JP4845395B2 - LED drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving circuit having high-practicality adjusting a property for every LED to suppress variations in a color and luminance, and to provide an LED driving circuit for a lighting system and a liquid crystal device with a miniaturization effect by utilizing the LEDs using a package incorporating a plurality of LED elements. <P>SOLUTION: A series parallel circuit wherein a plurality of series circuit branches with a plurality of LEDs connected with one light switch in series are connected in parallel or a parallel circuit wherein a plurality of series circuit branches with LEDs 11R, 11G, 11B, etc. connected with one light switch 21G, 21G, 21B, etc. in series are connected in series is formed, 3-inch 1 LED units 11, 12 , etc. are used and each LED is selected with the light switch to form a series circuit wherein lit LEDs are connected in series. Especially when a circuit wherein three 3-inch 1 LED units are connected in series is used, high quality black-and-white display without color breakup can be attained by switching with color display. <P>COPYRIGHT: (C)2006,JPO&amp;NCIPI

Description

The present invention relates to a LED drive circuit for driving the plurality of light-emitting diode (LED), about the light source blinking type LED driving circuit for selecting the display color in particular by selectively flashing three primary colors LED.

A lighting device using a light emitting diode (LED) is bright and has a long lifetime and can be flashed at high speed, and thus is used in various fields.
For example, there is an illumination device that directly illuminates an object with an LED. Patent Document 1 discloses an LED drive circuit in a device that illuminates a combination meter in a dashboard of an automobile. When the dashboard instrument is illuminated with LEDs, the number of LED elements is adjusted according to the size of the instrument so that the brightness is approximately the same. Therefore, when using a series circuit in which illumination LEDs are connected in series for each target meter, a method of adjusting the current with a resistor or the like is used for each series circuit in order to equalize the brightness of all meters. However, in a circuit that adjusts the current value with a resistor, when the power supply fluctuates, the light emission luminance changes for each series circuit, resulting in variations in illumination intensity.

  Therefore, the LED drive circuit disclosed in Patent Document 1 has a configuration in which a series connection circuit is connected in parallel, and a plurality of light emitting elements are arranged for each illumination target, and at least one for each illumination target. By distributing the light emitting elements so as to be separated and connected to another series connection circuit, at least one LED is always lit to ensure illumination of each illumination target meter even in the event of a disconnection accident. Further, the circuit characteristics are made uniform by equalizing the number of light emitting elements connected to the series connection circuit so that the entire combination meter is illuminated with the same brightness even when the power supply voltage fluctuates.

Citation 2 discloses an exposure apparatus for color photographic paper using three primary color LEDs.
Exposure to print color images of photographic film on photographic paper requires a strong light source for each of the colors red, green, and blue, and is obtained by measuring the sensitivity for each color to improve color reproducibility. The exposure must be adjusted. As the light source of the exposure apparatus, a light source in which several hundred LEDs whose luminance can be adjusted relatively easily by electrical means is arranged in a matrix is used. For example, a typical red LED is driven with a current of about 100 mA and a voltage of about 2.2 V. Therefore, a circuit in which LEDs are connected in parallel requires a current exceeding several tens of A, and a circuit in which the LEDs are connected in series is as high as 1000 V. A voltage is required. Furthermore, since it is required to switch the LEDs in units of several microseconds, actual handling becomes extremely difficult.

  Therefore, the invention disclosed in Patent Document 2 is configured such that an LED is connected by a circuit in which a series circuit and a parallel circuit are combined, and a means for adjusting the emitted light of the LED is provided, so that the current and voltage necessary for driving the LED are provided. Both are limited, and a practical control device is provided. The disclosed LED driving circuit may be configured such that a series circuit branch may be connected in parallel, or a parallel circuit branch in which multiple LEDs are connected in parallel may be connected in series. In addition, it is preferable to make the number of LEDs in the series circuit branches equal to make the light intensity distribution uniform.

The LED can be switched quickly and can be directly driven, so that the operability is excellent. Further, since it can be turned on / off by electrical control, a mechanical shutter for closing the exposure optical path can be omitted, and the cost can be reduced. Moreover, a special color filter can be omitted by using three primary color LEDs. It is also easy to compensate for the color sensitivity of the photographic paper.
However, since the LED group is turned on / off for each color and it is necessary to adjust the luminance for each color, only the LEDs of the same color are inevitably connected to one LED driving circuit.

Furthermore, the LED can be used for an illumination device in a liquid crystal device that performs monochrome display or color display. For example, as described in Patent Document 3, a light guide plate is disposed behind a liquid crystal shutter, a scattering plate is irradiated with an LED, and a light transmission shape that transmits light behind the liquid crystal shutter is controlled to control characters or A figure can be represented.
In such a liquid crystal device, it is possible to perform color display by switching LEDs having different emission colors on the time axis and selecting the emission color in accordance with the switching timing of the liquid crystal shutter. Further, in a field sequential type (or light source blinking type) color display device (also referred to as a field sequential color device), LEDs that emit light of the three primary colors are used in combination, and the colors are switched at high speed and mixed by the optical illusion. It is possible to select and display any color on the displayed characters / graphics.

