JP2012220928A - Liquid crystal display and driving method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further reduce current consumed by a pixel, and visually reduce image degradation resulting from a variation in a threshold voltage Vth of a transistor serving as a current supply source within a current mirror circuit.SOLUTION: A wiring B+ connected to a gate of a constant current load transistor Tr7 within a source follower buffer on a positive polarity side and a wiring B- connected to a gate of a constant current load transistor Tr8 within a source follower buffer on a negative polarity side are driven by a current mirror at one current reference source transistor Tr15 through a first CMOS switch consisting of transistors Tr16 and Tr17 and an inverter I1 and through a second CMOS switch consisting of transistors Tr18 and Tr19 and an inverter I2, respectively. Only one of the first and second CMOS switches is turned on by a signal CB+ and a signal CB-.

Description

  The present invention relates to a liquid crystal display device and a driving method thereof, and in particular, in each pixel, a positive video signal and a negative video signal are separately sampled and held in two holding capacitors, and then the holding voltages are alternately applied to the pixel electrodes. The present invention relates to a liquid crystal display device for applying an alternating current to a liquid crystal display element by applying the liquid crystal display device and a driving method thereof.

  In recent years, a liquid crystal display device of LCOS (Liquid Crystal on Silicon) type is often used as a central part for projecting images in projector devices and projection televisions. As this LCOS type liquid crystal display device, the present applicant has first made a plurality of sets of data lines including two data lines (column signal lines) and a plurality of gate lines (row scanning lines). Pixels are arranged in a matrix at each intersection, and positive and negative video signals are sampled and held separately in two holding capacitors at each pixel, and then the holding voltages are alternately applied to the pixel electrodes. A liquid crystal display device is proposed in which the liquid crystal display element is AC-driven by applying the voltage to the liquid crystal display (see, for example, Patent Document 1).

  FIG. 5 shows an equivalent circuit diagram of an example of one pixel of the liquid crystal display device. In the figure, one pixel has pixel selection transistors Tr1 and Tr2 for writing a positive-polarity video signal and a negative-polarity video signal, and two independent holdings that hold video signal voltages of respective polarities in parallel. Capacitors Cs1 and Cs2, transistors Tr3 to Tr8, and a liquid crystal display element LC. The liquid crystal display element LC has a well-known structure in which a liquid crystal layer (display body) LCM is sandwiched between a pixel electrode PE and a common electrode CE arranged to face each other.

  The pixel selection transistors Tr1 and Tr2 and the switching transistors Tr5 and Tr6 are N-channel MOS type field effect transistors (hereinafter referred to as NMOS transistors), and the transistors Tr3, Tr4, Tr7, and Tr8 are P-channel MOS type field effect transistors. It is assumed that it is a transistor (hereinafter referred to as a PMOS transistor). The transistors Tr3 and Tr7 and the transistors Tr4 and Tr8 are so-called source follower buffers. The transistors Tr3 and Tr4 function as source follower transistors, and the transistors Tr7 and Tr8 function as constant current source loads. The input resistance of the source follower buffer of the MOS transistor is almost infinite, and the charges accumulated in the holding capacitors Cs1 and Cs2 are held without leakage until a signal is newly written after one vertical scanning period.

  The pixel portion data lines are composed of a pair of positive data lines Di + and negative data lines Di− for each pixel, and video signals having different polarities sampled by a data line driving circuit (not shown). Is supplied. The drain terminals of the pixel selection transistors Tr1 and Tr2 are connected to the positive data line Di + and the negative data line Di-, respectively, and the gate terminals are connected to the row scanning line (gate line) Gj for the same row. Yes. Further, the wiring B is connected to the gates of the transistors Tr7 and Tr8. The wirings S + and S− are wirings for gate control signals, and are connected to the gates of the transistors Tr5 and Tr6, respectively. Further, the row scanning line Gj is commonly connected to the transistors Tr1 and Tr2 of a plurality of pixels in the same row.

  Next, an outline of the AC drive control of the pixel will be described with reference to the timing chart of FIG. 6A shows the vertical synchronization signal VD, and FIG. 6B shows the load characteristic control signal of the wiring B applied to the gates of the transistors Tr7 and Tr8 in the pixel of FIG. FIG. 6C shows the gate control signal of the wiring S + applied to the gate of the switching transistor Tr5 that transfers the positive side drive voltage in the pixel, and FIG. 6D shows the negative side in the pixel. Each signal waveform of the gate control signal of the wiring S− applied to the gate of the switching transistor Tr6 that transfers the drive voltage is shown.

  5, the positive polarity side switching transistor Tr5 is turned on while the gate control signal of the wiring S + shown in FIG. 6C is at a high level, and the load characteristic control signal supplied to the wiring B during this period is shown in FIG. When it is at a low level as shown in (B), the source follower buffer becomes active, and the pixel electrode PE node is charged to a positive video signal level. When the potential of the pixel electrode PE is fully charged, when the load characteristic control signal of the wiring B is set to high level and the gate control signal of the wiring S + is switched to low level at that time, the pixel electrode PE becomes floating, and a positive drive voltage is held in the liquid crystal capacitor.

  On the other hand, the negative polarity side switching transistor Tr6 is turned on while the gate control signal of the wiring S− shown in FIG. 6D is at a high level, and the load characteristic control signal supplied to the wiring B during this period is shown in FIG. ), The source follower buffer becomes active, and the pixel electrode PE node is charged to a negative video signal level. When the potential of the pixel electrode PE is fully charged, the load characteristic control signal of the wiring B is set to the high level, and the gate control signal of the wiring S- is switched to the low level at that time. PE becomes floating, and the negative drive voltage is held in the liquid crystal capacitor.

  Hereinafter, in synchronization with the switching in which the switching transistors Tr5 and Tr6 are alternately turned on, the operation of intermittently activating the transistors Tr7 and Tr8 according to the load characteristic control signal of the wiring B is repeated, whereby the liquid crystal display element LC A drive voltage VPE converted into an alternating current with each of the positive and negative video signals is applied to the pixel electrode PE as shown in FIG. The pixel shown in FIG. 5 does not directly transfer the retained charge to the pixel electrode PE, but supplies the voltage via the source follower buffer. Therefore, it is possible to realize driving without attenuation of the voltage level.

  Further, Vcom shown in FIG. 6F represents a voltage applied to the common electrode CE formed on the counter substrate of the liquid crystal display device. The substantial AC drive voltage of the liquid crystal layer LCM is a difference voltage between the applied voltage Vcom of the common electrode CE and the applied voltage of the pixel electrode PE. As shown in FIG. 6F, the applied voltage Vcom of the common electrode CE is inverted in synchronization with pixel polarity switching with respect to a reference level substantially equal to the inversion reference level Vc of the pixel electrode potential.

  Also, the positive and negative video signal voltages sampled and held in the holding capacitors Cs1 and Cs2, respectively, are read out via the high-input resistance source follower transistors Tr3 and Tr4, and FIG. As shown in FIG. 6B, the pixel electrodes PE are alternately selected by the switching transistors Tr5 and Tr6 which are turned on by the gate control signals supplied alternately to the wirings S + and S-, and are inverted to the positive polarity and the negative polarity. The drive voltage VPE shown in FIG. In the pixel shown in FIG. 5, once the positive and negative video signal voltages are written to the holding capacitors Cs1 and Cs2 once in one vertical scanning period (one frame), the video signal voltage of the next frame is obtained. The video signal voltage can be read from the holding capacitors Cs1 and Cs2 any number of times during one frame period until it is held, and the transistors Tr5 and Tr6 are alternately switched to drive the liquid crystal display element LC with AC. Therefore, the pixel shown in FIG. 5 can AC drive the liquid crystal display element LC at a high driving frequency without any restriction on the vertical scanning frequency independently of the video signal writing cycle.

  This AC drive frequency can be freely set in the inversion control cycle in the pixel circuit, regardless of the vertical scanning frequency. For example, it is assumed that the vertical scanning frequency is 60 Hz used for a general television image signal, and the configuration is composed of 1125 lines of full periodic high-definition vertical scanning lines. If the polarity of the pixel circuit is switched at a cycle of about 15 line periods, the AC drive frequency of the liquid crystal display element is 2.25 kHz (= 60 (Hz) × 1125 ÷ (15 × 2)), which is a conventional liquid crystal display. Compared with the apparatus, the liquid crystal driving frequency can be dramatically increased. As a result, image sticking can be prevented, and deterioration in display quality such as reliability, stability, and spots can be greatly improved as compared with the case where the AC drive frequency of the liquid crystal display element is low.

