PATENT DOCUMENT

Publication Number: US-11428987-B2
Application Number: US-202117203340-A
Country: US
Kind Code: B2

Title: Electronic device display with a backlight having light-emitting diodes and driver integrated circuits in an active area

Abstract:
A pixel array may be illuminated with backlight illumination from a backlight. The backlight may include a two-dimensional array of light-emitting diodes, with each light-emitting diode being placed in a respective cell. Different light-emitting diodes may have unique brightness magnitudes based on the content of the given display frame. Driver integrated circuits may control one or more associated light-emitting diodes to have a desired brightness level. The driver integrated circuits may be formed in an active area of the backlight. The driver integrated circuits may be arranged in groups that are daisy chained together. A digital signal (that includes information such as addressing information) may be propagated through the group of driver integrated circuits. To manage thermal performance of the backlight, the backlight may include a thermally conductive layer and/or a heat sink structure. To increase the efficiency of the backlight, the backlight may include one or more reflective layers.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a substrate having an upper surface; 
 a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; 
 an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and 
 driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes, wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, and wherein each driver integrated circuit is soldered to the upper surface of the substrate. 
 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the backlight has an active area and wherein each driver integrated circuit is positioned within the active area. 
     
     
       3. The electronic device defined in  claim 1 , wherein the array of light-emitting diodes comprises a two-dimensional array of light-emitting diodes and wherein the driver integrated circuits are interspersed amongst the two-dimensional array of light-emitting diodes. 
     
     
       4. The electronic device defined in  claim 1 , wherein the backlight further comprises:
 an additional reflective layer attached to a lower surface of the substrate. 
 
     
     
       5. The electronic device defined in  claim 4 , wherein the backlight further comprises:
 a thermally conductive layer attached to the additional reflective layer, wherein the additional reflective layer is interposed between the lower surface of the substrate and the thermally conductive layer. 
 
     
     
       6. The electronic device defined in  claim 4 , wherein the additional reflective layer is a reflective and thermally conductive layer having a thermal conductivity that is greater than 100 W/mK and a reflectance that is greater than 80%. 
     
     
       7. The electronic device defined in  claim 1 , wherein the substrate comprises a layer of white diffusive glass. 
     
     
       8. The electronic device defined in  claim 1 , wherein an upper surface of each driver integrated circuit has a reflectance that is greater than 80%. 
     
     
       9. The electronic device defined in  claim 1 , wherein each driver integrated circuit comprises an additional reflective layer that covers a respective top surface of that driver integrated circuit. 
     
     
       10. An electronic device comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a substrate having an upper surface; 
 a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; 
 an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and 
 driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes, wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, wherein the array of light-emitting diodes is a two-dimensional array of light-emitting diodes that is arranged in a two-dimensional array of respective cells, wherein each cell includes multiple light-emitting diodes, and wherein a spacing between adjacent light-emitting diodes within a given cell is smaller than a spacing between adjacent cells. 
 
 
     
     
       11. The electronic device defined in  claim 1 , wherein the array of light-emitting diodes is a two-dimensional array of light-emitting diodes that is arranged in plurality of zig-zag columns and a plurality of zig-zag rows. 
     
     
       12. An electronic device comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a substrate; 
 a two-dimensional array of light-emitting diodes mounted on the substrate; 
 driver integrated circuits mounted on the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the two-dimensional array of light-emitting diodes and has a respective top surface; and 
 a plurality of reflective layers, wherein each reflective layer is formed on the top surface of a respective driver integrated circuit. 
 
 
     
     
       13. The electronic device defined in  claim 12 , wherein the driver integrated circuits are interspersed amongst the two-dimensional array of light-emitting diodes. 
     
     
       14. The electronic device defined in  claim 12 , wherein each driver integrated circuit is soldered to the substrate. 
     
     
       15. The electronic device defined in  claim 12 , wherein each driver integrated circuit is a surface mount technology component. 
     
     
       16. The electronic device defined in  claim 12 , further comprising:
 a reflective layer that is formed separately from the plurality of reflective layers, wherein the reflective layer is formed on an upper surface of the substrate. 
 
     
     
       17. An electronic device comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a substrate having an upper surface; 
 a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; 
 an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and 
 driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes and wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, wherein each driver integrated circuit is a surface mount technology component.

