JP2006091138A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device in which the luminance of an LED is adjusted without forming a striped pattern on a screen nor wasting electric power. <P>SOLUTION: The liquid crystal display device is equipped with a liquid crystal panel, the LED 1 which irradiates the liquid crystal display panel with light from behind, a plurality of resistance elements R1 to Rn which are connected to the LED 1 in series and limit the current of the LED 1, and a selecting means of selecting at least one of the resistance elements R1 to Rn. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display device having a backlight.

  The liquid crystal display device includes a backlight that emits light from the back surface of the liquid crystal display panel. Recently, an LED (Light Emitting Diode) is used for the backlight. The liquid crystal display device is provided with an adjustment mechanism for adjusting the brightness of the screen, and the brightness of the screen is performed by adjusting the luminance of the LED.

  Generally, there are the following two methods for adjusting the luminance of the LED. These two methods will be described with reference to FIGS.

  In the first method, as shown in FIG. 5, a constant current circuit is provided between the current limiting resistance element and the LED, and the reference voltage value that is the source of the constant current circuit is varied to adjust the luminance of the LED. That's it.

  In the second method, as shown in FIG. 6, a switching element is provided between the current limiting resistance element and the LED, and the luminance of the LED is adjusted by controlling the duty ratio of the switching element.

  However, when an LED is used as the backlight of the liquid crystal display device and the brightness of the LED is adjusted by the above two methods in order to adjust the brightness of the screen, the following problems occur.

  In order to have a mechanism for adjusting the brightness of the screen in the liquid crystal display device, a certain amount of adjustment is required, and accordingly, the variable width of the luminance of the LED needs to be increased.

  In order to increase the variable range of luminance by the first method, the lower limit value of the reference voltage must be set to a small value or the upper limit value of the reference voltage must be set to a large value.

  If the lower limit value is set to a small value, the error of the current value flowing through the LED is large due to the error of the base-emitter voltage of the two transistors connected to the voltage variable power supply and the voltage variable power supply that makes the reference voltage variable. Become. Moreover, it becomes weak against noise. As a result, since the value of the current flowing through the LED varies, stable brightness cannot be obtained. On the other hand, if the upper limit value of the reference voltage is set to a large value, a circuit in which the power loss is increased correspondingly.

  In the second method, the above-mentioned problem does not occur, but a stripe pattern may occur on the screen due to the temporal change in the transmittance of the liquid crystal and the ON / OFF timing of the switching element.

  Next, a striped pattern generated on the screen will be described with reference to FIGS.

  In the TFT liquid crystal panel, wirings for forming a plurality of rows of scanning lines are arranged in the vertical (vertical) direction on the display surface, and wirings for forming a plurality of columns of signal lines are arranged in the horizontal (horizontal) direction. A pixel electrode is disposed at each intersection of the scanning line in each row and the signal line in each column.

  A thin film transistor (TFT) is provided between each pixel electrode and a corresponding signal line, and a corresponding scanning line is connected to the gate of each TFT. When a signal is applied to both a scanning line scanned for each row and a signal line corresponding to the scanning line for each frame period, a predetermined transmittance is generated in the pixel, and the image is transmitted from behind the TFT liquid crystal panel. Visible when illuminated by backlight.

  FIG. 7B shows (1) the transmittance when a signal is applied to both the scanning line and the signal line corresponding to the row, and (4) the signal corresponding to the scanning line and the row. The transmittance when a signal is applied to both of the lines and the state of turning on and off the backlight are shown. The refresh in FIG. 7B means that the TFT liquid crystal panel has a property that the transmittance of a pixel to which a signal is applied in a certain frame period decreases with the passage of time. This is an operation of applying a signal to the corresponding pixel again to recover the transmittance.

  As shown in FIG. 7B, the transmittance of the (1) row and the transmittance of the (4) row change with time lag, so if the backlight luminance is changed by duty ratio control, The brightness of the image when the light is lit changes between lines (1) and (4) (see FIG. 7A). However, since the cycle of turning on and off the backlight is very short, the difference in transmittance between the (1) line and the (4) line cannot be visually recognized by human eyes. For human eyes, for example, the difference between the average value of the transmittance a and the transmittance b in the row (1) and the average value of the transmittance c and the transmittance d in the row (4) is the brightness of the screen. It looks like a difference (see FIGS. 8A and 8B). In this way, a striped pattern is generated on the screen.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a liquid crystal display device that adjusts the brightness of an LED without causing striped patterns on the screen and consuming unnecessary power. is there.

