Patent Publication Number: US-2019172400-A1

Title: Display device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2017-233271 filed on Dec. 5, 2017. The entire contents of the priority application are incorporated herein by reference. 
     TECHNICAL FIELD 
     The technology described herein relates to a display device. 
     BACKGROUND 
     An example of a known display device is described in Japanese Unexamined Patent Application Publication No. 2005-071610. The display device disclosed in the publication includes a lighting device. The lighting device includes light sources and a light guide plate. Light emitted by the light sources to the light guide plate travels through the light guide plate and exits the light guide plate through the light output surface toward a liquid crystal panel. 
     SUMMARY 
     In recent years, a display device has been required to have a larger screen and to provide higher-definition images. A display device having a larger screen requires a lighting device to apply light over a larger area, increasing the power consumption of the lighting device. Furthermore, the wiring lines increase in the density for the higher definition, reducing the light transmittance of the display panel. This requires the lighting device to emit light having higher brightness, increasing the power consumption of the lighting device. In view of the above, a lighting device that consumes less power is required. 
     The technology described herein was made in view of the above circumstances. An object is to provide a display device including a lighting device that consumes less power. 
     To solve the above-described problems, a display device includes light sources spaced apart from each other, a light guide plate, a display panel, a light source controller, and a display controller. The light guide plate has a light input surface that faces the light sources to receive light from the light sources and a light output surface through which the light exits. The light input surface and the light output surface are, respectively, a side surface and a plate surface of the light guide plate. The display panel overlaps the light output surface and is configured to display an image by using light from the light output surface. The light source controller is configured to switch between a first lighting mode and a second lighting mode. The first lighting mode allows the light sources to be turned on. The second lighting mode allows at least one of the light sources to be turned on and another one of the light sources to be turned off. The display controller is configured to provide a black display in an end portion of a display area of the display panel adjacent to the light sources in the second lighting mode. 
     The second lighting mode, which allows the smaller number of light sources to be turned on than the first lighting mode, consumes less power than the first lighting mode. However, when only the at least one of the light sources is turned on, the light output surface of the light guide plate is relatively dark at the portions corresponding to the light sources that are not turned on, resulting in uneven brightness. Since the light radiates from the light source, the uneven brightness is likely to be seen in the display area at a portion close to the light sources. To solve the problem, the display controller provides a black display in the display area of the display panel over at least an end portion adjacent to the light sources when the light sources are in the second lighting mode. This reduces the power consumption and the uneven brightness in the light output surface, which is caused when only the at least one of the light sources is turned on. 
     According to the technology described herein, the power consumption is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating a liquid crystal display device according to a first embodiment. 
         FIG. 2  is a cross-sectional view taken along a long side of the liquid crystal display device. 
         FIG. 3  is a block diagram indicating an electrical configuration of the liquid crystal display device. 
         FIG. 4  is a plan view illustrating the liquid crystal display device in the first lighting mode. 
         FIG. 5  is a plan view indicating a brightness distribution in a display area of the liquid crystal display device in the second lighting mode. 
         FIG. 6  is a plan view illustrating an example of an image displayed in the display area of the liquid crystal display device in the second lighting mode. 
         FIG. 7  is a plan view illustrating another example of an image displayed in the display area of the liquid crystal display device in the second lighting mode. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment is described with reference to  FIGS. 1 to 7 . In this embodiment, a liquid crystal display device  10  is described as an example of the display device. As illustrated in  FIG. 1 , the liquid crystal display device  10  has a vertically long rectangular overall shape. The liquid crystal display device  10  includes a liquid crystal panel  20  (display panel) having a display surface  21  on which an image is displayed and a backlight device  40  (lighting device) that applies light for displaying to the liquid crystal panel  20 . The liquid crystal display device  10  according to the embodiment is used in a mobile information terminal, such as a smartphone. Thus, the liquid crystal panel  20  included in the liquid crystal display device  10  has a screen size categorized as a small size in general (a few inches, for example). 
     The liquid crystal panel  20  includes two substantially transparent substrates  22  and  23  bonded together with a predetermined gap therebetween. Liquid crystals (not illustrated) are sealed between the substrates  22  and  23 . One of the substrates  22  and  23  on the rear side is the array substrate  23 . The array substrate  23  has switching devices connected to source lines and gate lines, which are disposed perpendicular to each other, pixel electrodes connected to the switching devices, and an alignment film (none of them are illustrated), for example, thereon. The liquid crystal panel  20  has a display area A 1  on which an image is displayed and a non-display area A 2  on which an image is not displayed. The non-display area A 2  has a frame-like shape and surrounds the display area A 1 . As illustrated in  FIG. 2 , front and rear polarizing plates  24  are attached to the outer surfaces of the substrates  22  and  23 . The CF substrate  22  has a color filter, a counter electrode, and an alignment film (none of them are illustrated), for example, thereon. One end portion of the array substrate  23  protrudes outwardly from the CF substrate  22  in the long-side direction. An LCD controller  27  that controls the liquid crystal panel  20  is mounted on the protruded portion, for example. 
