Abstract:
A drive circuit for a display apparatus includes an input section for receiving a digital video signal, a voltage supply section for generating a common electrode voltage and a plurality of gradation voltages, a driving section for selecting one of the plurality of gradation voltages according to the input digital video signal and applying the selected gradation voltage to one of the pixel electrodes, and for applying the common electrode voltage to the common electrode, the common electrode voltage being shifted based on a certain relationship so that a central value of the common electrode voltage is different from central values of the gradation voltages.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a drive circuit for a display apparatus which operates in response to digital video signals, and more particularly to a drive circuit for a display apparatus whose pixels need to be driven by an a. c. voltage and are liable to deterioration or breakage if they are driven by a d.c. voltage, such as a liquid crystal display (LCD) apparatus. 
     2. Description of the Prior Art 
     Hereinafter, a TFT (thin film transistor) liquid crystal display (LCD) apparatus will be described as a typical example of a type of a display apparatus in which the driving circuit of the present invention can be used. 
     FIG. 6 shows a source driver which is a part of a drive circuit for a TFT liquid crystal display apparatus. Digital video signal data are input to the source driver. In this example, it is assumed that the input digital video signal data consist of two bits (D1, D0) having four distinct values &#34;0&#34; to &#34;3&#34;. The source driver selects one of the gradation voltages V 0  to V 3  which are supplied by voltage supply section 1 according to each value of the input digital video signal data, and outputs the selected voltage to source lines O n . 
     FIG. 7 shows a circuit corresponding to nth output portion of the source driver shown in FIG. 6. The circuit includes a D-type flip-flop (sampling flip-flop) M SMP  at a first stage and a flip-flop (holding flip-flop) M H  at a second stage which are provided for receiving each bit of the digital video signal data, a decoder DEC, and analog switches ASW 0  to ASW 3  for electrically connecting lines from the voltage supply section 1 with source lines O n . For the sampling of the digital video signal data, various circuits are available as alternatives to the D-type flip-flop. 
     The digital source driver shown in FIG. 7 operates as follows: 
     The sampling flip-flop M SMP  latches the digital video signal data (D1, D0) at the rising edge of a sample pulse T SMPn  corresponding to the nth pixel. When the sampling for one horizontal period is completed, an output pulse OE is fed to the holding flip-flop M H . Then, the data (D1, D0) held in the sampling flip-flop are moved to the holding flip-flop M H  and are simultaneously output to the decoder DEC. The decoder DEC decodes the 2-bit data (D1, D0) and recognizes a value of the data (D1, D0). In accordance with the value of the data (D1, D0), the decoder DEC makes one of the analog switches ASW 0  to ASW 3  conductive. As a result, the corresponding one of four gradation voltages V 0  to V 3  is output to the source line O n . 
     FIG. 8 shows waveforms of the gradation voltages V 0  to V 3 , and a common electrode voltage V COM  applied to a common electrode for a LCD panel. The gradation voltages V 0  to V 3  are higher in this order, and are applied to the pixels. This relationship is expressed as follows: 
     
         |V.sub.0 -V.sub.COM |≦|V.sub.1 -V.sub.COM |≦|V.sub.2 -V.sub.COM |≦|V.sub.3 -V.sub.COM |, where the reverse relationship can also be used. 
    
     As shown in FIG. 8, the gradation voltages V 0  to V 3  and the common electrode voltage V COM  alternately change between two voltage levels synchronously with a signal POL which is reversed for each output period. Alternatively, the common electrode voltage V COM  may be d.c. voltage. Each level of the gradation voltages is determined to be symmetrical with respect to a given voltage (a central voltage) VM. 
     FIG. 9 shows each level of the gradation voltages V 0  to V 3  seen from a common electrode to which the common electrode voltage V COM  is applied. 
     A particular pixel is charged with one of the gradation voltages shown in FIG. 9 when the pixel is selected by a gate driver (a scanning driver). If the pixel is selected at a beginning of a horizontal period when the gradation voltage is positive (i.e. the gradation voltage is higher than the common electrode voltage V COM ) and the positive gradation voltage is applied to the selected pixel during the horizontal period, then it is controlled so that a negative gradation voltage (i.e. the gradation voltage is lower than the common electrode voltage V COM ) corresponding to the positive voltage is applied to the selected pixel during the next horizontal period. 
     Thus, each pixel is changed with the gradation voltage which alternately changes between a positive voltage level and a negative voltage level, that is, an a.c. voltage, resulting in preventing a d.c. voltage from being applied to the pixel as an average value. 
     Under an ideal condition, the known drive circuit mentioned above protects the pixels against breakage or deterioration due to the application of d.c. voltage. However, an actual liquid crystal display apparatus can not be completely protected in such a manner because of the fact that the voltages applied to the pixels in a liquid crystal display panel are not the same as the gradation voltages V 0  to V 3  and the common electrode voltage V COM . The cause of this voltage difference is explained as follows: 
     FIGS. 12, 13 and 14 show equivalent circuits of a pixel portion including a pixel capacitance C LC  and an additional capacitance C S  which are connected in parallel to a common electrode COM. In FIG. 12, C gd  represents a parasitic capacitance present between the gate and drain of a thin film transistor (TFT) 10. 
     When the voltage of the gate line G n  is high (V GH ), the TFT 10 is turned on. As a result, a voltage V S  of the source line O n  is applied to the pixel. Then, the following equations are established among charges q 1 , q 2  and q 3  in the capacitances shown in FIG. 13: 
     