  However, when LEDs are used as the light source, the light emission efficiency and luminance of red, green, and blue LEDs are not uniform, so it is difficult to correct the color balance in white. In some cases, the background color may be selected as appropriate. Furthermore, blue LEDs are more expensive than other colors, which may reduce the number of uses.

Therefore, the invention disclosed in Patent Document 3 lights a plurality of color light sources that generate light of different wavelengths and can be controlled independently in different time zones (subfields), and display segments in subfields of each color. In the field sequential color display device which is turned on and off to display multiple colors, the time width of the subfield corresponding to each color light source is appropriately selected. The system described in Patent Document 3 is also called a field sequential color system (FSC system).
According to the invention of the cited document 3, the intensity of a certain color can be relatively emphasized by appropriately adjusting the subfield time width corresponding to each color, so that even if LEDs having different efficiencies are used, or Even if the number of specific color LEDs is reduced, white purity can be ensured.
However, the LED drive circuit controls a plurality of LEDs independently for each of the three primary colors and for each color, and it is necessary to adjust the light output characteristics according to the respective element characteristics. Alternatively, a specific LED driving circuit is used for each LED arrangement.

By the way, in a relatively large liquid crystal display device, a light guide plate is provided on the lower surface of the liquid crystal panel, and a plurality of LEDs are arranged as light sources around the light guide plate to form a backlight device. The brightness is made uniform. In order to prevent an increase in the size of a device that is required to be small, such as a cellular phone, the LED lighting device used for the backlight includes, for example, three color LEDs packed in a single package. The so-called 3 in 1 LED package used may be arranged and used in parallel.
The packaged LED generally has an anode connected as a common terminal for all LEDs, and a cathode drawn out as a terminal that can be connected independently for each LED element.

When a plurality of such LED packages are used, the anode common terminals are connected to connect the LEDs in parallel. For example, as shown in FIG. 9, 3 in 1 LED packages X1, X2, X3,... Are connected in parallel, and red LEDs R1, R2, R3,..., Green LEDs G1, G2, G3,. , Blue LEDs B1, B2, B3,... Are connected in series with a variable resistor and switches RR1, SR1; RG1, SG1; RB1, SB1; RR2, SR2; A parallel circuit connected in parallel is used. The current flowing in such a parallel circuit increases in proportion to the number of LED elements, and easily becomes several A to several tens A depending on the number of LED elements. In the field sequential color method (FSC method) described in Patent Document 3, since the LEDs are lit in a time-sharing manner, the current flowing through the entire LEDs can be reduced.
Further, since the characteristics of the LED vary from individual to individual, it is necessary to adjust the characteristics using a variable resistor or the like for each individual element in order to obtain a desired luminance and obtain a uniform brightness distribution. In particular, in an apparatus for performing color display using a package in which RGB elements are mounted together, a great deal of adjustment is required to ensure white color when all LEDs are different in brightness and chromaticity.

When monochrome display (B / W system) white display is performed using a light guide plate having a plurality of RGB 3 in 1 LED packages attached, power consumption cannot be ignored if RGB is always lit. When the battery capacity is limited, for example, in a mobile phone, a large power consumption becomes an obstacle when a sub panel that always displays monochrome is provided in addition to a main panel that displays color. As a countermeasure for this, it is conceivable to suppress power consumption by driving in order with the LED-R of the LED-R of the LED-B of the R-LED-B with a low duty by using the FSC driving method to display white. However, since the white display of the FSC method is performed by temporally mixing colors with human eyes, flickering, color deviation, so-called color breakup, etc. occur when the balance of colors entering the eyes is lost due to blinking or the like. Quality deteriorates.
This image quality deterioration phenomenon is obtained by using a 3 in 1 LED package as described above, sequentially lighting each of the three primary colors and mixing them in time to obtain white light, and with this white light, a color filter is provided via a light guide plate. This also occurs when the panel is illuminated.
JP-A-7-128093 JP 2000-081670 A JP-A-9-274471

The problem to be solved by the present invention is to provide a highly practical drive circuit that adjusts the characteristics of each LED to suppress variations in color and brightness, and by utilizing a package incorporating a plurality of LED elements. It is to fully exhibit the effect of downsizing using LEDs.
Another object of the present invention is to provide an illuminating device using an LED drive circuit that can be miniaturized, and to provide a liquid crystal device using such an illuminating device.
It is another object of the present invention to provide an illuminating device with low power consumption that can perform high-quality white display without color breakage by switching together with color display.

In order to solve the above-mentioned problems, the LED drive circuit of the present invention is configured by connecting in parallel three series circuit branches formed by connecting flashing switches in series with red, green, and blue LED elements. A parallel circuit branch is configured, and a plurality of parallel circuit branches are connected in series to form a parallel series circuit .
Further, the parallel circuit branches may be connected in series by a multiple of 3, and each of the three parallel circuit branches that are lit at the same timing may emit LED elements of different colors .

