JP2002050498A - Cold cathode tube lighting device, electronic device with cold cathode tube, and liquid crystal display device - Google Patents

Abstract

(57) [Problem] To provide a cold-cathode tube lighting device with reduced load fluctuation when lighting and dimming the cold-cathode tube, and an electronic device with a cold-cathode tube and a liquid crystal display device including the same. SOLUTION: A plurality of lighting circuits 5a, 5b are provided corresponding to a plurality of cold cathode tubes 6a, 6b, respectively. Pulse signals C1, C2 are supplied from the dimming control circuit 7a to the lighting circuits 5a, 5b.
Is output. The lighting circuits 5a and 5b output pulse signals C1,
When C2 is at a high level, a high-frequency voltage is applied to the cold-cathode tubes 6a and 6b, and when C2 is at a low level, the application of the high-frequency voltage is suspended. The dimming control circuit 7a outputs the pulse signals C1, C2
By adjusting the ratio between the application period of the high-frequency voltage and the idle period by changing the duty cycle, the dimming is performed. The dimming control circuit 7a matches the falling timing of the pulse signal C1 with the rising timing of the pulse signal C2 to chain the application periods of the lighting circuits 5a and 5b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-cathode tube lighting device for lighting and dimming a cold-cathode tube, and an electronic device with the cold-cathode tube and a liquid crystal display device having the same.

[0002]

2. Description of the Related Art Conventionally, electronic devices with a cold cathode tube, such as a liquid crystal display device using a cold cathode tube as a light source, include:
A cold-cathode tube lighting device that illuminates a cold-cathode tube and controls dimming is incorporated.

In the conventional liquid crystal display device shown in FIG. 7, power is supplied from a power supply circuit 2 to a drive circuit 3 and two lighting circuits 5a and 5b. When the liquid crystal panel 4 is driven by the drive circuit 3, the two lighting circuits 5
The two cold-cathode tubes 6a and 6b are turned on by a and 5b, respectively.

The lighting circuits 5a and 5b are constituted by inverter circuits having a switching operation frequency of about several tens of kHz. For dimming the cold cathode tubes 6a and 6b, a pulse signal C5 having a frequency of about several hundred Hz shown in FIG. 8 is simultaneously output from the dimming control circuit 7 to the two lighting circuits 5a and 5b. Lighting circuit 5 when pulse signal C5 is at high level
Since the high-frequency voltage is applied to the cold-cathode tubes 6a and 6b from the terminals a and 5b and the application of the high-frequency voltage to the cold-cathode tubes 6a and 6b is stopped when the level is low, the power supply circuit 2 supplies the lighting circuits 5a and 5b respectively. Currents i5 and i6 as shown in FIG. 8 flow. Assuming that the magnitude of the current flowing through each of the lighting circuits 5a and 5b is αA (ampere), the lighting circuits 5a and 5b
Is applying a high-frequency voltage to the cold-cathode tubes 6a, 6b, the current i flowing from the power supply circuit 2 to the lighting circuits 5a, 5b
The sum of 5, i6 is 2αA.

The light emission luminance of the cold cathode tubes 6a and 6b is controlled by pulse modulation of a pulse signal 5C in a dimming control circuit 7. If the high-level period of the pulse signal 5C is lengthened and the low-level period is shortened, the cold-cathode tubes 6a and 6b become brighter, and the high-level period of the pulse signal 5C is shortened and the low-level period is lengthened. For example, cold cathode tubes 6a and 6b
Darkens.

[0006]

In the conventional liquid crystal display device 1, the lighting circuits 5a and 5b are connected to the cold cathode tubes 6a and 6a.
The sum of the currents i5 and i6 flowing through the two lighting circuits 5a and 5b when the high-frequency voltage is applied to 6b is 2α
A. On the other hand, from the lighting circuits 5a and 5b, the cold cathode tubes 6
The sum of the currents i5 and i6 flowing when the application of the high-frequency voltage to a and 6b is stopped is zero. Therefore, the current flowing through the plurality of lighting circuits 5a and 5b greatly changes between 0 and 2αA. This large change in the current increases the load on the power supply circuit 2 and degrades the power supply performance of power supply to the drive circuit 3.

