This invention relates to active matrix display devices, in particular having a pixel configuration using a thin film transistor switching device.
This type of display typically comprises an array of pixels arranged in rows and columns. Each row of pixels shares a row conductor which connects to the gates of the thin film transistors of the pixels in the row. Each column of pixels shares a column conductor, to which pixel drive signals are provided. The signal on the row conductor determines whether the transistor is turned on or off.
In the case of a liquid crystal display, when the transistor is turned on, by a high voltage pulse on the row conductor, a signal from the column conductor is allowed to pass on to an area of liquid crystal material, thereby altering the light transmission characteristics of the material. An additional storage capacitor may be provided as part of the pixel configuration to enable a voltage to be maintained on the liquid crystal material even after removal of the row electrode pulse.
The frame (field) period for active matrix display devices requires a row of pixels to be addressed in a short period of time, and this in turn imposes a requirement on the current driving capabilities of the transistor in order to charge or discharge the liquid crystal material to the desired voltage level. In order to meet these current requirements, the gate voltage supplied to the thin film transistor needs to fluctuate between values separated by approximately 20-30 volts. For example, the transistor may be turned off by applying a gate voltage of around −8 volts, or even lower, (with respect to the source) whereas a voltage of around 15 volts, or even higher, may be required to bias the transistor sufficiently to provide the required source-drain current to charge or discharge the liquid crystal material sufficiently rapidly.
The gate voltage for the drive transistor when turned off also needs to be sufficiently low to ensure that charge does not leak away during the frame time.
The requirement for large voltage swings in the row conductors requires the row driver circuitry to be implemented using high voltage components. This results in larger IC devices and a more expensive integrated circuit. This also results in high power consumption, increased risk for metal track corrosion at higher voltages and increased (stress induced) degradation rate of the TFTs.
The required gate voltages depend on a number of factors including the materials used for the TFTs, the layout and external parameters such as light and temperature. It has been recognised that temperature has an effect on the threshold voltage of TFTs. In particular, the threshold voltage increases at lower temperatures and this requires an increased gate turn-on voltage. The leakage current increases at higher temperatures, and this requires the TFT to be turned harder off at higher temperatures (which requires a lower gate turn-off voltage).
Conventionally, these parameters are factored in to the turn-on and turn-off gate voltages so that satisfactory turn-on and turn-off performance is provided for all operating temperatures.
It has, however, also been proposed in US 2001/0040543 to make the gate turn-on voltage controlled in dependence on temperature, so that consistent pixel charging characteristics are obtained at different temperatures.
A further difficulty which arises in the design and control of liquid crystal displays results from so-called kickback. A kickback voltage results from the parasitic gate-source capacitance (CGS) in the drive transistor. When the gate voltage changes from the on to the off level, charge is transferred from the pixel storage capacitor (CS) and the LC cell capacitance (CLC) to the parasitic capacitance. This results in a change in voltage which alters the grey scale output of the pixel. This change in voltage is called the kickback voltage:
V
K
=C
GS/(CGS+CLC+CS) (VON−VOFF)
Where VON and VOFF are the on and off gate voltages. It is known to correct for the kickback effect, which gives rise to flicker, by using a DC compensation voltage to counter the kickback effect. This DC voltage is applied to the common electrode. There are various other more complicated methods of compensating for kickback which will be well known to those skilled in the art.
A problem with known schemes which manipulate the drive transistor gate voltages, for example in dependence on temperature, is that compensation schemes for kickback do not then operate correctly, or else more complicated compensation schemes are required. In particular, the kickback voltage itself is dependent on the control voltage levels applied to the drive transistor.
According to the invention, there is provided a display device comprising an array of pixels, each pixel comprising a thin film transistor switching device and a display element, the array being arranged in rows and columns, wherein each row of pixels shares a row conductor, which connects to the gates of the thin film transistors of the pixels in the row, wherein row driver circuitry provides row address signals for controlling the switching of the transistors of the pixels of the row, wherein the row address signals each comprise a waveform for providing an ON gate voltage and an OFF gate voltage to the drive transistor, wherein the device further comprises control circuitry for shifting the ON gate voltage and the OFF gate voltage in dependence on drive and/or environmental conditions, the control circuitry maintaining a constant difference between the ON gate voltage and the OFF gate voltage.
In this device, the gate control levels are shifted in response to drive and/or environmental conditions, and this allows the gap between the on and off voltages to be reduced, which results in power savings. In addition, the gap between the on and off gate control levels is maintained constant, so that the kickback voltage is constant, and therefore can be compensated in conventional manner.
A temperature sensor may be provided, and the control circuitry then shifts the ON gate voltage and the OFF gate voltage in dependence on temperature. In particular, the ON gate voltage and the OFF gate voltage are both higher for lower temperatures than for higher temperatures.
Alternatively or additionally, the control circuitry can shift the ON gate voltage and the OFF gate voltage in dependence on the display device refresh rate. In particular, the ON gate voltage and the OFF gate voltage are both higher for higher refresh rates than for lower refresh rates.
These compensation schemes allow power savings to be maximised.
Preferably, each column of pixels shares a column conductor to which pixel drive signals are provided, and wherein column address circuitry provides the pixel drive signals.
The display device of the invention can be used in a portable battery operated device, and the power savings provided then have particular benefit.
