JPH0746267B2 - LCD drive circuit - Google Patents

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 of an applied voltage for avoiding DC application to a liquid crystal used in a liquid crystal display device, and more particularly to a video signal drive circuit which is not influenced by the arrangement direction of a polarizing plate of a liquid crystal panel.

[0002]

2. Description of the Related Art A voltage applied to a liquid crystal display cell of a liquid crystal display device needs to be AC-driven to invert the polarity at a constant cycle in order to prevent deterioration of the liquid crystal.

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

In FIG. 6, a DC reproduced input video signal 1 is amplified by a stage amplifier 2 in accordance with a characteristic of a liquid crystal having a threshold value and having a non-linear light transmittance with respect to an applied voltage.

The in-stage amplifier 2 sends the amplified video signal to a pair of differential amplifiers A3 and B4 to which input terminals having opposite polarities are connected.

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

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

A positive video signal from the differential amplifier A3 or a negative video signal from the differential amplifier B4 is switched by a switch circuit by opening and closing a connection of the transmission gate 7.
An image signal for one row of the liquid crystal display cell 5 is accumulated in the sample hold 8 through the transmission gate 7.

The image signal from the sample hold 8 is applied to the TFT 9 in the liquid crystal display cell, changes according to 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 adjusting signal 11 is applied to the in-phase 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 is like this.

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

Since the drive circuit described above has a switch after the differential amplifier, there is a problem that the output impedance becomes large, the frequency characteristic is deteriorated, and the output amplitude is limited.

In order to prevent a change in brightness due to the normal difference between an image signal and a pixel signal, the applicant has previously applied a central voltage of an AC image signal and a voltage application to a counter electrode as a drive circuit of an active matrix type liquid crystal display device. A circuit for providing a potential difference between the voltage and a voltage (hereinafter, referred to as a counter voltage) has been proposed (Japanese Patent Laid-Open No. 61-116392).

Further, a normally black system 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 light incident side and the transmission side of the liquid crystal display cell, Light is applied to the liquid crystal display cell by a normally white method that intercepts light when a voltage is applied to the liquid crystal display cell by intersecting the linear polarization axes of the polarizing plates on the incident side and the transmission side by about 90 degrees. The polarities of the video signals are different.

Further, the counter voltage Vt applied to the counter electrode is a direct current DC voltage, or a rectangular wave is applied to the counter electrode so that the dynamic of the video signal can be secured even if the power supply voltage of the video signal drive circuit is lowered. The pedestal level of the video signal varies depending on whether the applied counter-polarity AC drive is applied.

If the driving method is different as described above, it is necessary to prepare a video driving signal corresponding to it.

FIG. 7 shows a liquid crystal color television C5-LC manufactured by Hitachi.
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 amplified by Tr with a voltage between the power supply voltage 16 and the GND 17, and the polarity is inverted at a predetermined cycle by the color and polarity switching analog switch circuit 18 to drive the liquid crystal display cell after the color and polarity switching. Converted to signal output 19.

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

In FIG. 8, if the color difference signals of three-axis demodulation are simplified into one signal, the luminance signal 20 is amplified by the Y amplifier 21, and the carrier color signal is color difference signal 23 by the color demodulation circuit 22.
Demodulated to.

The chroma video processing IC has a power supply voltage of 16, G
ND17, the input of the color difference signal 23 is amplified by Tr, and when SWA24 is OFF and SWB25 is ON, the liquid crystal display cell drive signal output 19 of the same phase as the input video signal is output, and when SWA24 is ON and SWB25 is OFF, it is inverted. The 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 level of the signal having the same phase as the input video signal and the signal having the inverted polarity can be set only differently.

In the drive circuits of the two liquid crystal display devices described above, both the video signal without polarity reversal and the video signal with polarity reversal are set to have a voltage of about 1/2 of the power supply voltage of the drive circuit as the central voltage Vc of the video signal. Since it can exist only above and below the center voltage with Vc sandwiched between them, it was only applicable to counter-polarity DC drive.

In another IC, a γ-correction IC for a liquid crystal television manufactured by Toshiba, model number TA8695F, the video signal is clamped at the bottom level of the sync signal, which is the lowest value of the input video signal. Polarity is specified,
It can only be used with a mari-white type panel.