Description:
This application claims the benefit of provisional patent application No. 63/029,048, filed May 22, 2020, provisional patent application No. 63/029,069, filed May 22, 2020, and provisional patent application No. 63/029,082, filed May 22, 2020, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This relates generally to electronic devices with displays, and, more particularly, to displays with backlights. 
     Electronic devices such as computers and cellular telephones have displays. Some displays such organic light-emitting diode displays have arrays of pixels that generate light. In displays of this type, backlighting is not necessary because the pixels themselves produce light. Other displays contain passive pixels that can alter the amount of light that is transmitted through the display to display information for a user. Passive pixels do not produce light themselves, so it is often desirable to provide backlight for a display with passive pixels. Passive pixels may be formed from a layer of liquid crystal material formed between two electrode layers and two polarizer layers. 
     In a typical backlight assembly for a display, an edge-lit light guide plate is used to distribute backlight generated by a light source such as a light-emitting diode light source. A reflector may be formed under the light guide plate to improve backlight efficiency. 
     Conventional backlight assemblies may cause visible artifacts, may not be robust, and may occupy an undesirably large amount of space within an electronic device. 
     It would therefore be desirable to be able to provide displays with improved backlights. 
     SUMMARY 
     A display may have an array of pixels for displaying images for a viewer. The array of pixels may be liquid crystal pixels formed from display layers such as a color filter layer, a liquid crystal layer, a thin-film transistor layer, an upper polarizer layer, and a lower polarizer layer. 
     The pixel array may be illuminated with backlight illumination from a backlight unit. The backlight unit may include an array of light-emitting diodes, with each light-emitting diode being placed in a respective cell. The brightness of each light-emitting diode may be changed in each display frame to optimize the viewing of the display. Different light-emitting diodes may have unique brightness magnitudes based on the content of the given display frame. 
     Driver integrated circuits may be used to control the light-emitting diodes of the backlight. Each driver integrated circuit may control one or more associated light-emitting diodes to have a desired brightness level. The driver integrated circuits may be formed in an active area of the backlight. For example, the light-emitting diodes may be mounted to the upper surface of a glass substrate. The driver integrated circuits may also be mounted to the upper surface of the glass substrate. The driver integrated circuits may be interspersed amongst the light-emitting diodes. 
     The driver integrated circuits may be arranged in groups that are daisy chained together. A digital signal (that includes information such as addressing information) may be propagated through the group of driver integrated circuits. Each driver integrated circuit may have a small number of input-output contacts (pins) for minimal complexity. The driver integrated circuits may have four pins, six pins, or nine pins, as examples. 
     To manage thermal performance of the backlight, the backlight may include a thermally conductive layer that is attached to a lower surface of the glass substrate for the light-emitting diodes. The glass substrate may also have exposed conductive layers that are coupled to heat sinks for additional heat dispersion. Sensors such as temperature sensors and/or optical sensors may be formed on the upper surface of the glass substrate. The sensors may provide real-time measurements to a controller such as a timing controller. The timing controller may, in turn, control operation of the light-emitting diodes in the backlight based at least partially on the sensor information. 
     To increase the efficiency of the backlight, the glass substrate may be formed from white diffusive glass. Additionally, a reflective layer may be formed on the upper surface of the glass substrate. A reflective layer may also be formed on a lower surface of the glass substrate. Reflective layers may be formed on the top surfaces of the driver integrated circuits to prevent a shadow from appearing in the active area of the display where the driver integrated circuits are present. The light-emitting diodes may be arranged in a non-square-grid layout to reduce periodicity and prevent mura. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a top view of an illustrative display in accordance with an embodiment. 
         FIG. 3  is a cross-sectional side view of an illustrative display in an electronic device that has a backlight and a pixel array in accordance with an embodiment. 
         FIG. 4  is a top view of an illustrative backlight having light-emitting diodes arranged in respective cells in accordance with an embodiment. 
         FIG. 5  is a top view of an illustrative display showing how different portions of the display may have different target brightness levels in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of an illustrative backlight having light-emitting diodes (LEDs) on an upper surface of a substrate and LED driver integrated circuits (ICs) on the upper surface of the substrate in an inactive area of the display in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of an illustrative backlight having light-emitting diodes (LEDs) on an upper surface of a substrate and LED driver integrated circuits (ICs) on a lower surface of the substrate in an active area of the display in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of an illustrative backlight having light-emitting diodes (LEDs) on an upper surface of a substrate and LED driver integrated circuits (ICs) on the upper surface of the substrate in an active area of the display in accordance with an embodiment. 
         FIG. 9  is a top view of an illustrative LED array that includes driver ICs distributed throughout the active area of the display in accordance with an embodiment. 
         FIG. 10  is a schematic diagram of an illustrative display with a timing controller that provides signals directly to LED driver ICs in the active area in accordance with an embodiment. 
         FIG. 11  is a schematic diagram of an illustrative display with a timing controller that provides signals to a backlight controller that then provides signals directly to LED driver ICs in the active area in accordance with an embodiment. 
         FIG. 12  is a schematic diagram of an illustrative LED array with driver ICs that have six pins in accordance with an embodiment. 
         FIG. 13  is a schematic diagram of an illustrative driver IC with nine pins for independently controlling different LED zones in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of an illustrative LED array with both LEDs and driver ICs soldered to a glass substrate in accordance with an embodiment. 
         FIG. 15  is a top view of an illustrative backlight showing how conductive layers on a glass substrate may be exposed and connected to heat sinks in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of the illustrative backlight of  FIG. 15  in accordance with an embodiment. 
         FIG. 17  is a top view of an illustrative backlight showing how temperature and optical sensors may be distributed across the active area of the backlight in accordance with an embodiment. 
         FIG. 18  is a cross-sectional side view of an illustrative backlight showing how a reflective layer may be attached to the lower surface of a glass LED substrate in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of an illustrative backlight showing how a reflective layer and a separate thermally conductive layer may be attached to the lower surface of a glass LED substrate in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of an illustrative backlight showing how a reflective and thermally conductive layer may be attached to the lower surface of a glass LED substrate in accordance with an embodiment. 
         FIG. 21  is a cross-sectional side view of an illustrative backlight showing how a substrate may be formed from white diffusive glass in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of an illustrative backlight showing how a reflective layer may be formed on an upper surface of an LED driver IC in accordance with an embodiment. 
         FIG. 23  is a top view of an illustrative LED array showing how the LEDs may be arranged in a zig-zag pattern in accordance with an embodiment. 
         FIG. 24  is a top view of an illustrative LED array showing how the LEDs may be arranged in a zig-zag pattern with increased zone-to-zone spacing in accordance with an embodiment. 
         FIG. 25  is a graph of an illustrative emission profile for LED groups in a backlight in accordance with an embodiment. 
         FIG. 26  is a top view of an illustrative LED array showing how a center LED may be driven at a higher current than surrounding, peripheral LEDs to achieve a desired emission profile in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a display is shown in  FIG. 1 . Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a display, a computer display that contains an embedded computer, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, or other electronic equipment. Electronic device  10  may have the shape of a pair of eyeglasses (e.g., supporting frames), may form a housing having a helmet shape, or may have other configurations to help in mounting and securing the components of one or more displays on the head or near the eye of a user. 
     As shown in  FIG. 1 , electronic device  10  may include control circuitry  16  for supporting the operation of device  10 . Control circuitry  16  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application-specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input resources of input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Input-output devices  12  may include one or more displays such as display  14 . Display  14  may be a touch screen display that includes a touch sensor for gathering touch input from a user or display  14  may be insensitive to touch. A touch sensor for display  14  may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements. A touch sensor for display  14  may be formed from electrodes formed on a common display substrate with the display pixels of display  14  or may be formed from a separate touch sensor panel that overlaps the pixels of display  14 . If desired, display  14  may be insensitive to touch (i.e., the touch sensor may be omitted). Display  14  in electronic device  10  may be a head-up display that can be viewed without requiring users to look away from a typical viewpoint or may be a head-mounted display that is incorporated into a device that is worn on a user&#39;s head. If desired, display  14  may also be a holographic display used to display holograms. 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14 . 
     Input-output devices  12  may also include one or more sensors  13  such as force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors (e.g., a two-dimensional capacitive touch sensor associated with a display and/or a touch sensor that forms a button, trackpad, or other input device not associated with a display), and other sensors. In accordance with some embodiments, sensors  13  may include optical sensors such as optical sensors that emit and detect light (e.g., optical proximity sensors such as transreflective optical proximity structures), ultrasonic sensors, and/or other touch and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, temperature sensors, proximity sensors and other sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices), optical sensors such as self-mixing sensors and light detection and ranging (lidar) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, and/or other sensors. In some arrangements, device  10  may use sensors  13  and/or other input-output devices to gather user input (e.g., buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc.). 