For example, one of the power supply performances required for the power supply circuit 2 is the stability of the output voltage with respect to a change in load current (load regulation). A difference in the load current causes a difference in the output voltage, and a dip, overshoot, ringing, or the like occurs in the output voltage when the load current changes. If the performance of the power supply circuit 2 is improved to keep the output voltage constant irrespective of a large change in the load current, the cost will increase.

The factors that determine the performance of the power supply circuit 2 include:
In addition to the performance of components such as the resistors constituting the power supply circuit 2, the drive circuit 3 and the lighting circuits 5a and 5b and the power supply circuit 2
And the impedance of a ground line for grounding the power supply circuit 2 and the power supply line connecting the power supply circuit 2 and the like. If there is a large change in current at a common impedance such as a power supply line and an earth line, the output voltage will fluctuate. Although the separation of only the power supply line is relatively easy to implement, the separation of the ground line complicates the structure of the power supply circuit 2 and leads to an increase in cost.

In the liquid crystal display device 1 shown in FIG. 7, the power supply circuit 2 supplies a single power supply line to the lighting circuits 5a and 5b and the drive circuit 3. However, in general, a multi-output power supply circuit also has a large number of power supply lines. As long as a circuit portion common to all output terminals is provided in the output power supply circuit, the stability (cross regulation) of the output voltage of the other output terminal against a change in the load current of one output terminal becomes a problem. For the improvement, for example, it is necessary to make the power supply circuits individually independent, which leads to an increase in cost.

Further, a large change in the load current means that the stress applied to the components of the power supply circuit 2 also increases. Further, it is difficult to remove the fluctuation of the output voltage of the power supply circuit 2 having a relatively low frequency of about several hundred Hz with a simple LC filter or the like.

An object of the present invention is to provide a cold-cathode tube lighting device with reduced load fluctuation when lighting and dimming the cold-cathode tube, an electronic device with a cold-cathode tube including the same, and a liquid crystal display device. It is.

[0012]

(First Invention) A cold cathode tube lighting device according to a first invention is a cold cathode tube lighting device for lighting and dimming a plurality of cold cathode tubes. A plurality of voltage applying means for applying a voltage to each of the cold cathode tubes,
Repeating the application and suspension of the voltage to the plurality of cold cathode tubes by the plurality of voltage applying units at a predetermined cycle, and adjusting the ratio between the application period of the voltage by each voltage applying unit and the suspension period within each cycle. Dimming control means for dimming a plurality of cold-cathode tubes, wherein the dimming control means uses a plurality of voltage applying means so that a total change amount of current consumption in the plurality of voltage applying means is minimized. The phases of the voltage application periods are shifted from each other.

In the cold-cathode tube lighting device of the present invention, a voltage is applied to each of the plurality of cold-cathode tubes by the plurality of voltage applying means, and the plurality of cold-cathode tubes are lit. And
The dimming control is performed by adjusting the ratio of the voltage application period by each voltage applying unit to the pause period in each cycle by the dimming control unit. At the time of dimming, the phases of the voltage application periods are shifted from each other by the dimming control unit so that the total change amount of the current consumption in the plurality of voltage applying units is minimized. As a result, it is possible to minimize the load fluctuation seen from the power supply circuit that supplies power to the plurality of application units in parallel, and to stabilize the output voltage of the power supply circuit that supplies power to the plurality of voltage application units. be able to.

(Second Invention) A cold-cathode tube lighting device according to a second invention is a cold-cathode tube lighting device according to the first invention, wherein the dimming control means includes a plurality of voltage applying means. The phase of the start timing of the application period is sequentially shifted by a predetermined time, and the predetermined time is a time corresponding to the voltage application period by each voltage application unit.

In this case, the phase of the start timing of the voltage application period by the plurality of voltage application units is sequentially shifted by the time corresponding to the voltage application period by each voltage application unit by the dimming control unit. As a result, when any of the plurality of voltage applying units stops applying the voltage, one of the remaining voltage applying units starts applying the voltage. As described above, the voltage application periods of the plurality of voltage application units can be linked by simple control of shifting the voltage application periods by the time corresponding to the voltage application period. Thus, the amount of change in the total current consumption of the plurality of voltage applying means can be minimized, and the configuration of the cold-cathode tube lighting device can be simplified.

