PATENT DOCUMENT

Publication Number: US-8884867-B2
Application Number: US-201113311474-A
Country: US
Kind Code: B2

Title: Efficient backlight short circuit protection

Abstract:
Systems, methods, and devices are provided for detecting short circuits in a backlight assembly without a resistor-based current sensor. For example, an electronic display according to the present disclosure may include a display panel and a backlight assembly to illuminate the display panel. The backlight assembly may drive a backlight element to illuminate the display panel and may include backlight short-circuit detection circuitry. The backlight short-circuit protection circuitry may detect a feedback voltage associated with the backlight element and determine when a short circuit has occurred based at least in part on the feedback voltage.

Claims:
What is claimed is: 
     
       1. An electronic display comprising:
 a display panel; and 
 a backlight assembly configured to illuminate the display panel at least partly by driving a backlight element, the backlight assembly comprising backlight short-circuit detection circuitry configured to detect a recent or current value of a feedback voltage associated with the backlight element and determine when a short circuit has occurred based at least in part on a comparison between the recent or current value of the feedback voltage and an expected value of the feedback voltage, wherein the expected value of the feedback voltage changes over time and the backlight assembly comprises configuration memory, wherein the backlight short-circuit detection circuitry is configured to determine the expected value of the feedback voltage based at least in part on a curve stored in the configuration memory, wherein the curve represents the expected value of the feedback voltage over time. 
 
     
     
       2. The electronic display of  claim 1 , wherein the feedback voltage comprises a voltage applied to the backlight element or a voltage-divided value of the voltage applied to the backlight element. 
     
     
       3. The electronic display of  claim 1 , wherein the backlight short-circuit detection circuitry is configured to determine when the short circuit has occurred based at least in part on a comparison between the recent or current value of the feedback voltage and a value of a backlight assembly input voltage. 
     
     
       4. Backlight driver circuitry configured to drive a backlight element, the backlight driver circuitry comprising:
 a power line FET configured to permit input power at a first input voltage to be received when the power line FET is activated; 
 boost converter circuitry configured to boost the first input voltage to a second input voltage at the backlight element; 
 an active current sink configured to draw current through the backlight element to drive the backlight element; and 
 control circuitry configured to control the power line FET, the boost converter circuitry, and the current sink to drive the backlight element, and detect whether a short circuit has occurred during a boost phase of operation based at least in part on a comparison between the second input voltage and an expected value of the second input voltage without using any resistor-based current measurement, wherein the expected value during the boost phase of operation is dynamic. 
 
     
     
       5. The backlight driver circuitry of  claim 4 , comprising first voltage sense circuitry configured to sense the first input voltage and second voltage sense circuitry configured to sense the second input voltage or a proportion of the second input voltage, wherein the control circuitry is configured to detect whether the short circuit has occurred during an inrush phase of operation based at least in part on a comparison between the first input voltage and the second input voltage or the proportion of the second input voltage. 
     
     
       6. The backlight driver circuitry of  claim 4 , comprising voltage sense circuitry configured to sense the second input voltage or a proportion of the second input voltage, wherein the control circuitry is configured to detect whether the short circuit has occurred during a normal phase of operation based at least in part on a comparison between the second input voltage or the proportion of the second input voltage and an expected value of the second input voltage or the proportion of the second input voltage. 
     
     
       7. The backlight driver circuitry of  claim 4 , wherein the control circuitry comprises a hardware state machine or a microcontroller, or a combination thereof, configured to detect whether the short circuit has occurred. 
     
     
       8. The backlight driver circuitry of  claim 4 , comprising a memory, wherein the control circuitry is configured to set a flag in the memory when a short circuit is detected to have occurred. 
     
     
       9. The backlight driver circuitry of  claim 4 , wherein the control circuitry is configured to deactivate the power line FET when the control circuitry detects that a short circuit has occurred. 
     
     
       10. An electronic device comprising:
 a processor configured to generate image data; and 
 an electronic display configured to display the image data, wherein the electronic display comprises a backlight assembly, the backlight assembly being configured to illuminate the electronic display by driving a backlight element and detect when a short circuit occurs in the backlight assembly during an inrush period, a boost soft start period, and a normal operation period occurring after the inrush period and the boost soft start period, wherein the backlight assembly is configured to detect when the short circuit occurs in the backlight assembly:
 during the inrush period, based at least in part on a comparison of a feedback voltage value associated with the backlight element and a value of a backlight assembly input voltage; and 
 during the boost soft start period and the normal operation period, based at least in part on a comparison of the feedback voltage value and an expected value of the feedback voltage value, wherein the expected value of the feedback voltage value is dynamic during the boost soft start period; 
 
 wherein the backlight assembly of the electronic display is configured to permanently cut off power to the backlight element when a short circuit is detected in the backlight assembly to protect the processor or other components of the electronic device. 
 
     
     
       11. The electronic device of  claim 10 , wherein the backlight assembly is configured to determine the expected value of the feedback voltage value based at least in part on the value of the backlight assembly input voltage. 
     
     
       12. A method comprising:
 sensing a backlight assembly input voltage during an inrush period of operation using backlight control circuitry; 
 sensing a feedback voltage associated with a voltage provided to a light-emitting element of the backlight assembly during operation using the backlight control circuitry; 
 determining in the backlight control circuitry whether a short circuit has occurred in the backlight assembly during the inrush period of operation before boosting the input voltage based at least in part on a comparison between the input voltage and the feedback voltage; 
 determining in the backlight control circuitry whether a short circuit has occurred in the back light assembly during a boost phase of operation based at least in part on a comparison between the feedback voltage and an expected value of the feedback voltage, wherein the expected value of the feedback voltage is non-static; and 
 when a short circuit has occurred, issuing a control signal from the backlight control circuitry to cut power to the backlight assembly. 
 
     
     
       13. The method of  claim 12 , wherein whether a short circuit has occurred during the inrush period is determined based at least in part on whether the feedback voltage has reached a threshold proportion of the input voltage. 
     
     
       14. The method of  claim 12 , comprising extrapolating the voltage provided to the light-emitting element of the backlight based at least in part on the feedback voltage using the backlight control circuitry, wherein whether the short circuit has occurred during the inrush period is determined based at least in part on whether the extrapolated value of the voltage provided to the light-emitting element of the backlight has reached a threshold proportion of the input voltage. 
     
     
       15. An article of manufacture comprising:
 non-transitory, tangible, machine-readable media configured to store processor-executable instructions, the instructions comprising:
 instructions to determine to test whether a short circuit has occurred in a backlight assembly of an electronic display; 
 instructions to receive a sensed feedback voltage value associated with a backlight element of the backlight assembly; 
 instructions to receive an expected value of the feedback voltage from memory; and 
 instructions to determine that the short circuit has occurred when the sensed feedback voltage value and the expected value of the feedback voltage differ by more than a threshold amount, wherein the expected value of the feedback voltage is non-static during a boost soft start period. 
 
 
     
     
       16. The article of manufacture of  claim 15 , wherein the instructions to determine to test whether the short circuit has occurred comprise instructions to set a timer when the backlight assembly enters a boost soft start phase and to determine to test whether the short circuit has occurred when the timer indicates a specified amount of time has passed. 
     