     Display  14  may be a liquid crystal display or may be a display based on other types of display technology (e.g., organic light-emitting diode displays). Device configurations in which display  14  is a liquid crystal display are sometimes described herein as an example. This is, however, merely illustrative. Any suitable type of display may be used, if desired. In general, display  14  may have a rectangular shape (i.e., display  14  may have a rectangular footprint and a rectangular peripheral edge that runs around the rectangular footprint) or may have other suitable shapes. Display  14  may be planar or may have a curved profile. 
       FIG. 2  is a top view of a portion of display  14  showing how display  14  may have an array of pixels  22 . Pixels  22  may have color filter elements of different colors such as red color filter elements R, green color filter elements G, and blue color filter elements B. Pixels  22  may be arranged in rows and columns and may form active area AA of display  14 . Pixels  22  may be formed form liquid crystal display layers, as one example. The rectangular shape of display  14  and active area AA in  FIG. 2  is merely illustrative. If desired, the active area AA may have a non-rectangular shape (e.g., a shape with one or more curved portions). For example, the active area may have rounded corners in one example. 
     A cross-sectional side view of display  14  is shown in  FIG. 3 . As shown in  FIG. 3 , display  14  may include a pixel array such as pixel array  24 . Pixel array  24  may include an array of pixels such as pixels  22  of  FIG. 2  (e.g., an array of pixels having rows and columns of pixels). Pixel array  24  may be formed from a liquid crystal display module (sometimes referred to as a liquid crystal display or liquid crystal layers) or other suitable pixel array structures. 
     During operation of display  14 , images may be displayed on pixel array  24 . Backlight unit  42  (which may sometimes be referred to as a backlight, backlight layers, backlight structures, a backlight module, a backlight system, etc.) may be used in producing backlight illumination  44  that passes through pixel array  24 . This illuminates any images on pixel array  24  for viewing by a viewer such as viewer  20  who is viewing display  14  in direction  21 . 
     Backlight unit  42  may have optical films  26 , a light diffuser such as light diffuser (light diffuser layer)  34 , and light-emitting diode (LED) array  36 . Light-emitting diode array  36  may contain a two-dimensional array of light sources such as light-emitting diodes  38  that produce backlight illumination  44 . Light-emitting diodes  38  may, as an example, be arranged in rows and columns and may lie in the X-Y plane of  FIG. 3 . 
     The light produced by each light-emitting diode  38  may travel upwardly along dimension Z through light diffuser  34  and optical films  26  before passing through pixel array  24 . Light diffuser  34  may contain light-scattering structures that diffuse the light from light-emitting diode array  36  and thereby help provide uniform backlight illumination  44 . Optical films  26  may include films such as dichroic filter  32 , phosphor layer  30 , and films  28 . Films  28  may include brightness enhancement films that help to collimate light  44  and thereby enhance the brightness of display  14  for user  20  and/or other optical films (e.g., compensation films, etc.). 
     Light-emitting diodes  38  may emit light of any suitable color. With one illustrative configuration, light-emitting diodes  38  emit blue light. Dichroic filter layer  32  may be configured to pass blue light from light-emitting diodes  38  while reflecting light at other colors. Blue light from light-emitting diodes  38  may be converted into white light by a photoluminescent material such as phosphor layer  30  (e.g., a layer of white phosphor material or other photoluminescent material that converts blue light into white light). If desired, other photoluminescent materials may be used to convert blue light to light of different colors (e.g., red light, green light, white light, etc.). For example, one layer  30  (which may sometimes be referred to as a photoluminescent layer or color conversion layer) may include quantum dots that convert blue light into red and green light (e.g., to produce white backlight illumination that includes, red, green, and blue components, etc.). Configurations in which light-emitting diodes  38  emit white light (e.g., so that layer  30  may be omitted, if desired) may also be used. 
     In configurations in which layer  30  emits white light such as white light produced by phosphorescent material in layer  30 , white light that is emitted from layer  30  in the downwards (−Z) direction may be reflected back up through pixel array  24  as backlight illumination by dichroic filter layer  32  (i.e., layer  32  may help reflect backlight outwardly away from array  36 ). In configurations in which layer  30  includes, for example, red and green quantum dots, dichroic filter  32  may be configured to reflect red and green light from the red and green quantum dots, respectively to help reflect backlight outwardly away from array  36 . By placing the photoluminescent material of backlight  42  (e.g., the material of layer  30 ) above diffuser layer  34 , light-emitting diodes  38  may be configured to emit more light towards the edges of the light-emitting diode cells (tiles) of array  36  than at the centers of these cells, thereby helping enhance backlight illumination uniformity. 
     In a configuration in which pixel array  24  is formed using a liquid crystal display, pixel array  24  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . Liquid crystal display structures of other types may be used in forming pixel array  24 , if desired. 
     Layers  56  and  58  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). Integrated circuits such as integrated circuit  62 A and/or flexible printed circuits such as flexible printed circuit  64  may be attached to substrate  58  in ledge region  66  (as an example). 
       FIG. 4  is a top view of an illustrative light-emitting diode array for backlight  42 . As shown in  FIG. 4 , light-emitting diode array  36  may contain rows and columns of light-emitting diodes  38 . Each light-emitting diode  38  may be associated with a respective cell (tile area)  38 C. The length D of the edges of cells  38 C may be 2 mm, 18 mm, 1-10 mm, 1-4 mm, 10-30 mm, more than 5 mm, more than 10 mm, more than 15 mm, more than 20 mm, less than 25 mm, less than 20 mm, less than 15 mm, less than 10 mm, or other suitable size. If desired, hexagonally tiled arrays and arrays with light-emitting diodes  38  that are organized in other suitable array patterns may be used. In arrays with rectangular cells, each cell may have sides of equal length (e.g., each cell may have a square outline in which four equal-length cell edges surround a respective light-emitting diode) or each cells may have sides of different lengths (e.g., a non-square rectangular shape). The configuration of  FIG. 4  in which light-emitting diode array  36  has rows and columns of square light-emitting diode regions such as cells  38 C (e.g., a two-dimensional array of cells  38 C) is merely illustrative. 
     In some cases, each cell  38 C may include a single light-emitting diode. Alternatively, each cell  38 C may have a light source that is formed form an array of light-emitting diode dies (e.g., multiple individual light-emitting diodes  38  arranged in an array such as a 2×2 group of light-emitting diodes or 3×3 group of light-emitting diodes in each cell  38 C). The diodes  38  in light source  38 ′ may be mounted on a common substrate, may be mounted on a printed circuit board substrate that extends across array  36 , may be mounted on a glass substrate that extends across array  36 , or may be mounted in array  36  using other desired arrangements. In general, each cell  38 C may include a single light-emitting diode, a pair of light-emitting diodes, 2-20 light-emitting diodes, at least 2 light-emitting diodes, at least 4 light-emitting diodes, at least 8 light-emitting diodes, fewer than 5 light-emitting diodes, between 4 and 12 light-emitting diodes, between 8 and 12 light-emitting diodes, between 8 and 10 light-emitting diodes, 9 light-emitting diodes, or other desired number of light-emitting diodes. 
     Light-emitting diodes  38  may be controlled in unison by control circuitry in device  10  or may be individually controlled. Controlling the light-emitting diodes individually may enable the electronic device to implement a local dimming scheme that helps improve the dynamic range of images displayed on pixel array  24  and that potentially reduces the power consumption of the backlight. The dynamic range of a display may be considered the ratio between the light of the highest intensity (e.g., the brightest light) that the display is capable of emitting and the light of the lowest intensity (e.g., the dimmest light) that the display is capable of emitting. 
     If all of the light-emitting diodes in backlight unit  42  are controlled in unison, the dynamic range of the display may be limited. Consider the example depicted in  FIG. 5 . In  FIG. 5 , objects such as objects  72 - 1  and  72 - 2  are displayed on display  14  (sometimes referred to as screen  14 ). In this example, object  72 - 1  may have a high brightness level. Object  72 - 2  may have an intermediate brightness level. The background of the display may have a low brightness level. If the light-emitting diodes providing backlight for display  14  in  FIG. 5  are controlled in unison, all of the light-emitting diodes may be set to a brightness that is optimized for object  72 - 1 . In this scenario, object  72 - 1  may be displayed with its intended brightness. However, the background of the display is also receiving backlight with a high brightness optimized for object  72 - 1 . Therefore, the background of the display may appear brighter than desired due to display limitations such as light leakage through the pixels or other limitations, and the dynamic range of the display is lower than desired. Alternatively, all of the light-emitting diodes may be set to a brightness that is optimized for the background of the display. In this scenario, the background may be displayed with its intended brightness. However, object  72 - 1  is also receiving backlight with a low brightness optimized for the background. Therefore, object  72 - 1  will appear dimmer than desired and the dynamic range of the display will be lower than desired. In yet another embodiment, the brightness of all of the light-emitting diodes may be set to a brightness that is optimized for object  72 - 2 . In this scenario, object  72 - 1  will appear dimmer than desired and the background will appear brighter than desired. 
     Additionally, controlling all of the light-emitting diodes in backlight unit  42  in unison may introduce power consumption limitations. The maximum allowable power consumption of the backlight unit may prevent all of the light-emitting diodes from being operated at a peak brightness level. For example, all of the light-emitting diodes may not be able to emit light with a desired brightness for bright object  72 - 1  while meeting power consumption requirements. 
     To summarize, operating all of the light-emitting diodes in the backlight in unison such that the backlight brightness is the same across the display forces trade-offs in the aesthetics of the displayed image. Portions of the display may be dimmer than desired or brighter than desired and the dynamic range of the display will be lower than desired. 
     To increase the dynamic range of the display (and to allow for peak brightness levels without exceeding power consumption requirements), the light-emitting diodes in backlight unit  42  may be controlled individually. For example, light emitting diodes in region  14 - 1  of the display may have a high brightness optimized for the high brightness of object  72 - 1 , light-emitting diodes in region  14 - 2  of the display may have a brightness optimized for the intermediate brightness of object  72 - 2 , and light-emitting diodes in region  14 - 3  of the display may have a low brightness optimized for the low brightness of the background of the display. In one example, the light-emitting diodes in region  14 - 1  may operate at a maximum brightness whereas the light-emitting diodes in background region  14 - 3  may be turned off (e.g., operate at a minimum brightness). Varying the brightness of the light-emitting diodes across the display in this manner increases the dynamic range of the display. 
