PATENT DOCUMENT

Publication Number: US-9380672-B2
Application Number: US-201313754640-A
Country: US
Kind Code: B2

Title: Methods and apparatus for improving backlight driver efficiency

Abstract:
An electronic device may be provided with display circuitry that includes a display timing controller, a backlight driver, a light source, and other associated backlight structures. The backlight control circuitry may generate a control signal having an adjustable duty cycle to the backlight driver. The backlight driver may include a boost converter, a current driver, and backlight control circuitry. The current driver may only be activated when the control signal is high. The backlight control circuitry may output an enable signal to the boost converter. The backlight control circuitry may activate the boost converter a predetermined amount of time before each rising clock edge in the control signal by asserting the enable signal for a longer period of time than when the control signal is high. The control signal and the enable signal may be deasserted at around the same times.

Claims:
What is claimed is: 
     
       1. A method for operating a display backlight unit having a voltage boost converter circuit and a current driver circuit, comprising:
 periodically enabling the voltage boost converter circuit using an enable signal having a first frequency; and 
 periodically activating the current driver circuit once a predetermined period of time has passed after each rising edge in the enable signal, wherein the current driver circuit is deactivated during the predetermined period of time, wherein enabling the voltage boost converter circuit comprises activating the voltage boost converter circuit using a boost converter switching control signal that exhibits a second frequency that is greater than the first frequency. 
 
     
     
       2. The method defined in  claim 1 , wherein the display backlight unit further includes a light source, and wherein activating the current driver circuit comprises periodically providing current to the light source. 
     
     
       3. The method defined in  claim 1 , wherein the display backlight unit further includes a light source, and wherein enabling the voltage boost converter circuit comprises periodically charging an output path that is coupled to the light source to an elevated voltage level. 
     
     
       4. The method defined in  claim 1 , wherein periodically activating the current driver circuit comprises periodically activating the current driver circuit using a control signal, the method further comprising:
 controlling a backlight level for the display backlight unit by performing duty cycle adjustments on the control signal, wherein the enable signal and the control signal toggle at a given frequency. 
 
     
     
       5. The method defined in  claim 4 , wherein the display backlight unit further includes control circuitry, and wherein enabling the voltage boost converter circuit comprises asserting the enable signal before each rising edge in the control signal with the control circuitry. 
     
     
       6. The method defined in  claim 5 , further comprising:
 with the control circuitry, deasserting the enable signal in response to falling edges in the control signal. 
 
     
     
       7. A method for operating a display backlight unit that includes a boost converter and a current driver, comprising:
 asserting a first control signal to activate the current driver, wherein the first control signal has a first duty cycle; and 
 asserting a second control signal to enable the boost converter, wherein the second control signal has a second duty cycle that is greater than the first duty cycle, wherein the first and second control signals toggle at a given frequency, wherein the first control signal is asserted after the second control signal is asserted, and wherein the first control signal is deasserted between when the second control signal is asserted and when the first control signal is asserted. 
 
     
     
       8. The method defined in  claim 7 , wherein the display backlight unit further includes control circuitry, the method further comprising:
 with the control circuitry, generating the second control signal based on the first control signal. 
 
     
     
       9. The method defined in  claim 8 , further comprising:
 with the control circuitry, periodically asserting the second control signal before each respective rising edge of the first control signal. 
 
     
     
       10. The method defined in  claim 8 , further comprising:
 with the control circuitry, periodically deasserting the second control signal in response to each falling edge of the first control signal. 
 
     
     
       11. The method defined in  claim 7 , wherein asserting the second control signal to enable the boost converter comprises periodically asserting the second control signal to enable the boost converter, the method further comprising:
 while the second control signal is asserted, continuously switching the boost converter on and off at a frequency that is at least two times greater than the given frequency. 
 
     
     
       12. The method defined in  claim 7 , wherein the display backlight unit further includes light source structures, the method further comprising:
 while the current driver is activated, providing current to the light source structures with the current driver; and 
 while the boost converter is activated, providing an elevated voltage to the light source structures with the boost converter. 
 
     
     
       13. The method defined in  claim 7 , further comprising:
 controlling a backlight level for the display backlight unit by performing duty cycle adjustments on the first control signal. 
 
