PIXEL DRIVING DEVICE, LIGHT EMITTING DEVICE, AND PROPERTY PARAMETER ACQUISITION METHOD IN A PIXEL DRIVING DEVICE

• US 20100134468A1
• Filed: 11/27/2009
• Published: 06/03/2010
• Est. Priority Date: 11/28/2008
• Status: Active Grant

• Current Assignee: Solas OLED Limited (Atlantic IP Services Limited)
• Original Assignee: Casio Computer Company Limited
• Inventors: TAKEI, Manabu, KASHIYAMA, Shunji, OGURA, Jun
• Granted Patent: US 8,269,759 B2
• Time in Patent Office: Days
• US Class Current: 345/211
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3291 in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting elements

Abstract

A pixel driving device in which, after a reference voltage exceeds a threshold voltage of a drive transistor is impressed through the signal lines on each pixel equipping a light emitting element and the drive transistor, set the signal lines in a state of high impedance, and acquires a voltage value of one end of the signal lines subsequent to a predetermined settling time elapsing, and acquires the threshold voltage of the drive transistor for each pixel and the current amplification factor of the pixel drive circuit as a first property parameter based on acquired voltage values at the time a plurality of first settling times longer than a predetermined value and acquires an irregularity parameter indicating the irregularity in the current amplification factor based on the value of the first property parameter and the measured voltage value acquired at the time shorter than the predetermined value.

Claims

1. A pixel driving device for driving a plurality of pixels, connected to each of a plurality of signal lines, and each comprising a light emitting element, and a pixel drive circuit having a drive transistor, which is for controlling the current supplied to the light emitting element, and whose one end of a current path is connected to one terminal of the light emitting element, the pixel drive circuit having holding capacity for storing charge by voltage impressed on a control terminal of the drive transistor, comprising:

a voltage impressing circuit for outputting a reference voltage;

a voltage measurement circuit equipped for connecting to each signal line;

a switching circuit for switching connection of one end of each signal line, between the voltage impressing circuit and the voltage measurement circuit; and

a property parameter acquisition circuit for acquiring the property parameters that relate to the electrical properties of each pixel;

wherein,

the reference voltage has an electric potential in which the electric potential difference of the one end with respect to the other end of the current path of the drive transistor is a value that exceeds a threshold voltage of the drive transistor; and

the switching circuit connects the one end of each signal line to the voltage impressing circuit and sets the connection between the one end of each signal line and the voltage impressing circuit being interrupted after impressing the reference voltage for a predetermined time on the one end of each signal line by the voltage impressing circuit, and connects one end of each signal line to each voltage measurement circuit subsequent to a predetermined settling time elapsing; and

each voltage measurement circuit acquires the voltage of the one end of each signal line as the measured voltage when being connected with one end of each signal line by the switching circuit; and

the settling time is set to a value of a first settling time group and a value of a second settling time;

the first settling time group is constituted with a plurality of different time values, each of which is larger than the ratio (C/β

0) where C is total capacity, which is, the sum of the parasitic capacity that is parasitic on a single signal line, the holding capacity, and the light emitting element capacity that is parasitic on the light emitting element, and β

0 is the reference value of the current amplification factor; and

the second settling time is constituted with the time value which is shorter than the ratio (C/β

0); and

the property parameter acquisition circuit acquires the threshold voltage of the drive transistor for each pixel and the current amplification factor of the pixel drive circuit as a first property parameter in the property parameters based on the values of the plurality of measured voltages acquired by the voltage measurement circuit for the first settling time group; and

the property parameter acquisition circuit acquires an irregularity parameter which indicates the irregularity of the current amplification factor as a second property parameter in the property parameters based on the value of the threshold voltage acquired for each pixel and the value of the measured voltage acquired by the voltage measurement circuit for the second settling time.

2. The pixel driving device according to claim 1, wherein

the reference value of the current amplification factor is a design value or a typical value for the current amplification factor.

3. The pixel driving device according to claim 1, wherein

the first property parameter and the second property parameter are acquired during an initial state in which the drive transistor of each pixel has the initial properties.

4. The pixel driving device according to claim 1, wherein

the property parameter acquisition circuit acquires the first property parameter, namely the Vth and β

for each pixel based on equation (1) when the first settling time group is t1 and t2, the measured voltage for the first settling time group are respectively Vmeas(t1) and Vmeas(t2), the threshold voltage is Vth, and the current amplification factor is β

.

