JP2008299182A - Liquid crystal display - Google Patents

Abstract

An object of the present invention is to provide a thinner liquid crystal display device.
A liquid crystal panel 120, a light guide plate 121 provided on the back surface of the liquid crystal panel 120, and a pair of LED mounting substrates 123 on the left and right side surfaces of the light guide plate 121 are disposed. A recess 101f is provided outside the side surface of the heat sink 101 connected to the LED non-mounting surface of the LED mounting substrate 123, and the LED driving circuit 125 is disposed there. The LED driving circuit 125 includes a plurality of LEDs 124 connected in series, and includes a plurality of lines connected in parallel, an on / off controller, and a current measuring unit that measures a current value flowing through the line. A switch that turns on and off the current flowing in the circuit is connected in series, and a constant voltage is applied from a constant voltage source. Based on the current value of each line measured by the current measuring means, the on / off controller turns on / off the switch of each line. Control the period.
[Selection] Figure 2

Description

  The present invention relates to a liquid crystal display device, and more particularly to a thin liquid crystal display device using a method in which a backlight light source is arranged on the side.

  Display devices can be broadly classified into light-emitting display devices such as CRT (Cathode Ray Tube) and plasma display panels, and non-light-emitting display devices such as liquid crystal display devices.

  Non-light-emitting display devices that use a reflective light modulation element that adjusts the amount of reflected light according to an image signal and a transmissive light modulation element that adjusts the amount of light transmitted according to an image signal Some use In particular, a liquid crystal display device using a liquid crystal display panel as a transmissive light modulation element and having an illumination device (also called a backlight) on the back is thin and lightweight, so that it can be used as a computer monitor or television receiver (TV). It is adopted as various display devices.

As the display principle of the liquid crystal panel, in addition to the mainstream TN (twisted nematic), IPS (in-plane switching) characterized by a wide viewing angle, MVA (multi-domain vertical alignment), etc. are used. Light is emitted from a backlight installed on the back surface of the area, and the liquid crystal panel forms an image by controlling the transmittance of the light emitted from the backlight.
In recent years, there has been a growing demand for higher brightness and wider color reproduction range for these liquid crystal display devices. In addition, with the spread of thin TVs, there is an increasing need for TVs that can be wall-mounted.
In order to reduce the thickness of the liquid crystal display device, a sidelight system has been proposed in which the backlight light source is not located on the back side of the liquid crystal panel but arranged on the side.

Patent Document 1 describes an example in which an LED (Light Emitting Diode) is disposed on the back surface of a liquid crystal panel and an LED drive circuit is disposed on the back surface of the LED (Patent Document 1, FIG. 8).
As a characteristic required for a backlight of a liquid crystal display device, it is desired that brightness and color do not change. However, since the LED has a characteristic that the light emission efficiency changes depending on the temperature, the brightness changes due to the effect of self-heating during lighting. Therefore, a compensation technique is required so that brightness and color do not change with respect to temperature changes.

  In Patent Document 2, as a technique for compensating for fluctuations in brightness of LEDs over time (hereinafter referred to as "brightness fluctuations"), an optical sensor is provided to detect the amount of light emitted from each LED, thereby satisfying the LED driving conditions. A technique for applying feedback to prevent LED brightness fluctuations is described (Patent Document 2, FIG. 14).

JP 2006-258972 A JP 2005-310997 A

  However, in recent years, liquid crystal TVs with large screens exceeding 32 inches have become widespread, and in order to realize this, it has become necessary to arrange a large number of LEDs almost uniformly on a wide surface. With the technique of Document 2, it is difficult to compensate for brightness variations of a large number of LEDs.

  For example, in a compensation technique using an optical sensor, when a large number of LEDs are used, the positional relationship between the LED and the optical sensor differs depending on the individual LED, so that the amount of light from each LED received by the optical sensor differs. . Therefore, by providing a correction table for correcting the positional relationship between the optical sensor and the LED and performing calculations, complicated processing such as estimating the amount of light emitted from each LED is required, resulting in an increase in circuit cost.

  In addition, it becomes necessary to use a large number of optical sensors, which increases the number of optical sensors and increases costs. In addition, when multi-primary light sources such as RGB (red, green, and blue) individual LEDs are used, a color filter is disposed in the optical sensor, and light for each color must be detected, which increases costs. There is.

  Further, in the prior art described in Patent Document 2, the LED drive circuit is complicated and the area occupied by the LED drive circuit is large, and it must be arranged on the back side of the liquid crystal display device. All the circuits and the like are arranged on the back side, and there is a problem that the liquid crystal display device becomes thick.

  The present invention has been made to solve such problems, and an object thereof is to provide a thinner liquid crystal display device.

In order to solve the above-described problems, a first invention includes a liquid crystal panel, a light guide plate disposed on the back surface of the liquid crystal panel, a pair of LED mounting substrates disposed opposite to both side surfaces of the light guide plate, and an LED mounting substrate. A plurality of LEDs mounted on the surface of the light guide plate, a first frame disposed on the liquid crystal panel side, and a back cover disposed on the back side of the light guide plate, and the pair of LED mounting substrates is a liquid crystal panel In the liquid crystal display device arranged in the left-right direction with respect to the display screen of
A heat sink provided on the surface of the LED mounting substrate opposite to the light guide plate, and a left and right outer side of the heat sink, and an LED disposed between the heat sink and the side surface of the first frame or the back cover. And an LED drive circuit that drives in series.

  According to the first aspect of the present invention, since the LED driving circuit that drives the LEDs in series is formed, the LED driving circuit becomes compact and can be disposed on the side end side of the liquid crystal display device. As a result, the number of circuits arranged on the back side of the liquid crystal display device can be reduced.

  Further, in the second invention, in addition to the configuration of the first invention, the LED drive circuit performs on / off control of energization of the line and a line in which a plurality of lines having the plurality of LEDs connected in series are connected in parallel. An on / off control means and a current measurement means for measuring a current value flowing through the line, and the line is configured by connecting in series a switch that is controlled by the on / off control means to turn on / off the current flowing through the LED. A predetermined voltage is applied from the voltage source, and the on / off control means controls the on / off period of each line switch based on the current value of each line measured by the current measuring means.

  ADVANTAGE OF THE INVENTION According to this invention, it can be set as the compact LED drive circuit which can control the light emission amount of LED also with respect to the temperature change of LED, and can suppress color unevenness and brightness unevenness.