  Note that the constant current load transistors Tr7 and Tr8 of the source follower buffer are not always active in consideration of current consumption in the liquid crystal display device, and are only active during a limited period of the conduction period of the switching transistors Tr5 and Tr6. Control to become. For example, even if the steady source follower circuit current per pixel circuit is a minute current of 1 μA, a large amount of current is consumed under the condition that all pixels of the liquid crystal display device constantly consume current. For example, in a liquid crystal display device with 2 million pixels of full high vision, the current consumption is estimated to reach 2A.

  Therefore, in the pixel shown in FIG. 5, the low level period of the load characteristic control signal B that becomes the gate bias of the constant current load transistors Tr7 and Tr8 is limited only to the transition period of the pixel voltage polarity switching, and the pixel electrode voltage VPE reaches the target level. Immediately after being charged and discharged, the current of the source follower buffer is stopped immediately by setting it to the high level. Therefore, it is possible to suppress a substantial current consumption even though the configuration includes a buffer for all pixels.

JP 2009-223289 A

  In the above liquid crystal display device, as shown in FIG. 5, since the gates of the constant current load transistors Tr7 and Tr8 are commonly wired as the wiring B in the row direction for the same row pixel, the PMOS transistor Tr3 is controlled by the load characteristic control signal. And Tr7 and two source follower buffers of PMOS transistors Tr4 and Tr8 are simultaneously turned on.

  However, since the signal applied by switching to the pixel electrode PE of the liquid crystal display element LC is always applied through one source follower buffer, the positive polarity signal is applied to the pixel electrode PE by turning on the switching transistor Tr5. During this period, the current flowing through the source follower buffer composed of the PMOS transistors Tr4 and Tr8 is simply discarded, and no signal is used. On the other hand, when the switching transistor Tr6 is turned on and a negative signal is applied to the pixel electrode PE, the current flowing through the source follower buffer composed of the PMOS transistors Tr3 and Tr7 is simply discarded. Is not used. Therefore, twice the current is flowing. Therefore, further reduction of current consumption is desired for the liquid crystal display device described in Patent Document 1.

  Therefore, the wiring connected to the gate of the constant current load transistor Tr7 in the source follower buffer for positive polarity is B +, and is connected to the gate of the constant current load transistor Tr8 in the source follower buffer for negative polarity. The wiring is set to B-, and current is supplied only to the source follower buffer on the side where reading is performed by turning on the switching transistors Tr5 and Tr6, and current is not supplied to the other source follower buffer. It is conceivable to further reduce current consumption by separately controlling load characteristic control signals supplied to the wirings B + and B−.

  Here, in the case of the liquid crystal display device having such a configuration, a first load characteristic control signal supplied to the wiring B + is generated by the first current mirror circuit and supplied to the wiring B−. It is conceivable that the signal is generated by the second current mirror circuit. However, since the wirings B + and B− are commonly connected to a plurality of pixels in the same row, the threshold voltage Vth of each current supply transistor in the first and second current mirror circuits varies. As a result, it is recognized as a horizontal line having a different luminance for each random line, and a little Vth variation component is noticeable.

  FIG. 7 shows an example of a display image in the liquid crystal display device having the gate lines B + and B− when the threshold voltage Vth of each current supply transistor in the first and second current mirror circuits varies. Indicates. As shown in the figure, the display image is an example in the case where a gray image of intermediate gradation is displayed on the entire screen, and the threshold voltage Vth of each current supply transistor in the first and second current mirror circuits. It can be seen that the voltage of the wirings B + and B− varies due to the variation, and the lines for one horizontal line are randomly displayed as lines having different luminances. The above Vth variation cannot be avoided due to manufacturing process problems.

  The present invention has been made in view of the above points. The positive voltage sampled and held in the positive holding capacitor and the negative voltage sampled and held in the negative holding capacitor are shorter than one vertical scanning cycle. It is an object of the present invention to provide a liquid crystal display device and a driving method thereof that can further reduce current consumption of a pixel configured to be alternately applied to a pixel electrode in a cycle.

  Another object of the present invention is to visually observe image degradation caused by variations in threshold voltage Vth of current supply transistors in a current mirror circuit that drives each constant current load transistor in two types of source follower buffers. An object of the present invention is to provide a liquid crystal display device capable of reducing and displaying a high-quality image and a driving method thereof.

In order to achieve the above object, the liquid crystal display device of the present invention has a plurality of data lines provided at intersections where a plurality of sets of data lines and a plurality of row scanning lines intersect each other. Each of the pixels
A display element in which a liquid crystal layer is sandwiched between an opposing pixel electrode and a common electrode, and a positive video signal supplied via one data line of a set of two data lines are sampled and fixed. The first sampling and holding means for holding in the first holding capacitor for the period and the negative polarity having the opposite polarity to the positive video signal supplied through the other data line of the set of two data lines Second sampling and holding means for sampling the video signal and holding it in the second holding capacitor for a certain period, a first source follower transistor, and a first having a drain connected to the source of the first source follower transistor. The first constant current load transistor is operated when the first load characteristic control signal is input through the first wiring connected to the gate of the first constant current load transistor. A second follower buffer, a second source follower transistor, and a second constant current load transistor having a drain connected to the source of the second source follower transistor, and connected to the gate of the second constant current load transistor A second source follower buffer that operates when a second load characteristic control signal is input through the second wiring that has been connected, and a first source follower buffer that is input through the first source follower buffer during operation A positive signal voltage corresponding to the positive video signal held in the holding capacitor and a negative electrode corresponding to the negative video signal held in the second holding capacitor inputted through the second source follower buffer during operation. A switching means for switching the polarity signal voltage with a predetermined cycle shorter than the vertical scanning cycle and alternately applying to the pixel electrode,
When the positive polarity signal voltage is applied to the pixel electrode by the switching means, the first wiring and a predetermined node are brought into conduction, and when the negative polarity signal voltage is applied to the pixel electrode by the switching means, the second polarity is applied. A wiring switching means for switching the wiring and the predetermined node to be in a conductive state, and a gate and a drain are connected to the predetermined node, and the predetermined node and the first wiring are in a conductive state by the wiring switching means. When the first constant current load transistor is used as a current copy side transistor to form a first current mirror circuit that supplies a first load characteristic control signal, a predetermined node and a second wiring are connected by the wiring switching means. When in the conductive state, the second current mirror that supplies the second load characteristic control signal using the second constant current load transistor as the current copy side transistor And having a same current mirror current reference source transistor constituting the road.

  Here, the first and second wirings, the wiring switching unit, and the current mirror current reference source transistor are provided in common for a plurality of pixels in the same row, and the first constant current load transistor The gate length and the gate width may be the same as the gate length and the gate width of the second constant current load transistor.

In order to achieve the above object, the liquid crystal display device of the present invention is provided at an intersection where a plurality of sets of data lines and a plurality of row scanning lines intersect each other. Each of the plurality of pixels
A display element in which a liquid crystal layer is sandwiched between an opposing pixel electrode and a common electrode, and a positive video signal supplied via one data line of a set of two data lines are sampled and fixed. The first sampling and holding means for holding in the first holding capacitor for the period and the negative polarity having the opposite polarity to the positive video signal supplied through the other data line of the set of two data lines Second sampling and holding means for sampling the video signal and holding it in the second holding capacitor for a certain period, a first source follower transistor, and a first having a drain connected to the source of the first source follower transistor. The first constant current load transistor is operated when the first load characteristic control signal is input through the first wiring connected to the gate of the first constant current load transistor. A second follower buffer, a second source follower transistor, and a second constant current load transistor having a drain connected to the source of the second source follower transistor, and connected to the gate of the second constant current load transistor A second source follower buffer that operates when a second load characteristic control signal is input through the second wiring that has been connected, and a first source follower buffer that is input through the first source follower buffer during operation A positive signal voltage corresponding to the positive video signal held in the holding capacitor and a negative electrode corresponding to the negative video signal held in the second holding capacitor inputted through the second source follower buffer during operation. A switching means for switching the polarity signal voltage with a predetermined cycle shorter than the vertical scanning cycle and alternately applying to the pixel electrode,
When the positive polarity signal voltage is applied to the pixel electrode by the switching means, the first wiring and a predetermined node are brought into conduction, and when the negative polarity signal voltage is applied to the pixel electrode by the switching means, the second polarity is applied. A source follower comprising: wiring switching means for switching between a predetermined wiring and a predetermined node so as to be conductive; a first transistor having a source connected to the predetermined node; and a resistor having one end connected to the predetermined node When a gate is connected to a circuit and a predetermined node, and the predetermined node and the first wiring are in a conductive state by the wiring switching means, the first constant current load transistor is used as a current copy side transistor through the source follower circuit. The first current mirror circuit for supplying the first load characteristic control signal is configured, and the predetermined node and the second wiring are connected by the wiring switching means. The same current mirror current reference source constituting the second current mirror circuit that supplies the second load characteristic control signal through the source follower circuit with the second constant current load transistor as the current copy side transistor when the current is in the through state And the gate of the first transistor is connected to the drain of the second transistor.