         q.sub.1 +q.sub.2 +q.sub.3 =constant 
    
     
         q.sub.1 /C.sub.LC =q.sub.2 /C.sub.S =V.sub.S 
    
     
         q.sub.3 =C.sub.gd ·(V.sub.S -V.sub.GH) 
    
     Accordingly, the following equation (1) is obtained: 
     
         (C.sub.LC +C.sub.S +C.sub.gd)·V.sub.S =constant+C.sub.gd ·V.sub.GH                                        ( 1) 
    
     On the other hand, when the voltage of the gate line G n  is low (V GL ), the TFT 10 is turned off. As a result, the following equations are established among charges q 1  &#39;, q 2  &#39; and q 3  &#39; in the capacitances shown in FIG. 14: 
     
         q.sub.1 &#39;+q.sub.2 &#39;+q.sub.3 &#39;=constant 
    
     
         q.sub.1 &#39;/C.sub.LC =q.sub.2 &#39;/C.sub.S =V.sub.S &#39; 
    
     
         q.sub.3 &#39;=C.sub.gd ·(V.sub.S &#39;-V.sub.GL) 
    
     Accordingly, the following equation (2) is obtained: 
     
         (C.sub.LC +C.sub.S +C.sub.gd)·V.sub.S &#39;=constant+C.sub.gd ·V.sub.GL                                        ( 2) 
    
     From the equations (1) and (2), the following equation (3) is derived. 
     
         V.sub.S -V.sub.S &#39;=C.sub.gd ·(V.sub.GH -V.sub.GL)/(C.sub.LC +C.sub.S +C.sub.gd)                                       (3) 
    