  According to the present invention, a thinner liquid crystal display device can be provided.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a perspective view illustrating a configuration of a liquid crystal display device according to an embodiment of the present invention, and FIG.
It is arrow sectional drawing. FIG. 3 is a layout view seen from the front side for explaining the connection layout between the LED mounting board and the LED drive circuit board of the liquid crystal display device.

"overall structure"
In the present embodiment, as shown in FIG. 1, for the purpose of explanation, the top, bottom, left, right, and front and back surfaces are defined based on the display screen of the liquid crystal panel 120. That is, when a user who watches the liquid crystal display device 1A faces the display screen of the liquid crystal panel 120, the front display screen side is “front”, the back side is “back”, and the right side toward the user Is defined as “right” and the left side as “left”.
The liquid crystal display device 1 </ b> A of the present embodiment includes a liquid crystal panel 120, a light guide plate 121, an LED 124, an LED mounting substrate 123, and a heat sink 101 as main components. As other components, the first frame 137, the first rubber cushion 131, the second rubber cushion 132, the second frame 138, the buffer 133, the optical sheet 134, the first reflective sheet 135, the first 2 reflection sheet 136, third frame 139, back cover 122, LED drive circuit 125 (see FIG. 2), circuit board 140 (see FIG. 2), and the like.

Next, the arrangement and function of each component will be described from the front side with reference to FIG.
(First frame, liquid crystal panel, and second frame)
The first frame 137 is disposed on the front surface of the liquid crystal panel 120 and has a function as a front cover of the liquid crystal display device 1A. The first frame 137 has a shape in which the display area of the liquid crystal display device 1A is opened. To fasten the second frame 138 to the screw hole (not shown) provided on the front surface of the housing portion 101a of the heat sink 101 with the fixing screw 201 on the front surface of the left and right end sides of the opening of the first frame 137. (Not shown) is provided.

The liquid crystal panel 120 is disposed on the back side of the first frame 137. The liquid crystal panel 120 has a structure in which liquid crystal is sandwiched between two substrates, and has a function as an optical shutter that controls transmission / blocking of light emitted from the light guide plate 121 by turning on / off the liquid crystal.
The liquid crystal panel 120 has a configuration in which a first rubber cushion 131 and a second rubber cushion 132 having a square frame shape are interposed between a first frame 137 and a second frame 138 having a square frame shape. is there.

Specifically, a first rubber cushion 131 is interposed between the back surface of the opening edge portion of the first frame 137 and the front edge portion of the liquid crystal panel 120. Similarly, the liquid crystal panel 120 A second rubber cushion 132 is interposed between the rear edge and the front edge of the second frame 138 to elastically press the liquid crystal panel 120 from both the front side and the rear side. And supported by the first frame 137 and the second frame 138.
The first and second frames 137 and 138 are made of, for example, a metal plate such as stainless steel or aluminum (Al). The first and second rubber cushions 131 and 132 also have a function as a cushioning material.

(Optical sheet, shock absorber)
The optical sheet 134 (see FIG. 1) is disposed on the back side of the second frame 138 with a rectangular frame-shaped buffer 133 interposed therebetween, and further uniformizes the light emitted from the light guide plate 121 or on the front surface. It has a directivity imparting function that improves the luminance in the lateral direction. In FIG. 1, one optical sheet 134 is representatively displayed, but in FIG. 2, the three optical sheets 134A, 134B, and 134C are arranged in this order from the front side. Here, the optical sheet 134A is a reflection deflecting plate that reflects light having a specific vibration surface of light and transmits others, the optical sheet 134B is a prism sheet that improves luminance in the front side direction, and the optical sheet 134C. Is a diffusion sheet for further uniforming the light in the surface.

  In the present embodiment, the above-described three optical sheets 134A, 134B, and 134C are used in combination as the optical sheet 134. However, the present invention is not limited to this, and one optical sheet has a multilayer structure and has a reflective deflection. An optical sheet having at least two functions of a plate function, a prism function, and a diffusion function may be used to reduce the number of optical sheets 134, or conversely, a plurality of diffusion sheets may be combined.

  It is preferable that the buffer 133 has elasticity, has sliding properties (low friction properties, easy slipping properties) with the optical sheet 134, and does not damage the surface of the optical sheet 134 due to sliding. For example, the inner core is made of urethane sponge, and the outer surface is covered with a nylon (Ag) coated nylon fiber cloth, which is further plated with nickel (Ni), or the surface of silicone (Si) rubber sponge is coated with fluororesin or fluororesin The one covered with tape is considered.

(First reflective sheet)
The first reflection sheet 135 is a resin sheet and is disposed on the back side of the optical sheet 134 (134A, 134B, 134C).
The first reflection sheet 135 is a square frame-like sheet having a square opening corresponding to the display area of the liquid crystal display device 1A, and the opening edge 135a around the opening rises from the left and right ends to the front side. The surface of the rising portion 135b on the LED mounting substrate 123 side is bonded to the LED mounting substrate 123 with an adhesive or an adhesive member 173 such as a double-sided tape.
The first reflection sheet 135 reflects the light that does not enter the incident surface 121a of the light guide plate 121 out of the light emitted from the LED 124 and makes it incident on the incident surface 121a, and the light exiting surface 121b of the light guide plate 121 near the LED 124. It has a function for returning the emitted light to the light guide plate 121 again.

In the vicinity of the LED 124, the emitted light from the RGB LEDs 124 is not sufficiently mixed and non-uniform, and this portion cannot be used as a display area portion. Therefore, by returning the light in the vicinity of the LED 124 to the light guide plate 121 by the first reflection sheet 135, it is possible to achieve uniform color and reduce light loss.
The first reflection sheet 135 has at least an expansion / contraction absorbing portion 135d on the left and right outer sides of a region overlapping the light guide plate 121 of the opening edge portion 135a. In the example of FIG. 2, the expansion / contraction absorption part 135d is obtained by attaching a wavy flexible ridge to the first reflection sheet 135 in advance to form the expansion / contraction absorption part 135d. The expansion / contraction absorbing portion 135d is not limited to a wavy flexible ridge, and a U-shaped bead ridge may be attached to the first reflective sheet 135 in advance.
Even if the light guide plate 121 expands and contracts in the left-right direction and the first reflection sheet is dragged in the left-right direction, the expansion / contraction absorbing portion 135d can absorb the expansion and contraction. As a result, the adhesive member 173 does not peel off when contracted, and a good reflection state in the vicinity of the LED 124 is maintained.

