The present invention relates to a liquid crystal display device, and more particularly relates to a liquid crystal display device having a function of switching a liquid crystal driving frequency.
A liquid crystal display device is used in various electronic appliances such as a television receiver and a personal computer. Generally, it is preferred that a liquid crystal display device is low in power consumption. In particular, a liquid crystal display device used in portable electronic appliances (e.g., a mobile telephone, a handheld computer) is strongly requested to achieve low power consumption.
As one of methods for reducing power consumption in a liquid crystal display device, there has been known a method for switching a liquid crystal driving frequency. For example, in a case where a liquid crystal display device is used in a handheld computer, when a state in which the handheld computer receives no manipulation input from a user continues for a predetermined time or more, a liquid crystal driving frequency may be set to be lower than that in a normal state. When the liquid crystal driving frequency is set to be low, power consumption is reduced considerably although a cycle of updating a screen becomes long.
On the other hand, a liquid crystal has a characteristic in that degradation takes place in a short time by application of a direct-current voltage. For this reason, the liquid crystal display device performs alternating-current driving for switching a polarity of a liquid crystal application voltage at every predetermined cycle. Moreover, when an effective value of the liquid crystal application voltage upon application of a positive polarity voltage (hereinafter, referred to as “in a positive polarity”) is different from an effective value of the liquid crystal application voltage upon application of a negative polarity voltage (hereinafter, referred to as “in a negative polarity”), flicker occurs at the screen. In order to prevent this flicker, a process of adjusting a voltage to be applied to a common electrode (hereinafter, referred to as a common voltage Vcom) is performed to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity.
With reference to
Thereafter, when a low-level voltage Vgl is applied to the gate line Gj, the TFT 12 turns into an OFF state. Since a parasitic capacitor is provided between the gate and the drain of the TFT 12, when the gate voltage Vg changes from Vgh to Vgl, the drain voltage Vd drops by a predetermined amount. A drop amount ΔV in such a case is referred to as a pull-in voltage or a feed-through voltage, and is expressed by the following equation (1).
ΔV=Vgp-p×Cgd/(Clc+Ccs+Cgd) (1)
In the equation (1), Vgp-p represents a gate voltage amplitude (=Vgh−Vgl), Clc represents a capacitance value of the liquid crystal capacitor 13, Ccs represents a capacitance value of the auxiliary capacitor 14, and Cgd represents a capacitance value of the parasitic capacitor between the gate and the drain of the TFT 12.
After the TFT 12 turns into the OFF state, a leak current flows through the TFT 12; therefore, the drain voltage Vd gradually rises or drops to approach the common voltage Vcom. This state continues until the high-level voltage Vgh is applied to the gate line Gj after one frame period.
In the pixel circuit 11, the liquid crystal capacitor 13 corresponds to a liquid crystal element. A transmittance of a liquid crystal panel is determined based on the effective value of the liquid crystal application voltage, that is, an effective value of a difference between the drain voltage Vd and the common voltage Vcom (a diagonally shaded portion in
In relation to the present invention, Patent Document 1 describes a technique of changing a common voltage or a signal voltage in accordance with a length of a write holding time. Moreover, Patent Document 2 describes a technique of changing both a gate-on voltage and a common voltage in accordance with a horizontal synchronous frequency.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-116739
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2001-13930
As described above, in order to prevent the flicker, the process of adjusting the common voltage Vcom is performed in the liquid crystal display device. Herein, the common voltage Vcom is adjusted such that the flicker becomes minimum when the screen is displayed at a specific liquid crystal driving frequency.
However, an off characteristic (an amount of leak current) in the positive polarity and an off characteristic in the negative polarity are different from each other in the TFT 12. Moreover, upper and lower substrates forming the liquid crystal capacitor 13 are different in characteristic from each other. In the liquid crystal element, therefore, the voltage holding ratio in the positive polarity and the voltage holding ratio in the negative polarity are different from each other. For this reason, even when the common voltage Vcom is adjusted such that the effective voltage in the positive polarity becomes equal to the effective voltage in the negative polarity at a certain liquid crystal driving frequency, both the effective voltages do not necessarily become equal to each other at a different liquid crystal driving frequency. Accordingly, when the liquid crystal driving frequency is switched, flicker occurs at the screen to degrade display quality. Moreover, when the liquid crystal driving frequency is switched, a direct-current voltage is applied to a liquid crystal, so that there arises a problem that the liquid crystal is degraded in a short time.