Vmeas(t)=Vth+(C/β

)/t 

 

(1)

where,

t=t1, t2

5. The pixel driving device according to claim 4, wherein

the property parameter acquisition circuit acquires the second property parameter, namely, the value of the irregularity parameter Δ

β

/β

3, based on equation (2) when the second settling time is t3, the deviation between pixels in the measured voltage for the second settling time is Δ

Vmeas(t3), the voltage value of the reference voltage is Vref, the threshold voltage of each pixel is Vth, the mean value of the ratio of capacity content in the plurality of pixels to the current amplification factor is <

C/β

>

, and the irregularity property is Δ

β

/β

.

$\begin{array}{cc}\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{Vmeas}<br><br>\left(t<br><br><br><br>3\right)=-\left[\frac{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{\beta <br><br>}}{\mathrm{\beta <br><br>}}\right]\times <br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}<br><br>\left\{1-\frac{2}{\mathrm{Vref}-\mathrm{Vth}}<br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}\right\}& \left(2\right)\end{array}$

6. A light emitting device, comprising:

a pixel array, which is constituted with a plurality of pixels arranged, connected to each of a plurality of signal lines, and each comprising a light emitting element, and a pixel drive circuit having a drive transistor for controlling the current supplied to the light emitting element with one end of the current path of the drive transistor connected to one terminal of the light emitting element as well as a holding capacity for storing charge by voltage impressed on a current control terminal of the drive transistor;

a voltage impressing circuit for outputting a reference voltage;

a plurality of voltage measurement circuits equipped respectively for connecting to each of the plurality of signal lines;

a switching circuit for switching connection of one end of each signal line, between the voltage impressing circuit and each voltage measurement circuit; and

a property parameter acquisition circuit for acquiring the property parameters that relate to the electrical properties of each pixel;

wherein,

the reference voltage has an electric potential in which the electric potential difference of the one end with respect to the other end of the current path of the drive transistor is a value that exceeds a threshold voltage of the drive transistor; and

the switching circuit connects the one end of each signal line to the voltage impressing circuit sets the connection between the one end of each signal line and the voltage impressing circuit being interrupted after impressing the reference voltage for a predetermined time on the one end of each signal line by the voltage impressing circuit, and connects one end of each signal line to each voltage measurement circuit subsequent to a predetermined settling time elapsing; and

each voltage measurement circuit acquires the voltage of the one end of each signal line as the measured voltage when being connected with one end of each signal line by the switching circuit; and

the settling time is set to respective value of a first settling time group and a second settling time;

the first settling time group is constituted with a plurality of different time values, each of which is larger than the ratio (C/β

0) where C is total capacity, which is, the sum of the parasitic capacity that is parasitic on a single signal line, the holding capacity, and the light emitting element capacity that is parasitic on the light emitting element, and β

0 is the reference value of the current amplification factor; and

the second settling time is constituted with the time value which is shorter than the ratio (C/β

0); and

the property parameter acquisition circuit acquires the threshold voltage of the drive transistor for each pixel and the current amplification factor of the pixel drive circuit as a first property parameter in the property parameters based on the values of the plurality of measured voltages acquired by the voltage measurement circuit for the first settling time group; and

the property parameter acquisition circuit acquires the irregularity parameter that indicates the irregularity of the current amplification factor as a second property parameter in the property parameters based on value of the threshold voltage acquired for each pixel and the value of the measured voltage acquired by the voltage measurement circuit for the second settling time.

7. The light emitting device according to claim 6, wherein

the plurality of signal lines in the pixel array are respectively arranged along a first direction;

the pixel array has at least one scan line arranged along a second direction orthogonal to the first direction;

the plurality of pixels are arranged in the vicinity of each intersecting point of the scan lines and the plurality of signal lines;

the light emitting device has a selection drive circuit for setting a selected state for each pixel connected to the scan line by impressing a selected signal on the scan line, and

the property parameter acquisition circuit acquires the first property parameter for each pixel which is in the selected state by the selection drive circuit.

8. The light emitting device according to claim 7, wherein

the pixel drive circuit comprises:

at least

a first thin film transistor to which a predetermined power voltage is impressed on one end of the current path of the first thin film transistor, and which has a connection point connecting the other end of the current path of the first thin film transistor to the one terminal of the light emitting element;

a second thin film transistor whose control terminal is connected to the scan line, one end of the current path is connected to the one end of the current path of the first thin film transistor, and the other end of the current path is connected to the control terminal of the first thin film transistor; and

a third thin film transistor whose the control terminal is connected to the scan line, one end of the current path is connected to the signal line, and the other end of the current path is connected to the connection point;

wherein,

the first thin film transistor corresponds to the drive transistor,

and when the pixel is in a selected state by the selection drive circuit, the second thin film transistor and the third thin film transistor enter an ON state, the one end of the current path of the first thin film transistor is connected with the control terminal of the first thin film transistor, and the signal line is connected to the connection point through the current path of the third thin film transistor whereby the reference voltage supplied from the voltage impressing circuit is impressed on the connection point through the third thin film transistor; and

the voltage measurement circuit acquires the voltage of the connection point of each pixel, arranged in the second direction in a selected state subsequent to each settling time elapsing via the third thin film transistor and each signal line as the measured voltage.

