally known configuration can be used as the voltage reduction circuit. The fact that the voltage reduction circuit can be implemented as a diode-wired transistor makes it convenient to manufacture typically the voltage reduction circuit and the driving circuit for driving the light emitting device by carrying out the same manufacturing process. For example, the voltage reduction circuit is designed as two diode-wired transistors connected to each other to form a series circuit. In this case, each of the diode-wired transistors and the device driving transistor can be designed as transistors of the same structure. In the case of a voltage reduction circuit designed as two diode-wired transistors connected to each other to form a series circuit, the voltage reduction circuit applies a voltage obtained as a result of subtracting twice the threshold voltage of the device driving transistor from the voltage of the video signal to the data line as the initialization voltage. The voltage reduction circuit designed as two diode-wired transistors connected to each other to form a series circuit offers a merit that the device driving transistor can be set in a turned-on state after the second-node electric-potential initialization process with a high degree of reliability.
A display apparatus according to the embodiment of the present invention and a display apparatus driven by adoption of a driving method according to the embodiment of the present invention are collectively referred to hereafter also as a display apparatus provided by the embodiment. It is possible to provide the display apparatus provided by the embodiment with a configuration in which the driving circuit further employs:
(F): a third switch circuit connected between the first node and a power-supply line conveying a driving voltage; and
(G): a fourth switch circuit connected between the other one of the source and drain areas of the device driving transistor and the specific one of the electrodes of the light emitting device.
In addition, it is possible to configure the driving method for driving the display apparatus provided by the embodiment to serve as a display apparatus including the desirable configurations described above to have the steps of:
(a): carrying out a second-node electric-potential initialization process of sustaining each of the first, third and fourth switch circuits in a turned-off state and applying the predetermined initialization voltage appearing on the data line to the second node by way of the second switch circuit put in a turned-on state and, then, putting the second switch circuit in a turned-off state in order to set an electric potential appearing on the second node at a reference electric potential determined in advance as the initialization voltage;
(b): carrying out a signal writing process of sustaining each of the second, third and fourth switch circuits in a turned-off state and putting the first switch circuit in a turned-on state to put the second node in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor so as to apply a video signal appearing on one of the data lines to the first node by way of the signal writing transistor put in a turned-on state by a signal appearing on one of the scan lines in order to change an electric potential appearing on the second node toward an electric potential obtained as a result of subtracting the threshold voltage of the device driving transistor from the video signal;
(c): applying a signal asserted on one of the scan lines to the gate electrode of the signal writing transistor later on in order put the signal writing transistor in a turned-off state; and
(d): carrying out a light emission process of putting the first switch circuit in a turned-off state, sustaining the second switch circuit in a turned-off state, applying a driving voltage determined in advance from the power-supply line to the first node by way of the third switch circuit which has already been put in a turned-on state and putting the other one of the source and drain areas of the device driving transistor in a state of being electrically connected to the specific one of the electrodes of the light emitting device by way of the fourth transistor put in a turned-on state so as to allow a driving current to flow from the device driving transistor to the light emitting device in order to drive the light emitting device.
In addition, it is possible to provide a configuration in which, between the steps (c) and (d), a second-node electric-potential correction process is carried out in order to change an electric potential appearing on the second node by applying the driving voltage as a voltage with a magnitude determined in advance to the first node for a period determined in advance with the first switch circuit sustained in a turned-on state and the third switch circuit put in a turned-on state.
In the display apparatus provided by the embodiment, it is possible to make use of a light emitting device emitting light at a luminance determined by the magnitude of a driving current flowing through the light emitting device to serve as the light emitting device employed in every light emitting unit included in the display apparatus. Typical examples of the light emitting device are an organic EL (Electro Luminescence) light emitting device, an inorganic EL light emitting device, an LED (light emitting diode) light emitting device and a semiconductor laser light emitting device. If construction of a color planar display apparatus is to be taken into consideration, it is desirable to make use of the organic EL light emitting device to serve as the light emitting device employed in every light emitting unit included in the display apparatus.
In the display apparatus provided by the embodiment, a reference voltage determined in advance is supplied to a specific one of the terminals of the capacitor. Thus, an electric potential appearing on the specific one of the terminals of the capacitor is sustained at the reference voltage determined in advance during an operation carried out by the display apparatus. The magnitude of the reference voltage determined in advance is not prescribed in particular. For example, it is also possible to provide a desirable configuration in which the specific one of the terminals of the capacitor is connected to a power line conveying the driving voltage and the driving voltage is applied as a reference voltage. As an alternative, it is also possible to provide a desirable configuration in which the specific one of the terminals of the capacitor is connected to a power line conveying a predetermined voltage to be applied to the other one of the electrodes of the light emitting device and the specific one of the terminals of the capacitor as a reference voltage.
In the display apparatus provided by the embodiment of the present invention as a display apparatus with the desirable configurations described above, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of a variety of lines such as the scan lines, the data lines and the power-supply lines. In addition, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of the light emitting device. To put it more concretely, if an organic EL light emitting device is used to serve as the light emitting device employed in every light emitting unit, typically, the organic EL light emitting device can be configured to include components such as an anode electrode, a hole transport layer, a light emitting layer, an electron transport layer and a cathode electrode. On top of that, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of a variety of circuits such as a scan circuit connected to the scan lines and a signal outputting circuit connected to the data lines.
The display apparatus provided by the embodiment of the present invention can have the configuration of the so-called monochrome display apparatus. As an alternative, the display apparatus provided by the embodiment of the present invention can have a configuration in which a pixel includes a plurality of sub-pixels. To put it more concretely, the display apparatus provided by the embodiment of the present invention can have a configuration in which a pixel includes three sub-pixels, i. e., a red-light emitting sub-pixel, a green-light emitting sub-pixel and a blue-light emitting sub-pixel. In addition, each of the three sub-pixels having types different from each other can be a set including an additional sub-pixel of a type determined in advance or a plurality of additional sub-pixels having types different from each other. For example, the set includes an additional sub-pixel for emitting light having the white color for increasing the luminance. As another example, the set includes an additional sub-pixel for emitting light having a complementary color for enlarging a color reproduction range. As a further example, the set includes an additional sub-pixel for emitting light having the yellow color for enlarging a color reproduction range. As a still further example, the set includes an additional sub-pixel for emitting light having the yellow and cyan colors for enlarging a color reproduction range.
Each of the signal writing transistor and the device driving transistor can be configured by making use of a TFT (Thin Film Transistor) of a p-channel type. It is to be noted that the signal writing transistor can be configured by making use of a TFT of an n-channel type. Each of the first, second, third and fourth switch circuits can be configured by making use of a commonly known switching device such as a TFT. For example, each of the first, second, third and fourth switch circuits can be configured by making use of a TFT of the p-channel type or a TFT of the n-channel type.
The capacitor employed in the driving circuit can be typically configured to include a specific electrode, another electrode and a dielectric layer sandwiched by the electrodes. The dielectric layer is an insulation layer. Each of the transistors and the capacitor, which compose the driving circuit, is created within a certain plane. For example, each of the transistors and the capacitor is created on a support body. If the light emitting device is an organic EL light emitting device for example, the light emitting device is created above the transistors and the capacitor composing the device driving transistor through the insulation layer. The other one of the source and drain areas of the device driving transistor is connected to a specific one of the electrodes of the light emitting device by way of another transistor. In the typical configuration shown in the diagram of FIG. 1, the specific electrode of the light emitting device is the anode electrode. Please be advised that it is possible to provide a configuration in which each of the transistors is created on a semiconductor substrate or the like.
The technical phrase ‘the specific one of the two source and drain areas of a transistor’ may be used to imply the source or drain area connected to a power supply in some cases. The turned-on state of a transistor is a state in which a channel has been created between the source and drain areas of the transistor. There is not raised a question as to whether a current is flowing from the specific one of the source and drain areas of the transistor to the other one of the source and drain areas of the transistor or vice versa in the turning-on state of the transistor. On the other hand, the turned-off state of a transistor is a state in which no channel has been created between the source and drain areas of the transistor. A particular one of the source and drain areas of a transistor is connected to a particular one of the source and drain areas of another transistor by creating the particular source and drain areas of the two transistors as areas occupying the same region. In addition, it is possible to create a source or drain area of a transistor from not only a conductive material, but also a layer made of substances of different kinds. Typical examples of the conductive material are poly-silicon and amorphous silicon which include impurities. The substances for making the layer include a metal, an alloy, conductive particles, a laminated structure of a metal, an alloy and conductive particles as well as an organic material (or a conductive polymer). In every timing chart referred to in the following description, the length of a time period along the horizontal axis representing the lapse of time is no more than a model quantity and does not necessarily represent a magnitude relative to a reference on the horizontal axis.
In the display apparatus provided by the embodiment of the present invention, the driving circuit further has a second switch circuit connected between the second node and the data line. The driving method can apply a predetermined initialization voltage to the second node. Thus, it is not necessary to separately provide a power-supply line for supplying the initialization voltage determined in advance. Thus, the number of power-supply lines can be reduced.
In accordance with a driving method provided by the embodiment of the present invention to serve as a method for driving the display apparatus provided by the embodiment of the present invention, the second switch circuit is put in a turned-on state with a timing adjusted to a period used for asserting the initialization voltage determined in advance on the data line whereas the signal writing transistor is put in a turned-on state with a timing adjusted to a period used for asserting the video signal on the data line. Thus, even if the separate power-supply line for applying the initialization voltage determined in advance to the second node is eliminated, the display apparatus can be driven without causing any problems.



