JP2006208501A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent occurrence of a striped pattern and flicker on a displayed picture even when prevention of occurrence of a high frequency noise resulting from discharge instability is difficult, and installing of a close shield for complete prevention of jumping of the high frequency noise into an image signal circuit is difficult. <P>SOLUTION: In a construction provided with a driving state switching means 2 to switch between a discharge tube driving state in which a discharge tube 7 is driven by a booster circuit 6 and a driving stopping state in which the driving is stopped, the driving state switching means 2 switches between the discharge tube driving state and the driving stopping state with an initial stage ready period in an initial driving time interval which is a time interval to start driving the discharge tube 7, wherein the initial stage ready period is set to be a period of reduction of a noise component produced by driving the discharge tube at a frequency which is a frequency of horizontal scanning frequency multiplied with an integer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display device having a drive state switching means for switching between a discharge tube drive state in which a booster circuit drives a discharge tube and a drive stop state in which the drive is stopped. The present invention relates to a liquid crystal display device that switches between a discharge tube drive state and a drive stop state at a cycle in which a noise component generated by driving a discharge tube decreases at an integral multiple of the horizontal scanning frequency at the start of driving. It is.

  In a liquid crystal display device that displays received commercial broadcast images, a backlight is provided behind the liquid crystal panel, and a cold cathode tube, which is a type of discharge tube, is used as the light source of the backlight. . FIG. 7 shows a booster circuit for driving the cold cathode tube. That is, when the backlight luminance control means 4 partially configured with the function of the microcomputer 1a sends an L level to the signal input terminal 13 of the booster circuit 6 via the second output terminal 12, the transistor Q1 is Turns off. For this reason, since a bias voltage is applied to the gates of the FETs 16 and 17, they are alternately turned on and off alternately. As a result, the current flowing through the primary coil L1 is switched, a high voltage output is generated in the secondary coil L2, and the cold cathode tube 7 is driven to light.

  On the other hand, when H level is sent to the signal input terminal 13 to turn on the transistor Q1, the gates of the FETs 16 and 17 are grounded via the diodes D6 and D7 and the transistor Q1, so that the bias voltage is Oscillation stops near 0V. As a result, the generation of the high voltage output in the secondary coil L2 is stopped, and the cold cathode tube 7 is turned off. Therefore, when the cold cathode tube 7 is lit at the maximum luminance, the backlight luminance control means 4 maintains the level of the signal sent to the signal input terminal 13 at the L level, and sets the cold cathode tube 7 to a duty ratio of 100%. It lights up. Further, when the cold cathode tube 7 is lit with a medium luminance, the ratio of the L level sent to the signal input terminal 13 is set to 70%. That is, the cold cathode tube 7 is turned on with a duty ratio of 70%. When the cold cathode tube 7 is lit with the minimum luminance, the ratio of the L level sent to the signal input terminal 13 is set to 50%. That is, the cold cathode tube 7 is turned on with a duty ratio of 50% (this is the first prior art).

Further, the following technique has been proposed (referred to as a second conventional technique). That is, in this technique, a high voltage for driving the discharge tube is generated using a piezoelectric transformer, and at the time of start-up, the piezoelectric transformer is used by using a predetermined frequency that is higher than the frequency at which the boost ratio of the piezoelectric transformer is maximized. Driving. And when the electric current which flows into the discharge tube turned on becomes more than a desired value, the drive frequency from which the electric current which flows into a discharge tube becomes a target value is detected by starting sweep of a driving frequency. Thereafter, the piezoelectric transformer is driven with the detected drive frequency. For this reason, the discharge tube is surely turned on without causing problems such as destruction of the piezoelectric transformer (for example, see Patent Document 1).
JP 2000-311792 A

  However, when the first prior art is used, the following problems have occurred. That is, since the power for lighting the cold cathode tube 7 is large, the level of the high frequency noise 611 radiated from the booster circuit 6 or the high frequency noise 711 generated from the current path 71 to the cold cathode tube 7 as shown in FIG. Is big. For this reason, in order to completely prevent the high frequency noise 611 and 711 from jumping into the video signal circuit 81 that performs predetermined processing on the received video signal, a strict shield is required and the configuration becomes large. It is difficult in the device. For this reason, even when the high-frequency noise 611 and 711 jumps into the video signal circuit 81, when the jumping level is set to a relatively suppressed level, the booster circuit 6 has no adverse effect on the image displayed on the liquid crystal panel. A switching frequency is selected.