     Next, the backlight device  40  is described. As illustrated in  FIG. 1 , the backlight device  40  is disposed on the rear side of the liquid crystal panel  20  and includes light emitting diodes (LEDs)  41 , an LED board  42  on which the LEDs  41  are mounted, a light guide plate  43  that guides the light from the LEDs  41 , an optical sheet  51  on the front side of the light guide plate  43 , and a light reflection sheet  46  on the rear side of the light guide plate  43 , and a frame-shaped frame  47  surrounding the LEDs  41 , the light guide plate  43 , and the optical sheet  51 . The backlight device  40  according to the embodiment is an edge-light type backlight device in which the light from the LEDs  41  enters the light guide plate  43  through only one side surface of the light guide plate  43 . The LEDs  41  are disposed at one of the ends in the long-side direction of the backlight device  40 . 
     The LED  41  (light source) has an LED chip sealed with a sealing material. The LED chip emits a single color of blue, for example, and the sealing material contains phosphors (yellow, green, and red phosphors) in a dispersed state. Thus, the LED  41  emits white light as a whole. The configuration of the LED  41  is not limited to this configuration and may be suitably changed. The LED board  42  is formed of a flexible insulating film (sheet). The LEDs  41  are disposed with a predetermined distance therebetween on the LED board  42 . The distance between the LEDs  41  is equal in this embodiment but is not limited to equal. 
     The light guide plate  43  is formed of a substantially transparent synthetic resin (an acrylic resin such as PMMA and polycarbonate, for example) and has a refractive index sufficiently higher than that of air. As illustrated in  FIG. 1 , the light guide plate  43  has a vertically long rectangular shape as the liquid crystal panel  20 . As illustrated in  FIG. 2 , one of four side surfaces of the light guide plate  43  is a light input surface  49  facing light emitting surfaces  48  of the LEDs  41 . The light input surface  49  extends linearly along the array of the LEDs  41  ( FIG. 1 ). As illustrated in  FIG. 2 , one of the major surfaces of the light guide plate  43  that faces the front side (liquid crystal panel  20 ) is a light output surface  50 . The light from the LEDs  41  enters the light guide plate  43  through the light input surface  49 . The light travels through the light guide plate  43  and then exits through the light output surface  50  toward the optical sheet  51 . 
     The optical sheet  51  includes a microlens sheet  52  that provides the light with isotropic light focusing effect, a prism sheet  53  that provides the light with anisotropic light focusing effect, and a reflective polarizing sheet  54  that polarizes and reflects the light. The microlens sheet  52 , the prism sheet  53 , and the reflective polarizing sheet  54  are stacked on top of another in this order from the bottom. The kind or the number of sheets included in the optical sheet  51  may be suitably changed. The light reflection sheet  46  has high light reflectance and reflects the light that has leaked from the light guide plate  43  through the surface opposite the light output surface  50  to the front side. The frame  47  has a white surface and is formed of a synthetic resin (polycarbonate, for example). The frame  47  collectively surrounds the LEDs  41  and the light guide plate  43 . The frame  47  is fixed to the liquid crystal panel  20  by a fixing tape  55  having light blocking effect. The liquid crystal panel  20  overlaps the light output surface  50  of the light guide plate  43  and provides an image by using the light from the light output surface  50 . 
     Next, controllers of the liquid crystal panel  20  and the backlight device  40  are described. As indicated in  FIG. 3 , the liquid crystal display device  10  according to the embodiment includes a display controller  25  that controls the liquid crystal panel  20  to display an image in the display area A 1  and an LED controller  60  (light source controller) that controls on and off states of the LEDs  41 . The display controller  25  includes an image data processor  26  that generates image signals based on the image data and an LCD controller  27  that controls the driver (not illustrated) of the liquid crystal panel  20  based on the generated image signals. With this configuration, the display controller  25  is able to drive the switching device on the array substrate  23  by controlling the driver, and thus the orientation of liquid crystals of the liquid crystal panel  20  is controlled for each pixel. Thus, a predetermined image is displayed in the display area A 1  of the liquid crystal panel  20 . The image data, which is the base of the image signals, is supplied from a device (not illustrated) connected to the display controller  25  (a memory that stores the image data, or a tuner configured to receive the image data, for example) to the display controller  25 . 