  Here, the first and second wirings, the wiring switching unit, the source follower circuit, and the second transistor that is the current mirror current reference source are provided in common for a plurality of pixels in the same row. The gate length and gate width of the first constant current load transistor may be the same as the gate length and gate width of the second constant current load transistor.

In order to achieve the above object, a driving method of a liquid crystal display device according to the present invention includes an intersection where a plurality of sets of data lines and a plurality of row scanning lines intersect each other. For a plurality of pixels including a display element in which a liquid crystal layer is sandwiched between a pixel electrode and a common electrode facing each other.
A positive video signal supplied via one data line of a set of two data lines is sampled and held in the first holding capacitor for a certain period, and at the same time, of the set of two data lines A sampling and holding step for sampling a negative polarity video signal having a polarity opposite to that of the positive polarity video signal, which is supplied via the other data line, and holding it in the second holding capacitor for a certain period; and a first source follower transistor And a first constant current load transistor having a drain connected to the source of the first source follower transistor, the first source follower buffer includes a first wiring at the gate of the first constant current load transistor A positive signal voltage corresponding to the positive video signal held in the first holding capacitor to be transferred when the first load characteristic control signal is supplied via A second source follower buffer consisting of a source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor is connected to the gate of the second constant current load transistor. The negative polarity signal voltage corresponding to the negative polarity video signal held in the second holding capacitor transferred when the second load characteristic control signal is supplied through the second wiring is shorter than the vertical scanning period. A switching step of switching at a predetermined cycle and alternately applying to the pixel electrode, and when the positive signal voltage is applied to the pixel electrode by the switching step, the first wiring and the predetermined node are brought into conduction, The first constant current load transistor is current copied from a current mirror current reference source transistor whose gate and drain are connected to the node. A first load characteristic control signal supply step for supplying a first load characteristic control signal as a side transistor, and a negative signal voltage applied to the pixel electrode by the switching step; And a second load characteristic control signal supply step of supplying a second load characteristic control signal from the current mirror current reference source transistor using the second constant current load transistor as a current copy side transistor. Features.

In order to achieve the above object, a driving method of a liquid crystal display device according to the present invention includes an intersection where a plurality of sets of data lines and a plurality of row scanning lines intersect each other. For a plurality of pixels including a display element in which a liquid crystal layer is sandwiched between a pixel electrode and a common electrode facing each other.
A positive video signal supplied via one data line of a set of two data lines is sampled and held in the first holding capacitor for a certain period, and at the same time, of the set of two data lines A sampling and holding step for sampling a negative polarity video signal having a polarity opposite to that of the positive polarity video signal, which is supplied via the other data line, and holding it in the second holding capacitor for a certain period; and a first source follower transistor And a first constant current load transistor having a drain connected to the source of the first source follower transistor, the first source follower buffer includes a first wiring at the gate of the first constant current load transistor A positive signal voltage corresponding to the positive video signal held in the first holding capacitor to be transferred when the first load characteristic control signal is supplied via A second source follower buffer consisting of a source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor is connected to the gate of the second constant current load transistor. The negative polarity signal voltage corresponding to the negative polarity video signal held in the second holding capacitor transferred when the second load characteristic control signal is supplied through the second wiring is shorter than the vertical scanning period. A switching step in which switching is performed at a predetermined cycle and alternately applied to the pixel electrode; and when the positive polarity signal voltage is applied to the pixel electrode by the switching step, a first transistor having a source connected to a predetermined node; The first wiring and a predetermined node are made conductive through a source follower circuit including a resistor having one end connected to the node. The first load characteristic with the first constant current load transistor as the current copy side transistor from the second transistor of the current mirror current reference source having the gate connected to the predetermined node and the drain connected to the gate of the first transistor When the negative signal voltage is applied to the pixel electrode by the first load characteristic control signal supply step for supplying the control signal and the switching step, the second wiring and a predetermined node are brought into conduction through the source follower circuit. And a second load characteristic control signal supply step of supplying a second load characteristic control signal from the second transistor of the current mirror current reference source as a current copy side transistor as a second constant current load transistor. Features.

  According to the present invention, the current consumption is further reduced, and the threshold voltage variation of the current supply transistor in the current mirror circuit that separately drives the gate lines of the constant current loads in the two types of source follower buffers. Therefore, it is possible to visually reduce the image degradation caused by the image quality and display a high-quality image.

It is a circuit diagram of the principal part of 1st Embodiment of the liquid crystal display device of this invention. 2 is a timing chart for explaining the operation of FIG. 1. It is a figure which shows the relationship from the black bell of a positive polarity video signal and a negative polarity video signal to a white level. It is a circuit diagram of the principal part of 2nd Embodiment of the liquid crystal display device of this invention. 10 is an equivalent circuit diagram of an example of one pixel of a liquid crystal display device described in Patent Document 1. FIG. 6 is a timing chart for explaining the operation of FIG. 5. FIG. 11 is a diagram showing an example of a display image when there is variation in threshold voltage Vth of a current supply transistor in a current mirror circuit in a liquid crystal display device having wirings B + and B−.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a circuit diagram showing a main part of a first embodiment of a liquid crystal display device according to the present invention. In the figure, the same components as those in FIG.

  The liquid crystal display device 10 of the present embodiment is provided at each intersection of a plurality of data lines each having two data lines (column signal lines) and a plurality of gate lines (row scanning lines). The pixels are arranged in a matrix, and in each of these pixels, the positive video signal and the negative video signal are separately sampled and held in two holding capacitors, and then the holding voltages are alternately applied to the pixel electrodes to liquid crystal The basic configuration for AC drive of the display element is the same as that of Patent Document 1, but each pixel is configured as a one-pixel circuit 11 shown in FIG. 1, and a bias current circuit 12 and a load characteristic control signal generation circuit. 13.

  The liquid crystal display device 10 performs digital-analog conversion of the digital video signal using a positive ramp signal and a negative ramp signal, and supplies the converted voltage to each set of data lines in units of one pixel. There are also signal generating means for generating various signals such as a means, a gate signal, a control signal, a switching signal, and a row selection signal, but these means are not directly related to the present invention and are described in Patent Document 1. Since it is the same as the liquid crystal display device, illustration and detailed description are omitted.

  The one-pixel circuit 11 shown in FIG. 1 includes a set of data lines including a positive-polarity data line Di + and a negative-polarity data line Di- in the i-th column of the pixel portion, and a j-th gate line (row scanning line). ) Equivalent circuit of one pixel arranged at the intersection with Gj, and the basic configuration of this one-pixel circuit 11 is the same as the pixel circuit of the liquid crystal display device shown in FIG. That is, in FIG. 1, the first sampling and holding means comprising the pixel selecting NMOS transistor Tr1 and the positive holding capacitor Cs1, and the second sampling and holding means comprising the pixel selecting NMOS transistor Tr2 and the negative holding capacitor Cs2. Holding means, liquid crystal display element LC, positive polarity source follower buffer comprising source follower transistor Tr3 and constant current load transistor Tr7, and negative polarity source follower buffer comprising source follower transistor Tr4 and constant current load transistor Tr8 And a positive polarity signal voltage corresponding to the positive polarity video signal held in the positive polarity holding capacitor Cs1 input through the positive polarity source follower buffer during operation, and the negative polarity source follower buffer during operation. Hold for input negative polarity Switching NMOS transistors Tr5 and Tr6 that constitute switching means for switching the negative polarity signal voltage corresponding to the negative polarity video signal held at the amount Cs2 at a predetermined cycle shorter than the vertical scanning cycle and alternately applying the negative polarity signal voltage to the pixel electrode PE. And have.

