Patent Publication Number: US-8976092-B2

Title: Display circuitry with dynamic pixel backlight and backlight sloping control

Description:
BACKGROUND 
     This relates generally to displays, and more particularly, to displays with backlights. 
     Displays such as liquid crystal displays and other displays sometimes include backlight units. A backlight unit may include an array of light-emitting diodes and a backlight control integrated circuit (sometimes referred to as a backlight driver) that directly controls the array of light-emitting diodes. Displays with backlight units may be incorporated into an electronic device such as a computer or cellular telephone or may be implemented as stand-alone units. 
     There is an increasing demand for electronic devices such as portable computers to be capable of supporting both a non-movie mode and a movie mode. For example, a laptop computer may be operated in the non-movie mode when a user is using the laptop computer to give a presentation or to run a text-editing application. The laptop computer may also be operated in the movie mode when the user is using the laptop computer to watch a movie. In either mode, the backlight unit is used to output a fixed backlight level such that the intensity of light generated by the light-emitting diodes remains constant from frame to frame. 
     Implementing the fixed backlight level in the movie mode, however, consumes an excessive amount of power. Consider, for example, a scenario in which two consecutive frames in the movie mode transition from a bright scene to a dark scene. In this example, the backlight unit would output a fixed backlight level in both the bright scene and the dark scene. Display pixels in the liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in each display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of each display pixel (i.e., changes in brightness from frame to frame is adjusted via control of the liquid crystal material without any adjustment to the backlight level). 
     It would therefore be desirable to be able to provide improved ways in which to control the brightness level of the display in the different display modes. 
     SUMMARY 
     A system may include storage and processing circuitry, a graphics controller, and a display. The graphics controller may generate video data to be displayed on the display. The display may include a display panel for displaying the video data, a backlight unit for providing the display panel with backlight, and a display timing controller for communicating with the graphics controller. 
     The display may be used in a first mode (e.g., a movie mode) and a second mode (e.g., a non-movie or “presentation” mode). A user of the system may request a mode switch event that causes the system to transition from one mode to the other. Mode switch events may be handled by software running on the storage and processing circuitry. 
     While transitioning from the movie mode to the non-movie mode, backlight sloping (ramping) control may be activated. The backlight sloping control may be handled internally by the backlight unit, by the display timing controller, or by the storage and processing circuitry (as examples). When the backlight sloping control is activated, the backlight unit may output backlight levels that change at predetermined rates. While transitioning from the non-movie mode to the movie mode, backlight sloping control may be deactivated. When the backlight sloping control is deactivated, the backlight unit may output backlight levels that change at rates that are substantially greater than the predetermined rates. 
     When the display is being used in the movie mode, the backlight unit may be operated in dynamic pixel brightness (DPB) mode. In the DPB mode, the backlight unit may be configured to output backlight levels that can vary from frame to frame (e.g., backlight brightness levels that change between consecutive frames that are being displayed on the display). When the display is being used in the non-movie mode, the backlight unit may be operated in a fixed backlight mode. In the fixed backlight (or non-DPB) mode, the backlight unit may be configured to output a backlight level that remains substantially constant. Backlight sloping control should not be activated when the display is in the movie mode. 
     In one suitable arrangement, the backlight unit may include a slope control circuit that internally handles backlight sloping control. For example, the display timing controller may provide a DPB enable signal to the slope control circuit. The slope control circuit, when activated, may be used to provide the backlight sloping function. The display timing controller may deassert the enable signal when the display is transitioning from the movie mode to the non-movie mode and may assert the enable signal when the display is transitioning from the non-movie mode to the movie mode. Asserting the control signal deactivates the slope control circuit, whereas deasserting the control signal activates the slope control circuit. 
     In another suitable arrangement, the backlight sloping control may be handled by the display timing controller. When the display is in non-movie mode and when the timing controller receives a request from the storage and processing circuitry to adjust backlight levels, the display timing controller may output to the backlight unit a control signal with duty cycles that change at a predetermined rate (e.g., the display timing controller may output a control signal with incrementally increasing pulse widths). When the display is placed in the movie mode, the graphics controller may direct the display timing controller to output to the backlight unit a control signal with duty cycles that change at a rate that is greater than the predetermined sloping rate. 