  In the LED drive circuit of the present invention, since at least one switch element is interposed in each DC circuit branch, several DC circuits are arranged by selecting a specific DC circuit branch for each stage. It is possible to select and drive a specific LED element among the LEDs that are present. Therefore, for example, only an R element can emit light at a certain moment among a large number of RGB elements, or RGB can be switched at an appropriate timing to emit light. Moreover, the series circuit which connected LED to conduct | electrically_connect in series can be formed for every moment.

  In addition, the LED driving circuit of the present invention has a configuration of a series-parallel circuit or a parallel-series circuit, so that the parallel circuit connecting one terminal of the LED has a configuration in which several stages are stacked in series. Thus, it is not necessary to connect all the LEDs in parallel. Therefore, the current flowing through the circuit is smaller than when all the LEDs are connected in parallel. Also, since a plurality of circuits in which LEDs are connected in parallel are inserted between the power supply and the ground, the power supply voltage can be significantly smaller than when all the elements are connected in series.

It should be noted that a plurality of constant current sources each capable of adjusting the output voltage and a power source switch may be provided so that the constant current source can be selected via the power source switch.
When an LED in each stage is selected according to a predetermined condition using a blinking switch, the selected LED is connected in series between the positive terminal and the negative terminal of the power supply, so that the characteristics of each LED element are integrated and a series circuit Circuit characteristics will appear. Therefore, in order to achieve uniform illumination, it is necessary to perform characteristic compensation for each series circuit thus selected. For the characteristic compensation, there is a method of adjusting by inserting a circuit element such as a resistor in a series circuit selected and established, but the power supply voltage may be adjusted separately or in combination with this. If the power supply voltage is used, it can be adjusted accurately and relatively easily.

  Therefore, even in the LED drive circuit of the present invention, several power supply devices adjusted for each circuit formed by selecting the LED are prepared, and the power supply device is selected and switched in accordance with the LED selection timing. The circuit characteristics can be adjusted by supplying various power supply voltages. In addition, several constant current power supplies with different supply currents are interposed in one power supply device, and the necessary current is supplied by selecting and switching the constant current devices to adjust the circuit characteristics. Also good.

In addition, it is preferable that LED connected to the LED drive circuit of this invention develops either red (R), green (G), or blue (B). Arbitrary colors can be obtained in addition to the three colors RGB by mixing the LEDs in the appropriate proportions using the three primary color LEDs.
In particular, in the case of configuring a light source flashing type liquid crystal device, each color is selectively turned on sequentially, and the color transmitted through each pixel of the liquid crystal device is selected by selecting the opening and closing of the liquid crystal shutter according to the lighting timing. Select to form any color.

In addition, LEDs connected in parallel are red, green, and blue LEDs arranged adjacent to each other, and in particular, one end is connected by a common terminal every predetermined number and formed as an integrated circuit. It is preferable that
In particular, in a liquid crystal display device using a light guide plate that introduces light from the end face and diffuses light over the entire surface, positioning of LEDs that emit the three primary colors is reduced so that the thickness of the light guide plate is reduced. It becomes an important management point. In the case of arranging a large number of three primary color LEDs, a large number of man-hours are required only for LED alignment.

In addition, when a plurality of single LEDs that emit each of the three primary colors are used, the distance between the LEDs increases, so that the portion where the light for each color reaches does not overlap completely, resulting in poor color mixing. .
For this reason, it is preferable to use a package in which three primary color LEDs are arranged adjacent to each other. In the packaged one, the distance between the LED elements is short, so that the color mixing between the colors such as RGB is good. Also, wiring is simplified. Furthermore, in what is formed as integrated circuits, such as what is called a 3 in 1 LED package, the distance between LED is small and color mixing property improves very much. In addition, the wiring area of the printed circuit board on which the LED element is mounted can be reduced, and the device can be further miniaturized.

Even for LEDs of the same color, the relationship between the threshold voltage (Vth) and the light emission luminance varies not only when the production lot is different but also within the same production lot, and the emission spectrum also varies. In the circuit in which the LEDs are connected in parallel, there is a problem that the brightness and chromaticity of the lighted LEDs are different due to variations in the LED characteristics.
LEDs vary greatly in individual characteristics in manufacturing, and products are usually ranked based on characteristics. LEDs classified into the same rank have characteristics that are substantially the same with respect to current, and that have substantially the same color and brightness with respect to a certain current value.
Therefore, in the LED drive circuit of the present invention, by configuring a circuit in which a series circuit branch in which a plurality of LEDs of the same color are connected in series is connected in parallel for each LED having a different color, the lighting brightness of the LEDs of the same color is distributed. It was possible to make it uniform regardless of the installation location.

