JP2002221701A - Light source driving device for liquid crystal display - Google Patents

Abstract

(57) [Summary] An object of the present invention is to provide a light source driving device for a liquid crystal display device capable of improving the quality of moving images on a liquid crystal display with a simple configuration. A one-chip microcomputer for intermittently operating an inverter power supply for causing a cold cathode fluorescent tube of a liquid crystal display device to emit light is provided. The microcomputer has an A / D converter built-in and controls the current flowing through the cold cathode fluorescent tube. A /
A / D conversion by D converter and measurement as digital value,
Lighting / intermittent operation in the intermittent operation of the inverter power supply is performed so that the measured digital value becomes a predetermined value set in advance.
For non-lighting, the duty ratio of pulse width modulation (PWM) is adjusted to adjust the lighting / non-lighting time ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermittent illumination device for illuminating a transmission type liquid crystal display device (liquid crystal display).

[0002]

2. Description of the Related Art In recent years, with the spread of liquid crystal displays and the development of high image quality technology and moving image support technology for display devices, high precision and high brightness are also required for liquid crystal displays. In particular, in the case of a liquid crystal display, as the number of cold cathode fluorescent tubes as a light source increases for higher brightness, the cost of an inverter, which is a power supply device for the light source of the cold cathode fluorescent tube, increases,
Deterioration of optical characteristics due to variation in current of a cold cathode fluorescent tube has become a problem.

On the other hand, in the so-called digital multimedia age, a liquid crystal display is required to have not only a still image display performance such as luminance and a viewing angle but also a good moving image display performance. In terms of video display quality, CR
T (Cathode Ray Tube) has an advantage, and the liquid crystal display does not have a display performance equal to or higher than that of a CRT.
It is known that the quality of a moving image of a liquid crystal display deteriorates due to a slow response time of the liquid crystal and blurring when a moving image is displayed. As methods for preventing such deterioration of moving image quality, there are an intermittent display method in which display light is intermittently turned on (blinking) and a double-speed display method in which display light is arranged at a screen position in accordance with the movement of many images. The double-speed display method is frequently used. However, the double-speed display method requires an image interpolation technique, and there has been a problem that a display system and a display circuit are complicated to realize the interpolation technique.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a light source driving device for a liquid crystal display device which can improve the quality of moving images on a liquid crystal display with a simple structure.

[0005]

SUMMARY OF THE INVENTION The present invention, which achieves this object, is characterized in that it comprises a one-chip microcomputer for intermittently operating an inverter power supply for emitting light from a cold cathode fluorescent tube of a liquid crystal display. The one-chip microcomputer has a built-in A / D converter, A / D converts the current flowing through the cold cathode fluorescent tube with the built-in A / D converter, and measures a digital value. Then, the intermittent operation is controlled so that the measured digital value becomes a predetermined value. The microcomputer performs pulse width modulation (P) on lighting / non-lighting in the intermittent operation of the inverter power supply.
WM), and the duty ratio of pulse width modulation is adjusted to adjust the lighting / non-lighting time ratio so that the digital value becomes a predetermined value. In this case, the microcomputer performs the pulse width modulation in synchronization with a field signal of the liquid crystal display device.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. 1 and 2 show a cold cathode fluorescent tube (CCFL (cold ca) as a light source driving device of a liquid crystal display device to which the present invention is applied.
FIG. 3 is a diagram illustrating an embodiment of a circuit configuration of a thode fluorescent lamp) driving device. This embodiment is composed of a digital control system 10 centered on a one-chip microcomputer (hereinafter, “MPU”) 11 and an analog system 50 including a resonance circuit including a cold cathode fluorescent tube 51 and a step-up transformer 53 and a drive circuit. It is configured. Although not shown,
This liquid crystal display device drives the liquid crystal display with a liquid crystal display illuminated by a cold cathode fluorescent tube 51 and image signals (R, G, B, luminance signals) of a predetermined vertical frequency and a field frequency input from, for example, a personal computer or the like. Liquid crystal drive circuit. The cold cathode fluorescent tube driving device of the present embodiment operates in synchronization with a field signal.