Patent Documents 1 and 2 describe the methods for preventing the flicker from occurring when the liquid crystal driving frequency is switched. According to these methods, however, the common voltage Vcom or the signal voltage is changed in accordance with the liquid crystal driving frequency. Consequently, it becomes difficult to perform a process of adjusting the common voltage Vcom or the signal voltage. In a liquid crystal display device that employs a direct-current voltage as a common voltage, particularly, when the common voltage Vcom is changed in accordance with a liquid crystal driving frequency, an effective value of a liquid crystal application voltage varies largely, resulting in change of brightness of a screen. Consequently, a signal voltage, which can not be adjusted with ease, must be changed in accordance with the liquid crystal driving frequency, in addition to the common voltage Vcom.
Hence, it is an object of the present invention to provide a liquid crystal display device that prevents flicker from occurring when a liquid crystal driving frequency is switched, by a method different from the conventional method.
A first aspect of the present invention has a feature in that a liquid crystal display device having a function of switching a liquid crystal driving frequency includes: a plurality of pixel circuits each including a liquid crystal element, the pixel circuits being arranged at intersections of a plurality of scanning signal lines and a plurality of data signal lines; a scanning signal line drive circuit for switching between a selection voltage and a non-selection voltage to apply the switched voltage to the scanning signal line; and a data signal line drive circuit for switching between a positive polarity voltage and a negative polarity voltage, both in accordance with display data, to apply the switched voltage to the data signal line, and herein, at least one of a difference between a first voltage corresponding to a selection voltage upon application of the positive polarity voltage and a second voltage corresponding to a non-selection voltage after application of the positive polarity voltage and a difference between a third voltage corresponding to a selection voltage upon application of the negative polarity voltage and a fourth voltage corresponding to a non-selection voltage after application of the negative polarity voltage changes in accordance with a driving frequency of the liquid crystal element.
A second aspect of the present invention has a feature in that, in the first aspect of the present invention, any one voltage among the first to fourth voltages changes in accordance with the driving frequency.
A third aspect of the present invention has a feature in that, in the first aspect of the present invention, a plurality of voltages among the first to fourth voltages change in accordance with the driving frequency.
A fourth aspect of the present invention has a feature in that, in the first aspect of the present invention, the third voltage is higher than the fourth voltage, and the difference between the third voltage and the fourth voltage becomes large as the driving frequency is low.
A fifth aspect of the present invention has a feature in that, in the fourth aspect of the present invention, the fourth voltage becomes low as the driving frequency is low.
A sixth aspect of the present invention has a feature in that, in the fourth aspect of the present invention, the third voltage becomes high as the driving frequency is low.
A seventh aspect of the present invention has a feature in that, in the first aspect of the present invention, the first voltage is higher than the second voltage, and the difference between the first voltage and the second voltage becomes large as the driving frequency is low.
An eighth aspect of the present invention has a feature in that, in the seventh aspect of the present invention, the second voltage becomes low as the driving frequency is low.
A ninth aspect of the present invention has a feature in that, in the seventh aspect of the present invention, the first voltage becomes high as the driving frequency is low.
A tenth aspect of the present invention has a feature in that, in the first aspect of the present invention, the liquid crystal display device further includes a voltage generation circuit for generating the selection voltage and the non-selection voltage to supply the generated voltage to the scanning signal line drive circuit, and herein, the voltage generation circuit separately supplies the first voltage and the third voltage, and/or the second voltage and the fourth voltage to the scanning signal line drive circuit.
An eleventh aspect of the present invention has a feature in that a method for driving a liquid crystal display device including a plurality of pixel circuits each including a liquid crystal element, the pixel circuits being arranged at intersections of a plurality of scanning signal lines and a plurality of data signal lines, includes the steps of: switching between a selection voltage and a non-selection voltage to apply the switched voltage to the scanning signal line; and switching between a positive polarity voltage and a negative polarity voltage, both in accordance with display data, to apply the switched voltage to the data signal line, and herein, at least one of a difference between a selection voltage upon application of the positive polarity voltage and a non-selection voltage after application of the positive polarity voltage and a difference between a selection voltage upon application of the negative polarity voltage and a non-selection voltage after application of the negative polarity voltage changes in accordance with a driving frequency of the liquid crystal element.