  It is to be noted that the expansion / contraction absorbing portion 135d is provided not only in the horizontal direction side but also in the vertical direction side of the opening edge portion 135a so as to be continuous and surround the front side of the light guide plate 121. It is easy to cope with.

<< Light guide plate, LED, second reflection sheet, heat sink, etc. >>
The light guide plate 121 is disposed on the back surface of the liquid crystal panel 120. A pair of LED mounting substrates 123 are arranged opposite to the left and right side surfaces of the light guide plate 121.
In addition, each LED mounting board | substrate 123 arrange | positioned at both right-and-left both sides does not need to be 1 piece, and may be divided | segmented.
A plurality of RGB LEDs 124 are mounted in a row in the longitudinal direction (vertical direction) on the LED mounting substrate 123. The heat sink 101 is bonded to the LED non-mounting surface of the LED mounting substrate 123, that is, the side opposite to the light guide plate 121 side using a heat conductive bonding member 171 (see FIG. 2). The plate-like second reflection sheet 136 is disposed on the back side of the light guide plate 121.
The detailed configuration and function of each will be described below.

(Light guide plate)
The light guide plate 121 is made of a transparent resin such as acrylic and has a function of converting light emitted from the LED 124 into a surface light source. The light incident on the incident surface 121 a of the light guide plate 121 propagates by being totally reflected in the light guide plate 121, is scattered by reflection dots (not shown) printed on the back surface of the light guide plate 121, and is emitted from the output surface 121 b of the light guide plate 121. It is taken out to the front side. A space in the horizontal direction is provided between the incident surface 121a of the light guide plate 121 and the lens 142 of the LED 124 described later, and it is an extension allowance when the light guide plate 121 is irradiated with the LED 124 and thermally expanded.

(Second reflection sheet)
The second reflection sheet 136 reflects light that is not directly incident on the incident surface 121a of the light guide plate 121 out of the light emitted from the LED 124 and makes it incident on the incident surface 121a. The light that is scattered by the reflective dots and deviates from the total reflection condition and exits to the back side of the light guide plate 121 is returned to the light guide plate 121 again. For this purpose, the light guide plate 121 is disposed adjacent to the back surface side, the front side of the side end portion 136a slides facing the back side end surface of the LED mounting substrate 123, and the side end portion 136a is further connected to the heat sink 101. It is inserted into a groove 101d to be described later and set so as not to hit the groove bottom surface 101e.

(LED)
The LED 124 has a function of emitting light for display.
The LED mounting substrate 123 is made of, for example, a ceramic substrate. The LED mounting substrate 123 mounts the LED 124 and supplies current / voltage to the LED through a wiring pattern formed on the LED mounting substrate 123. The LED mounting substrate 123 also has a function as a reflecting plate for efficiently guiding the light emitted from the LED 124 to the light guide plate 121. In addition, by using a low thermal resistance ceramic, it is possible to easily conduct the heat generated in the LED 124 to the outside.
The LED mounting substrate 123 is not limited to ceramic, and may be any resin plate as long as it has good light reflection efficiency, high thermal conductivity, and high electrical insulation.
The LEDs 124 arranged in a line in the vertical direction are molded into a substantially semi-cylindrical shape with a transparent resin, and the formed substantially semi-cylindrical transparent resin constitutes the lens 142.

The power to the RGB LEDs 124 arranged on the left and right LED mounting boards 123 is supplied from the LED driving circuit board (LED driving circuit) 125 arranged on the outer side of the left and right side surfaces of the heat sink 101 to the connectors 143A and FPC (flexible). Printed wiring board) 144A and connector 143B are supplied (see FIG. 3). Here, the FPC 144A and the FPC 144B described later have connectors at both ends.
The LED drive circuit 125 is a printed circuit board for an LED drive circuit in which various components are mounted on a printed circuit board. The LED drive circuit 125 is fixed to a recess 101f described later on the heat sink 101 via a heat insulating member 174 that also serves as an adhesive. Has been.
The LED drive circuit board 125 applies a predetermined DC voltage to the connector 143C, FPC 144B, and connector 143D from the LED power circuit 140a (see FIG. 3) of the circuit board 140 disposed on the back side of the third frame 139 shown in FIG. Is supplied through.
The circuit configurations of the LED drive circuit board 125 and the LED power supply circuit unit 140a will be described later in detail with reference to FIG.

(heatsink)
The heat sink 101 is made of a material such as copper or aluminum having excellent heat conductivity. For example, the heat sink 101 is brought into close contact with the LED mounting substrate 123 by a heat conductive adhesive member 171 such as a heat conductive tape or silver paste, thereby efficiently generating heat from the LED 124. It has a function to receive heat. The heat sink 101 is disposed in parallel to the left and right side edges of the liquid crystal display device 1A and serves as a structural member of the liquid crystal display device 1A. While holding the 3rd flame | frame 139, it is comprised from the holding base part 101b used also as the base part of the fin 101c for heat radiation.
A screw hole (not shown) corresponding to the fixing screw 201 is provided on the front surface side of the casing portion 101a. The second frame 138 and the first frame 137 are stacked on the front surface side in this order, and the fixing screw 201 is shared. It can be tightened.
In addition, a screw hole (not shown) corresponding to the fixing screw 201 is provided on the back side of the housing portion 101a so that the back cover 122 can be fixed by the fixing screw 201.

A groove 101d for receiving the side end portion 136a of the second reflective sheet 136 is provided in the vertical direction on the left and right inner surfaces of the housing portion 101a of the heat sink 101, and between the groove bottom surface 101e and the end surface of the side end portion 136a. There is a space for stretching.
In addition, concave portions 101f are provided on the left and right outer surface sides of the housing portion 101a so as to accommodate the LED drive circuit board 125, and the front and the left and right outer surface sides of the housing portion 101a are connected to a plurality of portions in the vertical direction up to the concave portion 101f. A communication groove 101g for passing the FPC 144A is provided (see FIG. 1).

(Third frame)
A third frame 139 is disposed on the back side of the second reflection sheet 136. The third frame 139 is made of, for example, a metal plate such as stainless steel or aluminum (Al). The third frame 139 is held on the front side of the holding base 101b, is pressed to the front side, presses the second reflection sheet 136 to the front side, and further includes the light guide plate 121 and the like disposed on the front side. Pressing to the front side prevents the light guide plate 121 from warping.
The third frame 139 is a rib extending from the upper and lower ends to the front side so as to give rigidity so that the deflection to the back side can be reduced and the reflection sheet 136, the light guide plate 121, etc. can be pressed uniformly. have.

