JP2002318565A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that a bias current has increased in order to speed up an amplifier circuit, resulting in increasing the power consumption. SOLUTION: The output circuit of a common driving amplifier is provided with an auxiliary capacitance and a switch circuit for controlling charge and discharge. In such a manner, a common charge time can be shorten, and the amplifier can apparently be speeded up. Further, there is no increase in power consumption.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for driving an active matrix type liquid crystal panel.
In particular, the present invention relates to a liquid crystal driving circuit that realizes a common AC driving method in which liquid crystal is AC driven by switching first and second common voltages applied to a common electrode of a liquid crystal panel in a frame cycle.

[0002]

2. Description of the Related Art In recent years, active matrix type liquid crystal displays have been widely used as displays for personal computers, but also as small displays for portable information terminals and mobile phones.
A secondary battery is mainly used as a power source of the portable information terminal and the portable telephone. Therefore, a display used in these devices is required to operate at an extremely low power consumption.

In a liquid crystal display, a data line and a gate line for driving each display pixel are arranged in a matrix on a glass substrate, and a thin film transistor (hereinafter, referred to as a TFT) is formed near an intersection of the data line and the gate line to form a liquid crystal. Display is performed by applying a drive voltage according to the display information.
The data lines and the gate lines are driven by a plurality of liquid crystal driving circuits (liquid crystal drivers). In order to operate such a liquid crystal display with low power consumption, it is necessary to reduce the power consumption of the liquid crystal driver. Especially for LCD drivers,
A large number of operational amplifier circuits are mounted to convert the liquid crystal driving voltage according to display information (gradation information), and reducing the power consumption is an important issue. In addition, since the liquid crystal display drives the liquid crystal sandwiched between two sheets of glass with a voltage, the liquid crystal itself has a very large capacitance, and the operational amplifier drives this large capacitive load. There is a need.

Therefore, a configuration of an operational amplifier capable of driving a large capacitive load with low power consumption for driving a liquid crystal panel is disclosed in Japanese Patent Application Laid-Open No. 9-27721. The differential input terminal of the amplifier is further provided with a differential amplifier circuit and a current supply circuit, which serve as an auxiliary buffer for assisting driving of a large load connected to the basic operational amplifier. This allows a large current to be supplied to the load,
It is stated that an operational amplifier with an improved slew rate, that is, capable of driving a large capacitive load, can be realized.

[0005]

In the above prior art, a large capacitive load can be driven by adding an auxiliary buffer to a basic operational amplifier. However, in order to operate the auxiliary buffer, a bias current is required. Is required, and the power consumption increases. The basic operational amplifier and the auxiliary buffer are both differential input amplifiers, and supply or sink current to the additional connected to the output according to the voltage difference between the differential inputs. Since the operational amplifier and the auxiliary buffer operate in parallel, a current supplied from the auxiliary buffer flows into the operational amplifier due to a difference in differential gain and an offset of each differential input. This can further increase power consumption. Therefore, a circuit device for preventing a shoot-through current is also disclosed in the above prior art,
Since the operation point of the differential input is shifted by the operational amplifier and the auxiliary buffer, it is necessary to pay close attention to the evaluation of the stability of the circuit system during the parallel operation of the respective circuits. In short, it is necessary to design a circuit in which the stability of the operation is ensured by considering the variation of the circuit elements for each circuit.

An object of the present invention is to drive a capacitive load at a high slew rate without increasing the power consumption of the circuit in a drive circuit for driving a very large capacitive load such as a liquid crystal panel common electrode. To provide a liquid crystal display device equipped with a common electrode drive circuit.

[0007]

In order to achieve the above object, a liquid crystal display device according to the present invention provides a liquid crystal display device in which a first and a second common voltage applied to a common electrode of a liquid crystal panel are switched at a frame period so that an alternating current is applied to the liquid crystal. In a liquid crystal display device to be driven, a common drive amplifier for supplying the first common voltage, an auxiliary capacitor for storing electric charge for switching the voltage of the common electrode from the second common voltage to the first common voltage, and A first switch circuit for short-circuiting or opening between the auxiliary capacitance and the common electrode; a second switch circuit for short-circuiting or opening between the common drive amplifier and the common electrode; A third switch circuit for short-circuiting or opening between auxiliary capacitances, and a short-circuiting or opening between the common capacitance and the second common voltage source; To constituted by a fourth switch circuit.