9. The light emitting device according to claim 6, wherein

the reference value of the current amplification factor is a design value or a typical value for a current amplification factor.

10. The light emitting device according to claim 6, wherein

the acquisition of the first property parameter and the second property parameter in the property parameter acquisition circuit is executed during the initial state in which the drive transistor of each pixel has the initial properties.

11. The light emitting device according to claim 6, wherein

the property parameter acquisition circuit acquires the first property parameter, namely the Vth and β

for each pixel based on equation (3) when the first settling time groups are t1 and t2, the measured voltage for the first settling time groups are respectively Vmeas(t1) and Vmeas(t2), the threshold voltage is Vth, and the current amplification factor is β

.

Vmeas(t)=Vth+(C/β

)/t 

 

(3)

where,

t=t1, t2

12. The light emitting device according to claim 6, wherein

the property parameter acquisition circuit acquires the second property parameter, namely, the value of the irregularity parameter Δ

β

/β

based on equation (4) when the second settling time is t3, the deviation between pixels in the measured voltage for the second settling time is Δ

Vmeas(t3), the voltage value of the reference voltage is Vref, the threshold voltage for each pixel is Vth, the mean value of the ratio of capacity content in the plurality of pixels to the current amplification factor is <

C/β

>

, and the irregularity property is Δ

β

/β

.

$\begin{array}{cc}\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{Vmeas}<br><br>\left(t<br><br><br><br>3\right)=-\left[\frac{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{\beta <br><br>}}{\mathrm{\beta <br><br>}}\right]\times <br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}<br><br>\left\{1-\frac{2}{\mathrm{Vref}-\mathrm{Vth}}<br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}\right\}& \left(4\right)\end{array}$

13. A property parameter acquisition method in a pixel driving device for driving a plurality of pixels, connected to each of a plurality of signal lines, and each comprising a light emitting element, and a pixel drive circuit having a drive transistor whose one end of the current path is connected to one terminal of the light emitting element for controlling the current supplied to the light emitting element, the pixel drive circuit having a holding capacity for storing charge determined by voltage impressed on the control terminal of the drive transistor, including;

a reference voltage impressing step for impressing a reference voltage on one end of each signal line so that the electric potential difference of one end to the other end of the current path of the drive transistor is a value that exceeds the threshold voltage of the drive transistor of one end of each signal line by connecting a voltage impressing circuit to one end of each signal line;

measurement voltage acquisition step that interrupts the connection between one end of each signal line and the voltage impressing circuit, then acquires a voltage of one end of each signal line as the measured voltage after passage of each of predetermined settling times after interrupting the connection;

a property parameter acquisition step for acquiring the property parameters that relate to the electrical properties of each pixel from the measured voltages;

wherein,

the settling time in the measured voltage acquisition step is set to respective values of a first settling time group and a second settling time;

the first settling time group is constituted with a plurality of different time values, each of which is larger than the ratio (C/β

0) C is total capacity, which is, the sum of the parasitic capacity that is parasitic on a single signal line, the holding capacity, and the light emitting element capacity that is parasitic on the light emitting element, and β

0 is the reference value of the current amplification factor, and the second settling time is constituted with the time value which is shorter than the ratio (C/β

0); and

the property parameter acquisition step, including;

a first property parameter acquisition step that acquires the threshold voltage of the drive transistor of each pixel and the current amplification factor of the pixel drive circuit as the first property parameters in the property parameters, based on the values of measured voltages for the first settling time group; and

a second property parameter acquisition step that acquires the irregularity parameter which indicates the irregularity of the current amplification factor as a second property parameter in the property parameters based on the value of the threshold voltage acquired for each pixel and the value of the measured voltage for the second settling time.

14. The property parameter acquisition method in a pixel driving device according to claim 13, wherein

a first property parameter acquisition step includes

a step that acquires the first property parameter, namely the Vth and β

for each pixel based on equation (5) when the first settling time groups are t1 and t2, the measured voltages for the first settling time groups are respectively Vmeas(t1) and Vmeas(t2), the threshold voltage is Vth, and the current amplification factor is β

.

