1. Field of the Invention
The present invention relates to a grayscale voltage generating circuit, a driver integrated circuit (IC) and a liquid crystal display apparatus, and more particularly relates to a liquid crystal display in which pixels are driven by a driver IC with a grayscale voltage generating circuit.
2. Description of the Related Art
In recent years, the number of grayscale levels in a color liquid crystal display (LCD) has been increased from 260,000 color levels in 6-bit representation to 16,700,000 color levels in 8-bit representation. Moreover, a product of 1,000,000,000 color levels in 10-bit representation has been developed. In such a situation, a grayscale voltage generating circuit is one of important basic circuits to generate voltages matched to γ characteristic of each liquid crystal panel. Conventionally, a grayscale power supply IC is provided independently from the IC for the LCD driver IC and is used to adjust the γ characteristic of the liquid crystal display driver (hereafter, to be referred to as an LCD driver).
However, the grayscale voltage generating circuit is built in each of a plurality of LCD driver ICs to reduce the cost of the liquid crystal display apparatus. In this case, the grayscale voltages outputted from the respective liquid crystal driver ICs indicate values different from each other, depending on offset voltages caused due to amplifiers of the grayscale power supply circuits. Thus, a problem of a display block unevenness is caused. In particular, in case that a LCD driver is stuck on COG (Chip On Glass) and wirings are formed, its wiring resistance is large. Thus, the γ characteristic changes for each LCD driver IC, depending on a current flowing through a γ resistance determining the γ characteristic. Therefore, this becomes a large factor involving the display block unevenness.
In operational amplifiers for a grayscale voltage generating circuit, typically, a 6-bit product has five amplifiers on a positive side and five amplifiers on a negative side. Also, an 8-bit product has nine amplifiers on a positive side and nine amplifiers on a negative side. In these amplifiers, a power supply efficiency is considered and its output voltage is in a range of a power supply voltage or a voltage close to the ground voltage (GND). Also, the grayscale voltage generating circuit is provided as a dedicated IC outside the LCD driver IC in many cases. However, there is a case that it is built in the LCD driver IC. In this case, since the amplifier must be composed of CMOS transistors, the driving performance of a driver is limited.
FIG. 1 is a block diagram showing the configuration of a conventional LCD source driver 1100A and a conventional LCD panel 1300. With reference to FIG. 1, the LCD source driver 1100A in the conventional example has a data register 11 for receiving 6-bit digital display data R, G and B, a latch circuit 12 for latching the digital display data in synchronization with a strobe signal ST, a D/A converter 13 composed of n-stage digital/analog converting circuits provided in parallel, a grayscale voltage generating circuit 14 for generating grayscale voltages having γ characteristic based on the characteristic of the LCD panel, and an output amplifier section 15 for buffering a voltage outputted from the D/A converter 13. Here, the output amplifier section 15 has n voltage followers 151 to 15n.
The LCD panel 1300 has thin film transistors (TFTs) 161 to 16n provided at the intersection regions between data lines and scanning lines. Also, pixel capacitances 171 to 17n are connected to the TFTs 161 to 16n. Here, gates of the TFTs 161 to 16n are connected to the scanning lines, and sources thereof are connected to the data lines. Also, ends of the pixel capacitances 171 to 17n on one side are connected to drains of the TFTs 161 to 16n, and ends on the other side are connected to a COM node. FIG. 1 shows the TFTs 161 to 16n connected to one scanning line and the pixel capacitances 171 to 17n. Usually, the LCD panel 1300 has a plurality of scanning lines. The TFTs 161 to 16n are connected to this scanning line and the data lines, and the pixel capacitances 171 to 17n are provided in the shape of an array. An LCD gate driver (not shown) sequentially drives the gates of the TFTs 161 to 16n connected to the scanning lines one by one. The D/A converter 13 performs D/A conversion on the 6-bit digital display data latched by the latch circuit 12 and sends to N voltage followers 151 to 15n. Then, the D/A converter 13 sends data signals through the TFTs 161 to 16n to pixels as the pixel capacitances 171 to 17n. Here, the grayscale voltage generating circuit 14 generates grayscale voltages as reference voltages for the data signal supplied on the data line. In the D/A converter 13, one of the grayscale voltages is selected by a decoder composed of a ROM switch (not shown). In a conventional grayscale voltage generating circuit disclosed in Japanese Patent No. 2590456 (first conventional example), a resistance ladder circuit is provided. This resistance ladder circuit is driven by voltage followers, in order to reduce impedance at an output node of each grayscale voltage and finely adjust the voltage value of the grayscale voltage.