     The LED controller  60  includes constant current circuits  61  and  62  that supply a constant current to the LEDs  41 . The middle LED  41  in the array of the LEDs  41  is connected to the constant current circuit  61  at the cathode. In the following description, the LED  41  connected to the constant current circuit  61  is referred to as the LED  41 A (at least one of the light sources) such that the LED  41 A is distinguished from the other LEDs  41 . In this embodiment, the total number of the LEDs  41  is an odd number. However, the total number may be an even number. In such a case, two middle LEDs  41  may be the LEDs  41 A, for example. 
     The LEDs  41  except for the LED  41 A are connected to the constant current circuit  62 . In the following description, the LEDs  41  that are connected to the constant current circuit  62  are referred to as the LEDs  41 B (the other light sources) such that the LEDs  41 B are distinguished from the LED  41 A. The LEDs  41 B (six LEDs  41 B in this embodiment) are connected in series. The LED  41 A and the LEDs  41 B are connected at the anodes to a common point. 
     The LED controller  60  is configured to dim the LEDs  41 . Examples of the dimming technique for the LEDs  41  by the LED controller  60  include PWM dimming and constant-current dimming. PWM dimming involves varying the time ratio (duty cycle) between ON time period and OFF time period of the circuit while applying a constant current to the LEDs  41 . Constant-current dimming involves controlling a value of current flowing through the LEDs  41 . Furthermore, the LED controller  60  is configured to switch between a first lighting mode in which all the LEDs  41  are turned on and a second lighting mode in which the LED  41 A is turned on and the LEDs  41 B are turned off. Specifically described, the LED controller  60  is configured to turn on and off each of the constant current circuits  61  and  62 . The LED controller  60  turns on both the constant current circuits  61  and  62  to allow the LEDs  41  to be in the first lighting mode, and turns on the constant current circuit  61  and turns off the constant current circuit  62  to allow the LEDs  41  to be in the second lighting mode. 
     The image data processor  26  of the display controller  25  and the LED controller  60  are electrically connected to each other. With this configuration, the LED controller  60  is able to switch between the first lighting mode and the second lighting mode based on the image signals generated by the image data processor  26 . In this embodiment, the first lighting mode and the second lighting mode are switched depending on the type of the image to be displayed in the display area A 1 . In the first lighting mode, the LED controller  60  turns on all the LEDs  41 . Thus, as illustrated in  FIG. 4 , in the first lighting mode, the brightness is substantially uniform over the entire light output surface  50  or the display area A 1 . 
     In the second lighting mode, the LED controller  60  turns on only the LED  41 A. The light guide plate  43  is designed to have uniform brightness over the entire light output surface  50  with all the LEDs  41  (seven LEDs  41  in this embodiment) being turned on. Thus, as illustrated in  FIG. 5 , in the second lighting mode, the portions of the light output surface  50  corresponding to the LEDs  41 B have lower brightness, resulting in uneven brightness. The output light from the LED  41 A radiates, and thus the brightness unevenness is less likely to be seen at a position far away from the LED  41 A and is likely to be seen at a position near the LEDs  41 A.  FIG. 4  and  FIG. 5  indicate the brightness distribution (corresponding to the brightness distribution on the light output surface  50 ) on the display area A 1  providing a white display. 
     To solve the problem, in this embodiment, an image is displayed in the entire display area A 1  when the LEDs  41  are in the first lighting mode and an image is displayed in a portion of the display area A 1  when the LEDs  41  are in the second lighting mode.  FIG. 6  illustrates a “time display” image  64  on a standby screen of a smartphone as an example of an image displayed in the second lighting mode. The image  64  is displayed in the middle of the display area A 1  in the Y-axis and X-axis directions, for example. In other words, in the second lighting mode, the display controller  25  allows the area around the image  64  to provide a black display, particularly, the end portion of the display area A 1  adjacent to the LEDs  41  (portion likely to have uneven brightness in the second lighting mode) to provide a black display. The image  64  in the display area A 1  overlaps the optical axis L 1  of the LED  41 A in plan view. The term “black display” used herein refers to a display with the minimum gray level but is not limited to a display with the minimum gray level and may be a display with a low gray level that is recognizable as a black display. The LED  41 A is positioned such that the optical axis L 1  thereof overlaps the center of the display area A 1  in the X-axis direction, for example. 
