DISPLAY APPARATUS

Abstract

A display including a plurality of pixels in matrix, wherein each pixel includes first and second power source lines; a switching device for controlling writing of a signal voltage supplied via a signal line; a capacitor for storing the signal voltage supplied via the switching device; an n-type driver device having a gate electrode and first and second electrodes for providing current corresponding to the signal voltage stored by the capacitor; a light emitting device having an anode and a cathode for emitting light in response to the current. The display makes use of storing a voltage related to the threshold voltage of the driving device during a second portion of the operating time.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a construction an embodiment of the present invention;

FIG. 2 is a timing chart of the embodiment shown in FIG. 1;

FIG. 3A is a view illustrating an initial state of the threshold voltage detection step (1) of FIG. 2;

FIG. 3B is a view illustrating a final state of the threshold voltage detection step (1) of FIG. 2;

FIG. 3C is a view illustrating a state of the writing step (2) of FIG. 2;

FIG. 3D is a view illustrating a state of the light emitting step (3) of FIG. 2;

FIG. 4 is a view illustrating another embodiment of the present invention;

FIG. 5 is a timing chart of the embodiment shown in FIG. 4;

FIG. 6A is a view illustrating an initial state of the threshold voltage detection step (1) of FIG. 5;

FIG. 6B is a view illustrating a final state of the threshold voltage detection step (1) of FIG. 5;

FIG. 6C is a view illustrating a state of the writing step (2) of FIG. 5;

FIG. 6D is a view illustrating a state of the light emitting step (3) of FIG. 5;

FIG. 7 is a view illustrating a configuration of a prior art pixel circuit;

FIG. 8 is a view illustrating an embodiment of the present invention;

FIG. 9 is a timing chart for FIG. 8; and

FIG. 10 is a view illustrating the state of voltage at each unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific aspects of the present invention will be described below with reference to the drawings. It is noted that the scope of the invention is not limited to the illustrated examples.

First Basic Configuration

FIG. 1 illustrates a circuit configuration for a display according to the present invention; and FIG. 2 illustrates a timing chart thereof.

The display is constituted of many pixel circuits 1 arranged in a matrix shape, and arranged in each pixel are an organic EL light emitting device (OLED), being a light emitting device, and a circuit for controlling light emission thereof.

A positive electrical power source supplying circuit 4 outputs a positive electrical power source voltage VDD, but performs switching at a predetermined timing to output a voltage Vp lower than a negative electrical power source voltage VSS to be supplied to each pixel. A signal line drive circuit 2 supplies a signal voltage Vdata to be displayed by each pixel to each signal line 15 arranged for each vertical line. A scanning line drive circuit 3 supplies a signal for driving a scanning line 16 arranged for each horizontal line. A negative electrical power source supplying circuit 5 supplies a negative electrical power source voltage VSS for causing a current to flow in the light emitting device.

In each pixel circuit, a positive electrical power source line 17 is connected to the positive electrical power source supplying circuit 4, and this positive electrical power source line 17 is connected to the anode electrode of a light emitting device 14 of each pixel circuit. The drain electrode of an n-type driver device 12 is connected to the cathode electrode of the light emitting device 14, and the source electrode of the driver device 12 is connected to a negative electrical power source line 18. A capacitor 13 is connected between the gate electrode and drain electrode of the driver device 12 is.

The source electrode of a switching device 11 is connected to the gate electrode of the driver device 12, and the drain electrode of the switching device 11 is connected to the signal line 15. The scanning line 16 is connected to the gate electrode of the switching device 11. In this embodiment, the signal line drive circuit 2, scanning line drive circuit 3 and switching device 11 constitute a data writing controller 6.

Here, an n-type TFT is used as the switching device 11, but a p-type TFT may also be used. When the type of TFT is changed, the polarity of a signal supplied to the scanning line 16 must be reversed. The driver device 12 is an n-type TFT.

The operation of the above described pixel circuit will be described with reference to the timing chart of FIG. 2 and to FIG. 3.

First, assume that (Vdata+Vt) is retained by the capacitor 13 at the gate electrode of the driver device 12 in the previous frame period. Vdata is the brightness data corresponding to the amount of light to be emitted by the light emitting device 14 of a particular pixel, and Vt is a threshold voltage of the driver device 12 of the particular pixel.

In this state, when writing timing of the particular pixel (the particular horizontal line) is reached, the scanning line 16 is changed to a potential (in this instance, H level) which causes the switching device 11 to turn on. Also, the potential of the signal line 15 is changed to a potential identical to potential VSS of the negative electrical power source line 18 to turn off the driver device 12.