  That is, when the switching frequency is a frequency in the vicinity of an integral multiple of the horizontal scanning frequency, a beat that deteriorates the image occurs between the horizontal scanning signal and the high frequency noise 611 and 711. For this reason, the switching frequency of the booster circuit 6 is, for example, as shown in FIG. 3A, three times the horizontal scanning frequency (15.734 KHz) (about 47.2 KHz) and four times the frequency (about The frequency is set to 56 KHz, which is a substantially intermediate frequency with 62.9 KHz).

  However, immediately after starting the operation of the cold cathode tube 7, since the temperature of the gas in the cold cathode tube 7 is low, the discharge is not stable and becomes unstable. For this reason, the current flowing through the cold-cathode tube 7 is unstable due to the instability of the discharge. As a result, when the cold-cathode tube 7 is lit with a duty ratio of 100%, the high-frequency noise 611 and 711 include a discharge in addition to the spectrum 51 corresponding to the switching frequency as shown in FIG. Spectra 52a and 52b are generated due to current disturbance due to instability. In addition, since the frequencies of the spectra 52a and 52b at this time are in the vicinity of twice the horizontal scanning frequency (about 31.5 KHz) or five times the frequency (about 78.7 KHz), the horizontal scanning signal and spectrum A beat occurs between 52a and 52b, and a striped pattern or flickering is generated in the displayed image.

  Further, by switching the level sent to the signal input terminal 13 between the H level and the L level with a predetermined period (about 3.3 mS), the cold cathode tube 7 can be set to a duty ratio of 70% or 50%. In the case of lighting, the state of unstable discharge of the cold cathode tube 7 changes from the state of instability when lighting with a duty ratio of 100%. Therefore, the spectrum generated due to current disturbance due to instability of discharge also changes from the spectrum 52a, 52b shown in FIG. 3B to the spectrum 53a, 53b shown in FIG. 3C. . However, even in this case, since the frequencies of the spectra 53a and 53b are in the vicinity of twice the horizontal scanning frequency (about 31.5 KHz) or five times the frequency (about 78.7 KHz), the horizontal scanning is performed. A beat occurs between the first signal and the spectrums 53a and 53b, causing a striped pattern or flickering in the displayed image.

  When the temperature inside the cold cathode tube 7 is increased by the discharge and the internal gas pressure is increased (for example, when 60 seconds have elapsed after the lighting of the cold cathode tube 7 is started), the discharge is stabilized. . Accordingly, at this time, the radiated high-frequency noises 611 and 711 are noises in which the spectra 52a and 52b or the spectra 53a and 53b have disappeared. That is, the noise includes only the spectrum 51 corresponding to the switching frequency. That is, regardless of whether the backlight has a maximum brightness, a medium brightness, or a minimum brightness, the cold cathode tube 7 is turned on after the power is turned on and the cold cathode tube 7 is turned on. During the period until the gas inside warms (for example, about 30 seconds), the screen is striped or flickering, and the displayed image is difficult to see.

  The second conventional technique is a technique for reliably lighting a discharge tube without causing a problem such as destruction of the piezoelectric transformer when a piezoelectric transformer is used as an element for boosting. For this reason, it is a technique that is difficult to apply from the viewpoint of preventing the problem that occurs when the first conventional technique is used, that is, the occurrence of stripes or flickering on the screen immediately after the power is turned on. ing.

  The present invention has been made to solve the above-described problems, and its purpose is that it is difficult to prevent the generation of high-frequency noise due to instability of discharge in a cold cathode tube, and the high-frequency noise is high. Even when it is difficult to provide a strict shield that completely prevents noise from jumping into the video signal circuit, striped patterns and flickering can be prevented and displayed in the displayed image. When preventing the occurrence of stripes and flickers in the image, the period during which the brightness of the image deviates from the set brightness can be shortened, and the occurrence of stripes and flickers in the displayed image It is another object of the present invention to provide a liquid crystal display device that can reduce the trouble of changing the device even when applied to a device that does not prevent the problem.

  It is another object of the present invention to provide a discharge tube drive state and a drive stop state with a period in which noise components generated by driving the discharge tube decrease at an integral multiple of the horizontal scanning frequency at the start of driving the discharge tube. It is difficult to prevent the generation of high frequency noise due to the unstable discharge of the cold cathode tube and to prevent the high frequency noise from jumping into the video signal circuit. An object of the present invention is to provide a liquid crystal display device capable of preventing the occurrence of a stripe pattern or flickering in a displayed image even when it is difficult to provide a shield.