     Having a two-dimensional array of independently controllable light sources such as light-emitting diodes  38  for producing backlight illumination  44  therefore may increase the dynamic range of the display. Backlights with two-dimensional arrays of light-emitting diodes may sometimes be referred to as two-dimensional backlights. These types of backlights may also sometimes be referred to as direct-lit backlights. The direct-lit backlights emit light vertically towards the pixel array, as opposed to backlights with edge-lit light guide plates (where light is emitted parallel to the plane of the pixel array and redirected vertically towards the pixel array by the light guide plate). 
     Driving circuitry may be included in display  14  to controlling the light-emitting diodes in backlight  42 . Driving circuitry for the LEDs may be formed from integrated circuits, thin-film transistor circuits, and/or other suitable circuitry. In one example, driving circuitry may be incorporated as thin-film transistor circuitry on a rigid printed circuit board (e.g., a printed circuit board with a plurality of layers of dielectric material such as polyimide and conductive layers). However, the costs associated with such an arrangement may be high, particularly in backlights with a high number of light-emitting diodes. An alternative arrangement for the LED driving circuitry is for driver integrated circuits (sometimes referred to as driver ICs) to be included in backlight  42 . Each driver IC may control one or more corresponding light-emitting diodes. In this way, the light emitting diodes may be controlled to have varying brightness magnitudes across the backlight. The driver integrated circuits may also be used in combination with a glass substrate in one example. In other words, instead of the light-emitting diodes and driver ICs being mounted on a rigid printed circuit board (e.g., formed using polyimide), the light-emitting diodes and driver ICs may be mounted on a glass substrate. The glass substrate may have conductive traces (e.g., copper traces) to allow signals to be transferred between components as necessary. 
     There are numerous options for how to mount the LEDs and corresponding driver ICs on a substrate. As shown in  FIG. 6 , LEDs  38  are mounted on an upper surface of substrate  84 . In the arrangement of  FIG. 6 , the driver ICs  82  are also mounted on the upper surface of substrate  84  in the inactive area IA. The area of the backlight with LEDs  38  corresponds to the active area AA of the display (e.g., the area of the display that emits light). The backlight  42  may be described as having an active area and inactive area, similar to the pixel array. The active area of the backlight and the active area of the pixel array may have the same footprint. The inactive area of the backlight and the inactive area of the pixel array may have the same footprint or may optionally have different footprints. The active and inactive areas of either the backlight or pixel array may sometimes simply be referred to as simply the active and inactive areas of the display. 
     As shown in  FIG. 6 , driver IC  82  is positioned at the periphery of the substrate in an inactive area (IA) of the display. The driver ICs may all be positioned at the periphery of substrate  84  and may each control a corresponding group of light-emitting diodes. This type of arrangement has numerous limitations, however. First, it is, in general, desirable to minimize the size of the inactive area. The inactive area takes up valuable space within the electronic device without contributing to the aesthetics of the display. Having the driver ICs in the periphery of the display undesirably increases the footprint of the display without increasing the light-emitting area of the display. Additionally, positioning driver ICs only in the inactive area of the display may make it challenging to increase the number of LEDs within the backlight. By only having the driver ICs at the periphery of the backlight, each driver IC may have to control a large number of light-emitting diodes (because the peripheral driver ICs have to control both the central LEDs and peripheral LEDs). 
     An alternate arrangement for the backlight is shown in  FIG. 7 . As shown, in this arrangement the light-emitting diodes  38  are positioned on an upper surface of substrate  84  whereas the driver IC  82  is positioned on a lower surface of substrate  84 . Having the driver IC positioned on the lower surface in this manner eliminates the need for the large inactive area of  FIG. 6 . In other words, the active area may extend virtually to the edge of substrate  84 . However, positioning the driver ICs on the lower surface of the substrate increases the total thickness  86  of the backlight unit. Additionally, more complex conductive routing (e.g., with conductive vias) may be required for driver IC  82  to properly control the light-emitting diodes on the opposing side of substrate  84 . 
       FIG. 8  is a cross-sectional side view of another possible arrangement where the driver ICs are positioned on the upper surface of substrate  84  within the active area. As shown, the driver ICs may be positioned between LEDs of the backlight. With this arrangement, the size of the inactive area may be minimized (because the driver ICs do not increase the size of the inactive area). Because the driver ICs are on the upper surface of substrate  84 , the thickness and complexity of the backlight may be mitigated. Additionally, because the driver ICs are positioned within the active area, each LED may have a corresponding driver IC. The driver ICs may therefore have a low complexity and size (because each driver IC only needs to control a small number of LEDs). Using low complexity driver ICs reduces the number of interconnects required and allows for the size of the backlight to be scaled to larger sizes (i.e., larger numbers of LEDs within the backlight). 
     Digital signals may be used to control the driver ICs  82  (sometimes referred to as LED driver ICs  82  or backlight driver ICs  82 ). Using digital control lines for the backlight may enable larger backlights on a single substrate, may reduce the total pin count for each driver IC, may reduce the number of interconnects within the backlight, and may increase the magnitude of drive currents enabled by the driver ICs. 
     As previously discussed, substrate  84  may optionally be a rigid printed circuit board (e.g., with a plurality of insulating layers formed from a dielectric material such as polyimide). Alternatively, to reduce the manufacturing cost and complexity of the LED array, substrate  84  (sometimes referred to as LED substrate  84 ) may formed from glass. Conductive traces (e.g., copper traces) may be deposited on the glass substrate to allow electrical connections between components mounted to the glass substrate. 
       FIG. 9  is a top view of an illustrative light-emitting diode array having driver ICs distributed throughout the active area of the display. As shown in  FIG. 9 , each driver IC  82  may control a corresponding LED group  102 . Each LED group  102  (sometimes referred to as LED zone  102 ) may include one or more light-emitting diodes. The light-emitting diodes may be connected in series between a power supply line  106  and the driver IC. In the example of  FIG. 9 , each LED zone  102  includes nine LEDs that are connected in series between power supply line  106  and the driver IC. 
     Herein, the term LED group (or LED zone) may be used to refer to an independently controllable group of LEDs. For example, first and second light-emitting diodes that are controlled separately would be referred to as first and second unique LED groups (even though there is only one LED per group). In contrast, nine light-emitting diodes that are controlled together is referred to as a single LED group. Each LED group has an associated LED cell, which may refer to the light-emitting area associated with that LED group. Because the LEDs emit light across a broad range of angles (as opposed to highly collimated light), the footprint of a light-emitting area associated with a given LED group will be larger than the footprint of the LED group itself. Because each LED group is controlled to have one brightness value, the LED cell associated with each group may have an associated single brightness value. In other words, the brightness may be, ideally, uniform across the LED cell. In practice, there may be some non-uniformities across the LED cell (e.g., caused by hotspots over the LEDs). Films  26  discussed in connection with  FIG. 3  may be designed to increase uniformity of light within each LED cell. 
     Power supply line  106  may provide a power supply voltage VLED (e.g., a positive power supply voltage) across the LED array. Each LED group may have a light-emitting diode with a first terminal (e.g., the anode) coupled to the power supply line. The second terminal (e.g., the cathode) of that LED is then connected to the first terminal of the subsequent LED. This chain may continue, with each LED having a first terminal coupled to the second terminal of the preceding LED and a second terminal coupled to the first terminal of the subsequent LED. In the example of  FIG. 9 , each LED has an anode coupled to the cathode of the preceding LED and a cathode coupled to the anode of the subsequent LED. The first LED in the group has an anode coupled to the supply line  106 . The last LED in the group has a cathode coupled to driver IC  82 . 
     This arrangement may be reversed if desired, with the first LED in the group having a cathode coupled to the supply line (e.g., a ground power supply line), the last LED in the group having an anode coupled to the driver IC, and the other LEDs having a cathode coupled to the anode of the preceding LED and an anode coupled to the cathode of the subsequent LED. 
     As shown in  FIG. 9 , the driver ICs may optionally be arranged in an array of rows and columns. Each row and column of driver ICs may include any desired number of driver ICs. Each driver IC  82  has input-output contacts referred to as pins. The pins are used by the driver IC to transmit and receive signals. 
     In  FIG. 9 , each driver IC has four pins (P 1 , P 2 , P 3 , and P 4 ). Various subsets of the driver ICs may be chained together in series (e.g., daisy-chained). In  FIG. 9 , each column of driver ICs are chained together. However, it should be noted that smaller larger groups of driver ICs may be chained together if desired. 
     Pin P 4  may sometimes be referred to as an input pin and pin P 1  may sometimes be referred to as an output pin. Pin P 4  for one of the driver ICs in a given column (e.g., driver IC  82 - 1  in  FIG. 9 ) may receive an input from control line  108 - 1 . The input from pin P 4  on driver IC  82 - 1  may subsequently be output at pin P 1  on driver IC  82 - 1 . The output from driver IC  82 - 1  is then received at input pin P 4  of driver IC  82 - 2  (e.g., the next driver IC in the column). 
     In other words, the output of each driver IC is provided as the input to the next driver IC in the chain (e.g., in the column in  FIG. 9 ). This means that output pin P 1  of each driver IC is electrically connected to input pin P 4  of an adjacent driver IC. Information provided via signal line  108 - 1  may therefore be propagated through the driver ICs in a given column. In one example, signal line  108 - 1  is a digital signal line that is configured to provide initialization information (e.g., address information) to the driver ICs. The initialization information is provided to driver IC  82 - 1  by signal line  108 - 1 . Driver IC  82 - 1  then passes the initialization information to the next driver IC ( 82 - 2 ), which passes the initialization information to the next driver IC, etc. 
     Each column of driver ICs may have a corresponding digital signal line for providing initialization information to input pin P 4  of at least one driver IC. As shown in  FIG. 9 , a second column of driver ICs may have a corresponding digital signal line  108 - 2  that provides information to pin P 4  of driver IC  82 - 3 . Driver IC  82 - 3  then passes the initialization information to the next driver IC ( 82 - 4 ), which passes the initialization information to the next driver IC, etc. 