     
       17. The article of manufacture of  claim 16 , wherein the specified amount of time comprises less than half of the time of the boost soft start phase. 
     
     
       18. The article of manufacture of  claim 15 , wherein the instructions to determine to test whether the short circuit has occurred comprise instructions to occasionally determine to test whether the short circuit has occurred while the backlight assembly is operating in a normal operation phase after an inrush phase and a boost soft start phase. 
     
     
       19. The article of manufacture of  claim 15 , wherein the instructions to determine to test whether the short circuit has occurred comprise instructions to determine to test whether the short circuit has occurred during an inrush phase and a boost soft start phase such that field effect transistors of the backlight assembly will not be operating outside of a safe operating area (SOA). 
     
     
       20. The article of manufacture of  claim 15 , wherein the instructions to determine to test whether the short circuit has occurred comprise instructions to determine to test whether the short circuit has occurred within approximately 10% of an inrush phase or within approximately 10% of a boost soft start phase, or both. 
     
     
       21. The article of manufacture of  claim 15 , wherein the instructions to determine that the short circuit has occurred comprise instructions to determine that the short circuit has occurred when the sensed feedback voltage value amounts to less than approximately 75% of the expected value of the feedback voltage. 
     
     
       22. The article of manufacture of  claim 15 , comprising instructions to set a flag in the memory to indicate that the short circuit has occurred when the short circuit is determined to have occurred. 
     
     
       23. The article of manufacture of  claim 15 , comprising instructions to set a hiccup timer and to retest whether the short circuit has occurred when the hiccup timer elapses. 
     