     In another suitable arrangement, the backlight sloping control may be handled directly using the storage and processing circuitry. When the display is in non-movie mode, the storage and processing circuitry may send incremental backlight level adjustment requests to the display timing controller via the graphics controller, where the requested levels change at a predetermined rate (e.g., an incrementally sloping rate). When the display is in movie mode, the storage and processing circuitry may send backlight level requests to the display timing controller via the graphics controller, where the requested levels change at a rate that is substantially greater than the predetermined rate (e.g., brightness changes may be effected instantaneously without sloping). 
     Further features of the present invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic diagram of an illustrative electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram showing how an electronic device may be operable in a non-movie mode and a movie most in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of an illustrative system with a display in accordance with an embodiment of the present invention. 
         FIG. 8  is a timing diagram showing how display brightness level changes when an electronic device transitions from a movie mode to a non-movie mode in accordance with an embodiment of the present invention. 
         FIGS. 9 and 10  are timing diagrams illustrating various slope control mechanisms for controlling changes in backlight brightness level in accordance with an embodiment of the present invention. 
         FIG. 11  is a flow chart of illustrative steps involved in using an electronic device that is operable in a dynamic pixel backlight display mode and a fixed pixel backlight display mode in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1 ,  2 , and  3 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have a display cover layer or other exterior layer that includes openings for components such as button  26 . Openings may also be formed in a display cover layer or other display layer to accommodate a speaker port (see, e.g., speaker port  28  of  FIG. 2 ). 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have a cover layer or other external layer with an opening to accommodate button  26  (as an example). 
     The illustrative configurations for device  10  that are shown in  FIGS. 1 ,  2 , and  3  are merely illustrative. In general, electronic device  10  may be 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, or other wearable or miniature device, a television, 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, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Displays for device  10  may, in general, include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels, liquid crystal display (LCD) components, or other suitable image pixel structures. In some situations, it may be desirable to use LCD components to form display  14 , so configurations for display  14  in which display  14  is a liquid crystal display are sometimes described herein as an example. It may also be desirable to provide displays such as display  14  with backlight structures, so configurations for display  14  that include a backlight unit may sometimes be described herein as an example. Other types of display technology may be used in device  10  if desired. The use of liquid crystal display structures and backlight structures in device  10  is merely illustrative. 
     A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . A display cover layer or other outer display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     Touch sensor components such as an array of capacitive touch sensor electrodes formed from transparent materials such as indium tin oxide may be formed on the underside of a display cover layer, may be formed on a separate display layer such as a glass or polymer touch sensor substrate, or may be integrated into other display layers (e.g., substrate layers such as a thin-film transistor layer). 
     A schematic diagram of an illustrative configuration that may be used for electronic device  10  is shown in  FIG. 4 . As shown in  FIG. 4 , electronic device  10  may include control circuitry  29 . Control circuitry  29  may include storage and processing circuitry for controlling the operation of device  10 . Control circuitry  29  may, for example, 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. Control circuitry  29  may include processing circuitry based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, etc. 
     Control circuitry  29  may be used to run software on device  10 , such as operating system software and application software. Using this software, control circuitry  29  may present information to a user of electronic device  10  on display  14 . When presenting information to a user on display  14 , sensor signals and other information may be used by control circuitry  29  in making adjustments to the strength of backlight illumination that is used for display  14 . 
     Input-output circuitry  30  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 circuitry  30  may include communications circuitry  32 . Communications circuitry  32  may include wired communications circuitry for supporting communications using data ports in device  10 . Communications circuitry  32  may also include wireless communications circuits (e.g., circuitry for transmitting and receiving wireless radio-frequency signals using antennas). 
     Input-output circuitry  30  may also include input-output devices  34 . A user can control the operation of device  10  by supplying commands through input-output devices  34  and may receive status information and other output from device  10  using the output resources of input-output devices  34 . 
     Input-output devices  34  may include sensors and status indicators  36  such as an ambient light sensor, a proximity sensor, a temperature sensor, a pressure sensor, a magnetic sensor, an accelerometer, and light-emitting diodes and other components for gathering information about the environment in which device  10  is operating and providing information to a user of device  10  about the status of device  10 . 
     Audio components  38  may include speakers and tone generators for presenting sound to a user of device  10  and microphones for gathering user audio input. 
     Display  14  may be used to present images for a user such as text, video, and still images. Sensors  36  may include a touch sensor array that is formed as one of the layers in display  14 . 