     Each driver IC also includes a pin P 3  that is coupled to a respective signal line. For example, pin P 3  of driver ICs  82 - 1  and  82 - 2  are coupled to signal line  104 - 1 . Pin P 3  of driver ICs  82 - 3  and  82 - 4  are coupled to signal line  104 - 2 . In other words, each column of driver ICs may have a corresponding signal line that is used to provide information to pin P 3  of the driver ICs. In this example, signal line  104 - 1  may be used to provide LED brightness values to the driver ICs. For example, signal line  104 - 1  indicates to driver IC  82 - 1  to update the brightness of its corresponding LED group  102  to a first given magnitude, indicates to driver IC  82 - 2  to update the brightness of its corresponding LED group  102  to a second given magnitude, etc. Signal line  104 - 2  indicates to driver IC  82 - 3  to update the brightness of its corresponding LED group  102  to a third given magnitude, indicates to driver IC  82 - 4  to update the brightness of its corresponding LED group  102  to a fourth given magnitude, etc. 
     Signal lines  104  and  108  may be digital signal lines that are used to convey digital signals. The signal lines may be used to convey data, instructions, or any other desired information. The signal lines may therefore sometimes be referred to as control lines, data lines, etc. Multiple signal lines may be part of a single bus  110 .  FIG. 9  shows an example where signal lines  104 - 1 ,  104 - 2 ,  108 - 1 , and  108 - 2  are part of bus  110 . 
     The LED array may include a plurality of busses, each of which provides signals to a corresponding subset of driver IC columns. In other words, the LED array may have a given number of busses (x). Each of those busses may provide one or more signals to a given number of driver IC columns (y). Each driver IC column may have a given number of driver ICs (z). Any desired values may be used for x, y, and z. In one illustrative example, there may be 24 busses, 2 driver IC columns per bus, and 27 driver ICs per column. This example is merely illustrative. In general, the LED array may include any desired number of busses (e.g., 1, 2, more than 2, more than 5, more than 10, more than 20, more than 30, more than 50, more than 100, more than 500, less than 100, less than 40, less than 30, less than 20, between 20 and 30, between 20 and 25, between 15 and 50, etc.). The LED array may include any desired number of driver IC columns per bus (e.g., 1, 2, 3, 4, more than 4, more than 8, less than 10, less than 5, between 1 and 4, etc.). The LED array may include any desired number of driver ICs per driver IC column (e.g., more than 5, more than 10, more than 20, more than 30, more than 50, more than 100, more than 500, less than 100, less than 40, less than 30, less than 20, between 20 and 30, between 25 and 30, between 20 and 50, etc.). Busses may also provide signals for partial columns of driver ICs in an arrangement where only part of one or more columns are chained together. 
     Pin P 2  in each driver IC may be coupled to ground (e.g., a ground power supply voltage). Therefore, each driver IC in  FIG. 9  sinks current through its LED group  102  to ground. In the example of  FIG. 9 , each driver IC is coupled between the cathode of the last LED in the chain and ground. This example is merely illustrative. In an alternative arrangement, each driver IC could be coupled between the anode of the first LED in the chain and the positive power supply line  106 . 
     The signal lines in  FIG. 9  (e.g.,  104 - 1 ,  104 - 2 ,  108 - 1 ,  108 - 2 , and  106 ) may be coupled to a connection area  104  of the LED array. Connection area  105  may be, for example, a connector that is coupled to a controller that is off of the LED substrate. This example is merely illustrative. In general, any desired connection scheme may be used to provide desired signals on the signal lines. 
     Each driver IC may have a length  212  and a width  214 . Reducing the complexity of the driver IC (e.g., by only having four pins, having each driver IC only control 1 LED group, etc.) may allow for the length and width of the driver IC to be reduced. The length and width of the driver IC may be any desired respective distances (e.g., less than 0.5 millimeters, less than 1.0 millimeters, less than 0.4 millimeters, less than 0.3 millimeters, less than 0.2 millimeters, greater than 0.1 millimeter, greater than 0.2 millimeters, greater than 0.3 millimeters, between 0.2 and 0.5 millimeters, between 0.30 and 0.35 millimeters, etc.). In one illustrative example, both the length  212  and width  214  may be less than 0.5 millimeters. Both the length  212  and width  214  may be between 0.30 and 0.35 millimeters. 
     During operation of LED array  36 , the driver ICs may be operable in an addressing phase (sometimes referred to as an initialization phase). During the addressing phase, signal lines  108  assign addresses (e.g., from an external controller) to the driver ICs. The addresses may be propagated through driver ICs within a given column. In other words, during the addressing phase each driver IC (except for the last driver IC in the chain) provides an output on pin P 1  that is received by an adjacent driver ICs pin P 4  (e.g., through a digital signal line that is coupled between the pins). In some embodiments, the same packet of information may be passed through the driver ICs. In other embodiments, the packet may be modified by a given driver IC before being passed to the next driver IC in the chain. 
     During the initialization phase, brightness values may be provided to the driver ICs using signal lines  104 . The brightness values may include a plurality of brightness values, with each brightness value corresponding to a respective LED zone  102 . The driver IC may receive a packet with brightness values, parse the packet to determine its corresponding brightness value, and update its target LED brightness to be equal to the newly received brightness value. The driver IC may select the appropriate brightness value out of multiple brightness values within the packet based on the assigned address received via signal path  108 . A single packet with the brightness values may be provided to the entire LED pixel array, different packets may be provided on each bus, or different packets may be provided to each display IC column. Having more unique packets may reduce the amount of data that needs to be included in each packet. 
     After the initialization is complete, the driver IC may switch from the initialization mode to a normal mode (sometimes referred to as a display mode). During the normal mode, each driver IC controls its associated LED zone  102  to emit light with the brightness that was received via the brightness data at pin P 3 . To control the brightness of the LED zone  102 , the display IC sinks a given amount of current to ground at pin P 2 . The display IC may include, for example, a drive transistor that controls the amount of current that is allowed to pass through the LEDs in zone  102  and therefore controls the brightness of the LEDs. 
     There are numerous control schemes that may be used to operate the LED array of  FIG. 9 . In one embodiment, a timing controller (TCON) may be used to control the LED array.  FIG. 10  is a schematic diagram of an illustrative electronic device with a timing controller that controls the LED array  36  of backlight  42 . As shown in  FIG. 10 , electronic device  10  may include a timing controller  122  (TCON) on a substrate such as circuit board  120 . Circuit board  120  may be a flexible printed circuit board or a rigid printed circuit board. Timing controller  122  may receive information from a graphics processing unit  132  (GPU) on main logic board  130 . Main logic board  130  may be a rigid printed circuit board in one example. GPU  132  may provide data for display  14  to the timing controller  122 . Timing controller  122  controls the pixel array  24  and LED array  36  (of the backlight) to display the data. 
     To control the pixel array  24 , the timing controller  122  may use display driver integrated circuits  128 . The display driver integrated circuits  128  (similar to display driver integrated circuits  62 A/ 62 B in  FIG. 3 ) may be configured to adjust the liquid crystal display pixels of pixel array  24  on a per-pixel basis. The pixels may be adjusted to pass different amounts of light to achieve a desired per-pixel transparency and corresponding brightness. Each display driver integrated circuit  128  may control a corresponding subset of the pixels in pixel array  24 . 
     Each display driver integrated circuit may be positioned on a respective flexible printed circuit  126 , as shown in  FIG. 10 . There may be one or more optional daughter boards  124  coupled between the flexible printed circuits  126  and circuit board  120 . In general, the depiction of printed circuits  126  and  124  in  FIG. 10  is merely illustrative. Any desired connection scheme (e.g., with any desired number of intervening circuit boards, connectors, signal lines, etc.) may be used to couple pixel array  24  to display driver integrated circuits  128 . Similarly, any desired connection scheme (e.g., with any desired number of intervening circuit boards, connectors, signal lines, etc.) may be used to couple the display driver integrated circuits  128  to timing controller  122 . 
     Timing controller  122  may control LED array  36  in unison with pixel array  24 . For example, for a given frame of image data, the timing controller  122  may send pixel values to display driver ICs  128  for pixel array  24  and may send LED brightness values to LED driver ICs  82  for LED zones  102 . In  FIG. 10 , timing controller  122  sends signals directly to driver ICs  82  on substrate  84 . A connecting structure (e.g., a flexible printed circuit)  136  may be coupled between circuit board  120  and LED substrate  84 . The connecting structure may pass signals from the timing controller  122  to the drivers  82  (and optionally from the drivers  82  back to the timing controller  122 ). 
       FIG. 10  also shows how main logic board  130  may include a boost converter  134  that is configured to provide a power supply voltage (e.g., VLED) to LED array  36 . The various printed circuits shown in  FIG. 10  may be electrically connected using solder, signal paths, vias, pins, etc. 
     The number of signal lines between timing controller  122  and LED array  36  may be proportional to the number of driver ICs included in the LED array. As the size and density of the LED array increases, the number of driver ICs included may increase. Increasing the number of driver ICs increases the number of signal paths required. Routing a high number of signal paths between the timing controller  122  and driver ICs may be challenging due to the limited space available to include all of the desired signal paths. When the number of LED driver ICs  82  is sufficiently small, the timing controller  122  may still send signals directly to the driver ICs. However, as the number of LED driver ICs increases, it may become preferred to provide a dedicated backlight controller for controlling the LED driver ICs  82 . 
       FIG. 11  is a schematic diagram of an illustrative electronic device with a backlight controller that controls the LED array  36  of backlight  42 . The arrangement of  FIG. 11  is the same as previously shown in  FIG. 10 , except for the presence of backlight controller  138  (BCON) between timing controller  122  and LED driver ICs  82 . The backlight controller  138  may be mounted on connecting structure  136  (e.g., a flexible printed circuit) and may receive signals from timing controller  122 . Based on the signals from timing controller  122 , backlight controller  138  provides signals to the driver ICs  82 . 
     The presence of backlight controller  138  may allow for a reduction of the number of signal paths between the timing controller and the LED driver ICs  82 . Between timing controller  122  and backlight controller  138 , the number of signal paths may be reduced. Backlight controller  138  may, based on the signals from the TCON, provide the full complement of signals to driver ICs  82 . However, the full signal path routing is only required in a smaller area (between the BCON and the driver ICs). This may mitigate routing and fan-out issues between the LED array  36  and timing controller  122 . 