BRIEF DESCRIPTION OF THE DRAWINGS
The innovations and features of the present invention will become clear from the following description of the preferred embodiments given with reference to the accompanying diagrams, in which:
FIG. 1 is a diagram showing an equivalent circuit of a driving circuit employed in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix of N×M light emitting units employed in a display apparatus according to a first embodiment;
FIG. 2 is a conceptual diagram showing the display apparatus according to the first embodiment;
FIG. 3 is a model cross-sectional diagram showing the cross section of a portion of the light emitting unit employed in the display apparatus shown in the conceptual diagram of FIG. 2;
FIG. 4 is a timing diagram showing a model of timing charts of signals involved in driving operations carried out by the display apparatus according to the first embodiment;
FIGS. 5A to 5D are model circuit diagrams showing turned-on and turned-off states of transistors in the driving circuit;
FIG. 6 is a diagram showing the equivalent circuit of a driving circuit included in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix of N×M light emitting units employed in a display apparatus according to a second embodiment;
FIG. 7 is a conceptual diagram showing the display apparatus according to the second embodiment;
FIG. 8 is a timing diagram showing a model of timing charts of signals involved in driving operations carried out by the display apparatus according to the second embodiment;
FIGS. 9A and 9B are model circuit diagrams showing turned-on and turned-off states of transistors in the driving circuit;
FIG. 10 is a diagram showing the equivalent circuit of a driving circuit included in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix of N×M light emitting units employed in a display apparatus according to a third embodiment;
FIG. 11 is a conceptual diagram showing the display apparatus according to the third embodiment;
FIG. 12 is a circuit diagram showing a model of a voltage conversion circuit employed in the third embodiment;
FIG. 13 is a model timing diagram showing timing charts referred to in explanation of operations carried out by the voltage conversion circuit as timing charts of turned-on and turned-off states of signal switching sections as well as turned-on and turned-off states of first and second transistors;
FIG. 14 is a model timing diagram showing timing charts of driving operations carried out by the display apparatus as timing charts to be referred to in explanation of a driving method according to the third embodiment;
FIG. 15 is a diagram showing the equivalent circuit of a driving circuit included in a light emitting unit located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix of N×M light emitting units employed in a display apparatus;
FIG. 16A is a model timing diagram showing timing charts of signals appearing on a scan line SCLm−1, a scan line SCLm and a third/fourth-transistor control line CLm; and
FIG. 16B to 16D are model circuit diagrams showing turned-on and turned-off states of transistors in the driving circuit.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the invention are explained by referring to diagrams as follows.
First Embodiment
A first embodiment implements a display apparatus provided by the present invention and a driving method provided by the present invention to serve as a method for driving the display apparatus. The display apparatus according to the first embodiment of the present invention is an organic EL (Electro Luminescence) display apparatus employing a plurality of light emitting units 10, which each have an organic EL light emitting device ELP and a driving circuit 11 for driving the organic EL light emitting device. In the following description, the light emitting unit is also referred to as a pixel circuit in some cases. First of all, an outline of the display apparatus is explained.
The display apparatus according to the first embodiment is a display apparatus employing a plurality of pixel circuits. Every pixel circuit is configured to include a plurality of sub-pixel circuits. Every sub-pixel circuit is the light emitting unit 10 which has a laminated structure composed of the driving circuit 11 and the light emitting device ELP connected to the driving circuit 11. FIG. 1 is a diagram showing an equivalent circuit of the driving circuit 11 employed in the light emitting unit 10 located at the intersection of an mth matrix row and an nth matrix column in a two-dimensional matrix in which N×M light emitting units 10 employed in a display apparatus are laid out to form a two-dimensional matrix composed of N columns and M rows where suffix or notation m denotes an integer having a value of 1, 2, . . . or M and notation n denotes an integer having a value of 1, 2, . . . or N. FIG. 2 is a conceptual diagram showing the display apparatus.
As shown in the conceptual diagram of FIG. 2, the display apparatus employs:
(1): N×M light emitting units 10 laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction;
(2): M scan lines SCL each stretched in the first direction; and
(3): N data lines DTL each stretched in the second direction.
Each of the scan lines SCL is connected to a scan circuit 101 whereas each of the data lines DTL is connected to a signal outputting circuit 102. The conceptual diagram of FIG. 2 shows 3×3 light emitting units 10 centered at a light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column. It is to be noted, however, that the configuration shown in the conceptual diagram of FIG. 2 is no more than a typical configuration. In addition, the conceptual diagram of FIG. 2 does not show the power-supply lines PS1 and PS2 shown in the diagram of FIG. 1 to serve as the first and second power-supply lines for conveying the power-supply voltage VCC and the cathode voltage VCat respectively.
In the case of a color display apparatus, the two-dimensional matrix composed of N matrix columns and M matrix rows has (N/3)×M pixel circuits. However, every pixel circuit is configured to include three sub-pixels, i. e., a red-light emitting sub-pixel, a green-light emitting sub-pixel and a blue-light emitting sub-pixel. Thus, the two-dimensional matrix has N×M sub-pixel circuits which are each the light emitting unit 10 described above. The light emitting units 10 are sequentially scanned by the scan circuit 101 in row units on a row-after-row basis at a display frame rate of FR times per second. That is to say, (N/3) pixel circuits (or N sub-pixel circuits each functioning as the light emitting unit 10) arranged along the mth matrix row are driven at the same time where suffix or notation m denotes an integer having a value of 1, 2, . . . or M. In other words, the light emission and no-light emission timings of the N light emitting devices 10 arranged along the mth matrix row are controlled in the same way.
The light emitting unit 10 employs a driving circuit 11 and a light emitting device ELP. The driving circuit 11 has a signal writing transistor TRW, a device driving transistor TRD, a capacitor C1 and a first switch circuit SW1 which is a first transistor TR1 to be described later. A driving current generated by the device driving transistor TRD flows to the light emitting device ELP. In the light emitting unit 10 located at the intersection of mth matrix row and the nth matrix column, a specific one of the source and drain areas of the signal writing transistor TRW is connected to the data line DTLn whereas the gate electrode of the signal writing transistor TRW is connected to the scan line SCLm. A specific one of the source and drain areas of the device driving transistor TRD is connected to the other one of the source and drain areas of the signal writing transistor TRW through a first node ND1. A specific one of the terminals of the capacitor C1 is connected to the first power-supply line PS1 for conveying a reference voltage determined in advance. In the case of the first embodiment shown in the diagram of FIG. 1, the reference voltage determined in advance is a predetermined driving voltage VCC to be described later. The other one of the terminals of the capacitor C1 is connected to the gate electrode of the device driving transistor TRD through a second node ND2.
Each of the device driving transistor TRD and the signal writing transistor TRW is a TFT of the p-channel type. The device driving transistor TRD is a depletion-type transistor. As will be described later, each of the first transistor TR1, the second transistor TR2, the third transistor TR3 and the fourth transistor TR4 is also a TFT of the p-channel type. It is to be noted that the signal writing transistor TRW can be implemented as a TFT of the n-channel type.
A commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of the scan circuit 101, the signal outputting circuit 102, the scan line SCL and the data line DTL. By the same token, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of the third/fourth-transistor controlling circuit 111 and the second-transistor controlling circuit 112 which will be described later.
In the same way as the scan circuit 101, the signal outputting circuit 102, the scan line SCL and the data line DTL, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of the third/fourth-transistor control line CS, the second-transistor control line CL2, the first power-supply line PS1 and the second power-supply line PS2 which will be described later.
FIG. 3 is a model cross-sectional diagram showing the cross section of a portion of the light emitting unit 10 employed in the display apparatus shown in the conceptual diagram of FIG. 2. As will be described later in detail, every transistor and the capacitor C1 which are employed in the driving circuit 11 of the light emitting unit 10 are created on a support body 20 whereas the light emitting device ELP is created over the transistors and the capacitor C1. Typically, a first inter-layer insulation layer 40 is sandwiched between the light emitting device ELP and the driving circuit 11 which employs the transistors and the capacitor C1. The organic EL light emitting device ELP has a commonly known configuration and a commonly known structure which include components such as an anode electrode, a hole transport layer, a light emitting layer, an electron transport layer and a cathode electrode. It is to be noted that the model cross-sectional diagram of FIG. 3 shows only the device driving transistor TRD while the other transistors are concealed and, thus, invisible. The other one of the source and drain areas of the device driving transistor TRD is connected to the anode electrode of the light emitting device ELP through the fourth transistor TR4 not shown in the model cross-sectional diagram of FIG. 3. A portion connecting the fourth transistor TR4 to the anode electrode of the light emitting device ELP is also concealed and, thus, invisible in the model cross-sectional diagram of FIG. 3.
The device driving transistor TRD is configured to include a gate electrode 31, a gate insulation layer 32 and a semiconductor layer 33. To put it more concretely, the device driving transistor TRD has a specific source or drain area 35 and the other source or drain-area 36 which are provided on the semiconductor layer 33 as well as a channel creation area 34. Sandwiched by the specific source or drain area 35 and the other source or drain area 36, the channel creation area 34 is a portion pertaining to the semiconductor layer 33. Each of the other transistors not shown in the model cross-sectional diagram of FIG. 3 has the same configuration as the device driving transistor TRD.
The capacitor C, has a capacitor electrode 37, a dielectric layer composed of an extension of the gate insulation layer 32 and another capacitor electrode 38. It is to be noted that a portion connecting the capacitor electrode 37 to the gate electrode 31 of the device driving transistor TRD and a portion connecting the capacitor electrode 38 to the second power-supply line PS2 are concealed and, thus, invisible.
The gate electrode 31 of the device driving transistor TRD, a portion of the gate insulation layer 32 of the device driving transistor TRD and capacitor electrode 37 of the capacitor C1 are created on the support body 20. Components such as the device driving transistor TRD and the capacitor Cl are covered by the first inter-layer insulation layer 40. On the first inter-layer insulation layer 40, the light emitting device ELP is provided. The light emitting device ELP has an anode electrode 51, a hole transport layer, a light emitting layer, an electron transport layer and a cathode electrode 53. It is to be noted that, in the model cross-sectional diagram of FIG. 3, the hole transport layer, the light emitting layer and the electron transport layer are shown as a single layer 52. On a portion pertaining to the first inter-layer insulation layer 40 as a portion on which the light emitting device ELP does not exist, a second inter-layer insulation layer 54 is provided. On the second inter-layer insulation layer 54 and the cathode electrode 53, a transparent substrate 21 is placed. Light emitted by the light emitting layer is radiated to the outside of the light emitting unit 10 by way of the transparent substrate 21. The cathode electrode 53 and the wire 39 serving as the second power-supply line PS2 are connected to each other by contact holes 56 and 55 provided on the second inter-layer insulation layer 54 and the first inter-layer insulation layer 40.
A method for manufacturing the display apparatus shown in the conceptual diagram of FIG. 2 is explained as follows. First of all, components are created properly on the support body 20 by adoption of an already known method. The components include lines such as the scan lines, the electrodes of the capacitor C1, the transistors each made of semiconductor layers, the inter-layer insulation layers and contact holes. Then, film-creation and patterning processes are carried out also by adoption of an already known method in order to form the light emitting device ELP. Subsequently, the support body 20 completing the processes described above is positioned to face the transparent substrate 21. Finally the surroundings of the support body 20 and the transparent substrate 21 are sealed in order to finish the process of manufacturing the display apparatus. Later on, if necessary, wiring to external circuits is provided.
Next, by referring to the diagrams of FIGS. 1 and 2, the following description explains the driving circuit 11 employed in the light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column. As described before, the other one of the source and drain areas of the signal writing transistor TRW is connected to the specific one of the source and drain areas of the device driving transistor TRD. On the other hand, the specific one of the source and drain areas of the signal writing transistor TRW is connected to the data line DTLn. Operations to put the signal writing transistor TRW in a turned-on and turned-off states are controlled by a signal asserted on the scan line SCLm connected to the gate electrode of the signal writing transistor TRW.
As will be described later in detail, the signal outputting circuit 102 asserts an initialization voltage VIni determined in advance or a video signal VSig for controlling the luminance of light emitted by the light emitting device ELP on the data line DTLn. The video signal VSig is also referred to as a driving signal or a luminance signal.
In a light emission state of the light emitting unit 10, the device driving transistor TRD is driven to generate a source-to-drain current Ids, the magnitude of which is expressed by Eq. (1) given below. In the light emission state of the light emitting unit 10, the specific one of the source and drain areas of the device driving transistor TRD is functioning as the source area whereas the other one of the source and drain areas of the device driving transistor TRD is functioning as the drain area. In order to make the following description easy to write just for the sake of convenience, in the following description, the specific one of the source and drain areas of the device driving transistor TRD is referred to as the source area whereas the other one of the source and drain areas of the device driving transistor TRD is referred to as the drain area in some cases. In Eq. (1) given below, reference notation μdenotes the effective mobility of the device driving transistor TRD whereas reference notation L denotes the length of the channel of the device driving transistor TRD. Reference notation W denotes the width of the channel of the device driving transistor TRD. Reference notation Vgs denotes a voltage applied between the source area of the device driving transistor TRD and the gate electrode of the same transistor. Reference notation Vth denotes the threshold voltage of the device driving transistor TRD. Reference notation C0X denotes a quantity expressed by the following expression:

