JP4859073B2 - Liquid crystal drive device - Google Patents

Description

  The present invention relates to a technique for reducing power consumption in a drive circuit of a matrix type liquid crystal display device.

  There is an active matrix liquid crystal display device that realizes display by applying a predetermined voltage to each of a plurality of drain electrodes, a gate electrode, and a common electrode on the opposite side of the liquid crystal layer. In this liquid crystal display device, a gradation voltage of a level corresponding to display data is applied to the drain electrode. As shown in FIG. 12, the configuration of the drain driving circuit that generates and outputs the gradation voltage is based on the display data for one line stored in the gradation voltage generation circuit and the data latch circuit. A configuration is known that includes a gradation voltage selection circuit that individually selects one of the voltage adjustment groups (V0 to V63). As shown in FIG. 13, the gradation voltage generating circuit is configured such that an input reference voltage group (Vref0 to Vref7) is received by a voltage follower circuit, and the output of this circuit is divided by a dollar resistor. A configuration for generating a regulated voltage group is known.

  The output of the grayscale voltage generation circuit shown in FIG. 13 is charged and discharged all at once by capacitive components including liquid crystal at the timing of an alternating operation described later. In order to compensate the phase against a sudden current change caused by this charging / discharging, a stabilizing capacitor of 0.1 to 10 μF is generally connected to the output of the voltage follower circuit. As described above, the stabilization capacitor has a large value, and even if the grayscale voltage generation circuit is integrated, it is necessary to provide the stabilization capacitor outside the IC. A first object of the present invention is to provide a voltage follower circuit that does not require a stabilizing capacitor, and to reduce the number of components.

  Next, in the liquid crystal display device, in order to prevent image quality deterioration similar to burn-in, an operation called a so-called alternating operation is required in which the polarity of the liquid crystal applied voltage is inverted at a constant period. At this time, for example, a so-called non-target drive method in which each level of the grayscale voltage group is different in the positive polarity and the negative polarity has an effect of reducing the amplitude of the common voltage and preventing image quality deterioration such as flicker. In order to realize this asymmetric drive with the gradation voltage generation circuit shown in FIG. 13, the level of the reference voltage may be made different between positive polarity and negative polarity. However, if the level of the reference voltage is changed periodically, the stabilizing capacity is charged and discharged, resulting in an increase in power consumption. The second object of the present invention is to suppress the charging / discharging current of the stabilizing capacitor during asymmetric driving and to avoid an increase in power consumption.

  Further, in the liquid crystal display device, it is assumed that the number of colors of the input display data is smaller than the number of levels of the gradation voltage group to be generated. At this time, unnecessary gradation voltages are also generated by the ladder resistor, and as a result, a useless steady current flows through the ladder resistor. A third object of the present invention is to eliminate the waste of steady current flowing through the ladder resistor in accordance with the number of display colors and to avoid an increase in power consumption.

  In order to solve the above problems, first, a voltage follower circuit that does not require a stabilization capacitor, which is the first object of the present invention, will be considered. First, if the stabilization capacitor is removed in a general voltage follower circuit, the most serious problem is that the phase margin is small and oscillation is likely to occur. In order to avoid this, it is effective to insert a resistor between the output of the voltage follower circuit and the load. However, when a resistance component is inserted, the time constant becomes large, so that there is a side effect that the recovery time from voltage fluctuation becomes long. Therefore, a voltage follower circuit has been devised to solve these problems. The voltage follower circuit of the present invention includes a differential circuit, first and second buffer circuits, a resistor, and two phase compensation capacitors. This circuit configuration is characterized in that the outputs of the first and second buffer circuits are connected to each other via a resistor, and at the same time, the output of the first buffer circuit is fed back to the input of the differential circuit. The output of the second buffer circuit is an output as a voltage follower circuit. Thus, by providing a resistor between the output of the voltage follower circuit and the feedback point of the differential circuit, the phase margin can be increased and the output operation is stabilized. At the same time, since the load can be directly driven by the output of the second buffer circuit, the time constant can be reduced and the recovery time from voltage fluctuation can be shortened.