Further, the liquid crystal display device of the present invention is a liquid crystal display device in which the liquid crystal is AC-driven by switching the first and second common voltages applied to the common electrode of the liquid crystal panel at a frame period. A first common drive amplifier for supplying a voltage, a first auxiliary capacitance for assisting the first common drive amplifier to charge the common electrode, A first switch circuit for short-circuiting or opening the circuit, a second switch circuit for short-circuiting or opening the first common drive amplifier and the common electrode, the first common drive amplifier and the first A third switch circuit that short-circuits or opens the auxiliary capacitor, a second common drive amplifier that supplies the second common voltage, and a second common drive amplifier A second auxiliary capacitance for assisting an operation for discharging from the common electrode, a fourth switch circuit for short-circuiting or opening the second auxiliary capacitance and the common electrode, and a second common drive amplifier And a fifth switch circuit for short-circuiting or opening between the common electrode and the second common drive amplifier and a sixth switch circuit for short-circuiting or opening between the second common drive amplifier and the second auxiliary capacitance. Is done.

It is desirable that the first, second, third, fourth, fifth, and sixth switch circuits be constituted by MOS transistors, respectively.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows an example of a liquid crystal display device to which the first embodiment of the present invention is applied. First, each part of FIG. 1 will be described. 1 is display data, 2 is a dot clock synchronized with the display data 1, 3 is a horizontal synchronization signal, 4 is a vertical synchronization signal, 5 is a data driver that stores display data 1 for one horizontal and outputs it to a liquid crystal panel, and 6 is a scanning driver. A driver 7 for a power supply circuit for generating a reference voltage required in the liquid crystal display device; 8 for a gray scale voltage line for providing a reference voltage for the gray scale generated by the power supply circuit 7; 9 for scanning generated by the power supply circuit 7 A scanning voltage line for applying a voltage, 26 is a common amplifier circuit for applying a reference voltage to a common electrode of the liquid crystal panel, 10 is a common voltage line for applying the common voltage generated by the common amplifier circuit 26 to the liquid crystal, and 11 is a common voltage line. A liquid crystal panel 12 is a liquid crystal display device. The liquid crystal panel 11 is a matrix type liquid crystal panel having 240 rows and 320 columns, and has 240 rows of row electrodes G1 to G.
240 is a scanning driver 6 having 320 columns of column electrodes D1 to D
320 is connected to the data driver 5.

FIG. 2 is an enlarged view of the circuit of the pixel on the m-th row and the n-th column of the liquid crystal panel 11, wherein 13 is a thin film transistor (hereinafter simply referred to as TFT) and 25 is a liquid crystal.
The gate terminal and the drain terminal of the TFT 13 are connected to the row electrode Gm of the m-th row and the column electrode Dn of the n-th column, respectively. The liquid crystal 14 is connected to the source terminal of the TFT 13. Further, the other terminal of the liquid crystal 14 is a common terminal of the liquid crystal, and is connected to the common voltage line 10. Further, the common voltage line 10 is connected to all liquid crystals arranged in a matrix of 240 rows and 320 columns. Therefore, the common voltage line 10 in FIG.
The liquid crystal of the pixel is connected, and the load capacitance of the liquid crystal viewed from the common voltage line 10 is a large capacitance of 240 × 320.

FIG. 3 shows an example of a driving waveform for driving the liquid crystal 25 in the first embodiment of the present invention.

FIG. 4 shows an example of a circuit for generating a drive voltage for the common voltage line 10 in the power supply circuit 7. Each part of FIG. 4 will be described. Reference numeral 13 denotes a liquid crystal (hereinafter simply referred to as a liquid crystal load) of 240 × 320 pixels connected to the common voltage line 10, reference numeral 14 denotes an amplifier for generating a first common voltage, and reference numeral 15 denotes an amplifier.
Is a storage capacitor, 16 is a switch for short-circuiting or opening between the auxiliary capacitance and the common voltage line 10, 17 is a switch for short-circuiting or opening between the output of the amplifier 14 and the common voltage line 10, and 18 is an output and an auxiliary for the amplifier 14 A switch for short-circuiting or opening between the capacitor 15 and 19 is a switch for short-circuiting or opening between the common voltage line 10 and the ground.
Reference numeral 23 denotes control lines for controlling short-circuiting or opening of the switches 16 to 19, respectively, and operates so as to be open at a low level and short-circuited at a high level. 24 is a first common voltage. This embodiment is an example in which the second common voltage is ground.