(Third invention) In a cold cathode tube lighting device according to a third invention, in the configuration of the cold cathode tube lighting device according to the first invention, the dimming control means includes a plurality of voltage applying means. The phase of the start timing of the application period is sequentially shifted by a predetermined time, and the predetermined time corresponds to a time obtained by equally dividing one cycle by the number of the voltage applying means.

In this case, the dimming control means shifts the phase of the start timing of the voltage application period by the plurality of voltage applying means in order of time obtained by equally dividing one cycle by the number of the plurality of voltage applying means. As a result, when the voltage application period is shorter than the equally divided time, the voltage is applied by one of the plurality of voltage applying means, and the voltage application period is the equally divided time n (n is 1). Each time the number of times becomes equal to or more than (integer) times, only a situation occurs in which n voltage applying means are simultaneously in the application period, or a situation in which (n + 1) voltage applying means are simultaneously in the application period. This makes it possible to minimize the amount of change in the total amount of current consumed by the plurality of voltage applying means and simplify the configuration of the cold-cathode tube lighting device by simple control of sequentially shifting the time by the equally divided time.

(Embodiment 4) An electronic device with a cold cathode tube according to the fourth invention is a plurality of cold cathode tubes and any one of the first to third embodiments for lighting and dimming the plurality of cold cathode tubes. A cold-cathode tube lighting device according to the invention, a power supply circuit for supplying power to a plurality of voltage applying means of the cold-cathode tube lighting device, and power supply from the power supply circuit in parallel with power supply to the plurality of voltage applying devices And an electronic circuit for receiving the signal.

Since the electronic device with a cold-cathode tube according to the present invention includes the cold-cathode tube lighting device according to any one of the first to third aspects, a power supply for supplying power in parallel to a plurality of voltage applying means is provided. Load fluctuations as seen from the circuit can be minimized, and the output voltage of the power supply circuit that supplies power to the plurality of voltage applying means can be stabilized. Thus, the output voltage output from the power supply circuit to the electronic circuit can be stabilized, and the electronic circuit can operate stably.

(Fifth Invention) A liquid crystal display device according to a fifth invention comprises a liquid crystal panel, driving means for driving the liquid crystal panel, a plurality of cold cathode tubes serving as a light source of the liquid crystal panel, and a plurality of cold cathodes. First to third lighting and dimming of the tube
And a power supply circuit for supplying electric power to the plurality of voltage applying means and the driving means of the cold-cathode tube lighting device in parallel.

Since the liquid crystal display device according to the present invention includes the cold-cathode tube lighting device according to any one of the first to third aspects, the liquid crystal display device is viewed from a power supply circuit that supplies power in parallel to a plurality of voltage applying means. The load fluctuation can be minimized, and the output voltage of the power supply circuit that supplies power to the plurality of voltage applying means can be stabilized. This makes it possible to stabilize the output voltage output from the power supply circuit to the driving means for driving the liquid crystal panel, and to perform stable display on the liquid crystal panel.

[0022]

(Embodiment 1) Hereinafter, a liquid crystal display device will be described as an example of an electronic device with a cold cathode tube according to Embodiment 1 of the present invention.

FIG. 1 is a block diagram showing the configuration of the liquid crystal display device of the first embodiment having a plurality of cold cathode tubes.

The liquid crystal display device 1a shown in FIG.
Power supply circuit 2, drive circuit 3, liquid crystal panel 4, lighting circuit 5
a, 5b, cold-cathode tubes 6a, 6b and dimming control circuit 7a
Is provided. Lighting circuits 5a, 5b and dimming control circuit 7a
Constitutes a cold-cathode tube lighting device 8a for lighting the cold-cathode tubes 6a and 6b for dimming.

The power supply circuit 2 includes a liquid crystal display device 1a
It supplies power to the whole. The drive circuit 3 operates by receiving power supply from the power supply circuit 2 and outputs a video signal to the liquid crystal panel 4 to drive a liquid crystal panel 4 described later.
Further, the drive circuit 3 outputs a control signal to a dimming control circuit 7a to be described later in order to set the brightness of the liquid crystal panel 4.
The liquid crystal panel 4 displays an image in response to the image signal output from the drive circuit 3.

