KR20030005801A - Driver circuit in lcd for converting viewing mode and its converting method - Google Patents

Abstract

PURPOSE: A viewing angle converting circuit for a liquid crystal display device and a method thereof are provided to freely carry out the conversion between wide viewing angle mode and narrow viewing angle mode while improving the image quality without modifying the existing LCD driving circuit arrangement largely. CONSTITUTION: A viewing angle converting circuit for a liquid crystal display device includes a DC/DC converter(40) for generating various voltages required for respective parts in the LCD from a power supply, a gamma power supply(42) for converting a liquid crystal driving voltage generated by the DC/DC converter by gamma reference, and a source drive IC for applying the converted liquid crystal driving voltage to a liquid cell array, wherein the DC/DC converter outputs two values(VDD1,VDD2) as the liquid crystal driving voltages with respectively proper for wide and narrow viewing angle modes, and a switch(41) is further provided to alternatively select one of the two valued liquid crystal driving voltages.

Description

DRIVER CIRCUIT IN LCD FOR CONVERTING VIEWING MODE AND ITS CONVERTING METHOD}

The present invention relates to a viewing angle switching drive circuit and a viewing angle switching method of a liquid crystal display.

Recently, various display devices with large screens and high definitions have been developed in addition to CRTs. Representative examples thereof include PDPs, LCDs, and projection displays. Among them, LCDs are widely used because they are relatively low in manufacturing cost, thin, and can be used for portable terminals as well as monitors and TVs.

LCD has many advantages as a display device, but has a small viewing angle and a relatively low screen brightness than other display devices. Therefore, the recent LCD development is focused on increasing the dash angle and transmittance.

For displays it is important to maintain a constant brightness throughout the screen. In the conventional TFT-LCD, since its transmittance-voltage characteristic (hereinafter, T-V characteristic) is different depending on the viewing angle, inversion of the screen occurs according to the viewing angle so that a good screen can be seen only within a specific viewing angle.

It is an object of the present invention to provide a liquid crystal display capable of freely switching between a wide viewing angle mode and a narrow viewing angle mode.

Another object of the present invention is to provide an LCD driving circuit capable of switching the viewing angle without significantly changing the existing LCD driving circuit.

Still another object of the present invention is to provide a liquid crystal display in which the image quality is not degraded even when the viewing angle mode is switched.

Other objects and features of the present invention will be described in detail in the configuration and claims of the following invention.

1 is a graph showing general T-V characteristics of a TFT-LCD using a TN liquid crystal.

2 is a graph showing another T-V characteristic of the TFT-LCD.

3A is a graph simulating the viewing angle characteristics of the driving voltage range A of FIG. 2.

3B is a graph simulating the viewing angle characteristics of the driving voltage range C of FIG. 2.

4 is a circuit diagram showing an embodiment of the present invention.

5 shows an example of a gamma power supply circuit used in a TFT-LCD.

6 is a graph showing a change in the actual output voltage of the gamma power supply according to the change in the drive voltage of the DC-DC converter.

FIG. 7A is a graph simulating the viewing angle characteristic of the driving voltage range A of FIG. 6.

FIG. 7B is a graph simulating the viewing angle characteristic of the driving voltage range C of FIG. 6.

8 is a circuit diagram showing another embodiment of the present invention.

9A is a graph showing the relationship between the low driving voltage waveform and Vcom.

9B is a graph showing the relationship between a large drive voltage waveform and Vcom.

10 is a circuit diagram showing another embodiment of the present invention.

*** Explanation of symbols for main parts of drawing ***

40: DC-DC converter 41: switch

42: gamma power supply

In the liquid crystal display, the liquid crystal driving voltage (V _DD ) input to the source drive IC from the DC / DC converter which generates various voltages necessary for each part inside the liquid crystal display from the power supply unit is a wide viewing angle mode and a narrow viewing angle mode. The present invention provides a viewing angle switching method of outputting two values so as to suit each of the two, and selectively determining the two driving voltage values through a switch.

The present invention also provides a DC / DC converter for generating various voltages required for each part of the liquid crystal display from the power supply unit, a gamma power supply unit for converting the liquid crystal driving voltage generated from the DC / DC converter by gamma correction, and conversion. In the LCD driving circuit comprising a source drive IC for receiving the input liquid crystal driving voltage applied to the liquid crystal cell array, the DC / DC converter has two types to match the liquid crystal driving voltage to the wide viewing angle mode and narrow viewing angle mode, respectively It provides a viewing angle switching LCD drive circuit characterized in that it comprises a switch for outputting a value, the two drive voltage values are selectively determined.

