JP4300810B2 - Discharge lamp lighting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in a discharge lamp lighting device that performs digital dimming control digitally in video equipment (for example, a liquid crystal television) using a cold cathode ray tube as a light source.
[0002]
[Prior art]
Generally, a cold cathode ray tube is used as a light source of a liquid crystal display panel. The light emission luminance is controlled by controlling the current flowing through the tube (tube current). As the control, the current peak value and the current average value are each performed by PWM control.
[0003]
Specifically, the power supply of the oscillation circuit is intermittently controlled at a frequency of about 100 kHz to control the peak value of the current flowing through the lamp, and the average current is controlled intermittently at a frequency of several hundred Hz.
[0004]
As one of the conventional tube current control methods, a configuration of a discharge lamp lighting circuit similar to that disclosed is shown in FIG. 7 (see Patent Document 1).
[0005]
In FIG. 7, 91 is a cold cathode ray tube, 89 is an inverter circuit configured by a Royer circuit and generates a high-voltage AC voltage, 87 is a switch circuit for intermittently operating the inverter circuit, and 90 is the negative resistance. A ballast capacitor which is an impedance for stabilizing the operation of the cathode ray tube 91, a tube current detection unit 92 which converts an alternating current flowing through the cold cathode ray tube into a voltage value, and performs rectification and smoothing, and 88 is the tube current detection unit A sampling and holding circuit that samples and holds the signal from 92, 86 is a reference voltage source that is a reference for tube current control, and 85 is an arithmetic operation that outputs the difference between the voltage from the sample and holding circuit and the signal from the sample and holding circuit 88 An amplifier, 84 is an adder for adding the output of the operational amplifier 85 and a luminance set value, and 81 is P A triangular wave generator 82 for generating a triangular wave serving as a reference for M control is for comparing the outputs of the triangular wave generator 81 and the adder 84 and controlling the switch circuit 87 and the sample hold circuit to perform burst dimming. It is a comparator which generates the PWM pulse.
[0006]
The operation of the conventional example will be described with reference to FIG. 8, (a) is an output waveform of the triangular wave generator, (b) is a luminance setting value for setting the light emission luminance of the discharge tube, (c) is an output of the operational amplifier 85, and (d) is an output of the adder 84. The output (e) is the output of the comparator 82.
[0007]
The inverter circuit 89 is operated by the primary power supply, and the operation is interrupted by the switch circuit 87 by the output of the comparator 82, the current as shown in (f) flows through the cold cathode ray tube 91, and the voltage is detected by the current detection unit 92. Converted, rectified and smoothed. The sample hold circuit 88 samples and holds the output of the tube current detector 92 with the output pulse of the comparator 82.
[0008]
The operational amplifier 85 takes the difference between the output voltage of the sample power supply circuit 86 and the reference power supply 86, which is the reference value of the tube current. If the output of the sample hold circuit is larger than that of the reference power supply 86, the output value is large. Accordingly, the output of the adder 84 also increases or decreases accordingly. As a result, if the tube current is larger than the luminance setting value, the operation time of the inverter circuit 89 is shortened. Thus, the average current flowing through the cold cathode ray tube 91 can be controlled to a value corresponding to the luminance setting value.
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-43088
[Problems to be solved by the invention]
In a liquid crystal TV, the backlight luminance is generally controlled in real time according to the display image. However, since the peak control of the tube current is based on an analog circuit, the response speed is accurately managed. Is difficult. As a means for accurately managing the tube current, digitization of control is conceivable. Usually, an LCD panel driving clock is used as a reference of PWM, and one PWM cycle is a fraction of that integer.
[0011]
As a general example, since the panel drive clock frequency is 30 MHz and the tube current control PWM frequency is 100 kHz, the duty control resolution is 300 steps. As a general characteristic of the lamp, the tube current becomes unstable when the duty ratio is smaller than a certain level, and saturates when the duty ratio increases to a certain level. Therefore, the actual controllable range is between about 60% and 80% duty, and the resolution between them. In the case of the above-described example, there is a problem that it becomes as rough as about 60 steps.
[0012]
In view of the above problems, the present invention provides a discharge lamp lighting device capable of digitizing tube current control and performing it with high accuracy.
[0013]
[Means for Solving the Problems]
In view of the above problems, the present invention performs control of a current flowing in a discharge tube using a PWM pulse by a digital circuit in a video device using a cold cathode ray tube as a light source, and a clock serving as a reference of the PWM is a PLL or the like. A discharge lamp that solves the problem by multiplying by a circuit, expanding the resolution of the PWM by a bit reduction circuit, or changing the period of the PWM in accordance with a control value and guaranteeing a certain ON period or more. A lighting device is provided.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The discharge lamp lighting device according to claim 1 of the present invention is characterized in that an LCD panel drive clock multiplied by an integer is used as a PWM reference clock used for peak value control of the current flowing in the discharge tube. is there.