  In addition to the above purpose, the period during which switching between the discharge tube drive state and the drive stop state is performed in a cycle in which noise components generated by driving the discharge tube decrease at an integer multiple of the horizontal scanning frequency. The brightness of the image is also set when preventing the occurrence of stripes and flickering in the displayed image by changing the time corresponding to the elapsed time from when the discharge tube was stopped in response to the instruction. An object of the present invention is to provide a liquid crystal display device capable of shortening the period in which the luminance deviates from the luminance.

  In order to solve the above problems, a liquid crystal display device according to the present invention is generated in a secondary coil of a step-up transformer by switching a discharge tube as a light source of a backlight and a current flowing in the primary coil of the step-up transformer. A booster circuit that drives the discharge tube with a high-voltage output; and a drive state switching means that switches between a discharge tube drive state in which the booster circuit drives the discharge tube and a drive stop state in which the booster circuit stops driving the discharge tube. This is applied to the liquid crystal display device provided. The booster circuit is provided with a signal input terminal to which a signal for switching between the discharge tube driving state and the driving stop state is input, a microcomputer having a first output terminal and a second output terminal, And a signal circuit for guiding both a signal sent from one output terminal and a signal sent from the second output terminal to the signal input terminal. The drive state switching means includes an initial drive control means for switching between the discharge tube drive state and the drive stop state with an initial corresponding period in the initial drive period, which is a period at the start of discharge tube drive, In this state, the booster circuit is set in the drive stop state, and in the power-on state, the ratio between the period in which the booster circuit is in the discharge tube drive state and the period in which the drive stop state is set is a ratio corresponding to the backlight brightness. A backlight luminance control means for lighting the discharge tube, and the initial corresponding period is a period in which the noise component generated by driving the discharge tube decreases at an integer multiple of the horizontal scanning frequency, and the initial drive period is turned off. In response to the instruction, the initial drive control means changes the signal corresponding to the elapsed time from when the discharge tube drive was stopped, and sends a signal transmitted through the first output terminal. Thus, switching between the discharge tube drive state and the drive stop state is performed, and the backlight luminance control means switches between the discharge tube drive state and the drive stop state by a signal sent through the second output terminal. It has become.

  That is, when the discharge of the discharge tube becomes unstable, the high frequency noise radiated from the path to the booster circuit or the discharge tube is caused by the unstable discharge in addition to the spectrum corresponding to the switching frequency of the booster circuit. The resulting spectrum is included. Further, the frequency of the spectrum generated due to the instability of the discharge changes when the cycle for switching between the discharge tube driving state and the driving stop state is changed. Therefore, there is a period in which the frequency of the spectrum caused by the instability of the discharge is a frequency separated from an integer multiple of the horizontal scanning frequency, and when this period is the initial corresponding period, the instability of the discharge The spectrum generated due to the noise becomes a noise component that does not affect the displayed image. Further, when the elapsed time from when the driving of the discharge tube is stopped in response to the instruction to turn off the power is short, the gas pressure in the discharge tube is not so low because the temperature of the discharge tube has not dropped to room temperature. For this reason, instability of discharge is eliminated within a short time. Therefore, when the elapsed time is short, even if the initial drive period is shortened, no stripe pattern or flickering occurs in the display image. In addition, when applied to the configuration including the backlight luminance control means, only the initial drive control means is provided, and the discharge tube has the initial corresponding period without changing the configuration of the backlight luminance control means. Switching between the driving state and the driving stop state can be performed.

  The liquid crystal display device according to the present invention includes a discharge tube serving as a light source of a backlight, a booster circuit that drives the discharge tube with a high-voltage output generated by switching an input current, and the booster circuit includes a discharge tube. The present invention is applied to a liquid crystal display device having a drive state switching means for switching between a discharge tube drive state to be driven and a drive stop state in which a booster circuit stops driving the discharge tube. In the initial drive period, which is the period at the start of driving, the discharge tube drive state and the drive stop state are switched in the initial corresponding period, and the noise component generated by driving the discharge tube in the initial corresponding period is scanned horizontally. The period decreases at a frequency that is an integral multiple of the frequency.

  That is, when the discharge of the discharge tube becomes unstable, the high frequency noise radiated from the path to the booster circuit or the discharge tube is caused by the unstable discharge in addition to the spectrum corresponding to the switching frequency of the booster circuit. The resulting spectrum is included. Further, the frequency of the spectrum generated due to the instability of the discharge changes when the cycle for switching between the discharge tube driving state and the driving stop state is changed. Therefore, there is a period in which the frequency of the spectrum caused by the instability of the discharge is a frequency separated from an integer multiple of the horizontal scanning frequency, and when this period is the initial corresponding period, the instability of the discharge The spectrum generated due to the noise becomes a noise component that does not affect the displayed image.