Next, the operation of the liquid crystal display device according to the first embodiment of the present invention will be described with reference to FIGS. In FIG.
The data driver 5 takes in the display data 1 sent in synchronization with the dot clock 2 and stores it as data for one row inside. Further, the data driver 5 outputs the stored display data for one row to the liquid crystal panel 11 as display voltages to the column electrodes D1 to D320 in accordance with the timing of the horizontal synchronization signal 3. On the other hand, in accordance with this operation, the scanning driver 6
According to the horizontal synchronizing signal 3 and the vertical synchronizing signal 4, the row electrode G
A gate-on voltage is output to one of the row electrodes 1 to G240. As a result, the TFT connected to the row electrode to which the gate-on voltage is applied is turned on, and the display voltage currently output by the data driver 5 is applied to each liquid crystal. As for the voltage applied to the liquid crystal, the difference voltage between the display voltage supplied from the data driver 5 and the common voltage supplied from the common voltage line 10 output from the power supply circuit 7 is applied to the liquid crystal itself. By this difference voltage, light and shade can be displayed for each pixel according to the characteristics of the liquid crystal. Further, the voltage applied to the liquid crystal needs to be subjected to an AC drive in which the polarity of the applied voltage is inverted at a predetermined cycle. We are implementing.

Further, a voltage driving waveform applied to the liquid crystal will be described with reference to FIG. FIG. 3 shows that the scanning driver 6 is m
The timing at which a gate-on voltage is applied to the row electrode Gm of the row and the data driver 5 accompanying the timing are applied to the column electrodes D1 to D3.
6 is a diagram illustrating a relationship between a timing at which a display voltage is applied to a common electrode 20 and a timing at which the common amplifier 14 applies a common voltage to a common electrode. After the scan driver 6 applies a gate-on voltage to the row electrode Gm, the data driver 5
The data driver 5 outputs display voltages corresponding to the respective gradation data to 1 to D320. On the other hand, common amplifier 2
6 outputs the first or second common voltage. FIG.
In the example, the first common voltage is output. Next, at the timing when the display voltage output from the data driver 5 and the common voltage are stabilized, the voltage of the row electrode Gm is set to the gate-off voltage. The display voltage given from the data driver 5 at the gate-off timing is held in the liquid crystal, and the display is maintained. At this time, the common voltage is the second common voltage, and the display voltage applied to the liquid crystal is a negative voltage with reference to the second common voltage. Therefore, in the example of FIG. 3, a negative voltage is applied to the pixel connected to the m-th row electrode Gm. At the timing when the gate-on voltage is applied to the next (m + 1) th row electrode Gm + 1, the common voltage is the first common voltage (ground in this embodiment), and based on this, the display voltage applied to the liquid crystal is Positive voltage. As described above, a driving method in which the polarity of the display voltage applied to the liquid crystal for each display line is switched from positive to negative and from negative to positive, and the common voltage is accordingly switched between the first common voltage and the second common voltage. Is called a common AC driving method. Since the common AC driving method is a driving method in which the common voltage of the liquid crystal is exchanged with the first and second voltages, the common amplifier 26 needs to drive the common voltage to a large load of the liquid crystal. In particular, since the display voltage is held in the liquid crystal at the gate-off timing, the common voltage of the liquid crystal needs to be stable at this timing. When this voltage fluctuates, the display voltage applied to the liquid crystal also fluctuates, which causes a significant reduction in display quality such as display unevenness and display flicker. Therefore, in the common AC driving method, it is necessary to increase the speed of the common voltage waveform in order to ensure display quality. Therefore, next, a detailed operation of the common amplifier 26 will be described.

FIG. 4 shows the detailed configuration of the common amplifier 26. FIG. 5 is a timing chart showing the operation of the common amplifier 26. An operation in which the common amplifier 26 switches the common electrode from the second common voltage (that is, ground) to the first common voltage will be described.
In the initial state, that is, before time A, the liquid crystal load 2
The state applied to 5 is the second common voltage (ground). At this time, the switch 19 is on, and the switches 16 and 17 are off. When the switch 18 is turned on, the first common voltage output from the amplifier 14 is charged in the auxiliary capacitance 15.

