JPH04280290A - Drive circuit for liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain a drive circuit having high output characteristics capable of coping with either of normally black type or normally white type device, and either of counter electrode DC drive or counter electrode AC drive. CONSTITUTION:A drive circuit comprises a subtracter 27 constituted of a differential amplifier having resistors R1, R2, R3 and R4 (resistance values R1=R3, R2=R4) and having an arbitrary amplification factor defined by a ratio R2/R1 (R4/R3), and a polarity reversal selector switch circuit 26 for alternately selecting and inputting a DC input video signal Vin 1 and DC voltage Vbl 28 fed back with input voltage V1 and V2 to the subtracter via a polarity select signal Vsw 29 having a suitable period.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC drive circuit for applying voltage that avoids the application of direct current to liquid crystals used in liquid crystal display devices, and more particularly to a drive circuit for video signals that is not affected by the orientation of polarizing plates in a liquid crystal panel.

0002

2. Description of the Related Art The voltage applied to a liquid crystal display cell of a liquid crystal display device needs to be driven by alternating current (AC) in which the polarity is reversed at regular intervals in order to prevent deterioration of the liquid crystal.

FIG. 6 shows a circuit diagram of a video signal polarity inversion circuit and a liquid crystal display cell of a conventional liquid crystal display device.

In FIG. 6, a DC-regenerated input video signal 1 is amplified by an initial stage amplifier 2 in accordance with the characteristics of a liquid crystal, which has a threshold value and whose light transmittance draws an S-shaped curve with respect to an applied voltage. Ru.

The initial stage amplifier 2 sends the amplified video signal to a pair of differential amplifiers A3 and B4 having input terminals of opposite polarity connected to each other.

On the other hand, a timing signal obtained from a constant periodic signal of the video signal is applied to shift registers 6 whose number corresponds to the number of column electrodes of the liquid crystal display cell 5.

The shift register 6 sends a sampling signal for the video signal to the transmission gate 7.

By opening and closing a series of transmission gates 7, a positive video signal from differential amplifier A3 or a negative video signal from differential amplifier B4 is transferred to a switch circuit,
The signal passes through the transmission gate 7 and is stored in the sample hold 8 as an image signal for one row of the liquid crystal display cell 5.

The image signal from the sample hold 8 is applied to the TFT 9 in the liquid crystal display cell, changes depending on the characteristics of the TFT 9 and the liquid crystal, and is applied to the pixel electrode 10 as a pixel signal.

An amplification factor adjustment signal 11 is applied to the initial amplifier 2 to adjust the contrast of the liquid crystal display cell 5, and a variable DC voltage signal 12 is applied to the differential amplifier B4 to adjust the brightness of the liquid crystal display cell 5. It looks like this.

The liquid crystal in a liquid crystal display cell is driven by an alternating current voltage that is the difference between a pixel voltage applied to a pixel electrode and a counter voltage applied to a counter electrode (Japanese Patent Publication No. 2-41039).

Since the above drive circuit has a switch after the differential amplifier, the output impedance becomes large, and only a low output signal can be supplied to the sample and hold.

[0013] In order to prevent brightness changes due to the difference between the image signal and the pixel signal, the applicant previously developed a drive circuit for an active matrix type liquid crystal display device in which the center voltage of the AC image signal and the opposing voltage of the opposing electrode are used. proposed a circuit that creates a potential difference between the
Publication No. 92).

Furthermore, there is a normally black method in which light is transmitted when a voltage is applied to the liquid crystal display cell by aligning the linear polarization axes of the polarizing plates on the incident side and the transmission side of the liquid crystal display cell, or When voltage is applied to the liquid crystal display cell with the linear polarization axes of the polarizing plates on the incident side and transmission side of the liquid crystal display cell intersecting each other by approximately 90 degrees, the voltage is applied using a normally white method that blocks light. The polarity of the video signal is different.

[0015] In addition, counter electrode DC drive is used in which the counter voltage Vt applied to the counter electrode is a DC voltage, or a rectangular wave is applied to the counter electrode so that the dynamic of the video signal can be ensured even if the power supply voltage of the video signal drive circuit is lowered. The pedestal level of the video signal differs depending on whether counter-pole AC drive is applied.

[0016] As described above, if the driving methods are different, video driving signals must be prepared in accordance with the different driving methods.

FIG. 7 shows a Hitachi LCD color television C5-LC.
1 shows an R, G, B video amplification analog switch circuit.