[0015]
The discharge lamp lighting device according to claim 2 of the present invention is a more specific example of the device according to claim 1.
[0016]
According to a third aspect of the present invention, there is provided a discharge lamp lighting device using an LCD panel drive clock as a PWM reference clock used for controlling a peak value of a current flowing in a discharge tube, and setting a duty value of a PWM pulse to a time by a bit reduction circuit. It is characterized by being dispersed in the direction.
[0017]
A discharge lamp lighting device according to a fourth aspect of the present invention is a more specific example of the device according to the third aspect.
[0018]
The discharge lamp lighting device according to claim 5 of the present invention uses an LCD panel drive clock as a PWM reference clock used for peak value control of the current flowing in the discharge tube, and when the tube current control value is larger than n, the PWM The ON period is increased while keeping the period constant, and when it is less than or equal to n, the ON period is fixed and the OFF period is increased so as to extend the period.
[0019]
A discharge lamp lighting device according to a sixth aspect of the present invention is a more specific example of the device according to the fifth aspect.
[0020]
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
[0021]
(Embodiment 1)
FIG. 1 shows a first embodiment of the present invention. In FIG. 1, 1 is an average current control PWM circuit, 2 is a first tube current control PWM circuit, 3 is a SW circuit, 4 is an AND circuit, and 5 is a comparator. , 6 is an A / D converter, 7 is a self-excited oscillation circuit, 8 is a ballast capacitor, 9 is a cold cathode ray tube, 10 is a tube current detection circuit, and 11 is an LPF circuit. FIG. 2 is a timing chart showing the operation of the first embodiment of the present invention.
[0022]
A specific operation will be described below. A PWM pulse is output from the average current control PWM circuit 1 in a form corresponding to a desired luminance setting value based on the input video synchronization signal. The desired luminance setting value is, for example, a duty ratio of PWM for controlling the brightness of the backlight that is open to the user, and arbitrarily sets Duty 0 to 100%. The PWM pulse is synchronized with the vertical frequency of the video synchronization signal in order to prevent interference fringes between the frame frequency of the normal video signal and the inverter.
[0023]
On the other hand, the tube current control PWM circuit 2 multiplies the clock for driving the liquid crystal panel, and generates a PWM pulse corresponding to the set tube current value. For example, when the panel drive clock frequency is 30 MHz and the tube current control PWM frequency is 100 kHz, the duty control resolution can be only 300 steps. However, by multiplying the panel drive clock by four, the duty control resolution is 1200 steps. Therefore, even between about 60% and 80% duty, the resolution during that time is 240 steps, which is sufficiently within the practical range.
[0024]
The PWM output of the average current control PWM circuit 1 and the PWM output of the tube current control PWM circuit 2 are calculated by the AND circuit 4 and supplied to SW3. In SW 3, the power supply voltage is controlled based on the PWM signal to drive the post-stage self-excited oscillation circuit 7, and the cold cathode ray tube 9 emits light through the ballast capacitor 8. The tube current that drives the cold cathode ray tube 9 is detected by the tube current detection circuit 10, smoothed by the LPF 11, and then digitized by the A / D converter 6 in synchronization with the period of the average current control PWM for comparison. A stable tube current can be obtained by obtaining the difference from the tube current setting with the device 5 and feeding back the difference value to the tube current set value.
[0025]
(Embodiment 2)
FIG. 3 shows a second embodiment of the present invention. In FIG. 3, reference numeral 12 denotes a second average current control PWM circuit. FIG. 4 is a timing chart showing the operation of the second embodiment of the present invention. A specific operation will be described below. In the first embodiment, the clock frequency at which the PWM operates is increased by multiplying the clock to increase the resolution.
[0026]
However, depending on the semiconductor, it may be difficult to incorporate a multiplier, or the cost may be increased. Therefore, the clock frequency may be the drive clock of the liquid crystal panel itself. Therefore, in the second embodiment, the apparent resolution is increased by modulating the duty of the tube current PWM to be controlled and performing bit reduction.
[0027]
That is, as shown in FIG. 4, when it is desired to realize Duty 50.25%, the PWM cycle is constant, and the Duty is controlled to be 50%, 50%, 50%, and 51%. Then, the average duty becomes (50% + 50% + 50% + 51%) / 4 = 50.25%, and the same effect as the increase in resolution can be obtained.
[0028]
(Embodiment 3)
FIG. 5 shows a third embodiment of the present invention. In FIG. 5, reference numeral 13 denotes a third average current control PWM circuit. FIG. 6 is a timing chart showing the operation of the second embodiment of the present invention. A specific operation will be described below. In the second embodiment, the duty of the tube current PWM is modulated in order to increase the apparent resolution, but in the third embodiment, the cycle of the tube current PWM is varied.