  In addition, an appropriate multiple of the three LED elements of the three primary colors RGB is connected in parallel to form a parallel circuit branch, and one of RGB is selected using a circuit in which the appropriate number of stages are connected in series, and a blinking switch is set. It is also possible to provide a circuit that is activated to electrically connect the LEDs of each stage. Even in this LED drive circuit, when three LEDs are mixed as a whole, only one of the LEDs is turned on when the LEDs are turned on. Therefore, when a predetermined color LED is selected for each stage to form a predetermined circuit, current is adjusted by placing an adjustment resistor in the circuit to adjust the current. All LEDs can have substantially the same emission color and brightness.

Since the field sequential color liquid crystal device has a structure in which only a certain color LED is lit at a certain moment, the LED driving circuit in which the RGB is selected and lit in the present invention is particularly suitable for the field sequential color liquid crystal device. When applied to a liquid crystal device, a great effect can be obtained.
Furthermore, even if the output voltage of the power supply device is adjusted, the LED current can be adjusted. When switching the LEDs of each color at high speed, prepare the required number of power supply devices with output voltage adjustment in advance, and switch the power supply device by operating the power supply changeover switch according to the color change, so that it is suitable for each color A current can be supplied.
Further, the whiteness of the white display can be made closer to a predetermined target by making the pulse width at the timing of illuminating the LED in a combination exhibiting the desired whiteness longer than the pulse width of the other timing. Using these techniques, the white color can be adjusted according to the product specifications of the display device.
Since the whiteness changes according to the mixing ratio of the three primary colors, it is possible to use a method of adjusting the light emission time of each LED element for each combination of the three primary colors to bring it closer to the desired whiteness.

  In particular, a package in which three primary color RGB LEDs are arranged adjacent to each other, a so-called 3-in-1 LED package, and a circuit in which three LEDs are connected in series so that a blinking switch is connected in series for each LED and can be selectively driven independently. By using the circuit described above, a field sequential color display system in which LEDs of the same color are simultaneously turned on in each package and color display is performed by synchronizing the opening and closing of the liquid crystal shutter at this timing, and three primary colors from each of the three LED packages. It may be possible to switch between and use a method of displaying white by simultaneously driving three LEDs selected one by one.

At the timing of color development, all three primary colors are colored synchronously and mixed in the light guide plate to display white, so that the light emitted from the light guide plate is always white. Therefore, for example, in contrast to white display by the field sequential color display system in which LEDs of the same color are all turned on simultaneously in the order of red field-green field-blue field-red field-. Therefore, a high-quality white display can be obtained.
Since the three primary color LEDs are connected in series, there is an effect that the voltage value required for driving is leveled, and the power supply device can be shared.
In addition, for example, white that is bluish can be improved in whiteness by shortening the lighting time of the blue LED. In this way, by adjusting the lighting times of the LEDs of the three primary colors, the color tone can be easily adjusted to the target whiteness. There are three types of white display. Of these, the white that is closest to the target can be made closer to the desired white by increasing the duty of the set and increasing the display time.

The illumination device of the present invention illuminates an object with the LED drive circuit of the present invention. If it is the illuminating device of this invention, even if it uses many LED elements, LED drive can be carried out using a suitable electric current and voltage.
Furthermore, the light source flashing liquid crystal device of the present invention is characterized in that the light guide plate is illuminated by the LED driving circuit of the present invention. The liquid crystal device of the present invention can drive a backlight using a current and voltage in a realistic range, and improve whiteness and suppress illuminance spots by appropriate characteristic adjustment.
In addition, a device using a drive circuit in which three 3-in-1 LED packages are connected in series can be used by easily switching between a field sequential color display method and a white display method, and a high-quality white display can be obtained.

The LED driving circuit, the lighting device, and the light source flashing type liquid crystal device of the present invention can be driven using a current and voltage in a practical range even when a large number of LED elements are driven simultaneously. In addition, the light emission characteristics can be easily adjusted, and the illumination spot is small. Furthermore, since wiring can be performed with a configuration in which one terminal of LEDs arranged in parallel is connected, the LED element unit packaged as an integrated circuit or the like can be used to easily attach to an object, and the position of the LED. Can be determined accurately.
In addition, by applying the LED driving circuit of the present invention, a liquid crystal display device capable of switching between color display and monochrome display can be configured. In particular, when performing monochrome display, it is possible to obtain a high-quality monochrome image with no color breakup and high whiteness of white display. In addition, since power consumption in monochrome display can be reduced, for example, when applied to a mobile phone, power consumption during standby display can be suppressed and battery life can be extended.

  Hereinafter, the present invention will be described in detail using examples.