(Specific dielectric constant of the gate insulation layer of the device driving transistor TRD)×(Vacuum dielectric constant)/(Thickness of the gate insulation layer of the device driving transistor TRD)

Reference notation k denotes an expression as follows:

k≡(1⁄2)*(W/L)*C0X 

Ids=k*μ*(Vgs−Vth)   (1)

The driving circuit 11 is provided with a first switch circuit SW1 connected between the second node ND2 and the other one of the source and drain areas of the device driving transistor TRD. The first switch circuit SW1 is implemented as the first transistor TR1. The specific one of the source and drain areas of the first transistor TR1 is connected to the second node ND2 whereas the other one of the source and drain areas of the first transistor TR1 is connected to the other one of the source and drain areas of the device driving transistor TRD. In the same way as the driving circuit described earlier by referring to the diagram of FIG. 15 in the section having a title of “BACKGROUND OF THE INVENTION,” in the case of the first embodiment, the gate electrode of the first transistor TR1 is connected to the scan line SCLm. Each of the first transistor TR1 and the signal writing transistor TRW is controlled by a signal asserted on the scan line SCLm.
In addition, the driving circuit 11 is also provided with a second switch circuit SW2 connected between the second node ND2 and the data line DTLn. The second switch circuit SW2 is implemented as the second transistor TR2. A specific one of the source and drain areas of the second transistor TR2 is connected to the data line DTLn whereas the other one of the source and drain areas of the second transistor TR2 is connected to the second node ND2. The gate electrode of the second transistor TR2 is connected to the second-transistor control line CL2m. The second-transistor control line CL2m is connected to the second-transistor controlling circuit 112. The second-transistor controlling circuit 112 applies a signal to the gate electrode of the second transistor TR2 by way of the second-transistor control line CL2m in order to control an operation to put the second transistor TR2 in a turned-on state or a turned-off state.
In addition, the driving circuit 11 is also provided with a third switch circuit SW3 connected between the first node ND1 and the first power-supply line PS1 for conveying the driving voltage VCC to be described later. On top of that, the driving circuit 11 is further provided with a fourth switch circuit SW4 connected between the other one of the source and drain areas of the device driving transistor TRD and a specific one of the electrodes of the light emitting device ELP. The third switch circuit SW3 is implemented as the third transistor TR3. A specific one of the source and drain areas of the third transistor TR3 is connected to the first power-supply line PS1 whereas the other one of the source and drain areas of the third transistor TR3 is connected to the first node ND1. The fourth switch circuit SW4 is implemented as the fourth transistor TR4. A specific one of the source and drain areas of the fourth transistor TR4 is connected to the other one of the source and drain areas of the device driving transistor TRD whereas the other one of the source and drain areas of the fourth transistor TR4 is connected to the specific one of the electrodes of the light emitting device ELP. The other electrode of the light emitting device ELP is the cathode electrode of the light emitting device ELP. The cathode electrode of the light emitting device ELP is connected to the second power-supply line PS2 for conveying a cathode voltage VCat to be described later. Reference notation CEL denotes the parasitic capacitance of the light emitting device ELP.
In the same way as the driving circuit described earlier in the section with a title of “BACKGROUND OF THE INVENTION” by referring to the diagram shown in FIG. 15, in the first embodiment, the gate electrodes of the third transistor TR3 and the fourth transistor TR4 are connected to the third/fourth-transistor control line CLm. The third/fourth-transistor control line CLm is connected to the third/fourth-transistor controlling circuit 111. The third/fourth-transistor controlling circuit 111 supplies a signal to the gate electrodes of the third transistor TR3 and the fourth transistor TR4 through the third/fourth-transistor control line CLm in order to put the third transistor TR3 and the fourth transistor TR4 in a turned-on state or a turned-off state.
In the explanation of the first an other embodiments, a variety of voltages and electric potentials have the following typical values even though the values shall be regarded as values merely used in the explanation and are not to be interpreted as limitations imposed on the voltages and the electric potentials. It is to be noted that, in the case of a third embodiment to be described later, a voltage with a magnitude varying in accordance with the video signal is used as the initialization voltage VIni. Thus, as described later, the initialization voltage VIni has a variety of magnitudes.
Reference notation VSig denotes a video signal for controlling the luminance of light emitted by the light emitting device ELP. The video signal VSig has a typical value in the range 0 volt representing the maximum luminance to 8 volts representing the minimum luminance.
Reference notation VCC denotes a driving voltage. The reference voltage VCC applied to the first power-supply line PS1 has a typical value of 10 volts.
Reference notation VIni denotes an initialization voltage serves as a voltage for initializing an electric potential appearing on the second node ND2. The initialization voltage VIni has a typical value of −4 volts.
Reference notation Vth denotes the threshold voltage of the device driving transistor TRD. The threshold voltage Vth has a typical value of 2 volts.
Reference notation VCat denotes a voltage applied to the second power-supply line PS2. The cathode voltage VCat has a typical value of −10 volts.
The following description explains driving operations carried out by the display apparatus on the light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column. In the following description, the light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column is also referred to simply as the (n, m)th light emitting unit 10 or the (n, m)th sub-pixel circuit. The horizontal scan period of the light emitting units 10 arranged along the mth matrix row is referred to hereafter simply as the mth horizontal scan period. To put it more concretely, the horizontal scan period of the light emitting units 10 arranged along the mth matrix row is the mth horizontal scan period of a currently displayed frame. The driving operations described below are also carried out on other embodiments to be described later.
A model of timing charts of signals involved in the driving operations carried out by the display apparatus is shown in the timing diagram of FIG. 4. FIGS. 5A and 5B are a plurality of model circuit diagrams referred to in description of driving operations carried out by the display apparatus. To be more specific, FIGS. 5A to 5D are model circuit diagrams showing turned-on and turned-off states of transistors in the driving circuit 11.
The driving method for the driving apparatus according to the first embodiment has a second-node electric-potential initialization process of applying a predetermined initialization voltage VIni appearing on the data line DTLn to the second node ND2 by way of the second switch circuit SW2 put in a turned-on state and, then, putting the second switch circuit SW2 in a turned-off state in order to set an electric potential appearing on the second node ND2 at a reference electric potential determined in advance. To put it more concretely, the second-node electric-potential initialization process is carried out during a period TP(1)0 shown in the timing diagram of FIG. 4.
The driving method according to the first embodiment has a signal writing process of changing an electric potential appearing on the second node ND2 toward an electric potential, which is obtained as a result of subtracting the threshold voltage Vth of the device driving transistor TRD from the voltage of a video signal VSig appearing the data line DTLn, by applying the video signal VSig to the first node ND1 by way of the signal writing transistor TRW which is put in a turned-on state by a signal appearing the scan line SCLm when the first switch circuit SW, is put in a turned-on state in order to put the second node ND2 in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD. It is to be noted that, after a second-node electric-potential initialization process has been completed, the signal writing process is carried out. To put it more concretely, the signal writing process is carried out during a period TP(1)1 shown in the timing diagram of FIG. 4.
As described above, in the case of the first embodiment, the initialization voltage VIni is a voltage having a fixed magnitude. Also in the case of a second embodiment to be described later, the initialization voltage VIni is a voltage having a fixed magnitude.