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

[0026]

SUMMARY OF THE INVENTION The present invention eliminates the above-mentioned problems, and if a DC reproduced video signal is input, the central voltage Vc of the video signal can be freely set within the power supply voltage without depending on the power supply voltage. In addition, it can be applied to the opposite polarity DC drive, opposite polarity AC drive, normally white method and normally black method, and the polarity inversion cycle and timing can be freely set, and the video signal that matches the characteristics of the liquid crystal display cell. The present invention provides a drive circuit for a liquid crystal display device, which can easily obtain the amplitude of.

[0027]

According to the present invention, a DC reproduced input video signal is input, the polarity of the input video signal is inverted in synchronization with a fixed periodic signal such as a horizontal sync signal or a vertical sync signal, and In a drive circuit of a liquid crystal display device that AC drives a liquid crystal display cell by using an output video signal and a counter electrode signal that are amplified according to the characteristics of the liquid crystal display cell, an input video signal and a DC voltage are synchronized with a constant periodic signal. The drive circuit of the liquid crystal display device is provided with a switch circuit that is switched by the polarity switching signal and a subtracter that is configured by a differential amplifier that receives the switched signal.

[0028]

By changing the DC voltage Vbl, the inverted and non-inverted output video signals Vout selected by the polarity of Vsw satisfy the condition that they are separated from the central voltage Vc of the video signal by the same voltage in the opposite direction. The pedestal level Vp changes, and the pedestal level Vp is set regardless of whether the pedestal level Vp is above or below Vc.

By switching the value of Vsw, whether the variable DC voltage Vbl is subtracted from the input video signal Vin or Vin is subtracted from Vbl, and the polarity of the output changes. For example, by setting the DC voltage Vbl near the pedestal level Vp (up to Vp) of the input video signal Vin, normally black type counter-polarity DC drive is performed.

By setting the DC voltage Vbl near the intermediate value Vvc of the input video signal Vin (up to Vvc), normally-black type counter-polarity AC drive is performed.

By setting the DC voltage Vbl near the white level Vw (up to Vw) of the input video signal Vin, normally-white type counter-polarity DC drive is performed.

By setting the DC voltage Vbl near the intermediate value Vvc of the input video signal Vin (up to Vvc), normally-white type counter-polarity AC drive is performed.

The brightness of the image on 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 and the phase of the output video signal Vout at the time of AC drive of the counter electrode are 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 R ₂ / R ₁ and is sent out to the sample hold circuit without passing through a switch, so that the output impedance is reduced and a high output drive circuit is obtained.

[0036]

1 is a circuit diagram of a drive circuit for a liquid crystal display device according to the present invention.

In FIG. 1, the drive circuit mainly comprises a polarity inversion switching switch circuit 26 for receiving the input video signal Vin and a subtracter 27 for outputting the output video signal Vout.

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

The polarity switching signal 29 is input to the polarity reversal switching switch circuit 26 as two signals which are inverted by the inverter 30 and is input to the four switch circuits SWA3.
1, SWB32, SWC33, SWD34.

The input video signal 1 is SWA31 and SWC33.
Variable DC signal 28 input to SWB32 and SWD
34 is input.

In FIG. 1, one set of SWA31 and SWD34 and one set of SWB32 and SWC33 in the polarity inversion switching switch circuit 26 are alternately selected to input 35 to the first subtractor and to subtract the second subtraction. An input 36 to the instrument is generated from the polarity inversion switching switch circuit 26.

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

Three inputs of V1 and V2 and a central voltage 37 which is a DC voltage for determining the median value of the output video signal are applied to the subtractor 27 from the polarity reversal switching circuit 26.

The subtractor 27 subtracts the voltage obtained by subtracting the input V1 to the first subtractor from the input V2 to the second subtractor and the resistance R ₁ 3
8 and the resistance value of the resistor R ₂ 39 are amplified by a ratio R ₂ / R ₁ and the voltage added to the central voltage 37 is output as the output video signal 42.

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

The amplification factor of the video signal can be freely set by the resistance ratio R ₂ / R ₁ according to the characteristics of the liquid crystal display device.

FIG. 2 is a circuit diagram of the drive circuit of the present invention.
The waveform diagram at the time of counter-polarity DC drive by a black system is shown.

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

When the DC voltage Vbl is brought close to the input video signal 1, the liquid crystal display image becomes dark, and when it is moved away, the liquid crystal display image becomes bright.

FIG. 3 is a circuit diagram of the drive circuit of the present invention.
The waveform diagram at the time of counter-polarity AC drive by a black system is shown.