(Back cover)
The back cover 122 is disposed on the back surface of the heat sink 101. As shown in FIG. 2, a space continuous in the vertical direction is provided between the third frame 139 and the back cover 122, and a large number of fins 101 c serving as heat dissipation portions of the heat sink 101 are provided in the space from the holding base portion 101 b. It is growing.
The back cover 122 is made of a resin plate and serves as a protective cover on the back of the liquid crystal display device 1A and a structural member of the liquid crystal display device 1A, and at the same time has a function of a heat insulating material that prevents heat conduction from the heat sink 101. is doing. Ventilation holes 107 are provided on the lower side surfaces of the first frame 137 and the back cover 122 for intake, and on the upper sides of the first frame 137 and the back cover 122, respectively.
In addition, a circuit board 140 is arranged at the center of the left and right of the space between the third frame 139 and the back cover 122 in the vertical direction, and is fixed to the back cover 122.
The circuit board 140 includes a power source circuit such as a DC power source supplied to the LED 124 and a DC power source for controlling the liquid crystal panel 120, a circuit for controlling the LED 124, a liquid crystal panel control circuit, and the like.

《Assembly order》
Next, the assembly order of the present liquid crystal display device 1A will be described with reference to FIGS.
(1) The circuit board 140 is fixed to the front surface side of the back cover 122, and the left and right heat sinks 101 to which the LED mounting substrate 123 is bonded are fixed to the back cover 122 with fixing screws 201, and the front surface is turned upward. Put it in a state.
At this time, one connector of the FPC 144B that supplies a DC voltage to the LED drive circuit board 125 from the LED power circuit part 140a (see FIG. 3) that is a part of the circuit board 140 is also connected to the LED power circuit part 140a. The other connector side is pulled out from the communication groove (not shown) of the housing 101a to the left and right side surfaces. After fixing the heat sink 101 and the back cover 122 with screws, the other connector of the FPC 144B from the LED power circuit part 140a (see FIG. 3) is connected to the connector 143D (see FIG. 3) of the LED drive circuit board 125.
(2) Next, the third frame 139 is placed on the front surface side of the holding base 101b, and the second reflective sheet 136 is inserted into the groove 101d with the second reflective sheet 136 facing the reflective surface up. And it mounts | wears so that the expansion allowance equal on both sides may be ensured from the groove bottom face 101e.
(3) Thereafter, the light guide plate 121 is disposed so as to be in the center of the left and right. Then, the first reflection sheet 135 is overlaid on the light guide plate 121. At this time, an adhesive is applied in advance to a region of the rising portion 135b facing the LED mounting substrate 123, and is pressed and adhered to the LED mounting substrate 123 side.
(4) Next, one connector of the FPC 144A is connected to the connector 143A of the LED drive circuit board 125, the other connector is connected to the connector 143B of the LED mounting board 123, and the FPC 144A is inserted into the communication groove 101g.

(5) Next, the three optical sheets 134C, 134B, and 134A are stacked on the first reflective sheet 135 in this order. Further, the buffer 133, the second frame 138, the second rubber cushion 132, the liquid crystal panel 120, and the first rubber cushion 131 are stacked in this order.
(6) Finally, the first frame 137 is covered so as to cover the front edge, the left and right side surfaces, and the top and bottom surfaces of the liquid crystal display device 1A, and is fixed from the front surface to a screw hole (not shown) provided on the front surface of the housing portion 101a. Insert the screw 201 and tighten.

As a result, the first rubber cushion 131, the liquid crystal panel 120, the second rubber cushion 132, the second frame 138, and the buffer 133 are provided between the front side of the holding base 101b and the back side of the first frame 137. The optical sheets 134A, 134B, 134C, the first reflection sheet 135, the light guide plate 121, the second reflection sheet 136, and the third frame 139 are pressed and held.
The first frame 137, the first rubber cushion 131, the liquid crystal panel 120, the second rubber cushion 132, and the second frame 138 are bonded together with an adhesive and assembled in advance to form a subassembly. There are many.
Further, instead of fixing the circuit board 140 to the front side of the back cover 122, it may be fixed to the back side of the third frame 139.

<< LED drive circuit >>
Next, the configuration of the LED drive circuit will be described in detail with reference to FIG.
FIG. 4 is a circuit diagram of a constant pressure power source supplied to the LED drive circuit and the LED drive circuit.
In FIG. 4, the LED driving circuit board 125 only on the left side of the LED mounting boards 123 respectively disposed on the left and right side surfaces of the light guide plate 121 (see FIG. 3), and an LED power supply circuit for supplying a predetermined constant voltage thereto. The part 140a is shown. The LED driving circuit board 125 is similarly arranged on the right side of the light guide plate 121. In FIG. 4, the LED 124 is distinguished from the LED 124R that emits red (R), the LED 124G that emits green (G), and the LED 124B that emits blue (B). When there is no need to distinguish between RGB, it is simply referred to as LED124.
In the present embodiment, the LED drive circuit board 125 includes block control units 20A and 20B, and the block control units 20A and 20B are arranged in RGB on the LED mounting board 123 in a single row in the vertical direction at a predetermined cycle. The LED 124 is vertically divided into two blocks BL1 and BL2, and the drive current of the LED 124 included in each of the blocks BL1 and BL2 is controlled.

(Constant voltage source for LED drive circuit)
The LED drive circuit board 125 is supplied with a direct current from an LED power supply circuit unit 140a that rectifies an alternating current power supply and supplies a direct current of a predetermined voltage. The LED power supply circuit part 140 a constitutes a part of the circuit board 140 and is disposed in a space between the third frame 139 and the back cover 122. The LED power supply circuit unit 140 a includes a transformer 11, a rectifier circuit 12, a smoothing circuit 13, a switching regulator 14, and a step-down chopper 15. A predetermined DC voltage V _GB is supplied from the switching regulator 14 to the predetermined DC voltage via the step-down chopper 15. and it outputs the V _R.

The LED power circuit 140 a increases the AC power input by the transformer 11 and generates a constant DC voltage by the rectifier circuit 12 and the smoothing circuit 13. The voltage is adjusted by the switching regulator 14 and used as a constant voltage source (voltage V _GB ) for G and B. The voltage V _GB of G and B is stepped down by a step-down chopper, and this is used as an _R constant voltage source (voltage V _R ). Thus, the constant voltage source _V GB plurality of primary colors, by sharing the _{V R,} thereby achieving the cost of the LED power supply circuit unit 140a.