The driving method according to the first embodiment is provided with a light emission process of allowing a driving current, which is generated by the device driving transistor TRD by application of a driving voltage VCC determined in advance to the first node ND1, to flow to the light emitting device ELP in order to drive the light emitting device ELP to emit light. It is to be noted that the light emission process is carried out after a signal writing process. To put it more concretely, the light emission process is carried out in a period TP(1)2 immediately following a period TP(1)1 allocated to the signal writing process as shown in a timing diagram of FIG. 4. The following description explains details of an operation carried-out in each period shown in the timing diagram of FIG. 4.
Period TP(1)−1 (With Reference to FIGS. 4 and 5A)
The period TP(1)−1 serving as the period of a light emission process is the period in which the light emitting unit 10 serving as the (n, m)th sub-pixel circuit is in an immediately preceding light emission state of emitting light at a luminance according to a video signal V′Sig written right before. Each of the third transistor TR3 and the fourth transistor TR4 is put in a turned-on state whereas each of the signal writing transistor TRW, the first transistor TR1 and the second transistor TR2 is conversely put in a turned-off state. Through the light emitting device ELP employed in the light emitting unit 10 serving as the (n, m)th sub-pixel circuit, the source-to-drain current I′dds expressed by Eq. (4) to be described later is flowing. Thus, the light emitting device ELP employed in the light emitting unit 10 serving as the (n, m)th sub-pixel circuit is emitting light with a luminance determined by the source-to-drain current I′ds.
In every horizontal scan period, the signal outputting circuit 102 asserts the initialization voltage VIni on a data line DTLn before the signal outputting circuit 102 asserts a video signal VSig on the same data line DTLn to serve as a substitute for the initialization voltage VIni. To put it more concretely, in the (m−1)th horizontal period, the signal outputting circuit 102 asserts the initialization voltage VIni on the data line DTLn before the signal outputting circuit 102 asserts a video signal VSig—m−1 for the (n, m−1)th sub-pixel circuit on the same data line DTLn. Reference notation VSig−−1 denotes a video signal for the (n, m−1)th sub-pixel circuit. A video signal for any other sub-pixel circuit is denoted by a reference notation having the same format as VSig—m−1. Since each of the signal writing transistor TRW and the first transistor TR1 is sustained in a turned-off state, the electric potential (or the voltage) appearing on each of the first node ND1 and the second node ND2 does not change even if the electric potential (or the voltage) appearing on the data line DTLn changes. In actuality, the electric potential (or the voltage) appearing on each of the first node ND1 and the second node ND2 may change due to an electrostatic coupling effect of a parasitic capacitor or the like. However, the change of the electric potential (or the voltage) appearing on each of the first node ND1 and the second node ND2 can normally be ignored. It is to be noted that, also in every horizontal scan period leading ahead of the (m−1)th horizontal period of the currently displayed frame, the signal outputting circuit 102 asserts the initialization voltage VIni on a data line DTLn before the signal outputting circuit 102 asserts a video signal VSig on the same data line DTLn to serve as a substitute for the initialization voltage VIni. However, the timing diagram of FIG. 4 does not show the operations.
Period TP(1)0 (With Reference to FIGS. 4 and 5B)
The period TP(1)0 serving as the period of the second-node electric-potential initialization process is the first half of the mth horizontal scan period of the currently displayed frame. During the period TP(1)0, each of the first switch circuit SW1, the third switch circuit SW3 and the fourth switch circuit SW4 is sustained in a turned-off state. After the initialization voltage VIni determined in advance is applied from the data line DTLn to the second node ND2 by way of the second switch circuit SW2 which has already been put in a turned-on state, the second switch-circuit SW2 is put in a turned-off state in order to set an electric potential appearing on the second node ND2 at a predetermined reference voltage. The process of setting the electric potential appearing on the second node ND2 at the initialization voltage VIni determined in advance is referred to as the second-node electric-potential initialization process.
To put it more concretely, each of the signal writing transistor TRW and the first transistor TR1 is sustained in a turned-off state whereas each of the third transistor TR3 and the fourth transistor TR4 is changed from a turned-on state to a turned-off state. Thus, the driving voltage VCC is not applied to the first node ND1. In addition, the light emitting device ELP is electrically disconnected from the device driving transistor TRD. As a result, the source-to-drain current Ids does not flow to the light emitting device ELP, putting the light emitting device ELP in a no-light emission state. In addition, the second transistor TR2 is changed from a turned-off state to a turned-on state so that the initialization voltage VIni determined in advance is applied from the data line DTLn to the second node ND2 by way of the second transistor TR2 put in a turned-on state. Then, the second transistor TR2 is typically put in a turned-off state before the video signal VSig—m is asserted on the data line DTLn. In this state, the driving voltage VCC is applied to a specific one of the terminals of the capacitor C1, and an electric potential appearing on the specific terminal of the capacitor C1 is put in a state of being sustained. Thus, the electric potential appearing on the second node ND2 is sustained at a predetermined level which is the level of the initialization voltage VIni of −4 volts.
Period TP(1)1 (With Reference to FIGS. 4 and 5C)
The period TP(1)1 serving as the period of the signal writing process is the second half of the mth horizontal scan period of the currently displayed frame. In the period TP1, each of the second switch circuit SW2, the third switch circuit SW3 and the fourth switch circuit SW4 is sustained in a turned-off state whereas the first switch circuit SW1 is conversely put in a turned-on state. With the first switch circuit SW1 put in a turned-on state, the second node ND2 is put in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD by way of the first switch circuit SW1. In this state, the video signal VSig—m asserted on the data line DTLn is supplied to the first node ND1 by way of the signal writing transistor TRW which has already been put in a turned-on state by a signal asserted on the scan line SCLm so that the electric potential appearing on the second node ND2 is raised toward a level obtained as a result of subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig—m. The process of raising the electric potential appearing on the second node ND2 toward such a level is referred to as the signal writing process.
To put it more concretely, each of the second transistor TR2, the third transistor TR3 and the fourth transistor TR4 is sustained in a turned-off state whereas each of the signal writing transistor TRW and the first transistor TR1 is put in a turned-on state by a signal asserted on the scan line SCLm. With the first transistor TR1 put in a turned-on state, the second node ND2 is put in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD through the first transistor TR1. In addition, the video signal VSig—m asserted on the data line DTLn is supplied to the first node ND1 by way of the signal writing transistor TRW which is put in a turned-on state by a signal asserted on the scan line SCLm so that the electric potential appearing on the second node ND2 is changed to a level obtained as a result of subtracting the threshold voltage Vth of the device driving transistor TRD from the video signal VSig—m.
That is to say, at the beginning of the period TP(1)1, the electric potential appearing on the second node ND2 has been initialized for putting the device driving transistor TRD in a turned-on state by carrying out the second-node electric-potential initialization process during the period TP0. In the period TP1, however, the electric potential appearing on the second node ND2 is raised toward the electric potential of the video signal VSig—m applied to the first node ND1. As the difference in electric potential between the gate electrode of the device driving transistor TRD and the specific one of the source and drain areas of the device driving transistor TRD attains the threshold voltage Vth of the device driving transistor TRD, however, the device driving transistor TRD is put in a turned-off state. In this state, the electric potential VND2 appearing on the second node ND2 becomes equal to about (VSig—m−Vth). That is to say, the electric potential VND2 appearing on the second node ND2 can be expressed by Eq. (2) given below. It is to be noted that, prior to the beginning of the (m+1)th horizontal scan period, a signal appearing on the scan line SCLm puts each of the signal writing transistor TRW and the first transistor TR1 in a turned-off state.