[0007] First, referring to FIG.
Will be described. This digital control system 10
An MPU 11 as a one-chip microcomputer incorporating an A / D converter 13 and a programmable logic circuit (hereinafter, “PLD”) 21 as external clock means
And The MPU 11 outputs an A / D converter 13 for measuring the current flowing through the cold cathode fluorescent tube 51 and a pulse width modulation (PWM) control signal output and system for intermittently lighting (intermittently operating) the cold cathode fluorescent tube 51. And a control system 15 having the above control function.

[0008] The PLD 21 has a system timing clock unit 23 and a trigger timing clock unit 25 built therein. The system timing clock unit 23 outputs a lamp clock for lighting the cold cathode fluorescent tubes 51, and the trigger timing clock unit 25 outputs a clock for setting a trigger (sampling) point for current measurement. On the other hand, the control system 15 outputs a PWM pulse that enables pulse width modulation. PW output by control system 15
The M control signal and the ramp clock output by the system timing clock unit 23 are input to the AND circuit 31,
The logical sum of these inputs is output from the AND circuit 31 as a lamp drive clock. That is, the control system 1
5 is outputting a high-level PWM control signal,
The ramp clock is sent from the AND circuit 31
Although output as a clock, a low-level signal is output from the AND circuit 31 regardless of the level of the ramp clock while the control system 15 is outputting the low-level PWM control signal. The MPU 11 sets the ramp clock output from the system timing clock unit 23 to PW
This is a configuration for performing M control.

In the embodiment of the present invention having the above configuration, the MPU 11 executes the lamp current measurement when the sampling trigger signal is output from the PLD 21 so that the optimum lamp current value can be obtained based on the measurement result. The lighting / non-lighting time of the cold cathode fluorescent tube 51 is adjusted by pulse width modulation to optimize the current measurement environment.

FIG. 2 shows the configuration of an embodiment of an analog system 50 as an inverter power supply device controlled by the digital control system 10. On the primary side of the step-up transformer 53, an analog power supply Vin is provided from a power supply (not shown),
Cold cathode fluorescent lamp drive clocks V + and V- are supplied. The drive clocks V + and V− correspond to the inverters Inv1 and Inv2 and the field effect transistors Tr1 and Tv.
It is supplied to a push-pull amplifier circuit connected to the primary side of the step-up transformer 53 via a switching device including r2, and drives the cold cathode fluorescent tubes 51 to be turned on with alternating positive and negative currents.

On the secondary side of the step-up transformer 53, a resonance circuit including a capacitor C1 and a cold cathode fluorescent tube 51 is formed. One terminal of the cold cathode fluorescent tube 51 is connected to the secondary side of the step-up transformer 53, the other terminal of the cold cathode fluorescent tube 51 is grounded via a resistor R2, and further detects the current of the cold cathode fluorescent tube 51. A resistor R1 for obtaining a current signal output in order to obtain a desired signal level is connected. With this resistor R1, a signal level (lamp current detection output) corresponding to the lamp current flowing through the cold cathode fluorescent tube 51 is detected as a voltage.

(1) Intermittent Operation In the embodiment of the present invention, the cold cathode fluorescent tube 51 is operated intermittently (intermittent lighting). FIG. 3 shows an embodiment of a timing chart relating to lighting of the cold cathode fluorescent tube 51. In this embodiment, in one field, lighting is performed at a predetermined lighting frequency during a predetermined lighting period, and light is turned off during a non-lighting period. That is, in the present embodiment, PWM
What is controlled is the ratio between the lighting time and the non-lighting time in one field cycle. In FIG. 3, the control system 15 keeps the PWM signal at the high level during the lighting period and at the low level during the non-lighting period.

The intermittent operation is realized by a lamp drive clock. The lamp drive clock determines a lighting cycle (lighting cycle) of intermittent lighting, and further determines a lighting period and a non-lighting period for intermittently lighting the cold cathode fluorescent tube 51 within one field frequency. That is, the cold cathode fluorescent tube 51 is configured to intermittently light in synchronization with the field frequency by the LAMP drive clock, and the ratio of lighting period / (lighting period + non-lighting period) within one field period, that is, duty (%) Is adjusted as Also, the duty of the field frequency, that is, 1
The ratio between the light-on period and the light-off period in the field period is variable, and the change of the duty is realized by the PWM signal output from the control system 15.