The lighting circuits 5a and 5b are constituted by small and highly efficient inverter circuits. Lighting circuit 5a, 5b
Operate by receiving power supply from the power supply circuit 2, and output pulse signals C1 and C1 output from a dimming control circuit 7a to be described later.
When 2 is at a high level, a high frequency voltage of several tens of kHz is applied to cold cathode tubes 6a and 6b described later. Dimming control circuit 7a
When the pulse signals C1 and C2 output from are low, the lighting circuits 5a and 5b are turned off to suspend the application of the high-frequency voltage to the cold cathode tubes 6a and 6b. Lighting circuits 5a, 5b, which apply a high-frequency voltage to the cold cathode tubes 6a, 6b,
5b, currents i1 and i2 of αA (ampere) are supplied from the power supply circuit 2 respectively.

The cold cathode tubes 6a and 6b are provided with lighting circuits 5a and 5b.
Illuminated by applying a high-frequency voltage to the liquid crystal panel 4. Here, two cold cathode tubes 6a and 6b are used for one liquid crystal panel 4, but the number of the cold cathode tubes is set according to the size of the liquid crystal panel 4, the application, and the like.

The dimming control circuit 7a performs on / off control of the inverter circuit operation of the lighting circuits 5a and 5b in accordance with the control signal output from the driving circuit 3 to control the cold cathode tubes 6a and 6b.
Adjust the light emission luminance of b. Therefore, the dimming control circuit 7a
Outputs pulse signals C1 and C2 of about several hundred Hz to the lighting circuits 5a and 5b, respectively.

The dimming control circuit 7a outputs the pulse signals C1, C
The second duty cycle can be changed.
That is, the dimming control circuit 7a performs pulse width modulation of the pulse signals C1 and C2 output to the lighting circuits 5a and 5b. The dimming control circuit 7a makes the timing when the pulse signal C1 changes from high level to low level coincide with the timing when the pulse signal C2 changes from low level to high level. Therefore, the dimming control circuit 7a includes, for example, a detection circuit that detects the rising timing and the falling timing of the pulse signal C1, and delays the pulse signal C2 based on the detection result of the detection circuit to generate the pulse signal C1.
And a delay circuit for matching the rising timing of the pulse signal C2 with the falling timing of the pulse signal C2.

The dimming control circuit 7a detects the timing when the pulse signal C1 goes high and the time during which the pulse signal C1 goes high based on the detection result of the detection circuit.
The delay circuit delays the rising timing of the pulse signal C2 from the rising timing of the pulse signal C1 by the time during which the pulse signal C1 is at the high level. Thereby, the rising timing of the pulse signal C1 and the falling timing of the pulse signal C2 can be matched.

The liquid crystal display device 1a according to the present embodiment
, The lighting circuits 5a and 5b correspond to a plurality of voltage applying units, the dimming control circuit 7a corresponds to a dimming control unit, and the driving circuit 3 corresponds to an electronic circuit and a driving unit.

FIGS. 2 and 3 show a liquid crystal display device 1.
It is a timing chart which shows operation | movement of a.

FIG. 2 shows the operation of the liquid crystal display device 1a when the low level period is longer than the high level period in one cycle of the pulse signals C1 and C2.

When the pulse signal C1 output from the dimming control circuit 7a is at a high level, a high-frequency voltage is applied from the lighting circuit 5a to the cold cathode tube 6a. Accordingly, a current i1 flows from the power supply circuit 2 to the lighting circuit 5a. Current i1
Is αA. At this time, the pulse signal C2 is at the low level, and the application of the high-frequency voltage from the lighting circuit 5b to the cold cathode tube 6b is suspended. Therefore, no current flows from the power supply circuit 2 to the lighting circuit 5b, and the current i2
Is 0. As a result, the lighting circuit 5a,
The total (i1 + i2) of the current flowing to 5b is αA.

Next, when the pulse signal C1 changes from the high level to the low level, the lighting circuit 5a outputs the cold cathode fluorescent lamp 6a.
The current i1 flowing from the power supply circuit 2 to the lighting circuit 5a becomes 0 as the application of the high frequency voltage to the power supply circuit is stopped. On the other hand, at the timing when the pulse signal C1 changes from the high level to the low level, the pulse signal C2 changes from the low level to the high level. At this time, the application of the high-frequency voltage from the lighting circuit 5b to the cold-cathode tube 6b is started, and a current i2 having a magnitude αA flows from the power supply circuit 2 to the lighting circuit 5b. Therefore, the total (i1 + i2) of the currents flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is αA.