  However, the one-pixel circuit 11 shown in FIG. 1 of the present embodiment is a source follower buffer for positive polarity as well as the PMOS transistor Tr3 which is a source follower transistor as compared with the pixel circuit described in Patent Document 1 shown in FIG. A constant current load PMOS transistor Tr7 that constitutes a positive current load characteristic control signal wiring B + is connected to the gate, and a negative current source follower buffer that constitutes a negative polarity source follower buffer together with the PMOS transistor Tr4 that is a source follower transistor It is characterized in that a negative polarity load characteristic control signal line B- is connected to the gate of the load PMOS transistor Tr8. One set of wiring B + and wiring B- is commonly connected to a plurality of pixels in the same row (1980 pixels in the case of full high-definition).

  The bias current circuit 12 is provided in common to a plurality of pixels in the i-th column of the pixel unit (1080 pixels in the case of full high-definition), and includes an NMOS transistor Tr10 and a PMOS transistor Tr11 whose gates are connected to each other. , And resistor R1. The PMOS transistor Tr11 has a drain connected to the resistor R1, and a source connected to the gate and drain of the NMOS transistor Tr10. The bias current circuit 12 can generate a bias current of a desired magnitude by adjusting the resistance value of the resistor R1.

  The load characteristic control signal generation circuit 13 uses positive polarity for a set of wirings B + and B− that are commonly connected to a plurality of pixels in the same row for each row (1980 pixels in the case of full high-definition). A circuit that generates and supplies a load characteristic control signal and a negative load characteristic control signal. The total number of rows in the pixel portion (the number of pixels in the vertical direction is 1080 pixels (line) in the case of full high-definition). Minutes are provided.

  The load characteristic control signal generation circuit 13 includes NMOS transistors Tr12 and Tr13, PMOS transistors Tr14 and Tr15, a PMOS transistor Tr16 that constitutes a first CMOS switch, an NMOS transistor Tr17, an inverter I1, and a PMOS that constitutes a second CMOS switch. The transistor Tr18, the NMOS transistor Tr19, the inverter I2, and the PMOS transistors Tr20 and Tr21. The NMOS transistor Tr12 has a gate connected to the gate and drain of the NMOS transistor Tr10 to form a current mirror circuit with the transistor Tr10, and the bias current created by the bias current circuit 12 is copied. In the current mirror circuit, the transistor Tr10 is a current reference source transistor, and the transistor Tr12 is a current copy side transistor.

  The NMOS transistor Tr13 has a source connected to the drain of the transistor Tr12 and a gate to which the buffer load control signal CC is applied together with the gate of the PMOS transistor Tr14. The drain of the NMOS transistor Tr13 is connected to the node A between the gate and drain of the PMOS transistor Tr15 and the drain of Tr14. Further, the node A is connected to the source of the PMOS transistor Tr20 and the wiring B + through the first CMOS switch, and is connected to the source of the PMOS transistor Tr21 and the wiring B− through the second CMOS switch. ing. The first CMOS switch and transistor Tr20 are controlled by a signal CB +, and the second CMOS switch and transistor Tr21 are controlled by a signal CB-.

  The transistor Tr14 serves to set the potential of the gate line of the transistor Tr15 to VDD when the buffer load control signal CC is at a low level. That is, when the buffer load control signal CC is set to the low level, if there is no transistor Tr14, the gate and source potentials of the transistor Tr15 become indefinite potentials, so that an inadvertent current may flow through the transistor Tr15. There is. Therefore, the transistor Tr14 is provided to reliably turn off the transistor Tr15.

  The feature of the liquid crystal display device 10 of the present embodiment is that two types are connected separately to the gates of the constant current load transistors Tr7 and Tr8 in two types of source follower buffers, a positive polarity side and a negative polarity side. The wiring B + and the wiring B- are driven in a current mirror manner by one current reference source transistor Tr15 via the first and second CMOS switches. The timings of the first and second CMOS switches are controlled so that one of the first and second CMOS switches is turned on by the signal CB + and the signal CB-.

  The gate lengths and gate widths of the constant current load transistors Tr7 and Tr8 are the same as the gate length and gate width of the current reference source transistor Tr15, respectively. In this way, the current value generated by the current reference source transistor Tr15 is the constant current load transistor used for the two types of source follower buffers by shifting the timing by controlling the signal CB + and the signal CB-. Copied to Tr7 and Tr8.

  Furthermore, the constant current load transistors Tr7 and Tr8 have the same layout of wiring and the like so as to have equivalent parasitic capacitance. As a result, in the constant current load transistors Tr7 and Tr8, since the voltage swing due to noise becomes the same, there is no bias in the copy current, and the constant current for biasing to the positive video signal and the negative video signal is reduced. It becomes possible to supply the same.

  When driving the wiring B +, timing control is performed so that the signal CB + is at a high level and the signal CB- is at a low level. As a result, the first CMOS switch is turned on, the transistor Tr20 is turned off, and the node A and the wiring B + are turned on. On the other hand, the second CMOS switch is turned off, and the transistor Tr21 is turned on. As a result, the node A and the wiring B− are disconnected, the wiring B− is raised to the power supply voltage VDD, and the constant current load transistor Tr8 is turned off.

  In this state, when the buffer load control signal CC is set to a high level by timing control, the current mirror circuits of the transistors Tr10, Tr12, Tr15, Tr7 are all turned on, and are created by the bias current circuit 12 and copied to the transistor Tr12. The current that has flowed further flows through the transistors Tr13 and Tr15, and the current that flows through the transistor Tr15 is copied to the constant current load transistor Tr7 using the transistor Tr15 as a current reference source transistor. As a result, the transistor Tr3 operates as a source follower, and the transistor Tr4 does not operate. At this time, the wiring S + becomes high level, the transistor Tr5 is turned on, and the source potential of the transistor Tr3 is applied to the pixel electrode PE. At this time, the wiring S- is at a low level and the transistor Tr6 is turned off, so that the source node of the transistor Tr4 and the pixel electrode PE become non-conductive.

  On the other hand, when driving the wiring B-, the timing control is performed so that the signal CB- is at a high level and the signal CB + is at a low level. As a result, the second CMOS switch is turned on, the transistor Tr21 is turned off, and the node A and the wiring B- are turned on. On the other hand, the first CMOS switch is turned off, and the transistor Tr20 is turned on. As a result, the node A and the wiring B + are disconnected, the wiring B + is raised to the power supply voltage VDD, and the constant current load transistor Tr7 is turned off.

  In this state, when the buffer load control signal CC is set to the high level by the timing control, the current mirror circuits of the transistors Tr10, Tr12, Tr15, Tr8 are all turned on, and are created by the bias current circuit 12 and copied to the transistor Tr12. The current that has flowed further flows through the transistors Tr13 and Tr15, and the current that flows through the transistor Tr15 is copied to the constant current load transistor Tr8 using the transistor Tr15 as a current reference source transistor. Thereby, Tr4 operates as a source follower, and the transistor Tr3 does not operate. At this time, the wiring S- becomes high level, the transistor Tr6 is turned on, and the source potential of the transistor Tr4 is applied to the pixel electrode PE. At this time, the wiring S + is at a low level and the transistor Tr5 is turned off, so that the source node of the transistor Tr3 and the pixel electrode PE become non-conductive. Thereafter, the driving operations for the wiring B + and the wiring B- are alternately repeated at a predetermined cycle shorter than one vertical scanning cycle.

  Next, the operation of the present embodiment will be further described with reference to the timing chart of FIG. 2A shows the vertical synchronization signal VD, FIG. 2B shows the buffer load control signal CC, FIGS. 2C and 2D show the signals CB + and CB−, and FIG. ), (F) are the positive polarity load characteristic control signals and negative polarity load characteristic control signals supplied to the wirings B +, B-, and (G), (H) are supplied to the wirings S +, S-. A positive polarity gate control signal and a negative polarity gate control signal are shown.

  First, a description will be given of pixel writing. When the row selection signal of the j-th row in one vertical scanning period supplied through the row scanning line Gj becomes high level for a predetermined period, the NMOS transistors Tr1 and Tr2 are simultaneously turned on for the predetermined period. On the other hand, at this time, both the load characteristic control signals of the wirings B + and B− are at the high level, and the transistors Tr7 and Tr8 are both in the off state.

