JP2002140040A - Bias voltage generating circuit for liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the value of ΔV in the bias voltage generating circuit of a liquid crystal display device and to reduce a display irregularity in the device. SOLUTION: In the voltage generating circuit 100, an input voltage (Vcc) is used as the voltage for V0 and a ground level is used as the voltage for V5. The voltage between V0-V5 is voltage divided by resistors R1, R2, R3, R4, and R5 to generate the voltages V1 to V4. Furthermore, the voltages V1 to V4 are outputted through amplifiers 11 to 14. A resistor R03 is connected between the Vcc (the highest level) and the output side of the amplifier 13 and a resistor R25 is connected between the output side of the amplifier 12 and the ground level (the lowest level).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bias voltage generating circuit for a liquid crystal display device, and more particularly to a bias voltage generating circuit for a liquid crystal display device used for driving a liquid crystal display device by a six-level driving method.

[0002]

2. Description of the Related Art As a driving method for driving a simple matrix type liquid crystal display device such as an STN liquid crystal display device, a driving method in which an AC drive is performed in a time-division manner is widely used. In the time-division AC driving method, a common electrode is line-sequentially selected for a liquid crystal display device having a row electrode group (common electrode group) and a column electrode group (segment electrode group) arranged in a matrix. Then, for example, a positive selection waveform and a negative selection waveform are alternately applied for each frame. In addition, an ON waveform or an OFF waveform is applied as a signal waveform to the segment electrode in synchronization with the selected waveform of the common electrode.

In order to realize a time-division AC driving method, for example, a 6-level driving method is used. In the 6-level driving method, 6-level voltages are generated. Hereinafter, the six-level voltages are referred to as V0, V1, V2, V3, V4, and V5.
>V1>V2>V3>V4> V5. FIG. 6A shows an example of a selection waveform applied to the common electrode.
(B) shows an example of a signal waveform applied to the segment electrode. FIG. 6B shows an example in which lighting and extinguishing are repeated in the pixels in the row direction.

FIG. 7 is a circuit diagram showing an example of a bias voltage generating circuit for generating a six-level voltage. FIG.
In the bias voltage generating circuit shown in FIG. 5, in the voltage generating circuit 100, the input voltage (Vcc) is used as the voltage of V0, and the ground level is used as the voltage of V5. Then, the voltage between V0 and V5 is equal to the resistance R1, R2, R3, R
4, the voltage is divided by R5 to generate voltages V1 to V4.
Further, the voltages V1 to V4 are supplied to the driving circuit side of the liquid crystal display device via the operational amplifiers 11 to 14 connected in a voltage follower.

[0005] As shown in FIG.
Since voltages of 0, V1, V4, and V5 are used, and voltages of V0, V2, V3, and V5 are used in the segment electrodes, V0, V2, V3, and V5 are used in the row driver (not shown) for driving the common electrodes in the drive circuit. The voltages V1, V4, and V5 are supplied, and the voltages V0, V2, V3, and V5 are supplied to column drivers (not shown) that drive the segment electrodes in the drive circuit. Note that the voltage generation circuit 100 is often implemented as an IC.

[0006]

However, as the display area of the liquid crystal display device increases and the duty ratio increases, the driving capability of the operational amplifiers 11 to 14 in the voltage generation circuit 100 relatively decreases. Insufficient driving capability of the operational amplifiers 11 to 14 causes waveform distortion of a driving waveform and a voltage shift of a bias voltage. Then, display unevenness 300 as illustrated in FIG. 8 occurs on the display screen due to the waveform distortion and the voltage shift of the bias voltage.

One of the causes of display unevenness is that the bias voltage fluctuates due to the offset voltages, VI characteristics, and load current characteristics of the operational amplifiers 11 to 14. It is desirable that the potential difference between V0-V1 and the potential difference between V1-V2 are the same, and the potential difference between V3-V4 and V1
It is desirable that the potential difference is equal to 4-V5. That is, it is desirable that ΔV according to the following equation is 0.

ΔV = | (| V0−V1 | − | V1−V2
|) | + | (| V3-V4 |-| V4-V5 |) |

However, the bias voltage fluctuates due to the offset voltages, VI characteristics, and load current characteristics of the operational amplifiers 11 to 14, and the bias voltage deviates from the optimum bias voltage calculated by the voltage averaging method. That is, FIG.
As shown in the figure, the drive voltage on the + side with respect to the reference level and-
The drive voltage on the side becomes unbalanced. As a result, the display of the liquid crystal display device becomes uneven. Note that the smaller the value of ΔV is, the more the display unevenness is reduced.

It is considered that display unevenness can be reduced by replacing the operational amplifiers 11 to 14 with high-performance ones. However, when the voltage generating circuit 100 is integrated into an IC, unless the IC itself is replaced, the operational amplifiers 11 to 14 are not changed. It is not possible to replace 14.

Accordingly, the present invention provides a bias voltage generating circuit for a liquid crystal display device configured to generate a plurality of levels of voltages for driving the liquid crystal display device using an operational amplifier by reducing the value of ΔV. It is another object of the present invention to reduce display unevenness of a liquid crystal display device.

