Liquid crystal display capable of compensating common voltage signal thereof

An exemplary liquid crystal display includes a liquid crystal panel having a plurality of pixel units arranged in rows, a scanning circuit configured to activate the pixel units row by row by outputting a plurality of corresponding scanning signals, a data circuit configured to provide data voltage signals to the activated pixel units, and a common voltage circuit. Each pixel unit includes a coupling member. When a row of pixel units is activated, all the coupling members in the row of pixel units cooperatively generate a coupling signal according to the data voltage signals applied to the activated row of pixel units, and superpose the coupling signal to the corresponding scanning signal so as to form a feedback signal. The common voltage circuit adjusts a reference voltage signal according to the feedback signal, and provides at least one common voltage signal to the pixel units.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to, and claims the benefit of, a foreign priority application filed in China as Ser. No. 200710074604.0 on May 25, 2007. The related application is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to liquid crystal displays (LCDs), and more particularly to an LCD capable of compensating a common voltage signal thereof.

GENERAL BACKGROUND

LCDs are widely used in various information products, such as notebooks, personal digital assistants, video cameras, and the like.

FIG. 3is an abbreviated circuit diagram of a conventional LCD. The LCD100includes a liquid crystal panel101, a scanning circuit102, and a data circuit103. The liquid crystal panel101includes n rows of parallel scanning lines110(where n is a natural number), m columns of parallel data lines120perpendicular to the scanning lines110(where m is also a natural number), and a plurality of pixel units140cooperatively defined by the crossing scanning lines110and data lines120. The scanning lines110are connected to the scanning circuit102, and the data lines120are connected to the data circuit130.

Each pixel unit140includes a thin film transistor (TFT)141, a pixel electrode142, and a common electrode143. A gate electrode of the TFT141is connected to a corresponding one of the scanning lines110, and a source electrode of the TFT141is connected to a corresponding one of the data lines120. Further, a drain electrode of the TFT141is connected to the pixel electrode142. The common electrodes143of all the pixel units140are connected together and further connected to a common voltage generating circuit (not shown). In each pixel unit140, liquid crystal molecules (not shown) are disposed between the pixel electrode142and the common electrode143, so as to cooperatively form a liquid crystal capacitor147.

In operation, the common electrodes143receive a common voltage signal from the common voltage generating circuit. The scanning circuit102provides a plurality of scanning signals to the scanning lines110sequentially, so as to activate the pixel units140row by row. The data circuit103provides a plurality of data voltage signals to the pixel electrodes142of the activated pixel units140. Thereby, the liquid crystal capacitors147of the activated pixel units140are charged. After the charging process, an electric field is generated between the pixel electrode142and the common electrode143in each pixel unit140. The electric field drives the liquid crystal molecules to control light transmission of the pixel unit140, such that the pixel unit140displays a particular color (red, green, or blue) having a corresponding gray level. The electric field is maintained by the liquid crystal capacitor147during a so-called current frame period, and accordingly the gray level of the color is maintained during the current frame period.

In the LCD100, each pixel unit140employs a capacitor structure (i.e. the liquid crystal capacitor147) to maintain the gray level of the color. In addition, a plurality of parasitic capacitors usually exist in the pixel unit140. Due to a so-called capacitor coupling effect, when the data voltage signal received by the pixel electrode142changes, an electrical potential of the common electrode143may be coupled and shift from the common voltage signal.

The shift of the electrical potential of the common electrode143may further bring on a change of the electric field between the pixel electrode142and the common electrode143. Thereby, the gray level of the color displayed by the pixel unit140is apt to change, and accordingly a so-called color shift phenomenon may be generated. Thus the display quality of the LCD100may be somewhat unsatisfactory.

What is needed is to provide an LCD that can overcome the above-described deficiencies.

SUMMARY

In a first aspect, a liquid crystal display includes a liquid crystal panel having a plurality of pixel units arranged in rows, a scanning circuit configured to activate the pixel units row by row by outputting a plurality of corresponding scanning signals, a data circuit configured to provide data voltage signals to the activated pixel units, and a common voltage circuit. Each pixel unit includes a coupling member. When a row of pixel units is activated, all the coupling members in the row of pixel units cooperatively generate a coupling signal according to the data voltage signals applied to the activated row of pixel units, and superpose the coupling signal to the corresponding scanning signal so as to form a feedback signal. The common voltage circuit adjusts a reference voltage signal according to the feedback signal, and provides at least one common voltage signal to the pixel units.