VND2≈(VSig—m−Vth)   (2)

Period TP(1)2 (With Reference to FIGS. 4 and 5D)

During a period TP(1)2 following the period TP(1)1, the first switch circuit SW1 is put in a turned-off state whereas the second switch circuit SW2 is sustained in a turned-off state and the driving voltage VCC determined in advance is applied to the first node ND1 by way of the third switch circuit SW3 which has already been put in a turned-on state. The fourth switch circuit SW4 put in a turned-on state puts the other one of the source and drain areas of the device driving transistor TRD in a state of being electrically connected the to a specific one of the electrodes of the light emitting device ELP. In this state, the device driving transistor TRD allows a source-to-drain current Ids to flow to the light emitting device ELP. The process of allowing the source-to-drain current Ids to flow to the light emitting device ELP is referred to as the light emission process.
To put it more concretely, as described above, prior to the start of the (m+1)th horizontal scan period, the first transistor TR1 is put in a turned-off state whereas the second transistor TR2 is sustained in a turned-off state. A signal asserted on the third/fourth-transistor control line CLm changes the state of the third transistor TR3 and the state of the fourth transistor TR4 from a turned-off state to a turned-on state. In these states, the predetermined reference voltage VCC is applied to the first node ND1 by way of the third transistor TR3 which has already been put in the turned-on state. In addition, by changing the state of the fourth transistor TR4 from a turned-off state to a turned-on state, the other one of the source and drain areas of the device driving transistor TRD is put in a state of being electrically connected to a specific one of the electrodes of the light emitting device ELP, allowing a source-to-drain current Ids generated by the device driving transistor TRD to flow to the light emitting device ELP to serve as a driving current for driving the light emitting device ELP to emit light.
Following Eq. (3) is derived from Eq. (2).

Vgs≈VCC−(VSig—m Vth)   (3)

Thus, Eq. (1) can be changed to following Eq. (4).




  
    
      
        
          
            
              
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As is obvious from Eq. (4) given above, the source-to-drain current Ids flowing to the light emitting device ELP is proportional to the square of an electric-potential difference (VCC−VSig−m In other words, the source-to-drain current Ids flowing to the light emitting device ELP is not dependent on the threshold voltage Vth of the device driving transistor TRD. That is to say, the luminance (or the light quantity) of light emitted by the light emitting device ELP is not affected by the threshold voltage Vth of the device driving transistor TRD. The luminance of light emitted by the light emitting device ELP employed in the (n, m)th light emitting unit 10 is a value determined by the source-to-drain current Ids flowing to the light emitting device ELP.
The light emission state of the light emitting device ELP is sustained till the (m−1)th horizontal scan period of the immediately following frame. That is to say, the light emission state of the light emitting device ELP is sustained till the end of the period TP(1)−1 of the immediately following frame.
At the end of the light emission state of the light emitting device ELP, the series of processes of driving the light emitting unit 10 serving as the (n, m)th sub-pixel circuit as described above is completed.
In the display apparatus according to the first embodiment, the predetermined initialization voltage VIni asserted on the data line DTLn is applied to the second node ND2 by way of the second switch circuit SW2. Thus, a separate power-supply line for supplying the initialization voltage VIni determined in advance is not required in particular. As a result, the number of power-supply lines can be reduced.
In accordance with the driving method for driving the display apparatus according to the first embodiment, the second switch circuit SW2 is put in a turned-on state with a timing adjusted to a period used for asserting the initialization voltage VIni determined in advance on the data line DTLn whereas the signal writing transistor TRW is put in a turned-on state with a timing adjusted to a period used for asserting the video signal on the data line DTLn. Thus, even if the separate power-supply line for applying the initialization voltage VIni determined in advance to the second node ND2 is eliminated, the display apparatus can be driven without causing any problems.
Second Embodiment
A second embodiment also implements the display apparatus provided by the present invention and the driving method for driving the display apparatus. The second embodiment is obtained by modifying the first embodiment. The display apparatus according to the second embodiment is different from the display apparatus according to the first embodiment in that, in the case of the display apparatus according to the second embodiment, the first switch circuit SW1 is controlled by a signal other than the signal asserted on the scan line SCLm and, in addition, the third switch circuit SW3 and the fourth switch circuit SW4 are controlled by signals different from each other.
The driving method according to the second embodiment is different from the driving method according to the first embodiment in that, in the case of the driving method according to the second embodiment, between the signal writing process and the light emission process, a second-node electric-potential correction process is carried out so as to change an electric potential appearing on the second node ND2 by applying a voltage with a magnitude determined in advance to the first node ND1 for a period determined in advance with the first switch circuit SW1 already put in a turned-on state in order to put the second node ND2 in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD.
It is to be noted that, in the case of the second embodiment, the driving voltage is applied to the first node ND1 as the-voltage with a magnitude determined in advance. To put it more concretely, between the signal writing process and the light emission process which are explained in the description of the first embodiment, the second-node electric-potential correction process is carried out by applying the driving voltage as the voltage with a magnitude determined in advance to the first node ND1 for a period determined in advance with the first switch circuit SW1 sustained in a turned-on state and the third switch circuit SW3 put in a turned-on state.
The display apparatus according to the second embodiment is also an organic EL (Electro Luminescence) display apparatus defined as a display apparatus employing light emitting units which each have an organic EL light emitting device and a driving circuit for driving the organic EL device. First of all, an outline of the organic EL display apparatus is explained. FIG. 6 is a diagram showing an equivalent circuit of the driving circuit 11 employed in the light emitting unit 10 at an intersection of the nth matrix column and the mth matrix row in a two-dimensional matrix of the display apparatus according to the second embodiment in which light emitting units are laid out to form the two-dimensional matrix. FIG. 7 is a conceptual diagram showing the display apparatus. The structure of the light emitting unit 10 employed in the second embodiment is identical with the structure of the light emitting unit 10 employed in the first embodiment.
The display apparatus according to the second embodiment is different from the display apparatus according to the first embodiment in that, in the case of the configuration of the display apparatus according to the second embodiment, the first switch circuit SW1 is controlled by a signal other than the signal asserted on the scan line SCLm and, in addition, the third switch circuit SW3 and the fourth switch circuit SW4 are controlled by signals different from each other. Otherwise, the configuration of the display apparatus according to the second embodiment is identical with the configuration of the display apparatus according to the first embodiment. In the second embodiment, configuration elements identical with their respective counterparts employed in the first embodiment are denoted by the same reference notations and reference numerals as the counterparts, and explanation of the identical configuration elements is not repeated in order to avoid duplications of descriptions.
In the same way as the first embodiment, the display apparatus according to the first embodiment employs:
(1): N×M light emitting units 10 laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction;
(2): M scan lines SCL each stretched in the first direction; and
(3): N data lines DTL each stretched in the second direction.
Each of the M scan lines SCL is connected to a scan circuit 101 whereas each of the N data lines DTL is connected to a signal outputting circuit 102. The conceptual diagram of FIG. 7 shows 3×3 light emitting units 10 centered at a light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column. It is to be noted, however, that the configuration shown in the conceptual diagram of FIG. 7 is no more than a typical configuration. In addition, the conceptual diagram of FIG. 7 does not show the first power-supply line PS1 and the second power-supply line PS2 which are shown in the diagram of FIG. 6 to serve as power-supply lines for conveying the driving voltage VCC and the cathode voltage VCat respectively.
In the case of the driving circuit according to the first embodiment described earlier, the first transistor TR, functioning as the first switch circuit SW1 is controlled by a signal asserted on the scan line SCLm. In the case of this second embodiment, on the other hand, the gate electrode of the first transistor TR1 is connected to a first-transistor control line CL1m. The first-transistor controlling circuit 121 supplies a signal to the gate electrode of the first transistor TR1 by way of the first-transistor control line CL1m in order to put the first transistor TR1 in a turned-on or turned-off state.
In the case of the first embodiment, each of the gate electrode of the third transistor TR3 serving as the third switch circuit SW3 and the gate electrode of the fourth transistor TR4 serving as the fourth switch circuit SW4 is connected to the control line CLm common to the third switch circuit SW3 and the fourth switch circuit SW4 so that the third switch circuit SW3 and the fourth switch circuit SW4 are control to enter a turned-on or turned-off state by the same control signal asserted on the control line CLm. In the case of the second embodiment, on the other hand, the gate electrode of the third transistor TR3 is connected to the third-transistor control line CL3m whereas the gate electrode of the fourth transistor TR4 is connected to the fourth-transistor control line CL4m.
In the case of the second embodiment, the third-transistor controlling circuit 123 supplies a signal to the gate electrode of the third transistor TR3 by way of the third-transistor control line CL3m in order to control transitions of the third transistor TR3 from a turned-on state to a turned-off state and vice versa. By the same token, the fourth-transistor controlling circuit 124 supplies a signal to the gate electrode of the fourth transistor TR4 by way of the fourth-transistor control line CL4m in order to control transitions of the fourth transistor TR4 from a turned-on state to a turned-off state and vice versa.
A commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of the first-transistor controlling circuit 121, the third-transistor controlling circuit 123 and the fourth-transistor controlling circuit 124. By the same token, a commonly known configuration and a commonly known structure can be used respectively as the configuration and structure of each of the first-transistor control line CL1, the third-transistor control line CL3 and the fourth-transistor control line CL4.
In the same way as the description of the first embodiment, the following description explains driving operations carried out by the display apparatus on the light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column.
A model of timing charts of signals involved in the driving operations carried out by the display apparatus is shown in the timing diagram of FIG. 8. FIGS. 9A and 9B are a plurality of model circuit diagrams referred to in description of driving operations carried out by the display apparatus. To be more specific, FIGS. 9A and 9B are model circuit diagrams showing turned-on and turned-off states of transistors in the driving circuit 11.
In the second embodiment, between the signal writing process and the light emission process, a second-node electric-potential correction process is carried out so as to change an electric potential appearing on the second node ND2 by applying a voltage with a magnitude determined in advance to the first node ND1 for a period determined in advance with the first switch circuit SW1 already put in a turned-on state in order to put the second node ND2 in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD. To put it more concretely, the signal writing process is carried out during a period TP(2)1 shown in the timing diagram of FIG. 8, the second-node electric-potential correction process is executed during a period TP(2)2 lagging behind the period TP(2)1 as shown in the same timing diagram whereas the light emission process is performed during a period TP(2)3 lagging behind the period TP(2)2 as shown in the same timing diagram. The following description explains details of an operation carried out in every period shown in the timing diagram of FIG. 8.
Period TP(2)−1 (With Reference to FIG. 8) As is the case with the period TP(1)−1 shown in the timing diagram of FIG. 4, the period TP(2)−1 serving as the period of a light emission process is the period in which the light emitting unit 10 serving as the (n, m)th sub-pixel circuit is in an immediately preceding light emission state of emitting light at a luminance according to a video signal V′Sig written right before. Each of the third transistor TR3 and the fourth transistor TR4 is put in a turned-on state whereas each of the signal writing transistor TRW, the first transistor TR1 and the second transistor TR2 is conversely put in a turned-off state. The turned-on and turned-off states of the transistors composing the driving circuit 11 are the same as those explained earlier by referring to the circuit diagram of FIG. 5A as the turned-on and turned-off states for the first embodiment. Through the light emitting device ELP employed in the light emitting unit 10 serving as the (n, m)th sub-pixel circuit, the source-to-drain current I′ds expressed by Eq. (7) to be described later is flowing. Thus, the light emitting device ELP employed in the light emitting unit 10 serving as the (n, m)th sub-pixel circuit is emitting light with a luminance determined by the source-to-drain current I′ds.

