Data signal driver for light emitting display

1. A display device comprising an array of light emitting display elements ( 2 ) arranged in rows and columns, with a plurality of pixels in a column being supplied with current from a respective column power supply line ( 26 ) and the pixels being addressed row by row, the addressing of all rows defining a field period, the device further comprising:

compensation circuitry for modifying target pixel drive currents corresponding to desired pixel brightness levels, to take account of the voltage on the column power supply line at each pixel resulting from the currents drawn from the column power supply line by the plurality of pixels in the column being supplied by the column power supply line for each row addressing cycle in a field period and the dependency of pixel brightness characteristics on a voltage on a row conductor at the pixel and independent changes in the drain and source voltages of the drive transistor.

2. A device as claimed in claim 1 , wherein the compensation circuitry comprises: means for applying an algorithm to the target pixel drive currents which represents a relationship between the currents drawn by the plurality of pixels in a column and the voltages on the column power supply line at the locations of the pixels and the dependency of the pixel brightness characteristics on the voltage on the row conductor.

3. A device as claimed in claim 2 , wherein the means for applying an algorithm derives values corresponding to multiplying a vector of the target pixel drive currents for a column of pixels by the inversion of a matrix M, in which:

M

=

[

-

2

1

1

-

2

1

⋰

⋰

⋰

1

-

2

1

1

-

2

]

,

and wherein the number of rows and columns of matrix M is equal to the number of pixels in the column.

4. A device as claimed in claim 2 , wherein each of the plurality of pixels in a column comprises a current sampling transistor ( 34 ) which samples an input current and provides a drive voltage to a drive transistor ( 22 ), and wherein the algorithm uses a value including terms derived from:

the voltage-current characteristics of the drive transistor ( 22 ); and

the voltage-current characteristics of the light emitting display element ( 2 ).

5. A device as claimed in claim 4 , wherein the drive transistor ( 22 ) and the light emitting display element ( 2 ) of each of the plurality of pixels in a column are in series between the column power supply line ( 26 ) and a common line.

6. A device as claimed in claim 2 , wherein the algorithm uses a value including a term (R) derived from a resistance of the column power supply line ( 26 ).

7. A device as claimed in claim 6 , wherein the algorithm uses a value Rλ/(1+λ/μ), where

R is the resistance of the column power supply line between adjacent pixels;

λ is the slope of the drain-source current vs. drain-source voltage curve of the drive transistor; and

μ is the slope of the current vs. voltage curve of the display element.

8. A device as claimed in claim 7 , wherein the value Rλ/(1+λ/μ) uses the slope of the drain-source current vs. drain-source voltage curve of the drive transistor and the slope of the current vs. voltage curve of the display element at a target pixel drive current.

9. A device as claimed in claims 3 , wherein the means for applying an algorithm derives values by a recursive operation

F

⁡

(

c

,

n

)

=

F

⁡

(

c

,

n

-

1

)

+

∑

j

=

0

n

-

1

⁢

I

av

⁡

(

c

,

j

)

+

F

⁡

(

c

,

0

)

in which:

F(c,n) is the nth term of the vector result of multiplying the vector of target pixel drive currents for the cth column of pixels by the inversion of the matrix M, F(c, 0) being the first term; and

I(c,j) is a target pixel drive current for the jth pixel in the cth column, the first pixel being j=0.

10. A device as claimed in claim 9 , wherein:

F

⁡

(

c

,

0

)

=

-

1

N

+

1

⁢

∑

j

=

0

N

-

1

⁢

(

N

-

j

)

⁢

⁢

I

av

⁡

(

c

,

j

)

,

in which:

N is the total number pixels in the column.

11. A device as claimed in claim 2 , wherein the means for applying an algorithm comprises a look up table ( 102 ).

12. A device as claimed in claim 11 , further comprising at least one pixel compensation module ( 110 , 112 , 114 ), and further comprising means for updating the values of the look up table to enable changes in pixel brightness characteristics over time to be modeled based on analysis of the characteristics of the pixel compensation module.

13. A circuit for generating pixel drive currents for the display elements of a light emitting display device having display elements arranged in rows and columns, with a plurality of pixels in a column being supplied with current from a respective column power supply line ( 26 ) and the plurality of pixels being addressed row by row, the addressing of all rows defining a field period, the circuit comprising:

means for receiving target pixel drive currents;

compensation circuitry for modifying target pixel drive currents to take account of the voltage on the column power supply line at each of the plurality of pixels in the column resulting from the currents drawn from the column power supply line by the plurality of pixels in the column for each row addressing cycle in a field period and the dependency of pixel brightness characteristics on the voltage on a row conductor at the pixel and independent changes in the drain and source voltages of the drive transistor.

14. A circuit as claimed in claim 13 , wherein the compensation circuitry comprises:

means for applying an algorithm to the target pixel drive currents which represents a relationship between the currents drawn by the pixels in a column and voltages on the column power supply line at the locations of the pixels in the column and the dependency of the pixel brightness characteristics on the voltage on the row conductor.

15. A circuit as claimed in claim 14 , wherein the means for applying an algorithm derives values corresponding to multiplying a vector of target pixel drive currents for a column of pixels by the inversion of a matrix M, in which:

M

=

[

-

2

1

1

-

2

1

⋱

⋱

⋱

1

-

2

1

1

-

2

]

,

and wherein the number of rows and columns of matrix M is equal to the number of pixels in the column.

16. A circuit as claimed in claim 15 , wherein the algorithm uses a value including a term (R) derived from the resistance of the column power supply line ( 26 ).

17. A circuit as claimed in claim 15 , wherein the means for applying an algorithm derives values by a recursive operation

F

⁡

(

c

,

n

)

=

F

⁡

(

c

,

n

-

1

)

+

∑

j

=

0

n

-

1

⁢

I

av

⁡

(

c

,

j

)

+

F

⁡

(

c

,

0

)

in which:

F(c,n) is the nth term of the vector result of multiplying the vector of target pixel drive currents for the cth column of pixels by the inversion of the matrix M, F(c,0) being the first term; and

I(c,j) is a target current for the jth pixel in the cth column, the first pixel being j=0.