FIG. 1 is an exploded perspective view of a main part of a liquid crystal device according to a first embodiment of the present invention, and FIG. 2 is a circuit diagram of an LED drive circuit used in the liquid crystal device of the first embodiment.
The liquid crystal device of this embodiment has a liquid crystal display panel 1 and a light guide plate 2. The liquid crystal display panel 1 includes a display surface 3 on which liquid crystal shutter elements are arranged in a matrix, and a shutter control circuit 4 opens and closes the liquid crystal shutter. The light guide plate 2 includes a reflective sheet on the opposite side of the liquid crystal display panel with respect to a substrate that introduces and transmits light, and includes a diffusion sheet and one or more prism sheets between the liquid crystal display panel, It has a function of guiding light generated from the LED, which is a light source, in the surface direction and emitting light in the direction of the liquid crystal display panel. The light guide plate 2 of this embodiment is used as a backlight which is an illumination device for color display. An LED package 5 called a 3 in 1 LED, in which three primary color red, green, and blue LED elements are arranged and accommodated on the side surface of the light guide plate. A plurality of LEDs are arranged, and the LED drive circuit 6 switches the color of the LED irradiated to the light guide plate 2. The LED drive circuit 6 includes a drive power supply.
Wirings between the liquid crystal display panel or LED package 5 and the LED drive circuit 6 shown in FIG. 1 are conceptually shown, and a specific circuit configuration is as described in detail below.

  The LED driving circuit of this embodiment drives LEDs of RGB three primary colors. As shown in FIG. 2, red LED elements 11R, 12R,..., Green LED elements 11G, 12G,. 11B, 12B,..., 21G, 22G,...; 21B, 22B,..., 21B, 22B,. Two or more parallel circuits 21G; 11B, 21B,... Connected in parallel are connected in series and connected to a battery, a DC power source or a constant current source 41. When selecting one of RGB based on the clock signal generated from the clock device 51 provided in the LED drive circuit 6, all of the parallel circuits in each stage are connected to the series circuit branch including the LED of the selected color. The blinking switch is short-circuited (lighted or turned on or conducted). Then, a circuit in which LEDs of the same color are connected in series is formed.

As illustrated in FIG. 2, for example, when red color is developed, the semiconductor switch connected to the red LEDs 11R, 12R,... In each of the first, second, third,. When 21R, 22R,... Are made conductive, a series circuit in which a plurality of red LEDs are connected in series is formed. In the figure, the LED that has become conductive at a certain point is shown in black.
The RGB LEDs are switched and blinked at high speed for each color by the LED drive circuit 6. At this time, when the liquid crystal shutter is opened and closed by the shutter control circuit 4 of FIG. 1 in accordance with the time when the LED of the selected color is turned on, the pixels of the liquid crystal display surface 3 are colored with red, green, blue, and a mixed color thereof. Thus, a color image can be displayed. If the three primary colors are switched in 16 ms or less, it is recognized as a natural color display for human eyes. A liquid crystal device that performs color display of characters and shapes in this way is called a light source flashing liquid crystal device.

  The light source flashing type liquid crystal device of this embodiment switches the three primary color LEDs at high speed and causes the LED to flash. Therefore, when all the changing colors of the light guide plate illuminated by the LED can be seen, the three primary colors are mixed into the eye to become white. appear. The more beautiful the white color is, the better. In order to reproduce the white color beautifully, the emission color and brightness of each LED must have an appropriate balance.

In the conventional apparatus shown in FIG. 9, when all the LED elements are connected in parallel, it is necessary to individually compensate the individual characteristics by providing an adjusting resistor for each LED element.
However, in this embodiment, as shown in FIG. 2, variable resistors 31R, 31G, 31B,... May be connected in series somewhere in the series circuit branch formed for each color. Even if there is a manufacturing variation in LED elements, the current characteristics of the ranked elements are almost the same, and a predetermined color tone and brightness can be exhibited by controlling the current. Therefore, when the circuit impedance is adjusted by the variable resistors 31R, 31G, 31B,..., The LEDs connected in series by RGB selection all have the same color and brightness by flowing the same current.

Since the LED drive circuit of this embodiment has a circuit configuration in which one of the terminals of the LED is connected, the 3-in-1 LED package can be used as it is. Compared with the case where the LED elements are positioned and connected one by one, the use of a 3 in 1 LED package significantly simplifies the mounting and wiring, and allows a reduction in size. Furthermore, since it is packaged, the dimensional reproducibility at the time of mounting is high compared to the case where individual LED elements are mounted and fully mounted. Even when the LED package is used for irradiation from the side surface, the light guide plate can be accurately arranged according to the design so that the color and illuminance of the light guide plate are not uneven. In addition, since the LED elements of RGB colors can be assembled in a very narrow range, the color mixing property is good.
When all the LED elements are connected in parallel, the current flowing through each LED changes, so that the brightness of each LED varies. In contrast, in the LED drive circuit of this embodiment, the LED elements that are lit are connected in series. The currents that flow through the lit LED elements are equal, and the brightness of the LEDs is also substantially equal.

  In the circuit shown in FIG. 2, 3in1 LED packages 11, 12,... In which three primary color LEDs are arranged in parallel are connected in series, and one LED element is selected for each stage to form a series circuit. The circuits configured as described above are connected in parallel, but a plurality of 3 in 1 LED packages may be connected in parallel to form a parallel circuit, and these parallel circuits may be connected in series. With such a configuration, the effect of the series connection can be obtained and the characteristics of the LED can be compensated, and the circuit can sufficiently cope with a failure due to disconnection.