     Next, the advantages of the embodiment are described. In this embodiment, the second lighting mode allows only the LED  41 A to be turned on, consuming less power than the first lighting mode. However, when only the LED  41 A is turned on, the light output surface  50  of the light guide plate  43  is relatively dark at the portions corresponding to the LEDs  41 B that are not turned on. This leads to uneven brightness. The output light radiates from the LED  41 A, and thus the uneven brightness is likely to be seen in the display area A 1  at the portion close to the LEDs  41 . To solve the problem, when the LEDs  41  are in the second lighting mode, the display controller  25  provides a black display in the display area A 1  of the liquid crystal panel  20  over at least the end portion adjacent to the LEDs  41 . This reduces the power consumption and also reduces the uneven brightness in the light output surface  50 , which is caused when only the LED  41 A is turned on. 
     When the LEDs  41  are in the second lighting mode, the display controller  25  allows the image  64  to be displayed on at least a portion of the display area A 1  overlapping the optical axis L 1  of the LED  41 A in plan view. Since the image  64  is displayed on the portion of the display area A 1  overlapping the optical axis L 1  of the LED  41 A, the image  64  is not displayed on a portion of the display area A 1  outside the light application area of the LED  41 A, reliably reducing the uneven brightness of the image  64 . 
     The LED  41 A, which is turned on in the second lighting mode, is the middle LED  41  in an array of LEDs  41 . Since the light radiates from the LED  41 , light from the middle LED  41 A of the LEDs  41  exits the light guide plate  43  through a large area of the light output surface  50  compared with light from the endmost LED  41 . This allows, in the second lighting mode, an image to be displayed in a larger area of the display area A 1 , because the light exits through a larger area of the light output surface  50 . 
     The number of LEDs  41 B of the LEDs  41  is plural and the LEDs  41 B are connected in series. If the LEDs  41 B are connected in parallel, the current passing through the LEDs  41 B may differ depending on individual variability, leading to uneven brightness. The LEDs  41 B connected in series reduces the possibility that the LEDs  41 B will have uneven brightness. 
     Other Embodiments 
     The technology disclosed herein is not limited to the embodiment described above and with reference to the drawings. The following embodiments are included in the technical scope, for example. 
     (1) The image displayed in the display area A 1  in the second lighting mode is not limited to the image described in the above-described embodiment and may be suitably changed. For examples, as illustrated in  FIG. 7 , in the second lighting mode, an image  164  of a clock indicating time may be displayed. An image displayed in the second lighting mode may be an image that informs the user that a message is received, for example. In the second lighting mode, at least the end portion of the display area A 1  adjacent to the LEDs  41  is required to provide a black display, and the position of the image and the display area may be suitably changed. For example, in the second lighting mode, the image may be concentrated in an area of the display area A 1  away from the LEDs  41 . 
     (2) The number of the LEDs  41  is not limited to that in the embodiment and may be suitably determined. The numbers of the LEDs  41 A and the LEDs  41 B may also be suitably determined. For example, multiple LEDs  41 A may be turned on in the second lighting mode. When the number of LEDs  41 A is smaller than the number of LEDs  41 B, the power consumption in the second lighting mode is reduced. 
     (3) In the above embodiment, the position of the LED  41 A in the X-axis direction corresponds to the center of the display area A 1 , but the position of the LED  41 A is not limited to this position. 
     (4) In the above embodiment, a liquid crystal panel is described as an example of the display panel but is not limited the display panel. The present technology is applicable to other display panels such as Microelectromechanical systems (MEMS) display panel. 
     (5) In the above embodiment, the image displayed in the second lighting mode overlaps the optical axis L 1  of the LED  41 A, but the position of the image is not limited to this position. 
     (6) The position in the X-axis direction of the LED  41  that is turned on in the second lighting mode is not limited to that in the above embodiment. The position may be suitably determined depending on the position of the image to be displayed. For example, when an image is displayed on one end portion of the display area A 1  in the X-axis direction, the LED  41  located adjacent to the one end of the display area A 1  in the X-axis direction may be turned on in the second lighting mode. 
     (7) The electrical configuration of the liquid crystal display device  10  is not limited to that indicated by the block diagram in  FIG. 3  and may be suitably designed. Any electrical configuration that is able to provide a black display in the display area A 1  over an end portion adjacent to the LEDs  41  when the LEDs  41  are in the second lighting mode may be employed. 
     (8) In the above embodiment, the LED controller  60  switches between the first lighting mode and the second lighting mode in accordance with image signals generated by the image data processor  26 , but the configuration for switching is not limited thereto. The LED controller  60  may send a signal to inform that the lighting mode is switched to the second lighting mode to the display controller  25  and the display controller  25  may provide a black display in the display area A 1  over the end portion adjacent to the LEDs  41  based on the signal.