In FIG. 3, in the period t1, the polarity switching signal Vsw
Is in the L (low) state, the switches SWB and SWC are turned on, Vin is input to V2 and Vbl is input to V1, and the period t
In 2 the switches SWA and SWD are turned on and V2 is Vbl
Vin is input to V1.

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

FIG. 4 shows the drive circuit of the present invention, which is normally
The waveform diagram at the time of counter-polarity DC drive by a white system is shown.

In FIG. 4, in the period t1, the polarity switching signal Vsw
Is in the H (high) state, the switches SWA and SWD are turned on, Vbl is input to V2 and Vin is input to V1, and the period t
In 2, the switches SWB and SWC are turned on, and V2 is Vin
However, Vbl is input to V1.

FIG. 5 is a circuit diagram of the drive circuit of the present invention.
The waveform diagram at the time of counter-electrode AC drive by the white method is shown.

In FIG. 5, in the period t1, the polarity switching signal Vsw
Is in the H (high) state, the switches SWA and SWD are turned on, Vbl is input to V2 and Vin is input to V1, and the period t
In 2, the switches SWB and SWC are turned on, and V2 is Vin
However, Vbl is input to V1.

Table 1 shows the states of signals and the open / closed 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 / closed states of the switches during the periods t1 and t2 in each display mode and each driving method, and the input signals serving as V1 and V2 signals.

The four switches in the switch circuit for polarity reversal switching of the present embodiment are a set of SWA and SWC, SWB.
, And SWD were used for one set of connections, but other combinations are possible.

As described above, in order to obtain the same output in the drive circuit of the present invention, Vin and Vbl can be freely set within the operating voltage range of the operational amplifier if the relative voltage relationship between the input video signal Vin and the DC voltage Vbl is the same. Can exist in.

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

[0064]

By using the drive circuit of the liquid crystal display device of the present invention, it is possible to support the opposite polarity DC drive, the opposite polarity AC drive, the normally black system and the normally white system, and to output. You can freely select the central voltage of the video signal, easily set the amplification factor of the video signal, and realize a high output drive circuit that can also adjust the brightness (brightness).

[Brief description of drawings]

FIG. 1 is a drive 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 counter electrode DC drive.

FIG. 3 is an input / output diagram in the case of normally-black type counter-polarity AC drive.

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

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

FIG. 6 is a drive circuit diagram for polarity inversion 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 drive circuit diagram for chroma video processing of a conventional liquid crystal display device.

[Explanation of symbols]

1 Input Video Signal 2 Stage 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 Ratio 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 Color and polarity switching analog switch circuit 19 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 Polarity Inversion Switching Switch Circuit 27 Subtractor 28 DC Voltage 29 Polarity Switching Signal 30 Inverter 31 SWA 32 SWB 33 SWC 34 SWD 35 Input to First Subtractor 36 Input to Second Subtractor 37 central voltage 38 resistor R ₁ 39 resistor R ₂ 40 resistor R ₃ 1 resistor R ₄ 42 output video signal 43 median median 46 white level 47 counter voltage Bae des barrel level 44 black level 45 input video signal

Claims (2)

[Claims] 1. An input video signal reproduced by direct current is input, the polarity of the input video signal is inverted in synchronization with a constant periodic signal such as a horizontal synchronizing signal or a vertical synchronizing signal, and the characteristics of the liquid crystal display cell are determined. In a drive circuit of a liquid crystal display device that AC-drives a liquid crystal display cell by using an output video signal and a counter electrode signal that are amplified together, a polarity switching signal that synchronizes the input video signal and the DC voltage with the constant periodic signal. A driving circuit for a liquid crystal display device, comprising a subtracter composed of a switch circuit for switching and a differential amplifier for inputting a switched signal. 2. Resistors R ₁ , R ₂ , R ₃ , R ₄ (resistance value R ₁ =
R ₃ and R ₂ = R ₄ ) and a subtracter having an arbitrary amplification factor defined by a ratio R ₂ / R ₁ (R ₄ / R ₃ ) of resistance values constituted by a differential amplifier and the subtraction. And a polarity reversing changeover switch circuit for alternately changing over and inputting the input video signal Vin and the direct current voltage Vbl, which have been reproduced as direct current, to the inputs V ₁ and V ₂ to the device. The drive circuit of the liquid crystal display device according to claim 1.