The LCD driving circuit and the switching method which can switch the viewing angle of the present invention can be driven by a line inversion method or a dot inversion method, and in the latter case, each common voltage is applied to two driving voltages output from a DC / DC converter. It is preferable that the electrode voltage Vcom is set.

In addition, the two driving voltages are respectively converted by gamma correction before being input to the source drive IC from the DC / DC converter, so that each gamma correction voltage corresponding to the driving voltage is sourced. It is preferable to input to the drive IC.

The present invention enables the inversion of an image to occur or not to occur at a particular viewing angle by changing the range of driving voltages. Therefore, it is possible to selectively determine the driving voltage range suitable for the wide viewing angle or the narrow viewing angle.

Particularly, in the present invention, the V _DD value, which is the liquid crystal driving voltage of the DC-DC converter of the TFT-LCD driving circuit part, is divided into two outputs, thereby simplifying the existing circuit part without making a large change in the existing drive-IC or bias circuit part. The change enables the change of the viewing angle.

Hereinafter, with reference to the drawings, the present invention will be described in more detail.

As shown in FIG. 1, the T-V characteristics of a TFT-LCD using a conventional twisted nematic (TN) liquid crystal show different curves depending on the viewing angle. A curve, a TV characteristic at a viewing angle of 0 °, and a b curve, a TV characteristic at a viewing angle of 50 °, are linearly inversely proportional to the voltage and transmittance in the range of approximately 1V to 2V. The range represents different types of TV characteristics. That is, in the case of the a curve, the transmittance is maintained as the voltage increases, but in the case of the b curve, the inversion phenomenon is increased again.

When the driving voltage is set within the range of A on the drawing, the difference in transmittance according to the viewing angle is small, and when the driving voltage is set within the range of B on the drawing, the inversion according to the viewing angle is severe. The driving voltage range of A is suitable for the wide viewing angle mode because the transmittance difference according to the viewing angle is small, and the driving voltage range of B is suitable for the narrow viewing angle mode because the inversion becomes severe when the viewing angle is increased.

If the driving voltage can be switched between the range of A and B, the two viewing angle modes can be used together with one LCD panel.

The present invention provides an LCD driving circuit and a driving method for easily switching between wide viewing angle and narrow viewing angle by simply changing the LCD driving circuit so as to selectively output these two driving voltage ranges.

The difference in transmittance according to the viewing angle will be described in more detail.

FIG. 2 shows transmittance-voltage characteristics in a normally white driving mode for LCDs using TN liquid crystals. As shown in the figure, the transmittance shows a linear inverse relationship with increasing voltage, but as the viewing angle increases, the reversal phenomenon increases at higher voltages.

When the driving voltage is 3.3V, that is, in the driving voltage range B, the transmittance is high near the driving voltage of 1V, so that the screen is white on the screen, and the screen is black near the driving voltage of 3.3V. As can be seen from the figure, as the viewing angle increases, the luminance of white decreases and the leakage of the black state increases. Therefore, as the viewing angle increases, contrast decreases, and gray inversion occurs, so that an accurate image cannot be viewed. For this reason, the effective viewing angle is limited to within a certain angle.

If the driving voltage range is A instead of B, the contrast ratio decreases at the front, i.e., the viewing angle of 0 degrees. However, even if the viewing angle is increased, the reduction of the CR and the degree of inversion are less. Could be.

On the contrary, when the LCD is driven using the driving voltage range C, there is almost no CR change in the front (viewing angle 0 degrees), and the effective viewing angle is narrowed as a whole because the degree of CR reduction and inversion increases with increasing viewing angle. .

3A and 3B show the viewing angle characteristics according to the driving voltage range.

3A illustrates a viewing angle characteristic using the driving voltage range A of FIG. 2, and it can be seen that the left and right viewing angles and the lower viewing angle are wide. On the other hand, referring to FIG. 3B, the viewing angle characteristics are shown using the driving voltage range C of FIG. 2, and the left and right and lower viewing angles are narrow when compared to the viewing angle characteristics of FIG. 3A.

In the present invention, the driving voltage range having a small difference in transmittance according to the viewing angle is set as the wide viewing angle mode, and when the viewing angle is increased, the driving voltage range in which the inversion is severe is set as the narrow viewing angle mode, so that switching between the wide viewing angle mode and the narrow viewing angle mode is possible. The conventional DC-DC converter outputs two liquid crystal driving voltage values (V _DD ). In addition, the selection switch is configured to select two liquid crystal driving voltages, and the output of the gamma power supply unit is changed according to the selected driving voltage range so that the final liquid crystal driving voltage range is determined.

