The present invention relates to methods of driving liquid crystal displays (LCDs); and more particularly to a method for driving an active matrix type LCD, which enables a resulting total luminous flux corresponding to gray scales of images to be displayed to be uniform across plural pixels.
Because LCD devices have the advantages of portability, low power consumption, and low radiation, they have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCD devices are considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
During a first frame, i.e. a period between a time t1 and a time t3, a gate electrode driving device (not shown) supplies a scanning voltage Vg to drive the gate electrode 1040 of the TFT 104. After the TFT 104 is turned on, a source electrode driving device (not shown) supplies a gray scale voltage Vd to the pixel electrode 103 through the source electrode 1041 and the drain electrode 1042 of the TFT 104. Thereby, the pixel electrode 103 is charged to a voltage Vp1 while the gray scale voltage Vd is maintained. When the time t is equal to t2, the TFT 104 is turned off by turning off the supply of the scanning voltage Vg, whereupon the capacitor 107 maintains the voltage Vp1 until the TFT 104 is turned on at t=t3.
Similarly, during a second frame, when t is equal to t3, the scanning voltage Vg is supplied to drive the TFT 104. The pixel electrode 103 is charged to a voltage Vp2 while the gray scale voltage Vd is maintained. At t=t4, the TFT 104 is turned off by turning off the supply of the scanning voltage Vg, whereupon the capacitor 107 maintains the voltage Vp2.
In the active matrix LCD 100, the gray scale voltage Vd corresponds to the gray scale of each of pixels that display images. That is, if the gray scales of all the pixels are equal, then the gray scale voltages Vd applied to the pixels should also be equal. However, liquid crystal molecules used in the liquid crystal layer of the active matrix LCD 100 are liable to be sticky, and normal manufacturing error is liable to result in the capacitors 107 of the pixels having slightly different capacitances. Therefore, even if the gray scale voltage Vd provided to all the pixel electrodes 103 is the same, this does not necessarily ensure that the voltages Vp maintained by the capacitors 107 are all equal. That is, the luminous flux in each pixel may differ from that in other pixels. As a result, the gray scales in the pixels may be different from each other, even when equal gray scale voltages Vd are provided thereto. This means the active matrix LCD 100 may not be able to provide clear, even images.
It is desired to provide a method for driving an active matrix LCD which can overcome the above-described deficiencies.
A method for driving a liquid crystal display includes: providing a liquid crystal display having a plurality of pixel units and a backlight; dividing a frame time into a plurality of sub-frames; defining each pixel unit to have two states, namely on or off, in each of the sub-frames; defining the backlight to have a gradation luminance and two states, namely on or off, in each of the sub-frames; and synchronously controlling the state of each pixel unit, a time period of the on state of each pixel unit, the gradation luminance of the backlight, and a time period of the on state of the backlight in each of the sub-frames to make a resulting total luminous flux in each pixel unit corresponding to a gray scale of an image to be displayed in the frame time to be the same as that of other pixel units.
Advantages and novel features of the method will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:
Reference will now be made to the drawings to describe preferred and exemplary embodiments of the present invention in detail.
The first substrate includes n rows of parallel scan lines 201 and m columns of parallel data lines 202. The data lines 202 are electrically insulated from and perpendicular to the scan lines 201. The first substrate further includes a plurality of thin-film transistors (TFTs) 204, which function as switching elements to drive respective pixel electrodes 203. Each of the TFTs 204 is positioned in the vicinity of the crossover of a corresponding scan line 201 and a corresponding data line 202. A gate electrode 2040 of the TFT 204 is electrically coupled to the scan line 201, and a source electrode 2041 of the TFT 204 is electrically coupled to the data line 202. Further, a drain electrode 2042 of the TFT 204 is electrically coupled to a corresponding pixel electrode 203. The second substrate includes a plurality of common electrodes 205 opposite to the pixel electrodes 203. In particular, the common electrodes 205 are formed on a surface of the second substrate facing the first substrate, and are made from a transparent material such as ITO (Indium-Tin Oxide) or the like. Each pixel electrode 203 and a respective common electrode 205 cooperatively form a capacitor 207. A pixel electrode 203, a common electrode 205 facing the pixel electrode 203, and liquid crystal molecules of the liquid crystal layer sandwiched between the two electrodes 203, 205 cooperatively define a single pixel unit.
An exemplary method for driving the active matrix LCD 200 includes the steps of: dividing a frame time into a plurality of sub-frames; defining each pixel unit to have two states, namely on or off, in each of the sub-frames, wherein a time period of the on state of each pixel unit may be less than, or equal to, or greater than a time period of the sub-frame; defining the backlight to have a gradation luminance and two states, namely on or off, in each of the sub-frames, wherein a time period of the on state of the backlight may be less than, or equal to, or greater than the time period of the sub-frame; and synchronously controlling the state of each pixel unit, the time period of the on state of each pixel unit, the gradation luminance of the backlight, and the time period of the on state of the backlight in each of the sub-frames to make a resulting total luminous flux in each pixel unit corresponding to a gray scale of an image to be displayed in the frame time to be the same as that of other pixel units.
Referring to
In the illustrated embodiment, for example, x and y are both defined as 8. Thus a frame is divided into 8 sub-frames T0˜T7, and the gradation luminance of the backlight is divided into 8 levels L0˜L7.