     User input may be gathered using buttons and other input-output components  40  such as touch pad sensors, buttons, joysticks, click wheels, scrolling wheels, touch sensors such as sensors  36  in display  14 , key pads, keyboards, vibrators, cameras, and other input-output components. 
     A cross-sectional side view of an illustrative configuration that may be used for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 , or  FIG. 3  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42  for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     In a configuration in which display layers  46  are used in forming a liquid crystal display, display layers  46  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 . 
     Layers  58  and  56  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 thin-film transistors and associated electrodes (display 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. 
     During operation of display  14  in device  10 , control circuitry  29  (e.g., one or more integrated circuits such as components  68  on printed circuit  66  of  FIG. 5 ) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed from circuitry  68  to display driver integrated circuit  62  using a signal path such as a signal path formed from conductive metal traces in flexible printed circuit  64  (as an example). 
     Display driver integrated circuit  62  may be mounted on thin-film-transistor layer driver ledge  82  or elsewhere in device  10 . A flexible printed circuit cable such as flexible printed circuit  64  may be used in routing signals between printed circuit  66  and thin-film-transistor layer  58 . If desired, display driver integrated circuit  62  may be mounted on printed circuit  66  or flexible printed circuit  64 . Printed circuit  66  may be formed from a rigid printed circuit board (e.g., a layer of fiberglass-filled epoxy) or a flexible printed circuit (e.g., a flexible sheet of polyimide or other flexible polymer layer). 
     Backlight structures  42  may include a light guide plate such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes. 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . 
     Light  74  that scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of white plastic or other shiny materials. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce hotspots, compensation films for enhancing off-axis viewing, and brightness enhancement films (also sometimes referred to as turning films) for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint in the X-Y plane of  FIG. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. 
     There is an increasing demand for portable/handheld electronic devices such as those described in  FIGS. 1 ,  2 , and  3  to support a normal non-movie mode and a movie mode. When device  10  is placed in non-movie mode, the user can operate device  10  to give presentations, to display text-editing applications, to display an Internet browser, etc. The non-movie mode may therefore sometimes be referred to as the “presentation” mode. When device  10  is placed in the movie mode, the user can operate device  10  to display a movie or other media having rapidly changing pictures to be displayed. 
     In the non-movie mode, it may be desirable to keep the display brightness level relatively constant to provide a comfortable user experience (e.g., the backlight unit may output a fixed backlight level during the presentation mode). In the movie mode, however, consecutive frames may vary drastically in brightness levels (e.g., a bright scene may be immediately followed by a dark scene or a dark scene may be immediately followed by a bright scene). As a result, it may be desirable to dynamically vary the display brightness level from frame to frame so as to optimize the power savings in the movie mode. For example, backlight unit  42  may be configured to output a first backlight level when displaying a bright scene with display  14  and may be configured to output a second backlight level that is lower than the first backlight level when displaying a relatively darker scene with the display. The backlight unit may therefore operate in a fixed pixel backlight mode during the non-movie mode and in a dynamic pixel backlight (DPB) mode during the movie mode (see, e.g.,  FIG. 6 ). 
     As shown in  FIG. 6 , device  10  may be operable in at least non-movie mode  100  and movie mode  102 . When device  10  is configured in non-movie mode  100 , the backlight unit may be configured to output a fixed backlight level and may have brightness ramping control enabled. In mode  100 , brightness ramping is needed to provide a smooth transition between different brightness levels. For example, consider a scenario in which a user of device  10  decides to adjust the display brightness level from 50% to 70%. If device  10  were to instantaneously adjust the backlight level from 50% to 70% in a single step, the user will observe an abrupt change in brightness or an unpleasant flash. The brightness ramping (sometimes referred to as a backlight sloping control) function, when engaged, will result in the backlight level gradually ramping up in incremental steps (e.g., from 50% to 51% at a first point in time, from 51% to 52% at a second point in time after the first point in time, from 52% to 53% at a third point in time after the second point in time, etc.). Implementing backlight level sloping in this way ensures a more pleasant user experience. 
     In response to a user request, device  10  may switch from non-movie mode  100  to movie mode  102  (as indicated by path  104 ). For example, the user may open a movie application or device  10  may automatically launch a movie application in response to insertion of a DVD or other media storage. When device  10  is configured in movie mode  102 , the backlight unit may be configured in DPB mode so that backlight levels can change from frame to frame to reduce power consumption. In mode  102 , however, brightness ramping can be disabled since backlight levels are already being varied from frame to frame while taking into account all the display brightness parameters (e.g., while taking into account desired display brightness as set by the user, ambient light sensor information, overall brightness level associated with the current scene or frame to be displayed, and other display information). 