  Next, suppression of charging / discharging of the stabilization capacitor during asymmetric driving, which is the second object of the present invention, can be realized by using the above-described circuit configuration that does not use the stabilization capacitor. Therefore, here, a means for suppressing charging / discharging will be described with respect to a conventional bolge follower circuit with a stabilizing capacity. The idea is that the potential applied to the stabilization capacitor is always constant even in asymmetric driving. Therefore, voltage follower circuits for positive polarity and negative polarity are prepared, and these are switched and used.

  Finally, as a third object of the present invention, as a means for eliminating the waste of the steady current flowing in the ladder resistor, the ladder resistor of the portion that generates the unnecessary gradation voltage is separated or switched to one having a high resistance. I decided to install a switch. With this configuration, the steady current flowing through the ladder resistor can be optimized in accordance with the number of colors of display data, so that an increase in power consumption can be avoided.

  According to the drain driving circuit of the present invention, the number of external stabilizing capacitors used can be reduced or eliminated, so that the cost can be reduced. Further, even when an external stabilization capacitor is provided, since the stabilization capacitor itself has a circuit configuration that does not charge / discharge, low power consumption can be achieved. Furthermore, since the necessary steady-state current can be controlled in accordance with the number of colors of the input display data, further reduction in power consumption can be achieved.

FIG. 1 is a block diagram showing a configuration of a voltage follower circuit according to the first embodiment of the present invention. FIG. 2 is a circuit diagram showing the configuration of the voltage follower circuit according to the first embodiment of the present invention. FIG. 3 is a circuit diagram showing the configuration of the voltage follower circuit according to the first embodiment of the present invention. FIG. 4 is a circuit diagram showing a configuration of the gradation voltage generation circuit according to the first embodiment of the present invention. FIG. 5 is a block diagram showing the configuration of the drain drive circuit according to the first embodiment of the present invention. FIG. 6 is an operation explanatory diagram of the drain drive circuit according to the first embodiment of the present invention. FIG. 7 is a timing chart showing the operation of the drain drive circuit according to the first embodiment of the present invention. FIG. 8 is a circuit diagram showing a configuration of a gradation voltage generation circuit according to the second embodiment of the present invention. FIG. 9 is a circuit diagram showing a configuration of a gradation voltage generation circuit according to the second embodiment of the present invention. FIG. 10 is a circuit diagram showing a configuration of a gradation voltage generation circuit according to the second embodiment of the present invention. FIG. 11 is a circuit diagram showing a configuration of a gradation voltage generation circuit according to the third embodiment of the present invention. FIG. 12 is a block diagram showing a configuration of a conventional drain drive circuit. FIG. 13 is a circuit diagram showing a configuration of a conventional gradation voltage generation circuit.

  FIG. 1 shows the basic configuration of the voltage follower circuit of the present invention. The voltage follower circuit 101 of the present invention includes a differential circuit 102, first and second buffer circuits 103 and 104, a resistor 105, and two phase compensation capacitors 106 and 107. This circuit configuration is characterized in that the outputs of the first and second buffer circuits 103 and 104 are connected to each other via a resistor 105, and at the same time, the output of the first buffer circuit 103 is connected to the differential circuit 102. The output of the second buffer circuit 104 is the output as the voltage follower circuit 101. Thus, by providing the resistor 105 between the output of the voltage follower circuit 101 and the feedback point of the differential circuit 102, the phase margin can be increased and the output operation is stabilized. At the same time, since the load can be directly driven by the output of the second buffer circuit 104, the time constant can be reduced and the recovery time from voltage fluctuation can be shortened.