     
     
       14. The method defined in  claim 7 , wherein asserting the first control signal to activate the current driver comprises asserting the first control signal a predetermined period of time after each rising edge in the second control signal. 
     
     
       15. Display backlight circuitry, comprising:
 a boost converter circuit that receives a first clock signal; and 
 a current driver circuit that receives a second clock signal, wherein the first and second clock signals are asserted at different times such that the boost converter circuit is turned on and off at least once between when the first clock signal is asserted and when the second clock signal is asserted. 
 
     
     
       16. The display backlight circuitry defined in  claim 15 , further comprising:
 backlight driver control circuitry that outputs the first clock signal to the boost converter circuit and that generates the second clock signal. 
 
     
     
       17. The display backlight circuitry defined in  claim 16 , wherein the first and second clock signals exhibit a given frequency, and wherein the backlight driver control circuitry is configured to assert the first clock signal before each rising clock edge in the second clock signal. 
     
     
       18. The display backlight circuitry defined in  claim 17 , wherein the backlight driver control circuitry is further configured to deassert the first clock signal when the second clock signal falls low. 
     
     
       19. The display backlight circuitry defined in  claim 16 , further comprising:
 light emitting structures interposed between the boost converter circuit and the current driver circuit, wherein the boost converter circuit is configured to provide a boosted voltage signal to the light emitting structures when the first clock signal is high, and wherein the current driver circuit is configured to provide current to the light emitting structures when the second clock signal is high.

Description:
This application claims priority to U.S. provisional patent application No. 61/734,906 filed Dec. 7, 2012, which is hereby incorporated by reference herein in its entirety. 
    
    
     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. 
     The backlight driver may include a boost converter circuit and a current driver circuit. The boost converter circuit is controlled using a first clock signal exhibiting a first frequency to periodically provide a boosted voltage to the array of light-emitting diodes when the first clock signal is high. The current driver circuit is controlled using a second clock signal exhibiting a second frequency to periodically provide a source of current for the light-emitting diodes when the second clock signal is high. The first frequency associated with the first clock signal that controls the boost converter is typically substantially greater than the second frequency associated with the second clock signal that controls the current driver circuit. When the second clock signal is low, the current driver circuit is turned off, thereby preventing the array of light-emitting diodes from emitting any light. 
     In conventional backlight drivers, the first clock signal continues to toggle during both high clock phases and low clock phases of the second clock. In other words, the boost converter circuit is being continuously switched on and switched off even when the current driver circuit is turned off. Operating the backlight driver in this way consumes more power than necessary. Since the power consumption associated with switching on/off the boost converter circuit does not scale with the amount of current that is being delivered using the current driver circuit, power efficiency degradation is exacerbated at lower loads when the current driver is being used to deliver lower average current levels (i.e., when the backlight driver is being used to produce lower backlight levels). 
     It would therefore be desirable to be able to provide improved ways for operating the backlight driver to improve power efficiency. 
    
    
     
       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 schematic diagram of illustrative display circuitry that includes a boost converter circuit and a current driver circuit in accordance with an embodiment of the present invention. 
         FIG. 7  is a circuit diagram of illustrative backlight driver circuitry in accordance with an embodiment of the present invention. 
         FIGS. 8 and 9  are timing diagrams showing conventional boost converter switching schemes. 
         FIG. 10  is a timing diagram showing illustrative boost converter switching schemes for improving power efficiency in accordance with an embodiment of the present invention. 
         FIG. 11  is a table illustrating different current driver loading scenarios in accordance with an embodiment of the present invention. 
         FIG. 12  is a plot of efficiency versus different load conditions for comparing a conventional boost converter switching scheme to an improved boost converter switching scheme in accordance with an embodiment of the present invention. 
         FIG. 13  is a flow chart of illustrative steps for operating backlight driver circuitry in accordance with an embodiment of the present invention. 
     