[0012]

According to the present invention, a bias voltage generating circuit for a liquid crystal display device according to the present invention has a maximum voltage (V).
0) A resistor is provided between the output and the output side of the operational amplifier that outputs the voltage V3, and a resistor is provided between the output side of the operational amplifier that outputs the voltage V2 and the lowest level voltage (V5) output. It is characterized by having. Particularly, when each operational amplifier in the bias voltage generating circuit is integrated in one integrated circuit, it is effective to provide the above two resistors.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing a configuration example of a bias voltage generation circuit of a liquid crystal display device according to the present invention. In the voltage generation circuit 100 shown in FIG. 1, the input voltage (V
cc) is used as the voltage of V0, and the ground level is used as the voltage of V5. Then, the voltage between V0 and V5 is divided by resistors R1, R2, R3, R4 and R5, and
Voltages of 1 to V4 are generated. Further, the voltages V1 to V4 are output via the operational amplifiers 11 to 14.

Then, six levels of voltages V0 to V5 are supplied to the drive circuit side of the liquid crystal display device. It is supplied to the drive circuit side of the liquid crystal display device. The relationship between the six levels of voltage is
V0>V1>V2>V3>V4> V5. Each of the operational amplifiers 11 to 14 is used as a non-inverting amplifier having an amplification degree of 1, that is, a voltage follower mainly for impedance conversion.

The feature of this embodiment is that the voltage V
A resistor R03 is connected between the output line of the voltage V0 (0) and the output line of the voltage V3, that is, between Vcc (highest level) and the output side of the operational amplifier 13, and the output line of the voltage V2 and the voltage V5. That is, the resistor R25 is connected between the output line, that is, between the output side of the operational amplifier 12 and the ground level (lowest level). Resistance R0
3, the resistance value of R25 is, for example, about 510 kΩ.

FIG. 2 shows the results of measuring the bias voltage and the value of ΔV in the case where there is no resistor (when the conventional configuration shown in FIG. 7 is used) and in the present embodiment (this example). FIG. In FIG. 2, each bias voltage value in the case of "theoretical value" is a voltage value for realizing the optimum bias based on the voltage averaging method, and each bias voltage value in the case of "character display" is actually stored in the liquid crystal display device. This is an actual measurement value when a predetermined character is displayed. Here, 1/78 duty ratio and 1/10 bias were used as driving conditions of the liquid crystal display device. FIG. 2 also shows the results of measuring the bias voltage and the value of ΔV in two comparative examples described later.

As shown in FIG. 2, in the present example, the voltage between V0 and V1 and V1
−V2 becomes smaller, and V3-
The difference between the voltage between V4 and the voltage between V4 and V5 is smaller. That is, the value of ΔV is greatly improved.

FIG. 1 shows that the difference between the voltage between V0 and V1 and the voltage between V1 and V2 and the difference between the voltage between V3 and V4 and the voltage between V4 and V5 are reduced. This is because the rising characteristics of the drive voltage applied to the liquid crystal display device are improved by connecting the resistors R03 and R25 to the positions where the resistors R03 and R25 are connected. That is, as a result of the improvement of the rise characteristic of the drive voltage, the actually measured value of the bias voltage when the liquid crystal display device actually performs display approaches the theoretical value, and the drive voltage on the + side with respect to the reference level as shown in FIG. And the degree of imbalance between the drive voltage on the negative side and the negative side is reduced. as a result,
Display unevenness is reduced, and the display quality of the liquid crystal display device is improved.

The difference between the voltage between V0 and V1 and the voltage between V1 and V2, and the voltage between V3 and V4 and V4 and V4
Since the difference between the on-state voltage and the off-state voltage is small, the margin between the on-state voltage and the off-state voltage is increased. Therefore, the adjustment work for setting the drive voltage to a value for obtaining the optimum contrast becomes easy. Furthermore, even if the Vth of the liquid crystal varies, good display quality can be maintained without changing the drive voltage. That is, the allowable range of the variation of Vth can be expanded.

The voltage generating circuit 100 is often integrated in an IC. When an IC incorporating the voltage generation circuit 100 is used, the driving capability of the operational amplifiers 11 to 14 incorporated in the IC cannot be increased in order to improve display unevenness or the like. However, according to the present invention, display unevenness can be improved by mounting the resistors R03 and R25 outside the IC. That is, the present invention is particularly useful when using the voltage generation circuit 100 formed as an IC.

The present invention can be applied to a case where an IC in which the voltage generation circuit 100 is integrated includes a circuit other than the voltage generation circuit 100 (for example, a common driver circuit or a segment driver circuit). For example, even when the voltage generation circuit 100, the common driver circuit, and the segment driver circuit are integrated on one chip, such an integrated circuit generally includes terminals for voltages V0 to V5. . Therefore, the resistors R03 and R25 can be connected to the terminals of the integrated circuit.