Next, at time A, switches 18 and 19
Is turned off and the switch 16 is turned on, so that the charge stored in the auxiliary capacitance 15 starts charging the liquid crystal load 25. Then, the potential of the common voltage line 10 rises and rises to the voltage Vas shown in the following equation.

[0019]

(Equation 1) Here, Cas is the capacitance of the auxiliary capacitance 15, CLC is the capacitance of the liquid crystal load 25, and V0 is the first common voltage. Since the rise to the voltage Vas is a movement of electric charge determined by the auxiliary capacitance 15 and the liquid crystal load 25, the amplifier circuit 1
4 has its potential determined irrespective of the driving capability of the device.

Next, at time B, the switch 16 is turned off and the switch 17 is turned on, so that the liquid crystal load 25 is turned on.
Then, the driving is started by the amplifier circuit 14 from the state where the voltage is charged to approximately the voltage Vas. The function of the rise in the potential of the common voltage line 10 at this time is shown by the following equation.

[0021]

(Equation 2) Here, t is the time from the time B as a base point, R is the sum of the output impedance of the amplifier circuit 14, the impedance of the common voltage line 10, and the various impedances inside the liquid crystal panel. Is the impedance related to the drive up to. By controlling the ON and OFF of the switches 16 to 19 in this manner, the switching operation from the second common voltage applied to the liquid crystal load 25 via the common voltage line 10 to the first common voltage can be performed.

Next, at time C, the switch 17 is turned off and the switch 18 is turned on, so that approximately Vas
The lower potential of the auxiliary capacitor 15 is charged by the amplifier circuit 14. The potential of the liquid crystal load 25 is discharged when the switch 19 is turned on, and is discharged to the ground (second
Common voltage) potential.

As described above, the amplifier circuit 14 according to the first embodiment of the present invention employs the common AC driving method in which the first and second common voltages are switched and driven at a constant period with respect to the liquid crystal common electrode. Auxiliary capacity 15 and switches 16-19
, A common AC drive waveform can be generated.

In the common AC driving method, a large liquid crystal load capacitance is driven at a constant period. Therefore, an amplifier circuit for driving the large load capacitance needs to have a large driving capability. The performance of the common AC drive when the amplifier circuit 14 according to the first embodiment of the present invention is applied will be described with reference to FIG. FIG. 6 shows the capacitance CLC of the liquid crystal load 25 and the capacitance Ca of the auxiliary capacitance 15.
The transition time required to switch from the second common voltage to the first common voltage with respect to the ratio of s is plotted on a graph based on the above equations (1) and (2). The transition time is defined as the time required to reach 99% of the target potential. The transition time when the capacitance Cas of the auxiliary capacitance 15 is “0” is set to “1” (unit time), and the transition time with respect to the ratio of the respective capacitances of the auxiliary capacitance 15 and the liquid crystal load 25 is obtained. FIG. 6 shows that the transition time can be greatly reduced by using the auxiliary capacitance 15. In particular, the transition time when the capacitance Cas of the auxiliary capacitance 15 is equal to the capacitance CLC of the liquid crystal load 25 is approximately 15
%. This is because the provision of the auxiliary capacitor 15 and the switch circuits 16 to 19 while maintaining the drive capability of the amplifier circuit 14 allows the common drive capability of the common amplifier 26 to be increased.

The capacitance Cas of the auxiliary capacitance 15 is desirably substantially equal to the capacitance CLC of the liquid crystal load 25. This is because the load fluctuation seen from the amplifier circuit 14 is minimized under this condition.
From the output of the amplifier circuit 14, the liquid crystal load 25 can be seen through the switch 17, and the auxiliary capacitance 15 can be seen through the switch 18. As can be seen from the timing chart of FIG. 5, the switches 17 and 18 are not turned on at the same time, and one of them is turned on or both are turned off. Since only one of the auxiliary capacitor 15 and the liquid crystal load 25 is visible from the viewpoint of the amplifier circuit 14, it is not necessary to drive both of them at once. Therefore, the degree of freedom in designing the amplifier circuit 14 is increased, and it is not necessary to change (increase) the driving capability of the amplifier circuit 14 when the auxiliary capacitance 15 is added.

The first embodiment of the present invention in which the auxiliary capacitance 15 is added
In the embodiment, the charge / discharge power of the liquid crystal load viewed from the common amplifier 26 is as follows.