     As is evident from the foregoing equations, while the voltage of the gate line G n  is high (V GH ), the pixel is charged with the voltage V S , and after the TFT 10 turns off, the voltage V S  is varied into a voltage V S  &#39;. The difference between V S  and V S  &#39; is represented by equation (3). 
     The difference V S  -V S  &#39; is observed as a variation of the pixel characteristics caused by the application of a positive and a negative gradation voltages to the pixels, when the positive and negative gradation voltages are actually applied to the driving terminal of a LCD panel. As a result, some of d.c. voltage components are applied to the pixels in a LCD panel according to the driving voltages shown in FIG. 8, which is described below in more detail. 
     FIG. 10 shows an exemplary relationship between a gradation voltage (an absolute value) input to a LCD panel and transmissivity characteristics of the pixels in a LCD panel. The gradation voltage is applied to the driving terminal of a LCD panel. In FIG. 10, a scale of the horizontal axis is determined so that a relationship between an absolute value of a negative gradation voltage and the transmissivity characteristics is represented as a straight line. 
     The V N   +  and V N   -  (where N=0, 1, 2, 3) represent positive gradation voltages and negative gradation voltages which are required to achieve the same transmissivity characteristics of pixels in a LCD panel respectively. For example, V 3   +  and V 3   - , as well as V 0   +  and V 0   - , which are required to achieve the same transmissivity characteristics of the pixel in a LCD panel are shown in FIG. 10. The positive and negative voltages are defined by the differences between the gradation voltages V N  and the common electrode voltage V COM , as mentioned above. ΔV N  represents a difference between the voltages V N   +  and V N   - . 
     FIG. 11 shows a relationship between a negative gradation voltage (an absolute value) applied to the driving terminal of a LCD panel and the difference ΔV N . FIG. 11 teaches that a positive gradation voltage V N   +  should be substantially equal to a voltage obtained by adding the difference ΔV N  to an absolute value of a negative gradation voltage V N   -  in order to achieve the same transmissivity characteristics of the pixel. 
     For example, a case where the gradation voltage is V 3  (N=3) will be described below. Assuming that a LCD panel is driven with the gradation voltage which has a waveform shown in FIG. 9, and that an absolute value of the difference between the gradation voltage V 3  and the common electrode voltage V COM  in FIG. 9 is substantially equal to a voltage represented by V 3  in FIG. 10. In such a case, a difference Δt 3  in the transmissivity characteristics occurs depending on which voltage is applied to a pixel, a positive gradation voltage or a negative gradation voltage, as shown in FIG. 10. 
     If the absolute value of the positive and negative gradation voltage levels actually applied to the pixels are different from each other, the pixel characteristics (for example, the transmissivity of liquid crystal in a case where a LCD panel is used) are varied, even though the display apparatus is not broken or deteriorated. The variation of the pixel characteristics causes a flicker effect of the image. 
     Further, the application of a d.c. voltage to the pixels causes not only a deterioration of the display apparatus but also a so-called &#34;after static image&#34; problem where a static image remains visible in the display even after the static image is deleted. In the display apparatus in which a static image is the primary mode of display, such as a terminal display device for a computer, the problem is serious. 
     SUMMARY OF THE INVENTION 
     The drive circuit of the present invention includes a section for receiving a digital video signal, a voltage supply section for generating a common electrode voltage and a plurality of gradation voltages, each of the plurality of gradation voltages alternately changing between two voltage levels, a driving section for selecting one of the plurality of gradation voltages according to the input digital video signal and applying the selected gradation voltage to one of the pixel electrodes, and for applying a common electrode voltage to the common electrode, the common electrode voltage being shifted based on a relationship between a driving voltage input to the display panel and the transmissivity characteristics of the pixels of a display panel so that a central value of the common electrode voltage is different from central values of the gradation voltages. 
     In another embodiment, the drive circuit of the present invention includes a section for receiving a digital video signal, a voltage supply section for generating a common electrode voltage and a plurality of gradation voltages, each of the plurality of gradation voltages alternately changing between two voltage levels, a driving section for selecting one of the plurality of gradation voltages according to the input digital video signal and applying the selected gradation voltage to one of the pixel electrodes, and for applying a common electrode voltage to the common electrode, at least one of the gradation voltages being shifted based on a relationship between a driving voltage input to a display panel and the transmissivity characteristics of the pixels of the display panel so that a central value of at least one of the gradation voltages is different from one of the central values of the remaining gradation voltages. 