     The arrangements of  FIGS. 10 and 11  are merely illustrative. In general, the components of LED array  36 , pixel array  24 , and the corresponding control circuitry (such as BCON  138 , TCON  122 , GPU  132 , boost converter  134 ) may be arranged on any desired number and type of substrates in any desired combination. The components may be electrically connected using any combination of solder, signal paths, vias, pins, etc. 
     In the example of  FIG. 9 , each LED driver integrated circuit has four pins. Minimizing the number of pins in each driver IC may advantageously minimize routing on the LED substrate  84 . Fewer pins in the driver ICs may also allow the driver ICs to be less complex, and therefore smaller and less expensive to manufacture. However, if desired, the number of pins in each driver IC may be increased for added functionality.  FIG. 12  is an example of an LED array where each driver IC  82  has six pins (P 1 , P 2 , P 3 , P 4 , P 5 , and P 6 ). Similar to as shown in  FIG. 9 , each driver IC controls the brightness of an associated LED zone  102 . 
     The function of pins P 1 -P 4  may be similar in  FIG. 12  as in  FIG. 9 . Pin P 1  may again serve as an output pin for each driver IC  82 . The light-emitting diodes of zone  102  are coupled between power supply line  106  and pin P 1  of the driver IC. The output pin P 1  is also coupled to the input pin P 4  of the next driver IC in the chain, as discussed in connection with  FIG. 9 . Pin P 2  may be coupled to ground. Pin P 4  may receive identification information (e.g., addressing information) from controller  138  (e.g., via signal line  108 ). P 3  may receive brightness values similar to as discussed in connection with  FIG. 9 . The example in  FIG. 12  of the controller being backlight controller  138  is merely illustrative. As shown in  FIG. 10 , the driver ICs  82  may instead be directly controlled by timing controller  122  if desired. 
     In  FIG. 12 , the driver ICs also have a pin P 5  that is coupled to a bi-directional data signal line. The bi-directional data signal line  142  may be used for providing control signals or data from controller  138  to the driver ICs. Alternatively, the bi-directional data signal line  142  may be used to convey feedback information from the driver ICs to controller  138 . For example, the driver ICs may send diagnostic information to the controller such as a flag indicating the presence of a short-circuit, a status of whether or not the driver IC is receiving sufficient voltage, etc. In some embodiments, controller  138  may convey brightness values to the driver ICs via bus  142  (e.g., the brightness values may be provided to pin P 5  instead of P 3 ). In this type of arrangement, the P 3  pin may optionally be omitted or may be used to receive a different type of signal. 
     Controller  138  may control the direction of transfer on bus  142 . Controller  138  may control the direction of signal transfer using, for example, a switch  146  that is coupled between the bus (and a resistor  148 ) and a bias voltage supply terminal. Controller  138  may control the state of the switch  146  to control the direction of signal transfer on bus  142 . 
     In  FIG. 12 , the driver ICs also have a pin P 6  that is coupled to a signal line  144 . Signal line  144  may be a digital signal line that is used to provide clock signals to the LED driver ICs. The clock signals may be used by controller  138  to control the timing of operation of the driver ICs. 
     The driver ICs of  FIG. 9  and  FIG. 12  each have one output pin for controlling LEDs (pin P 1 ). This example is merely illustrative. If desired, the driver IC may include additional output pins to allow independent control of multiple LED zones.  FIG. 13  is a schematic diagram of a driver IC with multiple output pins for control of multiple LED zones. As shown, LED driver IC  82  includes 9 pins (P 1 , P 2 , P 3 , P 4 , P 5 , P 6 , P 7 , P 8 , and P 9 ). Pins P 1 , P 2 , P 3 , P 4 , P 5 , and P 6  may have the same functions as discussed in connection with  FIGS. 9 and 12 . Pins P 7 , P 8 , and P 9  may serve as additional output pins for control of additional LED zones. For example, output pin P 1  may be used to control LED zone  102 - 1  (e.g., current is passed through the LEDs in zone  102 - 1  to ground through output pin P 1  and ground pin P 2 ). Additional output pin P 7  may be used to control LED zone  102 - 2  (e.g., current is passed through the LEDs in zone  102 - 2  to ground through output pin P 7  and ground pin P 2 ). Additional output pin P 8  may be used to control LED zone  102 - 3  (e.g., current is passed through the LEDs in zone  102 - 3  to ground through output pin P 8  and ground pin P 2 ). Additional output pin P 9  may be used to control LED zone  102 - 4  (e.g., current is passed through the LEDs in zone  102 - 4  to ground through output pin P 9  and ground pin P 2 ). 
     By including the additional output pins, a single LED driver IC is enabled to control multiple LED zones to have different brightness values. For example, in  FIG. 13 , LED zones  102 - 1 ,  102 - 2 ,  102 - 3 , and  102 - 4  may all have unique brightness magnitudes, as controlled by the driver IC  82 . Enabling multi-zone control in this way may reduce the number of total driver ICs required in the LED array. However, the complexity and size of each individual driver IC will increase. The number of pins selected for the LED driver ICs in a display may therefore depend on the particular design constraints for that display. 
     The example in  FIG. 9  and  FIG. 12  of each LED zone including 9 LEDs is merely illustrative. In general, each LED zone may include any desired number of LEDs (e.g., a single light-emitting diode, a pair of light-emitting diodes, 2-20 light-emitting diodes, at least 2 light-emitting diodes, at least 4 light-emitting diodes, at least 8 light-emitting diodes, fewer than 5 light-emitting diodes, between 4 and 12 light-emitting diodes, between 8 and 12 light-emitting diodes, between 8 and 10 light-emitting diodes, 9 light-emitting diodes, or other desired number of light-emitting diodes). Regardless of the number of LEDs in the LED zone, the LEDs may be connected in series as shown and discussed in connection with  FIG. 9  and  FIG. 12 . 
       FIG. 14  is a cross-sectional side view showing how light-emitting diodes and driver ICs may be mounted to the upper surface of the substrate. As shown, LED array  36  includes a substrate  84  (e.g., formed from glass). Substrate  84  may sometimes be referred to as glass substrate  84  or glass layer  84 . Circuitry layers  150  (sometimes referred to as thin-film circuitry may be formed on the glass layer  84 . Circuitry layers  150  may include one or more conductive layers that are deposited on glass layer  84 . The circuitry layers  150  may be patterned to form traces that follow desired paths (e.g., to form signal lines as in  FIG. 9 ). In one example, circuitry layers  150  includes first and second conductive layers with an intervening insulating layer. Glass layer  84  and circuitry layers  150  may sometimes collectively be referred to as thin-film layer  152 , thin-film glass  152 , thin-film circuitry layer  152 , thin-film circuitry glass  152 , glass substrate  152 , LED substrate  152 , glass LED substrate  152 , etc. In other words, the term glass substrate may be used to refer to both the individual layer of glass itself (e.g., glass substrate  84 ) and the collective combination of a glass layer and conductive layers to which the LEDs are mounted (e.g., glass substrate  152 ). 
     As shown in  FIG. 14 , circuitry layers  150  may include contact pads  154  (sometimes referred to as input-output contacts  154 , solder pads  154 , etc.). The contact pads may be electrically connected to mounted components by solder  160 . As shown in  FIG. 14 , light-emitting diode  38  is mounted on glass substrate  152  and is electrically connected to contact pads  154  by solder  160 . In particular, light-emitting diode  38  has input-output contacts  156  (e.g., pins, solder pads, etc.) that are attached to contact pads  154  with solder  160 . Driver integrated circuit  82  is mounted on glass substrate  152  and is electrically connected to contact pads  154  by solder  160 . In particular, driver IC  82  has input-output contacts  158  (e.g., pins, solder pads, etc.) that are attached to contact pads  154  with solder  160 . As shown in  FIG. 14 , driver IC  82  may be mounted to thin-film circuitry glass  152  between LEDs  38  in the active area of the display. 
     In the example of  FIG. 14 , LEDs  38  and driver ICs  82  may be surface mount technology (SMT) components. This example is merely illustrative, and other mounting techniques may be used to attach LEDs  38  and driver ICs  82  to thin-film circuitry glass  152 . One advantage of the arrangement of  FIG. 14  is that LEDs  38  and driver ICs  82  may be attached to the thin-film circuitry glass  152  in a single mounting step. In other words, because the LEDs  38  and driver ICs  82  have a similar size and are soldered to the thin-film circuitry glass in a similar manner, the attachment process for both LEDs  38  and driver ICs  82  may be performed simultaneously. This is advantageous for reducing the manufacturing cost and complexity associated with the LED array. 
     To improve the efficiency of the backlight, a reflective layer  162  may be formed on an upper surface of thin-film circuitry glass  152 . Reflective layer  162  may be patterned to fill portions of the upper surface of thin-film circuitry glass  152  not already occupied by LEDs  38  and driver ICs  82 . Said another way, the LEDs  38  and driver ICs  82  are formed in openings in the reflective layer  162 . 
     The reflective layer  162  may be formed from any desired material. As one example, the reflective layer may be formed from a diffusive white material (e.g., a white ink spray or a white tape). This example is merely illustrative. In general, reflective layer  162  may cause diffuse reflection and/or specular reflection. In diffusive reflection, an incident ray of light may be reflected in any direction. In specular reflection, an incident ray of light will be reflected at the same angle it strikes the reflective material. Reflective layer  162  may be formed form a metal coating that causes specular reflection, as another example. Reflective layer  162  may have a high reflectance of the light emitted by LEDs  38  (e.g., greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc.). The reflective layer  162  may have any desired thickness (e.g., greater than 1 micron, greater than 2 microns, greater than 3 microns, greater than 5 microns, greater than 10 microns, greater than 25 microns, less than 3 microns, less than 5 microns, less than 10 microns, less than 25 microns, less than 100 microns, between 3 and 15 microns, between 1 and 25 microns, etc.). Reflective layer  162  may sometimes be referred to as white overcoat layer  162 . 
     Thermal considerations may also be taken into account in backlight  42  with LED array  36 . In particular, the components of LED array  36  (e.g., the LEDs  38  and driver ICs  82 ) may generate heat during operation of the display. If care is not taken, the heat generation may adversely affect performance of the display. The glass substrate  84  may have a low thermal conductivity. Consequently, the heat generated by the components may not be evenly spread across the array. 
     To promote heat spreading across the backlight, a thermally conductive layer  164  may be attached to substrate  84 . Thermally conductive layer  164  may have a high thermal conductivity and thus more evenly spreads heat across the backlight. Thermally conductive layer  164  may be formed from any desired material. Thermally conductive layer  164  may have a thermal conductivity of greater than 100 W/mK, greater than 200 W/mK, greater than 300 W/mK, greater than 400 W/mK, between 100 W/mK and 400 W/mK, or another desired thermal conductivity. Examples of materials that may be used for forming thermally conductive layer  164  (sometimes referred to as heat spreading layer  164 ) include metal (e.g., copper, other metals, or combinations of copper and other metals), carbon nanotubes, graphite, or other materials that exhibit high thermal conductivity. If desired, heat spreading layer  164  may be formed from two or more thermally conductive layers of different types (e.g., a layer of copper attached to a layer of graphite, etc.). Polymer carrier films may also be incorporated in layer  164  (e.g., to support a layer of graphite). In one illustrative example, heat spreading layer  164  includes a layer of graphite interposed between two polymer carrier films. 
