Pixel driving device, light emitting device, and property parameter acquisition method in a pixel driving device

• US 8,269,760 B2
• Filed: 11/27/2009
• Issued: 09/18/2012
• 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: Ogura, Jun, Takei, Manabu, Kashiyama, Shunji
• Primary Examiner(s): Olson, Jason
• Application Number:
  US12/626,747
• Publication Number:
  US 20100134475A1
• Time in Patent Office: 1,026 Days
• Field of Search: None
• US Class Current: 345/211
• 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 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, the pixel drive circuit further having a holding capacity for storing charge by a voltage impressed on a control terminal of the drive transistor, the pixel driving device 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 property parameters that relate to electrical properties of the pixel;

wherein the reference voltage has an electric potential in which an 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;

wherein the switching circuit connects the one end of the signal line to the voltage impressing circuit and interrupts the connection between the one end of the signal line and the voltage impressing circuit 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 after a predetermined settling time has elapsed;

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

wherein the property parameter acquisition circuit acquires the threshold voltage of the drive transistor and a current amplification factor of the pixel drive circuit as the property parameters based on voltage values of a plurality of measured voltages acquired by the voltage measurement circuit for a plurality of settling times which are set to be different from each other.

2. The pixel driving device according to claim 1, wherein the 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 a parasitic capacity that is parasitic on the signal line, a holding capacity, and a light emitting element capacity that is parasitic on the light emitting element, and β

0 is a 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 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 substituting 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,

5. The pixel driving device according to claim 1, further comprising:

a signal correction circuit that corrects 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 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, the pixel drive circuit further having a 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, between the voltage impressing circuit and the voltage measurement circuit; and

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

wherein the reference voltage has an electric potential in which an 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,

wherein the switching circuit connects the one end of the signal line to the voltage impressing circuit and interrupts the connection between the one end of the signal line and the voltage impressing circuit 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 after a predetermined settling time has elapsed;

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

wherein the property parameter acquisition circuit acquires the threshold voltage of the drive transistor and a current amplification factor of the pixel drive circuit as the property parameters based on voltage values of a plurality of measured voltages acquired by the voltage measurement circuit for a plurality of settling times which are set to be different from each other.

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

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

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

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

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

the property parameter acquisition circuit acquires the property parameters for the plurality of pixels which are set in the selected state by the selection drive circuit.

8. The light emitting device according to claim 7, wherein the pixel drive circuit comprises:

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 a 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 a 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 a 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 the 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

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

9. The light emitting device according to claim 6, wherein the plurality of settling times are set to be a prescribed plurality of different values larger than (C/β

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

0 is a 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 for the 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,

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

a signal correction circuit that corrects 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.

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 current supplied to the light emitting element, the pixel drive circuit further having a holding capacity for storing charge by voltage impressed on a control terminal of the drive transistor, the method including:

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

a measurement voltage acquisition step that interrupts the connection between the one end of the signal line and the voltage impressing circuit, and then acquires voltage values, as a plurality of measured voltages, of the 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 a current amplification factor of the pixel drive circuit as property parameters based on the voltage values of the plurality of measured voltages acquired for the plurality of settling times.

14. The property parameter acquisition method in the pixel driving device according to claim 13, wherein the measurement voltage acquisition step includes a step for setting the 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 a parasitic capacity that is parasitic on the signal line, a holding capacity, and a light emitting element capacity that is parasitic on the light emitting element, and β

0 is a 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 a 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),

16. A light emitting device, comprising:

a pixel, connected to a signal line, and having a light emitting element, 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 electric current supplied to the light emitting element through the current path based on 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 property parameters that relate to 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 a threshold voltage of the drive transistor, when an elapsed time from the moment the impressed voltage is stopped by the existence of a high impedance state becomes a settling time t, and when C is total capacity, which is the sum of a holding capacity of the pixel connected by the signal line, a parasitic capacity that is parasitic on the signal line, and a light emitting element capacity that is parasitic on the light emitting element; and

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

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

)/t<

1,

17. The light emitting device according to claim 16, wherein the property parameter acquisition circuit acquires the property parameters by using equation (5),