  In addition to the above configuration, the initial drive period is changed in accordance with the elapsed time from when the discharge tube was stopped in response to an instruction to turn off the power. That is, when the elapsed time is short, since the temperature of the discharge tube does not drop to room temperature, the gas pressure in the discharge tube is not so low. For this reason, instability of discharge is eliminated within a short time. Therefore, when the elapsed time is short, even if the initial drive period is shortened, no stripe pattern or flickering occurs in the display image.

  According to the present invention, in the initial drive period in which a spectrum caused by discharge instability occurs, the frequency of the spectrum caused by discharge instability is an integral multiple of the horizontal scanning frequency. The operation is switched between the discharge tube driving state and the driving stop state with an initial corresponding period which is a period having a frequency separated from. Further, the period for switching between the discharge tube drive state and the drive stop state with the initial corresponding period is changed in accordance with the elapsed time from when the drive of the discharge tube was stopped according to the power-off instruction. In addition, since the output terminal different from the output terminal occupied by the backlight luminance control means for controlling the luminance of the backlight is used, the discharge tube driving state and the driving stop state are switched according to the initial corresponding period. Control by the initial correspondence period is possible without changing the configuration of the backlight luminance control means. For this reason, it is difficult to prevent the generation of high-frequency noise caused by the unstable discharge of the cold cathode tube, and a strict shield that completely prevents high-frequency noise from jumping into the video signal circuit is provided. It is possible to prevent the occurrence of stripes and flickering in the displayed image even when it is difficult, and the brightness of the image is also set when preventing the occurrence of stripes and flickering in the displayed image. It is possible to shorten the period when the luminance is deviated from the luminance, and to reduce the labor for changing the device even when applied to a device that does not prevent the occurrence of striped pattern or flicker in the displayed image. Can do.

  Further, according to the present invention, in the initial drive period in which a spectrum caused by instability of discharge occurs, the frequency of the spectrum caused by instability of discharge is equal to the horizontal scanning frequency. The operation is switched between the discharge tube drive state and the drive stop state with an initial corresponding cycle which is a cycle having a frequency separated from the integer multiple. For this reason, it is difficult to prevent the generation of high-frequency noise caused by the unstable discharge of the cold cathode tube, and a strict shield that completely prevents high-frequency noise from jumping into the video signal circuit is provided. Even when it is difficult, it is possible to prevent the occurrence of a stripe pattern or flicker in the displayed image.

  In addition, an initial drive period, which is a period for switching between the discharge tube drive state and the drive stop state with an initial corresponding period, corresponds to the elapsed time from when the discharge tube drive was stopped in response to a power-off instruction. To change. For this reason, when preventing the occurrence of a striped pattern or flickering in the displayed image, the period during which the luminance of the image deviates from the set luminance can be shortened.

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

  FIG. 2 is an explanatory diagram showing an outline of the mechanical configuration of an embodiment of the liquid crystal display device according to the present invention.

  In the figure, a diffuser plate 22 having a substantially identical shape to the liquid crystal panel 21 is provided behind the liquid crystal panel 21 having a substantially rectangular shape in plan view (downward in the drawing). A cold cathode tube (discharge tube) 7 serving as a light source is provided behind the diffusion plate 22. Further, the liquid crystal panel 21, the diffusion plate 22, and the cold cathode tube 7 are attached to the reflection plate 23. The cold cathode tubes 7 are U-shaped when viewed from the front side of the liquid crystal panel 21, and the number thereof is two. Further, the cold cathode tube 7, the diffusion plate 22, and the reflection plate 23 constitute a backlight according to the claims.

  FIG. 1 is a diagram showing an electrical configuration of the present embodiment. Elements and blocks that are the same as those in the prior art are assigned the same reference numerals as those in FIG. The present embodiment is used in a television receiver, and roughly includes a microcomputer 1, a signal circuit 5, a booster circuit 6, and a cold cathode tube 7.

  The microcomputer 1 controls main operations as a television receiver. That is, it controls the operation of a circuit unit (including the video signal circuit 81 shown in FIG. 2) for displaying the received commercial broadcast on the liquid crystal panel 21. Moreover, the drive state switching means 2 is comprised by a part of the function. The drive state switching unit 2 is a block for controlling the operation of the backlight of the liquid crystal display device, and includes an initial drive control unit 3 and a backlight luminance control unit 4.