Period TP(2)0 (With Reference to FIG. 8)

Much like the period TP(1)0 shown in the timing diagram of FIG. 4, the period TP(2)0 is the first half of the mth horizontal scan period of the currently displayed frame. The turned-on and turned-off states of transistors employed in the driving circuit 11 are shown in the circuit diagram of FIG. 5B referred to earlier in the description of the first embodiment. However, the display apparatus according to the second embodiment is different from the display apparatus according to the first embodiment in that, in the case of the configuration of the display apparatus according to the second embodiment, the first transistor TR1, the third transistor TR3 and the fourth transistor TR4 are controlled by a first-transistor controlling circuit 121, a third-transistor controlling circuit 123 and a fourth-transistor controlling circuit 124 respectively. Otherwise, operations carried out in the period TP(2)0 are identical with the operations carried out in the period TP(1)0 of the first embodiment. Thus, the operations carried out in the period TP(2)0 are not explained. As explained earlier in the description of the first embodiment, the initialization voltage VIni is used to set the electric potential appearing on the second node ND2 at a predetermined reference electric potential of −4 volts.
Period TP(2)1 (With Reference to FIG. 8)
Much like the period (1)1 shown in the timing diagram of FIG. 4, the period TP(2)1 serving as the period of the signal writing process is the second half of the mth horizontal scan period of the currently displayed frame. The turned-on and turned-off states of the transistors composing the driving circuit 11 are the same as those explained earlier by referring to the circuit diagram of FIG. 5C as the turned-on and turned-off states for the first embodiment.
Operations carried out in the period TP(2)1 are basically identical with the operations carried out in the period TP(1)1 of the first embodiment. In the case of the first embodiment, however, before the (m+1)th horizontal scan period is started, a signal asserted on the scan line SCLm puts the first transistor TR1 in a turned-off state. The display apparatus according to the second embodiment is different from the display apparatus according to the first embodiment in that, in the case of the display apparatus according to the second embodiment, the first transistor TR1 is sustained in a turned-on state till the end of a period TP(2)2 which will be described later. As explained earlier in the description of the first embodiment, the electric potential VND2 appearing on the second node ND2 is expressed by Eq. (2) given as follows.

VND2≈(VSig—m−Vth)   (2)

Period TP(2)2 (With Reference to FIGS. 8 and 9A)

The period TP(2)2 is the period of the second-node electric-potential correction process of changing an electric potential appearing on the second node ND2 by applying a voltage having a magnitude determined in advance to the first node ND1 for a time period determined in advance with the first switch circuit SW1 already put in a turned-on state in order to put the second node ND2 in a state of being electrically connected to the other one of the source and drain areas of the device driving transistor TRD. In the case of the second embodiment, the second-node electric-potential correction process is carried out by applying the driving voltage VCC to the first node ND1 as the voltage having a magnitude determined in advance.
To put it concretely, the first transistor TR1 is sustained in a turned-on state whereas the third transistor TR3 is put in a turned-on state in order to apply the driving voltage VCC to the first node ND1 as the voltage having a magnitude determined in advance for the period TP(2)2. It is to be noted that each of the second transistor TR2 and the fourth transistor TR4 is sustained in a turned-off state. As a result, if the mobility μ of the device driving transistor TRD is large, the source-to-drain current flowing through the device driving transistor TRD is also large, resulting in a large electric-potential change ΔV or a large electric-potential correction value ΔV. If the mobility μof the device driving transistor TRD is small, on the other hand, the source-to-drain current flowing through the device driving transistor TRD is also small, resulting in a small electric-potential change ΔV or a small electric-potential correction value ΔV. Since the second node ND2 is electrically connected to the drain area of the device driving transistor TRD, the electric potential VND2 appearing on the second node ND2 also rises by the electric-potential change ΔV or the electric-potential correction value ΔV. The equation for expressing the electric potential VND2 appearing on the second node ND2 is changed from Eq. (2) to Eq. (5) given as follows.

VND2≈(VSig—m−Vth)+ΔV   (5)

It is to be noted that the entire length t0 of the period TP(2)2 during which the second-node electric-potential correction process is carried out is determined in advance as a design value at the stage of designing the display apparatus. In addition, by carrying out the second-node electric-potential correction process, the source-to-drain current Ids is also compensated at the same time for variations in coefficient k which is expressed as follows: k≡(1⁄2)*(W/L)*C0X.
Period TP2(3) (With Reference to FIGS. 8 and 9B)
The period TP(2)3 is the period of the next light emission process of driving the light emitting device ELP to emit light.
To put it more concretely, at the beginning of the period TP(2)3, the first transistor TR1 is put in a turned-off state whereas the fourth transistor TR4 is put in a turned-on state. The second transistor TR2 is sustained in a turned-off state whereas the third transistor TR3 is sustained in a turned-on state. The driving voltage VCC determined in advance is applied to the first node ND1 by way of the third switch circuit SW3 sustained in the turned-on state whereas the fourth switch circuit SW4 put in a turned-on state puts the other one of the source and drain areas of the device driving transistor TRD in a state of being electrically connected to a specific one of the electrodes of the light emitting device ELP. In these states, a driving current generated by the device driving transistor TRD is flowing to the light emitting device ELP and driving the light emitting device ELP to emit light.
Following Eq. (6) is derived from Eq. (5).

Vgs≈VCC−((VSig—m−Vth)+ΔV)   (6)

Thus, Eq. (1) can be changed to following Eq. (7).




  
    
      
        
          
            