[0027]

(Equation 3) As shown in this formula, the auxiliary capacitance 15 does not affect the charge / discharge power of the liquid crystal load, and therefore, there is no increase in power consumption by applying this embodiment.

As described above, in the common AC driving method, it has already been described that speeding up of the common voltage waveform is important for ensuring display quality, but the common amplifier 26 of the first embodiment of the present invention is used. As a result, an amplifier circuit having an increased common drive capability can be realized. Therefore, a liquid crystal display device having improved display quality by the common AC driving method can be realized.

Next, a modification of the first embodiment of the present invention will be described with reference to FIG. FIG. 7 shows the switches 16 to 1 shown in FIG.
9 is a circuit example in which 9 is replaced with a MOS circuit. FIG. 7 (a)
7 shows switches 16 to 19, and FIG.
This is a circuit when it is replaced with an S circuit. FIG. 7 (a)
And the terminal symbols of ABC in (b) correspond to each other. In the MOS circuit shown in FIG. 7B, P and N channel MOS transistors are connected in parallel, and a switch circuit is formed between terminals A and B. A terminal C is a control terminal. When this terminal is at a low level, the MOS circuit is turned off, and when this terminal is at a high level, the MOS circuit is turned on. FIG.
The switches 16 to 19 shown in FIG.
By configuring the replacement amplifier circuit, the amplifier circuit itself can be integrated on a semiconductor chip.

Next, a second embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to FIG. FIG. 8 is another configuration example of the circuit of the common amplifier 26 of the liquid crystal display device of the present invention. The second embodiment is a specific example of the circuit of the common amplifier 26 when the second common voltage is not the ground but a different voltage from the first common voltage. First, each symbol will be described. The same parts as those in the first embodiment are denoted by the same reference numerals. 30 is a second common voltage, 31 is an amplifier circuit,
32 is an auxiliary capacitance, 33 to 35 are switches, and 36 to 38 are control lines for controlling the switches 33 to 35.

Next, the operation of the second embodiment of the present invention will be described with reference to FIG.

First, the operation of the common amplifier 26 for switching the common electrode from the second common voltage to the first common voltage will be described. In the initial state, that is, the state before time A, the voltage applied to the liquid crystal load 25 is the second common voltage. At this time, the switch 34
Is in the ON state, and the second common voltage 30 is applied to the liquid crystal load 25 via the amplifier circuit 31. Furthermore, switches 16 and 17 are off. When the switch 18 is turned on, the first common voltage output from the amplifier 14 is charged in the auxiliary capacitance 15. The switches 33 and 35 are off.

Next, at time A, the switch 18 is turned off and the switch 16 is turned on, whereby the auxiliary capacitance 15 is turned on.
The liquid crystal load 25 starts to be charged by the electric charge stored in the liquid crystal load 25. Then, the potential of the common voltage line 10 rises and rises to the voltage Vas shown in the equation 1 shown in the first embodiment. The rise to the voltage Vas is caused by the storage capacitor 15 and the liquid crystal load 2.
5, the potential is determined irrespective of the driving capability of the amplifier circuit 14. On the other hand, when the switch 34 is turned off and the switch 35 is turned on, the potential of the auxiliary capacitance 32 is discharged by the amplifier circuit 31 to the second common voltage. The states of the switches 34 and 35 continue until time C.

Next, at time B, the switch 16 is turned off and the switch 17 is turned on, so that the liquid crystal load 25 is turned on.
Then, the driving is started by the amplifier circuit 14 from the state where the voltage is charged to approximately the voltage Vas. Note that the potential of the common voltage line 10 at this time rises by the function shown in Equation 2 shown in the first embodiment and reaches the first common voltage. By controlling the on and off of the switches 16 to 19 in this manner, the second voltage applied to the liquid crystal load 25 via the common voltage line 10 is controlled.
The switching operation from the common voltage to the first common voltage can be performed.

Next, the operation of switching from the first common voltage to the second common voltage will be described. This operation starts at time C. As described above, the state before the time C is as described above, the switch 17 is on and the switches 16 and 18 are off, and the switches 33 and 34 are off and the switch 35 is off.
Are in the ON state, the auxiliary capacitor 15 is charged to the liquid crystal load 25, and the potential is reduced to Vas, and the auxiliary capacitor 32 is discharged to the second common voltage.