As an embodiment of the present invention, a circuit diagram including a DC-DC converter for generating two driving voltages is shown in FIG. 4. Two driving voltages V _DD 1 and V _DD 2 are output from the DC-DC converter 40. Any one of these two values is selected by the selection switch 41, and the selected driving voltage is the gamma power supply 42. Is entered.

It is desirable for the selector switch to extend and be exposed out of the LCD. Therefore, by operating the switch manually by the operator from the outside, it is possible to use two modes of wide viewing angle and narrow viewing angle with one LCD.

However, only by outputting two driving voltages V _DD from the DC-DC converter and selecting one of them, there is a risk of degrading the image quality appearing on the LCD. That is, the voltage range corresponding to the wide viewing angle mode and the narrow viewing angle mode as shown in FIG. 1 cannot be accurately selected. The reason is that since the circuit of the gamma power supply unit is used as it is, when the driving voltage V _DD is changed, all the gamma voltages corresponding thereto are changed.

This will be described in more detail as follows.

The liquid crystal drive voltage in the TFT-LCD is determined from the output of a DC-DC converter inside the source drive IC. On the other hand, the reference voltage for the output voltage of the DA converter is specified by the gamma power supply unit outside the source drive IC.

5 shows an example of a gamma power supply circuit used in a dot-inversion driving TFT-LCD. The voltage, denoted V _DD (50), is generated for the bias voltage of the source drive IC in the DC / DC converter. Since the gamma circuit part has a structure in which an output buffer is added to a simple voltage divider circuit composed of actual series resistors, when V _DD corresponding to the input voltage value changes, the output voltages GMA1 to GMA10 (51) are also proportional to V _DD . Will change.

Therefore, when the driving voltage V _DD generated from the DC-DC converter is changed between A and C as shown in FIG. 2, the output voltage of the gamma power supply unit is actually a driving voltage of A 'to C' as shown in FIG. 6. The range is shown.

In this case, the viewing angle characteristic according to the driving voltage is calculated through simulation and is shown in FIGS. 7A and 7B.

FIG. 7A corresponds to the case where the driving voltage range is A (wide viewing angle mode), and there is no significant change in viewing angle characteristics when compared to FIG. 3A, but in the case of FIG. 7B when the driving voltage range corresponds to C (narrow viewing angle mode). It can be seen that the viewing angle becomes narrower than in the case of FIG. 3B.

In addition, as shown in FIG. 5, the luminance of white is reduced. Therefore, it is preferable to separately provide a gamma circuit of the gamma power supply unit for each driving voltage generated from the DC-DC converter in order to prevent the reduction of the viewing angle and the luminance.

8 shows a circuit configuration including two gamma circuits as another embodiment of the present invention.

From the DC-DC converter 80 which outputs two driving voltages V _DD 1 and V _DD 2, any driving voltage selected by the selection switch 81 is input to the gamma power supply unit. The gamma power supply unit is input to the corresponding gamma circuit. If the selected driving voltage is V _DD 1, it is connected to gamma circuit 1 82a, and if the selected driving voltage is V _DD 2, it is connected to gamma circuit 2 82b.

The switch 81 selects the driving voltage and the corresponding gamma circuit at the same time so that each driving voltage is connected to the corresponding gamma circuit.

As shown in the figure, even with such a configuration, since the output buffer portions coming out of the two gamma circuit portions can be used together, only the parts added to the actual circuit portion are gamma resistors. Therefore, in the present embodiment, two gamma circuit units share one output buffer 83. The voltage value output through the output buffer is input to the drive IC 84.

As such, when two gamma circuit units are present, more precise gamma adjustment is possible for each of the wide viewing angle driving mode and the narrow viewing angle driving mode, thereby bringing not only the viewing angle switching but also the improvement of image quality.

On the other hand, in the dot inversion driving circuit, when the driving voltage V _DD generated from the DC-DC converter changes, the common electrode voltage V _com must also be changed. 9A and 9B schematically illustrate the reason why the V _com value in the dot-inversion driving should be changed when the driving voltage range is changed.

Using the low voltage V _DD value generated from the DC-DC converter, a driving voltage waveform 90 as shown in FIG. 9A is applied, and black (92, 96) and white (94) depending on the voltage difference from V _com. , 98) is determined. When a large V _DD value is used from the DC-DC converter, the voltage amplitude of the DC-DC converter is increased compared to that of FIG. 9A as shown in FIG. 9B, and thus the voltage difference from V _com is different from that of FIG. 9A. That is, when a + voltage Black (92 ') and white (94' with respect to) the voltage difference between the V _com becomes large, - the voltage of the case of the V _com for black (96 ') and white (98') The voltage difference becomes small. Thus, according to the V _DD value varies to be set to V _com accordingly by V _com '.