According to the first or eleventh aspect of the present invention, at least one of the voltage amplitude on the scanning signal line in the positive polarity and the voltage amplitude on the scanning signal line in the negative polarity is changed in accordance with the driving frequency of the liquid crystal element. Therefore, it is possible to set an effective value of a liquid crystal application voltage in the positive polarity to be equal to an effective value of a liquid crystal application voltage in the negative polarity irrespective of the liquid crystal driving frequency, and to prevent flicker from occurring when the liquid crystal driving frequency is switched.
According to the second aspect of the present invention, in accordance with the driving frequency of the liquid crystal element, any one voltage among the first to fourth voltages changes, so that the voltage amplitude on the scanning signal line in the positive polarity or the voltage amplitude on the scanning signal line in the negative polarity changes. Accordingly, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the simple circuitry.
According to the third aspect of the present invention, in accordance with the driving frequency of the liquid crystal element, a plurality of voltages among the first to fourth voltages change, so that at least one of the voltage amplitude on the scanning signal line in the positive polarity and the voltage amplitude on the scanning signal line in the negative polarity changes. Accordingly, it is possible to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity with higher accuracy, and to more suitably prevent flicker from occurring when the liquid crystal driving frequency is switched.
According to the fourth aspect of the present invention, in the case where the selection voltage is higher than the non-selection voltage, the voltage amplitude on the scanning signal line in the negative polarity is set to be large as the driving frequency of the liquid crystal element is low. Accordingly, it is possible to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity irrespective of the liquid crystal driving frequency, and to prevent flicker from occurring when the liquid crystal driving frequency is switched.
According to the fifth aspect of the present invention, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the simple circuitry. Moreover, when the liquid crystal driving frequency is low, the non-selection voltage in the negative polarity is set to be low. Thus, it is possible to reduce a leak current from the transistor in the pixel circuit, and to improve a voltage holding ratio of the liquid crystal element.
According to the sixth aspect of the present invention, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the simple circuitry.
According to the seventh aspect of the present invention, in the case where the selection voltage is higher than the non-selection voltage, the voltage amplitude on the scanning signal line in the positive polarity is set to be large as the driving frequency of the liquid crystal element is low. Accordingly, it is possible to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity irrespective of the liquid crystal driving frequency, and to prevent flicker from occurring when the liquid crystal driving frequency is switched.
According to the eighth or ninth aspect of the present invention, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the simple circuitry.
According to the tenth aspect of the present invention, by use of a voltage to be supplied from the voltage generation circuit to the scanning signal line drive circuit, at least one of the voltage amplitude on the scanning signal line in the positive polarity and the voltage amplitude on the scanning signal line in the negative polarity is changed in accordance with the driving frequency of the liquid crystal element. Thus, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched.
1 Liquid crystal display device
10 Pixel array
11 Pixel circuit
12 TFT
13 Liquid crystal capacitor
14 Auxiliary capacitor
20 Display control circuit
30 Gate line drive circuit
31 Shift register
32 Switch control circuit
33 Switch circuit
34 Flip-flop
35 Analog switch
40 Source line drive circuit
50 Gate voltage generation circuit
51 Vgh generation circuit
52 Vgl generation circuit
53 Resistance division circuit
54 Vgln selection circuit
Vgh Gate high voltage
Vglp Gate low voltage in positive polarity
Vgln Gate low voltage in negative polarity
Vcom Common voltage
The pixel array 10 includes m gate lines G1 to Gm, n source lines S1 to Sn, and (m×n) pixel circuits 11. The gate line is also referred to as a scanning signal line, and the source line is also referred to as a data signal line. The gate lines G1 to Gm are arranged in parallel with one another, and the source lines S1 to Sn are arranged in parallel with one another so as to be orthogonal to the gate lines G1 to Gm. The pixel circuits 11 are provided at intersections of the gate lines G1 to Gm and the source lines S1 to Sn. Each pixel circuit 11 is connected to one gate line and one source line. The pixel circuit 11 includes an N-channel type TFT 12, a liquid crystal capacitor 13, and an auxiliary capacitor 14 (see
The display control circuit 20 controls operations of the liquid crystal display device 1. More specifically, based on control signals provided from external (such as a horizontal synchronous signal HSYNC and a vertical synchronous signal VSYNC), the display control circuit 20 outputs control signals to the gate line drive circuit 30 and the source line drive circuit 40, and outputs display data DT provided from external to the source line drive circuit 40 at a suitable timing.