The invention also provides a row driver circuit for an active matrix display device for providing row address signals, in which device each pixel comprises a thin film transistor switching device and a display element, and the row address signals are provided to the gates of the thin film transistors of the pixels in the row, wherein row driver circuit comprises:
means for providing row address signals comprising a waveform for providing an ON gate voltage and an OFF gate voltage to the drive transistor,
an input for receiving a control signal dependent on drive and/or environmental conditions; and
means for shifting the ON gate voltage and the OFF gate voltage in response to the control signal, and maintaining a constant difference between the ON gate voltage and the OFF gate voltage.
The invention also provides a method of generating row address signals for an active matrix display device, the method comprising:
providing row address signals comprising a waveform for providing an ON gate voltage and an OFF gate voltage to the drive transistors of the pixels in a row, and
shifting the ON gate voltage and the OFF gate voltage in dependence on drive and/or environmental conditions whilst maintaining a constant difference between the ON gate voltage and the OFF gate voltage.
The shifting may again be in dependence on temperature and/or the display device refresh rate.
Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:
In order to drive the liquid crystal cell 16 to a desired voltage to obtain a required gray level, an appropriate signal is provided on the column conductor 12 in synchronism with a row address pulse on the row conductor 10. This row address pulse turns on the thin film transistor 14, thereby allowing the column conductor 12 to charge the liquid crystal cell 16 to the desired voltage, and also to charge the storage capacitor 20 to the same voltage. At the end of the row address pulse, the transistor 14 is turned off, and if a storage capacitor 20 is used then this maintains a voltage across the cell 16 when other rows are being addressed. The storage capacitor 20 reduces the effect of liquid crystal leakage and reduces the percentage variation in the pixel capacitance caused by the voltage dependency of the liquid crystal cell capacitance. The rows are addressed sequentially so that all rows are addressed in one frame period, and refreshed in subsequent field periods.
As shown in
In order to enable a sufficient current to be driven through the thin film transistor 14, which is typically implemented as an amorphous silicon thin film device, a high gate voltage must be used. In particular, the period during which the transistor is turned on is approximately equal to the total frame period within which the display must be refreshed, divided by the number of rows. The gate voltage for the on-state and the off-state may differ by 20-30 volts in order to provide the required small leakage current in the off-state, and sufficient current flow in the on-state to charge or discharge the liquid crystal cell 16 within the available time. As a result, the row driver circuitry 30 uses high voltage components.
The pulse height 39 must be sufficiently large that when the column carries the highest pixel drive signal, the peak gate voltage gives rise to a sufficient gate-source voltage above threshold to turn on the drive transistor. Similarly, the lowest gate drive voltage must be below the threshold voltage for the lowest pixel drive signal. In the circuit of
The voltage swing on the column electrode signal required by the drive scheme of
In
These drive schemes will be well known to those skilled in the art.
In this description and claims the term “ON gate voltage” and “OFF gate voltage” are used to refer to the effective gate voltages applied to the drive transistor to turn the drive transistor on and off. The effective gate voltage is the voltage relative to the voltages applied to the column, which determine the source and drain voltages of the drive transistor. In the case of the drive scheme of
The invention uses control circuitry to shift the (effective) ON gate voltage and the OFF gate voltage - in dependence on drive and/or environmental conditions. The control circuitry maintains a constant difference between the ON gate voltage and the OFF gate voltage, and thereby effectively shifts the complete row waveform up and down in dependence on conditions. Because the gap between the on and off gate voltages is maintained constant, the kickback voltage is constant and can be compensated in conventional manner.
In one example, the sensing/control circuitry 54 comprises a temperature sensor. The control circuitry then shifts the ON gate voltage and the OFF gate voltage in dependence on temperature,,in particular to higher values for lower temperatures than for higher temperatures.
The sensing/control circuitry may instead or additionally provide the display refresh rate to the circuitry 50. A display may have different refresh rates for different modes of operation. For example, a lower refresh rate may be used in a standby mode of operation or in other modes of operation when only slowly changing images are to be displayed. This may be a power saving technique, and this invention provides further power saving opportunities. The refresh rate may itself be controlled in dependence on temperature, for example a slower refresh rate may be acceptable at lower temperatures, when the LC response is slower and when leakage currents are lower.
The ON gate voltage and the OFF gate voltage will be higher for higher refresh rates than for lower refresh rates.
The adaptive control of the gate voltages using the invention enables the voltage height of the row address pulses to be reduced, whilst still allowing the pixel circuits to function satisfactorily for the prevailing environmental conditions and drive conditions.
The implementation of the circuitry shown in
The invention enables convention kickback compensation to be employed. For example, a DC offset may be applied to the common electrode waveform 48 of
The power savings enabled by the invention are of particular benefit in portable devices.
The invention can be applied to displays using many different technologies. Amorphous silicon drive transistors require particularly large voltage swings, but the invention may also be applied to displays using polycrystalline silicon pixel transistors. Furthermore, the invention can be applied to other display technologies and is not limited to liquid crystal displays.
The terms “row” and “column” are somewhat arbitrary in the description and claims. These terms are intended to clarify that there is an array of elements with orthogonal lines of elements sharing common connections. Although a row is normally considered to run from side to side of a display and a column to run from top to bottom, the use of these terms is not intended to be limiting in this respect.
The row and column circuits may be implemented as integrated circuits, and the invention also relates to the row circuit for implementing the display architecture described above.
Other features of the invention will be apparent to those skilled in the art.
Number | Date | Country | Kind |
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0402046.7 | Jan 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2005/050346 | 1/27/2005 | WO | 00 | 7/25/2006 |