In FIG. 7, the R primary color signal 13, the G primary color signal 14,
The B primary color signal 15 is a voltage between the power supply voltage 16 and GND 17 and is amplified by the transistor, and the polarity is inverted at a predetermined period by the color and polarity switching analog switch circuit 18, and the liquid crystal display cell is driven after the color and polarity switching. Signal output 1
Converted to 9.

FIG. 8 shows a basic RBG output polarity inversion circuit of a chroma video processing IC for liquid crystal color TV manufactured by NEC, model number μpc1472G.

When the color difference signals of three-axis demodulation are simplified into one signal in FIG.
is demodulated.

The chroma video processing IC has a power supply voltage of 16G.
The ND17 amplifies the input of the color difference signal 23 with the Tr, and outputs the liquid crystal display cell drive signal output 19 which is in phase with the input video signal when SWA24 is OFF and SWB25 is ON, and inverted when SWA24 is ON and SWB25 is OFF. A liquid crystal display cell drive signal output 19 is output.

The DC voltage level of the liquid crystal display cell drive signal output 19 is limited by the power supply voltage, and the DC voltage levels of a signal in phase with the input video signal and a signal with inverted polarity can only be set to be different.

The driving circuits of the two liquid crystal display devices described above use a voltage approximately 1/2 of the power supply voltage of the driving circuit as the center voltage Vc of the video signal for both the non-polarity-inverted video signal and the polarity-inverted video signal. Since they can only exist above and below the center voltage, sandwiching Vc, they can only support counter-electrode DC drive.

[0024] Another IC, Toshiba's gamma correction IC for LCD TVs, model number TA8695F, clamps the video signal at the bottom level of the synchronization signal, which is the lowest value of the input video signal. Polarity is defined and no
It is only compatible with Marie-White type panels.

Furthermore, in conventional drive circuits, the value of Vc is fixed depending on the power supply voltage, which also limits the degree of freedom in circuit configuration.

[0026]

[Problems to be Solved by the Invention] The present invention eliminates the above-mentioned problems, and allows the center voltage Vc of the video signal to be freely set within the power supply voltage without depending on the power supply voltage by inputting a DC-regenerated video signal. , and is compatible with opposite polarity DC drive, opposite polarity AC drive, normally white method, and normally black method.Furthermore, the cycle and timing of polarity reversal can be freely set to create images that match the characteristics of the liquid crystal panel. The present invention provides a driving circuit for a liquid crystal display device that can easily obtain the amplitude of a signal.

[0027]

[Means for Solving the Problems] The present invention receives a DC reproduced video signal as input, inverts the polarity of the video signal in synchronization with a constant periodic signal such as a horizontal synchronization signal or a vertical synchronization signal, and displays a liquid crystal display panel. In a circuit that AC drives a liquid crystal panel using a video signal and DC voltage that are amplified according to the characteristics of This is a drive circuit for a liquid crystal display device that includes a subtracter configured with a differential amplifier as an input.

[0028]

[Operation] Inverting and non-inverting output video signal Vo is selected depending on the polarity of Vsw by changing the DC voltage Vbl.
The pedestal level Vp changes while satisfying the condition that ut is separated from the center voltage Vc of the video signal by the same voltage in the opposite direction, and the pedestal level Vp is set regardless of whether it is above or below Vc.

[0029] By switching the value of Vsw, a variable DC voltage Vbl is subtracted from the input video signal Vin, or
By setting the DC voltage Vbl, which changes whether Vin is subtracted from Vbl and changes the polarity of the output, to around the pedestal level Vp of the input video signal Vin (~Vp),
Marie-Black type counter electrode DC driving is performed.

By setting the DC voltage Vbl near the intermediate value Vvc (~Vvc) of the input video signal Vin, normally black type counter-electrode AC driving is performed.

By setting the DC voltage Vbl near the white level Vw (~Vw) of the input video signal Vin, the normal
White type counter electrode DC driving is performed.

By setting the DC voltage Vbl near the intermediate value Vvc (~Vvc) of the input video signal Vin, normally white type opposite electrode AC driving is performed.

The brightness of the image of the liquid crystal display device is adjusted by adjusting the potential of the DC voltage Vbl with respect to the potential of the input video signal Vin.

The phase of the counter voltage Vt during counter polarity AC driving and the phase of the output video signal Vout can be matched by changing the phase of the polarity switching signal Vsw by 180°.