Based on the control signals output from the display control circuit 20, the gate line drive circuit 30 switches voltages at the gate lines G1 to Gm to a high level (a selection voltage: hereinafter, referred to as a gate high voltage Vgh) or a low level (a non-selection voltage: hereinafter, referred to as a gate low voltage Vgl). The gate voltage generation circuit 50 generates the gate high voltage Vgh and the gate low voltage Vgl, and supplies the generated voltages to the gate line drive circuit 30. One gate line among the gate lines G1 to Gm is selected by the action of the gate line drive circuit 30.
Based on the control signal SC and the display data DT output from the display control circuit 20, the source line drive circuit 40 controls voltages at the source lines S1 to Sn. By the action of the source line drive circuit 40, a voltage in accordance with the display data DT is written to the pixel circuit 11 connected to the gate line selected by the gate line drive circuit 30.
The liquid crystal display device 1 performs alternating-current driving for switching a polarity of a liquid crystal application voltage at every predetermined cycle. The liquid crystal display device 1 may perform frame inversion driving for switching a polarity of a liquid crystal application voltage on a frame period basis or may perform line inversion driving for switching the polarity of the liquid crystal application voltage on a line period basis. In order to perform the alternating-current driving, the source line drive circuit 40 switches between a positive polarity voltage and a negative polarity voltage, both in accordance with the display data DT, and applies the switched voltage to the source lines S1 to Sn. In the following, it is assumed that the source line drive circuit 40 switches between the positive polarity voltage and the negative polarity voltage at every one line period and at every one frame period, and applies the switched voltage to the source lines S1 to Sn.
The liquid crystal display device 1 has a function of switching a liquid crystal driving frequency. More specifically, the liquid crystal display device 1 receives a frequency selection signal FQS indicating a liquid crystal driving frequency, and the display control circuit 20 switches between an output timing of the control signal and an output timing of the display data DT in accordance with the frequency selection signal FQS. In the following, it is assumed that the liquid crystal display device 1 switches the liquid crystal driving frequency at four levels, that is, 60 Hz, 50 Hz, 40 Hz and 30 Hz. Moreover, it is assumed that the liquid crystal driving frequency is 60 Hz in a normal state.
In the liquid crystal display device 1, as will be described later, the gate low voltage Vgl after application of the negative polarity voltage changes in accordance with the liquid crystal driving frequency. Thus, a gate voltage amplitude in the negative polarity changes in accordance with the liquid crystal driving frequency. In the following, a gate low voltage after application of a positive polarity voltage is referred to as a gate low voltage Vglp in a positive polarity, a gate low voltage after application of a negative polarity voltage is referred to as a gate low voltage Vgln in a negative polarity, and differences between a gate high voltage Vgh and the respective gate low voltages are referred to as a gate voltage amplitude Vgp-p(p) in the positive polarity and a gate voltage amplitude Vgp-p(n) in the negative polarity.
The gate voltage generation circuit 50 includes a Vgh generation circuit 51, a Vgl generation circuit 52, a resistance division circuit 53, and a Vgln selection circuit 54. The Vgh generation circuit 51 generates a fixed gate high voltage Vgh, and the Vgl generation circuit 52 generates a fixed gate low voltage Vgl. The resistance division circuit 53 includes a plurality of resistors connected in series, and divides, using the resistors, the gate low voltage Vgl generated by the Vgl generation circuit 52 to output a plurality (four in this case) of voltages.
In accordance with the frequency selection signal FQS, the Vgln selection circuit 54 selects one voltage from the four voltages output from the resistance division circuit 53. As the liquid crystal driving frequency is low, the Vgln selection circuit 54 selects the low voltage. The voltage selected by the Vgln selection circuit 54 is supplied to the gate line drive circuit 30, as the gate low voltage Vgln in the negative polarity.