    
    
     SUMMARY 
     Embodiments of the present invention relate to reducing boost converter switching events during periods when a current driver is turned off and making appropriate predictions on when the current drivers will be turned on to precondition the boost converter. 
     An electronic device may include a display having a display backlight unit (sometimes referred to as display backlight circuitry). The display backlight unit may include a boost converter circuit, a current driver circuit, backlight driver control circuitry, light emitting structures (e.g., an array of light-emitting diodes, etc.), and other associated structures. The light emitting structures may be coupled in series between the boost converter circuit and the current driver circuit. The boost converter circuit may be used to provide a boosted voltage signal to the light emitting structures, whereas the current driver circuit may be used to provide current to the light emitting structures. 
     The current driver may be periodically activated using a first clock control signal, whereas the boost converter may be periodically activated using a second clock control signal. The first and second clock control signals may exhibit the same frequency. The control circuitry may output the first clock signal based on the second control signal. In particular, the control circuitry may have an input that receives the first clock control signal and an output on which the second clock control signal is provided. The backlight level of the display may be adjusted by tuning the duty cycle of the first clock control signal. 
     In one suitable arrangement, the control circuitry may assert the second control signal to activate the boost converter circuit before each respective rising edge in the first control signal. The control circuitry may then deassert the second control signal in response to detecting falling edges in the first control signal. In other words, the first and second control signals may exhibit different duty cycles (e.g., the second control signal may exhibit a duty cycle that is greater than that of the first control signal). 
     Further features of the present invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description. 
     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 (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. 
       FIG. 6  is a schematic diagram of display  14 . As shown in  FIG. 6 , display  14  may include a display panel such as display panel  100 , timing controller (ICON) circuitry such as display timing controller  102  (e.g., a ICON integrated circuit), and associated backlight structures. Display panel  100  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  104  (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 sometimes be referred to collectively as a backlight unit or as display backlight structures. 
     Display timing controller  102  may be used to provide data signals and control signals to display panel  100  via path  101 . As an example, timing controller  102  may provide data signals via data lines and gate control signals via gate lines to each corresponding display pixel in display panel  100  via path  101 . Control signals such as backlight enable control signal BE and display synchronization signals SYNC may be conveyed from display timing controller  102  to backlight driver  104  via paths  106  and  108 , respectively. When backlight enable signal BE is asserted, backlight driver  104  may be capable of turning 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 be unable to turn light source  72  on. 
     In the example of  FIG. 6 , backlight driver  104  may include a boost converter such as boost converter  120  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 a boosted voltage signal Vboost to light source  72  via path  124 ). 
     Backlight driver  104  may also include a current driver  122  that provides current to light source  72  via current path  126 . For example, current driver  122  may serve as a current sink that is periodically enabled using a pulse width modulated control signal PWM (shown in  FIG. 7 ). Signal PWM may be generated by the backlight driver control circuitry  200  acting on a brightness command BCMD from timing controller  102  received via path  110 . To achieve the desired backlight level, the pulse width of signal PWM is modulated (e.g., by adjusting the duty cycle of signal PWM). 
     For example, signal PWM may exhibit a first duty cycle during a first time period and may exhibit a second duty cycle that is different from the first duty cycle during a second time period following the first time period. A larger duty cycle may activate current driver  122  for a longer period of time to result in a higher backlight level, whereas a smaller duty cycle may activate current driver  122  for a relatively shorter period of time to result in a lower backlight level (e.g., the brightness of display  14  may be proportional to the duty cycle of signal PWM that controllers current driver  122 ). The use of PWM to control backlight brightness level may sometimes be referred to as PWM “dimming.” If desired, the backlight level can also be controlled by tuning the rate of current flow that is provided by current driver  122  when current driver  122  is activated. The use of current flow to control backlight brightness level may sometimes be referred to as “linear dimming.” 