     
       24. The article of manufacture of  claim 23 , wherein the hiccup timer lasts between approximately 10 ms and 10,000 ms.

Description:
BACKGROUND 
     The present disclosure relates generally to a backlight assembly for an electronic display and, more particularly, to a backlight assembly having efficient backlight short circuit protection. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Electronic displays, such as liquid crystal displays (LCDs), commonly appear in many different electronic devices. The brightness of an LCD depends on the amount of light provided by a backlight assembly. As the backlight assembly provides more light, the brightness of the LCD increases. Under certain circumstances, a short circuit may occur in the backlight assembly. When a short circuit occurs in the backlight assembly, other components of the electronic device could be damaged unless power to the backlight assembly is quickly cut off. As such, backlight assemblies typically include a resistor-based current sensor to detect when the amount of current drawn by the backlight assembly exceeds some threshold amount. Though a resistor-based current sensor may effectively indicate when a short circuit has occurred, the resistor-based current sensor continually wastes a portion of the power flowing to the backlight assembly as heat. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     Embodiments of the present disclosure relate to systems, methods, and devices for detecting short circuits in a backlight assembly without a resistor-based current sensor. For example, an electronic display according to the present disclosure may include a display panel and a backlight assembly to illuminate the display panel. The backlight assembly may drive a backlight element to illuminate the display panel and may include backlight short-circuit detection circuitry. The backlight short-circuit protection circuitry may detect a feedback voltage associated with the backlight element and determine when a short circuit has occurred based at least in part on the feedback voltage. 
     Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a schematic block diagram of an electronic device that incorporates a display with efficient backlight short-circuit protection, in accordance with an embodiment; 
         FIG. 2  is a perspective view of an example of the electronic device of  FIG. 1  in the form of a notebook computer, in accordance with an embodiment; 
         FIG. 3  is a front view of an example of the electronic device of  FIG. 1  in the form of a handheld electronic device, in accordance with an embodiment; 
         FIG. 4  is a front view of an example of the electronic device of  FIG. 1  in the form of a desktop computer, in accordance with an embodiment; 
         FIG. 5  is a schematic exploded view of various layers of the electronic display of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 6  is a schematic block diagram of a backlight assembly having backlight power and control circuitry with efficient backlight short-circuit protection, in accordance with an embodiment; 
         FIG. 7  is a circuit diagram representing a portion of the backlight assembly of  FIG. 6 , in accordance with an embodiment; 
         FIG. 8  is a flowchart describing a method for detecting a short-circuit condition in the backlight assembly during an inrush phase of operation of the backlight assembly, in accordance with an embodiment; 
         FIG. 9  is a flowchart describing a method for detecting a short-circuit condition in the backlight assembly during a boost soft start phase of operation of the backlight assembly, in accordance with an embodiment; 
         FIG. 10  is a plot modeling expected values of voltage supplied to a backlight element over the boost soft start phase, in accordance with an embodiment; and 
         FIG. 11  is a flowchart describing a method for detecting a short-circuit condition in the backlight assembly during a normal operation phase of the backlight assembly, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an example,” or the like, are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     As mentioned above, a liquid crystal display (LCD) may be illuminated by a backlight assembly. If a short circuit occurs in the backlight assembly, the LCD and/or other components of an electronic device in which the LCD is installed could be damaged. To detect and protect against short circuits without constantly wasting power as heat, a backlight assembly according to the present disclosure may not use a resistor-based current sensor. Rather, the backlight assembly may detect a short circuit based on a sensed value of a feedback voltage, which may be compared to an input voltage of the backlight assembly or an expected voltage value of the feedback voltage. 
     According to the present disclosure, a backlight assembly may include backlight short-circuit protection circuitry that can detect short-circuit conditions at various phases of the backlight assembly operation. During an inrush phase of operation, for example, the backlight short-circuit protection circuitry may compare an input voltage VIN to a feedback voltage related to the voltage supplied to a backlight element (e.g., a string of light emitting diodes (LEDs)). When the difference between the input voltage VIN and the backlight element fall outside of a threshold (e.g., the backlight element voltage is less than 75% of the input voltage VIN), a short-circuit condition may be understood to be occurring. Under these conditions, the backlight short-circuit protection circuitry may cause the power supply to the backlight assembly to be cut. The backlight short-circuit protection circuitry may set a flag in the memory indicating that a short circuit has been detected. 
     Likewise, during a boost soft start phase or during a normal operation phase of the backlight assembly, the backlight short-circuit protection circuitry may at certain times test for a short-circuit condition. To do so, the backlight short-circuit protection circuitry may obtain a sensed value of the feedback voltage. In addition, the backlight short-circuit protection circuitry may receive an expected value of the feedback voltage from memory accessible to the backlight short-circuit protection circuitry. This expected value of the feedback voltage may represent a voltage value that would be expected to be present under non-short-circuit conditions. When the sensed value of the feedback voltage differs from the expected value of the feedback voltage by more than some threshold, a short-circuit condition may be understood to be occurring. As such, the backlight short-circuit protection circuitry may cause the power supply to the backlight assembly to be cut. The backlight short-circuit protection circuitry may set a flag in the memory indicating that a short circuit has been detected. 
     With the foregoing in mind, a general description of suitable electronic devices that may employ electronic displays with backlight short-circuit protection capabilities will be provided below. In particular,  FIG. 1  is a block diagram depicting various components that may be present in an electronic device suitable for use with such a display.  FIGS. 2 ,  3 , and  4  illustrate various examples of suitable electronic devices in the form of a notebook computer, a handheld electronic device, and a desktop computer, respectively. 
     Turning first to  FIG. 1 , an electronic device  10  according to an embodiment of the present disclosure may include, among other things, one or more processor(s)  12 , memory  14 , nonvolatile storage  16 , a display  18  having backlight short-circuit protection circuitry  20 , input structures  22 , an input/output (I/O) interface  24 , network interfaces  26 , and a power source  28 . The various functional blocks shown in  FIG. 1  may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should be noted that  FIG. 1  is merely one example of a particular implementation and is intended to illustrate the types of components that may be present in electronic device  10 . 
     By way of example, the electronic device  10  may represent a block diagram of the notebook computer depicted in  FIG. 2 , the handheld device depicted in  FIG. 3 , the desktop computer depicted in  FIG. 4 , or similar devices. It should be noted that the processor(s)  12  and/or other data processing circuitry may be generally referred to herein as “data processing circuitry.” Such data processing circuitry may be embodied wholly or in part as software, firmware, hardware, or any combination thereof. Furthermore, the data processing circuitry may be a single contained processing module or may be incorporated wholly or partially within any of the other elements within the electronic device  10 . 
     In the electronic device  10  of  FIG. 1 , the processor(s)  12  and/or other data processing circuitry may be operably coupled with the memory  14  and the nonvolatile memory  16  to execute instructions to carry out various functions of the electronic device  10 . Among other things, these functions may include generating image data to be displayed on the display  18 . The programs or instructions executed by the processor(s)  12  may be stored in any suitable article of manufacture that includes one or more tangible, computer-readable media at least collectively storing the instructions or routines, such as the memory  14  and/or the nonvolatile storage  16 . The memory  14  and the nonvolatile storage  16  may represent, for example, random-access memory, read-only memory, rewritable flash memory, hard drives, and optical discs. Also, programs (e.g., an operating system) encoded on such a computer program product may also include instructions that may be executed by the processor(s)  12  to enable other functions of the electronic device  10 . 