Next, the pulse signal C2 changes from the high level to the low level. At this time, the current i2 becomes 0 as the application of the high-frequency voltage from the lighting circuit 5b to the cold cathode tube 6b is stopped. At this time, the pulse signal C1 is at a low level, that is, the lighting circuit 5a is
Since the application of the high-frequency voltage to a remains paused,
The current i1 flowing from the power supply circuit 2 to the lighting circuit 5a is 0. Therefore, the total (i1 + i2) of the currents flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is zero.

FIG. 3 shows the operation of the liquid crystal display device 1a when the low level period is shorter than the high level period in one cycle of the pulse signals C1 and C2.

The pulse signal C1 output from the dimming control circuit 7a changes from a low level to a high level. As a result, application of the high-frequency voltage from the lighting circuit 5a to the cold-cathode tube 6a is started, and a current i1 having a magnitude αA flows. At this time, the pulse signal C2 is at a high level and the cold cathode fluorescent lamp 6b
The lighting circuit 5b that applies a high-frequency voltage to the
A current i2 having a magnitude αA flows. Therefore, the sum (i) of the currents flowing from the power supply circuit 2 to the lighting circuits 5a and 5b
1 + i2) becomes 2αA.

Subsequently, the pulse signal C2 changes from the high level to the low level while the pulse signal C1 is at the high level. When the pulse signal C2 goes low, the application of the high-frequency voltage from the lighting circuit 5b to the cold cathode tube 6b is stopped, and the current i2 becomes zero. Therefore, the total (i1 + i2) of the current flowing from the current circuit 2 to the lighting circuits 5a and 5b is αA.

Next, when the pulse signal C1 changes from the high level to the low level, the lighting circuit 5a outputs the cold cathode fluorescent lamp 6a.
And the current i1 becomes zero. On the other hand, at the timing when the pulse signal C1 changes from the high level to the low level, the pulse signal C2 changes from the low level to the high level. As a result, the application of the high-frequency voltage from the lighting circuit 5b to the cold-cathode tube 6b is started, and a current i2 having a magnitude αA flows from the power supply circuit 2 to the lighting circuit 5b. Therefore, the total (i1 + i2) of the currents flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is αA.

As described above, when the low level period of the pulse signals C1 and C2 is longer than the high level period in one cycle, the total (i1 + i2) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is αA or 0. When the low-level period of the pulse signals C1 and C2 is shorter than the high-level period in one cycle, the total (i) of the currents flowing from the power supply circuit 2 to the lighting circuits 5a and 5b (i
1 + i2) is αA or 2αA, and the magnitude of the change in current is αA. As described above, the magnitude of the change in the current flowing from the power supply circuit 2 of the conventional liquid crystal display device 1 to the lighting circuits 5a and 5b is 2αA. The magnitude of the change in the current flowing through 5a, 5b is halved. As a result, it is possible to reduce the load fluctuation when the cold cathode tubes 6a and 6b are turned on, and to stabilize the output voltage of the power supply circuit 2 that supplies power to the drive circuit 3 and the lighting circuits 5a and 5b. it can.

In the description of the first embodiment, the liquid crystal display device 1a having two cold cathode tubes and two power supply systems has been described. However, three or more cold cathode tubes are provided. The present invention can also be applied to a liquid crystal display device having three or more power supply systems.

For example, n lighting circuits are provided for each of n (n ≧ 3) cold cathode tubes, and the n lighting circuits are connected to one power supply circuit. In order to control the n lighting circuits, the dimming control circuit outputs n types of pulse signals CC1 to CCn to the n lighting circuits. The dimming control circuit
The pulse signals CC2 to CCn are sequentially applied so that the pulse signals CC1 to CCn all have the same predetermined frequency and the falling of the pulse signal CC (ni) coincides with the rising of the pulse signal CC (ni + 1). The output is delayed so that the high-level periods of the pulse signals CC1 to CCn are linked. Here, i is an integer of 1 ≦ i ≦ n−1. Pulse signal CC for dimming by dimming control circuit
Although the duty cycles of 1 to CCn are changed, for example, the pulse signals CC1 to CCn are set so that only the phases are different.