In Figure 4, the constant voltage source V _GB, are displayed in a simplified as supplying a predetermined DC voltage to the line L1~L12 via the connector 143 shown in a broken line frame relative to V _R. A constant voltage source _V GB More specifically, as shown in FIG. 3, _{V R} is the LED power circuit unit 140a, the LED drive circuit board 125 of the pair disposed 101g recess of the heat sink 101, FPC144B (Figure 3 And is supplied to each of the lines L1 to L12 of the LED mounting substrate 123 via the connector 143A, the FPC 144A, and the connector 143B via a power supply wiring (not shown) in the LED driving circuit board 125.

(LED line configuration)
In the present embodiment, the LEDs 124 included in the two blocks BL1 and BL2 are connected in series to the LEDs 124R, 124G, and 124B for each primary color, and the serially connected lines L (L1 to L12 in FIG. 4). Includes a switch SW (indicated as SW1 to SW12 in FIG. 4) connected in series. The primary color lines L are connected in parallel.

In the example shown in FIG. 4, the LEDs 124R, 124G, and 124B in one block BL1 and BL2 are further divided into two lines L, respectively. Taking the block BL1 as an example, the LED 124R is divided into a line L1 and a line L4.
The block control unit 20A includes an on / off controller (on / off control means) 21, a shift register 22, a bypass switch 23, a current measuring resistor 25, an analog / digital converter (hereinafter referred to as an A / D converter) 26, and a connector 143B. (Refer to FIG. 3, simply indicated as a connector 143 in FIG. 4), the switch SW (SW1 to SW6) is included. Similarly, the block controller 20B includes an on / off controller 21, a shift register 22, a bypass switch 23, a temperature sensor 24, a current measurement resistor 25, an A / D converter 26, and a connector 143B (see FIG. 3 and FIG. 4). The switch SW (SW7 to SW12) is included.
Here, the current measuring resistor 25 and the A / D converter 26 constitute the current measuring means described in the claims.
Below, the internal electrical connection relationship and each function will be described with the block control unit 20A as a representative.

(On-off controller)
The on / off controller 21 is a microcomputer including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and other peripheral circuits, and executes a program stored in the ROM. Thus, on / off control of the switch SW (SW1 to SW12) and the bypass switch 23 is performed.
The on / off controller 21 has a condition table (condition information) 21a in the form of data stored in the ROM. The condition table 21a is for determining the light emission duty (on / off period) of the LED of each line L described later from the temperature signal from the temperature sensor 24 and the current value of each line measured by the A / D converter 26. . Details will be described later.

The on / off controller 21 receives the temperature signal from the temperature sensor 24 and the current value from the A / D converter 26. Then, the on / off controller 21 sends a gate signal (Hi voltage or Low voltage) to the switch SW (SW1 to SW6) via the shift register 22 and a gate signal (Hi voltage or Low voltage) directly to the bypass switch 23. Output.
The on / off controller 21 performs on / off control of the switches SW (SW1 to SW6) and the bypass switch 23 by a gate signal, measures the current value of each line during a current measurement period T _M (see FIG. 7) described later, and the line The light emission efficiency is detected and the light emission duty is calculated with reference to the condition table 21a.
Next, in the LED light emission period T _L (see FIG. 7) described later, the bypass switch 23 is turned on according to the calculated light emission duty, and the switch SW (SW1 to SW6) is controlled to turn on the switch SW (SW1 to SW6). The period is controlled to control the luminance.

The low potential sides of the switches SW (SW1 to SW6) connected in series to the low potential side of each line L are connected in common, and the current measurement resistor 25 is connected between this potential point P1 and the reference potential (GND) of the circuit. And a bypass switch 23 is connected in parallel to the current measuring resistor 25.
As the switches SW (SW1 to SW6) of each line L, semiconductor switches such as MOS transistors and bipolar transistors can be used. Here, MOSFETs are used for the switches SW (SW1 to SW6) of each line.

The current measuring resistor 25 is a highly accurate resistor, and its resistance value is known in advance. The potential at the potential point P <b> 1 is converted into a digital signal by the A / D converter 26 and input to the on / off controller 21.
Control signals to the switches SW (SW1 to SW6) output from the on / off controller 21 are once input to the shift register 22 and output from the shift register 22 to the gates of the individual switches SW (SW1 to SW6).
The bypass switch 23 is provided between the potential point P1 and the reference potential GND in parallel with the current measurement resistor 25, and is configured by, for example, two MOSFETs connected in parallel. The control signal Vb from the on / off controller 21 is also output to the gate of the bypass switch 23.

The bypass switch 23 and the current measuring resistor 25 are arranged in parallel as described above, and the on / off controller 21 controls the bypass switch 23 to be in an off state and only one of the switches SW1 to SW6 to be in an on state. Thus, the currents I _R1 , I _G1 , I _B1 , I _R2 , I _G2 , and I _B2 of each line L (L1 to L6) are converted into one current measuring means (current measuring resistor 25 and A / D converter 26). It becomes possible to measure with.
Incidentally, by measuring the voltage drop at the current measuring resistor 25 when flowing through the current measuring resistor 25 a current I _d by the A / D converter 26, previously known resistance of the current measuring resistor 25 by Ohm's law It can be calculated current value I _d from the value.

  In FIG. 4, the LED 124 and the temperature sensor 24 at the most downstream of each line L1 to L12 are simplified so as to be connected to the switches SW1 to SW12 or the on / off controller 21, respectively, via the connector 143 indicated by the broken line. Specifically, as shown in FIG. 3, the switches SW1 to SW12 in the LED drive circuit board 125 or the on / off controller 21 are connected via the connector 143B, FPC 144A, and connector 143A.

(Principle of control of light emission amount of LED)
The principle of controlling the light emission amount of the LED will be described below.
FIG. 5 is a graph showing an example of the dependence of the current characteristic with respect to the LED forward voltage on the LED junction temperature Tj. FIG. 6 is a graph showing the dependency of the LED light emission efficiency on the LED junction temperature Tj when a constant current flows.
In FIG. 5, it can be seen that when a certain voltage is applied, the current increases as the LED junction temperature Tj increases. That is, the LED junction temperature Tj can be known from the graph of FIG. 5 by applying a certain voltage to the LED 124 and measuring the flowing current. If the LED junction temperature Tj is known, the light emission efficiency can be known from FIG. 6, so that it is possible to provide input power corresponding to the light emission efficiency at the time of operation of the LED 124. The input power can be made variable by adjusting the application time under application of a constant voltage.