     An additional technique for distributing heat from the backlight is shown in  FIGS. 15 and 16 .  FIG. 15  is a top view of an illustrative backlight  42  with exposed conductive layers. Specifically, reflective layer  162  may be etched at the periphery of the backlight to expose underlying conductive layer  150 - 1  in circuitry layers  150 . LEDs  38  and driver ICs  82  may also be formed in recesses in reflective layer  162 , as shown in  FIG. 14 . The exposed portion of conductive layer  150 - 1  extends in a ring around reflective layer  162 . A conductive layer  150 - 3  may be separated from conductive layer  150 - 1  by insulating layer  150 - 2 . Conductive layer  150 - 1  and insulating layer  150 - 2  may be etched at the periphery of the backlight to expose underlying conductive layer  150 - 3 . The exposed portion of conductive layer  150 - 3  extends in a ring around conductive layer  150 - 1  and reflective layer  162 . 
     The exposed portions of conductive layers  150 - 1  and  150 - 3  may be coupled to heat sinks for additional heat dispersion. As shown in  FIG. 15 , conductive layer  150 - 3  is coupled to heat sink  166  and conductive layer  150 - 1  is coupled to heat sink  168 . Heat sinks  166  and  168  may be formed from any desired material or component (e.g., a component of the electronic device that serves an additional function such a metal housing component, a dedicated heat sink with fins, etc.). The heat sinks may be attached to conductive layers  150 - 1  and  150 - 3  with thermally conductive paste, in one example. Heat sinks  166  and  168  may sometimes be referred to as heat sink structures. The example of  FIG. 15  with conductive layers  150 - 1  and  150 - 3  connected to discrete heat sinks is merely illustrative. In another example, exposed conductive layers  150 - 1  and  150 - 3  of thin-film circuitry glass  152  may be coupled to a single heat sink structure. 
       FIG. 16  is a cross-sectional side view of the backlight shown in  FIG. 15 . As shown in  FIG. 16 , conductive layer  150 - 3  is deposited on an upper surface of glass layer  84 . Insulating layer  150 - 2  is deposited on conductive layer  150 - 3 . Conductive layer  150 - 1  is deposited on insulating layer  150 - 2 . Reflective layer  162  is deposited on conductive layer  150 - 1 . An exposed portion of conductive layer  150 - 3  is coupled to heat sink  168  and an exposed portion of conductive layer  150 - 1  is coupled to heat sink  166 . 
     In addition to the techniques of  FIGS. 14-16  for promoting heat spreading and dispersion, the backlight may include temperature sensors for active temperature sensing. As shown in  FIG. 17 , temperature sensors  170  may be distributed across the active area of the backlight on thin-film circuitry glass  152 . The temperature sensors may provide temperature data to backlight controller  138  using signal paths  174 . The backlight controller  138  may provide the temperature data to timing controller  122 . The temperature data from the temperature sensors across the backlight may allow for a 2D thermal profile to be determined for the backlight. The 2D thermal profile of temperature across thin-film circuitry glass  152  may be used to allow for real time optical compensation based on temperature. For example, the performance of the LEDs  38  in backlight  42  and the pixels in pixel array  24  may be dependent upon the operating temperature. Using the operating temperature from the 2D thermal profile, the LEDs and pixels may be operated to exhibit desired brightness values in the real-time temperature conditions. 
     Each temperature sensor may be formed using any desired technique. In one possible arrangement, the temperature sensor  170  may be a four-point resistive sensor that measures temperature based on changes in the resistance of thin-film traces on thin-film circuitry glass  152 . In other words, the temperature sensor may be formed from metal traces on the glass substrate (e.g., deposited using physical vapor deposition or other desired techniques). 
     In the example of  FIG. 17 , a temperature sensor  170  is formed between each group of four light-emitting diodes. This example is merely illustrative. In general, the temperature sensors  170  may be distributed across the LED array in any desired pattern. The temperature sensors may be distributed with a uniform density across the array or with a non-uniform density across the array. The ratio of LEDs to temperature sensors may be 4 to 1 (as in  FIG. 17 ) or any other desired ratio (e.g., 1 to 1, 2 to 1, 3 to 1, more than 4 to 1, more than 8 to 1, more than 10 to 1, more than 25 to 1, more than 50 to 1, less than 8 to 1, less than 10 to 1, less than 25 to 1, less than 50 to 1, less than 100 to 1, between 1 to 1 and 10 to 1, between 2 to 1 and 5 to 1, between 4 to 1 and 100 to 1, etc.). 
     The backlight may also include optical sensors for real time sensing of LED brightness and/or color. As shown in  FIG. 17 , optical sensors  172  may be distributed across the active area of the backlight on thin-film circuitry glass  152 . The optical sensors may provide optical data to backlight controller  138  using signal paths  174 . The backlight controller  138  may provide the optical data to timing controller  122 . The optical data from the optical sensors across the backlight may allow for a 2D profile of brightness and color to be determined for the backlight. The 2D optical profile across thin-film circuitry glass  152  may be used to allow for real time optical compensation. For example, the operation of the LEDs and pixels may account for the real time optical conditions. As one non-limiting example, the optical sensor may provide data indicating that a given LED has a brightness that is lower than expected. The timing controller may, in response, increase the brightness of that LED until the target brightness level is reached. Each optical sensor may include any desired components for measuring brightness levels. The optical sensor may have multiple color channels, different optical sensors may have different color channels, all of the optical sensors may have the same, single color channel, etc. 
     In the example of  FIG. 17 , an optical sensor  172  is formed between each group of four light-emitting diodes. This example is merely illustrative. In general, the optical sensors  172  may be distributed across the LED array in any desired pattern. The optical sensors may be distributed with a uniform density across the array or with a non-uniform density across the array. The ratio of LEDs to optical sensors may be 4 to 1 (as in  FIG. 17 ) or any other desired ratio (e.g., 1 to 1, 2 to 1, 3 to 1, more than 4 to 1, more than 8 to 1, more than 10 to 1, more than 25 to 1, more than 50 to 1, less than 8 to 1, less than 10 to 1, less than 25 to 1, less than 50 to 1, less than 100 to 1, between 1 to 1 and 10 to 1, between 2 to 1 and 5 to 1, between 4 to 1 and 100 to 1, etc.). 
     Also, in  FIG. 17  there is an equal number of temperature sensors  170  and optical sensors  172 . This example is merely illustrative. In general, the number and position of the temperature and optical sensors may be selected independently. There may therefore be different numbers of temperature and optical sensors if desired, and the temperature and optical sensors may be positioned in different patterns across the active area if desired. 
     In addition to a reflective layer ( 162 ) on the upper surface of thin-film circuitry glass  152 , a reflective layer may be included on the lower surface of thin-film circuitry glass  152 .  FIG. 18  is a cross-sectional side view of an illustrative backlight that includes a reflective layer attached to a lower surface of the thin-film circuitry glass. As shown, reflective layer  176  is attached to the lower surface of thin-film circuitry glass  152 . Reflective layer  176  may increase the efficiency of the backlight by reflecting light from light-emitting diode  38 . 
     As previously discussed in connection with  FIG. 14 , LED  38  may be attached to input-output contacts  154  in circuitry layer  150  by solder  160 . LED  38  may emit light in direction  182  (e.g., through pixel array  24  toward a viewer). However, LED  38  may also emit light in direction  184  (away from the pixel array and viewer). Without reflective layer  176 , the light emitted in direction  184  may be lost within the electronic device and fail to reach the viewer. To avoid this decrease in efficiency, reflective layer  176  may be present to redirect the light back in direction  182  through the pixel array. Reflective layer  176  is attached to the opposite side of glass substrate  84  as the LEDs  38  and driver ICs  82 . 
     Reflective layer  176  may be formed from any desired material. As one example, the reflective layer may be formed from a diffusive white material (e.g., a white ink spray or a white tape). This example is merely illustrative. Reflective layer  176  may be formed from a metal coating, as another example. In general, reflective layer  176  may cause diffuse reflection and/or specular reflection. Reflective layer  176  may have a high reflectance of the light emitted by LEDs  38  (e.g., greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc.). The reflective layer  176  may have any desired thickness (e.g., greater than 1 micron, greater than 2 microns, greater than 3 microns, greater than 5 microns, greater than 10 microns, greater than 25 microns, less than 3 microns, less than 5 microns, less than 10 microns, less than 25 microns, less than 100 microns, between 3 and 15 microns, between 1 and 25 microns, between 1 and 5 microns, etc.). Reflective layer  176  may be a coating or may be a layer of tape. Reflective layer  176  may sometimes be described as reflective coating  176  or reflective tape  176 . 
       FIG. 19  is a cross-sectional side view of a backlight that includes both a reflective layer and a thermally conductive layer attached to the lower surface of the glass substrate. As shown in  FIG. 19 , reflective layer  176  may be attached to the lower surface of glass substrate  84 . Reflective layer  176  may increase the optical efficiency of the backlight unit. Additionally, thermally conductive layer  164  (as discussed in connection with  FIG. 14 ) is attached to the reflective layer, such that the reflective layer  176  is interposed between the lower surface of glass substrate  84  and the thermally conductive layer  164 . Thermally conductive layer  164  may provide heat spreading benefits in addition to the efficiency benefits from reflective layer  176 . 
     In yet another example, shown in the cross-sectional side view of  FIG. 20 , a single reflective and thermally conductive layer  178  may be attached to the lower surface of glass substrate  84  (instead of separate reflective and thermally conductive layers  176 / 164  as in  FIG. 19 ). Reflective and thermally conductive layer  178  may have a high reflectivity to increase the efficiency of the backlight. For example, the reflective and thermally conductive layer  178  may have a reflectivity that is greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc. Additionally, the reflective and thermally conductive layer  178  may have a high thermal conductivity to achieve desired heat spreading properties. Thermally conductive layer  178  may have a thermal conductivity of greater than 100 W/mK, greater than 200 W/mK, greater than 300 W/mK, greater than 400 W/mK, between 100 W/mK and 400 W/mK, or another desired thermal conductivity. 
       FIG. 21  is a cross-sectional side view of an illustrative backlight showing how the glass substrate may be formed from white diffusive glass. Instead of clear glass having a high transparency, a white diffusive glass 84 W may be used as the substrate for thin-film circuitry glass  152 . The white diffusive glass 84 W may include dispersed particles  186  (e.g., scattering particles) that achieve the desired diffusion of light. The reflectance of white diffusive glass 84 W may be greater than 20%, greater than 30%, greater than 40%, greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc. 