     The display  18  may be a touch-screen liquid crystal display (LCD), for example, which may enable users to interact with a user interface of the electronic device  10 . By way of example, the display  18  may be a MultiTouch™ display that can detect multiple touches at once. The display  18  may include backlight short-circuit protection circuitry  20  to efficiently detect short-circuit conditions that may arise in a backlight assembly of the display  18 . Since a resistor-based current sensor could inefficiently dissipate substantial amounts of power over time, the backlight short-circuit protection circuitry  20  may detect short-circuit conditions in other ways. Namely, the backlight short-circuit protection circuitry  20  may, at certain times, detect voltage(s) relating to a backlight element of the display  18 . Based on these detected voltage value(s), the backlight short-circuit protection circuitry  20  may determine whether a short circuit has occurred without using feedback from a resistor-based current sensor. 
     The input structures  22  of the electronic device  10  may enable a user to interact with the electronic device  10  (e.g., pressing a button to increase or decrease a volume level). The I/O interface  24  may enable electronic device  10  to interface with various other electronic devices, as may the network interfaces  26 . The network interfaces  26  may include, for example, interfaces for a personal area network (PAN), such as a Bluetooth network, for a local area network (LAN), such as an 802.11x Wi-Fi network, and/or for a wide area network (WAN), such as a 3G or 4G cellular network. The power source  28  of the electronic device  10  may be any suitable source of power, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. 
     The electronic device  10  may take the form of a computer or other type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. By way of example, the electronic device  10 , taking the form of a notebook computer  30 , is illustrated in  FIG. 2  in accordance with one embodiment of the present disclosure. The depicted computer  30  may include a housing  32 , a display  18 , input structures  22 , and ports of an I/O interface  24 . The input structures  22 , such as a keyboard and/or touchpad, may be used to interact with the computer  30 . Via the input structures  22 , a user may start, control, or operate a GUI or applications running on computer  30 . 
     The display  18  of the computer  30  may be a backlit liquid crystal display (LCD). When the display  18  includes the backlight short-circuit protection circuitry  20 , the computer  30  may be largely protected from backlight short-circuit conditions that could damage other components of the computer  30  if not detected. As mentioned above, the display  18  may detect short-circuit conditions without relying on a resistor-based current sensor. Rather, the backlight short-circuit protection circuitry  20  of the display  18  may detect short circuits by occasionally sensing voltage(s) in the backlight assembly of the display  18 . Based at least partly on the sensed voltage(s), the backlight short-circuit protection circuitry  20  of the display  18  may determine if a short circuit has occurred. 
       FIG. 3  depicts a front view of a handheld device  34 , which represents one embodiment of the electronic device  10 . The handheld device  34  may represent, for example, a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. By way of example, the handheld device  34  may be a model of an iPod® or iPhone® available from Apple Inc. of Cupertino, Calif. In other embodiments, the handheld device  34  may be a tablet-sized embodiment of the electronic device  10 , which may be, for example, a model of an iPad® available from Apple Inc. 
     The handheld device  34  may include an enclosure  36  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  36  may surround the display  18 , which may display indicator icons  38 . The indicator icons  38  may indicate, among other things, a cellular signal strength, Bluetooth connection, and/or battery life. The I/O interfaces  24  may open through the enclosure  36  and may include, for example, a proprietary I/O port from Apple Inc. to connect to external devices. 
     User input structures  40 ,  42 ,  44 , and  46 , in combination with the display  18 , may allow a user to control the handheld device  34 . For example, the input structure  40  may activate or deactivate the handheld device  34 , the input structure  42  may navigate user interface  20  to a home screen, a user-configurable application screen, and/or activate a voice-recognition feature of the handheld device  34 , the input structures  44  may provide volume control, and the input structure  46  may toggle between vibrate and ring modes. A microphone  48  may obtain a user&#39;s voice for various voice-related features, and a speaker  50  may enable audio playback and/or certain phone capabilities. A headphone input  52  may provide a connection to external speakers and/or headphones. 
     Like the display  18  of the computer  30 , the display  18  of the handheld device  34  may be a backlit liquid crystal display (LCD). Backlight short-circuit protection circuitry  20  associated with the display  18  may protect other components of the handheld device  34  from damage that could occur from backlight short-circuit conditions. As mentioned above, the display  18  may detect short-circuit conditions without relying on a resistor-based current sensor. Rather, the backlight short-circuit protection circuitry  20  of the display  18  may detect short circuits by occasionally sensing voltage(s) in the backlight assembly of the display  18 . Based at least partly on the sensed voltage(s), the backlight short-circuit protection circuitry  20  of the display  18  may determine if a short circuit has occurred. 
     The electronic device  10  also may take the form of a desktop computer  56 , as generally illustrated in  FIG. 4 . In certain embodiments, the electronic device  10  in the form of the desktop computer  56  may be a model of an iMac®, Mac® mini, or Mac Pro® available from Apple Inc. The desktop computer  56  may include a housing  58 , a display  18 , and input structures  22 , among other things. The input structures  22 , such as a wireless keyboard and/or mouse, may be used to interact with the desktop computer  56 . Via the input structures  22 , a user may start, control, or operate a GUI or applications running on the desktop computer  56 . 
     The display  18  may be a backlit liquid crystal display (LCD). When the display  18  includes the backlight short-circuit protection circuitry  20 , the desktop computer  56  may be largely protected from backlight short-circuit conditions that could damage other components of the desktop computer  56  if not detected. As mentioned above, the display  18  may detect short-circuit conditions without relying on a resistor-based current sensor. Rather, the backlight short-circuit protection circuitry  20  of the display  18  may detect short circuits by occasionally sensing voltage(s) in the backlight assembly of the display  18 . Based at least partly on the sensed voltage(s), the backlight short-circuit protection circuitry  20  of the display  18  may determine if a short circuit has occurred. 
     Regardless of whether the electronic device  10  takes the form of the computer  30  of  FIG. 2 , the handheld device  34  of  FIG. 3 , the desktop computer  56  of  FIG. 4 , or some other form, the display  18  of the electronic device  10  may form an array or matrix of picture elements (pixels). By varying an electric field associated with each pixel, the display  18  may control the orientation of liquid crystal disposed at each pixel. The orientation of the liquid crystal of each pixel may permit more or less light emitted from a backlight to pass through each pixel. The display  18  may employ any suitable technique to manipulate these electrical fields and/or the liquid crystals. For example, the display  18  may employ transverse electric field modes in which the liquid crystals are oriented by applying an in-plane electrical field to a layer of the liquid crystals. Examples of such techniques include in-plane switching (IPS) and/or fringe field switching (FFS) techniques. 
     By controlling of the orientation of the liquid crystals, the amount of light emitted by the pixels may change. Changing the amount of light emitted by the pixels will change the colors perceived by a user of the display  18 . Specifically, a group of pixels may include a red pixel, a green pixel, and a blue pixel, each having a color filter of that color. By varying the orientation of the liquid crystals of different colored pixels, a variety of different colors may be perceived by a user viewing the display. It may be noted that the individual colored pixels of a group of pixels may also be referred to as unit pixels. 
     With the foregoing in mind,  FIG. 5  depicts an exploded view of different layers of a pixel of the display  18 . The pixel  60  includes an upper polarizing layer  64  and a lower polarizing layer  66  that polarize light emitted by a backlight assembly  68 . Although not visible in  FIG. 5 , the backlight assembly  68  includes the backlight short-circuit protection circuitry  20  discussed throughout this disclosure. A lower substrate  72  is disposed above the polarizing layer  66  and is generally formed from a light-transparent material, such as glass, quartz, and/or plastic. 
     A thin film transistor (TFT) layer  74  appears above the lower substrate  72 . For simplicity, the TFT layer  74  is depicted as a generalized structure in  FIG. 5 . In practice, the TFT layer may itself include various conductive, non-conductive, and semiconductive layers and structures that generally form the electrical devices and pathways that drive the operation of the pixel  60 . The TFT layer  74  may also include an alignment layer (formed from polyimide or other suitable materials) at the interface with a liquid crystal layer  78 . 