Moreover, the highest voltage among the four voltages output from the resistance division circuit 53 is supplied to the gate line drive circuit 30, as the gate low voltage Vglp in the positive polarity. The gate high voltage Vgh generated by the Vgh generation circuit 51 is also supplied to the gate line drive circuit 30. As described above, the gate voltage generation circuit 50 generates the fixed gate high voltage Vgh, the fixed gate low voltage Vglp in the positive polarity, and the gate low voltage Vgln in the negative polarity which changes in accordance with the liquid crystal driving frequency. A relation expressed by the following equation (2) is established among these voltages.
Vgln(30 Hz)<Vgln(40 Hz)<Vgln(50 Hz)<Vgln(60 Hz)=Vglp<Vgh (2)
The display control circuit 20 outputs a gate clock GCK, a gate start pulse GSP, and a gate voltage selection signal GVS to the gate line drive circuit 30. The gate clock GCK is a clock signal having a cycle corresponding to one line period, and the gate start pulse GSP is a signal which turns into the high level only for one line period within one frame period. The gate voltage selection signal GVS is a signal indicating whether a liquid crystal application voltage being written has a positive polarity or a negative polarity. The gate voltage selection signal GVS turns into the high level when the liquid crystal application voltage has the positive polarity, and turns into the low level when the liquid crystal application voltage has the negative polarity. A polarity inversion signal REV indicating a polarity of a voltage to be applied to the source lines S1 to Sn may be used in place of the gate voltage selection signal GVS. In a case of performing driving for switching the common voltage Vcom at the two levels, that is, the high and low levels, a common voltage control signal COM indicating a level of the common voltage Vcom may be used.
The gate line drive circuit 30 includes a shift register 31 having m stages, m switch control circuits 32, and m switch circuits 33. The switch control circuit 32 and the switch circuit 33 are provided in correspondence with each stage of the shift register 31. The gate start pulse GSP is input to the first stage of the shift register 31, the gate clock GCK is input to each stage of the shift register 31, and the gate voltage selection signal GVS is input to each switch control circuit 32.
In accordance with the gate clock GCK, the shift register 31 sequentially shifts the gate start pulse GSP. Ina case where Xj represents an output from the j-th stage of the shift register 31, an output X1 turns into the high level at a first line period in one frame period, and an output X2 turns into the high level at a subsequent line period. Similarly, the output Xj sequentially turns into the high level at every one line period.
The voltage at the gate line Gj changes as follows in accordance with the outputs Xj and Yj (see
Upon change from the high level to the low level, the voltage at the gate line changes to Vglp or Vgln in accordance with the preceding gate voltage selection signal GVS. Specifically, the gate voltage selection signal GVS is at the high level when the voltage at the gate line Gj is Vgh; therefore, the voltage at the gate line Gj changes to Vglp. The same thing holds true for the voltage at the gate line Gj+2. On the other hand, the gate voltage selection signal GVS is at the low level when the voltage at the gate line Gj+1 is Vgh; therefore, the voltage at the gate line Gj+1 changes to Vgln. As described above, the voltage at the gate line changes to Vglp in the positive polarity and changes to Vgln in the negative polarity.
Based on the equation (2), a relation expressed by the following equation (3) is established between the gate voltage amplitude Vgp-p(p)(=Vgh−Vglp) in the positive polarity and the gate voltage amplitude Vgp-p(n)(=Vgh−Vgln) in the negative polarity.
Vgp-p(n, 30 Hz)>Vgp-p(n, 40 Hz)>Vgp-p(n, 50 Hz)>Vgp-p(n, 60 Hz)=Vgp-p(p) (3)
As described above, in the liquid crystal display device 1, the gate low voltage Vgl in the negative polarity becomes low as the liquid crystal driving frequency is low. Thus, the gate voltage amplitude Vgp-p(n) in the negative polarity becomes large as the liquid crystal driving frequency is low.