When the step-up transformer 53 is operated by a digital signal, impedance cannot be matched because the digital signal cannot follow the digital signal at the beginning of the operation, and a so-called "transformer growl" is generated. In the present embodiment, in order to prevent the transformer humming, a soft start for gradually increasing the lamp current value is executed, thereby achieving the transformer humming and stabilizing the intermittent operation.

(2) Method of Measuring Lamp Current I CCFL of Cold Cathode Fluorescent Tube (CCFL) When the secondary load of the step-up transformer 53 is only the linear resistor R2, the current waveform is a sine wave. The current waveform when the fluorescent tube 51 is connected becomes non-linear,
With distortion. This distortion is caused by the fact that the impedance characteristic of the cold cathode fluorescent tube 51 is high impedance at the start of voltage application, but with time from the start of lighting, the secondary voltage drops to the discharge maintenance lower limit voltage, and the high-frequency impedance component decreases. It is to do. Such a current waveform is 4
When the current is measured by the A / D converter 13 built in the MPU 11, the built-in A / D
The A / D conversion speed of the D converter 13 may not be able to follow a fast change in current.

Therefore, in the present embodiment, a trigger point setting signal for current measurement is output from a trigger timing clock section 25 incorporated in the PLD 21 as an external clock means, and a trigger point is set so that current approximation is most appropriate. Next, the sampling timing of the A / D converter 13 is set. That is, a predetermined point (including a predetermined range) of the current waveform (phase) is sampled (integrated).
Then, the entire current is estimated from the sampled values (measured values). FIG. 4 shows a pulse current waveform and sampling timing. In this embodiment, a predetermined point is a predetermined time including a peak within one wavelength (t lamp clock) of the pulse current, and the current value within that time is measured (sampled and integrated), and the measured value is converted to an A / D value. Converting. Then, the PLD 21 executes the trigger point setting so that the peak current, which is the expected value, can be measured (FIG. 5).
In FIG. 5, the vertical axis represents the sampling current value Is,
The horizontal axis is the trigger point. In this embodiment, the measurement is performed at the point where the sampling current value Is is maximum. The pulse current value can be set by k × Is (where k is a constant).

Even if the trigger timing is set once, if the time elapses, the trigger timing may deviate from the peak. FIG. 6 shows that the cycle of the lamp current is 20 μs (50 kHz).
z), when the sampling time α is 2.42 μS, the sampling time is shifted from the peak by δ time. The variation in the measured value that occurs here is due to the measurement error Tole
rance can be represented by the approximate expression shown in Expression 1.

(Equation 1)

(3) Correction of Lamp Current (ICCFL) by Soft Start In this embodiment, in order to suppress the rush current at the start of discharge and reduce the hum of the step-up transformer 53, the cold cathode fluorescent tube 51 is used. Intermittent lighting is started by soft start.
That is, during the initial lighting period of the cold cathode fluorescent tube 51 (100 to 200 μS in the present embodiment), the width of the drive pulse (lamp clock) in duty dimming is controlled so that the lamp current ICCFL gradually increases. In such a soft start, when the duty of dimming is small, the duty ratio and the current ratio do not match, and it may not be possible to secure the linearity of dimming. Therefore, in the present embodiment, the number of drive pulses is increased or decreased by changing the duty of the PWM control. The variation of the lamp current I CCFL of the cold cathode fluorescent tube 51 can be corrected to an expected value by PWM control, and the correction can correct the variation of the lamp current I CCFL. The expected value is, for example, set in advance as a design value and written in a nonvolatile memory such as an EEPROM. ICCFL = Correction coefficient (Duty) × Pulse number (at 100%) × Pulse current Pulse number (Duty) = Correction coefficient A (Duty) × Pulse number (at 100%) A = ｛(D−Is / Ia) / (1-Is / Ia)｝ × (1-
Ts / Ta) + Ts / Ta where the number of pulses: the number of pulses in one field period, Is: total integrated current of Ia, Ia: total integrated current, Ts: soft start time, Ta: field period, D: expected duty, A: Correction coefficient for Ta.