  Hereinafter, the first embodiment of the present invention will be described with reference to FIGS. The first embodiment of the present invention is an example of a voltage follower circuit that does not require the stabilization capacitor described above, and its circuit configuration is shown in FIGS. In FIG. 2, 201 is a voltage follower circuit, 202 to 211 are MOS transistors, 212 is a resistor, 213 to 214 are phase compensation capacitors, and corresponding to FIG. 1 described above, the differential stage circuit is a MOS transistor. The portion composed of 202 to 205, the first buffer circuit corresponds to the portion composed of the MOS transistor 206, and the second buffer circuit corresponds to the portion composed of the MOS transistors 207 to 211. Since the configurations of the differential circuit and the buffer circuit are the same as the configuration of a widely used class AB amplifier, details of the operation and the like are omitted. As described above, the output of the first buffer circuit is fed back to the input of the differential circuit at the same time as the outputs of the first and second buffer circuits are connected to each other via a resistor as described above. Thus, the output of the second buffer circuit is an output as a voltage follower circuit.

  A voltage follower circuit 301 shown in FIG. 3 is obtained by replacing the voltage follower circuit 201 shown in FIG. Here, considering the range of voltage levels that can be output, the voltage follower circuit 201 is the output range near GND, and the voltage follower circuit 301 is the output range near VDD that is the power supply voltage. Therefore, it is desirable to use both according to the level of the gradation voltage to be output. A voltage follower circuit with a stabilizing capacity based on a conventional so-called class A amplifier has a wider output range than the voltage follower circuit of the present invention. Therefore, a conventional voltage follower circuit with a stabilizing capacitor may be used only for gradation voltages at levels closer to VDD and GND.

  An example of a circuit configuration that generates 64 types of gradation voltages based on this concept will be described. FIG. 4 is a diagram showing the configuration of the gradation voltage generating circuit 401. Reference numerals 402 to 403 and 408 to 409 are voltage follower circuits with stabilizing capacitors based on conventional class A amplifiers, and 404 to 405 are the present invention described above. These voltage follower circuits 301 and 406 to 407 are the voltage follower circuit 201 of the present invention. As can be seen from FIG. 4, the voltage follower circuits 402 to 409 are arranged so that the voltage level output in this order decreases. Reference numeral 410 denotes a ladder resistor, which, like the ladder resistor shown in FIG. 13, is for dividing the output of the voltage follower circuit to generate a gradation voltage. In the gradation voltage generation circuit 401 of the present invention, the reference voltage Vref is not input from the outside, but a two-level reference voltage (VH, VL) is input and divided by the ladder resistor 411. Therefore, it was decided to generate Vref. Thereby, the number of wirings with the outside can be reduced. With the circuit configuration described above, the gradation voltage generation circuit 401 can generate 64 types of gradation voltages.

  Next, the configuration and operation of the drain drive circuit including the gradation voltage generation circuit 401 will be described by taking as an example a case where so-called common inversion drive is performed. FIG. 5 is a block diagram of the drain driving circuit 501, 502 is a data latch circuit, 503 is a data inverting circuit, and 504 is a gradation voltage circuit. First, the drain drive circuit 501 receives CL1 indicating one scanning period, EN indicating the transfer period of effective display data, M indicating the polarity of alternating current, CL2 indicating the transfer clock of display data, and display data from the external liquid crystal controller. Each signal of DATA shown is input. In this embodiment, DATA has 6-bit gradation information for each pixel. First, in the drain driver circuit 501, the data latch circuit 502 stores DATA during a period when EN is high (= 1), stores CL1 as a clock for one line, and stores the stored display data all at once in synchronization with CL1. The operation of outputting as LD is repeated. The data inversion circuit 503 inputs LD and M. When M is low (= 0), the LD is unchanged, and when M is high (= 1), the LD is inverted and output as PD. The gradation voltage selection circuit 504 selects one of the inputted gradation voltages V0 to V63 according to the value of PD and outputs it as VD. An example of this operation is shown in FIG. Next, a timing chart summarizing the operation of the drain drive circuit 501 is shown in FIG. As can be seen from FIG. 7, a voltage level corresponding to the display data is output according to CL1, and a drive waveform of a general common inversion drive is realized.