  In order to solve the above problems, in claim 1, a liquid crystal display panel, a light emitting element for irradiating light from the back surface of the liquid crystal display panel, and a light emitting element connected in series to limit a current of the light emitting element. And a selection means for selecting at least one of the resistance elements.

  With this configuration, the switching element is controlled by the duty ratio to adjust the luminance of the light emitting element, as in the prior art, and the resistance element is selected by the selecting means and the current flowing through the light emitting element is changed to increase the luminance. Since the adjustment is performed, it is possible to adjust the luminance of the light emitting element without causing a striped pattern on the screen and consuming unnecessary power.

  In particular, in the second aspect, the selection unit includes a switching element and a control device that drives the switching element, and the switching elements are connected in series to a plurality of resistance elements one by one. The resistor element and the switching element are connected in parallel to the light emitting element.

  With this configuration, it is possible to control the current flowing through the light emitting element and adjust the luminance simply by adding a resistance element and a switching element connected thereto.

  According to a third aspect of the present invention, the selection unit includes a switching element and a control device that drives the switching element, and the switching element is connected in parallel to a plurality of resistance elements one by one, and the resistance connected in parallel The element and the switching element are connected in series to the light emitting element.

  With this configuration, by selecting a combination of resistance elements connected in series, compared to the case where each resistance element is connected in parallel, the combined resistance value is changed at a constant interval with a smaller number of resistance elements, It becomes possible to change the brightness | luminance of LED by the intensity | strength of a fixed space | interval.

  According to a fourth aspect of the present invention, the light emitting element is supplied with electric power from a constant voltage source or a constant voltage circuit. According to a fifth aspect of the present invention, a constant current circuit is connected to the light emitting element.

  For example, a 12V battery is used as a power source installed in a vehicle. The output voltage of the battery generally varies in a voltage range of 9 to 16V. In order to stabilize the luminance of the light emitting element, it is conceivable to make the voltage or current supplied to the light emitting element constant.

  On the other hand, according to the description of claim 4 or claim 5, since the constant voltage source, the constant voltage circuit, or the constant current circuit is provided, the voltage or current supplied to the LED even if the battery output voltage fluctuates Can be made constant, and the luminance of the light emitting element can be stabilized.

  According to a sixth aspect of the present invention, the liquid crystal display device of the present invention is used for in-vehicle use.

  The brightness of the passenger compartment varies depending on the brightness situation around the vehicle. Depending on the situation, it may be necessary to change the brightness of the liquid crystal screen mounted on the vehicle to make the screen easier to see, such as day and night, during tunnel travel, during fine weather, and during rainy weather. Therefore, it is particularly preferable to mount the liquid crystal display device of the present invention on a vehicle.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a backlight luminance adjustment circuit (hereinafter referred to as a luminance adjustment circuit) of a liquid crystal display device showing a first embodiment of the present invention.

  The luminance adjustment circuit 10 is connected in series to a constant voltage power supply 2, a light emitting element (hereinafter referred to as an LED) 1 connected in series, a plurality of resistance elements R1 to Rn, and each resistance element R1 to Rn. Switching elements (hereinafter referred to as transistors) T1 to Tn, and a microcomputer 4 that controls ON / OFF of the transistors T1 to Tn. The ON / OFF control of the transistors T1 to Tn in the present embodiment does not repeat lighting and extinguishing by controlling the duty ratio of the transistor as performed in the prior art, but a plurality of resistance elements R1 connected to the LED 1 This is for selecting at least one of ˜Rn.

  The constant voltage power source 2 is composed of a constant voltage circuit that makes the output voltage from the battery constant, and is connected to the anode terminal of the LED 1. Since the output voltage of the on-vehicle battery fluctuates in the range of 9 to 16V, when the output voltage of the battery is directly supplied to the LED 1, the luminance changes with the fluctuation of the voltage. By providing the constant voltage power supply 2, the luminance of the LED 1 can be stabilized.

  Resistance elements R1 to Rn for limiting the current flowing through the LED 1 are connected in parallel to the cathode terminal of the LED 1, respectively. The collector terminals of the transistors T1 to Tn are connected in series to the resistor elements R1 to Rn at a terminal different from the terminal connected to the LED1 of the resistor elements R1 to Rn.

  A signal output terminal of the microcomputer 4 is connected to a base terminal which is a signal input terminal of the transistors T1 to Tn. The remaining emitter terminals of the transistors T1 to Tn are grounded.

  Next, the operation of the luminance adjustment circuit 10 in the first embodiment will be described. Here, a case where the selected resistance element is R1 will be described.