     The liquid crystal layer  78  includes liquid crystal particles or molecules suspended in a fluid or gel matrix. The liquid crystal particles may be oriented or aligned with respect to an electrical field generated by the TFT layer  74 . The orientation of the liquid crystal particles in the liquid crystal layer  78  determines the amount of light transmission through the pixel  60 . Thus, by modulation of the electrical field applied to the liquid crystal layer  78 , the amount of light transmitted though the pixel  60  may be correspondingly modulated. 
     Disposed on the other side of the liquid crystal layer  78  from the TFT layer  74  may be one or more alignment and/or overcoating layers  82  interfacing between the liquid crystal layer  78  and an overlying color filter  86 . The color filter  86  may be a red, green, or blue filter, for example. Thus, each pixel  60  corresponds to a primary color when light is transmitted from the backlight assembly  68  through the liquid crystal layer  78  and the color filter  86 . 
     The color filter  86  may be surrounded by a light-opaque mask or matrix, represented here as a black mask  88 . The black mask  88  circumscribes the light-transmissive portion of the pixel  60 , delineating the pixel edges. The black mask  88  may be sized and shaped to define a light-transmissive aperture over the liquid crystal layer  78  and around the color filter  86 . In addition, the black mask  88  may cover or mask portions of the pixel  60  that do not transmit light, such as the scanning line and data line driving circuitry, the TFT, and the periphery of the pixel  60 . In the example of  FIG. 5 , an upper substrate  92  may be disposed between the black mask  88  and color filter  86  and the polarizing layer  64 . The upper substrate  92  may be formed from light-transmissive glass, quartz, and/or plastic. 
     The backlight assembly  68  provides light to illuminate the display  18 . As seen in  FIG. 6 , the backlight assembly  68  may include, among other things, one or more backlight elements  100  such as light emitting diode (LED) strings  102 . Although the backlight elements  100  in  FIG. 6  are shown to be LED strings  102 , additionally or alternatively, any other suitable light-emitting backlight elements  100  may be employed. For example, one or more cold cathode lighting elements may be used in lieu of, or in addition to, the LED strings  102 . Moreover, although the LED strings  102  of the backlight assembly  68  schematically appear to be disposed in discrete locations apart from one another, the LED strings  102  may be interleaved among one another. 
     Backlight driver circuitry, here illustrated as backlight power and control circuitry  104 , may drive the LED strings  102  to emit light  106 . In the example of  FIG. 6 , the backlight assembly  68  is shown to be edge-lit. That is, the backlight elements  100  may be located at the edge of a diffuser  108 , rather than directly underneath. The light  106  may enter the light diffuser  108 , which may cause the light  106  to be diffused substantially evenly. Additionally, the light diffuser  108  may cause the light to pass up through the other layers of the display  18 , which have been generally discussed above with reference to  FIG. 5 . While the backlight assembly  68  of  FIG. 6  is represented as an edge-lit backlight assembly  68 , other arrangements are possible. Indeed, the backlight elements  100  may be disposed in any suitable arrangement, including being disposed beneath or behind the backlight diffuser  108 . 
     The backlight power and control circuitry  104  may control the brightness of the display  18  by varying the amount of light  106  emitted by the LED strings  102 . For example, the backlight power and control circuitry  104  may employ any suitable form of pulse width modulation (PWM) to drive the LED strings  102 . By varying the duty cycle over which the LED strings  102  are driven over PWM clock cycles, the light perceived by a user of the display may be increased or decreased. 
     On rare occasion, the power used to drive the LED strings  102  could result in a short circuit. Such a short circuit could damage other components of the electronic device  10 . As such, the backlight power and control circuitry  104  may include the backlight short-circuit protection  20 . When a short circuit occurs in the backlight assembly  68 , the backlight short-circuit protection circuitry  20  may detect the short circuit and shut off power to the backlight elements  100 . Although doing so will render the electronic device  10  in which the display  18  is installed effectively unusable, other components of the electronic device  10  will be protected, and it may be possible to repair the electronic device  10  in the future. 
     A circuit diagram of  FIG. 7  illustrates a relationship between the backlight power and control circuitry  104  and circuitry used to control the backlight elements  100  (e.g., the LED strings  102 ). A hardware state machine (HSM) and/or microcontroller (μC)  110  may generally govern the operation of the backlight power and control circuitry  104 . The HSM and/or μC  110  may also include the backlight short-circuit protection circuitry  20 , the general operation of which will be discussed further below. To enable the HSM and/or μC  110  to control the manner in which the backlight elements  100  are driven with power, the backlight power and control circuitry  104  may include current sinks  112 , voltage sense circuitry  114 , a boost block  116 , current sink  118 , and configuration memory  120 . 
     These components may enable the backlight power and control circuitry  104  to control three distinct phases of backlight assembly  68  operation: an inrush phase, in which input power initially enters the power supply circuitry of the backlight assembly  68  at an input voltage VIN; a boost soft start phase, in which the boost block  116  boosts the voltage to a level sufficient to drive the backlight elements  100  (e.g., the LED string  102 ); and a normal operation phase, in which the current sink  118  drives the backlight elements  100  (e.g., the LED strings  102 ) by drawing current through them according to some pattern (e.g., a pulse width modulation (PWM) duty cycle). Since short circuits could potentially occur during any of these phases of operation, the backlight short-circuit protection circuitry  20  may take different measures to detect short-circuit conditions. In each case, the backlight short-circuit protection circuitry  20  may not rely on a resistor-based current sensor, which is not present in the example of the backlight assembly  68  of  FIG. 7 . Instead, the backlight short-circuit protection circuitry  20  may use sensed voltage value(s) and/or expected voltage values stored in the configuration memory  120  to determine whether a short circuit has occurred. 
     The inrush phase may begin when the HSM and/or μC  110  cause the current sinks  112  to activate a power line field effect transistor (FET) PLF 1 . The current sinks  112  may be used by the HSM and/or μC  110  to control the slew rate of the power line FET PLF 1  and, by extension, to control the length of time of the inrush phase. Specifically, by applying a gate current I G1 , I G2 , and/or I G3  from the current sinks  112  to the gate of the power line FET PLF 1 , the HSM and/or μC  110  may control the slew rate of the power line FET PLF 1 . It should be understood that the resistor R 1  shown in  FIG. 7  may optionally be present, but may not be present in other embodiments. When the power line FET PLF 1  is activated, an input voltage VIN from an external power supply may be supplied to the backlight assembly  68 . As a result, an inrush current may enter the circuitry beyond power line FET PLF 1  into a capacitance C 1  and through an inductance L 1  toward a LED string  102 . The amount of time required to complete this inrush phase, also referred to herein as the inrush period Tinrush, may depend upon the slew rate of the power line FET PLF 1 . In general, the HSM and/or μC  110  may select which of the current sinks  112  to apply based on a programmed value of Tinrush stored in the configuration memory  120 . For example, a value set in the configuration memory  120  may set the inrush period Tinrush to one of a variety of suitable values (e.g., 5 ms, 50 ms, 100 ms, or 500 ms, or the like). Depending on the programmed value of Tinrush, the HSM and/or μC  110  may select different of the current sinks  112 , varying the slew rate of the power line FET PLF 1  and, accordingly, the inrush current. The HSM and/or μC  110  may also deactivate the power line FET PLF 1  by grounding the gate of the power line FET PLF 1 , a condition selectable from among the current sinks  112 . 
     During the inrush phase, power may flow to the inputs of all the LED strings  102  of the backlight assembly  68 . It should be noted, however, that  FIG. 7  illustrates circuitry to drive only one of the LED strings  102 . For clarity, like circuitry may be used to drive the other LED strings  102 , the start of which is generally represented at numeral  122 . In particular, the circuitry associated with the boost block  116  and the current sink  118  shown in  FIG. 7  may operate exclusively with a single one of the LED strings  102 . That is, the inductance L 1 , a diode D 1 , a switching FET SWF 1 , resistors R 2 , R 3 , and R 4 , a current sink FET CSF 1 , and certain functionalities of the boost block  116  and current sink  118  shown in  FIG. 7  may be associated exclusively with driving a single one of the LED strings  102 . For clarity, only one LED string  102  and its associated driving circuitry are shown in  FIG. 7 . It should be understood, however, that an actual implementation may employ additional like circuitry from numeral  122  to drive each of the other LED strings  102  of the backlight assembly  68 . 