When the resistors R03 and R25 are mounted outside of the IC incorporating the voltage generating circuit 100, FIG.
As shown in FIG.
1 in which the voltage generation circuit 100 is built in a circuit board (generally a rigid printed wiring board) 203 connected through
When C is mounted, the resistors R03 and R25 can be mounted on the circuit board 203. Flexible board 2 between liquid crystal panel 200 and circuit board 203
When an IC incorporating the voltage generating circuit 100 is mounted on the circuit board 04, as shown in FIG.
In FIG. 3C, resistors R03 and R25 are mounted.
As shown in FIG.
3, R25 can be mounted. FIG. 3 (d)
As shown in the figure, when an IC incorporating the voltage generation circuit 100 is mounted on the liquid crystal panel 200, the circuit board 2 connected to the liquid crystal panel 200 via the cable 201
03, resistors R03 and R25 can be mounted.

Comparative Example 1 As shown in FIG.
A resistor R02 is connected between the output line of the operational amplifier 12 and the output side of the operational amplifier 12 that outputs the voltage V2, and a resistor R0 is connected between the output line of the voltage V0 and the output side of the operational amplifier 14 that outputs the voltage V4. did. Furthermore, a resistor R15 is connected between the output side of the operational amplifier 11 that outputs the voltage V1 and the output line of the voltage V5, and the operational amplifier 1 that outputs the voltage V3.
A resistor R35 is connected between the output side of the output terminal 3 and the output line of the voltage V5.
Connected. And each bias voltage was measured. The measured values are shown as “character display” in Comparative Example 1 in FIG. As shown in FIG. 2, the value of ΔV is not improved.

Comparative Example 2 As shown in FIG.
The resistor R12 is connected between the output side of the operational amplifier 11 that outputs the voltage V2 and the output side of the operational amplifier 12 that outputs the voltage V2, and the output side of the operational amplifier 13 that outputs the voltage V3 and the operational amplifier that outputs the voltage V4. 14 and the output R
34 were connected, and each bias voltage was measured. The measured value is shown as “character display” in Comparative Example 2 in FIG. FIG.
As shown in the figure, the value of ΔV is not improved.

The method shown in FIG. 4 or FIG.
This is effective when the current driving is performed by only one of the source and the sink, but is not effective when the current driving is performed by both the source and the sink. In the method according to the present invention shown in FIG. 1, even when the operational amplifiers 11 to 14 are driven by both the source and the sink,
The value of ΔV can be improved.

[0026]

As described above, according to the present invention, the bias voltage generating circuit of the liquid crystal display device is connected between the output of the highest level voltage (V0) and the output of the operational amplifier for outputting the voltage V3. Is provided, and a resistor is provided between the output side of the operational amplifier that outputs the voltage V2 and the lowest level voltage (V5) output, so that the value of ΔV is reduced and the display unevenness of the liquid crystal display device is reduced. Has the effect of being able to reduce

When the operational amplifiers in the bias voltage generating circuit are integrated in one integrated circuit, the display unevenness can be obtained without making any changes to the elements such as the operational amplifier built in the integrated circuit. Can be improved. That is, even if the existing integrated circuit is used as it is, the display unevenness of the liquid crystal display device can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of a bias voltage generation circuit of a liquid crystal display device according to the present invention.

FIG. 2 is an explanatory diagram showing a result of measuring a bias voltage and a value of ΔV in a case without a resistor and in the present example.

FIG. 3 is an explanatory diagram exemplifying an installation position of a resistor.

FIG. 4 is a circuit diagram showing a configuration of a bias voltage generation circuit of the liquid crystal display device of Comparative Example 1.

FIG. 5 is a circuit diagram showing a configuration of a bias voltage generation circuit of a liquid crystal display device of Comparative Example 2.

FIG. 6 is a waveform diagram showing an example of using a voltage of a 6-level driving method.

FIG. 7 is a circuit diagram showing a configuration of a bias voltage generation circuit of a conventional liquid crystal display device.

FIG. 8 is an explanatory diagram showing the appearance of display unevenness.

FIG. 9 is a waveform chart showing an unbalanced drive voltage.

[Explanation of symbols]

 11-14 Operational amplifier 100 Voltage generation circuit R03, R25 Resistance

Claims (2)

[Claims] An input voltage is divided into six levels of voltages (V
0, V1, V2, V3, V4, V5: V0>V1> V2
>V3>V4> V5), and comprises four intermediate levels of voltages (V1, V2, V3,
V4) via the operational amplifier, in the bias voltage generating circuit of the liquid crystal display device, wherein the highest level voltage output and the middle two level voltage (V2,
V3), a resistor is provided between the output side of the operational amplifier that outputs the lower-level voltage (V3) and the higher of the middle two-level voltages (V2, V3). A bias voltage generating circuit for a liquid crystal display device, wherein a resistor is provided between an output side of the operational amplifier that outputs a level voltage (V2) and a lowest level voltage output. 2. The circuit according to claim 1, wherein each operational amplifier is integrated in one integrated circuit.