  In this state, the positive video signal supplied via the positive data line Di + is sampled by the NMOS transistor Tr1 and held in the positive holding capacitor Cs1. In parallel with this, a negative polarity video signal having the same video information as the positive polarity video signal but having a reverse polarity is input via the negative polarity data line Di-, and is sampled by the NMOS transistor Tr2 to be negative polarity. Is held in the holding capacitor Cs2.

  FIG. 3 shows the relationship from the black bell to the white level of the positive video signal and the negative video signal. In the figure, the positive polarity video signal a is the black level of the minimum gradation when the level is the minimum, and the white level of the maximum gradation when the level is the maximum, whereas the level of the negative polarity video signal b is the minimum. Is the white level of the maximum gradation, and the black level of the minimum gradation when the level is the maximum. The positive polarity video signal a and the negative polarity video signal b have opposite polarities, and their inversion centers are indicated by c.

  The positive video signal a and the negative video signal b supplied to the data lines Di + and Di- are voltages obtained by digital-analog conversion of the digital video signal in units of pixels. The method and the holding method itself in the holding capacity are performed as follows in the same manner as in Patent Document 1.

  When the digital video signal is 10 bits, the positive polarity of the 1H cycle increases monotonically within one horizontal scanning period (1H) from the digital data value “000” (black level) to the digital data value “3FF” (white level). Pixels of the ramp signal RAMP1 + and a negative ramp signal RAMP1- with a 1H cycle that monotonically decreases within 1H from the digital data value “000” (black level) to the digital data value “3FF” (white level) are pixels of one line. The number of sets corresponding to the number is simultaneously supplied to each video switch. Here, each set of video switches includes a positive video switch supplied with a positive ramp signal RAMP1 + and a negative video switch supplied with a negative ramp signal RAMP1-.

  Then, after all sets of video switches are simultaneously turned on every time the horizontal scanning period starts, a counter value (reference gradation data) indicating a gradation obtained by counting clocks synchronized with the ramp signals RAMP1 + and RAMP1- with a counter. ) And the pixel value of the digital video signal in units of pixels of one line, a coincidence pulse is output when they coincide. Then, a pair of video switches provided corresponding to the comparators that output coincidence pulses are turned off at the same time, and a pair of data lines Di + and Di- connected to the set of video switches that are turned off are connected. The voltage of the ramp signals RAMP1 + and RAMP1- is sampled at one pixel connected to the data lines Di + and Di- through the sampling lines and supplied to the positive holding capacitor Cs1 and the negative holding capacitor Cs2. Then, sampling and holding of the signal voltage obtained by converting the digital video signal into the analog video signal is performed.

  Subsequently, the row selection signal of the gate line Gj becomes low level, and the following pixel reading operation is performed during that time.

  First, an operation in the period T1 in which the signal CB + illustrated in FIG. During this period T1, the signal CB- is at a low level as shown in FIG. Accordingly, the first CMOS switch including the transistors Tr16 and Tr17 is turned on, the transistor Tr20 is turned off, and the node A and the wiring B + are in a conductive state.

  In this state, the positive polarity side switching transistor Tr5 is turned on during a predetermined period T2 (<T1) in which the positive polarity gate control signal of the wiring S + is at a high level as shown in FIG. Within the period T2, the buffer load control signal CC is set to a high level as shown in FIG. As a result, as described above, the transistor Tr13 is turned on, and a current mirror circuit is formed in which the transistor Tr15 is a current reference source transistor and the constant current load transistor Tr7 is a current copy side transistor, and between the drain and source of the transistor Tr15. The flowing current is copied to the constant current load transistor Tr7 through the wiring B + as a laurel positive load characteristic control signal as shown in FIG.

  As a result, the positive-side source follower buffer including the transistors Tr3 and Tr7 becomes active, and the positive-polarity video signal (digital-analog conversion voltage) held in the positive-polarity holding capacitor Cs1 is positive-side source follower buffer. Through the pixel electrode PE to be charged. At this time, even if the buffer load control signal CC is set to the high level, only the positive load characteristic control signal of the wiring B + is set to the low level.

  When the potential VPE of the pixel electrode PE is fully charged as shown in FIG. 2 (I) by the positive video signal as described above, the buffer load control signal CC is shown in FIG. 2 (B). As shown, the transistor Tr14 is turned on, and the potential of the node A, that is, the positive load characteristic control signal of the wiring B + is set to the high level as shown in FIG. Further, after a period T2, the positive polarity gate control signal of the wiring S + is switched to the low level as shown in FIG. As a result, the positive polarity side source follower buffer is turned off and the pixel electrode PE is floated, and the positive polarity driving voltage is held in the liquid crystal capacitor.

  In the period T2 and the subsequent period, the negative polarity load characteristic control signal of the wiring B- remains at a high level as shown in FIG. 2F, and the negative polarity side source follower buffer including the transistors Tr4 and Tr8 Since no current flows and the negative polarity gate control signal of the wiring S− remains at a low level and the switching transistor Tr6 is off as shown in FIG. 5H, the pixel electrode PE is connected to the negative polarity through the transistor Tr4. The holding voltage of the sexual holding capacitor Cs2 is not applied.

  Subsequently, the signal CB− becomes high level during the period T3 (= T1) as shown in FIG. 2D, and the signal CB + becomes low level as shown in FIG. 2C. Accordingly, the second CMOS switch including the transistors Tr18 and Tr19 is turned on, the transistor Tr21 is turned off, and the node A and the wiring B− are turned on.

  In this state, the negative polarity side switching transistor Tr6 is turned on during a predetermined period T4 (<T3) in which the negative polarity gate control signal of the wiring S− is at a high level as shown in FIG. Within the period T4, the buffer load control signal CC is set to the high level as shown in FIG. As a result, as described above, the transistor Tr13 is turned on, and a current mirror circuit in which the transistor Tr15 is a current reference source transistor and the constant current load transistor Tr8 is a current copy side transistor is configured, and between the drain and source of the transistor Tr15. The flowing current is copied to the constant current load transistor Tr8 through the wiring B− as a low level negative load characteristic control signal as shown in FIG.

  As a result, the negative polarity side source follower buffer composed of the transistors Tr4 and Tr8 becomes active, and the negative polarity video signal (digital-analog conversion voltage) held in the negative polarity holding capacitor Cs2 becomes negative polarity side source follower buffer. Through the pixel electrode PE to be charged. At this time, even if the buffer load control signal CC is set to the high level, only the negative load characteristic control signal of the wiring B- is set to the low level.

  When the potential VPE of the pixel electrode PE is completely charged as shown in FIG. 2 (I) by the negative video signal as described above, the buffer load control signal CC is shown in FIG. 2 (B). As shown, the transistor Tr14 is turned on, and the potential of the node A, that is, the negative load characteristic control signal of the wiring B- is set to the high level as shown in FIG. Further, after the period T4, the negative polarity gate control signal of the wiring S- is switched to the low level as shown in FIG. As a result, the negative polarity side source follower buffer is turned off and the pixel electrode PE is floated, and the negative polarity drive voltage is held in the liquid crystal capacitor.

  In the period T4 and the subsequent period, the positive load characteristic control signal of the wiring B + remains at the high level as shown in FIG. 2E, and the positive polarity side source follower buffer including the transistors Tr3 and Tr7 Since no current flows and the positive gate control signal of the wiring S + remains at a low level and the switching transistor Tr5 is off as shown in FIG. 5G, the pixel electrode PE is connected to the positive electrode through the transistor Tr5. The holding voltage of the sexual holding capacitor Cs1 is not applied.

  Thereafter, as shown in FIG. 2, the above operation is alternately repeated at a predetermined cycle shorter than one vertical scanning cycle. As a result, the drive voltage VPE shown in FIG. 2 (I) that is converted into an alternating current with positive and negative video signals is applied to the pixel electrode PE.

  On the other hand, as shown in FIG. 2 (J), the common electrode CE of the liquid crystal display element LC is inverted in synchronization with pixel electrode voltage polarity switching with respect to a reference level substantially equal to the reference level Vc of pixel electrode potential. A common voltage Vcom is applied. As a result, the absolute value of the potential difference between the common voltage Vcom and the drive voltage VPE of the pixel electrode PE is always the same. Since the voltage applied to the liquid crystal layer LCM of the liquid crystal display element LC is a potential difference between the common voltage Vcom and the drive voltage VPE of the pixel electrode PE, the switching of the direct current component as shown in FIG. There is no AC voltage VLC. In this manner, in the pixel shown in FIG. 1, the amplitude of the drive voltage VPE can be reduced by switching the common voltage Vcom applied to the common electrode CE in the opposite phase to the drive voltage VPE of the pixel electrode PE. The transistor breakdown voltage and power consumption on the circuit side can be reduced.