       FIG. 7  is a circuit diagram of backlight driver  104 . As shown in  FIG. 7 , backlight driver  104  may include boost converter circuitry  120 , current driver circuitry  122 , and associated backlight driver control circuitry  200 . Boost converter circuitry  120  may include a boost converter control circuit  130 , a transistor such as gating transistor  132  (e.g., an n-channel transistor), a diode such as diode  136 , a capacitive element such as capacitor  138 , an inductive element such as inductor  134 , and resistive elements such as resistors  140 ,  142 , and  144 . Transistor  132  may have a drain terminal that is coupled to a positive power supply terminal  196  (e.g., a power supply terminal on which power supply voltage Vin is provided) via inductor  134 , a source terminal that is coupled to a ground power supply terminal via resistor  144 , and a gate terminal that receives voltage boost converter gating control signal BST_GATE. The source terminal of transistor  132  may also be coupled back to boost converter control circuit via a feedback path  146 . 
     The drain terminal of transistor  132  may further be coupled to a boost converter output path  124  via diode  136 . Boost converter circuitry  120  may be used to generate boosted voltage Vboost on output path  124 . Resistors  140  and  142  may be coupled in series between output path  124  and the ground terminal. In particular, resistors  140  and  142  may be connected at an intermediate node that is coupled back to boost converter control circuit  130  via another feedback path  148 . Capacitive element  138  may be coupled to output path  124  in a shunt configuration. 
     Boost converter control circuit  130  may be configured to toggle control signal BST_GATE at a first frequency. When signal BST_GATE is asserted, transistor  132  is turned on so that current may flow through diode  136  to charge up capacitor  124  towards a regulated voltage level. Consider an example in which power supply voltage is equal to 12 V. By choosing appropriate values for the different passive components in circuitry  120 , circuitry  120  may be configured to provide a Vboost of 60 V on output path  124  (as an example). When signal BST_GATE is deasserted, transistor  132  is turned off and output path  124  may be left floating such that output path  124  is not actively being driven. In this floating state, the voltage level on output path  124  may be temporarily stored on capacitive element  138 . 
     Current driver circuitry  122  may include a current driver control circuit  150 , a transistor such as gating transistor  152  (e.g., an n-channel transistor), and a resistive element such as resistor  154 . In particular, transistor  152  may have a drain terminal that is coupled to current sink path  126 , a source terminal that is coupled to the ground line via resistor  154 , and a gate terminal that receives gating control signal LED_GATE. The source terminal of transistor  152  may also be coupled back to current driver control circuit  150  via feedback path  156 . 
     The string of LEDs in light source  72  may be coupled in series between output path  124  and current sink path  126 . Path  126  may also be coupled to boost converter control circuit  130  via path  127  to help ensure that sufficient headroom is provided to current driver  122  (e.g., the voltage on path  126  may serve as headroom information LED_HDR that is fed back to boost converter control circuit  130  as a control signal). Connected using this arrangement, light source  72  may be configured to emit light when transistor  152  is activated and may be turned off when transistor  152  is deactivated. Transistor  152  may be activated when signal LED_GATE is asserted and may be deactivated when signal LED_GATE is deasserted. The amount of current that is delivered by current driver circuitry  122  may therefore depend on the frequency and/or duration with which signal LED_GATE is asserted. 
     Current driver control circuit  150  may receive signal PWM from backlight driver control circuitry  200  via path  111 . The frequency and duration with which signal LED_GATE is asserted may be directly proportional to the frequency and duty cycle of signal PWM, as controlled by backlight driver control circuitry  200 . In other words, light source  72  may only be capable of emitting light when signal LED_GATE or signal PWM is asserted. Generally, signal PWM may be toggled at a second frequency that is substantially less than the first frequency at which signal BST_GATE is being toggled. As an example, signal BST_GATE may toggle at 400 kHz whereas signal PWM may only toggle at 20 kHz. 
     Boost converter circuitry  120  and current driver circuitry  122  of  FIG. 7  are merely illustrative and do not serve to limit the scope of the present invention. If desired, other suitable types of voltage boosting circuit and other types of current source/sink circuit may be used in backlight driver integrated circuit  104 . 
