Patent Publication Number: US-6211866-B1

Title: Grayscale voltage generating circuit

Description:
FIELD OF THE INVENTION 
     This invention relates to a liquid crystal display device and, more particularly, to a grayscale voltage generating circuit used in circuitry for driving a liquid crystal display device which presents a multi-level grayscale (graduation) display. 
     BACKGROUND OF THE INVENTION 
     A grayscale voltage generating circuit for a liquid crystal display device of this type has been disclosed in the specification of Japanese Patent Kokai Publication JP-A-6-348235, by way of example. Specifically, the specification proposes a grayscale voltage generating circuit comprising a plurality of fixed resistors serially connected between a high-potential reference voltage and a low-potential reference voltage, and voltage varying means capable of varying the voltages at the nodes between the fixed resistors between the high-potential reference voltage and the low-potential reference voltage, with the voltages at the nodes between the fixed resistors being used as grayscale signals. 
     FIG. 5 illustrates part of this prior-art grayscale voltage generating circuit. Though there are eight outputs of positive polarity and eight outputs of negative polarity according to the description given in the above-mentioned Japanese Patent Kokai Publication JP-A-6-348235 (see FIG. 1 of the specification), here the grayscale voltage generating circuit will be described as a circuit having three outputs of each of the positive and negative polarities. 
     As shown in FIG. 5, the prior-art grayscale voltage generating circuit includes fixed resistors R 1  to R 4  and a variable resistor VR 3  serially connected between a reference voltage VH and a reference voltage VL, variable resistors VR 1 , VR 2  serially connected between the reference voltage VH and reference voltage VL, an amplifier circuit (a voltage-follower operational amplifier) A 1  connected between the node of the fixed resistors R 1 , R 2  and the variable resistor VR 1 , and an amplifier circuit A 2  connected between the node of the fixed resistors R 3 , R 4  and the variable resistor VR 2 . 
     The reference voltages VH and VL in this grayscale voltage generating circuit are output as is as a high-potential grayscale voltage V 0 H on the side of positive polarity and a high-potential grayscale voltage V 0 L on the side of negative polarity, respectively. A halftone grayscale voltage V 1 H and a low-potential grayscale voltage V 2 H on the side of positive polarity as well as a low-potential grayscale voltage V 2 L and a halftone grayscale voltage V 1 L on the side of negative polarity can be adjusted by adjusting the resistance values of the variable resistors VR 1 , VR 2  connected to the input side of the amplifier circuits A 1 , A 2 , respectively, and the resistance value of the variable resistor VR 3  located between the grayscale voltage V 2 H and grayscale voltage V 2 L. 
     The above-described grayscale voltage generating circuit makes it possible to improve upon a change in grayscale characteristic brought about by a change in viewing angle, namely the angle from which a liquid crystal display monitor is viewed. This change in grayscale characteristic due to a change in viewing angle is one characteristic of a liquid crystal display monitor primarily of the twisted nematic type. 
     Owing to the characteristics of the liquid crystal in a liquid crystal monitor, it is required that identical positive and negative voltages be applied in AC drive. Furthermore, it is necessary to correct for a shift (referred to as a “feed-through characteristic”) in ground potential of the liquid crystal caused by a difference in the voltages applied to the liquid crystal (e.g., as when 1 V is applied and when 5 V is applied). This correction is referred to as a “feed-through correction.” 
     In the above-described grayscale voltage generating circuit, feed-through correction values are decided by the potential dividing ratio of the serially connected fixed resistors R 2 , R 3  and variable resistor VR 3  and the adjustment of the resistance values of the variable resistors VR 1 , VR 2 . Consequently, even if the variable resistors VR 1 , VR 2  are varied to obtain the optimum grayscale characteristic and optimize the feed-through correction values of the grayscale voltages V 1 H, V 1 L in order to improve upon the grayscale characteristic based upon the viewing angle, the feed-through correction values of the low-potential grayscale voltages V 2 H, V 2 L are decided solely by the potential dividing ratio determined by the fixed resistors R 2 , R 3  and variable resistor VR 3  between the halftone-level grayscale voltages V 1 H, V 1 L. In other words, adjusting the variable resistor VR 3  merely changes the potential difference between the low-potential grayscale voltages V 2 H and V 2 L. This means that these feed-through correction values cannot be adjusted individually for each of the positive and negative polarities. 