     Thus, the invention described herein makes possible the advantages of (1) providing a drive circuit which can reduce the ratio of d.c. voltage components to be applied to the pixels, (2) providing a drive circuit which can reduce a flicker effect of the image, and (3) providing a drive circuit which can improve an &#34;after static image&#34; problem. 
     These and other advantages of the present invention will become apparent upon reading and understanding the following detailed description made with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing the voltage supply section in Example 1. 
     FIG. 2 is a diagrammatic view showing waveforms of the gradation voltages and the common electrode voltage. 
     FIG. 3 is a circuit diagram showing the voltage supply section in Example 2. 
     FIG. 4 is a circuit diagram showing the voltage supply section in Example 3. 
     FIG. 5 is a circuit diagram showing an operational amplifier of the voltage supply section. 
     FIG. 6 is a circuit diagram showing a driving circuit for a liquid crystal display apparatus. 
     FIG. 7 is a circuit diagram showing a part of the driving circuit shown in FIG. 6. 
     FIG. 8 is a diagrammatic view showing the waveforms of the gradation voltages and the common electrode voltage. 
     FIG. 9 is a diagrammatic view showing the waveforms of the gradation voltages and the common electrode voltage when viewed from the common electrode. 
     FIG. 10 is a graph showing the relationship between a voltage input to a LCD panel and a transmissivity of the pixels of a LCD panel. 
     FIG. 11 is a graph showing the relationship between a negative voltage input to a LCD panel and a difference between V N   +  and V N   - . 
     FIG. 12 is an equivalent circuit of the pixel portion. 
     FIG. 13 is an equivalent circuit wherein the switching element (TFT) is in on-state. 
     FIG. 14 is an equivalent circuit wherein the switching element (TFT) is in off-state. 
     FIG. 15 is a circuit diagram showing the voltage supply section in Example 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is described in detail by way of the following examples. 
     EXAMPLE 1 
     In this example, ΔV N  in FIG. 10 is compensated by adjusting the common electrode voltage V COM . 
     FIG. 2 shows waveforms of the gradation voltage V 3  and the common electrode voltage V COM  &#39; which has been adjusted based on the relationship shown in FIG. 10. As shown in FIG. 2, the common electrode voltage V COM  is shifted so that a central value VM COM  of the common electrode voltage V COM  &#39; is different from the central voltage VM by ΔVM. As a result, a positive gradation voltage V 3   +  may be higher than a negative gradation voltage V 3   -  by ΔV 3 . This makes it possible to apply the gradation voltages V 3   +  and V 3   -  as shown in FIG. 10 to the driving terminal of a LCD panel. 
     FIG. 1 shows an exemplary circuit for the voltage supply section 1. The drive circuit for a display apparatus shown in FIG. 6 is exemplary of just one type of circuit which may be used in Example 1 through 5 where the voltage supply section 1 is varied. The present invention is not limited by this specific type of drive circuit. Other drive circuits may be used for the voltage supply section 1 to generate the common electrode voltage and the gradation voltages as defined by this invention. 
     The circuit shown in FIG. 1 includes an operational amplifier OP C  for generating the common electrode voltage V COM , and operational amplifiers OP 0  to OP 3  for generating the gradation voltages V 0  to V 3 . The operational amplifiers OP C , OP 0 , and OP 1  receive a signal POL at their inverted inputs, and the operational amplifiers OP 2  and OP 3  receive the signal POL at their inverted inputs through an inverter INV. Each operational amplifier OP C , OP 0  to OP 3  receives a respective output of resistance type potential dividers PD C , PD 0  to PD 3  at their non-inverted inputs. Operational amplifiers OP C , OP 0  to PO 3  generate voltages V COM  and V 0  to V 3  which altenately change between two voltage levels synchronously with the signal POL, and a central value of the two voltage levels is substantially equal to the voltage at the non-inverted inputs of each operational amplifier. Further, phases of the voltages V COM , V 0  and V 1  are opposite to those of the voltages V 2  and V 3  as shown in FIG. 8. The amplitudes of these voltages depend upon the amplification factors of the operational amplifiers. 
     Each resistance type potential divider PD C , and PD 0  to PD 3  has two fixed resistances, one being connected at one end to a power source V dd  having a plus potential, and the other being connected at one end to a power source V ss  having a ground potential. As a result, the voltage VR C  at the junction of the fixed resistances R C1  and R C2  of the resistance type potential divider PD COM  becomes {R C2  /(R C1  +R C2 )}V dd . This voltage VR C  is applied to the non-inverted input of the operational amplifier OP C  as an output voltage of the resistance type potential divider PD COM . Similarly, the voltage VR 0  to VR 3  at the respective junctions of the fixed resistances R 01  and R 02 , R 11  and R 12 , R 21  and R 22 , and R 31  and R 32  become }R 02  /(R 01  +R 02 )}V dd , {R 12  /(R 11  +R 12  )}V dd , {R 22  /(R 21  +R 22 )}V dd , and {R 32  /(R 31  +R 32 )}V dd . 
     Accordingly, the values of the output voltages VR C , VR 0  to VR 3  are determined by setting resistance ratios of fixed resistances. In this example, the resistance ratios are set to satisfy the following relationship: 
     