In a second aspect, a liquid crystal display includes a plurality of pixel units arranged in rows and cooperatively defined by a plurality of scanning lines and a plurality of data lines, a scanning circuit configured to activate the pixel units row by row via the scanning lines, a data circuit configured to provide data voltage signals to an activated row of the pixel units via the data lines, and a common voltage circuit. Each pixel unit comprises a pixel electrode, a common electrode, and a coupling member, the coupling members transfer electrical potential shifts of the common electrodes to a corresponding one of the scanning lines when the data voltage signals are applied to the pixel electrodes of the activated row of pixel units, and the common voltage circuit generates at least one common voltage signal according to a feedback signal obtained from the corresponding scanning line.

Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe preferred and exemplary embodiments of the present invention in detail.

FIG. 1is an abbreviated circuit diagram of an LCD according to an exemplary embodiment of the present invention. The LCD200includes a liquid crystal panel201, a scanning circuit202, a data circuit203, a common voltage circuit205, and a power supply circuit206.

The liquid crystal panel201includes n rows of parallel scanning lines210(where n is a natural number), n rows of parallel common lines230alternately arranged with the scanning lines210, m columns of parallel data lines220perpendicular to the scanning lines210and the common lines230(where m is also a natural number), and a plurality of pixel units240cooperatively defined by the crossing scanning lines210and data lines220. Thus, the pixel units240are arranged in a matrix having n rows and m columns.

Each pixel unit240includes a TFT241, a pixel electrode242, a common electrode243, a storage capacitor248, and a coupling capacitor245. A gate electrode of the TFT241is connected to a corresponding one of the scanning lines210, and a source electrode of the TFT241is connected to a corresponding one of the data lines220. Further, a drain electrode of the TFT241is connected to the pixel electrode242. The common electrode243is generally opposite to the pixel electrode242, with a plurality of the liquid crystal molecules (not shown) sandwiched therebetween. The common electrode243, the pixel electrode242, and the liquid crystal molecules cooperatively form a liquid crystal capacitor247. The coupling capacitor245is connected between the pixel electrode242and the corresponding scanning line210. The storage capacitor248is connected between the pixel electrode242and the corresponding common line230. In particular, a capacitance of the coupling capacitor245is the same as a sum of capacitances of the corresponding storage capacitor248and liquid crystal capacitor247.

The power supply circuit206is configured to provide power voltage to the scanning circuit202, the data circuit203, and the common voltage circuit205. The power supply circuit206includes a first power output terminal261for outputting a low level power voltage to the scanning circuit202, a second output terminal262for outputting a high level power voltage to the scanning circuit202, a third power output terminal263for outputting a digital power voltage DVCCto the data circuit203, and a fourth power output terminal264for outputting an analog power voltage AVCCto the common voltage circuit205.

The scanning circuit202is configured for providing a plurality of scanning signals to activate the pixel units240row by row. The scanning circuit202includes a first input terminal221for receiving the low level power voltage, a second input terminal for receiving the high level power voltage, a feedback terminal223for outputting a feedback signal VFBto the common voltage circuit205, and a plurality of pulse output terminals224for outputting the scanning signals to the scanning lines210respectively.

The data circuit203is configured for providing a plurality of data voltage signals to the corresponding pixel units240. The data circuit203includes a plurality of data voltage output terminals232, each of which is connected to a respective one of the data lines220.

The common voltage circuit205is configured for providing common voltage signals for the pixel units240. The common voltage circuit205includes a feedback input terminal251, a power input terminal252, a first common voltage output terminal253, and a second common voltage output terminal254. The feedback input terminal251is configured for receiving the feedback signal VFB. The power input terminal252is configured for receiving the analog power voltage AVCC. The first common voltage output terminal253and the second common voltage output terminal254are respectively connected to the common lines230and the common electrodes243of the pixel units240.