Next, at time C, the switch 35 is turned off and the switch 33 is turned on, so that the liquid crystal load 25 is turned on.
From the current potential (first common voltage) to the auxiliary capacitor 32 starts. Therefore, the potential of the liquid crystal load 25 falls,
The potential of the auxiliary capacitor 32 rises, and each potential becomes Vas2. This potential is the capacitance C of the liquid crystal load 25.
The potential is determined by LC and the capacitance Cas2 of the auxiliary capacitor 32, and the relationship is the potential shown by Expression 4.

[0037]

(Equation 4) Here, Cas2 is the capacitance of the auxiliary capacitance 32, CLC is the capacitance of the liquid crystal load 25, and V2 is the second common voltage. Since the discharge up to the voltage Vas2 is a movement of electric charge determined by the auxiliary capacitance 15 and the liquid crystal load 25, the potential is determined irrespective of the driving capability of the amplifier circuit 31.

Next, at a time D, the switch 33 is turned off and the switch 34 is turned on, so that the liquid crystal load 25 is turned on.
Then, the driving by the amplifier circuit 31 is started from a state where the voltage is discharged to the voltage Vas2. The function of the potential of the common voltage line 10 at this time is shown by the following equation.

[0039]

(Equation 5) Here, t is the time from the time D, R is the total of the output impedance of the amplifier circuit 31, the impedance of the common voltage line 10, and the various impedances inside the liquid crystal panel. Is the impedance related to the drive up to. By controlling the ON / OFF of the switches 33 to 35 in this manner, the switching operation from the first common voltage to the second common voltage applied to the liquid crystal load 25 via the common voltage line 10 can be performed.

As described above, the amplifier circuit 26 according to the second embodiment of the present invention is different from the common AC driving method in which the first and second common voltages are switched and driven at a constant cycle with respect to the liquid crystal common electrode. Auxiliary capacitors 15, 32 and switch 16
, And 33 to 35, a common AC drive waveform can be generated.

The performance of the common AC drive when the common amplifier 26 of the second embodiment of the present invention is applied is the same as that described in the first embodiment, and is the same as the characteristic shown in FIG. The storage capacity of the auxiliary circuits 15, 32 and the switching circuits 16-1
8, 33 to 35, the common drive capability of the common amplifier 26 can be increased. Furthermore,
Since the auxiliary capacitors 15 and 32 do not affect the charge / discharge power of the liquid crystal load, there is no increase in power consumption by applying this embodiment. Furthermore, switch circuits 16 to 18, 33
By configuring 35 with a MOS circuit as shown in FIG. 7, the amplifier circuit itself can be integrated on a semiconductor chip.

As described above, by using the common amplifier 26 of the second embodiment of the present invention, it is possible to realize an amplifier circuit having an increased common driving capability, and therefore, a liquid crystal display device having improved display quality by the common AC driving method. realizable.

Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, each switch and auxiliary capacitance of the common amplifier 26 described above are replaced with a polysilicon TFT.
This is an example of a structure formed by using the following method. As a TFT disposed in each pixel of an active matrix liquid crystal as in the present invention, an amorphous TFT is currently widely used. In recent years, a polysilicon TFT has been employed as a TFT for driving each pixel of a liquid crystal. Things are being commercialized. Polysilicon TFTs have significantly improved electrical characteristics compared to amorphous TFTs, and not only TFTs arranged in each pixel but also other peripheral driving circuits and the like can be formed on the same glass. As a result, the number of parts can be reduced, so that significant cost reduction can be expected.

Next, each part of the third embodiment of the present invention will be described. The same parts as those in the first and second embodiments are denoted by the same reference numerals. 40 is a reference voltage line for outputting a first common voltage, 42 to 44 are switch circuits composed of MOS transistors, 45 is an auxiliary capacitor, 46 to 48 are control signals for driving the switch circuits 42 to 44, and 41 is 240
This is a polysilicon liquid crystal panel in which switch circuits 42 to 44 and an auxiliary capacitor 45 are formed on the same glass substrate in addition to pixels of rows × 320 columns.

