Patent Publication Number: US-7916107-B2

Title: Gamma voltage output circuit and liquid crystal display having same

Description:
FIELD OF THE INVENTION 
     The present invention relates to voltage output circuits, and more particularly to a gamma voltage output circuit for driving a liquid crystal display (LCD) and a liquid crystal display having the same. 
     BACKGROUND 
     LCDs are commonly used as display devices for compact electronic apparatuses, because they not only provide good quality images with little power but also are very thin. In general, an LCD includes a liquid crystal panel and a backlight module for illuminating the liquid crystal panel. 
     In an active matrix liquid crystal display (AM-LCD) system, the character curve in  FIG. 5  which shows the transmittance of the liquid crystal versus the applied gamma driving voltage in an actual AM-LCD, is a non-linear curve, not linear curve. In operation, the liquid crystal display changes the optical transmittance of the liquid crystal molecular through changing the applied driving voltage between the upper and the lower substrates, for displaying image. The applied driving voltage is named gamma voltage. 
       FIG. 6  is a character curve, which shows the transmittance of the liquid crystal versus the gray level. The character curve is named gamma curve. The relationship between the transmittance of liquid crystal and the gray level maintains the following function:
 
 T=T max*( G/G max) r  
 
wherein T represents transmittance of liquid crystal; Tmax Represents the maximal transmittance of liquid crystal; G represents gray level; Gmax represents the maximal gray level corresponding to the maximal transmittance of liquid crystal; r represents gamma value. In  FIG. 6 , the gamma value of the gamma curve equals to 1.0. When the relationship between the transmittance of liquid crystal and the gray level maintains the gamma curve of  FIG. 6 , the liquid crystal display has an ideal vision effect for human eyes. However, actually, the transmittance of liquid crystal and the gray level has a non-linear relationship. Thus, a special circuit for a liquid crystal display is needed to output corresponding gamma voltage to make the relationship between the transmittance of liquid crystal and the gray level maintains the gamma curve of  FIG. 6 , i.e. linear relationship.
 
     Referring to  FIG. 7 , a typical gamma voltage output circuit is shown. The gamma voltage output circuit  1  is capable of outputting gamma voltage signals to display gray scale images with sixty-four levels. That is, the gamma voltage output circuit  1  can output sixty-four gamma voltages V 1 ˜V 64 . 
     The gamma voltage output circuit  1  includes: a resistor string  11  connected between an analog electrical source (AVDD) and ground. The resistor string  11  includes sixty-five resistors R 0 ˜R 64  connected in series. 
     However, movable or portable display are usually operated under different external environment, such as cloudy day, sun day, or night, et. Under different external environment, the display images produce different color bias if only single gamma curve is used in the movable or portal display. That is the transmittance corresponding to the gray level cannot be properly displayed under different external environments. Thus, different gamma curves are needed for different external environments. Referring to  FIG. 8 , three different gamma curves are shown corresponding to three different external environments, which respectively represent gamma values of 1.0, 2.0 and 3.0. 
       FIG. 9  shows another typical gamma voltage output circuit which can provide three gamma voltages. The gamma voltage output circuit  2  includes a first resistance string  21 , a second resistance string  22  and a third resistance string  23 , respectively connecting in series between a power supply AVDD and ground. The first resistor string  21  has sixty-five resistors R 0 _ 1 ˜R 64 _ 1  and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V 1 _ 1 ˜V 64 _ 1 . The second resistor string  22  has sixty-five resistors R 0 _ 2 ˜R 64 _ 2  and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V 1 _ 2 ˜V 64 _ 2 . The third resistor string  23  has sixty-five resistors R 0 _ 3 ˜R 64 _ 3  and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V 1 _ 3 ˜V 64 _ 3 . By adjusting the resistance value of each resistor, three gamma curves shown on  FIG. 8  can be attained. 
     When three gamma curves is needed, the gamma voltage output circuit  2  has thrice the number of the resistors of the gamma voltage output circuit  2 . However, when eight or ten or more gamma curves are needed, the number of the resisors of the gamma voltage output circuit can be enormous. For designing or manufacturing an integrated circuit (IC), more resisotrs, more cost. 
     In the gamma voltage output circuit  1 , the voltage output from the analog electrical source is distributed to the resistors R 0 ˜R 14  of the resistor string  11 , and the capacitors have a function of wave filtering. Each operational amplifier  12  improves the capability of equipping loads. The gamma voltage output from the output port of each operational amplifier  12  is equal to the voltage signal inputted into the non-inverting input port of the same operational amplifier  12 . Thus, each gamma voltage can be calculated according to the following equations:
 
 V 1 =AVDD *( R 1 +R 2 + . . . +R 14)/( R 0 +R 1 +R 2 + . . . +R 14)
 
 V 2 =AVDD *( R 2 + . . . +R 14)/( R 0 +R 1 +R 2 + . . . +R 14)
 
. . .
 