Note that the applied voltage may be adjusted instead of or in addition to adjusting the characteristic unevenness of the LED with the resistor.
FIG. 3 is a circuit diagram conceptually illustrating another characteristic compensation method in the LED drive circuit of the present embodiment. As shown in FIG. 3, constant current sources 42R, 42G, and 42B connected to the LED circuits of the respective colors via a high-speed changeover switch 61 are connected to a power supply device 41, and the constant current sources are selected in accordance with the color switching. Can be switched. The constant current sources 42R, 42G, 42B are adjusted so that a predetermined current flows with respect to the characteristics of the LED series connection circuit formed for each of the three primary colors by the action of the blinking switches 21, 22,. .
Since the power supply voltage can be adjusted accurately and relatively easily, according to the method of switching the constant current source, it is possible to accurately adjust the emission color and brightness of the LED.

  Also, since the LED has a voltage threshold Vth that varies depending on the color, if the power supply voltage is set according to the highest blue LED, power is wasted when driving a red or green LED, and a resistor is used in the adjustment circuit However, in the case where the constant current source is switched and used, characteristics are adjusted by switching the power source for each color, so that such power consumption can be suppressed.

In the above embodiment, the case where a parallel circuit is formed by connecting the anode terminals of the LEDs is illustrated with the intention of using a 3in1 LED package that is currently commercially available, but the cathode terminals of the LEDs are connected. It goes without saying that the same effect can be obtained even with such a circuit.
In addition, if the LED connected to the anode terminal and the LED connected to the cathode terminal are used in pairs, the blinking switch can be saved. FIG. 4 is a partial circuit showing a part of an LED driving circuit using the two types of LEDs in pairs. In this circuit, as shown in FIG. 4, the cathode terminal of one LED of the LED element 71 connected to the anode terminal, the anode terminal of one LED of the LED element 72 connected to the cathode terminal, and these One flashing switch 81 is connected in series so as to be sandwiched between terminals, thereby forming a series circuit branch. In this series circuit branch, two LED elements can be controlled by one blinking switch. If a series parallel circuit in which such series circuit branches are connected in parallel is used alone or connected to a constant current source as a circuit in which some are connected in series, circuit elements can be saved.

Furthermore, in this embodiment, the LED elements connected in parallel need not be three primary colors, and are not limited to three, and may be any specific color or single color LED.
Further, it goes without saying that one or more LEDs may be connected in series to the other terminal of the LEDs connected in parallel to the terminal of the above embodiment to increase the brightness or increase the number of illumination points. .

The second embodiment of the present invention is an LED driving device used for a lighting device of a liquid crystal display device such as a mobile phone or a digital camera. FSC for color display using a circuit in which three 3-in-1 LED packages with three primary color RGB LEDs arranged adjacent to each other are connected in series, and a blinking switch is connected in series for each LED so that it can be selectively driven independently. It was made possible to switch between the method and the B / W method of white illumination by selecting three primary color LEDs from each of the three LED packages and driving the three primary color LEDs simultaneously.
The LED drive circuit of the present embodiment has the same configuration as the circuit in the first embodiment, and the operation method is added to produce the effects of the present embodiment.
FIG. 5 is a circuit diagram of an LED driving circuit according to a second embodiment of the present invention, FIG. 6 is an operation principle diagram when the FSC system is executed in the driving circuit of the second embodiment, and FIG. 7 is a white display. FIG. 8 is a conceptual diagram showing an example of a liquid crystal display device using the LED driving circuit of the second embodiment. The elements in FIG. 5 have the same functions as the elements shown in FIG. 2. Therefore, the elements having the same functions are denoted by the same reference numerals to simplify the description.

As illustrated in FIG. 5, the LED drive circuit of this embodiment is a series of three RGB 3 primary colors 3 in 1 LED packages connected in series. Each LED in the unit is provided with a blinking switch, and a pulse given from the clock device 51. The on / off state of each blinking switch can be controlled by a signal.
Usually, as shown in the part surrounded by the dotted line in FIG. 5, in the 3in1 LED package, one terminal of each color LED element is connected to a common terminal in the unit, and the other terminal is connected to an independent terminal. ing.

  In the LED drive circuit of this embodiment, the blinking switches 21R, 21G, 21B, 22R, and 22G are connected to the independent terminals connected to the LED elements 11R, 11G, 11B, 12R, 12G, 12B, 13R, 13G, and 13B of the respective colors. , 22B, 23R, 23G, and 23B are connected in series, and the other terminal of the blinking switch is connected to each other and connected to the common terminal of the LED unit at the next stage. The common terminal of the first stage LED unit 11 is connected to the high potential side terminal of the power supply 41, and the flashing switches 23 R, 23 G, and 23 B of the third stage LED unit 13 are connected to the low potential side terminal of the power supply 41. It is connected.