  This embodiment is made by paying attention to the fact that the LED junction temperature Tj can be indirectly measured by measuring the electrical characteristics of the LED 124 as described above. If this method is used, it becomes possible to estimate the luminous efficiency of the LED regardless of the spatial position difference between the LED and the optical sensor as in the prior art described in Patent Document 2.

(Explanation of LED drive circuit operation)
The operation of the LED drive circuit of FIG. 4 will be described below with reference to FIG. FIG. 7 is a diagram showing an on / off control sequence of each line L of the LED drive circuit shown in FIG. 4 in the present embodiment.
In the present embodiment, it is divided into a current measurement period _TM and an LED light emission period _TL . Current measurement period T _M is the time period for measuring the current value of each line L, LED emission period T _L is the time to operate as a backlight to emit light the LED package 124. Off controller 21 repeats the current measurement period of a predetermined time T _M, the control at a constant period a combination of a LED emission period of the predetermined time T _L. For example, it repeats at 60 Hz or more (about 16.6 milliseconds period or less). This is to prevent the human eyes from seeing that the LEDs 124 of each line L are intermittently lit. If this frequency is lower than 60 Hz, blinking of the LED 124 will be detected as flickering in human eyes.

The current measurement period _TM is 100 microseconds. In the current measurement period T _M, but to measure the current value by applying a current in a time division as described later to each line L, thus found that this period glowing long as the human eye by one line . Therefore, in order to prevent the human eye from seeing this, a current is passed for a very short time to measure the current value.

The time required to measure the current value of each line L may be the time constant of the circuit, that is, the time until the current settles and the time for the A / D converter 26 to convert the analog signal into a digital signal. Several tens of microseconds is sufficient. In this embodiment, the current of each line L is measured, for example, in 20 microseconds. Thus the current measurement period T _M of measuring the three lines of the current is 60 microseconds.

Here, SW1 to SW6 in FIG. 7 indicate ON / OFF states of the switches SW (SW1 to SW6) of the respective lines L (L1 to L6), and when the voltage becomes Hi voltage, the switches SW (SW1 ... To SW6) are turned on and the switch SW (SW1 to SW6) is turned off when the voltage becomes Low. Also, I _d is the current value measured by the A / D converter 26, I _t represents the current flowing when the bypass switch 23 is on.

The first switch SW (SW4 to SW6) of the period of the current measurement period _{T M} on the switch SW of the lines L1~L3 the (SW1 to SW3) and only one turned on the other two lines and line L4~L6 OFF state Then, the current values I _R1 , I _G1 , and I _B1 of the lines L1 to L3 are measured. Then, the light emission duty (on / off period ratio) of the lines _L1 to _L3 in the LED light emission period _TL is determined based on the measured current values I _R1 , I _G1 , and I _B1 . Further, the switch SW of the lines L4 to L6 as only one turn-on to the current measurement period _{T M} of the next period, to determine the current value of each line L4 to L6, to determine the light emission duty of the line L4 to L6. In this case, the light emission duty of half of the lines L1 to L6 (L1 to L3 or L4 to L6) of the lines L does not change between two consecutive cycles. As described above, the number of lines to be measured in the current measurement period T _M may be freely selected. For example, only one current value in the lines L (L1 to L6) in one cycle of the current measurement period T _M may be selected. Measurement may be performed.

I _t in FIG. 7 shows a current flowing through the bypass switch. A large current flows through the bypass switch 23 during the LED light emission period _TL . Therefore, the voltage drop generated by a current flows through the bypass switch 23, the DC voltage V _GB supplied to the line L (L1 to _L6), it is necessary to lower the negligible compared to V _R. For this reason, the on-resistance of the bypass switch 23 is required to be very low. In the present embodiment, a MOSFET is used as the bypass switch 23, but two MOSFETs are connected in parallel to reduce the on-resistance. By so doing, heat generation by the measuring resistor 25 during the LED emission period is further suppressed, and the voltage drop of the bypass switch 23 is reduced.

The on / off controller 21 has a condition table 21a, and the current values I _R1 , I _G1 , I _B1 , I _R2 , I _G2 , and I _B2 of each line L are used to determine the LED 124 included in each line L (L1 to L6). The brightness is compensated for by converting the efficiency and controlling the light emission duty of each line L (L1 to L6) in the LED light emission period _TL . That is, the time average power is adjusted so that the product of efficiency and supplied power is always constant, and the same brightness is always obtained.

Here, the light emission duty is the ratio of the ON time in the LED light emission period _TL shown in FIG. 7, and is defined as shown in FIG. Thus, the current measurement period T _M in one cycle emission duty is even 100%, no light emission as a function of the original backlight. By shortening the current measurement period T _M, the user of the liquid crystal display apparatus 1A need not feel blinking by eye image phenomenon.
The on / off controller 21 turns on the bypass switch 23 by setting the control signal Vb to the Hi voltage during the LED light emission period _TL , thereby bypassing the current. Thereby, the heat_generation | fever by the current measurement resistor 25 during LED light emission period _TL can be prevented.

  In the present embodiment, the on / off controller 21 serially transmits the on / off information of each line L1 to L6 to the shift register 22, and the shift register 22 converts the on / off information into parallel to convert the switch SW () of each line L (L1 to L6). SW1 to SW6) are controlled on and off. By using the shift register 22, the number of pins of the on / off controller 21 can be reduced. This is effective when the number of lines is very large. When the number of lines controlled by one on / off controller is small, the shift register 22 may be omitted and the gate voltage of the switch SW (SW1 to SW6) may be directly controlled by the on / off controller 21.

(Condition table)
Here, the setting method of the condition table 21a is demonstrated using FIG.9, FIG10 and FIG.11.
FIG. 9 is a graph showing changes in current and LED junction temperature Tj when a constant voltage is applied. FIG. 10 is a graph showing the amount of light emitted from the LED 124 at each current value when a constant voltage is applied.
As shown in FIG. 9, the junction temperature Tj of the LED 124 gradually increases after the on-power of the liquid crystal display device 1A, is saturated at a certain temperature, and the current value is also saturated. As can be seen from FIGS. 9 and 10, when the LED junction temperature Tj reaches the saturation temperature, the current value is maximum and the amount of emitted light is minimum. In the example of FIG. 10, as the LED junction temperature Tj rises, emission light intensity is reduced to 70% of the time of 100% at a current value I _A at the normalized relative values of the current values I _B, also the current value It has been an increase in _{I B} from _{I a} in the meantime.