  Here, the drain drive circuit 501 was made into an IC, and actual characteristics were measured. First, regarding the output range, the voltage follower circuits 404 to 405 of the present invention were (VDD−0.6 V) or less, and the voltage follower circuits 406 to 407 of the present invention were (GND + 0.8 V) or more. In addition, as a result of connecting 120 × 160 pixels and 2 inch TFT liquid crystal at the tip of the drive circuit and performing common inversion drive at a frame frequency of 60 Hz, all gradation voltages have no problems such as oscillation, and a good display is obtained. Obtained.

  From the above, the voltage follower circuit of the present invention can obtain good characteristics even without a stabilizing capacitor, so that it is possible to reduce the number of parts of the stabilizing capacitor compared to the conventional drain driving circuit. It is. In the gradation voltage generation circuit 401 of the present invention, a conventional voltage follower circuit with a stabilizing capacitor is combined, but the present invention is not limited to this configuration. For example, if the vicinity of VDD and GND is not used, the voltage follower circuits 201 and 301 of the present invention can be used alone.

  Next, a second embodiment of the present invention will be described with reference to FIGS. The second embodiment of the present invention shows the configuration of a gradation voltage generation circuit that realizes suppression of charging / discharging of a stabilization capacitor during asymmetric driving. As described above, the voltage follower circuit for positive polarity and negative polarity is prepared and used as described above. In this embodiment, the present invention shown in FIG. This will be improved on the basis of the gradation voltage generation circuit 401 according to the first embodiment.

  First, in FIG. 8, 801 and 802 are switches for switching between positive and negative grayscale voltages, 803 is a ladder resistor that generates positive grayscale voltages, and 804 is a negative grayscale voltage. The ladder resistor to be generated and other components are the same as those of the grayscale voltage generation circuit 401 shown in FIG. This circuit configuration is characterized in that two types of voltage follower circuits with ladder resistors and stabilizing capacitors are prepared for positive polarity and negative polarity, and a switch is provided for switching them with an AC signal. Here, regarding the voltage follower circuit with the stabilizing capacitor, the output of this circuit is switched by a switch. For example, the switch 801 selects the reference voltage generated by the positive-polarity ladder resistor 803 when the AC signal is low (= 0), and for the negative electrode when the AC signal is high (= 1). The reference voltage generated by the ladder resistor 804 is selected. With this configuration, the output of the voltage follower does not fluctuate, and charging / discharging of the stabilization capacitor can be avoided. On the other hand, regarding the voltage follower circuit of the present invention that does not require a stable capacitor, a switch 802 is provided in front of the amplifier input. This is because the number of amplifiers used can be reduced as compared with a configuration in which a switch is provided at the subsequent stage of amplifier input. Note that the operation of the switch 802 is the same as that of the previous switch 801, and is omitted here.

  Next, another configuration of the grayscale voltage generation circuit according to the second embodiment of the present invention will be described with reference to FIG. In FIG. 9, 901 is a switch for selectively switching the connection between two stabilizing capacitors and a voltage follower circuit, and 902 selects a reference voltage generated by a ladder resistor for positive polarity and negative polarity. It is a switch to do. As can be seen from FIG. 9, in this circuit configuration, only a stabilizing capacitor is prepared for each of the positive polarity and the negative polarity, and a switch for selectively switching the connection with the voltage follower circuit is provided. As a result, as compared with the circuit shown in FIG. 8, only one voltage follower circuit is required per reference voltage, so that the circuit scale can be reduced.

  Next, another configuration of the grayscale voltage generation circuit according to the second embodiment of the present invention will be described with reference to FIG. In FIG. 10, 1001 is a positive resistance, 1002 is a negative positive resistance, and 1003 is a switch that selectively switches the connection between a ladder resistor that generates a reference voltage and the above resistor. The purpose of the circuit configuration shown in FIG. 10 is to unify the ladder resistor that generates the reference voltage. If the values of the resistor 1001 and the resistor 1002 are different values, the positive polarity and the negative polarity have different levels. It is possible to generate a gradation voltage. In addition, it is desirable to provide this configuration on the upper side and the lower side of the ladder resistor because the degree of freedom of adjustment becomes higher.