     As a consequence of the foregoing, the feed-through correction values of the grayscale voltages V 2 H, V 2 L differ from the appropriate values. 
     SUMMARY OF THE DISCLOSURE 
     In the course of the investigations toward the present invention the following problems have been encountered in the prior art. 
     First, the variable resistances which decide the grayscale voltages (grayscale characteristic) on the side of positive and negative polarity are required to be adjusted by identical ratios at adjusting points provided on both the positive- and negative-polarity sides, and this necessitates the use of measuring equipment such as a voltmeter. Consequently, it is difficult for the user of the liquid crystal monitor to adjust the grayscale characteristic without using measuring equipment. The reason for this is that in order to operate a liquid crystal panel, it is necessary to drive the liquid crystal by an AC voltage and apply the grayscale voltages on the sides of positive and negative polarity at identical ratios with respect to the ground potential of the applied voltage. 
     The second problem is that when a grayscale voltage is varied, the feed-through correction must be performed again. For example, if grayscale voltage is varied in the grayscale voltage generating device shown in FIG. 5, the ideal feed-through correction values to be applied to the liquid crystal panel will no longer be obtained. The reason for this is as follows: In the grayscale voltage generating circuit shown in FIG. 5, the high-potential grayscale voltages V 0 H, V 0 L serve as the references and the high-potential feed-through correction values need not be varied to accomplish the feed-through correction. The feed-through correction of the halftone levels can be performed by adjusting the variable resistor VR 1  in such a manner that the grayscale voltage V 1 H will agree with a grayscale voltage to which a feed-through correction value on the positive polarity side has been added and adjusting the variable resistor VR 2  in such a manner that the grayscale voltage V 1 L will agree with a grayscale voltage to which a feed-through correction value on the negative polarity side has been added. However, since the low-potential feed-through correction is decided by the potential dividing ratio of the potential dividing resistors R 2 , R 3 , VR 3  located between the grayscale voltages V 1 H and V 1 L, the correction is influenced by a change in the halftone-level grayscale voltages V 1 H, V 1 L and low-potential feed-through correction values that should be constant levels are no longer obtained. 
     Accordingly, the present invention seeks to solve the aforementioned problems and, it is an object thereof to provide a grayscale voltage generating circuit for a liquid crystal display device in which, by adjusting a variable resistor at only one location without use of measuring equipment, halftone-level grayscale voltages on the positive and negative polarity sides can be varied simultaneously over identical voltage ranges without affecting high- and low-potential grayscale voltages. 
     It is another object to provide a grayscale generating circuit in which ideal halftone-level feed-through correction values to be applied to the liquid crystal panel can be varied simultaneously on the sides of positive and negative polarity without affecting the high- and low-potential grayscale voltages and the feed-through correction values. 
     Further objects of the present invention will become apparent in the entire disclosure. 
     According to an aspect of the present invention, the foregoing object is attained by providing a grayscale voltage generating circuit comprising means for generating a halftone-level reference voltage by resistance-type potential division of a potential between positive-polarity high- and low-potential grayscale voltages that are outputs obtained by amplifying voltages produced by potential division of a reference power supply voltage, first amplifying means for generating a variable halftone-level grayscale voltage from the halftone-level reference voltage using a variable resistor as a feedback resistor, and second amplifying means for inverting all positive-polarity grayscale voltages with respect to a liquid crystal ground potential, which has been obtained by potential division and amplification of the reference power supply voltage, amplifying the inverted grayscale voltages by the same ratio and outputting the amplified voltages as negative-polarity grayscale voltages. 