In this case, when the duty cycle of the pulse signals CC1 to CCn is 1 / n or less, the current output from the power supply circuit to the n lighting circuits is one lighting circuit or zero. When the value is not less than 1 / n and not more than 2 / n, the current output from the power supply circuit to the n lighting circuits is equivalent to two lighting circuits or one lighting circuit. When the value is equal to or more than i for n and not more than (i + 1) for n, the current output from the power supply circuit to the n lighting circuits is equivalent to the lighting circuit i + 1 or i.
It is individual. In either case, the magnitude of the current fluctuation is
The magnitude of the fluctuation of the current caused by the application of the high-frequency voltage and the suspension of the application in one lighting circuit is the same as that of the lighting circuit, and the load fluctuation of the power supply circuit when lighting and dimming the cold-cathode tube can be reduced. In addition, the output voltage of the power supply circuit that supplies power to the n lighting circuits and the driving circuits can be stabilized.

Second Embodiment Next, a liquid crystal display device according to a second embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to FIG.

FIG. 4 is a block diagram showing a configuration of the liquid crystal display device of the second embodiment having a plurality of cold cathode tubes.

The liquid crystal display device 1b shown in FIG.
Power supply circuit 2, drive circuit 3, liquid crystal panel 4, lighting circuit 5
a, 5b, cold-cathode tubes 6a, 6b and dimming control circuit 7b
Is provided. Lighting circuits 5a, 5b and dimming control circuit 7b
Constitutes a cold-cathode tube lighting device 8b for lighting the cold-cathode tubes 6a and 6b for dimming.

The configuration of the liquid crystal display device 1b according to the second embodiment shown in FIG.
Except that the function is different from that of the dimming control circuit 7a of the liquid crystal display device 1a according to the first embodiment described above, and the description of the components other than the dimming control circuit 7b is omitted.

The dimming control circuit 7b performs on / off control of the operation of the inverter circuits of the lighting circuits 5a and 5b in accordance with the control signal output from the driving circuit 3 to control the cold cathode tubes 6a and 6b.
Adjust the light emission luminance of b. Therefore, the dimming control circuit 7b
Outputs pulse signals C3 and C4 of about several hundred Hz to the lighting circuits 5a and 5b, respectively.

The dimming control circuit 7b outputs pulse signals C3 and C
4 can be changed.
That is, the dimming control circuit 7b performs pulse width modulation of the pulse signals C3 and C4 output to the lighting circuits 5a and 5b. The timing at which the pulse signal C4 output from the dimming control circuit 4b rises is delayed by a half cycle with respect to the timing at which the pulse signal C3 rises. That is, the phase of the pulse signal C4 is delayed by 180 ° with respect to the phase of the pulse signal C3.

FIGS. 5 and 6 show the liquid crystal display device 1.
It is a timing chart which shows operation | movement of b.

FIG. 5 shows the operation of the liquid crystal display device 1b when the low level period is longer than the high level period in one cycle for the pulse signals C3 and C4.

When the pulse signal C3 output from the dimming control circuit 7b changes from a low level to a high level, application of a high-frequency voltage from the lighting circuit 5a to the cold-cathode tube 6a is started, and the power supply circuit 2 turns on the lighting circuit. Current i3 flowing to 5a
Becomes αA. At this time, the pulse signal C4 is at the low level, the application of the high-frequency voltage from the lighting circuit 5b to the cold-cathode tube 6b is suspended, and the current i4 is 0. As a result, the total (i3 + i4) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b becomes αA.

Next, when the pulse signal C3 changes from the high level to the low level, the lighting circuit 5a outputs the cold cathode fluorescent lamp 6a.
The application of the high-frequency voltage to is stopped, and the current i3 becomes zero. At this time, since the pulse signal C4 is at the low level, the application of the high-frequency voltage from the lighting circuit 5b to the cold-cathode tube 6b is suspended, and the current i4 is 0. as a result,
The total (i3 + i4) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is 0A.

Next, the pulse signal C4 changes from the low level to the high level, the application of the high-frequency voltage from the lighting circuit 5b to the cold-cathode tube 6b is started, and the current i4 becomes αA.
At this time, the current i3 is 0 because the pulse signal C3 is at the low level. As a result, from the power supply circuit 2 to the lighting circuit 5
The sum (i3 + i4) of the currents flowing to a and 5b is αA.