         R.sub.C2 /R.sub.C1 &lt;R.sub.02 /R.sub.01 =R.sub.12 /R.sub.11 =R.sub.22 /R.sub.21 =R.sub.32 /R.sub.31 
    
     Therefore, the output voltages satisfy the following relationship. 
     
         VR.sub.C &lt;VR.sub.0 =VR.sub.1 =VR.sub.2 =VR.sub.3 
    
     As shown in FIG. 2, the central value VM COM  of the output voltage V COM  of the operational amplifier OP C  is shifted away from the central voltage VM by Δ VM toward the low voltage side. The central voltage VM is substantially equal to the central value of the output voltages V 0  to V 3  of the other operational amplifiers OP 0  to OP 3 . 
     For simplicity, FIG. 2 shows only the voltages V COM  and V 3 . By shifting the voltage V COM  as mentioned above, the positive gradation voltage V 3   +  becomes substantially equal to a voltage obtained by adding the difference Δ VM to the voltage V 3 . On the other hand, the negative gradation voltage V 3   -  becomes substantially equal to a voltage obtained by subtracting the difference Δ VM from the voltage V 3 . The difference Δ VM can be determined by setting appropriately the output voltages VR C  and VR 3  (i.e. the values of the fixed resistances in the resistive potential dividers). 
     Accordingly, the determination of the values of the fixed resistances makes it possible to obtain the voltages V 3   +  and V 3   -  which do not result in direct application of a d.c. voltage to the pixels, and keep the tramsmissivity constant irrespective of the positive and negative gradation voltages. 
     This can be effectively applied to other voltages applied to the pixels on the basis of voltages V 0  to V 2  to avoid applying a d.c. voltage directly to the pixels. The difference Δ VM may be adjusted to be an optimum value between the lines L0 and L3 in FIG. 11 so that d.c. componets applied to the pixels are minimized. 
     EXAMPLE 2 
     In this example, Δ V N  in FIG. 10 is compensated by adjusting the common electrode voltage V COM  for each display apparatus. 
     FIG. 3 shows an exemplary circuit for a voltage supply section 1 used in another embodiment. The circuit shown in FIG. 3 includes a potentiometer PM COM  in place of the resistance type potential divider PD COM  described in Example 1. In this structure, a value of the output voltage VR C  of the potentiometer PM COM  which is applied to the non-inverted inputs of the operational amplifier OP C  can be adequately adjusted. This means that the difference Δ VM can be adjusted so that optimum display characteristics of each display apparatus can be obtained. 
     EXAMPLE 3 
     In this example, Δ V N  in FIG. 10 is compensated by adjusting the graduation voltages V 0  to V 3  independently. 
     FIG. 4 shows an exemplary circuit for a voltage supply section 1 used in another embodiment. The circuit shown in FIG. 4 includes potentiometers PM 0  to PM 3  in place of the resistance type potential dividers PD 0  to PD 3  set forth in Example 1. Values of the output voltages VR 0  to VR 3  of the potentiometer PM 0  to PM 3  which are applied to the non-inverted inputs of the operational amplifiers OP 0  to OP 3  can be independently adjusted. In this structure, each of the gradation voltages V 0  to V 3  is shifted so that each central value of the gradation voltages V 0  to V 3  is different from the central voltage VM shown in FIG. 2. The distance shifted away from the central voltage VM may be different among the gradation voltages V 0  to V 3 . Thus, since all of the gradation voltages are independently adjusted, optimum display characteristics in each gradation level of a display apparatus can be obtained so that d.c. components applied to the pixels are minimized. 
     EXAMPLE 4 
     In this example, Δ V N  in FIG. 10 is compensated by adjusting the common electrode voltage V COM  and the gradation voltages V 0  to V 3 , independently. 
     As is evident from the combination of the circuit in Example 2 with the circuit in Example 3, it is possible to obtain a circuit for the voltage supply section 1 in which all of the resistance type potential dividers PD C , and PD 0  to PD 3  in shown in FIG. 1 have been replaced by potentiometers. Such a circuit allows independent adjustment of the common electrode voltage V COM  and the gradation voltages V 0  to V 3 . 
     EXAMPLE 5 
     FIG. 15 shows an exemplary circuit for the voltage supply section 1 used in another embodiment. According to this circuit, the common electrode voltage V COM  output from the operational amplifier OP C  is any d.c. voltage. This configuration can be applied to any circuits for the voltage supply section 1 mentioned above. 
     The operational amplifiers OP C , OP 0  to OP 3  mentioned above may have a configuration as shown in FIG. 5. In FIG. 5, the output of the operational amplifier OP is amplified by means of a bidirectional current amplifier circuit including two transistors Q 1  and Q 2 . This configuration can be applied to any circuits for the voltage supply section 1 mentioned above. When the output of the current amplifier circuit are used as the common electrode voltage V COM  and the gradation voltages V 0  to V 3 , the same effect set forth can be obtained. Since a current capacity of the operational amplifier OP may be small enough to drive the transistors Q 1  and Q 2 , it makes possible to use an operational amplifier having a small current capacity. 
     According to the present invention, the ratio of a d.c. component applied to the pixels of a LCD panel can be reduced. As a result, the display apparatus is not likely to be broken or deteriorated, and also the &#34;after static image&#34; problem can be much improved. 
     Further, since the same transmissivity characteristics of the pixels is achieved regardless of the application of the positive or negative gradation voltages to the pixels, the flicker effect of the image can be reduced. 
     Based on the observation of the flicker effect of the image for each gradation level, the voltages supplied by the voltage supply section 1 may be adjusted so that the flicker effect of the image be minimized. In the inspection for adjustment, it is preferable that distinct reference patterns are prepared for each gradation level and a reference pattern which causes the flicker effect most radically is used. 
     For reasons mentioned above, a display apparatus having much improved reliability and quality can be obtained. 
     Various other modifications will be apparant to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.