  Similarly to the liquid crystal display device described in Patent Document 1, the liquid crystal display device 10 according to the present embodiment is not configured to transfer the held charge directly to the pixel electrode, but to transfer the voltage via the source follower buffer. Even if charging / discharging is repeated with positive and negative polarity, there is no problem of charge neutralization, and driving without attenuation of voltage level can be realized.

  Further, according to the liquid crystal display device 10 of the present embodiment, a current is supplied only to the source follower buffer on the reading side of the positive side source follower buffer and the negative side source follower buffer, and the other side Since the current follower buffer is controlled so that no current flows, the current consumption can be reduced to ½ compared to the liquid crystal display device described in Patent Document 1.

  Furthermore, in the liquid crystal display device 10 of the present embodiment, both the wiring B + and the wiring B- are driven by the same current reference source transistor Tr15 constituting the current mirror circuit at different timings. A good display image in which random horizontal lines as shown can not be seen can be obtained.

  For example, when the current value of the transistor Tr15 is slightly increased due to the variation in Vth and a current of 1.05 μA flows with respect to the target of 1 μA, among the constant current load transistors Tr7 and Tr8, A current of 1.05 μA flows (when the gate length and the gate width of the transistor Tr15 and the constant current load transistors Tr7 and Tr8 are the same). At this time, a current of 1.05 μA also flows through the source follower transistor in the on state among the source follower transistors Tr3 and Tr4.

  The source follower transistor shifts the source voltage in the VDD direction when a large amount of current flows and the source voltage shifts in the GND direction when a small amount of current flows. Therefore, the source voltages of the source follower transistors Tr3 and Tr4 are both in the VDD direction in the above case. Will shift to. At this time, as shown in FIG. 3, the pixel electrode voltage VPE shifts in the white level direction for the positive video signal and in the black level direction for the negative video signal.

  However, in the liquid crystal display device 10 according to the present embodiment, as described above, the positive-side source follower buffer including the source follower transistor Tr3 and the constant current load transistor Tr7, the source follower transistor Tr4, and the constant current load transistor Tr8. The negative polarity side source follower buffer is turned on alternately in synchronization with the switching of the polarity switching switching transistors Tr5 and Tr6, so that the positive polarity video signal shifted in the level direction is applied to the pixel electrode PE. Since the negative video signal is alternately applied, the average video signal canceled by the human eye is recognized, and a normal copy current of 1 μA alternately flows through the transistors Tr7 and Tr8 in the display image. Video with the same brightness as the The line is no longer visible.

  Contrary to the above, when the current value of the transistor Tr15 slightly decreases due to the variation in Vth and a current of 0.95 μA flows against the target of 1 μA, the constant current load transistors Tr7 and Tr8 are in the on state. A current of 0.95 μA flows through the current load transistor (when the gate length and the gate width of the transistor Tr15 and the constant current load transistors Tr7 and Tr8 are the same). At this time, a current of 0.95 μA also flows through the source follower transistor in the on state among the source follower transistors Tr3 and Tr4.

  The source follower transistor shifts the source voltage in the VDD direction when a large amount of current flows, and the source voltage shifts in the GND direction when a small amount of current flows. Therefore, in both cases, the source voltages of the source follower transistors Tr3 and Tr4 are in the GND direction. Will shift to. At this time, as shown in FIG. 3, the pixel electrode voltage VPE shifts in the black level direction for the positive video signal and in the white level direction for the negative video signal.

  However, in the liquid crystal display device 10 of the present embodiment, the positive video signal and the negative video signal shifted in the above-described level direction are alternately applied to the pixel electrode PE, so that they are canceled out by human eyes. The average video signal is recognized, and the display image becomes an image having the same luminance as a line in which normal copy currents 1 μA flow alternately in the transistors Tr7 and Tr8, and random horizontal lines are not visible.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. FIG. 4 shows a circuit diagram of a main part of the second embodiment of the liquid crystal display device according to the present invention. In the figure, the same components as those in FIG.

  The liquid crystal display device 20 according to the present embodiment is provided at each intersection of a plurality of data lines each including two data lines (column signal lines) and a plurality of gate lines (row scanning lines). The pixels are arranged in a matrix, and in each of these pixels, the positive video signal and the negative video signal are separately sampled and held in two holding capacitors, and then the holding voltages are alternately applied to the pixel electrodes to liquid crystal The basic configuration for AC drive of the display element is the same as that of Patent Document 1, but each pixel is represented by a single pixel circuit 11 shown in FIG. 4, and a bias current circuit 12 and a load characteristic control signal generation circuit are provided. 21. The liquid crystal display device 20 of the present embodiment is different from the liquid crystal display device 10 shown in FIG. 1 in the configuration of the load characteristic control signal generation circuit 21 from the load characteristic control signal generation circuit 13 of FIG.

  As in the liquid crystal display device 10, the liquid crystal display device 20 performs digital-analog conversion of the digital video signal using the positive polarity ramp signal and the negative polarity ramp signal, and converts the converted voltage to the data lines of each set. There are means for supplying pixels in units of one line and signal generating means for generating various signals such as gate signals, control signals, switching signals, and row selection signals, but these means are directly related to the present invention. Further, since it is the same as the liquid crystal display device described in Patent Document 1, illustration and detailed description thereof are omitted.

  The liquid crystal display device 20 of the present embodiment is characterized by two types of wiring B connected to the gates of the constant current load transistors Tr7 and Tr8 used for a total of two types of source follower buffers on the positive polarity side and the negative polarity side. A current mirror circuit for supplying positive load characteristic control signals and negative load characteristic control signals to the + and B− via the first and second CMOS switches is speeded up using a source follower circuit. In the point.

  In FIG. 4, the load characteristic control signal generation circuit 21 is connected to a set of wirings B + and B− that are commonly connected to a plurality of pixels (1980 pixels in the case of full high-definition) in the same row for each row. A circuit that generates and supplies a load characteristic control signal for positive polarity and a load characteristic control signal for negative polarity, respectively. The total number of rows in the pixel portion (the number of pixels in the vertical direction, 1080 pixels in the case of full high-definition) ))).

  The load characteristic control signal generating circuit 21 includes NMOS transistors Tr12 and Tr13, PMOS transistors Tr22, Tr23 and Tr24, PMOS transistor Tr16, NMOS transistor Tr17 and inverter I1 constituting the first CMOS switch, and a second CMOS switch. It comprises a PMOS transistor Tr18, NMOS transistor Tr19 and inverter I2, and PMOS transistors Tr20 and Tr21.

  The NMOS transistor Tr12 has a gate connected to the gate and drain of the NMOS transistor Tr10 to form a current mirror circuit with the transistor Tr10, and the bias current created by the bias current circuit 12 is copied. In the current mirror circuit, the transistor Tr10 is a current reference source transistor, and the transistor Tr12 is a current copy side transistor.

  The NMOS transistor Tr13 has a source connected to the drain of the transistor Tr12 and a gate to which the buffer load control signal CC is applied together with the gate of the PMOS transistor Tr22. Further, the PMOS transistor Tr23 is a current reference transistor of a current mirror circuit similarly to the PMOS transistor Tr15 of FIG. 1, but unlike the Tr15, the gate and the drain are not connected, and the gate of the Tr23 is a PMOS transistor. The transistor Tr24 is connected to a connection point (node A) between the source of the transistor Tr24 and the resistor R2, and the drain of Tr23 is connected to the drain of the NMOS transistor Tr13 and the gate of the PMOS transistor Tr24. The node A incorporates a source follower circuit composed of a transistor Tr24 and a resistor R2.

  Further, the node A is connected to the source of the PMOS transistor Tr20 and the wiring B + through the first CMOS switch, and is connected to the source of the PMOS transistor Tr21 and the wiring B− through the second CMOS switch. ing. The first CMOS switch and transistor Tr20 are controlled by a signal CB +, and the second CMOS switch and transistor Tr21 are controlled by a signal CB-. Unlike the transistor Tr14 of FIG. 1, the drain of the transistor Tr22 controlled by the buffer load control signal CC is connected to the gate of the transistor Tr24.