In particular, the common voltage circuit205further includes a reference voltage generator257and a compensating circuit258therein. The reference voltage generator257is capable of providing a reference voltage signal VREFto the compensating circuit258. The compensating circuit258is capable of adjusting the reference signal VREFaccording to the feedback signal VFB, so as to generate the common voltage signals.

Referring toFIG. 2, the compensating circuit258includes an input terminal301, a filter capacitor302, a first compensating branch310, and a second compensating branch320. The first compensating branch310and the second compensating branch320have a common terminal303. The input terminal301is connected to the feedback input terminal251of the common voltage circuit205, such that the feedback signal VFBcan be applied to the compensating circuit258. The filter capacitor302is configured as a filter member for filtering a direct current (DC) component from the feedback signal VFB, and is connected between the input terminal301and the common terminal303.

Circuit structures of the first compensating branch310and the second compensating branch320are the same. Each of the first and second compensating branches310,320includes a voltage adjusting circuit319and an output circuit314. The voltage adjusting circuit319includes an integrated operational amplifier (IOA)311connected in a negative feedback arrangement between the common terminal303and the output circuit314. In particular, an inverting terminal of the IOA311is connected to the common terminal303via a first resistor312, and is connected to an output terminal of the IOA311via a second resistor313. A non-inverting terminal of the IOA311is configured to receive the reference voltage signal VREFfrom the reference voltage generator257. The output circuit314employs a so-called complementary circuit, such that an output resistance of the first compensating branch310is diminished. Moreover, the output circuits314of the first and second compensating branches310,320are respectively connected to the first common voltage output terminal253and the second common voltage output terminal254.

Typical operation of the LCD200is as follows.

The power supply circuit206provides a low level power voltage and a high level power voltage to the scanning circuit202, and simultaneously provides a digital power voltage DVCCand an analog power voltage AVCCto the data circuit203and the common voltage circuit205respectively.

The reference voltage generator257generates and outputs a reference voltage signal VREFto the IOAs311of the first and second compensating branches310,320. The reference voltage signal VREFis treated as a predetermined common voltage signal by each of the IOAs311, and is transmitted to the corresponding output circuit314. The predetermined common voltage signal is then outputted to the common lines230and the common electrodes243of the pixel unit240.

The scanning circuit202provides a plurality of scanning signals, and outputs the scanning signals to the scanning lines210sequentially via the pulse output terminals224. Thereby, the TFTs241of the corresponding pixel units240are switched on, so as to activate the corresponding pixel units240. In particular, each of the scanning signals is a pulse signal. A high level voltage of the pulse signal is determined by the high level power voltage, and a low level voltage of the pulse signal is determined by the low level power voltage.

The data circuit203provides a plurality of data voltage signals, and outputs the data voltage signals to the pixel electrodes242of the corresponding activated pixel units240via the data lines220and the corresponding TFTs241. Once the data voltage signal is received by each corresponding pixel electrode242, due to a capacitor coupling effect, a first interference voltage signal VIF1is correspondingly generated in the common electrode243by the liquid crystal capacitor247and the storage capacitor248. Thereby, an electrical potential of the common electrode243is coupled and shifts.

The first interference voltage signal VIF1is an alternating current (AC) voltage signal. Assuming that the data voltage signal applied to the pixel electrode242of the pixel unit240in the current frame period is VN, and a data voltage signal applied to the pixel electrode242of the pixel unit240in the previous frame period is VN-1, a primary value of the first interference voltage signal VIF1can be calculated by the equation ΔV=VN−VN-1(i.e. a change of the data voltage signal applied thereto), and the absolute value of the first interference voltage signal VIF1drops gradually in an exponential manner. That is, the first interference voltage signal VIF1can be expressed by the following equation VIF1=ΔV*(1−e−t/τ), where the symbol t represents a period of time, and the symbol r represents a time constant.

Because all the common electrodes243in the activated row of pixel units240are connected together, electrical potentials of these common electrodes243shift simultaneously. That is, each of the common electrodes243has a respective first interference voltage signal VIF1generated therein. All the first interference voltage signals VIF1cooperatively form a first coupling signal VCP1. The first coupling signal VCP1superposes the predetermined common voltage signal, such that a first superposing signal is formed in all the common electrodes243.