Subsequently, the potential of the positive electrical power source line 17 is changed to Vp lower than VSS as illustrated in FIG. 3A. If the voltage drop of the light emitting device 14 is Voled, when the potential of the positive electrical power source line 17 is VDD, the potential of the drain electrode of the driver device 12 will be VDD-Voled, and when the potential of the positive electrical power source line 17 changes from VDD to Vp, the difference therebetween is distributed to a capacitance component Coled of the light emitting device 14 and a capacitance component Cs of the capacitor 13. A first electrode 19 of the capacitor 13 is connected to the gate electrode of the driver device 12 and a second electrode 20 of the capacitor 13 is connected to the drain electrode of the driver device 12.

Consequently, at the moment the potential of the positive electrical power source line 17 changes to Vp, the potential of the drain electrode of the driver device 12 is VDD−Voled+{Coled/(Cs+Coled)}(Vp−VDD). Here, if the maximum value of the range of threshold voltage of the driver device 12 to be compensated is Vt (TFT) (>0), Vp is set such that

VSS−Vt(TFT)>=VDD−Voled×{Coled/(Cs+Coled)}(Vp−VDD)  (formula 2)

That is, the setting is performed such that the drain voltage of the driver device 12 lower than a value obtained by subtracting Vt (TFT) from VSS being the gate and source voltage of the driver device 12.

Consequently, starting just after the positive electric power source line 17 changes to Vp, threshold voltage detection step (1) of the driver device 12 is started. Then, as illustrated in FIG. 3A, a current flows from the source of the driver device 12 to the drain thereof, and potential VSS−Vt occurs at the drain electrode of the driver device 12 (FIG. 3B). This threshold voltage detection step (1) is executed simultaneously at all the pixels.

Subsequently, the scanning line 16 is changed to a level (in this instance, L level) to turn off the switching device 11, entering step (2) for writing a pixel signal into each pixel. That is, after changing the potential of the signal line 15 to Vdata, the scanning line 16 is set again so as to turn on the switching device 11, and the gate potential of the driver device 12 is changed to Vdata (<VSS). Accordingly, the gate voltage of the driver device 12 changes from VSS to Vdata, and the amount of change thereof is distributed to capacitance Cs of the capacitor 13 and capacitance Coled of the light emitting device 14. As a result, the potential of the drain electrode of the driver device 12 changes from VSS−Vt to VSS−Vt+{Cs/(Cs+Coled)} (Vdata−VSS)(FIG. 3C).

Consequently, at this time, the capacitor 13 is charged by

Vdata−(VSS−Vt+{Cs/(Cs+Coled)}(Vdata−VSS)).

This writing step (2) is executed line-sequentially as illustrated in FIG. 2. In this case, data writing may be performed simultaneously with respect to one horizontal line, or may be performed point-sequentially.

Subsequently, the positive electrical power source line 17 is changed to VDD so that the voltage applied to the light emitting device 14 becomes sufficiently higher than the threshold voltage of the light emitting device 14. Accordingly, the drain voltage of the driver device 12 changes to VDD−Voled. Consequently, the gate voltage of the driver device 12 changes to a value obtained by adding

Vdata −(VSS−Vt+{Cs/(Cs+Coled)}(Vdata−VSS))=(1−{Cs/(Cs+Coled)})(Vdata−VSS)+Vt

(the charge voltage of the capacitor 13) to VDD−Voled.

Accordingly, at this time, the potential difference between the gate and source electrodes of the driver device 12 changes to

Vgs=VDD−Voled−VSS+(Vdata−VSS){Coled/(Cs+Coled)}+Vt  (formula 3) (FIG. 3D)

Thus, current id flowing in the driver device 12 is expressed as follows.

$\begin{array}{cc}\begin{array}{c}\mathrm{id}=\left(\beta /2\right){\left(\mathrm{Vgs}-\mathrm{Vt}\right)}^2\\ =\left(\beta /2\right)(\mathrm{VDD}-\mathrm{Voled}-\mathrm{VSS}+\\ {\left(\mathrm{Vdata}-\mathrm{VSS}\right)\left\{\mathrm{Coled}/\left(\mathrm{Cs}+\mathrm{Coled}\right)\right\})}^2\end{array}& \left(\mathrm{formula}4\right)\end{array}$

This current id is supplied to the light emitting device 14. The current id is not dependent on Vt and thus the threshold voltage of the light emitting driver device 12 of the light emitting device 14 is compensated.