 V 14 =AVDD*R 14/( R 0 +R 1 R 2 + . . . +R 14)
 
     In order to increase the precision of the resistors R 0 ˜R 14 , the configuration of the resistor string  11  can usually be varied. Referring to  FIG. 4 , the resistors R 01  and R 02  are connected in parallel, and a resistance of the parallel connected resistors R 01  and R 02  is equal to that of the resistor R 0 . The resistors R 11  and R 12  are connected in parallel, and a resistance of the parallel connected resistors R 11  and R 12  is equal to that of the resistor R 1 . In other words, each pair of resistors Rm 1  and Rm 2  are connected in parallel, and a resistance of the parallel connected resistors Rm 1  and Rm 2  is equal to that of the resistor Rm (0≦m≦14). Thus the resistance of the resistors R 0 ˜R 14  can be suitably configured by controlling the resistances of the resistors Rm 1 ˜Rm 2 . 
     When the gamma voltages need to be modulated, the resistances of the corresponding resistors need to be adjusted. For example, when the gamma voltage V 2  needs to be modulated, then the resistance of the resistors R 2  (R 21  and R 22 ) needs to be adjusted. However, according to the equations shown above, when the resistance of one of the resistors is varied, the value of other output gamma voltages also varies. That is, the gamma voltages output from the gamma voltage output circuit  1  affect one another, and cannot be adjusted individually. 
     Accordingly, what is needed is a gamma voltage output circuit that can overcome the above-described deficiencies. 
     SUMMARY 
     An exemplary gamma voltage output circuit for a liquid crystal display has an internal resistor string, which has a plurality of resistors and a plurality of nodes; at least one external resistor string, which has a plurality of resistors and a plurality of nodes; a plurality of switching circuit, each switching circuit having at least one input end and at least one output end. The internal and the at least one external resistor strings connect in series between the power source AVDD and ground, respectively. Each node outputs a gamma voltage. The nodes of internal and the at least one external resistor strings respectively are connected to the at least one output end and the at least one input end, the resistors of the internal resistor string parallel connecting to corresponding resistors of the at least one external resistor string through the corresponding switching circuit. 
     A exemplary liquid crystal display has a printed circuit board, which has a driving IC (not shown) and a gamma voltage output circuit. The gamma voltage output circuit for a liquid crystal display has an internal resistor string, which has a plurality of resistors and a plurality of nodes; at least one external resistor string, which has a plurality of resistors and a plurality of nodes; a plurality of switching circuit, each switching circuit having at least one input end and at least one output end. The internal and the at least one external resistor strings connect in series between the power source AVDD and ground, respectively. Each node outputs a gamma voltage. The nodes of internal and the at least one external resistor strings respectively are connected to the at least one output end and the at least one input end, the resistors of the internal resistor string parallel connecting to corresponding resistors of the at least one external resistor string through the corresponding switching circuit. 
     Other novel features and advantages will become apparent from the following detailed description of preferred and exemplary embodiments when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an abbreviated diagram of a gamma voltage output circuit according to an exemplary embodiment of the present invention, which includes a switching circuit, a first resistors string, a second resistors string. 
         FIG. 2  is a abbreviated diagram of the switching circuit of the gamma voltage output circuit of  FIG. 1 . 
         FIG. 3  is an abbreviated diagram showing the parallel connection between the first resistors string and the second resistors string of the gamma voltage output circuit of  FIG. 1 . 
         FIG. 4  is an abbreviated equivalent circuitry of the parallel connection between the first resistors string and the second resistors string of the gamma voltage output circuit of  FIG. 3 . 
         FIG. 5  is a diagram showing the transmittance of the liquid crystal versus the applied driving voltage. 
         FIG. 6  is a diagram showing the transmittance of the liquid crystal versus the gray level. 
         FIG. 7  is a schematic diagram, showing a conventional gamma voltage output circuit. 
         FIG. 8  is a diagram, showing three gamma curves of transmittance of the liquid crystals versus the gray level, having gamma values of 1.0, 2.0, 3.0. 
         FIG. 9  is a schematic diagram, showing an another conventional gamma voltage output circuit. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings to describe preferred and exemplary embodiments in detail. 
     Referring to  FIG. 1 , a gamma voltage output circuit of a liquid crystal display according to an embodiment of the present invention is shown. The liquid crystal display includes a printed circuit board (not shown), which has a driving IC (not shown) and a gamma voltage output circuit  3 . The gamma voltage output circuit  3  includes a first resistor string  31 , a second resistor string  32 , a third resistor string  33  and a fourth resistor string  34 , respectively connecting in series between the power source AVDD and ground, and a plurality of switching circuits  35 . The first resistor string  31  is disposed in the driving IC, named as internal resistor strings, which includes sixty-five resistors R′ 0 ˜R′ 64  and sixty-four nodes, the sixty-four nodes corresponding to sixty-four gamma voltages V′ 1 ˜V′ 64 . The second, third and fourth resistor strings  32 ,  33 ,  34  and the plurality of switching circuits  35  are formed out of the driving IC, named as external resistor string. The second resistor string  32  includes fifteen resistors R′ 0 _ 1 ˜R′ 14 _ 1  and fourteen nodes; the third resistor string  33  includes fifteen resistors R′ 0 _ 2 ˜R′ 14 _ 2  and fourteen nodes; and the fourth resistor string  34  includes fifteen resistors R′ 0 _ 3 ˜R′ 14 _ 3  and fourteen nodes. The number of the plurality of switching circuit  35  is fourteen. 