Similarly, due to the coupling capacitor245, a second interference voltage signal VIF2is also generated in the gate electrode of the TFT241of the corresponding activated pixel unit240. Thereby, an electrical potential of the gate electrode of the TFT241is also coupled and shifts. Because the second interference voltage signal VIF2also results from the changing of the data voltage signal applied to the pixel unit240, it is substantially equal to the first interference voltage signal VIF1.

Because the pixel units240are activated row by row via the corresponding scanning lines210in sequence, electrical potentials of the gate electrodes of the TFTs241in the activated row of pixel units240shift simultaneously. That is, each of the gate electrodes has a respective second interference voltage signal VIF2generated therein. All the second interference voltage signals VIF2cooperatively form a second coupling signal VCP2that is equal to the first coupling signal VCP1. The second coupling signal VCP2further superposes the corresponding scanning signal, such that a second superposing signal that is substantially equal to the first superposing signal is formed in the corresponding scanning line210. The second superposing signal is then sampled by the scanning circuit202from the scanning line210. The sampling signal obtained by the scanning circuit202serves as the feedback signal VFB, and is outputted to the compensating circuit258via the feedback terminal223.

In the compensating circuit258, the filter capacitor302filters the feedback signal VFB, so as to remove the DC component thereof (i.e. the scanning signal). Thereby, the second coupling signal VCP2is extracted from the feedback signal VFB, and is then outputted to the first and second compensating branches310,320. In the first compensating branch310, the IOA311compares the reference voltage signal VREFwith the second coupling signal VCP2, and further adjusts the reference voltage signal VREFaccording to a result of the comparison, so as to generate a first adjusted common voltage signal. The second compensating branch320carries out a similar operation simultaneously, and accordingly generates a second adjusted common voltage signal substantially equal to the first adjusted common voltage signal. The first and second adjusted common voltage signals operate to replace the predetermined common voltage signal, and are respectively outputted to the common lines230and the common electrodes243of the pixel units240.

The data voltage signals, together with the first adjusted common voltage signal, charge the storage capacitors248of the activated row of pixel units240. In addition, the data voltage signals, together with the second adjusted common voltage signal, charge the corresponding liquid crystal capacitors247. Thereby, an electric field is generated between the pixel electrode242and the common electrode243in each pixel unit240after the charging process. The electric field drives the liquid crystal molecules of the pixel unit240to control the light transmission of the pixel unit240, such that the pixel unit240displays a particular color (e.g., red, green, or blue) having a corresponding gray level. Moreover, the gray level of the color displayed by the pixel unit240is maintained by cooperation of the storage capacitor248and liquid crystal capacitor247. The aggregation of colors displayed by all the pixel units240simultaneously constitutes an image viewed by a user of the LCD200.

In summary, in the LCD200, a plurality of coupling capacitors245are provided in the pixel units240of the liquid crystal panel201. Due to the coupling capacitors245, an electrical potential coupling in the common electrode243of each pixel unit240is transferred to the gate electrode of the corresponding TFT241, and the shift of the common voltage signal is transferred to a shift of the scanning signal. The common voltage circuit205adjusts the reference voltage signal according to a feedback signal VFBobtained by sampling the scanning signal, such that the shift of the common voltage signal is compensated. Thereby, the electric field between the pixel electrode342and the common electrode343of each pixel unit340is stable during the current frame period. Moreover, because the feedback signal VFBis obtained from the scanning signal that is provided by the scanning circuit202, the feedback signal VFBis independent of other voltage signals, including the adjusted common voltage signal. By employing such feedback signal VFB, the compensation of the common voltage signal is more reliable. Therefore, the gray level of the color displayed by the pixel unit340is stable. Accordingly, any color shift phenomenon that might otherwise be induced because of the capacitor coupling effect is diminished or even eliminated, and the display quality of the LCD200is improved.

In alternative embodiments, the coupling capacitor245in each pixel unit240can employ a parasitic capacitor between the gate electrode and drain electrode of the corresponding TFT241. The compensating circuit258can include only one compensating branch, with the single compensating branch outputting an adjusted common voltage signal to the common lines230and the common electrodes243of the pixel unit240. The compensating circuit258can include three or more compensating branches, with the compensating branches respectively outputting adjusted common voltage signals generated therein to predetermined regions of the pixel units240in the liquid crystal panel201.

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 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.