During the sub-frame T0, a gate electrode driving device (not shown) supplies a scan voltage Vg to drive the gate electrode 2040 of the TFT 204 at a time t0. Thereby, the TFT 204 is turned on. In addition, a source electrode driving device (not shown) supplies a gray-scale voltage Vs to the pixel electrode 203 through the source electrode 2041 and the drain electrode 2042. The pixel electrode 203 is charged to a voltage Vp because of the gray-scale voltage Vs supplied. When the scan voltage Vg is turned off to turn off the TFT 204 at a time t0′, the capacitor 207 maintains the voltage Vp of the pixel electrode 203. The backlight is turned on to provide light beams at the L0 level of the gradation luminance during the sub-frame T0. Then the pixel unit is switched from an off state to an on state by the voltage Vp of the pixel electrode 203.
During the sub-frame T1, the gate electrode driving device supplies the scan voltage Vg to drive the gate electrode 2040 of the TFT 204 at a time t1. Thereby, the TFT 204 is turned on. In addition, the source electrode driving device supplies the gray-scale voltage VS to the pixel electrode 203 through the source electrode 2041 and the drain electrode 2042. The pixel electrode 203 is charged to the voltage Vp because of the gray-scale voltage Vs supplied. When the scan voltage Vg is turned off to turn off the TFT 204 at a time t1′, the capacitor 207 maintains the voltage Vp of the pixel electrode 203. The backlight is turned on to provide light beams at the L1 level of the gradation luminance during the sub-frame T1. Then the pixel unit is maintained in the on state.
During the sub-frame T2, the gate electrode driving device supplies the scan voltage Vg to drive the gate electrode 2040 of the TFT 204 at a time t2. Thereby, the TFT 204 is turned on. In addition, the source electrode driving device supplies a restoring voltage Vh to the pixel electrode 203 through the source electrode 2041 and the drain electrode 2042. The pixel electrode 203 is charged to a voltage Vh′ because of the restoring voltage Vh supplied. When the scan voltage Vg is turned off to turn off the TFT 204 at a time t2′, the capacitor 207 maintains the voltage Vh′ of the pixel electrode 203. The backlight is turned on to provide light beams at the L2 level of the gradation luminance during the sub-frame T2. Though the backlight provides light beams during the sub-frame T2, the pixel unit is switched from the on state to the off state by the restoring voltage Vh′ of the pixel electrode 203.
During the sub-frame T3, the gate electrode driving device supplies the scan voltage Vg to drive the gate electrode 2040 of the TFT 204 at a time t3. Thereby, the TFT 204 is turned on. In addition, the source electrode driving device supplies the gray-scale voltage Vs to the pixel electrode 203 through the source electrode 2041 and the drain electrode 2042. The pixel electrode 203 is charged to the voltage Vp because of the gray-scale voltage Vs supplied. When the scan voltage Vg is turned off to turn off the TFT 204 at a time t3′, the capacitor 207 maintains the voltage Vp of the pixel electrode 203. The backlight is turned on to provide light beams at the L3 level of the gradation luminance during the sub-frame T3. Then the pixel unit is switched from the off state to the on state by the voltage Vp of the pixel electrode 203.
The same kind of process continues during the sub-frame T3, the sub-frame T4, the sub-frame T5, and the sub-frame T6. Then during the sub-frame T7, the gate electrode driving device supplies the scan voltage Vg to drive the gate electrode 2040 of the TFT 204 at a time t7. Thereby, the TFT 204 is turned on. In addition, the source electrode driving device supplies the gray-scale voltage Vs to the pixel electrode 203 through the source electrode 2041 and the drain electrode 2042. The pixel electrode 203 is charged to the voltage Vp because of the gray-scale voltage Vs supplied. When the scan voltage Vg is turned off to turn off the TFT 204 at a time t7′, the capacitor 207 maintains the voltage Vp of the pixel electrode 203. The backlight is turned on to provide light beams at the L7 level of the gradation luminance during the sub-frame T7. Then the pixel unit is maintained in the on state until the end of the frame.
The above-described method for driving the active matrix LCD 200 requires that the liquid crystal molecules have fast response capability. In particular, ferroelectric liquid crystal having a response time in the order of microseconds is preferred. Either of the following two types of ferroelectric liquid crystal may for example be used: surface stabilized ferroelectric liquid crystal (SSFLC), and soft mode ferroelectric liquid crystal (SMFLC).
According to the above-described method, an integral of total luminous flux (IF) in each pixel unit corresponding to a gray scale of an image to be displayed can be obtained by controlling the following four parameters: the state of each pixel unit (A), the time of an on state of each pixel unit (t), the gradation luminance of the backlight (L), and the time of an on state of the backlight (T). The integral of the total luminous flux may be expressed by the following equation:
IF ==∫L·T·A·tdt
In summary, each pixel unit in a sub-frame has only two states: on or off. When the pixel unit is in the off state, the driving voltage is zero. On the other hand, when the pixel unit is in the on state, only a driving voltage greater than the threshold voltage of the pixel unit is needed to turn the pixel unit on. Therefore the driving voltage need only have two states. Thus even when normal manufacturing error results in the capacitors 207 of the pixel units having different capacitances from each other, when the above-described method for driving the active matrix LCD 200 is used, all of the pixel units may have a same luminous flux corresponding to a same gray scale. That is, images displayed by the active matrix LCD 200 operating according to the exemplary driving method are clear and even.
In alternative embodiments, for example, one or several sub-frames may be used as a black insertion period Tr. During the period Tr, one or both of the pixel units and the backlight may be turned off. In addition, a time period divided into several sub-periods is not limited to being a frame. The dividend time period may be a period of time needed for driving a row or a column of the pixel units, or a period of time needed for driving a plurality of rows or a plurality of columns of the pixel units.
It is to be further understood that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of structures and functions associated with the embodiments, the disclosure is illustrative only, and changes may be made in detail (including in matters of shape, size, and arrangement of parts) within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
Number | Date | Country | Kind |
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94135973 | Oct 2005 | TW | national |