To this end, two V _com output terminals are required, and an embodiment of the circuit configuration in this case is shown in FIG.

Any one of the driving voltages selected by the selection switch 101 from the DC-DC converter 100 which outputs two driving voltages V _DD 1 and V _DD 2 is not only input to the corresponding gamma power supply unit, but also the V _com circuit. Is also entered.

If the driving voltage is V _DD 1, it is connected to the gamma circuit 1 102a and V _com 1 105a, and if the selected driving voltage is V _DD 2, it is connected to the gamma circuit 2 102b and V _com 2 105b. The switch 101 simultaneously selects the driving voltage and the corresponding gamma circuit so that each driving voltage is connected to the corresponding gamma circuit and the corresponding V _com circuit. The two gamma circuit units share one output buffer 103, and the voltage value output through the output buffer is input to the source drive IC 104, and the voltage output from the selected V _com circuit is applied to the common electrode 106. do.

Since the V _com circuit uses V _DD as an input, the corresponding V _com can be selected without adding a switch depending on whether the driving voltage V _DD is selected.

In the above embodiment, two gamma circuits are connected in parallel with the corresponding V _com circuits, but one gamma circuit is the same as the embodiment shown in FIG. 4, and each V for two driving voltages V _DD is provided. The circuit configuration selected by the switch to the _com circuit is also possible.

Through the circuit configuration of various embodiments of the present invention as described above, it is possible to configure an LCD that can switch between the wide viewing angle and the narrow viewing angle without reducing the viewing angle or the luminance.

According to the present invention, it is possible to freely switch between the wide viewing angle mode and the narrow viewing angle mode in the liquid crystal display. In addition, the present invention provides an LCD driving circuit capable of switching the viewing angle without significantly changing the existing LCD driving circuit. In addition, it is possible to provide a liquid crystal display capable of improving image quality even by switching the viewing angle mode.

Claims (14)

In liquid crystal display, Outputs the liquid crystal driving voltage input to the source drive IC from the DC / DC converter which generates various voltages necessary for each part of the liquid crystal display from the power supply unit in two values so as to be suitable for the wide viewing angle mode and the narrow viewing angle mode, respectively. And a method of selectively determining the two driving voltage values through a switch. The viewing angle switching method of a liquid crystal display according to claim 1, wherein the display device is driven by a line inversion method. The viewing angle switching method of a liquid crystal display according to claim 1, wherein the driving method is driven by a dot inversion method. 4. The method of claim 3, wherein the common electrode voltages are set for two driving voltages. The method of claim 4, wherein each common electrode voltage is selectively determined together with a corresponding driving voltage by the switch. The method of claim 1, wherein the two drive voltages are respectively converted by gamma correction before being input from the DC / DC converter to the source drive IC so that each gamma correction voltage corresponding to the drive voltage is converted into a source drive IC. Viewing angle switching method of the liquid crystal display, characterized in that the input. 7. The method of claim 6, wherein each gamma correction voltage is selectively determined together with a corresponding driving voltage by the switch. 7. The method according to claim 6, wherein the common electrode voltages are set for two driving voltages. The method of claim 1, wherein the selection switch is exposed to the outside of the LCD to enable manual operation. A DC / DC converter for generating various voltages necessary for each part of the liquid crystal display from a power supply unit; A gamma power supply unit converting the liquid crystal driving voltage generated from the DC / DC converter by gamma correction; In the LCD driving circuit comprising a source drive IC for receiving the converted liquid crystal driving voltage applied to the liquid crystal cell array, The DC / DC converter outputs liquid crystal driving voltages in two values so as to be suitable for a wide viewing angle mode and a narrow viewing angle mode, respectively. And a switch in which the two driving voltage values are selectively determined. The driving angle switching driver circuit of claim 10, wherein the DC / DC converter outputs common electrode voltages to two driving voltages. 12. The driving angle switching driving circuit of claim 11, wherein each common electrode voltage is selectively determined by the switch together with a corresponding driving voltage. The viewing angle switching driving circuit of claim 10, wherein the gamma power supply unit outputs gamma correction voltages corresponding to two driving voltages and applies them to a source drive IC. 14. The switching circuit of a viewing angle of a liquid crystal display according to claim 13, wherein each gamma correction voltage is selectively determined together with a corresponding driving voltage by the switch.