The amplification factor of the video signal is set by the value of R2/R1 and is sent to the sample and hold circuit without going through a switch, so the output impedance is reduced and a high output drive circuit is achieved.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit diagram of a driving circuit for a liquid crystal display device according to the present invention.

In FIG. 1, the drive circuit mainly uses the input video signal Vin.
It is composed of a polarity inversion switch circuit 26 that receives the input video signal Vout, and a subtracter 27 that outputs the output video signal Vout.

The input video signal 1 , a variable DC voltage 28 , and a polarity switching signal 29 are input to the polarity inversion switching switch circuit 26 .

The polarity switching signal 29 is input to the polarity inversion switching switch circuit 26 as two signals inverted from each other by the inverter 30, and the four switching circuits SWA3
1, applied to SWB32, SWC33, and SWD34.

Input video signal 1 is SWA31 and SWC33
The variable DC signal 28 is input to the SWB 32 and the SWD.
34.

In FIG. 1, one set of SWA31 and SWD34 and one set of SWB32 and SWC33 in the polarity reversal switching circuit 26 are alternately selected, and the input 35 to the first subtracter and the second subtraction An input 36 to the device originates from a polarity reversal switch circuit 26.

When the polarity switching signal 29 is H (high), the switch circuits SWA31 and SWD34 are turned on, and the input video signal Vin becomes the input V1 to the first subtracter, and the variable DC voltage Vbl becomes the second subtractor. This becomes the input V2 to the calculator.

When the polarity switching signal 29 is L (low), the switch circuits SWB32 and SWC33 are turned on, and the variable DC voltage Vbl becomes the input V1 to the first subtracter, and the input video signal Vin becomes the second subtracter. This becomes the input V2 to the subtracter.

Three inputs are applied to the subtracter 27: V1 and V2 from the polarity inversion switch circuit 26, and a center voltage 37 which is a DC voltage that determines the center value of the output video signal.

The subtracter 27 outputs the voltage obtained by subtracting the input V1 to the first subtracter from the input V2 to the second subtracter to the resistor R1.
38 and the resistance value of resistor R239 is amplified by the ratio R2/R1,
A voltage obtained by adding this to the central voltage 37 is output as an output video signal 42.

The counter voltage Vt is set in accordance with the characteristics of the liquid crystal display device using the previously proposed circuit with reference to the center voltage Vc of the output video signal Vout.

The amplification factor of the video signal can be freely set by adjusting the resistance value ratio R2/R1 in accordance with the characteristics of the liquid crystal display device.

FIG. 2 shows a normal drive circuit according to the present invention.
A waveform diagram in the case of counter electrode DC driving in the black method is shown.

In FIG. 2, during period t1, the polarity switching signal Vs
w is in the L (low) state, switches SWB and SWC are on, Vin is input to V2, Vbl is input to V1,
During period t2, switches SWA and SWD are turned on, and Vbl is input to V2 and Vin is input to V1.

When the DC voltage Vbl is brought closer to the input video signal 1, the liquid crystal display image becomes darker, and when it is moved away from it, the liquid crystal display image becomes brighter.

FIG. 3 shows a normal drive circuit using the drive circuit of the present invention.
A waveform diagram in the case of counter electrode AC driving in the black method is shown.

In FIG. 3, during period t1, the polarity switching signal Vs
w is in the L (low) state, switches SWB and SWC are on, Vin is input to V2, Vbl is input to V1,
During period t2, switches SWA and SWD are turned on, and Vbl is input to V2 and Vin is input to V1.

The counter voltage Vc alternates between two voltages, and the median value 47 of the counter voltages is set to be smaller than the median voltage 37 of the output video signal 42.

FIG. 4 shows a normal drive circuit using the drive circuit of the present invention.
A waveform diagram is shown in the case of counter electrode DC driving using the white method.

In FIG. 4, during period t1, the polarity switching signal Vs
w is in the H (high) state, switches SWA and SWD are turned on, Vbl is input to V2, Vin is input to V1,
During period t2, switches SWB and SWC are turned on, and Vin is input to V2 and Vbl is input to V1.

FIG. 5 shows a normal drive circuit using the drive circuit of the present invention.
A waveform diagram in the case of counter electrode AC driving in the white method is shown.