     Backlight driver control circuitry  200  may have an input configured to receive brightness information (e.g., brightness control command BCMD) from ICON and an output on which it will generate a PWM signal and another output on which a boost converter enable signal BST_EN is provided. Backlight driver control circuitry  200  may serve to toggle BST_EN based on when signal PWM rises high and when signal PWM falls low. When signal BST_EN is asserted, boost converter circuitry  120  may be allowed to toggle signal BST_GATE at the first frequency (i.e., at a boost converter switching frequency) to periodically charge up output voltage Vboost. When signal BST_EN is deasserted, boost converter circuitry  120  may be placed in an idle state such that BST_GATE is held low (e.g., output path  124  may remain floating while signal BST_EN is deasserted). 
     In conventional backlight drivers, the boost converter circuitry may continue to switch on and off during both high and low clock phases of signal PWM (see,  FIG. 8 ). As shown in  FIG. 8 , signal BST_GATE continues to toggle during the high clock phases  300  and the low clock phases  302  of PWM. Voltage Vboost is kept at a regulated voltage level Vreg. In other words, boost converter  120  is being continuously switched on and switched off even when current driver  122  is turned off. Operating the backlight driver in this way consumes more power than necessary. 
     In an effort to lower power consumption, techniques have been developed to reduce boost converter switching events during periods when the current driver is turned off.  FIG. 9  shows an approach in which signal BST_GATE is prevented from toggling during the low clock phases  312  of signal PWM (i.e., BST_GATE is only allowed to toggle when signal PWM is asserted during high clock phases  310 ). Operating the boost converter in this way may, however, result in some voltage droop in Vboost at the rising edges of PWM. In the example of  FIG. 9 , voltage Vboost may suffer a voltage offset Vdroop from the desired voltage level Vreg when signal PWM is turned back on at time t1. It may take some time before Vboost is recharged up to Vreg (at time t2). During this time when Vboost is being charged back up to Vreg (i.e., from time t1 to t2), the current driver may not have sufficient headroom and may not be capable of providing an accurate current to the array of backlight LEDs. 
     In accordance with an embodiment of the present invention, boost converter circuitry  120  may be activated prior to each rising edge in signal PWM (see, e.g.,  FIG. 10 ). As shown in  FIG. 10 , the enabling of signal BST_GATE may be controlled using boost converter enabling signal BST_EN (e.g., boost converter switching signal BST_GATE can only toggle when signal BST_EN is asserted). Backlight driver control circuitry  200  may assert BST_EN a predetermined number of cycles before PWM is asserted. In the example of  FIG. 10 , signal BST_EN is deasserted in response to a corresponding falling clock edge in PWM (at time t1 towards the end of PWM high clock phase  320 ). signal BST_EN may be asserted (at time t2) two boost converter switching cycles before the corresponding rising clock edge of PWM (e.g., BST_EN may be asserted two BST_GATE clock cycles before time t3 when PWM is raised high). This is merely illustrative. 
     Signal BST_EN may be asserted any suitable number of boost converter switching cycles before each respective rising edge of signal PWM. In this way, backlight driver control circuitry  200  may assert BST_EN for time periods  324  that are greater than the high clock phases  320  of PWM (e.g., the duty cycle of BST_EN may be greater than the duty cycle of PWM). Generally, enough time should be allowed for boost converter  120  to recharge capacitor  138  on which Vboost is provided so as to eliminate any existing voltage droop before the rising edge of PWM. The number of boost converter switching cycles that should be allowed before the rising edge of PWM is a function of various circuit component parameters associated with the boost converter circuitry. The number of boost converter switching cycles may be adjustable/programmable for use in various applications. 
     For example, consider a scenario in which signal PWM has a frequency of 20 kHz and the boost converter is switching at a frequency of 400 kHz (e.g., signal BST_GATE is toggling at 400 kHz). If the desired LED backlight level is set at 50%, then the display timing controller may adjust signal PWM to exhibit a 50% duty cycle (e.g., PWM may be a square wave with a 25 μs high clock phase and a 25 μs low clock phase). During one complete PWM cycle, the boost converter may switch up to 20 times if no deactivation of boost converter switching is implemented (i.e., 10 times during the high PWM clock phase and 10 times during the low PWM clock phase). 
     In accordance with an embodiment of the present invention, by preventing boost converter from switching during the PWM low clock phases  322  and by starting the PWM switching two cycles before the rising edge of signal PWM, eight switching events are eliminated and thus reduces switching loses by 40% (as an example). The amount of power savings generally increases for lower backlight levels (e.g., for lower loads or PWM duty cycles). 
     Deactivating the boost converter switching activity in this way may serve to improve the power efficiency of display  14 , in particular at “mid” to “light” load conditions.  FIG. 11  is a table illustrating different types of loading conditions. In the example of  FIG. 11 , current driver  122  may be configured to provide up to 60 mA of current when the duty cycle of PWM is set between zero and 30 percent, to provide between 60 mA and 140 mA of current when the duty cycle of PWM is set between 30 and 70 percent, and to provide up to 200 mA of current when the duty cycle of PWM is set between 70 and 100 percent. 