FIG. 2 is a block diagram showing the configuration of the conventional grayscale voltage generating circuit 14. With reference to FIG. 2, the grayscale voltage generating circuit 14 is provided with a resistance ladder circuit 1102 built in an LCD source driver 1100A, an external resistance ladder circuit 1401 provided outside the LCD source driver 1100A; a buffer amplifier section 1101 having a plurality of operational amplifiers OP1 to OPn functioning as voltage followers; and a constant voltage generating circuit for outputting a reference voltage Vr. Here, the built-in resistance ladder circuit 1102 has resistances R1 to Rn−1 connected in series and respectively connected to the output ends of the operational amplifiers OP1 to OPn. Also, the external resistance ladder circuit 1401 has the constant voltage generating circuit and resistances R0′ to Rn−1, connected in series. The resistances R0′ to Rn−1, are connected to non-inversion input terminals of the operational amplifiers OP1 to OPn.
The operational amplifiers OP1 to OPn output grayscale voltages Vg1 to Vgn based on tap voltages of the resistances R0′ to Rn−1, in the external resistance ladder circuit 1401. Here, the resistances R0′ to Rn−1′ in the external resistance ladder circuit 1401 are variable resistances. By changing those resistance values, the tap voltages applied to the operational amplifiers OP1 to OPn are adjusted. At this time, the voltages applied to the operational amplifiers OP1 to OPn are adjusted such that the grayscale voltages Vg1 to Vgn outputted from the external resistance ladder circuit 1401 are the optimal voltages for the characteristic of the LCD panel 1300.
The reference voltage Vr is supplied to the grayscale voltage generating circuit 14. The reference voltage Vr is generated by a stable external constant voltage generating circuit such as a band gap reference. The grayscale voltages Vgn, Vgn−1, Vgn−2, - - - , Vg2 and Vg1 are finally determined based on the ladder resistances R0′, R1′, R2′, - - - , Rn−2′ and Rn−1′, respectively. That is, the grayscale voltages Vgn, Vgn−1, Vgn−2, - - - , Vg2 and Vg1 are determined as follows.Vgn=VrVgn−1=Vr{(Rn−2′+Rn−3′+ . . . +R0′)/(Rn−1′+Rn−2′+Rn−3′,+ . . . +R0′)}, . . . ,Vg1=Vr{R0′/(Rn−1′,+Rn−2′+Rn−3′+ . . . +R0′)}Here, if a resistance ratio of the resistances R1 to Rn−1 to determine the grayscale voltages Vg1 to Vgn in an LCD source driver 10 and a resistance ratio of the resistances R1′ to Rn−1′ to determine the grayscale voltages Vg1 to Vgn are equal to each other, the output currents of the operational amplifiers OP2 to OPn−1 become zero.
However, an output current In in the n-th operation amplifier OPn (the operational amplifier that outputs the maximum grayscale voltage Vgn) is given by the following equation (1) in a discharge directionIn=(Vgn−Vg1)/(R1+R2+ . . . +Rn−1  (1)Also, an output current I1 of the first operational amplifier OP1 (the operational amplifier that outputs the minimum grayscale voltage Vg1) is given by the following equation (2) in the discharge direction.I1=(Vgn−Vg1)/(R1+R2+ . . . +Rn−1)  (2)Thus, the operation amplifier OPn and the operation amplifier OP1 need to be designed as the output stages that can output the output currents In and I1, respectively. In particular, when they are designed by using MOS transistors, a mutual conductance gm of the MOS transistor which determines a drive performance is small as compared with a bipolar transistor. Therefore, attention should be paid thereto.
Also, Japanese Laid Open Patent Application (JP-A-Heisei 10-142582: second conventional example) discloses a technique in which reduction in an output dynamic range of an operational amplifier is improved in a liquid crystal grayscale voltage generating circuit.
Also, an LCD driver in which a plurality of LCD driver ICs are connected in parallel to increase the number of grayscale levels to be displayed on a liquid crystal is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-119744: third conventional example). FIG. 3 is a block diagram showing the configuration of an LCD source driver 1100B using two LCD source driver ICs, each of which has a built-in grayscale voltage generating circuit. With reference to FIG. 3, the LCD source driver 1100B has a first LCD source driver IC 110-1 and a second LCD source driver IC 110-2. The first LCD source driver IC 110-1 is provided with a grayscale voltage generating circuit 14′-1, a data register 11-1, a latch circuit 12-1, a D/A converter 13-1 and an output amplifier section 15-1. The grayscale voltage generating circuit 14′-1 is provided with a negative side grayscale resistance group 142-1 composed of a group of resistances R1-1 to R(n/2)−1-1 and a positive side grayscale resistance group 141-1 composed of a group of resistances R(n/2)+1-1 to Rn−1-1; operational amplifiers 1431-1 and 1432-1 which are connected to the negative side grayscale resistance group 142-1; and operational amplifiers 1433-1 and 1434-1 which are connected to the positive side grayscale resistance group 141-1. The configuration of the second LCD source driver IC 110-2 is similar to that of the first LCD source driver IC 110-1. The reference numerals of the similar components are used in which an additional number “1” of the component of the first LCD source driver IC 110-1 is replaced with “2”.