When the FSC method is selected, the LED packages at each stage are controlled at the timing shown in FIG. In the figure, the horizontal axis represents time t, the vertical axis represents the on / off state of the LED packages (LED1, LED2, LED3), and the letters R, G, B represent the colors of the elements that are in the on state of the LED package. LED1, LED2, and LED3 in the figure correspond to the LED packages 11, 12, and 13 in FIG. 5, respectively, but it goes without saying that the order may be changed.
As can be seen from the figure, the switch control circuit 52 provided in the LED drive circuit 6 generates a control signal for selecting one of RGB based on the clock signal transmitted from the clock device, and selects it for each parallel circuit of each stage. The blinking switch connected in series with the colored LED is turned on (conductive state). Then, a circuit in which the LEDs of the selected color are connected in series is formed, the LEDs of the selected color emit light, and the light guide plate 2 (see FIG. 1) is dyed in that color.
For example, when red color is developed, the semiconductor switches 21R, 22R, and 23R connected to the red LEDs 11R, 12R, and 13R in the first stage LED unit 11, the second stage LED unit 12, and the third stage LED unit 13 are turned on. By doing so, a series circuit in which three red LEDs are connected in series is formed.
Since the three primary color LEDs are sequentially switched at high speed by the LED driving circuit 6 and blinking, when the liquid crystal shutter is opened and closed in accordance with the lighting time of the LED of the color selected by the shutter control circuit 4, the pixels on the liquid crystal display surface 3 are displayed. Can be colored with red, green, blue, or a mixed color thereof to display a color image.
The red LED emits light during time t1, the green LED emits light during t2, and the blue LED emits light during t3. Usually, t1, t2, and t3 are about 5.5 ms, respectively.

When switching to the B / W driving method, the LED elements at each stage are controlled at the timing shown in FIG. The meaning of the display in FIG. 7 is the same as that in FIG.
As can be seen from FIG. 7, each of the three 3-in-1 LED packages turns on LEDs of RGB colors in order. In addition, the pulse lighting of each stage displayed in the vertical direction in FIG. 5 is synchronized, and each stage of the 3-stage 3-in-1 LED package that lights up at the same timing emits LED elements of different colors, so any time during lighting The three primary colors are aligned and light is emitted, and the light guide plate 2 becomes white.
Since there is a degree of freedom in the combination of LED elements that are lit simultaneously, a combination of the three primary colors with good white color development is selected.
In the B / W method in the present embodiment, the light guide plate always emits white light at the lighting timing, so even if the incident light to the eyes is interrupted in time such as blinking, it can always be recognized as white, so color breakage, etc. A high-quality white display can be obtained without any obstacles.
Also, the LED elements are not always lit, but as shown in FIG. 7, the three LEDs combined in the three primary colors are pulsed within the times t1 ′, t2 ′, and t3 ′ as the combination changes. Illuminated. At this time, for example, since the lighting time is shortened with a duty of about 10%, the power consumption is reduced, and there is an advantage that the battery life is prolonged particularly when applied to a battery type portable display device.

Note that, according to the B / W method LED driving method of the present embodiment, as shown in FIG. 5, the three primary color RGB LED elements are connected in series when displaying white. FIG. 5 shows a state in which the first stage LED unit 11 is energized with the red LED element 11R, the second stage LED unit 12 is energized with the green LED element 12G, and the third stage LED unit 13 is energized with the blue LED element 13B to display white. . In the figure, black light-emitting diodes indicate LEDs that emit light when energized.
Although the LED has a voltage threshold Vth that differs depending on the color, according to the B / W method LED driving method of this embodiment, the LED elements of the three primary colors are connected in series at the timing of color development. Therefore, almost the same drive voltage is sufficient for any combination. In addition, since the individual differences in the color development performance with respect to the drive current are small for the LED elements, the whiteness of white display is relatively stable in this embodiment in which three LED elements are connected in series and all are controlled for color development by the same drive current. Yes.

Since the LED elements have individual differences, there arises a problem that whiteness varies depending on how the three primary color LED elements are combined. In this embodiment, since there are many possibilities of combining the three primary color LED elements, the problem can be solved by selecting a combination close to the target whiteness. Furthermore, the whiteness of white display can be made closer to a predetermined target by making the pulse width at the timing of illuminating the LED in a combination exhibiting the desired whiteness longer than the pulse width of other timings. Using these techniques, the white color can be adjusted according to the product specifications of the display device.
Since the whiteness changes according to the mixing ratio of the three primary colors, it is possible to use a method of adjusting the light emission time of each LED element for each combination of the three primary colors to bring it closer to the desired whiteness. For example, in a combination exhibiting white with excellent bluishness, the whiteness can be improved by shortening the lighting time of the blue LED. In this way, by adjusting the lighting times of the LEDs of the three primary colors, the color tone can be easily adjusted to the target whiteness.
Needless to say, the white display light guide plate can also be applied to a color filter type liquid crystal display device that performs color display using a color filter.