  A predetermined voltage is supplied from the constant voltage power supply 2 to the LED 1. When the microcomputer 4 selects the resistance element R1, the microcomputer 4 transmits an ON signal to the base terminal of the transistor T1. When the selected transistor T1 is turned on, a current flows through the resistance element R1 and the LED1 connected to the transistor T1. The current flowing through the LED 1 is determined by the resistance value of the resistance element R1. The LED 1 shines with a luminance corresponding to the current value.

  In the present embodiment, any one of the resistance elements R1 to Rn is selected, but a plurality of resistance elements may be selected at a time. The resistance value (synthetic resistance value) at that time is the reciprocal of the sum of the reciprocal numbers of the selected resistance elements because the resistance elements R1 to Rn are connected in parallel. If a plurality of resistance elements are selected at the same time, finer current control and brightness adjustment of the LED 1 are possible.

  According to this configuration, the resistance elements R1 to Rn are selected by controlling ON / OFF of the transistors T1 to Tn without adjusting the luminance of the LED 1 by controlling the duty ratio of the switching element as in the prior art. Since the luminance is adjusted by changing the current flowing through the LED 1, the luminance of the LED 1 can be adjusted without generating a stripe pattern on the screen. Further, since the brightness is not adjusted by changing the reference voltage of the constant current circuit, useless power is not consumed unlike the conventional case.

  Further, in the present embodiment, the luminance can be adjusted by controlling the current flowing through the light emitting element simply by adding the resistance elements R1 to Rn and the transistors T1 to Tn connected thereto.

  In the present embodiment, the resistance elements R1 to Rn are selected by a microcomputer. According to this, for example, by attaching a sensor for measuring the amount of light around the liquid crystal display device to the microcomputer, the brightness of the liquid crystal display suitable for the surrounding brightness can be automatically set. It is also possible to interlock with the operation of the headlight.

  The brightness adjustment circuit 10 is particularly effective when the liquid crystal display device is mounted on a vehicle. The brightness of the passenger compartment is often affected by the location (tunnel travel, night travel, etc.) and the weather (clear weather, rainy weather, etc.). By changing the brightness of the liquid crystal display device according to the surrounding conditions, it is possible to make it easier for the passenger to see the screen.

  In the present embodiment, since the constant voltage power supply 2 is connected to the anode terminal of the LED 1, even if the power supply voltage fluctuates, such as when the liquid crystal display device is used for in-vehicle use, a stable voltage is applied to the LED 1. Can be supplied.

  Next, Modification 1 will be described with reference to the drawings. FIG. 2 is a circuit diagram of a backlight luminance adjustment circuit showing a first modification of the present invention.

  As shown in FIG. 2, a power source 21 is connected to the anode terminal of the LED 1, and a constant current circuit 3 is connected to the cathode terminal. The constant current circuit 3 is a general one and includes a constant voltage source, a resistance element that divides the constant voltage source, a separate power source, a resistance element for voltage drop, and a transistor. According to this configuration, a stable current can be supplied to the LED 1 even when the power supply voltage supplied to the LED 1 is unstable.

  Next, Modification 2 will be described with reference to the drawings. FIG. 3 is a circuit diagram of a backlight luminance adjustment circuit showing a second modification of the present invention.

  As shown in FIG. 3, the LED 1 is composed of two rows of LEDs 11 and 12, and each row of LEDs 11 and 12 is connected in parallel to the power source 21. A power supply 21 is connected to the anode terminals of the LEDs 11 and 12 in each column, and a constant current circuit 3 is connected to the cathode terminals.

Further, resistance elements R1 to Rn and R1 (2) to Rn (2) corresponding to the LEDs 11 and 12 of each column are connected to the constant current circuit 3 of the LEDs 11 and 12 of each column. The resistance elements R1 to Rn and R1 (2) to Rn (2) are connected in parallel.
Further, as shown in FIG. 3, the terminals on the LED 12 side of the resistance elements R1 (2) to Rn (2) on the opposite side of the LED 12 are connected to the LED 11 side resistance elements R1 to Rn. Transistors T1 to Tn are connected downstream of the connection point.

  Even with this configuration, it is possible to adjust the luminance of the LED 1 without generating a stripe pattern on the screen.

  A second embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a circuit diagram of a backlight luminance adjustment circuit of the liquid crystal display device showing the second embodiment of the present invention.

As shown in FIG. 4, the constant voltage power source 2 is connected to the anode terminal of the LED 1, and the resistance elements R <b> 1 to Rn are connected to the cathode terminal.
The resistance elements R1 to Rn are connected in series to the LED 1, and the transistors T1 to Tn are connected in parallel to the resistance elements R1 to Rn one by one.

  Next, the operation of the brightness adjustment circuit 10 in the second embodiment will be described. Here, a case where the selected resistance element is R1 will be described.