  When driving the wiring B +, timing control is performed so that the signal CB + is at a high level and the signal CB- is at a low level. Accordingly, the first CMOS switch is turned on and the transistor Tr20 is turned off, so that the node A and the wiring B + are brought into conduction. On the other hand, since the second CMOS switch is turned off and the transistor Tr21 is turned on, the node A and the wiring B- are disconnected, the wiring B- is raised to the power supply voltage VDD, and the constant current load transistor Tr8 is turned off. Become.

  In this state, when the buffer load control signal CC is set to the high level by timing control, the transistor Tr13 is turned on and the transistor Tr22 is turned off, and the current mirror circuit having the source follower circuit configuration is activated. As a result, the positive load characteristic control signal of the wiring B + becomes low level, and the constant current load transistor Tr7 is turned on, so that the transistor Tr3 operates as a source follower and the transistor Tr4 does not operate.

  On the other hand, when driving the wiring B-, the timing control is performed so that the signal CB- is at a high level and the signal CB + is at a low level. Accordingly, the second CMOS switch is turned on and the transistor Tr21 is turned off, so that the node A and the wiring B− are brought into conduction. On the other hand, since the first CMOS switch is turned off and the transistor Tr20 is turned on, the node A and the wiring B + are disconnected, the wiring B + is raised to the power supply voltage VDD, and the constant current load transistor Tr7 is turned off. Become.

  In this state, when the buffer load control signal CC is set to a high level by timing control, the transistor Tr13 is turned on and the transistor Tr22 is turned off as described above, and the current mirror circuit having the source follower circuit configuration is activated. As a result, the negative load characteristic control signal of the wiring B− becomes a low level and the constant current load transistor Tr8 is turned on, so that the transistor Tr4 operates as a source follower and the transistor Tr3 does not operate.

  When the buffer load control signal CC is set to the low level, the transistor Tr13 is turned off and the transistor Tr22 is turned on, so that the transistor Tr24 is turned off. As a result, the node A is charged to the power supply voltage VDD by the resistor R2. The At this time, if the signal CB + is at a high level and the signal CB- is at a low level, the wiring B + becomes high because it is electrically connected to the node A, and the transistor Tr7 is controlled to be off. At this time, if the signal CB + is at a low level and the signal CB- is at a high level, the wiring B- becomes conductive because it is electrically connected to the node A, and the transistor Tr8 is controlled to be off.

  The timings of the gate control signals to the wiring S + and the wiring S-, the load characteristic control signals to the wiring B + and the wiring B-, the buffer load control signal CC, the signal CB + and the signal CB- The operation of the liquid crystal display device 20 shown in FIG. 4 is the same as that described with reference to the timing chart of FIG.

  In the liquid crystal display device 20 of the present embodiment, the current mirror circuit has a source follower configuration, so that the current mirror circuit has a normal current mirror configuration as in the liquid crystal display device 10 of the first embodiment. This has the advantage of speeding up the operation.

  That is, the current mirror circuit shown in FIG. 1 has a function of copying a current. For example, when attempting to copy a current of 1 μA, the gate length and gate of both the current reference source transistor and the current copy side transistor When the width is the same, only 1 μA of the current value to be copied flows between the source and drain of the current reference source transistor.

  The transistor Tr15, which is the current reference source transistor of FIG. 1, has a gate and a drain connected, and the current of 1 μA until the selected wiring B + or wiring B− reaches a predetermined voltage (gate voltage for copying 1 μA). Therefore, it is necessary to charge the capacitance of the wiring B + or the wiring B− (capacity of the node A), so that it takes time to reach a predetermined voltage, and it takes time for the copy current to reach 1 μA.

  For example, the capacitance of the wiring B + or the wiring B− in the case of full high vision includes a parasitic capacitance between the wirings and a gate MOS capacitance for 1980 pixels in the horizontal direction, and is about 3 pF in total. The gate bias of 1 μA flowing between the drain and source of the transistor Tr15 which is a PMOS transistor is about 4.3V. In addition, since the transistor Tr15 is a PMOS transistor, the gate voltage for charging is 1.2V, and if a capacitor of 3 pF is charged with 1 μA, 3.6 μsec is required from the relationship of C × V = I × T. . In this case, the speed can be increased by changing the ratio of the gate width of the current reference source transistor to the current copy side transistor. However, when it is desired to increase the gate width by 100 times, the gate width of the current reference source transistor is set to the current copy side transistor. It must be 100 times the gate width, which is not practical due to the layout area problem.

  On the other hand, in the current mirror circuit using the source follower circuit composed of the transistor Tr24 and the resistor R2 in the liquid crystal display device 20 of the present embodiment shown in FIG. 4, the transistor Tr23 which is a current reference source transistor has a node A and a drain. Are not connected, the node A cannot be charged with 1 μA flowing between the source and drain of the current reference transistor Tr23. Therefore, current is supplied from the power supply VDD connected to the source follower circuit until the node A reaches a predetermined voltage (gate voltage for copying 1 μA) after the buffer load control signal CC is turned on (high level). Charging, the charging time is short.

For example, the capacitance of the selected gate wiring B + (capacity of the node A) when the buffer load control signal CC is turned on (high level) and the signal CB + is high level is the parasitic capacitance between the wirings. There is a gate MOS capacity of 1980 pixels in the horizontal direction in full high vision, and the total is about 3 pF. At this time, for example, if the resistance value of the resistor R2 of the source follower circuit is 4 kΩ, the time required for the node A to reach a predetermined voltage (gate voltage for copying 1 μA) is 0.012 μsec (= 3 × 10 ^−12). × 4 × 10 ³ ), which is 300 times faster than the configuration of FIG.

  However, in the current mirror circuit having this source follower circuit, the elements such as the Vth variation of the current reference source transistor Tr23, the Vth variation of the source follower transistor Tr24, and the resistance value variation of the resistor R2 are summed, and a current mirror having a normal configuration is obtained. The variation increases compared to the circuit. However, in the same way as the operation of the current mirror circuit of the normal configuration in the first embodiment, the liquid crystal display device 20 of the present embodiment also displays the positive video signal and the negative video signal alternately. This has the effect of canceling the luminance variation due to the wiring B + or the wiring B-, and as a result, the expected effect that the horizontal lines with different luminance on the screen as shown in FIG. can get.

  The present invention is not limited to the above embodiment. For example, the channel of each transistor in the one pixel circuit 11, the bias current circuit 12, and the load characteristic control signal generation circuits 13 and 21 is different from the embodiment. A reverse channel (that is, an N channel in the case of the P channel and a P channel in the case of the N channel) may be used. However, in this case, for example, the VDD wiring as the power supply wiring is a GND wiring, and the GND wiring is a VDD wiring.

10, 20 Liquid crystal display device 11 One pixel circuit 12 Bias current circuit 13, 21 Load characteristic control signal generation circuit Di +, Di- Data line (column signal line)
S + Positive gate control signal wiring S- Negative gate control signal wiring B + Positive load characteristic control signal wiring B- Negative load characteristic control signal wiring Tr1, Tr2 Pixel selection NMOS transistors Tr3, Tr4, Tr24 Source follower PMOS transistor Tr5, Tr6 Switching NMOS transistor Tr7, Tr8 Constant current load PMOS transistors Tr10, Tr12, Tr13 NMOS transistors Tr11, Tr14, Tr20, Tr21, Tr22, Tr24 PMOS transistors Tr15, Tr23 Current reference source PMOS transistor Tr16 Tr18 CMOS switch PMOS transistor Tr17, Tr19 CMOS switch NMOS transistor I1, I2 Inverter Cs1 Positive holding capacitor (first holding capacitor)
Cs2 Negative polarity holding capacity (second holding capacity)
LC liquid crystal display element PE pixel electrode CE common electrode LCM display body (liquid crystal layer)

Claims (6)