     In the grayscale voltage generating circuit according to the present invention, a halftone-level reference voltage (VA in FIG. 1) is generated by a resistance-type potential divider circuit from a potential between a high-potential grayscale voltage and a low-potential grayscale voltage on the side of positive polarity, and a halftone-level grayscale voltage (V 1 H in FIG.  1 ), which is capable of being varied to be an intermediary, is generated from the halftone reference voltage by an amplifier (A 3  in FIG.  1 ), which uses a variable resistor (VR 1  in FIG. 1) as a feedback resistor. All grayscale voltages on the side of positive polarity are inverted with respect to the ground potential (VB in FIG. 1) of a liquid crystal panel and amplified by the same ratio by amplifiers (A 5  to A 7  in FIG.  1 ). 
     Further, on both the sides of positive and negative polarity, high- and low-potential feed-through correction values are rendered constant, and a halftone-level feed-through correction value is varied automatically in dependence upon a change in the halftone-level grayscale voltage on the side of positive polarity, whereby there are obtained ideal feed-through correction values to be applied to the liquid crystal cell. 
     In the present invention, a halftone-level grayscale voltage (V 1 L in FIG. 1) on the side of negative polarity also is capable of being varied by the amplifier that inverts and amplifies the positive-polarity halftone-level grayscale voltage made variable by the variable resistor (VR 1  in FIG.  1 ). Thus, by varying a resistor at a single location, positive- and negative-polarity grayscale voltages can be adjusted at the same time. 
     Further, by continuously varying the halftone-level ground potential of the liquid crystal panel, the variable halftone-level grayscale voltage can always be made one which is the result of adding on a suitable feed-through correction value. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating the circuitry of a first embodiment of a grayscale voltage generating circuit according to the present invention; 
     FIG. 2 is a diagram useful in describing adjustment of voltage in the first embodiment of the grayscale voltage generating circuit according to the present invention; 
     FIG. 3 is a circuit diagram illustrating the circuitry of a second embodiment of a grayscale voltage generating circuit according to the present invention; 
     FIG. 4 is a diagram useful in describing adjustment of voltage in the second embodiment of the grayscale voltage generating circuit according to the present invention; and 
     FIG. 5 is a diagram illustrating the circuitry of a grayscale voltage generating circuit according to the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Modes for practicing the present invention will be described below. 
     FIG. 1 is a circuit diagram illustrating the circuitry of a first embodiment of a grayscale voltage generating circuit according to the present invention. 
     The circuit components for generating grayscale voltages on the side of positive polarity will be described first. As shown in FIG. 1, a voltage produced from a reference voltage +V by a potential dividing circuit composed of resistors R 1  and R 2  is buffered by an amplifier A 1 , which outputs a grayscale voltage V 0 H on the high-potential side. A voltage produced from the reference voltage +V by a potential dividing circuit composed of resistors R 3  and R 4  is buffered by an amplifier A 2 , which outputs a grayscale voltage V 2 H on the low-potential side. The potential between the grayscale voltages produced by the amplifiers A 1  and A 2  is divided by a potential dividing circuit composed of resistors R 5  and R 6 , whereby a voltage VA is obtained. A halftone-level grayscale voltage V 1 H, whose voltage value is capable of being adjusted, is produced from the voltage VA by an amplifier A 3  using a variable resistor VR 1  as a feedback resistor. 
     The circuit components for generating grayscale voltages on the side of negative polarity will be described next. A voltage produced from the reference voltage +V by a potential dividing circuit composed of resistors R 8  and R 9  is buffered by an amplifier A 4 , which produces a voltage VB serving as the ground level of a liquid crystal panel. The ground-level voltage VB is used as the bias voltage of amplifiers A 5 , A 6  and A 7 , which invert and amplify the positive-polarity grayscale voltages V 0 H, V 1 H and V 2 H to negative-polarity grayscale voltages V 0 L, V 1 L and V 2 L, respectively. 