  The backlight luminance control means 4 has the same configuration as that of the prior art described with reference to FIG. That is, when the power is turned off, the H level is sent to the signal input terminal 13 through the second output terminal 12 and the signal circuit 5, thereby stopping the state of the booster circuit 6 and the generation of the high voltage output. To a driving stop state (a state in which the driving of the cold cathode tube 7 is stopped). Further, in the power-on state, the state of the booster circuit 6 is set to the operation state (state in which the cold cathode tube 7 is lit). In the operation state, the luminance of the cold cathode tube 7 is controlled by controlling the ratio between the period of the discharge tube driving state in which the high voltage output is generated and the period of the driving stop state in which the generation of the high voltage output is stopped. (Backlight brightness) is set to the target brightness.

  That is, when the cold cathode tube 7 is lit at the maximum luminance (when the luminance of the backlight is maximized), the ratio of the discharge tube driving period is set to 100% (the period during which the L level is output is 100%). To do). When the cold cathode tube 7 is lit at normal luminance, the ratio of the discharge tube driving state period is 70%, and the ratio of the driving stop state period is 30% (the period for outputting the L level is 70%). To do). Further, when the cold cathode tube 7 is lit at the lowest luminance, the ratio of the discharge tube driving state period is 50%, and the ratio of the driving stop state period is 50% (the period during which the L level is output is 50%). To do). Note that the cycle of switching between the discharge tube driving state and the driving stop state is about 3.3 mS.

  The initial drive control means 3 is a booster circuit having an initial corresponding period (for example, 2 mS) in an initial drive period which is a period at the start of driving of the cold cathode tube 7 (a period immediately after the power-on state is shifted). 6 is switched between the discharge tube drive state and the drive stop state. In addition, the initial drive period is changed in a range of, for example, 0 seconds to 1 minute in accordance with the elapsed time from when the drive of the cold cathode tube 7 is stopped in response to an instruction to turn off the power.

  The initial drive control means 3 sends the H level or the L level to the signal input terminal 13 via the first output terminal 11 which is an output terminal provided in the microcomputer 1 and the signal circuit 5. Thus, the operation of the booster circuit 6 is controlled. Further, the driving state switching means 2 sends H level or L level to the signal input terminal 13 via the second output terminal 12 which is an output terminal provided in the microcomputer 1 and the signal circuit 5. Thus, the operation of the booster circuit 6 is controlled.

  The booster circuit 6 drives the cold cathode tube 7 with a high-voltage output generated by switching an input current (supplied as a DC power supply P +). For this purpose, it includes a step-up transformer 14, FETs 16 and 17, transistors Q1 and Q2, Zener diode D3, diodes D6 and D7, resistors R1 to R7, and a capacitor C1.

  Specifically, a 13V DC power supply P + is led to the collector of the transistor Q2, and a Zener diode D3 is connected between the base of the transistor Q2 and the ground level. Further, a resistor R3 for supplying a current to the Zener diode D3 is connected between the collector and base of the transistor Q2. For this reason, the transistor Q2 stabilizes the voltage of the DC power supply P + to a predetermined voltage (for example, 10 V) and outputs it from the emitter.

  One terminal of the secondary coil L2 wound around the step-up transformer 14 is connected to the cold cathode tube 7 via the capacitor C1, and the other terminal of the secondary coil L2 is directly connected to the cold cathode tube 7. ing. The DC power source P + is led to the middle point terminal of the primary coil L1 wound around the step-up transformer 14, and one end terminal of the primary coil L1 is connected to the drain of the FET 16, and the end of the primary coil L1 is connected. The other of the partial terminals is connected to the drain of the FET 17. Further, one terminal of the feedback coil L3 wound around the step-up transformer 14 is connected to the gate of the FET 16, and the other terminal of the feedback coil L3 is connected to the gate of the FET 17. The sources of the FETs 16 and 17 are grounded.

  Between the gates of the FETs 16 and 17 and the ground level, resistors R6 and R7 for suppressing an increase in gate impedance and performing voltage division are connected. Further, a stabilized DC voltage sent from the transistor Q2 is led as a bias voltage to the respective gates of the FETs 16 and 17 via the resistors R4 and R5. The gates of the FETs 16 and 17 are connected to the anodes of the diodes D6 and D7. The cathodes of the diodes D6 and D7 are connected to each other and to the collector of the transistor Q1. The base of the transistor Q1 is connected to the signal input terminal 13 via the resistor R1. Further, a resistor R2 for suppressing an increase in base impedance is connected between the base of the transistor Q1 and the ground level.

  The signal circuit 5 guides the signal sent from the first output terminal 11 and the signal sent from the second output terminal 12 to the signal input terminal 13. For this reason, a diode D1 having an anode connected to the first output terminal 11 and a diode D2 having an anode connected to the second output terminal 12 are provided. The cathode of the diode D1 and the cathode of the diode D2 are Are connected to each other and led to the signal input terminal 13.

  The signal circuit 5 and the booster circuit 6 are configured as described above. Therefore, the transistor Q1 is turned on when at least one of the level sent from the first output terminal 11 or the level sent from the second output terminal 12 is H level, and the first output Only when both the level sent from the terminal 11 and the level sent from the second output terminal 12 are at the L level, the off state is established.

  In the booster circuit 6, when the transistor Q1 is turned off, a bias voltage is applied to the gates of the FETs 16 and 17, so that the FETs 16 and 17 are alternately turned on and off to switch the current flowing through the primary coil L1. Therefore, a high voltage output is generated in the secondary coil L2. On the other hand, when the transistor Q1 is turned on, the gate voltages of the FETs 16 and 17 are close to 0V, and the FETs 16 and 17 are turned off. Therefore, when the transistor Q1 is turned off, the cold cathode tube 7 is driven (discharge tube driving state), and when the transistor Q1 is turned on, the driving of the cold cathode tube 7 is stopped (driving stopped state).

  Prior to the description of the operation of the embodiment configured as described above, the relationship between the switching cycle and the spectrum of the radiated high frequency noise when the booster circuit 6 is alternately switched between the discharge tube drive state and the drive stop state will be described. .

  As already described, at the start of lighting of the cold cathode tube 7, the cold cathode tube 7 is cooled, and the discharge becomes unstable because the internal gas pressure is low. When the discharge becomes unstable, the high-frequency noise 611 and 711 radiated from the path 71 to the booster circuit 6 and the cold cathode tube 7 include the spectrum 51 corresponding to the switching frequency of the booster circuit 6 and the discharge failure. Includes spectra caused by stability.

  The spectrum caused by the instability of discharge is shown in FIG. 3 (when the cold cathode tube 7 is driven at a duty ratio of 100%) when the booster circuit 6 performs a continuous switching operation. The spectrums 52a and 52b shown in FIG. Further, when switching between the discharge tube driving state and the driving stop state at a cycle of about 3.3 mS in order to reduce the luminance of the cold cathode tube 7, the above-described spectrum becomes the spectrum shown by 53a and 53b. That is, when the booster circuit 6 continues to drive the cold cathode tube 7 and when the booster circuit 6 intermittently drives the cold cathode tube 7, the frequency of the spectrum caused by the instability of the discharge is high. Change.

  In addition, it has been confirmed that the spectrum caused by the instability of discharge has the following characteristics from the experimental results of actual machines.

  That is, when the configuration of the booster circuit 6 (the configuration of the step-up transformer 14, the value of the capacitor C1, the configuration of the path to the cold cathode tube 7, etc.) and the type of the cold cathode tube 7 to be used are the same, When the tube 7 is driven intermittently, the spectrum frequency becomes the same when the intermittent cycle is the same. When the intermittent period is changed, the spectrum frequency changes. That is, a certain relationship is established between the intermittent period and the spectrum frequency.

  In other words, when the discharge of the cold-cathode tube 7 becomes unstable, a spectrum caused by the instability of the discharge occurs. The spectrum at this time becomes a spectrum with a random frequency according to the occasion. Instead, the frequency changes with a substantially constant relationship with respect to the condition of intermittent period. This means that when the intermittent cycle is set appropriately, the frequency of the spectrum caused by the instability of discharge can be set to a frequency separated from an integral multiple of the horizontal scanning frequency. That is, it means that the deterioration of the image quality can be prevented even when the discharge of the cold cathode tube 7 becomes unstable.

  FIG. 3D shows a spectrum of high-frequency noises 611 and 711 when the cycle of intermittently lighting the cold cathode tube 7 is about 2 mS. That is, at this time, the frequencies of the spectra 54a and 54b generated due to the instability of the discharge are both frequencies separated from an integral multiple of the horizontal scanning frequency. Therefore, even when the high-frequency noise 611 and 711 jumps into the video signal circuit 81, image degradation is suppressed to a level that can be ignored.

  FIG. 4 is a flowchart showing main operations of the embodiment, and FIG. 5 is an explanatory diagram showing waveforms of main signals. The operation of the embodiment will be described with reference to these drawings as necessary.

  In the power-off state before time T1, the backlight luminance control unit 4 sends the H level to the second output terminal 12 to turn on the transistor Q1, thereby stopping the switching operation of the FETs 16 and 17. Yes. That is, the cold cathode tube 7 is maintained in the extinguished state (in this case, the level that the initial drive control means 3 sends to the first output terminal 11 may be either the H level or the L level. In the embodiment, L level is output).

  At time T1, when a power-on instruction is input using a remote controller (not shown) or the like, the initial drive control means 3 checks the elapsed time from the last power-off (step S1, S2). When the elapsed time exceeds 30 seconds, the initial drive period is calculated based on the elapsed time (FIG. 6 shows the initial drive period calculated corresponding to the elapsed time) (step S3 , S4).

  Next, the initial drive control means 3 changes the level sent to the second output terminal 12 from the H level to the L level by giving an instruction to the backlight luminance control means 4 (step S5). After that, by alternately outputting H level and L level to the first output terminal 11, the cold cathode tube 7 is driven and stopped (switching between the discharge tube driving state and the driving stop state). Repeat with a period of 2 mS. That is, in the period t1, which is the initial drive period, the initial drive control means 3 sends the L level to the first output terminal 11 during the 1.6 mS period (indicated by t2L) (set to the discharge tube drive state). Then, the operation of sending the H level (setting the drive stop state) is repeated in the period of 0.4 mS (indicated by t2H).

  Therefore, in the initial driving period t1, since the temperature of the cold cathode tube 7 is low and the internal gas pressure is low, the discharge becomes unstable, and a spectrum having a frequency different from the switching cycle of the booster circuit 6 is generated. However, since the frequency of this spectrum is a frequency separated from an integral multiple of the horizontal scanning frequency (see FIG. 3D), a stripe pattern or flickering that makes it difficult to see the image may occur in the liquid crystal panel 21. Is prevented (steps S6 and S7).

  Then, when the temperature in the cold cathode tube 7 is increased, the gas pressure is increased, and the time T2 when the discharge is stabilized, that is, when the initial drive period t1 has passed (time T2), the initial drive control means. 3 maintains the level sent to the first output terminal 11 at the L level. Further, the backlight luminance control means 4 is instructed to perform predetermined control.

  The backlight luminance control means 4 to which this instruction is given is set to 100% by maintaining the level sent to the second output terminal 12 at the L level when the luminance is set to be maximum. The cold cathode tube 7 is turned on with a duty ratio (indicated by 12a). Further, when the luminance is set to be medium, the L level ratio is 70% (indicated by t3L and t3H) with a period of about 3.3 mS (indicated by t3). The level to be sent to the second output terminal 12 is switched (indicated by 12b). For this reason, the cold cathode tube 7 is lit with a duty ratio of 70%. When the luminance is set to be minimum, the L level ratio is 50% (indicated by t4L and t4H) with a period of about 3.3 mS (indicated by t4). The level to be sent to the second output terminal 12 is switched (indicated by 12c). For this reason, the cold cathode tube 7 is lit with a duty ratio of 50%.

  That is, the backlight luminance control means 4 switches between the discharge tube drive state and the drive stop state with a duty ratio of 100%, 70%, or 50% after time T2. Further, the switching cycle is about 3.3 mS. However, after time T2, the gas pressure in the cold cathode tube 7 has increased and the discharge is stable, so the spectrum of the high frequency noise 611, 711 jumping into the video signal circuit 81 is shown in FIG. It becomes a spectrum. Therefore, the image displayed on the liquid crystal panel 21 is an easy-to-see image that does not have a stripe pattern or flicker.

  When it is determined in step S3 that the elapsed time is shorter than 30 seconds, it is determined that the discharge is stable because the temperature in the cold cathode tube 7 is high and the gas pressure is not lowered. For this reason, the operation proceeds from step S3 to step S8. That is, the drive control of the cold cathode tube 7 by the backlight luminance control means 4 is immediately started without the drive control of the cold cathode tube 7 by the initial drive control means 3 being performed.

  In the following supplementary explanation, the microcomputer 1 measures the elapsed time from when the driving of the cold cathode tube 7 is stopped in response to the power-off instruction. In this timing, if the elapsed time is within 2 minutes, the elapsed time is measured and stored internally. However, when the initial drive period exceeds 2 minutes, which is the period set to the longest value, a flag indicating that the initial drive period has exceeded 2 minutes is set, and then the subsequent timing is stopped. Therefore, when the power is turned off, the microcomputer 1 can be put into a standby state after two minutes have elapsed, and the power consumption in the standby state as an apparatus can be reduced.

  Note that the booster circuit 6 may be configured to perform a switching operation when the level input to the signal input terminal 13 becomes H level (in this configuration, the configuration of the signal circuit 5 is changed). At the same time, when the initial drive control means 3 and the backlight luminance control means 4 are set to the discharge tube drive state, the H level is sent out).

  Further, the case where the initial correspondence period is set to 2 mS has been described, but the initial correspondence period has a characteristic that it changes according to the type of the cold cathode tube 7 to be used and the configuration of the booster circuit 6. Value.

  Further, the duty ratio in the initial drive period has been described as being 80%, but it can be set to an arbitrary value such as 90% or 70%.

It is a figure which shows the electrical constitution of one Embodiment of the liquid crystal display device based on this invention. It is explanatory drawing which shows the outline of the mechanistic structure of embodiment. It is explanatory drawing which shows the spectrum of the high frequency noise which generate | occur | produces at the time of the drive of a cold cathode tube (discharge tube). It is a flowchart which shows the main operation | movement of embodiment. It is explanatory drawing which shows the waveform of the main signal in embodiment. It is explanatory drawing which shows the relationship between the elapsed time from the last power-off, and an initial stage drive period. It is a figure which shows the electrical structure of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Microcomputer 2 Drive state switching means 3 Initial drive control means 4 Backlight brightness control means 5 Signal circuit 6 Booster circuit 7 Cold cathode tube (discharge tube)
11 First output terminal 12 Second output terminal 13 Signal input terminal 14 Step-up transformer L1 Primary coil L2 Secondary coil t1 Initial drive period t2 Initial corresponding period t2L Discharge tube drive state period t2H Drive stop state period

Claims (3)

A discharge tube as a light source of the backlight;
A step-up circuit that drives the discharge tube with a high-voltage output generated in the secondary coil of the step-up transformer by switching the current flowing through the primary coil of the step-up transformer;
In a liquid crystal display device comprising: a discharge tube drive state in which the booster circuit drives the discharge tube; and a drive state switching means for switching between a drive stop state in which the booster circuit stops driving the discharge tube.
The booster circuit is provided with a signal input terminal for inputting a signal for switching between the discharge tube driving state and the driving stop state,
A microcomputer having a first output terminal and a second output terminal;
A signal circuit for guiding both a signal sent from the first output terminal and a signal sent from the second output terminal to the signal input terminal;
The drive state switching means is
An initial drive control means for switching between a discharge tube drive state and a drive stop state with an initial corresponding period in an initial drive period that is a period at the start of driving of the discharge tube;
When the power supply is turned off, the booster circuit is set in a drive stop state. In the power-on state, the ratio between the period during which the booster circuit is in the discharge tube drive state and the period in which the drive stop state is set corresponds to the luminance of the backlight. Backlight luminance control means for lighting the discharge tube as a ratio,
The initial corresponding period is a period in which the noise component generated by driving the discharge tube decreases at an integer multiple of the horizontal scanning frequency,
Change the initial driving period according to the elapsed time from when the driving of the discharge tube was stopped according to the power off instruction,
The initial drive control means switches between the discharge tube drive state and the drive stop state with a signal sent through the first output terminal,
The backlight luminance control means switches between a discharge tube driving state and a driving stop state by a signal sent through the second output terminal. A discharge tube as a light source of the backlight;
A booster circuit that drives the discharge tube with a high-voltage output generated by switching the input current;
In a liquid crystal display device comprising: a discharge tube drive state in which the booster circuit drives the discharge tube; and a drive state switching means for switching between a drive stop state in which the booster circuit stops driving the discharge tube.
The drive state switching means performs switching between the discharge tube drive state and the drive stop state with an initial corresponding period in the initial drive period, which is a period at the start of driving of the discharge tube,
A liquid crystal display device characterized in that an initial corresponding period is a period in which a noise component generated by driving a discharge tube decreases at a frequency that is an integral multiple of a horizontal scanning frequency.   3. The liquid crystal display device according to claim 2, wherein the initial driving period is changed in accordance with an elapsed time from when the driving of the discharge tube is stopped in accordance with an instruction to turn off the power.

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