FIG. 8 is a side view showing a state in which the side portion of the frame is cut away in an example in which the LED driving circuit of this embodiment is applied to a portable liquid crystal display device.
In the liquid crystal display device, a light guide plate 91 of the present embodiment, a liquid crystal panel 93 for a main screen, and a liquid crystal panel 96 for a sub screen are incorporated in a frame 95. The light guide plate 91 is mounted with 3 in 1 LED packages 92 that are light sources in the vicinity of the light guide plate 91 in three or multiples of three. Illumination light emitted from the light source enters the light guide plate 91 and is emitted from the light emission surfaces on the upper and lower surfaces of the light guide plate to illuminate the liquid crystal panels 93 and 96.
A transflective layer 94 is formed between the light guide plate 91 and the sub-screen liquid crystal panel 96.

A large opening is formed on the main screen side of the frame 95 to form a main screen display window 97.
When the 3-in-1 LED package 92 is driven by the FSC method, the LED elements of the same color in each LED unit emit light at the same time, and the light guide plate 91 is light corresponding to the color of the image to be displayed on the liquid crystal panel 93 for the main screen. Illuminate with. The three color LED elements are sequentially turned on in synchronization. The light directly projected from the light guide plate 91 and the light incident after being reflected by the transflective layer 94 are transmitted through the liquid crystal panel 93, reach the main screen display window 97, and are observed as a color image by FSC driving.

When the display device is driven by the B / W method, different one of the three primary colors is selected from each stage of the three 3-in-1 LED packages 92 and lighted simultaneously. The RGB three primary color lights that are simultaneously turned on are mixed in the light guide plate 91 and emitted as white light from the light guide plate 91, transmitted through the semi-transmissive reflective layer 94, and incident on the sub-screen liquid crystal panel 96 to become a backlight.
At this time, it is preferable that the liquid crystal panel 93 for the main screen is in a state in which light does not pass to improve the light use efficiency. Needless to say, even if the liquid crystal panel 93 for main screen is in a light transmitting state, it can be displayed on the sub screen.

  By applying the LED driving circuit of this embodiment, a liquid crystal display device capable of switching between color display and monochrome display can be configured. In particular, when performing monochrome display, it is possible to obtain a high-quality monochrome image with no color breakup and high whiteness of white display. In addition, since power consumption in monochrome display can be reduced, for example, when applied to a mobile phone, power consumption during standby display can be suppressed and battery life can be extended.

It is a disassembled perspective view of the principal part of the liquid crystal device of 1st Example which concerns on this invention. It is a circuit diagram of the LED drive circuit used for the liquid crystal device of 1st Example. It is a circuit diagram explaining another characteristic compensation method in the LED drive circuit of 1st Example. It is a partial circuit diagram explaining the case where it uses as a pair LED of anode terminal connection and cathode terminal connection in the LED drive circuit of 1st Example. It is a circuit diagram of the LED drive circuit used for the liquid crystal device of 2nd Example which concerns on this invention. It is an operation | movement principle figure of the FSC system in 2nd Example. It is an operation | movement principle figure of the white display in 2nd Example. It is a conceptual diagram which shows the example of the liquid crystal display device using the LED drive circuit of 2nd Example. It is a circuit diagram of the LED drive circuit of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal display panel 2 Light guide plate 3 Display surface 4 Shutter control circuit 5 LED package 6 LED drive circuit 11, 12, 13, ... 3 in 1 LED package 11R, 12R, 13R, ... Red LED element 11G, 12G, 13G, ... Green LED element 11B, 12B, 13B, ... Blue LED element 21G, 21G, 21B; 22R, 22G, 22B; ... Flashing switch 31R, 31G, 31B; ... Variable resistor 41 Power supply 42R , 42G, 42B Constant current source 51 Clock device 52 Switch control circuit 61 Changeover switch 71 Anode terminal connection LED element 72 Cathode terminal connection LED element 81 Flashing switch 91 Light guide plate 92 3 in 1 LED package 93 Liquid crystal panel for main screen 94 Transflective layer 95 Frame 96 LCD panel for sub screen 97 Main screen display window

Claims (3)

A parallel circuit branch is formed by connecting three series circuit branches, each of which is formed by connecting a blinking switch in series with each of the LED elements of red, green, and blue, in parallel. A parallel series circuit is configured by being connected in series ,
The parallel circuit branches are connected in series by a multiple of 3, the parallel circuit branches every third to light at the same timing you characterized in that emit different colors of the LED elements to one another L ED Driving circuit. LED driving circuit according to claim 1 Symbol mounting, characterized in that the flashing switch comprises two or more of the LED elements in the series circuit branches are pinched serially connected between the LED elements. 3. The LED drive according to claim 1, wherein the LED elements connected in parallel have a configuration in which one end is connected by a common terminal every predetermined number and is formed as an integrated circuit. circuit.

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