A predetermined voltage is supplied from the constant voltage power supply 2 to the LED 1. The microcomputer 4 transmits an ON signal to the base terminals of the transistors T2 to Tn other than the transistor T1 in order to select the resistance element R1.
When the transistors T2 to Tn other than the transistor T1 are turned on, a current flows only through the resistance element R1 and the LED1. At this time, since the transistors T2 to Tn other than the transistor T1 are ON, no current flows through the resistance elements R2 to Rn connected in parallel to the transistors T2 to Tn. The current flowing through the LED 1 is determined by the resistance value of the resistance element R1. The LED 1 shines with a luminance corresponding to the current value.

  In the present embodiment, one resistance element is selected from among the resistance elements R1 to Rn, but a plurality of resistance elements may be selected at a time. The resistance value at that time is the sum of the selected resistance elements because the resistance elements R1 to Rn are connected in series. If a plurality of resistance elements are simultaneously selected, finer current control of the LED 1 is possible.

  For example, when the resistance element R1 is 1Ω, the resistance element R2 is 2Ω, and the resistance element R3 is 4Ω, the combined resistance value can be changed at a constant interval by a combination of the resistance elements R1 to R3. When only the resistance element R1 is selected, the combined resistance value is 1Ω. When only the resistance element R2 is selected, the combined resistance value is 2Ω. When the resistance elements R1 and R2 are selected, the combined resistance value is 3Ω. When only the resistor element R3 is selected, the combined resistance value is 4Ω. When the resistance elements R1 and R3 are selected, the combined resistance value is 5Ω. When the resistance elements R2 and R3 are selected, the combined resistance value is 6Ω. When the resistance elements R1, R2, and R3 are selected, the combined resistance value is 7Ω.

  In this way, by connecting the resistance elements in series and changing the combination of the resistance elements, the combined resistance value can be set at a constant interval with a smaller number of resistance elements than when the resistance elements are connected in parallel. It is possible to change the luminance of the LED at a constant interval.

  As shown in this embodiment, there is provided a liquid crystal display device that adjusts the brightness of an LED without causing a striped pattern on the screen and consuming unnecessary power even when the resistance elements R1 to Rn are connected in series. The object of the present invention can be achieved.

  Also, constant current control can be performed in combination with the embodiment shown in FIG.

FIG. 3 is a circuit diagram of a backlight luminance adjustment circuit of the liquid crystal display device according to the first embodiment of the present invention. It is a circuit diagram of the backlight brightness adjusting circuit showing a first modification of the present invention. It is a circuit diagram of the backlight brightness adjustment circuit showing the modification 2 of the present invention. It is a circuit diagram of the backlight brightness adjusting circuit of the liquid crystal display device showing the second embodiment of the present invention. It is a circuit diagram of the backlight brightness adjustment circuit of a prior art. It is a circuit diagram of the backlight brightness adjustment circuit of a prior art. (A) is a figure which shows the scanning row | line | column of a TFT liquid crystal panel, (b) is a graph which shows the transmittance | permeability for every scanning row | line | column, lighting of a backlight, and a light extinction timing. (A) is a figure which shows the appearance of a TFT liquid crystal panel, (b) is a graph which shows the brightness and position of a liquid crystal panel.

Explanation of symbols

10 Backlight brightness adjustment circuit 1 LED (light emitting element)
2 Constant voltage power supply 21 Power supply 3 Constant current circuit 4 Microcomputers R1 to Rn Resistance elements T1 to Tn Transistors (switching elements)

Claims (6)

A liquid crystal display panel;
A light emitting element for irradiating light from the back of the liquid crystal display panel;
A plurality of resistance elements connected in series to the light emitting element for limiting the current of the light emitting element;
A liquid crystal display device comprising: selection means for selecting at least one of the resistance elements. The selection means includes a switching element and a control device that drives the switching element,
The switching elements are connected in series to the plurality of resistance elements one by one,
The liquid crystal display device according to claim 1, wherein the resistance element and the switching element connected in series are connected in parallel to the light emitting element. The selection means includes a switching element and a control device that drives the switching element,
The switching elements are connected in parallel to the plurality of resistance elements one by one,
The liquid crystal display device according to claim 1, wherein the resistance element and the switching element connected in parallel are connected in series to the light emitting element.   The liquid crystal display device according to claim 1, wherein a constant voltage source is connected to the light emitting element.   The liquid crystal display device according to claim 1, wherein a constant current circuit is connected to the light emitting element.   The liquid crystal display device according to any one of claims 1 to 5, wherein the liquid crystal display device is used for in-vehicle use.

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