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

• US 20100134475A1
• 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: Kashiyama, Shunji, Takei, Manabu, OGURA, JUN
• Granted Patent: US 8,269,760 B2
• Time in Patent Office: Days
• US Class Current: 345/213
• CPC Class Codes: G09G 2300/0819 used for counteracting undesired variations, e.g. feedback or autozeroing
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/04 Maintaining the quality of display appearance
• 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 has a voltage impressing circuit that outputs a reference voltage that exceeds a threshold voltage of a drive transistor, a voltage measurement circuit, and a property parameter acquisition circuit that acquires a property parameter related to an electronic property of a pixel. The pixel driving device impresses the reference voltage on the pixel that has a light emitting element and the drive transistor. The voltage measurement circuit acquires voltage of a signal line, as measured voltages, after each of a plurality of the settling times elapsing from the time when the reference voltage is cut. The property parameter acquisition circuit acquires, as property parameters, the threshold voltage and a current amplification factor of drive transistor based on values of a plurality of measured voltages acquired by the voltage measurement circuit.

Claims

1. A pixel driving device for driving a pixel, connected to a signal line, and 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 a 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 a voltage impressed on a control terminal of the drive transistor, comprising;

a voltage impressing circuit for outputting a reference voltage;

a voltage measurement circuit;

a switching circuit for switching connection of one end of the 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 the pixel;

wherein,

the reference voltage has an electric potential in which the electric potential difference between 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 the signal line to the voltage impressing circuit and sets the connection between the one end of the signal line and the voltage impressing circuit being interrupted after impressing the reference voltage for a predetermined time on the one end of the signal line by the voltage impressing circuit, and connects the one end of the signal line to the voltage measurement circuit subsequent to a predetermined settling time elapsing; and

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

the property parameter acquisition circuit acquires the threshold voltage of the drive transistor and the current amplification factor of the pixel drive circuit as property parameters based on the values of the plurality of measured voltages acquired by the voltage measurement circuit for the plurality settling times.

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

a plurality of settling times are set to be a larger value than (C/β

0) where C is total capacity, which is the sum of the parasitic capacity that is parasitic on a 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.

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

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

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

the property parameter acquisition circuit acquires the threshold voltage and the current amplification factor by subtracting each of the plurality of settling times and each of the values of the plurality of measured voltages into equation (1) where the measured voltage is Vmeas(t), the threshold voltage is Vth, and the current amplification factor is β

when the settling time is t.

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

5. A pixel driving device according to claim 1, further comprises:

a signal correction circuit that corrects the supplied image data and generates a corrected gradation signal based on the property parameters acquired by the property parameter acquisition circuit; and

a drive signal impressing circuit for generating a drive signal based on the corrected gradation signal and impressing the drive signal on the one end of the signal line.

6. A light emitting device, comprising:

at least one pixel, connected to at least one signal line, and 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 a current path of the drive transistor connected to one terminal of the light emitting element as well as holding capacity for storing charge by a voltage impressed on a current control terminal of the drive transistor;

a voltage impressing circuit for outputting a reference voltage;

a voltage measurement circuit;

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

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

wherein,

the reference voltage has an electric potential in which the electric potential difference between the one end and 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 the signal line to the voltage impressing circuit and sets the connection between the one end of the signal line and the voltage impressing circuit being interrupted after impressing the reference voltage for a predetermined time on the one end of the signal line by the voltage impressing circuit, and connects the one end of the signal line to the voltage measurement circuit subsequent to a predetermined settling time elapsing; and

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

the property parameter acquisition circuit acquires the threshold voltage of the drive transistor and the current amplification factor of the pixel drive circuit as property parameters based on the values of the plurality of measured voltages acquired by the voltage measurement circuit for the plurality settling times.

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

a plurality of the signal lines are arranged along a first direction;

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

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

the light emitting device has a selection drive circuit for setting a selected state for the plurality of pixels connected to the scan line by impressing a selected signal on the scan line; and

the property parameter acquisition circuit acquires the property parameters for the plurality of pixels 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 on whose one end of a current path a predetermined power voltage is impressed, and which has a connection point connecting the other end of the current path 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 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 control terminal is connected to the scan line, one end of the current path is connected to the signal line, and 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 the 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 settling time is set to be a prescribed plurality of different values larger than (C/β

0) where C is total capacity, which is the sum of the parasitic capacity that is parasitic on a 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.