              
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As is obvious from Eq. (7) given above, the source-to-drain current Ids flowing to the light emitting device ELP is proportional to the square of a difference between an electric-potential difference (VCC−VSig—m) and the electric-potential correction value ΔV which is determined by the mobility μ of the device driving transistor TRD. In other words, the source-to-drain current Ids flowing to the light emitting device ELP is not dependent on the threshold voltage Vth of the device driving transistor TRD. That is to say, the luminance (or the light quantity) of light emitted by the light emitting device ELP is not affected by the threshold voltage Vth of the device driving transistor TRD. The luminance of light emitted by the light emitting device ELP employed in the (n, m) light emitting unit 10 is a value determined by the source-to-drain current Ids flowing to the light emitting device ELP.
In addition, the larger the mobility μ of the device driving transistor TRD, the larger the electric-potential correction value ΔV. Thus, the larger the mobility μof the device driving transistor TRD, the smaller the value of the expression ((VCC−VSig—m)−ΔV)2 included in Eq. (7) or the smaller the magnitude of the source-to-drain current Ids. As a result, the source-to-drain current Ids can be compensated for variations in mobility μ from transistor to transistor. That is to say, if a video signal VSig—m having the same value is applied to different light emitting units 10 employing device driving transistors TRD having different values of the mobility μ, the source-to-drain currents Ids generated by the device driving transistors TRD have magnitudes about equal to each other. As a result, the source-to-drain current Ids flowing to the light emitting device ELP as a driving current for controlling the luminance of light emitted by the light emitting device ELP can be made uniform. Thus, it is possible to eliminate the effects of variations in mobility μ or the effects of variations in coefficient k, and it is therefore possible to eliminate the effects of variations of the luminance of light emitted by the light emitting device ELP.
The light emission state of the light emitting device ELP is sustained till the (m−1)th horizontal scan period of the immediately following frame. That is to say, the light emission state of the light emitting device ELP is sustained till the end of the period TP(2)−1 of the immediately following frame.
At the end of the light emission state of the light emitting device ELP, the series of processes of driving the light emitting unit 10 serving as the (n, m)th sub-pixel circuit as described above is completed.
Third Embodiment
A third embodiment also implements a display apparatus and a driving method for driving the display apparatus. The third embodiment is also obtained by modifying the first embodiment. In the case of the first embodiment, a voltage with a fixed magnitude is used as the initialization voltage. In the case of the third embodiment, on the other hand, a voltage with a magnitude varying in accordance with the video signal is used as the initialization voltage. Thus, the display apparatus according to the third embodiment is provided with a voltage conversion circuit 131 having a voltage reduction circuit 132. The third embodiment is different from the first embodiment in these points.
The display apparatus according to the third embodiment is also an organic EL (Electro Luminescence) display apparatus defined as a display apparatus employing light emitting units which each have an organic EL light emitting device and a driving circuit for driving the organic EL device. First of all, an outline of the display apparatus is explained. FIG. 10 is a diagram showing an equivalent circuit of the driving circuit 11 employed in the light emitting unit 10 at an intersection of the nth matrix column and the mth matrix row in a two-dimensional matrix of the display apparatus according to the third embodiment in which light emitting units are laid out to form the two-dimensional-matrix. FIG. 11 is a conceptual diagram showing the display apparatus. The structure of the light emitting unit 10 employed in the second embodiment is identical with the structure of the light emitting unit 10 employed in the first embodiment.
As described above, the display apparatus according to the third embodiment is provided with a voltage conversion circuit 131 having a voltage reduction circuit 132. The input side of the voltage conversion circuit 131 is connected to the signal outputting circuit 102 and the output side of the voltage conversion circuit 131 is connected to the data line DTL. The third embodiment is much different from the first embodiment in that, in the case of the third embodiment, the signal outputting circuit 102 outputs only a video signal VSig in both the first and second halves of the horizontal scan period. Except for the differences described above, the third embodiment otherwise has a configuration basically identical with the configuration of the first embodiment. Configuration elements employed in the third embodiment as elements identical with their respective counterparts included in the first embodiment are denoted by the same reference notations and the same reference numerals as the counterparts, and the explanation of the configuration elements identical with the counterparts is not repeated in order to avoid duplications of descriptions.
In the same way as the first embodiment, the display apparatus according to the third embodiment employs:
(1): N×M light emitting units 10 laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction;
(2): M scan lines SCL each stretched in the first direction; and
(3): N data lines DTL each stretched in the second direction.
The scan line SCL is connected to the scan circuit 101. As described above, the display apparatus according to the third embodiment is provided with a voltage conversion circuit 131 having a voltage reduction circuit 132. The input side of the voltage conversion circuit 131 is used for receiving a video signal VSig from the signal outputting circuit 102 and the output side of the voltage conversion circuit 131 is connected to the data line DTL. The conceptual diagram of FIG. 11 shows 3×3 light emitting units 10 centered at a light emitting unit 10 located at the intersection of the mth matrix row and the nth matrix column. It is to be noted, however, that the configuration shown in the conceptual diagram of FIG. 11 is also no more than a typical configuration. In addition, the conceptual diagram of FIG. 11 does not show the first power-supply line PS1 and the second power-supply line PS2 which are shown in the diagram of FIG. 10 to serve as power-supply lines for conveying the driving voltage VCC and the cathode voltage VCat respectively.
As described above, the input side of the voltage conversion circuit 131 is used for receiving a video signal VSig from the signal outputting circuit 102. In addition, in the second-node electric-potential initialization process, the voltage reduction circuit 132 employed in the voltage conversion circuit 131 asserts a voltage obtained as a result of subtracting a voltage having a fixed magnitude from the voltage of the video signal VSig to the data line DTL as the initialization voltage. Except for the differences described above, operations of processes carried out in accordance with the driving method for driving the display apparatus according to the third embodiment are otherwise basically identical with the operations of the processes carried out in accordance with the driving method for driving the display apparatus according to the first embodiment and the explanation of the operations of the processes carried out in accordance with the driving method for driving the display apparatus according to the third embodiment is not repeated.
As shown in a diagram of FIG. 10, the voltage conversion circuit 131 is provided with a voltage reduction circuit 132 as well as signal switching sections 133A and 133B for every data line DTL. The voltage reduction circuit 132 as well as the signal switching sections 133A and 133B are configured as transistors which are provided on the support body 20 by carrying out the same manufacturing process as the driving circuit 11. The signal switching sections 133A and 133B are properly subjected to switching control to put the switching sections 133A and 133B alternately in turned-on and turned-off states with timings which are determined by a control clock signal not shown in the diagram of FIG. 10 as will be described later. The input side of the voltage reduction circuit 132 receives the video signal VSig from the signal outputting circuit 102 whereas the output side of the voltage reduction circuit 132 asserts a voltage obtained as a result of subtracting a voltage VD having a fixed magnitude from the voltage of the video signal VSig to the data line DTL as the initialization voltage VIni to be described later.
As described above, in the case of the third embodiment, a voltage with a magnitude varying in accordance with the video signal VSig is used as the initialization voltage VIni. To put it more concretely, the initialization voltage VIni is a voltage expressed by (VSig−VD).
Next, the configuration of the voltage reduction circuit 132 is explained. FIG. 12 is a circuit diagram showing a model of the voltage conversion circuit 131. As shown in the circuit diagram, the voltage reduction circuit 132 employed in the third embodiment is configured to include diode-wired transistors. To put it more concretely, the voltage reduction circuit 132 has two diode-wired transistors 132A and 132B which are connected to each other to form a series circuit. In this case, each of the diode-wired transistors 132A and 132B and the device driving transistor TRD can be designed as transistors of the same configuration. To put it more concretely, each of the diode-wired transistors 132A and 132B and the device driving transistor TRD is a TFT of the p-channel type. Thus, from the design point of view, each of the diode-wired transistors 132A and 132B has a threshold voltage equal to the threshold voltage Vth of the device driving transistor TRD. Therefore, in the voltage reduction circuit 132 shown in the diagram of FIG. 12, from the design point of view, the voltage VD is equal to 2×Vth (that is, VD=2×Vth). That is to say, in the third embodiment, since threshold voltage Vth of the device driving transistor TRD is 2 volts, the voltage VD is 4 volts.
FIG. 13 is a model timing diagram showing timing charts referred to in explanation of operations carried out by the voltage conversion circuit 131. The timing diagram shows timing charts of turned-on and turned-off states of the signal switching sections 133A and 133B as well as turned-on and turned-off states of the first and second transistors TR1 and TR2.
As shown in the timing diagram of FIG. 13, the signal switching section 133A is controlled to sustain the signal switching section 133A in a turned-on state during the first half of every horizontal scan period and a turned-off state during the second half of every horizontal scan period. On the other hand, the signal switching section 133B is controlled to conversely sustain the signal switching section 133B in a turned-off state during the first half of every horizontal scan period and a turned-on state during the second half of every horizontal scan period. Typically, each of the signal switching sections 133A and 133B is controlled by properly making use of a clock signal for generating a scan signal in the scan circuit 101.
During the first half of the mth horizontal scan period, the signal switching section 133A is sustained in a turned-on state but the signal switching section 133B is conversely sustained in a turned-off state. The first half of the mth horizontal scan period is a period TP(1)0 which has been explained earlier in the description of the first embodiment. Thus, the initialization voltage VIni—m asserted on the data line DTLn in the mth horizontal scan period is expressed as follows:

VIni—m=VSig—m−VD 

To put it more concretely, the initialization voltage VIni—m asserted on the data line DTLn in the mth horizontal scan period is expressed as follows:

VIni—m=VSig—m−2×Vth 

where reference notation VSig—m denotes the video signal VSig supplied to the voltage conversion circuit 131 in the mth horizontal scan period.