Next, the operation of the third embodiment of the present invention will be described. In FIG. 10, the operations of the data driver 5 and the scanning driver 6 are the same as those described in the first embodiment, and thus the description will be omitted. Further, the necessity of AC driving of the liquid crystal and the driving waveform are basically the same as those in the first embodiment, and the voltage applied to the liquid crystal is supplied from the data driver 5 by the operation of the data driver 5 and the scanning driver 6. The liquid crystal itself receives a difference voltage between the displayed voltage and the common voltage supplied from the common voltage line 10 output from the power supply circuit 7. By this difference voltage, light and shade can be displayed for each pixel according to the characteristics of the liquid crystal. In this embodiment, the driving method of the liquid crystal is the common AC driving method, and it is necessary to drive the common voltage line to a large load of the liquid crystal. Further, similarly, the common voltage of the liquid crystal needs to be stable, and if this voltage fluctuates, the display voltage applied to the liquid crystal also fluctuates, thereby causing a significant reduction in display quality such as display unevenness and display flicker. Therefore, as in the first embodiment, in the common AC driving method, it is necessary to increase the speed of the common voltage waveform in order to ensure display quality. Therefore, the operation of the circuit of each unit for realizing the common AC driving method will be described.

First, the operation of switching the common electrode from the second common voltage (ie, ground) to the first common voltage will be described. In the initial state, the state applied to the common voltage line 10 is the second common voltage (ground). At this time, switch 4
Reference numeral 9 denotes an ON state, and switches 42 and 43 are OFF. When the switch 44 is in the ON state, the power supply circuit 7
The auxiliary capacitance 45 is added to the reference voltage of the first common voltage
Is charged.

Next, at the timing when the switching to the first common voltage is started, the switches 44 and 49 are turned off and the switch 42 is turned on, so that the electric charge stored in the auxiliary capacitance 45 causes the common voltage line 10 Charging to the common electrode starts. Then, the potential of the common voltage line 10 rises and the voltage Vas shown in the equation (1) shown in the first embodiment.
To rise.

Then, by turning off the switch 42 and turning on the switch 43, the driving by the power supply circuit 7 is started from the state where the common voltage line 10 is charged to approximately the voltage Vas. The function of the rise in the potential of the common voltage line 10 at this time is the same as the equation (2) shown in the first embodiment.

By controlling the ON / OFF of the switches 42 to 44 and 49 in this manner, the switching operation from the second common voltage applied to the common electrode of the liquid crystal through the common voltage line 10 to the first common voltage is performed. be able to. Further, the potential of the auxiliary capacitance 45 reduced to about Vas is charged by the amplifier circuit 14 by turning on the switch 44.

Further, with respect to the operation of switching from the first common voltage to the second common voltage (that is, the ground), the switch 43 is turned off and the switch 49 is turned on, so that the potential of the common voltage line 10 is discharged to the ground. (Second common voltage) potential.

As described above, the liquid crystal display device according to the third embodiment of the present invention is different from the common AC driving method in which the first and second common voltages are switched and driven at a constant period with respect to the liquid crystal common electrode. By realizing the auxiliary capacitance 45 and the switches 42 to 44 and 49 on the liquid crystal panel 41 with low-temperature polysilicon TFTs, a common AC drive driving method can be realized.

The performance of the common AC drive when the third embodiment of the present invention is applied has already been described for the first and second common drive.
In the same manner as the characteristic shown in FIG. 6, the driving capability of the amplifier
5 and the switch circuits 42 to 44 and 49, the common drive capability can be increased. Furthermore,
Since the auxiliary capacitor 45 does not affect the charge / discharge power of the liquid crystal load, there is no increase in power consumption by applying this embodiment. Furthermore, in the third embodiment, the switch circuits 42 to 44 and 49 together with the auxiliary capacitance 45 are connected to the M as shown in FIG.
Since it is composed of an OS circuit, a polysilicon liquid crystal panel formed on the same glass substrate can be easily realized, and since these drive circuits can be formed on the same glass, the number of parts is reduced and cost is significantly reduced. Liquid crystal display device can be realized.

[0053]

As described above, according to the liquid crystal display device of the present invention, a liquid crystal display device with improved display quality by the common AC driving method can be realized, and the common driving capability required for the common AC driving method is increased. An amplifier circuit can be easily realized. Furthermore, by using a MOS circuit, the amplifier circuit itself can be integrated on a semiconductor chip.

By applying the present invention to a polysilicon liquid crystal panel that can be formed on the same glass substrate as the liquid crystal pixels, a liquid crystal display device in which the number of parts is reduced and the cost is greatly reduced can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is a detailed configuration diagram of a liquid crystal panel 11.