Next, when the pulse signal C4 changes from the high level to the low level, the lighting circuit 5b sends the cold cathode fluorescent lamp 6b
The application of the high frequency voltage to is stopped, and the current i4 becomes zero. At this time, the current i3 is 0 because the pulse signal C3 is at the low level. As a result, the total (i3 + i4) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is 0 A
become.

FIG. 6 shows the operation of the liquid crystal display device 1b when the low level period is shorter than the high level period in one cycle for the pulse signals C3 and C4 output from the dimming control circuit 7b. ing.

When the pulse signal C3 output from the dimming control circuit 7b changes from a low level to a high level, application of a high-frequency voltage from the lighting circuit 5a to the cold cathode tube 6a is started, and the current i3 becomes αA. At this time, the pulse signal C
Since 4 is at the high level, the current i4 is αA. As a result, the total (i3 + i4) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is 2αA.

Subsequently, when the pulse signal C4 changes from the high level to the low level, the lighting circuit 5b switches the cold cathode fluorescent lamp 6 from the lighting circuit 5b.
The application of the high-frequency voltage to b is stopped, and the current i4 becomes zero. At this time, since the pulse signal C3 is at the low level, the current i3 is αA. As a result, the total (i3 + i4) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is α
Become A.

Next, when the pulse signal C4 changes from the low level to the high level, the lighting circuit 5b switches the cold cathode tube 6b from the lighting circuit 5b.
When the application of the high-frequency voltage to the current is started, the current i4 becomes αA. At this time, since the pulse signal C3 is at the high level, the current i3 is αA. As a result, the total current (i3 + i4) flowing from the power supply circuit 2 to the lighting circuits 5a and 5b is 2
αA.

Subsequently, when the pulse signal C3 changes from the high level to the low level, the current i3 becomes 0. At this time, since the pulse signal C4 is at the high level, the current i4 is αA. As a result, the lighting circuit 5a,
The total (i3 + i4) of the current flowing to 5b is αA.

As described above, when the low level period of the pulse signals C3 and C4 is shorter than the high level period in one cycle, the total current (i3 + i4) flowing from the power supply circuit 2 to the lighting circuits 5a and 5b becomes αA or 0. When the low-level period of the pulse signals C3 and C4 is shorter than the high-level period in one cycle, the total (i) of the current flowing from the power supply circuit 2 to the lighting circuits 5a and 5b (i
3 + i4) is 2αA or αA, and the magnitude of the change in current is αA. As described above, the magnitude of the change in the current flowing from the power supply circuit 2 of the conventional liquid crystal display device 1 to the lighting circuits 5a and 5b is 2αA. The magnitude of the change in the current flowing through 5a, 5b is halved. As a result, load fluctuations when the cold cathode tubes 6a and 6b are turned on can be reduced, and the output voltage of the power supply circuit 2 that supplies power to the drive circuit 3 and the lighting circuits 5a and 5b can be stabilized. Can be.

In the above description of the second embodiment, the liquid crystal display device 1b having two cold cathode tubes and two power supply systems has been described. However, three or more cold cathode tubes are provided. The present invention can also be applied to a liquid crystal display device having three or more power supply systems.

For example, n lighting circuits are provided for each of n (n ≧ 3) cold cathode tubes, and the n lighting circuits are connected to one power supply circuit. In order to control the n lighting circuits, the dimming control circuit outputs n types of pulse signals CC1 to CCn to the n lighting circuits. The dimming control circuit
The pulse signals CC1 to CCn all have the same predetermined frequency, and are delayed in order of 1 / n cycle from the rise of the pulse signal CC (n-i) to the rise of the pulse signal CC (n-i + 1). The pulse signals CC1 to CCn are sequentially delayed so that the 360 ° / n phase is delayed. Here, i is an integer of 1 ≦ i ≦ n−1. The pulse signal CC1 to CCn is used for dimming by the dimming control circuit.
For example, the pulse signals CC1 to CCn are set so that only the phases are different.

In this case, when the duty cycle of the pulse signals CC1 to CCn is 1 / n or less, the current output from the power supply circuit to the n lighting circuits is one lighting circuit or 0. When the value is not less than 1 / n and not more than 2 / n, the current output from the power supply circuit to the n lighting circuits is equivalent to two lighting circuits or one lighting circuit. In either case,
The magnitude of the current fluctuation is the same as the magnitude of the current fluctuation caused by the application of the high-frequency voltage and the suspension of the application in one lighting circuit, and the power supply circuit for lighting and dimming the cold cathode tubes. Load fluctuation can be reduced, and the output voltage of the power supply circuit that supplies power to the n lighting circuits and the drive circuits can be stabilized.