[0029]
That is, the PWM High period is fixed and the Low period is modulated as shown in FIG. In FIG. 6, c1 represents the period of the original tube current PWM, and c2 represents a case where the Low period is modulated. By doing so, the apparent resolution can be improved as in the second embodiment. Whether to modulate the duty or the period may be selected and used according to factors such as the inverter circuit or the liquid crystal panel.
[0030]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a discharge lamp lighting device capable of accurately managing the tube current value because the peak current control flowing through the discharge tube can be digitized while ensuring sufficient accuracy. Become.
[Brief description of the drawings]
1 is a configuration diagram of a discharge lamp lighting device according to a first embodiment of the present invention. FIG. 2 is a waveform diagram for explaining the operation of FIG. 1. FIG. 3 is a configuration of a discharge lamp lighting device according to a second embodiment of the present invention. 4 is a waveform diagram for explaining the operation of FIG. 2. FIG. 5 is a block diagram of a discharge lamp lighting device according to Embodiment 3 of the present invention. FIG. 6 is a waveform diagram for explaining the operation of FIG. Configuration diagram of conventional discharge lamp lighting device [FIG. 8] Waveform diagram for explaining the operation of FIG.
DESCRIPTION OF SYMBOLS 1 Average current control PWM circuit 2 1st tube current control PWM circuit 3 SW circuit 4 AND circuit 5 Comparator 6 A / D converter 7 Self-excited oscillation circuit 8 Ballast capacitor 9 Cold cathode ray tube 10 Tube current detection circuit 11 LPF circuit 12 Second tube current control PWM circuit 13 Third tube current control PWM circuit

Claims (3)

A discharge lamp lighting device used for a display device,
An average current control PWM circuit that generates a PWM pulse to obtain a luminance according to a desired luminance setting value using an input video synchronization signal as a trigger;
A tube current control PWM circuit that outputs a PWM pulse according to the value of the tube current setting register set by multiplying the clock for driving the liquid crystal panel;
An AND circuit that calculates an AND of the average current PWM pulse and the output of the tube current PWM pulse;
A SW circuit for controlling a power supply voltage based on an output signal of the AND circuit;
A self-oscillation circuit that constitutes an inverter based on the output of the SW circuit;
A cold-cathode tube connected to the self-excited oscillation circuit via a ballast and a capacitor for stable operation;
A tube current detection circuit for detecting a tube current for driving the cold cathode ray tube;
An LPF circuit for smoothing the tube current value, and an A / D converter for digitizing the tube current smoothed by the LPF circuit;
A comparator that compares a desired tube current setting value with the output of the A / D converter, sets the value in the tube current setting register so that both are the same, and controls the output of the tube current control PWM circuit;
A discharge lamp lighting device comprising: A discharge lamp lighting device used for a display device,
An average current control PWM circuit that generates a PWM pulse to obtain a luminance according to a desired luminance setting value using an input video synchronization signal as a trigger;
A tube current control PWM circuit for outputting a PWM pulse obtained by modulating the duty based on the value of the tube current setting register set with the clock for driving the liquid crystal panel as a clock;
An AND circuit that calculates an AND of the average current PWM pulse and the output of the tube current PWM pulse;
A SW circuit for controlling a power supply voltage based on an output signal of the AND circuit;
A self-oscillation circuit that constitutes an inverter based on the output of the SW circuit;
A cold-cathode tube connected to the self-excited oscillation circuit via a ballast and a capacitor for stable operation;
A tube current detection circuit for detecting a tube current for driving the cold cathode ray tube;
An LPF circuit for smoothing the tube current value, and an A / D converter for digitizing the tube current smoothed by the LPF circuit;
A comparator that compares a desired tube current setting value with the output of the A / D converter, sets the value in the tube current setting register so that both are the same, and controls the output of the tube current control PWM circuit;
A discharge lamp lighting device comprising: A discharge lamp lighting device used for a display device,
An average current control PWM circuit that generates a PWM pulse to obtain a luminance according to a desired luminance setting value using an input video synchronization signal as a trigger;
A tube current control PWM circuit that outputs a PWM pulse obtained by modulating the control period of the tube current based on the value of the tube current setting register set with the clock for driving the liquid crystal panel as a clock;
An AND circuit that calculates an AND of the average current PWM pulse and the output of the tube current PWM pulse;
A SW circuit for controlling a power supply voltage based on an output signal of the AND circuit;
A self-oscillation circuit that constitutes an inverter based on the output of the SW circuit;
A cold-cathode tube connected to the self-excited oscillation circuit via a ballast and a capacitor for stable operation;
A tube current detection circuit for detecting a tube current for driving the cold cathode ray tube;
An LPF circuit for smoothing the tube current value, and an A / D converter for digitizing the tube current smoothed by the LPF circuit;
A comparator that compares a desired tube current setting value with the output of the A / D converter, sets the value in the tube current setting register so that both are the same, and controls the output of the tube current control PWM circuit;
A discharge lamp lighting device comprising:

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