The condition table 21a is created based on the characteristics shown in FIG. That is, the luminance (brightness) of the LED 124 is proportional to the product of the emitted light amount and the light emission duty, and therefore the light emission duty is maximized at the measurement current value at which the emitted light amount is minimum, and the measurement current value at which the emitted light amount is maximum. Sets the light emission duty at each measured current value so that the light emission duty is minimized. FIG. 11 is a graph illustrating a condition table for determining the light emission duty with respect to the measured current value, where the vertical axis represents the light emission duty and the horizontal axis represents the measured current value. In the example of FIG. 11, the light emission duty when the measured current I _A to 70%, by the light emission duty when the measured current value I _B to 100%, the amount of emitted light 70% as shown by a broken line in FIG. 10 Corresponding luminance is always obtained, and fluctuations in brightness can be prevented.

On the other hand, although the efficiency decreases as the LED junction temperature Tj rises, there is an LED in which the amount of emitted light increases because the current increases. When such an LED is used, control is performed such that the light emission duty is reduced when the current is large and the light emission duty is increased when the current is small.

The LED junction temperature Tj gradually increases immediately after the backlight is turned on, and the temperature is saturated when the amount of heat generated by the LED 124 itself and the heat dissipation of the LED mounting substrate 123 on which the LED 124 is mounted are balanced, It is also affected by the environmental temperature where the liquid crystal display device 1A is placed.
In the present embodiment, the light emission duty at each temperature is set based on the efficiency of the LED 124 at the saturation point of the LED junction temperature Tj. Therefore, since the saturation temperature can be predicted by measuring the environmental temperature with the temperature sensor 24 in the on / off controller 21, it is possible to set an optimum light emission duty according to various environmental temperatures.

  The location of the temperature sensor 24 is preferably the center in the vertical direction on the light guide plate 121 side of each of the blocks BL1 and BL2 of the LED mounting substrate 123 from the viewpoint of reducing the estimation error. And the saturation temperature of LED junction temperature Tj of LED124 contained in each block BL1 and BL2 can be estimated by investigating the correlation of the saturation temperature of LED junction temperature Tj and the detection temperature of the temperature sensor 24 beforehand. .

(Function, effect)
In this LED drive circuit board 125, a current measuring resistor 25 and a bypass switch 23 are provided in a portion P1 where currents of all lines L included in the respective blocks BL1 and BL2 are concentrated. With such a configuration, the current values of all the lines L (L1 to L6) can be individually measured by one current measurement means, that is, the current measurement resistor 25 and the A / D converter 26. It becomes possible.
FIG. 12 shows an LED drive circuit for one line in which a plurality of LEDs 124 in a comparative example are connected in series. A low voltage source of constant DC voltage is supplied to the LED 124 in series via a choke coil and a Zener diode, and a current measuring resistor 25 is connected downstream of the LED 124 and connected to the GRN. A capacitor is connected in parallel with the series LED 124. A MOSFET switch SW is connected between the connection point of the choke coil and the Zener diode and GND. The gate of the switch SW is controlled by the switching controller 21A. The potential P2 on the upstream side of the current measurement resistor 25 is input to the switching controller 21A. The switching controller 21A is a microcomputer, calculates the current value of the series LED 124 based on the potential P2, calculates the light emission duty from the current value, and controls the on / off of the gate of the switch SW, thereby controlling the brightness of the series LED 124. To control. Therefore, in the comparative example, a switching controller is required for each line, and the LED driving circuit for the entire liquid crystal display device requires an area.
According to the present embodiment, unlike the comparative example shown in FIG. 12, a complicated circuit for monitoring the current flowing in each line and controlling the light emission duty is not required.
Further, only the LED power supply circuit portion 140a which is a constant voltage source is a common portion, and is thus arranged on the circuit board 140 on the back side. As a result, the LED drive circuit board 125 can be made compact and can be disposed on the side surface of the liquid crystal display device 1A.

  Since the circuit board 140 on the back surface of the third frame 139 does not include the LED drive circuit board 125, the back surface side can be thinned and a space for heat dissipation can be secured. And the thickness of the entire liquid crystal display device 1A can be reduced.

Since the distance between the LED mounting substrate 123 and the LED drive circuit substrate 125 is short, the FPC 144A that is a source of high-frequency noise can be shortened, and noise can be suppressed.
Further, the switching of the switch SW is short as described above and is as fast as 20 microseconds, and harmonic noise is likely to occur. Since the LED drive circuit board 125 is housed in the recess 101f of the heat sink 101 and covered with the first frame 137, the noise is shielded from the circuit board 140 and the image of the liquid crystal drive circuit that controls the display of the liquid crystal panel 120. The influence of noise on the signal is suppressed.

(Modification of the embodiment)
In this embodiment, on-off controller 21, the current measurement period T _M, for example, to the other switches SW in the OFF state and only the switch SW of a line L in the ON state of the line L1~ line L6 Although the current value of only one line L is measured, the present invention is not limited to this. In the current measurement period _{T M,} as shown in FIG. 13, the bypass switch 23 is turned off, first, all of the switches SW (SW1 to SW6) is turned on and measures the current value _{I all,} switch SW (SW1 to the next To SW6), only one switch SW is turned off, and the other SW is turned on to measure the current. For example, when only the switch SW1 is off and the switches SW2 to SW6 are on, a current value (I _G1 + I _B1 + I _R2 + I _G2 + I _B2 ) is obtained. The on / off controller 21 can calculate the current value I _R1 by taking the difference between the current value I _all and the current value (I _G1 + I _B1 + I _R2 + I _G2 + I _B2 ). Thus, one only lines L are off of the current value I _all the line L contained in each block BL1, BL2 in the case where all the line L contained in each block BL1, BL2 are energized The current value of each line L can be obtained by measuring the current value and taking the difference.

The method for calculating the light emission duty of each line L after the current value of each line L is obtained, and the control of the light emission duty are the same as in the embodiment. By such control, the current measurement period T _M number of line L does not emit light can be made to reduce the, when the number of switches SW to turn on and off controlled by a single on-off controller 21 is large, one cycle of The change in the brightness of the backlight can be reduced.
In FIG. 13, in one current measurement period T _M, as is shown in the example of the case of measuring the lines L1~ line L6 respective current values, as shown in FIG. 7 of the embodiment, the line from measuring the current value of L1 to L6, it may be performed in two of the current measurement period T _M.