Particularly, according to the present basic configuration, a capacitor is arranged between the gate electrode and drain electrode of the driver device 12 used for causing the light emitting device 14 to emit light, to detect a threshold voltage between the gate electrode and drain electrode of the driver device 12 used for causing the light emitting device 14 to emit light. Then, a voltage lower than the potential supplied to the gate electrode of the driver device 12 at the time of the threshold voltage detection is applied as the pixel signal, whereby brightness data Vdata can be superimposed on the gate voltage of the driver device 12 in the signal writing step without losing threshold voltage Vt of the driver device 12 retained by the capacitor 13.

Second Basic Configuration

FIG. 4 illustrates a circuit configuration of another display to which the present invention is applied, and FIG. 5 illustrates a timing chart thereof.

The display includes: a light emitting device 24 having the cathode electrode thereof connected to a negative electrical power source line 18; a driver device 22 having the drain electrode thereof connected to the anode electrode of the light emitting device 24 and the source electrode thereof connected to a positive electrical power source line 17; a capacitor 23 connected between the gate electrode and drain electrode of the driver device 22 (A first electrode 29 of the capacitor 23 is connected to the gate electrode of the driver device 22 and a second electrode 30 of the capacitor 23 is connected to the drain electrode of the driver device 22.); and a switching device 21 having the source or drain electrode thereof connected to the gate electrode of the driver device 22, the drain or source electrode thereof connected to a signal line 15, and the gate electrode thereof connected to a scanning line 16. The switching device 21 is an n-type or p-type TFT, and the driver device 22 is a p-type TFT. In this embodiment, the switching device 21 and driver device 22 constitute a current value controller 31.

The operation of the above described pixel circuit will be described with reference to the timing chart of FIG. 5 and to FIG. 6. Assume that (Vdata−Vt) is retained by the capacitor 23 at the gate electrode of the driver device 22 in the previous frame period.

First the scanning line 16 is changed to a potential (in this instance, H level) which causes the switching device 21 to turn on. Also, the potential of the signal line is changed to a potential identical to a potential VDD of the positive electrical power source line 17 to turn off the driver device 22. Subsequently, the potential of the negative electrical power source line 18 is changed to Vp higher than VDD as illustrated in FIG. 6A. At the moment the potential of the negative electrical power source line 18 changes to Vp, the potential of the drain electrode of the driver device 22 is Voled+{Coled/(Cs+Coled)}(Vp-VSS). Here, if the range of threshold voltage of the driver device 22 to be compensated is Vt (TFT) (<0), Vp is set such that

VDD−Vt(TFT)<=Voled+{Coled/(Cs+Coled)}(Vp−VDD)  (formula 5)

Starting just after the negative electric power source line 18 changes to Vp, threshold voltage detection step (1) of the driver device 22 is started. Then, potential VDD-Vt occurs at the drain electrode of the driver device 22 (FIG. 6B).

Subsequently, the scanning line 16 is changed to a level (in this instance, L level) to turn off the switching device 21, entering step (2) of writing a pixel signal into each pixel. That is, after changing the potential of the signal line 15 to Vdata, the scanning line 16 is set again to a level (in this instance, H level) so as to turn on the switching device 21, and the gate potential of the driver device 22 is changed to Vdata (>VDD). As a result, the potential of the drain electrode of the driver device 22 changes to VDD+{Cs/(Cs+Coled)} (Vdata−VDD)−Vt (FIG. 6C).

Subsequently, the negative electrical power source line 18 is changed to VSS so that the voltage applied to the light emitting device 24 becomes sufficiently lower than the threshold voltage of the light emitting device 24, and at the same time, the switching device 11 is turned off by the scanning line 16. Accordingly, the drain voltage of the driver device 22 changes to

VSS+Voled and thus the gate voltage of the driver device 22 changes to Vss+Voled+(1−{Cs/(Cs+Coled)}(Vdata−VDD)+Vt.

Consequently, at this time, the potential difference between the source and gate electrodes of the driver device 22 is

Vsg=VDD−Voled−VSS+(Vdata−VDD) {Coled/(Cs+Coled)}−Vt  (formula 6) (FIG. 6D).

Thus, the current flowing in the driver device 22 is expressed as follows.

id=(β/2)(Vsg+Vt)²=(β/2)(VDD−Voled−VSS+(Vdata−VDD){Coled/(Cs+Coled)})²  (formula 7).

Accordingly, the threshold voltage of the driver device 22 is compensated.