FIG. 3 is a driving waveform diagram for driving the liquid crystal panel 11.

FIG. 4 is a detailed configuration diagram of a common amplifier 26.

FIG. 5 is a timing chart illustrating the operation of the common amplifier 26.

FIG. 6 is a graph showing a charging time reduction effect according to the first embodiment.

FIG. 7 is a relationship diagram between a switch circuit and a MOS transistor circuit.

FIG. 8 is a configuration diagram of a common amplifier 26 according to a second embodiment of the present invention.

FIG. 9 is a timing chart illustrating the operation of the common amplifier 26 according to the second embodiment.

FIG. 10 is a configuration diagram of a liquid crystal display device according to a third embodiment of the present invention.

[Explanation of symbols]

1 display data, 2 dot clocks, 3 horizontal synchronization signals, 4 vertical synchronization signals, 5 data drivers, 6 scanning drivers, 7 power supply circuits, 8 gradation voltage lines, 9 scanning voltage lines, 10: common voltage line, 11: liquid crystal panel, 12 ...
Liquid crystal display device, 13 ... 240 × 320 pixel liquid crystal, 14
... Amplifier circuit, 15 ... Auxiliary capacitance, 16-19 ... Switch, 20-23 ... Control line, 24 ... First common voltage, 2
6: common amplifier circuit, 30: second common voltage, 31
... Amplifier circuit, 32 ... Auxiliary capacitance, 33-35 ... Switch, 36-38 ... Control line, 40 ... Reference voltage line, 41 ... Polysilicon liquid crystal panel, 42-44 ... Switch circuit, 4
5 ... Auxiliary capacity, 46-48 ... Control signal.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) G09G 3/20 624 G09G 3/20 624C 624P 680 680G (72) Inventor Toshio Miyazawa 3300 Hayano, Mobara-shi, Chiba F-term in Display Group, Hitachi Ltd. DD08 DD25 DD26 DD27 FF11 JJ02 JJ03 JJ04 JJ05

Claims (5)

[Claims] 1. A liquid crystal display device for alternating current driving a liquid crystal by switching a first and a second common voltage applied to a common electrode of a liquid crystal panel at a frame cycle, comprising: a common drive amplifier for supplying the first common voltage; An auxiliary capacitor for storing a charge for switching the voltage of the common electrode from the second common voltage to the first common voltage; and a first switch circuit for short-circuiting or opening between the auxiliary capacitor and the common electrode. A second switch circuit for short-circuiting or opening between the common drive amplifier and the common electrode; a third switch circuit for short-circuiting or opening between the common drive amplifier and the auxiliary capacitance; A liquid crystal display device comprising a fourth switch circuit for short-circuiting or opening the second common voltage source. 2. A liquid crystal display device in which liquid crystal is AC-driven by switching first and second common voltages applied to a common electrode of a liquid crystal panel in a frame cycle, wherein the first common voltage for supplying the first common voltage is provided. A drive amplifier, a first auxiliary capacitance that assists the first common drive amplifier in charging the common electrode, and a first short-circuit or release between the first auxiliary capacitance and the common electrode. And a second circuit for short-circuiting or opening between the first common drive amplifier and the common electrode.
A switch circuit for short-circuiting or opening between the first common drive amplifier and the first storage capacitor; a second common drive amplifier for supplying the second common voltage; A second auxiliary capacitance for assisting the second common drive amplifier to discharge from the common electrode, and a fourth switch circuit for short-circuiting or opening between the second auxiliary capacitance and the common electrode. A fifth switch circuit for short-circuiting or opening between the second common drive amplifier and the common electrode; and a fifth switch circuit for short-circuiting or opening between the second common drive amplifier and the second auxiliary capacitance. 6. A liquid crystal display device comprising the switch circuit of 6. 3. The device according to claim 1, wherein each of the first, second, third, fourth, fifth, and sixth switch circuits includes a MOS transistor. Liquid crystal display device. 4. The liquid crystal display device according to claim 1, wherein the first, second, third, and fourth switch circuits and the auxiliary capacitance are formed on a liquid crystal panel on which pixels are arranged. 5. The liquid crystal display device according to claim 2, wherein said first, second, and thirty-fourth switch circuits and said auxiliary capacitance are formed on a liquid crystal panel on which pixels are arranged.