The non-inversion input terminals of the operational amplifiers 1434-1 and 1434-2 are connected to a first constant voltage source VH+ and the non-inversion input terminals of the operational amplifiers 1433-1 and 1433-2 are connected to a second constant voltage source VL+ for supplying a voltage lower than the first constant voltage source VH+. Thus, the operational amplifier 1434-1 supplies the highest voltage to the positive side grayscale resistance group 141-1. Similarly, the operational amplifier 1434-2 supplies the highest voltage to the positive side grayscale resistance group 141-2. Also, the operational amplifier 1433-1 supplies the lowest voltage to the positive side grayscale resistance group 141-1. Similarly, the operational amplifier 1433-2 supplies the lowest voltage to the positive side grayscale resistance group 141-2. Moreover, the non-inversion input terminals of the operational amplifiers 1432-1 and 1432-2 are connected to a third constant voltage source VH−, and the non-inversion input terminals of the operational amplifiers 1431-1 and 1431-2 are connected to a fourth constant voltage source VL− for supplying a voltage lower than the third constant voltage source VH−. Thus, the operational amplifier 1432-1 supplies the highest voltage to the negative side grayscale resistance group 142-1. Similarly, the operational amplifier 1432-2 supplies the highest voltage to the negative side grayscale resistance group 142-2. Also, the operational amplifier 1431-1 supplies the lowest voltage to the negative side grayscale resistance group 142-1. Similarly, the operational amplifier 1431-2 supplies the lowest voltage to the negative side grayscale resistance group 142-2. Also, when the two or more LCD source driver ICs are used, the non-inversion input terminals of the operational amplifiers are commonly connected to the power supply voltages, respectively.
In the first to fourth constant voltage sources VH+, VL+, VH− and VL−, since they are usually constituted to use resistance division, their impedances are high. Thus, buffer amplifiers are required. In this example, the operational amplifiers 1431 to 1434 carry out the roles as the buffer amplifiers. The LCD panel changes the brightness in response to the output from the LCD source driver 1100B having such a configuration. For example, in the LCD panel of a normally white type, the values of the first to fourth constant voltage sources VH+, VL+, VH− and VL− are set such that the high voltage side of the positive side grayscale corresponds to a black level, the low voltage side corresponds to a white level, the low voltage side of the negative side grayscale corresponds to the black side, and the high voltage side corresponds to the white level.
As mentioned above, in the conventional technique, the LCD source driver contains the plurality of LCD source driver ICs. In this case, a variation in the ladder resistances is caused in each LCD source driver IC. Accordingly, the grayscale characteristic is different among the respective driver ICs, and a problem of the display block unevenness is caused. Moreover, the difference in the offset voltage of the operational amplifier for the grayscale voltage generating circuit, which is built in the LCD driver, causes the generation of the grayscale voltage that is different between the LCD source driver ICs. Therefore, there is a possibility that the problem of the display block unevenness is caused. In detail, the grayscale voltage is determined based on resistance division in each LCD source driver IC. A resistance division ratio is varied for each LCD source driver IC, although this is natural. As a result, the grayscale characteristics of the first LCD source driver IC 110-1 and second LCD source driver IC 110-2 are different. In this case, if the two driver ICs are arranged systematically and the liquid crystal panel is driven in response to the data signals based on the respective grayscale voltages, the boundary between the LCD panels driven by the respective driver ICs can be recognized by a human's eye. It should be noted that the human's eye is said to be possible to recognize the difference of 10 mV in the voltage applied to the liquid crystal, as the different grayscale.
In order to solve the above problems, the outputs of the grayscale power supply operational amplifiers are considered to be commonly connected. However, in the conventional technique, the offset voltages of the respective operational amplifiers are different. Thus, if the outputs are short-circuited, the power supply operational amplifier is abnormally operation. For this reason, it is difficult to connect the outputs of the grayscale power supply operational amplifiers to each other. Therefore, in the conventional examples, it is difficult to commonly connect the LCD driver ICs in which the grayscale voltage generating circuits are built.