     FIG. 2 is a diagram useful in describing adjustment of voltage according to the first embodiment of the invention. Reference will be had to FIG. 2 to describe the operation of the grayscale voltage generating circuit shown in FIG.  1 . 
     The high-potential grayscale voltage V 0 H on the side of positive polarity is rendered constant by the amplifier A 1  functioning as a buffer. Similarly, the low-potential grayscale voltage V 2 H on the side of positive polarity is rendered constant by the amplifier A 2  functioning as a buffer. 
     The halftone-level grayscale voltage V 1 H, for which the reference voltage is the voltage VA obtained potential-dividing the grayscale voltages V 0 H and V 2 H, is capable of being varied from VA+V 2 H to VA×(1+VR 1 /R 7 )+V 2 H by using the variable resistor VR 1  serving as the feedback resistor. 
     Using the voltage VB, which is the ground potential of the liquid crystal panel produced by the amplifier A 4 , as a bias voltage, the amplifiers AS, A 6  and A 7  invert and amplify the positive-polarity grayscale voltages V 0 H, V 1 H, V 2 H, respectively, by an amplification factor of one. As a result, the grayscale voltages V 0 L, V 1 L and V 2 L output by the respective amplifiers A 7 , A 6  and A 5  have identical potentials as the grayscale voltages V 0 H, V 1 H and V 2 H, respectively, with respect to the voltage VB of the ground potential of the liquid crystal panel. 
     EXAMPLES 
     Preferred embodiments of the present invention will now be described in greater detail with reference to the drawings. 
     The positive-polarity side of the circuitry according to the first embodiment will be described first. As shown in FIG. 1 illustrating the first embodiment, fixed resistors R 1 , R 2  are serially connected between the reference power supply voltage +V and ground, thereby potential-dividing the reference voltage. The resulting voltage is input to the + (non-inverting) input terminal of the amplifier A 1 , the − (inverting) input terminal and output terminal of which are connected together. As a result, the amplifier A 1  outputs the high-potential grayscale voltage V 0 H. Fixed resistors R 3 , R 4  are serially connected between the reference power supply voltage +V and ground, thereby potential-dividing the reference voltage. The resulting voltage is input to the + input terminal of the amplifier A 2 , the − input terminal and output terminal of which are connected together so that amplifier A 2  outputs the low-potential grayscale voltage V 2 H. Fixed resistors R 5 , R 6  are serially connected between the high-potential grayscale voltage V 0 H and the low-potential grayscale voltage V 2 H, thereby producing the voltage VA by potential division. The voltage VA is input to the + input terminal of the amplifier A 3 , which amplifies this voltage but does not invert it. The − input terminal of amplifier A 3  is connected to the grayscale voltage V 2 H via resistor R 7  as well as to the output terminal via the variable feedback resistor VR 1 . As a result, the amplifier A 3  outputs the variable halftone-level grayscale voltage V 1 H. 
     Next, with regard to the circuitry that produces the grayscale voltages on the side of negative polarity, fixed resistors R 8 , R 9  are serially connected between the reference power supply voltage +V and ground, thereby potential-dividing the reference voltage. The resulting voltage is input to the + input terminal of the amplifier A 4 , the − input terminal and output terminal of which are connected together. As a result, the amplifier A 4  produces the voltage VB serving as the ground potential of the liquid crystal panel. 
     The voltage VB serving as the ground potential of the liquid crystal panel is input to the + input terminal of the amplifier AS, which is constructed as an inverting amplifier, the low-potential grayscale voltage V 2 H on the side of positive polarity is input to the − input terminal via a fixed resistor R 10 , and the − input terminal of amplifier A 5  is connected to the output terminal via a feedback resistor R 11 , whereby the amplifier A 5  outputs the low-potential grayscale voltage V 2 L. 