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

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

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

the property parameter acquisition circuit acquires the threshold voltage and the current amplification factor by calculating the plurality of settling times t and the values of the plurality of measured voltages Vmeas(t) represented in equation (2) when the measured voltage is Vmeas(t), the threshold voltage is Vth, and the current amplification factor is β

when the settling time is t.

$\begin{array}{cc}\mathrm{Vmeas}<br><br>\left(t\right)=\mathrm{Vth}+\frac{\left(C/\mathrm{\beta <br><br>}\right)}{t}& \left(2\right)\end{array}$

12. The light emitting device according to claim 6, further comprises:

a signal correction circuit that corrects the supplied acquired image data and generates a corrected gradation signal based on the property parameters acquired by the property parameter acquisition circuit; and

a drive signal impressing circuit for generating a drive signal based on the corrected gradation signal and impressing the drive signal on the one end of the signal line.

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

a reference voltage impressing step for impressing a reference voltage on one end of the 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 transistorby connecting a voltage impressing circuit to the one end of the signal line;

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

a property parameter acquisition step that acquires the threshold voltage of the drive transistor and the current amplification factor of the pixel drive circuit as property parameters based on the values of the plurality of measured voltages acquired for the plurality of settling times.

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

a measurement voltage acquisition step includes a step for setting a plurality of settling times to be a prescribed plurality of values larger than (C/β

0) where C is the total capacity, which is the sum of the parasitic capacity that is parasitic on a 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.

15. The property parameter acquisition method according to claim 14, wherein

the property parameter acquisition step includes,

a step for substituting each of the plurality of settling times t and the values of the plurality of measured voltages into Vmeas(t) represented in equation (3) where the measured voltage is Vmeas(t), the threshold voltage is Vth, and the current amplification factor is β

where the settling time is t; and

a step for acquiring the threshold voltage and the value of the current amplification factor by performing a calculation based on the settling times and the values of the plurality of measured voltages represented in equation (3).

$\begin{array}{cc}\mathrm{Vmeas}<br><br>\left(t\right)=\mathrm{Vth}+\frac{\left(C/\mathrm{\beta <br><br>}\right)}{t}& \left(3\right)\end{array}$

16. A light emitting device, comprising:

a pixel, connected to a signal line, having a light emitting element, a drive transistor having a current path and 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, and a holding capacity for storing charge determined by the voltage impressed on the drive transistor;

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

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

wherein,

the voltage measurement circuit acquires the voltage value of the voltage of the one end of the signal line indicated in equation (4), as the measured voltage, after voltage is impressed between both ends of the current path of the drive transistor via the one end of the signal line so as to exceed the threshold voltage of the drive transistor when the elapsed time from the moment the impressed voltage is stopped by the existence of a high impedance state becomes the settling time t, and when C is total capacity, which is the sum of the holding capacity of the pixel connected by the signal line, the parasitic capacity that is parasitic on the signal line, and the light emitting element capacity that is parasitic on the light emitting element; and

the property parameter acquisition circuit acquires the threshold voltage of the drive transistor and the (C/β

) value as property parameters based on the plurality of measured voltages acquired by the measurement circuit when the settling time t is a plurality of differing values that satisfy the condition of (C/β

)/t<

1.

$\begin{array}{cc}\mathrm{Vmeas}<br><br>\left(t\right)=\mathrm{Vth}+\frac{1}{\frac{t}{\left(C/\mathrm{\beta <br><br>}\right)}+\frac{1}{\mathrm{Vref}-\mathrm{Vth}}}& \left(4\right)\end{array}$

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+Ce1)

Cs;

Holding capacity

Cp;

Wiring parasitic capacity

Ce1;

Light emitting element capacity

β

;

Constant

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

the property parameter acquisition circuit acquires the property parameters by using equation (5) modified from equation (4).

$\begin{array}{cc}\mathrm{Vmeas}\; <li></li>\left(t\right)\approx <br><br>\mathrm{Vth}+\frac{\left(C/\mathrm{\beta <br><br>}\right)}{t}& \left(5\right)\end{array}$