Further, since the luminance control is performed on the left and right LED drive circuit boards 125 in such a manner that the current measurement period _TM and the LED light emission period _TL are combined in this way, the cycle in the left and right LED drive circuit boards 125 is controlled. May be controlled not to synchronize. By doing so, the change in the luminance of the liquid crystal display device 1A is reduced.

  In the present embodiment, the LED 124 disposed on the side surface of the liquid crystal display device 1A has been described as an example divided into two blocks. However, the present invention is not limited to two blocks, and may be more finely divided. One block may be sufficient.

In the present embodiment, the heat sink 101 is shown as having the fins 101c, but is not limited thereto.
As shown in FIG. 14, in the small liquid crystal display device 1B, the third frame 139A may be fixed to the heat sink 101A and the third frame 139A itself may dissipate heat. In the example of FIG. 14, the circuit board 140 is not drawn on the assumption that the display control circuit of the liquid crystal panel 120 is disposed below the display area.

It is a perspective view which shows the structure of the liquid crystal display device which concerns on embodiment of this invention. It is XX transverse cross section of FIG. It is a figure explaining the connection arrangement | positioning of the LED mounting board | substrate and LED drive circuit board | substrate of a liquid crystal display device, and is the arrangement | positioning figure seen from the front side. FIG. 4 is a circuit diagram of a constant pressure power source supplied to the LED drive circuit and the LED drive circuit. It is a graph which shows an example of the LED junction temperature Tj dependence of the characteristic of the electric current with respect to the forward direction constant voltage of LED. It is a graph which shows the LED junction temperature Tj dependence of the luminous efficiency of LED when a fixed electric current flows. FIG. 7 is a diagram showing an on / off control sequence of each line L of the LED drive circuit shown in FIG. 4 in the present embodiment. It is a figure explaining a light emission duty. It is a graph which shows the change of an electric current and LED junction temperature Tj when a fixed voltage is applied. It is a graph which shows the emitted light quantity of LED124 in each electric current value when a fixed voltage is applied. It is a graph explaining the condition table which determines light emission duty. It is a LED drive circuit diagram with respect to the multiple one line in the comparative example in a comparative example. It is a figure which shows the sequence of on-off control of each line L of the LED drive circuit in the modification of this embodiment. It is a figure equivalent to the XX cross section of FIG. 1 in the liquid crystal display device of the modification of this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal display device 20A, 20B Block control part 21 On-off controller (on-off control means)
21a Condition table (condition information)
22 Shift register 23 Bypass switch 24 Temperature sensor 25 Current measurement resistor (current measurement means)
26 Analog-digital converter (current measuring means)
101 heat sink 101a housing part 101b holding base part 101c fin 101d groove 101e groove bottom face 101f communication groove 101g recess 107 vent hole 120 liquid crystal panel 121 light guide plate 121a incident surface 121b emitting surface 122 back cover 123 LED mounting substrate 124 LED
125 LED drive circuit board 131 First rubber cushion 132 Second rubber cushion 134, 134A, 134B, 134C Optical sheet 135 First reflective sheet 135a Open edge 135b Rising part 135d Stretching and absorbing part 136 Second reflective sheet 136a Side end portion 137 First frame 138 Second frame 139 Third frame 140 Circuit board 140a LED power supply circuit portion 142 Lens 143, 143A, 143B Connector 144A, 144B Flexible printed wiring board 171 Thermal conductive adhesive member 173 Adhesive member 174 Thermal insulation member 201 Fixing screw

Claims (10)

A liquid crystal panel, a light guide plate disposed on the back surface of the liquid crystal panel, a pair of LED mounting substrates disposed opposite to both side surfaces of the light guide plate, and a plurality of LEDs mounted on the light guide plate side surface of the LED mounting substrate The LED, a first frame disposed on the front side of the liquid crystal panel, and a back cover disposed on the back side of the light guide plate, and the pair of LED mounting substrates with respect to the display screen of the liquid crystal panel In the liquid crystal display device arranged in the left-right direction,
A heat sink provided on a surface of the LED mounting substrate opposite to the light guide plate;
An LED driving circuit that drives the LEDs arranged in series on the left and right outer sides of the heat sink and between the heat sink and the side surface of the first frame or the back cover. A liquid crystal display device. The LED drive circuit is
A plurality of lines having a plurality of the LEDs connected in series and connected in parallel;
On / off control means for on / off controlling energization of the line;
Current measuring means for measuring a current value flowing through the line,
The line is configured by connecting in series a switch for controlling the current flowing through the LED to be turned on and off by the on / off control means, and a predetermined voltage is applied from a constant voltage source,
The liquid crystal display device according to claim 1, wherein the on / off control unit controls an on / off period of a switch of each line based on a current value of each of the lines measured by the current measuring unit. The on / off control means has condition information for determining the on / off period ratio based on the current value of the line,
The liquid crystal display device according to claim 2, wherein an on / off period is controlled according to the condition information.   4. The liquid crystal display device according to claim 2, wherein the current measuring unit includes a current measuring resistor, and measures a voltage drop when a current flows through the current measuring resistor. The LED drive circuit has a bypass switch controlled by the on / off control means connected in parallel with the current measuring device,
The on / off control means controls the light emission period of the LED that turns on the bypass switch and the current measurement period of the current measurement means that turns off the bypass switch. Item 5. A liquid crystal display device according to item 4.   6. The liquid crystal display device according to claim 5, wherein the switches of the plurality of lines are turned on in a time division manner during the current measurement period.   In the current measurement period, a current value when one switch of the plurality of lines is turned off in a time-sharing manner is measured by the current measurement unit, and all lines when all the switches of the plurality of lines are turned on are measured. 6. The liquid crystal display device according to claim 5, wherein a current value of one line in which the switch is turned off is obtained based on a difference from a current value of a minute. The plurality of lines are:
A red line configured by connecting a red LED and the switch in series;
A green line configured by connecting a green LED and the switch in series;
A blue line configured by connecting a blue LED and the switch in series;
Including two or more primary color lines
The liquid crystal display device according to claim 2, wherein the same constant voltage source is used for the two or more primary color lines. The plurality of lines are:
Including the red line, the green line, and the blue line;
9. The liquid crystal display device according to claim 8, wherein the same constant voltage source is used for the green line and the blue line. All of the plurality of lines are commonly connected at a low potential point,
10. The liquid crystal display device according to claim 2, wherein the current measuring unit is arranged between the commonly connected low potential point and the lowest potential point of the circuit. 11.

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