     The voltage VB serving as the ground potential of the liquid crystal panel is input to the + input terminal of the amplifier A 6 , which is constructed as an inverting amplifier, the halftone-level grayscale voltage V 1 H on the side of positive polarity is input to the − input terminal via a fixed resistor R 12 , and the − input terminal of amplifier A 6  is connected to the output terminal via a feedback resistor R 13 , whereby the amplifier A 6  outputs the low-potential grayscale voltage V 1 L. 
     The voltage VB serving as the ground potential of the liquid crystal panel is input to the + input terminal of the amplifier A 7 , which is constructed as an inverting amplifier, the high-potential grayscale voltage V 0 H on the side of positive polarity is input to the − input terminal via a fixed resistor R 14 , and the − input terminal of amplifier A 7  is connected to the output terminal via a feedback resistor R 15 , whereby the amplifier A 7  outputs the high-potential grayscale voltage V 0 L. 
     The operation of the first embodiment will now be described with reference to FIG.  2 . 
     According to a technique for driving a normally white liquid crystal panel, the output terminal of the amplifier Al delivers the positive-polarity fixed grayscale voltage V 0 H that is necessary for displaying black (which corresponds to a high potential), the output terminal of the amplifier A 2  delivers the positive-polarity fixed grayscale voltage V 2 H that is necessary for displaying white (which corresponds to a low potential), the output terminal of the amplifier A 5 , which inverts and amplifies its input by a factor of one, delivers the negative-polarity fixed grayscale voltage V 2 L necessary for displaying white, and the output terminal of the amplifier A 7 , which inverts and amplifies its input by a factor of one, delivers the negative-polarity fixed grayscale voltage V 0 L necessary for displaying black. 
     Thus, the potentials applied during the white and black displays are fixed, whereby a constant contrast is obtained. 
     The output terminal of the amplifier A 3 , which is constructed as a non-inverting amplifier, delivers the positive-polarity grayscale voltage V 1 H necessary for displaying halftones. The grayscale voltage V 1 H is capable of being varied over the voltage amplification range 
     
       
         VA+V 2 H to VA×(1+VR 1 /R 7 )+V 2 H 
       
     
     decided by the fixed resistor R 7  and the variable resistor VR 1 . The output terminal of the amplifier A 6 , which inverts and amplifies its input by a factor of one, delivers the negative-polarity grayscale voltage V 1 L necessary for displaying halftones. The grayscale voltage V 1 L is capable of being varied over the same voltage amplification range as that indicated above. Thus, the grayscale voltages necessary for displaying halftones can be varied by identical amounts on the sides of positive and negative polarity with respect to the ground potential of the liquid crystal cell, as a result of which a variable grayscale characteristic (halftone contrast) is obtained. 
     A second embodiment of the present invention will now be described with reference to FIGS. 3 and 4, in which FIG. 3 is a circuit diagram illustrating the circuitry of the grayscale voltage generating circuit and FIG. 4 is a diagram useful in describing adjustment of voltage. 
     The purpose of the second embodiment of the invention is to optimize feed-through correction values as well when the halftone-level grayscale voltages are varied. According to this embodiment, the section comprising the fixed resistors R 8 , R 9  and amplifier A 4  that is for generating the ground potential of the liquid crystal panel is changed as set forth below. 
     First, the fixed resistors R 8 , R 9  are serially connected between the power supply voltage +V and ground, thereby potential-dividing the reference voltage. The resulting voltage is input to the + input terminal of the amplifier A 4 , the − input terminal and output terminal of which are connected together. As a result, the amplifier A 4  produces the potential VB serving as the ground potential of the liquid crystal panel. This is a comparatively low potential. The low ground potential VB is connected to the + input terminal of the amplifier A 7 , which performs an inverting amplifying function. 