Vmeas(t)=Vth+(C/β

)/t 

 

(5)

where,

t=t1, t2

15. The property parameter acquisition method in a pixel driving device according to claim 13, wherein

a second property parameter acquisition step includes

a step that acquires the second property parameter, namely, the value of the irregularity parameter Δ

β

/β

for each pixel based on equation (6) when the second settling time is t3, the deviation between pixels in the measured voltage for the second settling time is Δ

Vmeas(t3), the voltage value of the reference voltage is Vref, the threshold voltage for each pixel is Vth, the mean value of the ratio of capacity content in the plurality of pixels to the current amplification factor is <

C/β

>

, and the irregularity property is Δ

β

/β

.

$\begin{array}{cc}\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{Vmeas}<br><br>\left(t<br><br><br><br>3\right)=-\left[\frac{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{\beta <br><br>}}{\mathrm{\beta <br><br>}}\right]\times <br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}<br><br>\left\{1-\frac{2}{\mathrm{Vref}-\mathrm{Vth}}<br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t<br><br><br><br>3}\right\}& \left(6\right)\end{array}$

16. A light emitting device, comprising:

a plurality of pixels, connected to each of a plurality of signal lines, each comprising

a light emitting element, and

a drive transistor having a current path and a control terminal, which connects one end of the current path to one terminal of the light emitting element and which controls the electric current supplied to the light emitting element through the current path based on the voltage data written between the control terminal and the one end of the current path;

a voltage measurement circuit for acquiring a voltage value as a measured voltage of one end of each signal line;

a property parameter acquisition circuit for acquiring the property parameters that relate to the electrical properties of each pixel;

wherein,

the voltage measurement circuit acquires the voltage value of the one end of each signal line, denoted as Vmeas(t) in equation (7), as a plurality of first measured voltages for a plurality of different settling times whose respective values to satisfy the condition (C/β

)/t<

1 and as a second measured voltage, for a settling time whose value satisfies the condition (C/β

)/t≧

1, where the settling time is defined as the time elapsed from the time when an impressed voltage is stopped after the voltage is impressed on the one end of the current path of the drive transistor via the one end of each signal line so that the electric potential difference between one end and the other end of the current path of the drive transistor exceeds the threshold voltage of the drive transistor, and then the one end of each signal line is set in a state of high impedance; and

the property parameter acquisition circuit acquires an irregularity parameter (Δ

β

/β

) that indicates the irregularity of the voltage/current properties of the drive transistor of the plurality of pixels, as the property parameter, based on the plurality of first measured voltage values, and the value of the second measured voltage, acquired by the voltage measurement circuit.

$\begin{array}{cc}\mathrm{Vmeas}<br><br>\left(t\right)=\mathrm{Vth}+\frac{1}{\frac{t}{\left(C/\mathrm{\beta <br><br>}\right)}+\frac{1}{\mathrm{Vref}-\mathrm{Vth}}}& \left(7\right)\end{array}$

where, t;

settling time

Vmeas(t);

the measured voltage acquired by the voltage measurement circuit at the elapsed settling time t

Vth;

the threshold voltage of the drive transistor

Vref;

Reference voltage

C;

Total Capacity(C=Cs+Cp+Cel)

Cs;

Holding capacity

Cp;

Wiring parasitic capacity

Cel;

Light emitting element capacity

β

;

Constant

17. The light emitting device according to claim 16, wherein

a property parameter acquisition circuit acquires

the Vth and the β

, as the property parameters for the drive transistor, by substituting the plurality of first measured voltages acquired by the voltage measurement circuit for each pixel in equation (8) while the (C/β

) value and the initial threshold voltage of the drive transistor are the property parameters; and

acquires

the irregularity parameter (Δ

β

/β

) by substituting the acquired property parameters of the drive transistor, and the deviation Δ

Vmeas(t) at the plurality of pixels for the second measured voltage acquired by the voltage measurement circuit, in equation (9).

$\begin{array}{cc}\mathrm{Vmeas}<br><br>\left(t\right)=\mathrm{Vth}+\frac{\left(C/\mathrm{\beta <br><br>}\right)}{t<br><br>}<br><br><br><br>\mathrm{where},\left(C/\mathrm{\beta <br><br>}\right)<br><br>K<<br><br>t& \left(8\right)\\ \mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{Vmeas}<br><br>\left(t\right)=-\left[\frac{\mathrm{\Delta <br><br>}<br><br><br><br>\mathrm{\beta <br><br>}}{\mathrm{\beta <br><br>}}\right]\times <br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t}<br><br>\left\{1-\frac{2}{\mathrm{Vref}-\mathrm{Vth}}<br><br>\frac{<<br><br>C/\mathrm{\beta <br><br>}><br><br>}{t}\right\}& \left(9\right)\end{array}$

where, Δ

Vmeas(t) is the deviation at the plurality of pixels for the measured voltage acquired by the voltage measurement circuit at the elapsed settling time t

Δ

β

is the deviation of β

at the plurality of pixels.