The initialization voltage VIni—m asserted on the data line DTLn in the first half of each of periods other than the mth horizontal scan period is the same as that asserted on the data line DTLn in the first half of the mth horizontal scan period.
During the second half of the mth horizontal scan period, on the other hand, the signal switching section 133A is sustained in a turned-off state but the signal switching section 133B is conversely sustained in a turned-on state. The second half of the mth horizontal scan period is a period TP(1)1 which has been explained earlier in the description of the first embodiment. Thus, the video signal VSig—m is asserted on the data line DTLn in the mth horizontal scan period is as it is. The video signal VSig—m is asserted on the data line DTLn as it is in the second half of each of periods other than the mth horizontal scan period.
FIG. 14 is a model timing diagram showing timing charts of driving operations carried out by the display apparatus as timing charts to be referred to in explanation of a driving method according to the third embodiment. The timing diagram of FIG. 14 corresponds to the timing diagram of FIG. 4 referred to in the description of the first embodiment. If the video signal VSig—m has a typical voltage of 6 volts for example, the initialization voltage VIni—m(=VSig—m−2×Vth) is 2 volts in the third embodiment since the threshold voltage Vth of the device driving transistor TRD (or the diode-wired transistors 132A and 132B) is 2 volts. As explained earlier in the description of the first embodiment, during the period TP(1)1, the signal writing process is carried out.
In the timing diagram of FIG. 4 referred to in the description of the first embodiment, the initialization voltage VIni is −4 volts. If the video signal VSig—m has a typical voltage of 6 volts for example, during the period TP(1)1, the electric potential appearing on the second node ND2 must rise from −4 volts to 4 volts (=VSig—m−Vth=6 volts−2 volts). Otherwise, the signal writing process is not completed normally. In accordance with specifications of the display apparatus, however, the period TP(1)1 must be shortened in some cases, raising a problem that the period TP(1)1 is undesirably ended before the electric potential appearing on the second node ND2 attains the level of 4 volts (=VSig—m−Vth=6 volts−2 volts).
In the case of the driving method according to the third embodiment, on the other hand, if the video signal VSig—m has a typical voltage of 6 volts for example, the initialization voltage VIni—m(=VSig—m−2×Vth) in the third embodiment is 2 volts as described above. Thus, in the case of the third embodiment, the electric potential appearing on the second node ND2 needs to be raised by only an increase equal to the threshold voltage Vth of 2 volts during the period TP(1)1. If the electric potential appearing on the second node ND2 can be raised by 2 volts, the signal writing process can be completed normally. The increase of 2 volts is equal to the threshold voltage Vth of the device driving transistor TRD and independent of the magnitude of the video signal VSig—m. Thus, the driving method according to the third embodiment of the present invention has a merit that the length of the period TP(1)1 required for completing the signal writing process normally can be reduced.
The present invention has been exemplified above by taking each of the preferred embodiments as a typical example. However, implementations of the present invention are by no means limited to this preferred embodiment. That is to say, the configuration and structure of each component employed in the driving circuit and the light emitting device which are included in the light emitting unit of the display apparatus according to the preferred embodiment as well as the processes of the method for driving the light emitting device are typical examples and can thus be changed properly.
For example, the display apparatus according to the second embodiment can be changed to a display apparatus having a configuration in which the initialization voltage has a magnitude varying in accordance with the voltage of the video signal, that is, a configuration further provided with the voltage conversion circuit having the voltage reduction circuit as is the case with the third embodiment described above.
The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2008-119839 filed in the Japan Patent Office on May 1, 2008 and Japanese Priority Patent Application JP 2008-319828 filed in the Japan Patent Office on Dec. 16, 2008, the entire contents of which are hereby incorporated by reference.
It should be understood by those skilled in the art that a variety of modifications, combinations, sub-combinations and alterations may occur, depending on design requirements and other factors as far as they are within the scope of the appended claims or the equivalents thereof.

## Claims

1. A driving method for driving a display apparatus, said display apparatus including N×M light emitting units laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction; M scan lines each extending in said first direction; N data lines each extending in said second direction; a driving circuit provided for each of said light emitting units and having a signal writing transistor, a device driving transistor, a capacitor and a first switch circuit; and a light emitting device provided for each of said light emitting units to serve as a device for emitting light at a luminance according to a driving current output by said device driving transistor to said light emitting device, wherein in each of said light emitting units, first current electrode of said signal writing transistor is connected to one of said data lines, a gate electrode of said signal writing transistor is connected to one of said scan lines, a first current electrode of said device driving transistor is connected to a second current electrode of said signal writing transistor through a first node, a first terminal of said capacitor is connected to a power-supply line for conveying a reference voltage determined in advance, a second terminal of said capacitor is connected to a gate electrode of said device driving transistor through a second node, a first terminal of said first switch circuit is connected to said second node, a second terminal of said first switch circuit is connected to a second current electrode of said device driving transistor, and said driving circuit further has a second switch circuit connected between said second node and said one of said data lines, said method comprising:
a second-node electric-potential initialization process of applying a predetermined initialization voltage appearing on said data line to said second node by way of said second switch circuit put in a turned-on state and, then, putting said second switch circuit in a turned-off state in order to set an electric potential appearing on said second node at a reference electric potential determined in advance.

2. The driving method in accordance with claim 1, further comprising
a signal writing process of changing an electric potential appearing on said second node toward an electric potential, which is obtained as a result of subtracting the threshold voltage of said device driving transistor from the voltage of a video signal appearing on one of said data lines, by applying said video signal to said first node by way of said signal writing transistor which is put in a turned-on state by a signal appearing on one of said scan lines when said first switch circuit is put in a turned-on state in order to put said second node in a state of being electrically connected to said second current electrode of said device driving transistor,
said signal writing process being carried out after said second-node electric-potential initialization process has been completed.

3. The driving method in accordance with claim 2, wherein said initialization voltage is a voltage having a constant magnitude.

4. The driving method in accordance with claim 2, wherein said initialization voltage is a voltage having a magnitude which varies in accordance with said video signal.

5. The driving method in accordance with claim 4, wherein:
said display apparatus is provided with a voltage conversion circuit having a voltage reduction circuit; and
said video signal is supplied to said voltage conversion circuit and, in said second-node electric-potential initialization process, said voltage reduction circuit employed in said voltage conversion circuit applies a voltage obtained as a result of subtracting a voltage having a constant magnitude from the voltage of said video signal to said data line as said initialization voltage.

6. The driving method in accordance with claim 5, wherein said voltage reduction circuit includes a diode-wired transistor.

7. The driving method in accordance with claim 6, wherein:
said voltage reduction circuit includes two diode-wired transistors connected to each other to form a series circuit; and
each of said diode-wired transistors has the same configuration as said device driving transistor.

8. The driving method in accordance with claim 2, further comprising a light emission process of supplying a driving current from said device driving transistor to said light emitting device by applying a driving voltage determined in advance to said first node in order to drive said light emitting device to emit light, whereby said light emission process is carried out after a signal writing process has been completed.

9. The driving method in accordance with claim 8, further comprising a second-node electric-potential correction process carried out between said signal writing process and said light emission process so as to change an electric potential appearing on said second node by applying a voltage having a magnitude determined in advance to said first node for a time period determined in advance with said first switch circuit already put in a turned-on state in order to put said second node in a state of being electrically connected to the second current electrode of said device driving transistor.

10. The driving method in accordance with claim 9, wherein said driving voltage is applied to said first node as said voltage having a magnitude determined in advance.

11. The driving method in accordance with claim 1, wherein said driving circuit provided for each of said light emitting units employed in said display apparatus further includes a third switch circuit connected between said first node and said power-supply line conveying a driving voltage, and a fourth switch circuit connected between said second current electrode of said device driving transistor and a specific one of the electrodes of said light emitting unit, and said driving method further comprises:
(a): carrying out a second-node electric-potential initialization process of sustaining each of said first, third and fourth switch circuits in a turned-off state and applying said predetermined initialization voltage appearing on said data line to said second node by way of said second switch circuit put in a turned-on state and, then, putting said second switch circuit in a turned-off state in order to set an electric potential appearing on said second node at a reference electric potential determined in advance as said initialization voltage;
(b): carrying out a signal writing process of sustaining each of said second, third and fourth switch circuits in a turned-off state and putting said first switch circuit in a turned-on state to put said second node in a state of being electrically connected to said second current electrode of said device driving transistor so as to apply a video signal appearing on one of said data lines to said first node by way of said signal writing transistor put in a turned-on state by a signal appearing on one of said scan lines in order to change an electric potential appearing on said second node toward an electric potential obtained as a result of subtracting said threshold voltage of said device driving transistor from said video signal;
(c): applying a signal asserted on one of said scan lines to said gate electrode of said signal writing transistor later on in order put said signal writing transistor in a turned-off state; and
(d): carrying out a light emission process of putting said first switch circuit in a turned-off state, sustaining said second switch circuit in a turned-off state, applying a driving voltage determined in advance from said power-supply line to said first node by way of said third switch circuit which has already been put in a turned-on state and putting said second current electrode of said device driving transistor in a state of being electrically connected to said specific one of said electrodes of said light emitting device by way of said fourth transistor put in a turned-on state so as to allow a driving current to flow from said device driving transistor to said light emitting device in order to drive said light emitting device.

12. The driving method in accordance with claim 11, wherein between said steps (c) and (d), a second-node electric-potential correction process is carried out in order to change an electric potential appearing on said second node by applying said driving voltage as a voltage with a magnitude determined in advance to said first node for a period determined in advance with said first switch circuit sustained in a turned-on state and said third switch circuit put in a turned-on state.

13. The driving method in accordance with claim 1, wherein said light emitting device is an organic electro luminescence light emitting device.

14. A display apparatus comprising:
N×M light emitting units laid out to form a two-dimensional matrix composed of N matrix columns oriented in a first direction and M matrix rows oriented in a second direction;
M scan lines each extending in said first direction;
N data lines each extending in said second direction;
a driving circuit provided for each of said light emitting units and having a signal writing transistor, a device driving transistor, a capacitor and a first switch circuit; and
a light emitting device provided for each of said light emitting units to serve as a device for emitting light at a luminance according to a driving current output by said device driving transistor to said light emitting device, wherein in each of said light emitting units,
a first current electrode of said signal writing transistor is connected to one of said data lines,
a gate electrode of said signal writing transistor is connected to one of said scan lines,
a first current electrode of said device driving transistor is connected to a second current electrode of said signal writing transistor through a first node,
a first terminal of said capacitor is connected to a power-supply line for conveying a reference voltage determined in advance,
a second terminal of said capacitor is connected to a gate electrode of said device driving transistor through a second node,
a first terminal of said first switch circuit is connected to said second node,
a second terminal of said first switch circuit is connected to a second current electrode of said device driving transistor, and
said driving circuit further has a second switch circuit connected between said second node and said one of said data lines.

15. The display apparatus in according to claim 14 wherein said light emitting device is an organic electro luminescence light emitting device.