     In response to a user request, device  10  may switch from movie mode  102  back to non-movie mode  100  (as indicated by path  106 ). Problems may arise when transitioning between these two modes. For example, consider a scenario in which a user switches from the movie mode to the non-movie mode while manually adjusting the desired display brightness. During the transition, if the brightness ramping function is not enabled fast enough, the user may observe sudden changes in brightness and other visual artifacts. It may therefore be desirable to provide a way for seamlessly transitioning between the two modes while always ensuring pleasant user experience. 
       FIG. 7  shows an illustrative system diagram of device  10  or a network of devices. As shown in  FIG. 7 , system  10 ′ may include a graphics controller such as graphics controller  114  that is coupled to display structures  14 . Graphics controller  114 , which may sometimes be referred to as a video card or video adapter, may be used to provide video data and control signals to display  14 . The video data may include text, graphics, images, moving video content, or other content to be presented on display  14 . 
     Graphics controller  114  may receive video data to be displayed on display  14  from control circuitry  29 . Control circuitry  29  may include processing circuitry  110  and storage  112 . Processing circuitry  110  may include one or more processors such as central processing units (CPUs), microprocessors, microcontrollers, digital signal processors, application-specific integrated circuits, or other processing circuits. Storage  112  may include random-access memory, read-only memory, solid state memory in a solid state hard drive, magnetic storage, and other volatile and/or nonvolatile memory. 
     Input-output components such as sensors, touch sensor arrays, keyboards, buttons, microphones that receive voice input and other audio input, speakers that provide audio output, vibrators, status indicator lights, wireless and wired communications circuits for communicating with external equipment, and other components may be used for receiving input from a user or other external source and/or for conveying output to a user or other external destination. 
     Display  14  may include a display panel such as display panel  198 , timing controller (ICON) circuitry such as display timing controller  120  (e.g., a ICON integrated circuit), and associated backlight structures. Display panel  198  may be a liquid crystal display module containing an array of display pixels, an electrophoretic display, an electrowetting display, or display structures using other types of display technologies. The backlight structures may include light guide plate  78 , light source  72  (e.g., an array of light-emitting diodes), and backlight control circuitry such as backlight controller  122  (sometimes referred to as a backlight driver integrated circuit) that is used to control light source  72 . Light guide plate  78 , light source  72 , backlight controller  122 , and other associated circuitry may therefore sometimes be referred to collectively as a backlight unit. 
     Communications path  130  may be used to convey information between graphics controller  114  and display  14 . Communications path  130  may, for example, serve as a video data path or display port for conveying video data bits and other control signals from graphics controller  114  to display timing controller  120 . 
     The components of system  10 ′ may be integrated into a single piece of electronic equipment or multiple pieces of electronic equipment. For example, system  10 ′ may be implemented as a single electronic device such as a portable computer, a tablet computer, a cellular telephone, a media player, a computer display that includes an embedded computer, a television, or other stand-alone electronic equipment. In this type of configuration, communications path  130  may be formed from an internal bus. 
     If desired, system  10 ′ may include a first piece of equipment such as a desktop computer, set-top box, or other equipment (formed from input-output circuitry  34 , control circuitry  29 , and graphics controller  114 ) that is coupled by path  130  to a second piece of equipment (e.g., a display such as display  14  that is mounted in a display housing to form a stand-alone computer display or other monitor). In this type of configuration, path  130  may be formed as part of a cable (e.g., a display cable). The display cable may be pigtailed to the first piece of equipment, may be pigtailed to the second piece of equipment, or may be a stand-alone cable having a first end coupled to the first piece of equipment and an opposing second end coupled to the second piece of equipment. Configurations for system  10 ′ that include more than two pieces of equipment or that include components that are embedded into kiosks, automobiles, or other systems may also be used, if desired. 