     The circuit configuration of each switching circuit  35  is shown in  FIG. 2 , which has an enabling signal input end (EN)  350 , a first controlling signal input end (A 0 )  351 , a second controlling signal input end (A 1 )  352 , a first input end (S 1 )  353 , a second input end (S 2 )  354 , a third input end (S 3 )  353 , and an output end (OUT)  356 . The switching circuit  35  in the embodiment employs analog switch AD7502. The first input ends (S 1 )  353  of the fourteen switching circuits  35  respectively electrically connect with the fourteen nodes of the second resistor string  32 . The second input ends (S 2 )  354  of the fourteen switching circuits  35  respectively electrically connect with the fourteen nodes of the third resistor string  33 . The third input ends (S 3 )  355  of the fourteen switching circuits  35  respectively electrically connect with the fourteen nodes of the fourth resistor string  34 . The output ends (OUT)  356  of the fourteen switching circuits  35  respectively electrically connect with fourteen nodes of the sixty-four nodes of the first resistor string  31 . For different gamma voltage output circuits, the fourteen nodes can be chosen according to different needs. But, for a certain gamma voltage output circuit, the fourteen nodes are changeless. 
     In operation, when the driving IC sends a high level signal to the enabling signal input end (EN)  350  of the switching circuit  35 , the switching circuit  35  starts to work. When the first and the second controlling signal input ends (A 0 , A 1 )  351 ,  352  respectively receive a low level signal, the first input end (S 1 )  353  electrically connects with the output end (OUT)  356 . That is, the resistors of the first resistor string  31  parallel connect to the corresponding resistor of the second resistors string  32 . The corresponding resistor of the second resistors string  32  can be chosen according to different needs.  FIG. 3  provides one parallel connecting circuitry  41 . In  FIG. 3 , the resistor R′ 0  parallel connects to the resistor R′ 0 _ 1 ; the resistor R′ 1  parallel connects to the resistor R′ 1 _ 1 ; the resistor R′ 2  parallel connects to the resistor R′ 2 _ 1 ; the resistor R′ 63  parallel connects to the resistor R′ 13 _ 1 ; the resistor R′ 34  parallel connects to the resistor R′ 14 _ 1 . In addition, each six continuous resistors of resistor R′ 3 ˜R′ 62  parallel connect to one resistor of R′ 3 _ 1 ˜R′ 12 _ 1 , such as resistors R′(3+6n)˜R′(8+6n) (0≦n≦9) parallel connect to R′(3+n)_ 1 . The sixty-four nodes of the first resistor string  31  respectively output gamma voltages V″ 1 ˜V″ 64 . 
       FIG. 4  shows the equivalent scheme  51  of the parallel circuitry  41  of  FIG. 3 , the equivalent scheme  51  has sixty-five equivalent resistance R″ 0 ˜R″ 64  and sixty-four nodes. Each node output one gamma voltage. A serial voltages V″ 1 ˜V″ 64  of the equivalent scheme  51  correspond to one gamma curve. 
     When the driving IC sends a high level signal to the enabling signal input end (EN)  350  of the switching circuit  35 , and the first and the second controlling signal input ends (A 0 , A 1 )  351 ,  352  of the switching circuit  35  respectively receive a high level signal and a low level signal, the second input end (S 2 )  354  electrically connects with the output end (OUT)  356 . That is, the resistors of the first resistor string  31  parallel connect to the corresponding resistor of the third resistors string  33 , similar to the second resistors string  32 . when the driving IC sends a high level signal to the enabling signal input end (EN)  350  of the switching circuit  35 , and the first and the second controlling signal input ends (A 0 , A 1 )  351 ,  352  of the switching circuit  35  respectively receive a low level signal and a high level signal, the second input end (S 3 )  355  electrically connects with the output end (OUT)  356 . That is, the resistors of the first resistor string  31  parallel connect to the corresponding resistor of the fourth resistors string  34 , similar to the second resistors string  32 . When the driving IC sends a low level signal to the enabling signal input end (EN)  350  of the switching circuit  35 , the switching circuit  35  turns off. 
     In the gamma voltage output circuit  3 , the numbers of the second, third, fourth resistors strings  32 ,  33 ,  34  can also be others. And, the number of the plurality of switching circuits  35  can be determined according to the numbers of the second, third, fourth resistors strings  32 ,  33 ,  34 . 
     Comparing to prior arts, the gamma voltage output circuit  3  does not need change the internal circuit configuration of the driving IC, which just add a quantity of resistors at an external peripheral region of the driving IC to realize gamma voltages adjusting according to different needs. Thus, a good displaying characteristics can be attained even in different external environments. 
     When the LCD needs to be operated in more different external environments, the number of the external resistors string needs to be added. However, the internal circuitry configuration does not need to be changed. 
     When the driving IC of the LCD is eight bit or ten bit, the number of the internal resistors string is two hundred fifty-six or one thousand twenty-four. However, the number of each external resistors string does not need to be changed or just change a small quantities, such as add to twenty or thirty. 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.