     The backlight short-circuit protection circuitry  20  may use sensed feedback voltage values during the inrush phase to protect the backlight assembly  68 , the display  18  in which the backlight assembly  68  is installed, and/or the electronic device  10  in which the display  18  is installed from short-circuit conditions arising in the backlight assembly  68 . Specifically, the backlight short-circuit protection circuitry  20  may receive a value of the input voltage VIN from the voltage sense block  114  and a feedback voltage VFB value from the boost block  116 . It should be understood that the feedback voltage VFB may directly relate to the LED string  102  input voltage Vstring through a voltage divider circuit formed by the resistors R 3  and R 4 . For example, when the resistors R 3  and R 4  are of the same value, the feedback voltage VFB will be approximately half the value of the LED string  102  input voltage Vstring. As will be discussed below with reference to  FIG. 8 , a comparison between the feedback voltage VFB and the LED string  102  input voltage Vstring may indicate whether the a short-circuit condition has occurred. When the backlight short-circuit protection circuitry  20  detects a short circuit, the HSM and/or μC  110  may cut off power to the backlight assembly  68  by deactivating the power line FET PLF 1 . In some embodiments, if the configuration memory  120  indicates that a hiccup timer has been enabled, the backlight short-circuit protection circuitry  20  may allow the backlight assembly  68  to restart operation and retest for short circuits after a period of time. 
     The boost soft start phase may begin after the inrush phase. During the boost soft start phase, the boost block  116  may boost the voltage from the input voltage VIN to a voltage high enough to drive the LED string  102 . Specifically, the boost block  116  may vary a switching signal SGD supplied to the switching FET SWF 1 . The current may flow through the inductance L 1 , the switching FET SWF 1 , and the resistor R 2  at a higher rate than otherwise. Because of the inductance L 1 , this higher rate of current will continue to flow even when the switching FET SWF 1  is switched off, flowing through the diode D 1  and the resistors R 3  and R 4  at this higher rate and increasing the LED string  102  input voltage Vstring accordingly. The boost block  116  may determine the duty cycle of the switching signal SGD by sensing the feedback voltage VFB that occurs between the resistors R 3  and R 4  and the current sink block  118 . Since the feedback voltage VFB correlates with the LED string  102  input voltage Vstring, and the switching signal SGD duty cycle impacts the degree to which the voltage is boosted, the boost block  116  may vary the switching signal SGD frequency based on the feedback voltage VFB to achieve a desired LED string  102  input voltage Vstring. 
     The backlight short-circuit protection circuitry  20  may also use the feedback voltage VFB sensed by the boost block  116  to ascertain whether a short circuit has occurred during the boost soft start phase. Specifically, as will be discussed below with reference to  FIGS. 9 and 10 , the backlight short-circuit protection circuitry  20  may compare the feedback voltage VFB (or an estimate of the LED string  102  input voltage Vstring determined from the feedback voltage VFB) to an expected voltage value stored in the configuration memory  120  to determine whether a short circuit has occurred. When the backlight short-circuit protection circuitry  20  detects a short circuit, the HSM and/or μC  110  may cut off power to the backlight assembly  68  by deactivating the power line FET PLF 1 . In some embodiments, if the configuration memory  120  indicates that a hiccup timer has been enabled, the backlight short-circuit protection circuitry  20  may allow the backlight assembly  68  to restart operation and retest for short circuits after a period of time. 
     Following the boost soft start phase, the LED string  102  input voltage Vstring may be sufficiently high to drive the LED string  102  during a normal operation phase. As such, the backlight assembly  68  may enter a phase of normal operation, during which the LED string  102  may be driven according to varying patterns (e.g., pulse width modulation (PWM) duty cycles) to achieve corresponding brightness levels. To cause the LED string  102  to emit light, the current sink  118  may activate a current sink FET CSF 1  using a power voltage VP signal and draw a string current Istring through to ground. While the current sink  118  has activated the current sink FET CSF 1  and is drawing the string current Istring through the LED string  102 , the LED string  102  will emit light, illuminating the electronic display  18 . By varying the ratio of time the LED string  102  is on and emitting light to the time the LED string  102  is off and is not emitting light (i.e., the duty cycle of the LED string  102 ), the current sink  118  may set the perceived brightness of the display  18  to various dimming levels. 
     During the normal operation phase, the backlight short-circuit protection circuitry  20  may occasionally (e.g., periodically or when desired) test for short circuits using the feedback voltage VFB sensed by the boost block  116 . Specifically, as will be discussed below with reference to  FIG. 11 , the backlight short-circuit protection circuitry  20  may compare the feedback voltage VFB (or an estimate of the LED string  102  input voltage Vstring determined from the feedback voltage VFB) to an expected voltage value stored in the configuration memory  120  to determine whether a short circuit has occurred. When the backlight short-circuit protection circuitry  20  detects a short circuit, the HSM and/or μC  110  may cut off power to the backlight assembly  68  by deactivating the power line FET PLF 1 . In some embodiments, if the configuration memory  120  indicates that a hiccup timer has been enabled, the backlight short-circuit protection circuitry  20  may allow the backlight assembly  68  to restart operation and retest for short circuits after a period of time. 
     As mentioned above, the operation of the backlight power and control circuitry  104  may be influenced by values stored in the configuration memory  120 , which may represent any suitable memory to store operational parameters of the backlight power and control circuitry. For example, the configuration memory  120  may represent electrically erasable programmable read only memory (EEPROM), flash memory, read only memory (ROM), random access memory (RAM) programmed by a component of the electronic device  10 , or any other suitable form of memory. By way of example, operational parameters of the backlight assembly  68  that may be stored in the configuration memory  120  may include a selectable inrush period Tinrush (e.g., 5 ms, 50 ms, 100 ms, and/or 500 ms, or the like), a hiccup timer enable flag, and/or a selectable hiccup timer setting (e.g., 100 ms or 1000 ms, or the like). As will be described further below, the hiccup timer may be used to ensure that a false positive detection of a short circuit does not cause the backlight assembly  68  to be permanently disabled by the backlight short-circuit protection circuitry  20 . 
     The backlight short-circuit protection circuitry  20  of the HSM and/or μC  110  may detect and protect against short-circuit conditions in somewhat different ways during the various phases of operation described above. As will be discussed below,  FIG. 8  represents a manner of detecting short circuits during the inrush phase,  FIGS. 9 and 10  represent a manner of detecting short circuits during the boost soft start phase, and  FIG. 11  represents a manner of detecting short circuits during the phase of normal operation. It should be understood that the methods of the flowcharts of  FIGS. 8 ,  9 , and  11  may be carried out by the backlight short-circuit protection circuitry  20  as hard-coded in hardware as a hardware state machine (HSM) or as instructions running on a microcontroller (μC). 
     Turning to  FIG. 8 , a flowchart  130  represents a manner of detecting short-circuit conditions in the backlight assembly  68 . The flowchart  130  may begin as the inrush phase of operation of the backlight assembly  68  begins (block  132 ). This inrush phase may start when the HSM and/or μC  100  causes the power line FET PLF 1  to be activated, causing current to rush into some of the backlight assembly  68  circuitry. The backlight short-circuit protection circuitry  20  may begin a timer measuring the time elapsed since the start of the inrush period (block  134 ). After some amount of time has passed, represented here as 10% of the inrush period Tinrush, has completed (decision block  136 ), the backlight power and control circuitry  104  may sense the input voltage VIN and the feedback voltage VFB (block  138 ). Although the decision block  136  illustrates the value of 10% of the inrush period Tinrush, any other suitable value may be contemplated. In general, since a short circuit is likely to become apparent relatively early in the inrush phase, it may be useful to sense the voltages VIN and VFB earlier in the inrush phase rather than later. The value 10% has been provided as one example that ensures the FETs of the backlight assembly  68  operate within a safe operating area (SOA). It should be appreciated that this value may vary in an actual implementation, but generally may be selected such that the FETs of the backlight assembly  68  operate within an SOA. 