     Display timing controller  120  may be used to provide data signals and control signals to display panel  198  via path  148 . As an example, timing controller  120  may provide data signals via data lines and gate control signals via gate lines to each corresponding display pixel in display panel  198  via path  148 . Control signals such as backlight enable control signal BE and display synchronization signals SYNC may be conveyed from display timing controller  120  to backlight driver  122  via paths  136  and  138 , respectively. When backlight enable signal BE is asserted, backlight driver  122  may turn light source  72  on to illuminate the display panel via light guide plate  78 . When backlight enable signal BE is deasserted, backlight driver  122  may turn light source  72  off. In the example of  FIG. 7 , backlight driver  122  may include a boost converter such as boost converter  144  for providing elevated voltage signals that are used to drive the array or chain of light-emitting diodes in light source  72  (e.g., by providing the boosted voltage signals to light source  72  via path  150 ). 
     The DPB mode may be implemented using a dynamic pixel backlight control circuit  134  that is part of controller  120  (e.g., control circuit  134  may be used to run a DPB algorithm that dynamically controls the backlight level based on the current frame to be displayed). As shown in  FIG. 7 , DPB control circuit  134  may provide a pulse width modulated control signal PWM to backlight driver  122  via path  140 . Control circuit  134  may dynamically adjust the desired backlight level by modulating the pulse width of signal PWM (e.g., by adjusting the duty cycle of signal PWM). For example, during movie mode, signal PWM may exhibit a first duty cycle during a first display cycle and may exhibit a second duty cycle that is different from the first duty cycle during a second display cycle following the first display cycle. A larger duty cycle may result in a higher backlight level, whereas a smaller duty cycle may result in a lower backlight level. During non-movie mode, however, signal PWM may exhibit relatively constant duty cycles. 
     In one suitable arrangement, the backlight brightness level ramping control may be implemented using a slope control circuit  146  that is part of backlight driver  122 . When slope control circuit  146  is activated, any request to instantaneously change backlight levels by a noticeable amount will result in a gradual (sloped) brightness change that is pleasant to the user (e.g., the backlight unit will output backlight levels that change at a predetermined rate when backlight sloping control is activated). When slope control circuit  146  is deactivated, any request to instantaneously change backlight levels will be immediately effected (e.g., backlight driver  122  will control light source  72  to output at the currently requested brightness level based on the duty cycle of signal PWM). In other words, the backlight unit will output backlight levels that change at a rate that is substantially greater than the predetermined rate when the backlight sloping control is deactivated. Brightness sloping/ramping should not be activated when the backlight unit is placed in DPB mode. 
     One way of activating and deactivating slope control circuit  146  involves sending a DPB enable signal DPB_En from display timing controller  120  to backlight driver  122  via path  142 . As an example, signal DPB_En should be asserted when display  14  is placed in DPB mode, whereas signal DPB_En should be deasserted when display  14  is placed in fixed backlight mode. Signal DPB_En asserted/deasserted in this way may therefore serve as a control signal for selectively enabling slope control circuit  146  (e.g., an asserted DPB_En may turn off the brightness sloping function, whereas a deasserted DPB_En may turn on the brightness sloping function). Control of the brightness ramping functionality in this way incurs negligible latency since DPB control circuit  134  is responsible for enabling/disabling DPB mode in the first place and any mode switch that enters or exits DPB mode will be immediately handled by control circuit  134 . Mode switching events may be handled using software running on processor  110  in response to user request (e.g., processor  110  may be configured to pass appropriate mode information to display timing controller  120  through graphics controller  114  via paths  130  and  196 ). Graphics controller  114  used in this way should not perform any brightness control. 
       FIG. 8  is a timing diagram that illustrates control of the backlight level when display  14  transitions from the movie mode to the non-movie mode. As shown in  FIG. 8 , the image content to be presented by display  14  may vary dramatically from frame to frame during movie mode and may remain relatively stable during non-movie (presentation) mode. During movie mode, the backlight unit may be placed in DPB mode such that the backlight level dynamically tracks the brightness level of the image content from frame to frame (see, e.g., dynamically varying backlight levels  302 ). During movie mode, signal DPB_En may be asserted to deactivate slope control circuit  146 . 
     At time t 1 , a request to switch from movie mode to non-movie may be received from the user (e.g., the user may exit a movie application or open a text-editing application). At this time, signal DPB_En may be deasserted to activate slope control circuit  146 . 