In the case where the liquid crystal driving frequency is 60 Hz (
In contrast to this, in the case where the liquid crystal driving frequency is 40 Hz (
Hereinafter, description will be given of effects of the liquid crystal display device 1 according to this embodiment, in comparison with a conventional liquid crystal display device in which a gate voltage amplitude Vgp-p is fixed. In the liquid crystal display device 1, when the liquid crystal driving frequency is 60 Hz, the drain voltage at the TFT 12 changes as shown in
The TFT 12 has a current characteristic shown in
Also in the conventional liquid crystal display device, when a liquid crystal driving frequency is 60 Hz, flicker can be prevented by the method described above. In the conventional liquid crystal display device, when the liquid crystal driving frequency is 40 Hz, a drain voltage at a TFT changes as shown in
However, a common voltage Vcom is adjusted with respect to the case where the liquid crystal driving frequency is 60 Hz. Consequently, in a case where the drop rate in a positive polarity is slower than the rise rate in a negative polarity with regard to the drain voltage during the TFT off period, if the liquid crystal driving frequency is 40 Hz, an effective voltage Vrms(p) in the positive polarity becomes higher than an effective voltage Vrms(n) in the negative polarity. In the conventional liquid crystal display device, accordingly, when the liquid crystal driving frequency is 40 Hz, flicker occurs at a screen. In order to prevent this flicker, the common voltage Vcom needs to be changed such that the effective voltage Vrms(p) in the positive polarity becomes equal to the effective voltage Vrms(n) in the negative polarity.
In contrast to this, in the liquid crystal display device 1, when the liquid crystal driving frequency is 40 Hz, the drain voltage at the TFT 12 changes as shown in
In the liquid crystal display device 1, moreover, the gate low voltage in the negative polarity at the time when the liquid crystal driving frequency is 40 Hz is lower than the gate low voltage in the negative polarity at the time when the liquid crystal driving frequency is 60 Hz (Vgln (40 Hz)<Vgln (60 Hz)). As shown in
In the same manner, the gate low voltage Vgln (30 Hz) in the negative polarity at the time when the liquid crystal driving frequency is 30 Hz is determined so that the effective voltage Vrms(p) in the positive polarity becomes equal to the effective voltage Vrms(n) in the negative polarity at the time when the liquid crystal driving frequency is 30 Hz. The gate low voltage Vgln (50 Hz) in the negative polarity at the time when the liquid crystal driving frequency is 50 Hz is determined so that the effective voltage Vrms(p) in the positive polarity becomes equal to the effective voltage Vrms(n) in the negative polarity at the time when the liquid crystal driving frequency is 50 Hz. Thus, it is possible to prevent flicker and to improve an off characteristic of the TFT 12 in the negative polarity even when the liquid crystal driving frequency is 30 Hz or 50 Hz.
As described above, the liquid crystal display device 1 according to this embodiment changes the gate voltage amplitude Vgp-p(n) in the negative polarity in accordance with the liquid crystal driving frequency to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity irrespective of the liquid crystal driving frequency. Thus, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched. Moreover, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the common voltage Vcom fixed. Further, as the liquid crystal driving frequency is low, the gate low voltage Vgln in the negative polarity is set to be low. Thus, it is possible to reduce the leak current from the TFT 12 and to improve the voltage holding ratio of the liquid crystal element.
With regard to the liquid crystal display device 1, the following modification examples may be configured. In a liquid crystal display device that performs alternating-current driving, at least one of a difference between a selection voltage upon application of a positive polarity voltage (a first voltage) and a non-selection voltage after application of the positive polarity voltage (a second voltage) and a difference between a selection voltage upon application of a negative polarity voltage (a third voltage) and a non-selection voltage after application of the negative polarity voltage (a fourth voltage) may change in accordance with a liquid crystal driving frequency. In order to achieve this configuration, any one voltage among the four voltages or a plurality of voltages among the four voltages may change in accordance with the liquid crystal driving frequency.
In a case where the selection voltage is higher than the non-selection voltage, the difference between the third voltage and the fourth voltage may become large as the liquid crystal frequency is low. In order to achieve this configuration, the fourth voltage may become low or the third voltage may become high as the liquid crystal driving frequency is low. Alternatively, the difference between the first voltage and the second voltage may become large as the liquid crystal driving frequency is low. In order to achieve this configuration, the second voltage may become low or the first voltage may become high as the liquid crystal driving frequency is low.