In the first and second embodiments, power is supplied from one output terminal of the power supply circuit 2 to the drive circuit 3 and the lighting circuits 5a and 5b.
The driving circuit 3 and the lighting circuits 5a, 5
The present invention can be applied to a multi-output power supply circuit that supplies power to b. Even in such a case, it is possible to reduce the load fluctuation of the power supply circuit at the time of lighting and controlling the dimming of the cold-cathode tube, and to stabilize the output voltage of the power supply circuit that supplies power to the lighting circuit and the drive circuit. Can be.

[0067]

According to the cold-cathode tube lighting device, the electronic device with the cold-cathode tube or the liquid crystal display device of the present invention, the dimming control means minimizes the total change in the current consumption in the plurality of voltage applying means. Thus, the phases of the voltage application periods are shifted from each other. As a result, it is possible to minimize the load fluctuation seen from the power supply circuit that supplies power to the plurality of application units in parallel, and to stabilize the output voltage of the power supply circuit that supplies power to the plurality of voltage application units. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a liquid crystal display device according to Embodiment 1 of the present invention.

FIG. 2 is a timing chart for explaining the operation of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 3 is a timing chart for explaining the operation of the liquid crystal display device according to the first embodiment of the present invention.

FIG. 4 is a block diagram illustrating a configuration of a liquid crystal display device according to a second embodiment of the present invention.

FIG. 5 is a timing chart for explaining an operation of the liquid crystal display device according to the second embodiment of the present invention.

FIG. 6 is a timing chart for explaining the operation of the liquid crystal display device according to the second embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a conventional liquid crystal display device.

FIG. 8 is a timing chart for explaining the operation of a conventional liquid crystal display device.

[Explanation of symbols]

 1a, 1b Liquid crystal display device 2 Power supply circuit 3 Drive circuit 4 Liquid crystal panel 5a, 5b Lighting circuit 6a, 6b Cold cathode tube 7a, 7b Dimming control circuit 8a, 8b Cold cathode tube lighting device

Claims (5)

[Claims] 1. A cold-cathode tube lighting device for lighting and dimming a plurality of cold-cathode tubes, comprising: a plurality of voltage applying means for applying a voltage to each of the plurality of cold-cathode tubes; Means for applying and suspending the voltage to the plurality of cold-cathode tubes at a predetermined cycle, and adjusting the ratio between the voltage application period and the pause period by each voltage applying means within each cycle. Dimming control means for dimming the cold-cathode tubes, wherein the dimming control means comprises a plurality of the voltage applying means such that a total change amount of current consumption in the plurality of voltage applying means is minimized. Wherein the phases of the voltage application periods are shifted from each other. 2. The dimming control unit shifts a phase of a start timing of a voltage application period by the plurality of voltage application units in order of a predetermined time, and the predetermined time corresponds to a voltage application period by each voltage application unit. 2. The cold-cathode tube lighting device according to claim 1, wherein the time is a time for performing the operation. 3. The dimming control unit shifts the phase of the start timing of the voltage application period by the plurality of voltage application units in order of a predetermined time, and the predetermined time corresponds to one cycle of the number of the plurality of voltage application units. 2. The cold-cathode tube lighting device according to claim 1, wherein the time corresponds to the time divided equally. 4. A plurality of cold-cathode tubes, and lighting and dimming of the plurality of cold-cathode tubes.
3. A cold-cathode tube lighting device according to any one of claims 3, a power supply circuit that supplies power to the plurality of voltage applying units of the cold-cathode tube lighting device, and a power supply to the plurality of voltage applying units. An electronic circuit provided with power from the power supply circuit. 5. A liquid crystal panel, driving means for driving the liquid crystal panel, a plurality of cold cathode tubes serving as a light source of the liquid crystal panel, and lighting and dimming of the plurality of cold cathode tubes.
3. The cold-cathode tube lighting device according to any one of (3), and a power supply circuit that supplies power to the plurality of voltage applying means and the driving means of the cold-cathode tube lighting device in parallel. Liquid crystal display device.