A display according to an embodiment of the present invention will now be described. FIG. 8 illustrates a circuit configuration thereof, and FIG. 9 illustrates a timing chart thereof. This circuit configuration is identical to that of FIG. 4 described above, but the configuration of FIG. 1 may be employed.

As described above, a signal supplied to each signal line 15 has inserted therein period B during which 0 (zero) V, being a threshold voltage detection reference voltage, is provided between periods of signal voltage (data) corresponding to the brightness of pixel on each line. More specifically, a threshold voltage detection reference voltage is inserted between n-th data and (n+1)th data.

The scanning line 16 (n) arranged for each line changes to H level V_gH starting just after the threshold voltage detection step is initiated, and changes to L level V_gL when the data writing is finished. Accordingly, the switching device 21 is in a conduction state during the threshold voltage detection step.

Similarly to the basic configuration described above, when the positive electrical power source line 17 is changed from VDD to 0 V, the threshold voltage detection step is initiated. During this threshold voltage detection step (period), data of each line and the threshold voltage detection reference voltage are alternately supplied to the signal line 15, and in data writing periods (period B₁, B₂, . . . , B_j) corresponding to j-number of lines, a threshold voltage of the driver device 22 is detected. More specifically, as illustrated in FIG. 10, the drain electrode voltage V_N2 is gradually set to a voltage lower than the gate electrode voltage V_N1 by threshold voltage V_T.

Here, in this period, potential V_N2 of the drain electrode of the driver device 22 connected to the light emitting device 24 is preferably lower than the threshold voltage of the light emitting device 24. The maximum value of the potential of the drain electrode of the driver device 22 is preferably lower than the threshold voltage of the light emitting device 24. The condition for this is expressed as formula 8. Here, N1, N2 and N3 denote the gate electrode, drain electrode and source electrode of the driver device 22, respectively.

V _OLED,th >V _N2,max  (formula 7)

Under this condition, the threshold voltage of the driver device 22 is recorded on the drain electrode of the driver device 22.

In this example, when the voltage of the positive electrical power source line 17 is 0 V and in this state, the signal voltage of the signal line 16 changes to 0 V, then the gate voltage V_N1 and source voltage V_N3 of the driver device 22 change to 0 V, and on the other hand, voltage V_N2 of the drain electrode N2 changes to a voltage of −V_T lower than 0 V by the threshold voltage V_T.

V_N1=0

V _N2 =−V _T

V_N3=0

On the other hand, when formula 8 is not satisfied, the threshold voltage detection reference voltage will be set to V_P(<0), and the potential of the signal line 15 and the positive electrical power source line 17 shall be set to not 0 V but V_P.

Accordingly, instead of formula 8, when V_P is determined so as to satisfy

V _OLED,th >V _data,max −V _T +V _P

then threshold voltage V_T of the driver device 22 is recorded on the drain electrode N2 of the driver device 22.

Thereafter, when the switching device 21 is in an on state with the potential of the scanning line 16 being V_gH and the potential of the signal line 15 being V_data(n), the potential of the gate electrode, source electrode and drain electrode of the driver device 22 is expressed as follows.

V _N1 =V _data>0

$V_{N2}=\frac{C_s}{C_s+C_{\mathrm{OLED}}}V_{\mathrm{data}}-V_T$ V_N3=0

Thus, on the capacitor 23, there is recorded the following;

$V_{N1}-V_{N2}=\frac{C_{\mathrm{OLED}}}{C_s+C_{\mathrm{OLED}}}V_{\mathrm{data}}+V_T$

In this state, when the scanning line 16 (n) is changed to L level to turn off the switching device 21, this state is established.

Thereafter, the potential of the positive electrical power source line 17 is changed from the threshold voltage detection reference voltage to VDD, whereby a transition to the light emitting step is made. At this time, the following formula is provided;

$V_{N1}=\frac{C_{\mathrm{OLED}}}{C_s+C_{\mathrm{OLED}}}V_{\mathrm{data}}+V_T+V_{\mathrm{OLED}}^{\prime }$ V _N2 =V′ _OLED(≠V_OLFD^o)

V_N3=V_DD

Thus,

$\begin{array}{c}{V}_{\mathrm{sg}}=V_{N3}-V_{N1}\\ =V_{\mathrm{DD}}-\frac{C_{\mathrm{OLED}}}{C_s+C_{\mathrm{OLED}}}V_{\mathrm{data}}-V_T-V_{\mathrm{OLED}}^{\prime }\end{array}$

Finally, the following formula is provided;

$\begin{array}{c}{I}_{\mathrm{sd}}=\frac{\beta }{2}{\left(V_{\mathrm{sg}}+V_T\right)}^2\\ =\frac{\beta }{2}{\left(V_{\mathrm{DD}}-\frac{C_{\mathrm{OLED}}}{C_s+C_{\mathrm{OLED}}}V_{\mathrm{data}}-V_{\mathrm{OLED}}^{\prime }\right)}^2\end{array}$

Therefore, a current not dependent on the threshold voltage of the driver device 22 flows.