In FIG. 5, during period t1, the polarity switching signal Vs
w is in the H (high) state, switches SWA and SWD are turned on, Vbl is input to V2, Vin is input to V1,
During period t2, switches SWB and SWC are turned on, and Vin is input to V2 and Vbl is input to V1.

Table 1 shows the signal states and the open/close states of the switches in each period of the drive circuit of the liquid crystal display device of the present invention.

[0059]

[Table 1]

Table 1 describes the open/close states of the switches during periods t1 and t2 in each display mode and each driving method, and the input signals that become the signals V1 and V2.

The four switches in the polarity reversal switching circuit of this embodiment are one set of SWA and SWC, and one set of SWB.
Although one set of connections of SWD and SWD was used, other combinations are also possible.

In this way, in order to obtain the same output in the drive circuit of the present invention, the input video signal Vin and the DC voltage Vb
If the relative voltage relationship of l is the same, Vin and Vbl can freely exist within the operating voltage range of the operational amplifier.

Therefore, there is an advantage that the degree of freedom in designing the driving circuit of the liquid crystal display device is increased.

[0064]

[Effects of the Invention] By using the drive circuit of the liquid crystal display device of the present invention, it is possible to support counter-polar DC drive, counter-polar AC drive, normally black method and normally white method, and output The center voltage of the video signal can be freely selected, the amplification factor of the video signal can be easily set, and the brightness can be adjusted to create a high-output drive circuit.

[Brief explanation of the drawing]

FIG. 1 is a driving circuit diagram of a liquid crystal display device of the present invention.

FIG. 2 is an input/output diagram in the case of normally black type opposite electrode DC drive.

FIG. 3 is an input/output diagram in the case of normally black type opposite electrode AC drive.

FIG. 4 is an input/output diagram in the case of normally-white counter-electrode DC driving.

FIG. 5 is an input/output diagram in the case of normally-white counter-electrode AC driving.

FIG. 6 is a diagram of a drive circuit for polarity reversal of a conventional liquid crystal display device.

FIG. 7 is a drive circuit diagram of an analog switch of a conventional liquid crystal display device.

FIG. 8 is a diagram of a driving circuit for chroma video processing of a conventional liquid crystal display device.

[Explanation of symbols]

1 Input video signal 2 Initial amplifier 3 Differential amplifier A 4 Differential amplifier B 5 Liquid crystal display cell 6 Shift register 7 Transmission gate 8 Sample hold 9 TFT 10 Pixel electrode 11 Amplification factor adjustment signal 12 Variable DC voltage signal 13 R primary color signal 14 G primary color signal 15 B primary color signal 16 Power supply voltage 17 GND 18 Analog switch circuit for color and polarity switching 1
9 Liquid crystal display cell drive signal output 20 Luminance signal 21 Y amplifier 22 Color demodulation circuit 23 Color difference signal 24 SWA 25 SWB 26 Switch circuit for polarity inversion 27 Subtractor 28 DC voltage 29 Polarity switching signal 30 Inverter 31 SWA 32 SWB 33 SWC 34 SWD 35 Input to the first subtractor 36 Input to the second subtractor 37 Central voltage 38 Resistor R1 39 Resistor R2 40 Resistor R3 41 Resistor R4 42 Output video signal 43 Pedestal level 44 Black level 45 Input Median value of video signal 46 White level 47 Median value of opposing voltage

Claims (2)

[Claims] [Claim 1] Inputs a DC-regenerated video signal,
The polarity of the video signal is inverted in synchronization with a constant periodic signal such as a horizontal synchronization signal or a vertical synchronization signal, and the video signal and DC voltage are amplified according to the characteristics of the liquid crystal panel. The driving circuit includes a subtracter configured of a switch circuit that switches the video signal and the DC voltage using a polarity switching signal synchronized with the constant periodic signal, and a differential amplifier that receives the switched signal as input. A drive circuit for a liquid crystal display device. Claim 2: An arbitrary amplifier defined by the resistance value ratio R2/R1 (R4/R3) formed by a differential amplifier equipped with resistors R1, R2, R3, and R4 (resistance values R1=R3, R2=R4). A subtracter with an amplification factor and inputs V1 and V to the subtracter
2, the input video signal Vin and the DC voltage Vb that are reproduced as DC
2. The drive circuit for a liquid crystal display device according to claim 1, further comprising a polarity reversal switch circuit which alternately switches and inputs the polarity 1 using a polarity switching signal Vsw having an appropriate period.