     The feedback voltage VFB correlates directly with the LED string  102  input voltage Vstring. Thus, the backlight short-circuit protection circuitry  20  may estimate the LED string  102  input voltage Vstring from the feedback voltage VFB (block  140 ). For example, when the resistors R 2  and R 3  are equivalent, Vstring will be approximately equal to double the feedback voltage VFB. Alternatively, the backlight short-circuit protection circuitry  20  may not estimate the LED string  102  input voltage Vstring. Thus, the backlight short-circuit protection circuitry  20  may compare the input voltage VIN and the LED string  102  input voltage Vstring (block  142 ) or, alternatively, the input voltage VIN and the feedback voltage VFB, to determine whether a short-circuit condition is present. 
     Under non-short-circuit conditions, the input voltage VIN and the string voltage Vstring should be approximately equal, less any efficiency losses due to circuit elements between the input voltage VIN and Vstring (e.g., a voltage reduction due to the diode D 1 ). When a short circuit is occurring, however, the input voltage VIN may be significantly higher than the LED string  102  input voltage Vstring. Thus, when a difference between the input voltage VIN and the LED string  102  input voltage Vstring is within some threshold value (decision block  144 ), it may be understood that a short circuit is not occurring. That is, depending on the efficiency losses of the circuitry from VIN to Vstring, when Vstring is within some threshold value (e.g., is approximately 75%) of VIN, a short circuit may be understood not to be occurring. The threshold value may be a programmable value stored in the configuration memory  120  or may be, for example, hard-coded in the backlight short-circuit protection circuitry  20 . 
     When the difference between the input voltage VIN and the LED string  102  input voltage Vstring is within the threshold value (decision block  144 ), the backlight short-circuit protection circuitry  20  may determine that a short circuit is not occurring. Accordingly, the backlight power and control circuitry  104  may continue as normal in the inrush phase in preparation for the boost soft start phase (block  146 ). When the difference between the input voltage and the string voltage is not within the threshold value (decision block  144 ), it may be understood that a short-circuit condition has been detected, and thus the backlight power and control circuitry  104  may be directed by the backlight short-circuit protection circuitry  20  to disconnect the power line FET PLF 1  (block  148 ). Doing so could render the display  18  unusable, given that the backlight assembly  68  will no longer illuminate the display  18 . Disconnecting the power line FET PLF 1  will, however, protect other components of the electronic device  10 , making it possible to refurbish the electronic device  10  in the future. In addition to disconnecting the power line FET PLF 1 , the backlight short-circuit protection circuitry  20  may also cause the configuration memory  120  to store an indication that a short-circuit condition was detected during the inrush phase of the operation of the backlight assembly  68 . 
     Although the flowchart  130  of  FIG. 8  illustrated a manner of detecting short circuits by comparing the input voltage VIN and the LED string  102  input voltage Vstring, alternative manners of detecting short circuits during the inrush phase may be employed. For example, the backlight short-circuit protection circuitry  20  may test the LED string  102  input voltage Vstring or the feedback voltage VFB against an expected voltage value Vexp. The value of Vexp may be stored in the configuration memory  120  and may represent the value of the LED string  102  input voltage Vstring or the value of the feedback voltage VFB expected at, for example, 10% of the inrush period. Additionally, the method described in the flowchart  130  of  FIG. 8  may involve subsequent retesting for short-circuit conditions using a hiccup timer, as generally discussed below with reference to  FIGS. 9 and 11 . 
     Assuming no short circuits are detected during the inrush period, the backlight short-circuit protection circuitry  20  may continue to monitor whether a short-circuit condition occurs during the boost soft start phase, as generally represented by a flowchart  160  of  FIG. 9 . The flowchart  160  may begin when the backlight power and control circuitry  104  causes the backlight assembly  68  to enter the boost soft start phase (block  162 ). As mentioned above, the boost soft start phase may involve increasing the voltage of the LED string  102  input voltage Vstring via the boost block  116 . Some amount of time after the boost soft start phase begins, such as 10% of the boost soft start period (decision block  164 ), the value of the feedback voltage VFB may be sensed (block  166 ). Although the decision block  164  of  FIG. 9  indicates that the feedback voltage VFB is sensed after 10 percent of the boost soft start phase has begun, the feedback voltage VFB may be sensed any suitable amount of time after the boost soft start phase has begun. In general, since a short circuit is likely to become apparent relatively early in the boost soft start phase, it may be useful to sense the feedback voltage VFB earlier in the boost soft start phase rather than later. 
     As mentioned above, the feedback voltage VFB correlates directly with the LED string  102  input voltage Vstring. Thus, the backlight short-circuit protection circuitry  20  may estimate the LED string  102  input voltage Vstring from the feedback voltage VFB (block  168 ). For example, when the resistors R 2  and R 3  are equivalent, Vstring will be approximately equal to double the feedback voltage VFB. Alternatively, the backlight short-circuit protection circuitry  20  may not estimate the LED string  102  input voltage Vstring. The backlight short-circuit protection circuitry  20  may compare the LED string  102  input voltage Vstring to an expected voltage value Vexp (representing an expected value of Vstring) (block  170 ) or, alternatively, the feedback voltage VFB to an expected voltage Vexp (representing an expected value of VFB), to determine whether a short-circuit condition is present. 
     Specifically, during the boost soft start phase, the LED string  102  input voltage Vstring gradually increases as it is boosted to the voltage level sufficient to drive the LED string  102 . The LED string  102  input voltage Vstring generally increases in a predictable manner. Thus, an expected voltage Vexp can be stored in the configuration memory  120 . As mentioned above, the backlight short-circuit protection circuitry  20  may compare the estimated value of the string voltage Vstring to the expected voltage value Vexp (representing an expected value of Vstring) (block  170 ) or, alternatively, the feedback voltage VFB to an expected voltage Vexp (representing an expected value of VFB), to determine whether a short-circuit condition is present. If a difference between the expected voltage Vexp and the string voltage Vstring (or a difference between the expected voltage Vexp and the feedback voltage VFB) is less than some threshold value (decision block  172 ), a short-circuit condition has not been detected. As such, the backlight power and control circuitry  104  may prepare for a normal phase of operation (block  174 ). The threshold value may be a programmable value stored in the configuration memory  120  or may be, for example, hard-coded in the backlight short-circuit protection circuitry  20 . The threshold referred to in  FIG. 9  may be a different threshold than the threshold referred to in  FIG. 8 . 
     If the difference between the expected voltage Vexp and the LED string  102  input voltage Vstring (or the difference between the expected voltage Vexp and the feedback voltage VFB) does exceed the threshold (decision block  172 ), it may be understood that a short-circuit condition is occurring in the backlight assembly  68 . Thus, the backlight short-circuit protection circuitry  20  may disconnect the power line FET PLF 1  to cut power to the backlight assembly  68  (block  176 ). Additionally, the backlight short-circuit protection circuitry  20  may cause the configuration memory  120  to store an indication that a short-circuit condition was detected during the boost soft start phase of the operation of the backlight assembly  68 . 
     In the example illustrated in  FIG. 9 , the backlight short-circuit protection circuitry  20  may seek to avoid false positive short-circuit conditions by entering a hiccup mode, if enabled in the configuration memory  120  (decision block  178 ). As mentioned briefly above, the hiccup mode may allow for retesting whether a short circuit has actually occurred. If the hiccup mode has not been enabled (decision block  178 ), the power line FET PLF 1  may remain disconnected (block  180 ). Otherwise, the backlight power and control circuitry  104  may set a hiccup timer for a particular amount of time (block  182 ). The hiccup timer may last any suitable length of time, and may be a programmable setting in the configuration memory  120 . For example, the hiccup timer may be a value between approximately 10 ms to 10,000 ms. In one example, the hiccup timer may last either 100 ms or 1000 ms depending on a setting in the configuration memory  120 . When the hiccup timer ends, the backlight power and control circuitry  104  may restart the operation of the backlight assembly  68  starting from the inrush phase (block  184 ). If the short circuit is not determined to occur again, the previously determined short-circuit condition may be understood to have been a false positive. 