Each of a plurality of pixels provided at intersections where a plurality of sets of data lines and a plurality of row scanning lines intersect each other with two data lines as one set,
A display element in which a liquid crystal layer is sandwiched between a pixel electrode and a common electrode facing each other;
First sampling and holding means for sampling a positive video signal supplied via one of the two data lines in a set and holding it in a first holding capacitor for a certain period;
The negative polarity video signal having the opposite polarity to the positive polarity video signal supplied through the other data line of the set of the two data lines is sampled and held in the second holding capacitor for a certain period. Second sampling and holding means;
A first source follower transistor and a first constant current load transistor having a drain connected to the source of the first source follower transistor, the first constant current load transistor being connected to the gate of the first constant current load transistor A first source follower buffer that operates when a first load characteristic control signal is input through the wiring of
A second source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor, and a second constant current load transistor connected to the gate of the second constant current load transistor. A second source follower buffer that operates when a second load characteristic control signal is input through the wiring of
A positive signal voltage corresponding to the positive video signal held in the first holding capacitor, which is input through the first source follower buffer during operation, and the second source follower buffer during operation Switching means for alternately switching the negative polarity signal voltage corresponding to the negative polarity video signal held in the second holding capacitance inputted through the switching circuit at a predetermined cycle shorter than the vertical scanning cycle, to the pixel electrodes. When,
With
When the positive polarity signal voltage is applied to the pixel electrode by the switching means, the first wiring and a predetermined node are brought into conduction, and the negative polarity signal voltage is applied to the pixel electrode by the switching means. A wiring switching means for switching the second wiring and the predetermined node to be in a conductive state;
When a gate and a drain are connected to the predetermined node and the predetermined node and the first wiring are in a conductive state by the wiring switching unit, the first constant current load transistor is connected to a current copy side transistor. As a first current mirror circuit for supplying the first load characteristic control signal, and when the predetermined node and the second wiring are in a conductive state by the wiring switching means, The same current mirror current reference source transistor constituting a second current mirror circuit for supplying the second load characteristic control signal using a constant current load transistor as a current copy side transistor;
A liquid crystal display device comprising: The first and second wirings, the wiring switching unit, and the current mirror current reference source transistor are provided in common to a plurality of the pixels in the same row,
2. The liquid crystal display device according to claim 1, wherein a gate length and a gate width of the first constant current load transistor are the same as a gate length and a gate width of the second constant current load transistor. Each of a plurality of pixels provided at intersections where a plurality of sets of data lines and a plurality of row scanning lines intersect each other with two data lines as one set,
A display element in which a liquid crystal layer is sandwiched between a pixel electrode and a common electrode facing each other;
First sampling and holding means for sampling a positive video signal supplied via one of the two data lines in a set and holding it in a first holding capacitor for a certain period;
The negative polarity video signal having the opposite polarity to the positive polarity video signal supplied through the other data line of the set of the two data lines is sampled and held in the second holding capacitor for a certain period. Second sampling and holding means;
A first source follower transistor and a first constant current load transistor having a drain connected to the source of the first source follower transistor, the first constant current load transistor being connected to the gate of the first constant current load transistor A first source follower buffer that operates when a first load characteristic control signal is input through the wiring of
A second source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor, and a second constant current load transistor connected to the gate of the second constant current load transistor. A second source follower buffer that operates when a second load characteristic control signal is input through the wiring of
A positive signal voltage corresponding to the positive video signal held in the first holding capacitor, which is input through the first source follower buffer during operation, and the second source follower buffer during operation Switching means for alternately switching the negative polarity signal voltage corresponding to the negative polarity video signal held in the second holding capacitance inputted through the switching circuit at a predetermined cycle shorter than the vertical scanning cycle, to the pixel electrodes. When,
With
When the positive polarity signal voltage is applied to the pixel electrode by the switching means, the first wiring and a predetermined node are brought into conduction, and the negative polarity signal voltage is applied to the pixel electrode by the switching means. A wiring switching means for switching the second wiring and the predetermined node to be in a conductive state;
A source follower circuit including a first transistor having a source connected to the predetermined node and a resistor having one end connected to the predetermined node;
When the gate is connected to the predetermined node and the predetermined node and the first wiring are in a conductive state by the wiring switching means, the first constant current load transistor is used as a current copy side transistor as the source. When a first current mirror circuit that supplies the first load characteristic control signal through a follower circuit is configured and the predetermined node and the second wiring are in a conductive state by the wiring switching unit, the first current mirror circuit is supplied. A second transistor of the same current mirror current reference source constituting a second current mirror circuit that supplies the second load characteristic control signal through the source follower circuit using two constant current load transistors as current copy side transistors; ,
Have
The liquid crystal display device, wherein the gate of the first transistor is connected to the drain of the second transistor. The first and second wirings, the wiring switching unit, the source follower circuit, and the second transistor that is the current mirror current reference source are provided in common for the plurality of pixels in the same row. ,
4. The liquid crystal display device according to claim 3, wherein a gate length and a gate width of the first constant current load transistor are the same as a gate length and a gate width of the second constant current load transistor. A liquid crystal layer is sandwiched between a pixel electrode and a common electrode provided at intersections where a plurality of data lines and a plurality of row scanning lines intersect each other, each having two data lines. For a plurality of pixels including a display element,
The positive video signal supplied through one data line of the set of the two data lines is sampled and held in the first holding capacitor for a certain period, and at the same time, the set of the two data lines A sampling and holding step for sampling a negative polarity video signal having a polarity opposite to that of the positive polarity video signal and holding the negative polarity video signal in a second holding capacitor for a certain period, which is supplied via the other data line,
A first source follower buffer comprising a first source follower transistor and a first constant current load transistor having a drain connected to the source of the first source follower transistor comprises the first constant current load transistor. A positive signal voltage corresponding to the positive video signal held in the first holding capacitor to be transferred when a first load characteristic control signal is supplied to the gate of the first via a first wiring; A second source follower buffer comprising a second source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor. When the second load characteristic control signal is supplied to the gate via the second wiring, the gate holds the second holding capacitor. Wherein the negative polarity signal voltage corresponding to the negative polarity video signal, a switching step of switching a short predetermined period from the vertical scanning period is alternately applied to the pixel electrodes,
When the positive polarity signal voltage is applied to the pixel electrode by the switching step, the first wiring and a predetermined node are brought into conduction, and a current mirror having a gate and a drain connected to the predetermined node A first load characteristic control signal supply step for supplying the first load characteristic control signal from a current reference source transistor using the first constant current load transistor as a current copy side transistor;
When the negative signal voltage is applied to the pixel electrode by the switching step, the second wiring and the predetermined node are brought into conduction, and the second constant current is supplied from the current mirror current reference source transistor. A second load characteristic control signal supply step for supplying the second load characteristic control signal using a load transistor as a current copy side transistor;
A method for driving a liquid crystal display device, comprising: A liquid crystal layer is sandwiched between a pixel electrode and a common electrode provided at intersections where a plurality of data lines and a plurality of row scanning lines intersect each other, each having two data lines. For a plurality of pixels including a display element,
The positive video signal supplied through one data line of the set of the two data lines is sampled and held in the first holding capacitor for a certain period, and at the same time, the set of the two data lines A sampling and holding step for sampling a negative polarity video signal having a polarity opposite to that of the positive polarity video signal and holding the negative polarity video signal in a second holding capacitor for a certain period, which is supplied via the other data line,
A first source follower buffer comprising a first source follower transistor and a first constant current load transistor having a drain connected to the source of the first source follower transistor comprises the first constant current load transistor. A positive signal voltage corresponding to the positive video signal held in the first holding capacitor to be transferred when a first load characteristic control signal is supplied to the gate of the first via a first wiring; A second source follower buffer comprising a second source follower transistor and a second constant current load transistor having a drain connected to the source of the second source follower transistor. When the second load characteristic control signal is supplied to the gate via the second wiring, the gate holds the second holding capacitor. Wherein the negative polarity signal voltage corresponding to the negative polarity video signal, a switching step of switching a short predetermined period from the vertical scanning period is alternately applied to the pixel electrodes,
When the positive polarity signal voltage is applied to the pixel electrode by the switching step, the source includes a first transistor having a source connected to a predetermined node and a resistor having one end connected to the predetermined node. A second current mirror current reference source in which the first wiring and the predetermined node are rendered conductive through a follower circuit, a gate is connected to the predetermined node, and a drain is connected to the gate of the first transistor. A first load characteristic control signal supply step of supplying the first load characteristic control signal from the transistor using the first constant current load transistor as a current copy side transistor;
When the negative signal voltage is applied to the pixel electrode by the switching step, the second wiring and the predetermined node are brought into conduction through the source follower circuit, and the second current source of current mirror current is referred to. A second load characteristic control signal supply step of supplying the second load characteristic control signal from the transistor using the second constant current load transistor as a current copy side transistor;
A method for driving a liquid crystal display device, comprising: