JP2009294294A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device capable of reducing power consumption of a backlight when external signal is not input without complicating a system. <P>SOLUTION: A backlight driving part 18 includes: a PWM light control circuit 30 that generates a PMW signal having a prescribed duty based on a control signal from a control part; and tube current adjustment circuits 34, 35 containing resistors R1 to R6 for adjusting a tube current of a backlight 17. In an external input mode of inputting an image signal from external equipment, when such a state that no image signal is input from the external equipment while an OSD image is displayed on a liquid crystal panel continues for a constant time, the backlight driving part 18 changes resistances of the tube current adjustment circuits 34, 35 based on the PMW signal and reduces the tube current of the backlight 17. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device used for a liquid crystal television receiver or the like, and more particularly to a technology for power saving of a backlight.

  A liquid crystal television receiver (hereinafter referred to as “liquid crystal television”) includes a backlight for back lighting a liquid crystal panel. As a backlight, a discharge lamp having a large power consumption such as a cold cathode tube is generally used, and the power consumption of the backlight accounts for more than half of the power consumption of the entire liquid crystal television.

  On the other hand, in a liquid crystal television, a normal mode for displaying video based on a video signal received by a tuner and an external for displaying video based on a video signal input from an external device such as a video deck or a DVD (Digital Versatile Disc) recorder. A function to switch between input modes is provided.

  When the external input mode is selected, even if no video signal is input from the external device, the OSD (On Screen Display) function allows specific characters on the screen to be recognized as being in the external input mode. And figures are displayed. Therefore, despite the no-signal state, the backlight is in a lighting state and continues to consume useless power.

  As prior art regarding power saving of a backlight, there exist some which are shown by the following patent documents 1-3, for example.

  Patent Document 1 discloses a liquid crystal in which a backlight other than the backlight arranged opposite to characters and figures displayed on the OSD is turned off when no video signal is input for a certain period of time in the external input mode. A television is disclosed.

  Patent Document 2 discloses a PWM (Pulse Width Modulation) dimming circuit unit that performs dimming control by controlling the timing of turning on and off the backlight, and dimming by controlling the current of the backlight. There is disclosed a backlight drive circuit that includes a current dimming circuit unit that performs control, and controls the dimming level of the backlight by both dimming circuit units to expand the dimming range.

  In Patent Document 3, in the external input mode, it is detected whether or not a connector is inserted into the external input terminal, and if not, a dimming instruction control signal is transmitted from the microcomputer to the dimming circuit. A liquid crystal image display device is disclosed in which a backlight is turned on with a duty based on an on / off timing signal output from a dimming circuit.

JP 2006-13942 A Japanese Patent Laid-Open No. 10-284282 JP 2003-316325 A

  According to the liquid crystal television of Patent Document 1, it is possible to suppress unnecessary power consumption by the backlight when there is no video signal input from an external device. Since the light is turned off, for example, the upper side of the screen is bright and the lower side is dark, giving the viewer a sense of discomfort. In addition, since there are backlights that are lit when there is no signal and backlights that are not lit, there is a problem that the lifetime of the backlight becomes non-uniform.

  Therefore, as a countermeasure, it is conceivable to reduce the tube current of the backlight in the absence of an external signal by making the resistance value for setting the tube current in the backlight drive circuit variable. More specifically, a tube current setting resistor is constituted by a series circuit of two resistors, a transistor is connected in parallel to one resistor, and ON / OFF of this transistor is turned on by H (High) / L (from a microcomputer) By controlling with a Low signal, the resistance value is changed to reduce the tube current when there is no signal. According to this, since the brightness of the entire backlight is lowered, a state where a bright portion and a dark portion exist on the screen does not occur, and the lifetime of the backlight can be made uniform.

  However, when the above configuration is adopted, the microcomputer needs to generate a control signal for turning on / off the transistor, and a port for outputting the control signal is required. There is a problem of complexity. Further, when the number of ports of the microcomputer is limited, there is a problem that the degree of freedom in design is lowered. The above-mentioned patent documents 1 to 3 do not show a solution to such a problem.

  The present invention solves the above-described problems, and an object of the present invention is to provide a liquid crystal display capable of reducing the power consumption of the backlight when no external signal is input without complicating the system. To provide an apparatus.

  A liquid crystal display device according to the present invention includes a backlight that is a light source of a liquid crystal panel, a backlight driving unit that drives the backlight, a control unit that controls the backlight driving unit, and an OSD that is displayed on the liquid crystal panel. (On Screen Display) A liquid crystal display device including an OSD processing unit that generates an image, wherein the backlight driving unit is a PWM (Pulse Width Modulation) having a predetermined duty based on a control signal from the control unit It has a PWM dimming circuit for generating a signal and a tube current adjusting circuit including a resistor for adjusting the tube current of the backlight. Then, in the external input mode in which the video signal is input from the external device, the control unit continues the PWM signal when the state in which the video signal is not input from the external device continues for a certain period while the OSD video is displayed on the liquid crystal panel. The tube current of the backlight is decreased by changing the resistance value of the tube current adjusting circuit based on the above.

  In this way, since the resistance value of the tube current adjusting circuit can be changed using the existing dimming PWM signal, it is necessary to generate a control signal for turning on / off the transistor in the microcomputer. In addition, a port for outputting this control signal becomes unnecessary. Therefore, the power consumption of the backlight when no external signal is input can be reduced without complicating the system. Further, even when the number of ports of the microcomputer is limited, it is possible to cope with it.

  In the present invention, the tube current adjustment circuit includes two resistors connected in series and a transistor connected in parallel to one of these resistors, and controls ON / OFF of the transistor based on the PWM signal. Can be adopted. According to this, the tube current of the backlight can be controlled with a simple circuit configuration using a resistor and a transistor.

  In this case, an integration circuit that integrates the PWM signal and a comparator that compares the output voltage of the integration circuit with a reference voltage and outputs a signal for turning on / off the transistor based on the comparison result are further provided. By turning off the transistor, the resistance value of the tube current adjusting circuit may be increased to reduce the tube current of the backlight. According to this, ON / OFF control of the transistor based on the PWM signal can be easily realized.

  According to the present invention, since the tube current can be controlled using the existing PWM signal for dimming, it is not necessary to generate a control signal separately from the PWM signal in the microcomputer, and the output port of the control signal Is also unnecessary. As a result, the power consumption of the backlight when no external signal is input can be reduced without complicating the system.

  Hereinafter, an embodiment in which the present invention is applied to a liquid crystal television will be described with reference to the drawings.

  FIG. 1 is a front view showing an appearance of a liquid crystal television according to an embodiment of the present invention. The liquid crystal television 100 includes a main body 1 and a base 2 that supports the main body 1. A liquid crystal panel 16 is provided on the front surface of the main body 1, and a backlight 17 that is a light source of the liquid crystal panel 16 is disposed on the back side of the liquid crystal panel 16. The backlight 17 is composed of, for example, four cold cathode tubes 17a to 17d, and irradiates the liquid crystal panel 16 with light from the back. Reference numeral 24 denotes an operation unit such as a power switch provided on the front surface of the main body 1.

  FIG. 2 is a block diagram showing an electrical configuration of the liquid crystal television 100. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals. 10 is an antenna that receives broadcast radio waves, 11 is a tuner that extracts a signal of a predetermined frequency from the broadcast radio waves received by the antenna 10, and 12 is a signal from an external device 26 such as a video deck or a DVD (Digital Versatile Disc) recorder. Is an external input terminal.

  A signal processing unit 13 switches a signal from the tuner 11 and a signal from the external input terminal 12 and performs predetermined processing on the switched signal. Reference numeral 14 denotes a video circuit that performs processing for combining and displaying the video based on the video signal output from the signal processing unit 13 and the OSD video generated by the OSD processing unit 15.

  Reference numeral 16 denotes a liquid crystal panel that displays an image based on the output of the video circuit 14, and 17 denotes a backlight of the liquid crystal panel 16. These are the same as those shown in FIG. Reference numeral 18 denotes a backlight drive unit for driving the backlight 17, and details thereof will be described later.

  Reference numeral 19 denotes an audio circuit that amplifies the audio signal output from the signal processing unit 13, and 20 denotes a speaker that outputs audio based on the output of the audio circuit 19.

  Reference numeral 21 denotes a microcomputer (hereinafter referred to as “microcomputer”) as a control unit that controls the operation of the liquid crystal television 100, and includes a CPU, a memory, and the like. Reference numeral 22 denotes a remote control receiver that receives a signal transmitted from the remote controller 23. Reference numeral 24 denotes an operation unit provided in the main body 1 of the liquid crystal television 100 as shown in FIG.

  A power source 25 supplies necessary power to the backlight drive unit 18 and other parts of the liquid crystal television 100.

  FIG. 3 is a diagram showing an example of a specific circuit of the backlight driving unit 18 in FIG. In the figure, the same parts as those in FIGS. 1 and 2 are denoted by the same reference numerals. Reference numeral 30 denotes a PWM dimming circuit that performs dimming control for adjusting the luminance of the backlight 17. The PWM dimming circuit 30 is composed of, for example, a known circuit as shown in Patent Document 2 described above, and generates a PWM signal having a predetermined duty based on a control signal from the control unit 21 (FIG. 2). An oscillation circuit 31 is provided. This PWM signal is given to the switching circuit 32 and also to an integration circuit 36 described later.

  Reference numeral 32 denotes a switching circuit that performs an ON / OFF operation at a timing corresponding to the duty of the PWM signal output from the PWM dimming circuit 30. The switching circuit 32 is also composed of a known circuit as disclosed in Patent Document 2, and has a switching element (not shown) such as a transistor or FET.

  Reference numeral 33 denotes a step-up transformer (hereinafter simply referred to as “transformer”) that constitutes an inverter together with the switching circuit 32, and includes a pair of primary windings Wa1, Wb1 and a pair of secondary windings Wa2, Wb2. ing. One terminal b of the secondary winding Wa2 is connected to one terminal p of the backlight 17, and one terminal c of the secondary winding Wb2 is connected to the other terminal q of the backlight 17. Therefore, the voltage applied to the terminal p of the backlight 17 and the voltage applied to the terminal q of the backlight 17 are in an opposite phase relationship.

  Diodes D1 and D2, a capacitor C1, resistors R1 to R3, and a transistor Q1 are connected to the other terminal a of the secondary winding Wa2. The cathode of the diode D1 is connected to the terminal a, and the anode is connected to the ground. The anode of the diode D2 is connected to the terminal a, and the cathode is connected to one end of the capacitor C1. The other end of the capacitor C1 is connected to the ground. The resistor R1 is connected in parallel with the capacitor C1. The resistors R2 and R3 are connected in series, and this series circuit is connected in parallel with the resistor R1. The transistor Q1 is connected in parallel with the resistor R3. The resistors R1 to R3 and the transistor Q1 constitute a tube current adjusting circuit 34 for adjusting the tube current of the backlight 17.

  Diodes D3 and D4, a capacitor C2, resistors R4 to R6, and a transistor Q2 are connected to the other terminal d of the secondary winding Wb2. The cathode of the diode D3 is connected to the terminal d, and the anode is connected to the ground. The anode of the diode D4 is connected to the terminal d, and the cathode is connected to one end of the capacitor C2. The other end of the capacitor C2 is connected to the ground. The resistor R4 is connected in parallel with the capacitor C2. The resistors R5 and R6 are connected in series, and this series circuit is connected in parallel with the resistor R4. The transistor Q2 is connected in parallel with the resistor R6. The resistors R4 to R6 and the transistor Q2 constitute a tube current adjusting circuit 35 for adjusting the tube current of the backlight 17.

  The outputs of the comparator 37 are commonly supplied to the bases of the transistors Q1 and Q2 of the tube current adjusting circuits 34 and 35, respectively. In the comparator 37, the output of the integrating circuit 36 is input to the positive input terminal, and the reference voltage B is input to the negative input terminal. The integrating circuit 36 includes a resistor R7 and a capacitor C3, and receives the PWM signal generated by the oscillation circuit 31 of the PWM dimming circuit 30.

  The integrating circuit 36 integrates the PWM signal and outputs a DC voltage corresponding to the duty of the PWM signal. The PWM signal is a binary pulse signal of “H” and “L”, and the output voltage of the integration circuit 36 increases as the ratio of the “H” period in one cycle, that is, the ON duty increases. Hereinafter, the term “duty” refers to “ON duty”. The comparator 37 outputs an “H” signal if the voltage input from the integration circuit 36 exceeds the reference voltage B, and outputs an “L” signal if the voltage input from the integration circuit 36 does not exceed the reference voltage B. Is output. The output of the comparator 37 is given to the bases of the transistors Q1 and Q2. The transistors Q1 and Q2 are turned on when the output of the comparator 37 is “H”, and turned off when the output of the comparator 37 is “L”. Become.

  Next, a dimming operation in the liquid crystal television 100 will be described with reference to FIGS. When the user operates the remote controller 23 to adjust the brightness of the screen of the liquid crystal panel 16, a signal is transmitted from the remote controller 23 to the remote control receiver 22. In response to this signal, the microcomputer 21 sends a dimming control signal to the PWM dimming circuit 30. In the PWM dimming circuit 30, based on this control signal, a PWM signal having a duty corresponding to a command from the remote controller 23 is generated by the oscillation circuit 31, and the PWM signal is output to the switching circuit 32. At this time, the PWM dimming circuit 30 does not output a PWM signal to the integrating circuit 36 (however, it may be output as described later).

  In the switching circuit 32, the transistor performs an ON / OFF operation at a timing according to the duty of the PWM signal from the PWM dimming circuit 30. The AC voltage generated by this ON / OFF operation is boosted by the transformer 33 and supplied to the secondary side of the transformer 33 as the drive voltage of the backlight 17. This drive voltage is in accordance with the duty of the PWM signal. If the duty is large, the drive voltage is increased, and the luminance of the backlight 17 is increased. If the duty is small, the drive voltage is decreased. The brightness decreases.

  The PWM dimming operation described above is the same as the conventional one and is not particularly new.

  Next, the power saving operation by the tube current control of the backlight 17, which is a feature of the present invention, will be described with reference to FIGS. 2 and 3.

  In FIG. 2, when an external device 26 is connected to the external input terminal 12 and the external input mode is set by the remote controller 23, the OSD processing unit 15 causes the screen of the liquid crystal panel 16 to display a plain screen (for example, the external input mode). A screen with characters such as external input displayed on a blue screen) is displayed. At this time, the backlight 17 is in a lighting state. Thereafter, when the external device 26 is operated and a signal from the external device 26 is input from the external input terminal 12, an image based on the signal is displayed on the screen of the liquid crystal panel 16, and a normal reproduction operation is performed. Is called. However, when the external device 26 is not operated and a signal is not input from the external input terminal 12 even after a predetermined time has elapsed, the microcomputer 21 instructs the PWM dimming circuit 30 to shift to the power saving operation. A control signal is sent.

  When the PWM dimming circuit 30 receives this control signal from the microcomputer 21, the oscillation circuit 31 generates a PWM signal with a small duty and outputs the PWM signal to the integration circuit 36. At this time, the PWM dimming circuit 30 outputs the same PWM signal to the switching circuit 32 (however, it does not have to be output as described later).

  In the integrating circuit 36, the PWM signal output from the PWM dimming circuit 30 is integrated by the resistor R7 and the capacitor C3, and a DC voltage of a level corresponding to the duty of the PWM signal is output from the integrating circuit 36. This output voltage is compared with the reference voltage B in the comparator 37. As described above, if the output voltage exceeds the reference voltage B, an "H" signal is output from the comparator 37, and the output voltage exceeds the reference voltage B. If not, the comparator 37 outputs an “L” signal. In this case, since the duty of the PWM signal is small, the signal output from the comparator 37 is an “L” signal.

  This “L” signal is applied in common to the base of the transistor Q1 of the tube current adjusting circuit 34 and the base of the transistor Q2 of the tube current adjusting circuit 35, whereby both the transistors Q1 and Q2 are turned OFF.

  When the aforementioned power saving transition command is not given from the microcomputer 21 to the PWM dimming circuit 30, a PWM signal having a large duty or a continuous “H” signal (duty) is sent from the PWM dimming circuit 30 to the integrating circuit 36. Is 100% PWM signal). For this reason, an “H” signal is output from the comparator 37, and the transistors Q1 and Q2 are both turned on, so that the resistors R3 and R6 are short-circuited.

  When the transistor Q1 is turned off by the “L” signal from the comparator 37, the short circuit of the resistor R3 is released, and as a result, the resistor R2 and the resistor R3 are connected in series. The current flowing through R2 decreases. Similarly, when the transistor Q2 is turned off by the “L” signal from the comparator 37, the short circuit of the resistor R6 is released and the resistor R5 and the resistor R6 are connected in series. As a result, the transistor Q2 is turned on. As a result, the current flowing through the resistor R5 decreases. As a result, the tube current of the backlight 17 decreases and the luminance of the backlight 17 decreases. Therefore, in the external input mode, the power consumption of the backlight 17 when no signal is input can be reduced.

  Further, since the PWM signal having a small duty at this time is also supplied to the switching circuit 32, the driving voltage of the backlight 17 supplied from the switching circuit 32 via the transformer 33 is reduced, and the luminance of the backlight 17 is lowered. To do. Therefore, in addition to the reduction in power consumption by the above-described tube current control, the power consumption can be reduced by the dimming control, so that the power consumption of the backlight 17 can be greatly reduced.

  For example, when the normal value of the tube current of the backlight 17 is set to 7 mA, the power consumption of the liquid crystal television 100 is about 140 W. However, the tube current is reduced to 6 mA by performing the tube current control by the PWM signal. By performing dimming control, the power consumption of the liquid crystal television 100 is about 61 W. Therefore, it is possible to reduce power of about 79 W.

  FIG. 4 is a flowchart showing the procedure of the power saving operation by the tube current control described above. This procedure is executed by the CPU constituting the microcomputer 21.

  In step S1, it is determined whether or not the external input mode is selected on the remote controller 23. When the external input mode is not selected, that is, in the normal mode for viewing the broadcast received by the tuner 11 (step S1: NO), the process proceeds to step S8, and when the external input mode is selected (step S1: YES), the process proceeds to step S2.

  In step S2, it is determined whether or not OSD display is performed on the screen of the liquid crystal panel 16. If OSD display is not performed (step S2: NO), the process proceeds to step S8, and if OSD display is performed (step S2: YES), the process proceeds to step S3.

  In step S <b> 3, the OSD video generated by the OSD processing unit 15 is displayed on the screen of the liquid crystal panel 16.

  In the next step S4, it is determined whether or not a synchronization signal is detected. When the external device 26 is operated to reproduce a video tape, a DVD, or the like, the signal processing unit 13 detects a synchronization signal included in the video signal input from the external input terminal 12. However, if the external device 26 is not operated, the video signal is not input from the external input terminal 12, and therefore the synchronization signal is not detected. If a synchronization signal is detected (step S4: YES), the process proceeds to step S7. If a synchronization signal is not detected (step S4: NO), the process proceeds to step S5.

  In step S7, it is determined whether or not the remote controller 23 has been operated. If operated (step S7: YES), the process proceeds to step S8. If not operated (step S7: NO), the process returns to step S4.

  In step S5, it is determined whether or not a predetermined time (for example, 10 minutes) has elapsed since the mode was switched in step S1. If the predetermined time has not elapsed (step S5: NO), the process proceeds to step S7. If the predetermined time has elapsed (step S5: YES), the process proceeds to step S6.

  In step S6, the transition to the power saving mode by the tube current control is performed, and as described above, the tube current of the backlight 17 is decreased based on the PWM signal from the PWM dimming circuit 30, and the screen brightness is increased. Reduce.

  In step S8, the power saving mode is canceled when the external input mode is not selected in step S1, the OSD display is not performed in step S2, or the remote control 23 is operated in step S7.

  After executing Steps S6 and S8, the above-described series of processing ends.

  According to the embodiment described above, the resistance values of the tube current adjustment circuits 34 and 35 can be changed using the existing dimming PWM signal, and therefore the transistors Q1 and Q2 are turned on / off in the microcomputer 21. Therefore, it is not necessary to generate a control signal for generating the control signal, and a port for outputting the control signal is not necessary. Therefore, the power consumption of the backlight 17 when no external signal is input can be reduced without complicating the system. Further, even when the number of ports of the microcomputer 21 is limited, it is possible to cope with it.