(4) Variation correction of lamp current I CCFL As described above, the lamp current I CCFL is
It varies depending on the step-up transformer 53 or the operating environment (temperature, voltage, aging, etc.). Therefore, the MPU 11 compares the measured current with, for example, an expected value stored in advance in a nonvolatile memory, and corrects the PWM duty from the initial value so that the measured current becomes equal to the expected value. That is, the MPU 11 feeds back to the control system 15 based on the correction coefficient A and the number of pulses (at the time of duty) obtained by the above equation so that the measured lamp current becomes a predetermined value, and sets the duty of the PWM signal.

FIG. 7 is a graph showing how the variation of the lamp current ICCFL converges to the expected value by changing the duty. Thus, in the embodiment of the present invention,
The number of intermittently driven pulses is changed, that is, the duty is adjusted so that the lamp current ICCFL becomes an expected value (predetermined value). FIG. 8 shows the relationship between the duty and the lamp current ICCFL before correction and the lamp current ICCF after correction. By performing the correction in this manner, the deviation (error with respect to the expected value) of the lamp current ICCFL can be minimized, and the deviation can be suppressed to about the accuracy error of the sampling. As described above, the embodiment of the present invention has been described with reference to the accompanying drawings, but the present invention is not limited to this embodiment.

[0021]

As is clear from the above description, the present invention
Since a one-chip microcomputer for controlling a power supply device for intermittently emitting light from the cold-cathode fluorescent tube of the liquid crystal display device is provided, it is possible to control the light emission of the cold-cathode fluorescent tube with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a circuit configuration of a control system of a cold cathode fluorescent tube driving device which is a light source driving device of a liquid crystal display device to which the present invention is applied.

FIG. 2 is a diagram showing an embodiment of an analog circuit configuration of the cold cathode fluorescent tube driving device.

Fig. 3 Lamp driver of the same cold cathode fluorescent tube driving device
FIG. 3 is a diagram showing a timing chart of a clock.

FIG. 4 is a graph showing a pulse current waveform and sampling timing.

FIG. 5 is a graph showing a pulse current waveform and a sampling timing of the cold cathode fluorescent tube driving device.

FIG. 6 is a graph showing a state where sampling of the cold cathode fluorescent tube driving device deviates from a peak of a lamp current.

FIG. 7 is a graph showing a state of correcting a lamp current ICCFL variation in the cold cathode fluorescent tube driving device.

FIG. 8 is a diagram illustrating a relationship between a duty and a lamp current ICCFL before correction and a lamp current ICCF after correction.

[Explanation of symbols]

 Reference Signs List 10 Digital control system 11 One-chip microcomputer (MPU) 15 Control system 21 Programmable logic circuit (PLD) 23 System timing clock unit 25 Trigger timing clock unit 50 Analog system 51 Cold cathode fluorescent tube 53 Boost transformer

Claims (6)

[Claims] 1. A light source driving device for a liquid crystal display device, comprising a one-chip microcomputer for intermittently operating an inverter power supply device for lighting a cold cathode fluorescent tube of the liquid crystal display device. 2. The one-chip microcomputer has a built-in A / D converter, and A / D converts a current flowing through the cold-cathode fluorescent tube with the built-in A / D converter and measures a digital value. 2. A light source driving device for a liquid crystal display device according to claim 1. 3. The microcomputer according to claim 1, wherein the microcomputer has a built-in A / D converter.
2. A light source drive of a liquid crystal display device according to claim 1, wherein the intermittent operation is controlled so that the digital value is A / D converted by a D converter and measured as a digital value, and the measured digital value becomes a predetermined value. apparatus. 4. The microcomputer controls lighting / non-lighting in intermittent operation of the inverter power supply by pulse width modulation (PWM), and changes a duty of the pulse width modulation so that the digital value becomes a predetermined value. 4. The light source driving device for a liquid crystal display device according to claim 3, wherein the lighting / non-lighting time ratio is adjusted by adjusting. 5. The light source driving device for a liquid crystal display device according to claim 5, wherein the microcomputer performs the pulse width modulation in synchronization with a field signal of the liquid crystal display device. 6. A light source driving device for a liquid crystal display device according to claim 5, further comprising an external clock means for outputting a trigger pulse for setting an A / D conversion operation timing of said A / D converter.