     At time t 2 , device  10  may receive manual input from the user to increase the backlight level from level Bi to Bf. Since the slope control circuit  146  is enabled, the backlight level will increase from Bi to Bf with a predetermined slope as shown in sloping portion  304 . The smooth transition  304  as shown in  FIG. 8  is merely illustrative. If desired, brightness sloping may be implemented in multiple smaller steps (as shown in  FIG. 9 ), as a linear ramp with sharp transition points  306  (as shown in  FIG. 10 ), or using other suitable sloping techniques. Sloping control circuit  146  may be used to provide any desired amount of backlight sloping (e.g., sloping control circuit  146  may provide a predetermined ramp rate or programmable ramp rate). If desired, decreases in backlight level or other combinations of backlight level adjustments can be handled in this way. 
     In another suitable arrangement, the backlight brightness ramping control may be implemented by directly sloping the duty cycle of signal PWM using display timing controller  120 . Display timing controller  120  may receive mode information from processor  110  via graphics controller  114 . When an instantaneous brightness change is requested during non-movie mode, direct display timing controller  120  may output signals PWM with gradually increasing pulses. In other words, sloping is implemented by gradually increasing the duty cycle of signal PWM from one cycle to another. For example, the display timing controller may output signal PWM with duty changes that change at a predetermined rate during non-movie mode and to output PWM with duty cycles that change at a rate that is greater than the predetermined rate during movie mode. Slope control circuit  146  in backlight driver  122  need not be used in this arrangement. Control of the brightness ramping function in this way may be comparably slower than the alternate arrangement that uses the DPB enable signal, because the control of sloping is performed via multiple PWM cycles using display timing controller  120 . 
     In another suitable arrangement, the brightness ramping control may be implemented directly using processor  110 . For example, consider a first scenario in which display  14  is in movie mode and the current backlight level is set to 50%. In this scenario, a user request to change the backlight level from 50% to 70% may be instantaneously effected (e.g., processor  110  may send a request that directs display timing controller  120  to output signal PWM having a duty cycle corresponding to a 70% backlight output level). Consider a second scenario in which display  14  is in non-movie mode and the current backlight level is set 40%. In this second scenario, a user request to change the backlight level from 40% to 60% may be gradually effected (e.g., processor  110  may output a first request that directs controller  120  to output PWM having a first duty cycle corresponding to a 41% backlight level, a second request that directs controller  120  to output PWM having a second duty cycle corresponding to a 42% backlight level, a third request that directs controller  120  to output PWM having a third duty cycle corresponding to a 43% backlight level, etc.). Control of the brightness ramping function in this way may be comparably slower than the alternate arrangement that uses the DPB enable signal, because the control of sloping is performed using processor  114 , which is several layers removed from backlight driver  122 . If desired, the brightness ramping control may also be implemented by sending commands from processor  110  directly to backlight driver  122  via an inter-integrated circuit (I 2 C) link  132 . 
       FIG. 11  is a flow chart of illustrative steps involved in operating a device  10  or system  10 ′ having a display operable in a movie mode and a non-movie mode in accordance with an embodiment of the present invention. At step  400 , display unit  14  may be powered up. 
     At step  402 , display  14  may be placed in non-movie (presentation) mode. In the non-movie mode, the backlight unit may be placed in non-DPB mode such that backlight driver  122  will output a current backlight level based on the user-selected brightness input and associated sensor information. If the requested backlight level is sufficiently greater than a previous backlight level, backlight level sloping may be implemented (e.g., backlight brightness level may be sloped if the difference between the request backlight level and the previous backlight level exceeds a predetermined threshold). 
     When the user wants to switch to movie mode, backlight level sloping may be disabled (step  406 ). As examples, signal DPB_En may be asserted to disable slope control circuit  146 , PWM can freely change duty cycles from frame to frame as requested by processor  110 , processor  110  may send as-requested backlight level commands directly to display timing controller  120 , or other ways of toggling the backlight level sloping functionality can be used. 
     At step  404 , display  14  may be placed in movie mode. In the movie mode, the backlight unit may be placed in DPB mode such that backlight driver  122  will output whatever backlight level that is being requested by the current frame to be displayed while taking into account the user-selected brightness input and any associated sensor data. Since backlight level sloping is disabled, no sloping should be performed when changing among different brightness levels. 
     When the user wants to switch back to non-movie mode, backlight level sloping may be enabled (step  408 ). As examples, signal DPB_En may be deasserted to enable slope control circuit  146 , PWM sloping may be performed by gradually increasing/decreasing duty cycle from frame to frame), processor  110  may send incremental backlight level commands to display timing controller  120  that direct controller  120  to output sloping backlight levels, or other ways of toggling the backlight level sloping functionality can be used. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.