  The tube current adjusting circuits 34 and 35 include two resistors R2, R3 and R5, R6 connected in series, and transistors Q1, Q2 connected in parallel to one of these resistors R3, R6, Since the ON / OFF of the transistors Q1 and Q2 is controlled based on the PWM signal, the tube current of the backlight 17 can be controlled with a simple circuit configuration using resistors and transistors.

  Further, an integrating circuit 36 for integrating the PWM signal, and a comparator 37 for comparing the output voltage of the integrating circuit 36 with the reference voltage B and outputting a signal for turning on / off the transistors Q1 and Q2 based on the comparison result. Since the transistors Q1 and Q2 are turned off by the output of the comparator 37, the resistance values of the tube current adjusting circuits 34 and 35 are increased, and the tube current of the backlight 17 is reduced. The ON / OFF control of the transistors Q1 and Q2 can be easily realized.

  The present invention can adopt various embodiments other than those described above. For example, in the above embodiment, during the dimming operation, the PWM signal generated by the oscillation circuit 31 is not output to the integration circuit 36. However, as described above, the PWM signal may be output to the integration circuit 36. Good. In this case, in addition to the luminance adjustment of the backlight 17 by normal dimming control, the luminance adjustment by tube current control is also performed, so that the dimming range can be expanded.

  In the above embodiment, the PWM signal is also output to the switching circuit 32 during tube current control. However, as described above, the PWM signal may not be output to the switching circuit 32. In this case, reduction of power consumption by dimming control cannot be expected, but considerable power consumption can be reduced only by tube current control.

  In the above-described embodiment, an example in which the transistors Q1 and Q2 are provided in the tube current adjustment circuits 34 and 35 has been described. However, the transistors may be other switching elements such as FETs.

  In the above-described embodiment, an example in which a cold cathode tube is used as the backlight 17 is shown. However, a discharge tube such as an EEFL (External Electrode Fluorescent Lamp) may be used instead of the cold cathode tube. Good.

  Furthermore, in the above-described embodiment, an example in which the present invention is applied to the liquid crystal television 100 has been described. However, the present invention can also be applied to an apparatus such as a personal computer capable of connecting an external video device.

It is a front view which shows the external appearance of the liquid crystal television which concerns on embodiment of this invention. It is a block diagram which shows the electric constitution of a liquid crystal television. It is the figure which showed an example of the specific circuit of a backlight drive part. It is a flowchart showing the procedure of the power saving operation | movement by tube current control.

Explanation of symbols

12 External input terminal 15 OSD processing unit 16 Liquid crystal panel 17 Backlight 18 Backlight drive unit 21 Microcomputer (control unit)
25 Power supply 26 External device 30 PWM dimming circuit 31 Oscillation circuit 32 Switching circuit 33 Step-up transformer 34, 35 Tube current adjustment circuit 36 Integration circuit 37 Comparator 100 LCD television R1-R6 Resistor Q1, Q2 Transistor

Claims (3)

A backlight that is the light source of the liquid crystal panel;
A backlight driving unit for driving the backlight;
A control unit for controlling the backlight driving unit;
An OSD processing unit for generating an OSD (On Screen Display) image displayed on the liquid crystal panel;
In a liquid crystal display device comprising:
The backlight driving unit is configured to generate a PWM (Pulse Width Modulation) signal having a predetermined duty based on a control signal from the control unit, and to adjust a tube current of the backlight. A tube current adjusting circuit including a resistor,
In the external input mode in which a video signal is input from an external device, the control unit is in a state where the video signal is not input from the external device for a certain time while the OSD video is displayed on the liquid crystal panel. A liquid crystal display device, wherein the tube current of the backlight is reduced by changing a resistance value of the tube current adjusting circuit based on the PWM signal. The liquid crystal display device according to claim 1.
The tube current adjusting circuit includes two resistors connected in series and a transistor connected in parallel to one of these resistors,
A liquid crystal display device that controls ON / OFF of the transistor based on the PWM signal. The liquid crystal display device according to claim 2,
An integrating circuit for integrating the PWM signal;
A comparator that compares the output voltage of the integration circuit with a reference voltage and outputs a signal for turning on / off the transistor based on the comparison result; and
A liquid crystal display device characterized in that the tube current of the backlight is reduced by increasing the resistance value of the tube current adjusting circuit by turning off the transistor by the output of the comparator.

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