Here, COLED denotes a capacitance of the light emitting device 24, Cs a capacitance of the capacitor 23, Isd a source-drain current of the driver device 22, Vsg a source-drain voltage of the driver device 22, and V′OLED a voltage drop of the light emitting device 24 when current Isd flows and thus light is being emitted.

As such, according to the present embodiment, a threshold voltage detection reference voltage is inserted into a signal voltage sent to each pixel supplied to the signal line 15, so it is possible to set the threshold voltage at the drain electrode of the driver device 22 with the switching device 21 being in a conducting state.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

PARTS LIST

• • 1 pixel circuit
  • 2 line drive circuit
  • 3 line drive circuit
  • 4 supplying circuit
  • 5 supplying circuit
  • 6 data writing controller
  • 11 switching device
  • 12 driver device
  • 13 capacitor
  • 14 light emitting device
  • 15 signal line
  • 16 scanning line
  • 17 source line
  • 18 source line
  • 21 switching device
  • 22 driver device
  • 23 capacitor
  • 24 light emitting device
  • 30 current value controller
  • 101 switching device
  • 102 driver device
  • 103 capacitor
  • 104 light emitting device
  • 105 signal line
  • 106 scanning line
  • 107 source line
  • 108 source line

Claims

1. A display including a plurality of pixels in matrix, wherein each pixel comprising: first and second power source lines;a switching device for controlling writing of a signal voltage supplied via a signal line;a capacitor for storing the signal voltage supplied via the switching device;an n-type driver device having a gate electrode and first and second electrodes for providing current corresponding to the signal voltage stored by the capacitor;a light emitting device having an anode and a cathode for emitting light in response to the current wherein the driver device and the light emitting device are arranged in series between the first and second power source lines such that the first electrode of the driver device is connected between the light emitting device and the second electrode of the driver device and the cathode of the light emitting device is connected between the anode of the light emitting device and the driver device and the one side of the capacitor is connected to the gate electrode of the driver device and other side of the capacitor is connected between the first electrode of the driver device and the light emitting device;first means for causing light emission during a first portion of the operating time; andsecond means for detecting and storing a voltage related to the threshold voltage of the driving device during a second portion of the operating time.

2. The display according to claim 1 where the second means causes the switching device to be in a conducting state during detection of the drive threshold voltage.

3. The display according to claim 1 where the second means causes the light emitting device to be in a non-conducting state during detection of the drive threshold voltage.

4. The display according to claim 1, wherein the first means supplies a first voltage which is positive between the first and second power source lines for causing light emission and the second means supplies a second voltage between the first and second power supply lines for detecting the voltage related to the threshold voltage of the driving device where the second voltage is lower than the first voltage.

5. A display including a plurality of pixels in matrix, wherein each pixel comprising: first and second power source lines;a switching device for controlling writing of a signal voltage supplied via a signal line;a capacitor for storing the signal voltage supplied via the switching device;a p-type driver device having a gate electrode and first and second electrodes for providing current corresponding to the signal voltage stored by the capacitor;a light emitting device having an anode and a cathode for emitting light in response to the current wherein the driver device and the light emitting device are arranged in series between the first and second power source lines such that the first electrode of the driver device is connected between the light emitting device and the second electrode of the driver device and the anode of the light emitting device is connected between the cathode of the light emitting device and the driver device and the one side of the capacitor is connected to the gate electrode of the driver device and other side of the capacitor is connected between the first electrode of the driver device and the light emitting device;first means for causing light emission during a first portion of the operating time; andsecond means for detecting and storing a voltage related to the threshold voltage of the driving device during a second portion of the operating time.

6. The display according to claim 5 where the second means causes the switching device to be in a conducting state during detection of the drive threshold voltage.

7. The display according to claim 5 where the second means causes the light emitting device to be in a non-conducting state during detection of the drive threshold voltage.

8. The display according to claim 5, wherein the first means supplies a first voltage which is negative between the first and second power source lines for causing light emission and the second means supplies a second voltage between the first and second power source lines for detecting the voltage related to the threshold voltage of the driving device where the second voltage is higher than the first voltage.