  The grayscale voltage generation circuit according to the second embodiment of the present invention described above can avoid charging / discharging of the stabilization capacitor connected to the voltage follower circuit even when so-called non-target driving is performed. Is possible. Therefore, a drain driving circuit with lower power consumption can be provided.

  Hereinafter, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment of the present invention, a method for optimizing the steady current flowing through the ladder resistor in accordance with the number of colors of display data will be described in order to eliminate the waste of the steady current flowing through the ladder resistor. In the present embodiment, there are cases where the number-of-colors information included in the display data is 6 bits or 1 bit, and information regarding the state is given from an external liquid crystal controller. Further, when the number-of-colors information included in the display data is 6 bits, all 64 types of gradation voltages (V0 to V63) are used, and when the display data is 1 bit, gradation voltages (V0 and V63) at both ends are used. Only shall be used.

  FIG. 11 is a grayscale voltage generation circuit according to the third embodiment of the present invention based on the grayscale voltage generation circuit shown in FIG. 9. 1101 is a 6-bit resistor, 1102 is a 1-bit resistor, Reference numeral 1103 denotes a switch for switching the connection between the ladder resistor for generating the gradation voltage and the above resistor according to the color number information. For example, if the color number information is high (= 1), the switch 1103 recognizes the 6-bit display mode and selects the 6-bit resistor 1101, and if low (= 0), recognizes the 1-bit display mode and 1 bit. Resistor 1102 is selected. Here, the value of the 1-bit display resistor is determined in advance so as to be sufficiently larger than that of the 6-bit resistor 1101, and when this resistor is selected, the current flowing through the ladder resistor is reduced. As described above, in the display mode in which the number of colors is 1 bit, only the gradation voltages (V0 and V63) at both ends are used, so even if the level of other gradation voltages fluctuates, It uses the fact that it does not affect the display.

  As described above, the gradation voltage generation circuit according to the third embodiment of the present invention can control the value of the current flowing through the gradation voltage generation ladder resistor according to the information on the number of colors. . Therefore, a drain driving circuit with lower power consumption can be provided. In the third embodiment of the present invention, two types of 6-bit resistors and 1-bit resistors are prepared. However, the present invention is not limited to this. For example, in the 1-bit display mode, V0 and V63 are set as follows. It may be completely separated from the ladder resistance.

  In the first to third embodiments of the present invention, the common inversion driving has been described as an example. However, the present invention is not limited to this, and other known inversion driving methods such as dot inversion driving and column-by-column inversion are known. The same concept can be applied to driving.

101, 201, 301 ... Voltage follower circuit 102 ... Differential circuit 103, 104 ... Buffer circuit 105 ... Resistor 106, 107 ... Phase compensation capacitor 401 ... Gradation voltage generation circuit 410 ... Ladder resistor 501 ... Drain drive circuit 502 ... Data latch Circuit 503: Data inverting circuit 504: Gradation voltage selection circuit 801 ... Switch 802 ... Switch 803 ... Positive polarity ladder resistor 804 ... Negative polarity ladder resistor 901 ... Switch 902 ... Switch 1001 ... Positive polarity resistance 1002 ... Negative polarity positive use Resistor 1003... Switch 1101... 6-bit resistor 1102... 1-bit resistor 1103.

Claims (2)

A display voltage is applied to each of the plurality of drain electrodes, the gate electrode, and the common electrode on the opposite side of the liquid crystal layer by applying a predetermined voltage to the active matrix liquid crystal panel. A liquid crystal driving device that applies a gradation voltage of a corresponding voltage level,
A first ladder resistor that generates a plurality of levels of reference voltage from an input power supply voltage; a voltage follower circuit that stabilizes the reference voltage; and a second ladder resistor that generates the gradation voltage from the reference voltage. Including
The second ladder resistor is a liquid crystal driving device that changes the resistance value according to the information on the number of colors input from the outside. The liquid crystal driving device according to claim 1.
A liquid crystal driving device in which the resistance value of the second ladder resistor is higher when the number of colors is small.

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