     Further, fixed resistors R 10 , R 11  are serially connected between the reference power supply voltage +V and ground, thereby potential-dividing the reference voltage. The resulting voltage is input to the + input terminal of an amplifier A 5 , the − input terminal and output terminal of which are connected together. As a result, the amplifier A 5  produces a comparatively high ground potential VC of the liquid crystal panel. The high ground potential VC is connected to the + input terminal of the amplifier A 9 , which performs an inverting amplifying function. Fixed resistors R 12 , R 13  are serially connected between the halftone-level grayscale voltage V 1 H on the side of positive polarity and the voltage produced by the potential dividing operation of the fixed resistors R 10 , R 11 , thereby producing a voltage by potential division. This voltage is connected to the − input terminal of the amplifier A 6 , which performs an inverting amplifying function, via a resistor R 14 . The − input terminal of this amplifier is connected to the output terminal via a feedback resistor R 15 . The voltage produced by the fixed resistors R 8 , R 9  by potential division is connected to the + input terminal of the amplifier A 6 . As a result, the amplifier A 6  outputs a ground potential VD of the liquid crystal panel. This is a halftone-level potential. The halftone-level potential VD is connected to the + input terminal of the inverting amplifier A 8 . 
     According to the technique for driving a normally white liquid crystal panel, a feed-through value increases when the voltage applied to the liquid crystal is increased and decreases when the applied voltage is reduced. This is a characteristic of the liquid crystal. With the arrangement of the second embodiment described above, a feed-through correction value VE is produced between the liquid crystal ground potential VC (used as the reference) when black is displayed (which corresponds to high potential) and the liquid crystal ground potential VB when white is displayed (which corresponds to low potential). 
     In regard to the liquid crystal ground potential when a halftone is displayed, it is required that the liquid crystal ground potential VD be low when the halftone-level grayscale voltage V 1 H is high and that the liquid crystal ground potential VD be high when the halftone-level grayscale voltage V 1 H is low. First, if an amount of change in feed-through value that corresponds to a fluctuation in applied voltage when the grayscale voltage V 1 H is varied is generated by the voltage dividing circuit composed of the fixed resistors R 12 , R 13  and inverting amplification by a factor of one is performed with respect to the liquid crystal ground potential VB, which is on the high-potential side, of the fixed-output ground potentials VB, VC, then the halftone-level liquid crystal ground potential VD can be produced. In a case where the liquid crystal ground potential VC for the black display (high potential) is used as the reference, a feed-through correction value VF will be produced. 
     Further, by varying the variable resistor VR 1 , which is for adjusting the grayscale characteristic, so as to vary the halftone-level grayscale voltages, the feed-through correction value VF will vary in dependence upon the varied grayscale voltages and the halftone-level grayscale voltages V 1 H, V 1 L will become grayscale voltages of the same potential with respect to the liquid crystal ground potential VD to which the feed-through correction value VF has been reflected. 
     Thus, in accordance with the second embodiment of the present invention, by varying the single variable resistor VR 1  for adjustment of the grayscale voltage characteristic, halftone-level grayscale voltages on the sides of both positive and negative polarity can be adjusted while contrast between all white and all black is held constant, and it is possible to set feed-through correction values to optimum values automatically by varying the halftone-level grayscale voltages. 
     The present invention has a number of advantages. 
     First, the grayscale characteristic can be adjusted by adjusting a variable resistor at a single location. As a result, the end user of the liquid crystal monitor is capable of adjusting the grayscale characteristic with ease without using measuring equipment such as a voltmeter. The reason for this is that, unlike the prior art, the grayscale voltages on the sides of positive and negative polarity need not be adjusted separately over identical voltage ranges. 
     Second, long term image sticking of the liquid crystal monitor screen, which occurs when halftone-level grayscale voltages are varied, can be reduced. The reason for this is that when the halftone-level grayscale voltages are varied, the halftone-level feed-through correction values can be optimized automatically within the grayscale voltage generating circuit. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.