     LED driver integrated circuits  82  are distributed across the active area of the backlight between LEDs  38 . Driver ICs  82  do not emit light and cover the otherwise reflective treatments on thin-film circuitry glass  152  (e.g., driver ICs  82  may prevent light from reaching reflective layer  162 , reflective layer  176 , etc.). If care is not taken, the driver ICs may be visible (e.g., as a shadow on the display when a purely white image is otherwise desired). To prevent the driver ICs from causing visible artifacts in the display and to increase the efficiency of the backlight, the driver ICs may have a reflective upper surface. 
       FIG. 22  is a cross-sectional side view of an illustrative backlight having a driver IC with a reflective upper surface. As shown in  FIG. 22 , a reflective layer  180  may be formed on the upper surface  188  of driver IC  82 . Reflective layer  180  may be formed from any desired material. As one example, the reflective layer  180  may be formed from a diffusive white material (e.g., a white ink spray or a white tape). Reflective layer  180  may be a metal coating, as another example. In general, reflective layer  180  may cause diffuse reflection and/or specular reflection. Reflective layer  180  may have a high reflectance (e.g., greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc.). The reflective layer  180  may have any desired thickness (e.g., greater than 1 micron, greater than 2 microns, greater than 3 microns, greater than 5 microns, greater than 10 microns, greater than 25 microns, less than 3 microns, less than 5 microns, less than 10 microns, less than 25 microns, less than 100 microns, between 3 and 15 microns, between 1 and 25 microns, etc.). 
     The example in  FIG. 22  of a separate reflective layer  180  being formed over driver IC  82  is merely illustrative. In another illustrative example, the upper surface  188  of the IC may itself be polished to increase the reflectivity of the upper surface. In this type of arrangement, the reflectance of upper surface  188  may be greater than 50%, greater than 70%, greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, greater than 99%, less than 99%, etc. When driver IC  82  does have a separate reflective layer  180 , the upper surface of the reflective layer  180  may be considered to be the upper surface of the driver IC (e.g., because the reflective layer effectively forms the upper surface). 
       FIG. 23  is a top view of an illustrative LED array showing a possible LED layout. As shown, the LEDs  38  may be arranged according to a zig-zag grid (e.g., a non-square-grid), instead of a uniform square grid. In a uniform square grid, the LEDs may be arranged in straight columns and rows (similar to as shown in  FIG. 4 , for example). This type of arrangement, however, may result in visible artifacts such as grid mura during operation of the display. The visible artifacts may be caused by the periodicity of the uniform square grid. Therefore, arranging the LEDs in a non-square grid as in  FIG. 23  may reduce periodicity and mitigate visible artifacts. 
     As shown in  FIG. 23 , the LEDs may be arranged according to zig-zag grid lines  190 . The LEDs are arranged in a number of rows (‘R’) and columns (‘C’). Within a given row, the grid line defining the placement of the LEDs may follow a zig-zag pattern (e.g., instead of a straight line, the grid line has a plurality of segments at angles relative to each other). The grid lines that define rows of LEDs may be referred to as horizontal grid lines. Similarly, within a given column, the grid line defining the placement of the LEDs may follow a zig-zag pattern (e.g., instead of a straight line, the grid line has a plurality of segments at angles relative to each other). The grid lines that define columns of LEDs may be referred to as vertical grid lines. 
     The resulting pattern has horizontal and vertical grid lines that intersect at different angles relative to each other. For example, at some points, the grid lines are at a right angle (e.g., angle  194 ) relative to each other. At other points, however, the grid lines are at an acute angle (e.g., acute angle  192 ) relative to each other. At other points, the grid lines are at obtuse angles (e.g., obtuse angle  196 ) relative to each other. Angles  192  and  196  may be supplementary angles. 
     In  FIG. 23 , the horizontal and vertical grid lines are in a zig-zag pattern. The LEDs may therefore be referred to as being arranged in a non-square grid or non-rectangular grid (e.g., the grid lines do not form rectangles). The rows of LEDs may therefore be referred to as zig-zag rows, rows following a zig-zag pattern, or non-linear rows. Similarly, the columns of LEDs may therefore be referred to as zig-zag columns, columns following a zig-zag pattern, or non-linear columns. As a result of the zig-zag pattern of  FIG. 23 , there may be numerous different distances between adjacent LEDs within the display. For example, some of the LEDs may be separated by a diagonally opposite LED (e.g., in an immediately adjacent row and immediately adjacent column) by distance D 1 . Other LEDs may be separated from diagonally opposite LEDs by distance D 2  or D 3 . D 3  may be smaller than D 1 , which may be smaller than D 2 . This is in contrast to a square grid, where the distance between each LED and a diagonally opposite LED is uniform across the LED array. 
     The example of zig-zag grid lines in  FIG. 23  to mitigate periodicity is merely illustrative. If desired, the grid lines and LEDs may have a hexagonal arrangement, octagonal arrangement, or any other desired arrangement. Dithering may also be used to add variance to the positions of the LEDs across the array. 
       FIG. 24  is a top view of an illustrative LED array with another possible LED layout. In  FIG. 24 , each 3×3 group of LEDs (sometimes referred to as an LED zone  102 ) has a reduced layout footprint relative to in  FIG. 23 . Similar to as in  FIG. 23 , grid lines  190  in  FIG. 24  include horizontal zig-zag grid lines and vertical zig-zag grid lines. However, in  FIG. 23  the LEDs are positioned at the intersection points between the horizontal and vertical zig-zag grid lines. In  FIG. 24 , only the center LED  38 E is positioned at the intersection point between the horizontal and vertical zig-zag grid lines. The peripheral LEDs of the group ( 38 P) are moved in direction  198  from the grid line intersection towards the central LED  38 E. 
     Arranging the LEDs in this way effectively decreases the surface area of the footprint of each LED group. Consequently, the distance  200  between each adjacent LED group is greater (e.g., distance  200  between groups is greater in  FIG. 24  than in  FIG. 23 ). Having smaller LED zones may improve backlight performance by mitigating halo effect. Halo effect may refer to the phenomenon that occurs when a small area on the display is intended to have a high brightness and be surrounded by a low brightness region (e.g., a star in a night sky). Ideally, the low brightness region would be controllable totally independently from the high brightness region. However, if both the intended high and low brightness regions occupy the area of one LED zone, there will be a bright ‘halo’ in the intended low brightness region (because the LED zone is set to a high brightness for the high brightness region on the display). Reducing the area of each LED zone may mitigate this halo effect (as there is more resolution to have the intended backlight brightness levels only in intended areas). 
     The LED zones of LED array  36  may be optimized to have a target energy profile.  FIG. 25  is a graph of an illustrative energy profile, showing brightness as a function of position for two LED zones. As shown, the brightness follows profile  202 , with one peak for each corresponding LED zone. The distance between the peaks for the adjacent LED zones is shown as pitch  206 . Another relevant property of the profile is distance  204 , which is the width of the peak at a brightness that is half of the maximum brightness of the peak (referred to herein as full width half maximum or FW). The ratio of the pitch  206  (P) to the distance  204  (FW) may be a key property of the LED zones in a backlight. P/FW for the LED zones in backlight  42  (e.g., the zones of  FIG. 9 ,  FIG. 12 ,  FIG. 23 ,  FIG. 24 , etc.) may be less than 1.3, less than 1.2, less than 1.1, greater than 1, between 1.05 and 1.2, between 1.05 and 1.15, between 1.01 and 1.2, etc. The profile shape depicted in  FIG. 25  is merely illustrative. The brightness profile of a given LED zone may follow any desired shape. 
     To achieve a desired emission profile, the center LED of a given LED zone may be driven with more current than the peripheral LEDs.  FIG. 26  is a top view of an illustrative LED array showing how a first LED in a first zone  102 -A may be driven with a different current than the LEDs in a second zone  102 -B. Having the center LED (‘A’) be driven with a higher current may optimize the emission profile of the 3×3 group of LEDs. A single driver IC  82  may be used to drive both zones  102 -A and  102 -B, or two discrete driver ICs may be used to drive the two zones. Although the LEDs in zones  102 -A and  102 -B are driven with different currents and may therefore be referred to as different zones, the 3×3 group of LEDs may still be designed to operate together to achieve a desired emission profile. Therefore, the 3×3 group may sill be referred to as a unitary LED group or LED cell. 
     In the LED group formed by LED zones  102 -A and  102 -B, the ratio of current between the peripheral LEDs (‘B’) and the central LED (‘A’) may be constant. In other words, the LED group still may have a single target brightness value, and the driver IC may apply currents per a predetermined ratio to achieve the target brightness and the optimized emission profile. The example in  FIG. 26  of the central LED having a different current (brightness) than the peripheral LEDs is merely illustrative. In general, any of the LEDs within the group may have a unique brightness to help tune the emission profile as desired. 
     Herein, the LED array with driver ICs in the active area is described as serving as a backlight for a pixel array (e.g., a liquid crystal pixel array). It should be noted that, if desired, an arrangement of the type shown herein may be used to form a stand-alone display (e.g., without the external LCD pixels). The LEDs may form display pixels that are controlled by driver ICs in the active area. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20210316
Publication Date: 20220830
Grant Date: 20220830
Priority Date: 20200522
Inventors: GU, MINGXIA
QI, JUN
LI, YANMING
CHEN, JINGDONG
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/133603", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133601", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L25/0753", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": true, "first": true, "tree": "[]"}, {"code": "Y02B20/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0646", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B47/105", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B47/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133628", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/36", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133605", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133606", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133628", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133601", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/882", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/856", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/882", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/856", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133606", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133628", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B47/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133601", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B47/105", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133605", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y02B20/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/22", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0646", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/36", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133628", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133612", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133601", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 78608867