     As mentioned above, the LED string  102  input voltage Vstring may increase over the boost soft start phase. The feedback voltage VFB may increase accordingly. Thus, the value of the expected voltage Vexp used by the backlight short-circuit protection circuitry  20  to determine whether a short circuit has occurred may be set to take these changes into account. Depending on what point during the boost soft start phase the feedback voltage VFB is tested, the expected voltage Vexp may be higher or lower. 
     One example of how the expected voltage Vexp may vary with time appears in a plot  190  of  FIG. 10 . An ordinate  192  of the plot  190  represents the expected voltage Vexp value of either the LED string  102  input voltage Vstring or the feedback voltage VFB. An abscissa  194  represents time over the boost soft start phase. Between the start of the boost soft start phase  196  and the end of the boost soft start phase  198 , a curve  200  representing the change in the expected voltage Vexp of the LED string  102  input voltage Vstring or the feedback voltage VFB gradually increases from a starting voltage Vstart to an ending voltage Vend. The precise manner in which the curve  200  changes over time may vary depending on the design of the backlight assembly  68  and may be determined experimentally or through modeling. 
     The value of the expected voltage Vexp compared to Vstring or VFB by the backlight short-circuit protection circuitry  20  may be stored in the configuration memory  120  as a fixed value or as a mathematical formula, or may be hard-coded into the backlight short-circuit protection circuitry  20 . For example, when the feedback voltage VFB is to be sensed at approximately 10% of the boost soft start phase (e.g., at a time  202  shown in  FIG. 10 ), the configuration memory  120  may store a corresponding expected voltage Vexp value (e.g., as illustrated at numeral  204 ). Alternatively, the configuration memory  120  may store a mathematical function representing the increase in the expected voltage Vexp over time through the boost soft start phase (e.g., the curve  200 ). Under such conditions, the backlight short-circuit protection circuitry  20  may determine the value of Vexp to be tested based on the mathematical function and the time through the boost soft start phase in which the feedback voltage VFB is sensed. In some embodiments, the value of the expected voltage Vexp may be determined at least partly by the sensed value of the input voltage VIN. That is, since the starting voltage Vstart and the ending voltage Vend may depend on the input voltage VIN, the backlight short-circuit protection circuitry  20  may determine Vexp as a function of VIN. Thus, in some embodiments, the value of Vexp may be stored in the configuration memory  120  as a function of VIN. 
     Following the boost soft start phase, the backlight short-circuit protection circuitry  20  may continue to detect and protect against short circuits through the normal operation phase, as shown by a flowchart  210  of  FIG. 11 . The flowchart  210  of  FIG. 11  may begin after the boost soft start phase has ended and the LED string  102  input voltage Vstring has reached the level sufficient to drive the LED string  102 . The boost block  116  may maintain this boosted voltage level of the LED string input voltage Vstring through the normal operation phase. Indeed, as long as short circuits are not occurring, Vstring should remain relatively constant. Thus, the backlight short-circuit protection circuitry  20  may test the feedback voltage VFB to ensure that the LED string  102  input voltage Vstring has not dropped dramatically, which could indicate a short-circuit condition (block  214 ). For example, the backlight short-circuit protection circuitry  20  may test whether a short circuit is occurring every 1 ms, 10 ms, 100 ms, 1000 ms, 10,000 ms, or even longer, depending on a confidence in the backlight assembly  68 . 
     When the backlight short-circuit protection circuitry  20  tests for short-circuit conditions, the value of the feedback voltage VFB may be sensed (block  216 ). As mentioned above, the feedback voltage VFB correlates directly with the LED string  102  input voltage Vstring. Thus, the backlight short-circuit protection circuitry  20  may estimate the LED string  102  input voltage Vstring from the feedback voltage VFB (block  218 ). For example, when the resistors R 2  and R 3  are equivalent, Vstring will be approximately equal to double the feedback voltage VFB. Alternatively, the backlight short-circuit protection circuitry  20  may not estimate the LED string  102  input voltage Vstring. The backlight short-circuit protection circuitry  20  may compare the LED string  102  input voltage Vstring to an expected voltage value Vexp (representing an expected value of Vstring) (block  220 ) or, alternatively, the feedback voltage VFB to an expected voltage Vexp (representing an expected value of VFB), to determine whether a short-circuit condition is present. It should be noted that the particular expected voltage Vexp discussed with reference to  FIG. 11  will likely be different (higher) than the particular expected voltage Vexp discussed with reference to  FIG. 9 . 
     Specifically, after the boost soft start phase, the LED string  102  input voltage Vstring will remain substantially constant at a boosted level absent a short circuit. Thus, this expected voltage Vexp can be stored in the configuration memory  120 . As mentioned above, the backlight short-circuit protection circuitry  20  may compare the estimated value of the string voltage Vstring to the expected voltage value Vexp (representing an expected value of Vstring) (block  220 ) or, alternatively, the feedback voltage VFB to an expected voltage Vexp (representing an expected value of VFB), to determine whether a short-circuit condition is present. If a difference between the expected voltage Vexp and the string voltage Vstring (or a difference between the expected voltage Vexp and the feedback voltage VFB) is less than some threshold value (decision block  222 ), a short-circuit condition has not been detected. As such, the backlight power and control circuitry  104  may continue with the normal phase of operation (block  212 ). It should be noted that threshold value may be a programmable value stored in the configuration memory  120  or may be, for example, hard-coded in the backlight short-circuit protection circuitry  20 . The threshold referred to in  FIG. 11  may be a different threshold than the thresholds referred to in  FIGS. 8 and 9 . 
     If the difference between the expected voltage Vexp and the LED string  102  input voltage Vstring (or the difference between the expected voltage Vexp and the feedback voltage VFB) does exceed the threshold (decision block  222 ), it may be understood that a short-circuit condition is occurring in the backlight assembly  68 . Thus, the backlight short-circuit protection circuitry  20  may disconnect the power line FET PLF 1  to cut power to the backlight assembly  68  (block  224 ). Additionally, the backlight short-circuit protection circuitry  20  may cause the configuration memory  120  to store an indication that a short-circuit condition was detected during the normal operation phase of the operation of the backlight assembly  68 . 
     In the example illustrated in  FIG. 11 , the backlight short-circuit protection circuitry  20  may seek to avoid false positive short-circuit conditions by entering a hiccup mode, if enabled in the configuration memory  120  (decision block  226 ). As mentioned briefly above, the hiccup mode may allow for retesting whether a short circuit has actually occurred. If the hiccup mode has not been enabled (decision block  226 ), the power line FET PLF 1  may remain disconnected (block  228 ). Otherwise, the backlight power and control circuitry  104  may set a hiccup timer for a particular amount of time (block  230 ). The hiccup timer may last any suitable length of time, and may be a programmable setting in the configuration memory  120 . For example, the hiccup timer may be a value between approximately 10 ms to 10,000 ms. In one example, the hiccup timer may last either 100 ms or 1000 ms depending on a setting in the configuration memory  120 . When the hiccup timer ends, the backlight power and control circuitry  104  may restart the operation of the backlight assembly  68  starting from the inrush phase (block  232 ). If the short circuit is not determined to occur again, the previously determined short-circuit condition may be understood to have been a false positive. 
     Technical effects of the present disclosure include, among other things, the efficient detection of short circuits in a backlight assembly without the use of a resistor-based current sensor. Since a resistor-based current sensor may constantly dissipate power in the form of waste heat, detecting short circuits according to the present disclosure may result in a very substantial savings in power. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

Metadata:
Filing Date: 20111205
Publication Date: 20141111
Grant Date: 20141111
Priority Date: 20111205
Inventors: HUSSAIN ASIF
PANDYA MANISHA P.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05B45/50", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B45/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B47/10", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B45/50", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/50", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B45/38", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/325", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/325", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/38", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y02B20/30", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02B20/30", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 47178854