Active matrix display devices and methods of driving the same
View Patent ↗An active matrix display device has a column driver circuit for providing pixel drive signals to columns of pixels, and comprising current source circuits. Each current source circuit has a supply switch ( 78 ) for controlling the time during which the current source supplies current to or drains current from the column. A mapping means ( 74 ) derives from a pixel drive level a digital value which represents a time period for the control of the supply switch ( 78 ) of each current source circuit. The mapping means ( 74 ) implements a single mapping function for use in providing the digital values for all current source circuits. Having a current source circuit for each column facilitates the application of inversion patterns. The conversion of pixel drive levels to values representing time is carried out in a shared manner, so that the required area is kept to a minimum.
1. An active matrix liquid crystal display device comprising:
an array of pixels arranged in rows and columns, each pixel comprising a liquid crystal cell;
a column driver circuit for providing pixel drive signals to the columns of pixels, wherein the column driver circuit comprises an array of current source circuits, a respective current source circuit being provided for each column of pixels, wherein each current source circuit comprises:
a current source; and
a supply switch for controlling the time during which the current source supplies current to or drains current from the column, and
wherein the device further comprises a mapping means for deriving a digital value from the transmittance of the liquid crystal cell, wherein the digital value represents a time period for the control of the supply switch of each current source circuit, the mapping means implementing a single mapping function for use in providing the digital values for all current source circuits;
wherein the column driver circuit is adapted to apply a polarity inversion scheme such that the current source is controlled to supply current to or drain current from the column thereby generating a voltage corresponding to a positive gamma curve and a voltage corresponding to a negative gamma curve on a capacitor of the liquid crystal cell alternately.
2. A device as claimed in claim 1 , wherein the mapping means comprises a look-up table.
3. A device as claimed in claim 1 , wherein each current source circuit further comprises a counter for converting the digital value into a time value.
4. A device as claimed in claim 1 , wherein each current source circuit comprises a precharge switch for connecting the column to a precharge voltage.
5. A device as claimed in claim 1 , wherein the current source of each current source circuit comprises a unipolar current source.
6. A device as claimed in claim 1 , wherein the current source of each current source circuit comprises a bidirectional current source.
7. A device as claimed in claim 1 , wherein the current source of each current source circuit supplies or drains a constant current over time.
8. A device as claimed in claim 1 , further comprising means for identifying at least one calibration pixel for use in calibrating the current sources of the current source circuits.
9. A device as claimed in claim 2 , wherein the look-up table stores digital values representing time having more bits than the number of bits corresponding to the number of colour levels of the display device.
10. A device as claimed in claim 4 , wherein the precharge voltage is below the lowest pixel drive voltage or above the highest pixel drive voltage.
11. A device as claimed in claim 4 , wherein the precharge voltage is between the lowest pixel drive voltage and the highest pixel drive voltage.
12. A device as claimed in claim 8 , wherein the calibration pixel is identified as one which is driven to a predetermined drive level.
13. A device as claimed in claim 8 , further comprising sample-and-hold circuitry for storing a drive voltage which results in the calibration pixel after addressing the pixel.
14. A device as claimed in claim 8 , wherein the means for identifying is for identifying a plurality of calibration pixels, and wherein each respective calibration measurement is for controlling the current source circuits of a respective group of columns.
15. A device claimed in claim 8 , wherein the calibration pixel or pixels comprise a dedicated calibration pixel or pixels outside a normal pixel display area.
16. A device as claimed in claim 12 , wherein the predetermined drive level is a maximum or minimum drive level.
17. A device as claimed in claim 13 , further comprising means for adjusting the current output of the current sources of the current source circuits in response to the drive voltage.
18. A method of driving the pixels of a liquid crystal active matrix display device comprising an array of pixels arranged in rows and columns, the method comprising:
deriving a digital value representing a time period for each column from the transmittance of the liquid crystal cell for each column using a common mapping of the transmittance of the liquid crystal cell to digital values;
driving an array of current source circuits, with a respective current source circuit for each column of pixels, each pixel comprising a liquid crystal cell, each current source circuit being driven for the time value corresponding to the respective digital value; and
applying a polarity inversion scheme when driving the array of current source circuits such that the current source is controlled to supply current to or drain current from the column thereby generating a voltage corresponding to a positive gamma curve and a voltage corresponding to a negative gamma curve on a capacitor of the liquid crystal cell alternately.
19. A method as claim in claim 18 , wherein deriving a digital value comprises addressing a look-up table.
20. A method as claimed in claim 18 , further comprising, before driving the array of current source circuits, connecting the column to a precharge voltage.
21. A method as claimed in claim 18 , wherein driving the array of current source circuits comprising supplying or draining a constant current over time.
22. A method as claimed in claim 18 , further comprising:
identifying at least one calibration pixel for use in calibrating the current source circuits.
23. A method as claimed in claim 22 , wherein the calibration pixel is identified as one which is driven to a predetermined drive level.
24. A method as claimed in claim 22 , further comprising storing a drive voltage which results in the calibration pixel after addressing the pixel.
25. A method as claimed in claim 22 , further comprising adjusting the current output of current source circuits in response to the drive voltage.
26. A method as claimed in claim 22 , comprising identifying a plurality of calibration pixels, and further comprising controlling the current source circuits of a respective group of columns based on each calibration pixel.
27. A method as claimed in claim 23 , wherein the predetermined drive level is a maximum or minimum drive level.