     When the amount of current that is delivered to light source  72  is within the first range (e.g., between zero and 60 mA), the current driver may be considered as being used to drive a “light” load, which corresponds to a relatively low backlight brightness level. When the amount of current that is delivered to light source  72  is within the second range (e.g., between 60 mA and 140 mA), the current driver may be considered as being used to drive a “medium” load, which corresponds to an intermediate backlight brightness level. When the amount of current that is delivered to light source  72  is within the third range (e.g., between 140 mA and 200 mA), the current driver may be considered as being used to drive a “heavy” load, which corresponds to a relatively high backlight brightness level. The different types of load conditions directly affect the amount of power consumed by current driver circuitry  122 . 
     Part of the power that is consumed by boost converter circuitry  120  may be independent of the current load condition. Specifically switching losses are for the most part independent of current load condition. The boost converter may, as an example, consume 5 mA of current whether or not the current driver is being used to drive a light load, medium load, or heavy load. The overall power efficiency of display  14  may depend on the combined power efficiency of the boost converter and the current driver. Since the amount of savings provided by reducing boost converter switching activity is fixed, the improvement offered by this approach is magnified at light to mid load conditions as the amount of power savings represents a larger percentage of the total power consumption at lighter load levels. 
       FIG. 12  is a diagram plotting display power efficiency versus different load conditions. Curve  400  may correspond to conventional boost converter switching schemes in which the boost converter is allowed to continuously switch during current driver down times (e.g., signal BST_GATE continues to toggle when PWM is deasserted), whereas curve  402  may correspond to the improved boost converter switching scheme that is described in connection with  FIG. 10 . In the example of  FIG. 12 , current levels that are less than 0.1 A may be considered as light load; current levels that are between 0.1 A and 0.25 A may be considered to be medium load; whereas current levels that are greater than 0.25 A may be considered as heavy load. As shown in  FIG. 12 , the amount of power efficiency improvement is enhanced at light to mid loading conditions (e.g., the gap between curves  400  and  402  is most pronounced when Iload is less than 0.25 A). 
       FIG. 13  is a flow chart of illustrative steps for operating the display circuitry of  FIG. 6 . At step  500 , backlight driver control circuitry  200  may generate signal PWM with a selected duty cycle to direct the backlight unit (e.g., backlight driver  104 , light source  72 , light guide plate  78 , and other associated backlight structures) to output the desired backlight level. 
     At step  502 , backlight driver control circuitry  200  may assert signal BST_EN a predetermined number of cycles before each rising edge of PWM to ensure that voltage Vboost recovers from any voltage droop (e.g., so that Vboost is properly reestablished before PWM clocks high). While BST_EN is asserted, boost converter control circuit  130  may pulse BST_GATE at the boost converter switching frequency to periodically activate boost converter circuitry  120  (at step  504 ). In other words, boost converter circuitry  120  may be continuously switched on/off when BST_EN is asserted. 
     In response to signal PWM rising high, current driver  122  may be enabled to drive the desired load (step  506 ). The amount of load that is currently being driven may depend on the duty cycle of PWM as determined by display timing controller  102 . For example, the duty cycle of PWM may be increased to drive a higher load when outputting higher backlight levels. 
     In response to signal PWM falling low, backlight driver control circuitry  200  may deassert BST_EN to prevent boost converter circuitry  104  from switching (at step  508 ). In other words, boost converter circuitry activity may be temporarily halted while BST_EN is deasserted. Processing may then loop back to step  500 , as indicated by path  510 . 
     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.

Metadata:
Filing Date: 20130130
Publication Date: 20160628
Grant Date: 20160628
Priority Date: 20121207
Inventors: HUSSAIN ASIF
CHEN JINGDONG
NAVABI-SHIRAZI MOHAMMAD JAFAR
PANDYA MANISHA
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G3/3406", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B45/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/062", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/062", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/062", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B33/086", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B33/0863", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 50880218