In the liquid crystal display device 1, specifically, the gate low voltage Vgln in the negative polarity becomes low as the liquid crystal driving frequency is low. Thus, the gate voltage amplitude Vgp-p(n) in the negative polarity becomes high as the liquid crystal driving frequency is low. Instead of this configuration, the gate high voltage Vghn in the negative polarity may become high as the liquid crystal driving frequency is low. Alternatively, the gate low voltage Vglp in the positive polarity may become low or the gate high voltage Vghp in the positive polarity may become high as the liquid crystal driving frequency is low. Thus, the gate voltage amplitude Vgp-p(p) in the positive polarity may become large as the liquid crystal driving frequency is low.
Moreover, the gate high voltage Vghn in the negative polarity and the gate low voltage Vgln in the negative polarity may change in accordance with the liquid crystal driving frequency. Thus, the gate voltage amplitude Vgp-p(n) in the negative polarity may become large as the liquid crystal driving frequency is low. Further, the gate high voltage Vghp in the positive polarity and the gate low voltage Vglp in the positive polarity may change in accordance with the liquid crystal driving frequency. Thus, the gate voltage amplitude Vgp-p(p) in the positive polarity may become large as the liquid crystal driving frequency is low. In addition, a plurality of voltages among the voltages Vghp, Vghn, Vglp and Vgln may change in accordance with the liquid crystal driving frequency. Thus, both the gate voltage amplitude Vgp-p(p) in the positive polarity and the gate voltage amplitude Vgp-p(n) in the negative polarity may change in accordance with the liquid crystal driving frequency.
As in the liquid crystal display device 1, the liquid crystal display device according to these modification examples allows prevention of flicker occurring when the liquid crystal driving frequency is switched. In particular, one voltage is changed in accordance with the liquid crystal driving frequency. Thus, it is possible to prevent flicker from occurring when the liquid crystal driving frequency is switched, with the simple circuitry. Moreover, a plurality of voltages are changed in accordance with the liquid crystal driving frequency. Thus, it is possible to set the effective value of the liquid crystal application voltage in the positive polarity to be equal to the effective value of the liquid crystal application voltage in the negative polarity with higher accuracy, and to more suitably prevent flicker from occurring when the liquid crystal driving frequency is switched.
The liquid crystal display device may switch the liquid crystal driving frequency at levels other than four levels or may continuously switch the liquid crystal driving frequency. In place of the Vgln selection circuit 54, moreover, the liquid crystal display device may include a gate voltage selection circuit for selecting a voltage to be applied to a gate line based on a signal indicating a liquid crystal driving frequency, for example, a dot clock given to the liquid crystal display device. The liquid crystal display device may perform driving for switching the common voltage Vcom at two levels, that is, the high and low levels or may perform driving for switching the common voltage Vcom at three or more levels. Moreover, the common voltage Vcom in the liquid crystal display device may be adjusted with respect to a liquid crystal driving frequency other than the normal state (e.g., 30 Hz).
The liquid crystal display device may perform dot inversion driving for switching a polarity of a liquid crystal application voltage on a dot basis. In the liquid crystal display device that performs the dot inversion driving, the positive polarity voltage and the negative polarity voltage are simultaneously applied to the source line. Therefore, the gate high voltage Vgh and the gate low voltage Vgl can not be separated in the positive polarity and the negative polarity, so that the relations of Vghp=Vghn and Vglp=Vgln are always satisfied. Also in this case, when both the gate high voltage Vghp in the positive polarity and the gate high voltage Vghn in the negative polarity change in accordance with the liquid crystal driving frequency, when both the gate low voltage Vglp in the positive polarity and the gate low voltage Vgln in the negative polarity change in accordance with the liquid crystal driving frequency, or when the voltages Vghp, Vghn, Vglp and Vgln change in accordance with the liquid crystal driving frequency while satisfying the relations of Vghp=Vghn and Vglp=Vgln, flicker can be prevented from occurring when the liquid crystal driving frequency is switched, as in the liquid crystal display device
According to the liquid crystal display device described above, since no flicker occurs at the screen even when the liquid crystal driving frequency is switched, it is possible to display the screen with low power consumption while keeping display quality favorably. Moreover, since a direct-current voltage is prevented from being applied to the liquid crystal, it is possible to prevent the liquid crystal from being degraded.
The liquid crystal display device of the present invention has a feature in that no flicker occurs at a screen even when a liquid crystal driving frequency is switched, and therefore is applicable to various electronic appliances such as a mobile telephone and a handheld computer.
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WO2008/139656 | 11/20/2008 | WO | A |
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