Local display backlighting systems and methods

Aspects of the subject technology relate to control circuitry for light-emitting diodes. The control circuitry includes a two-dimensional light-emitting diode (LED) array. The control circuitry may include a single LED array operable by a common driver or multiple LED arrays each operable by a dedicated LED matrix driver. Each matrix driver may receive a synchronization signal from a common controller and may include a programmable phase lock loop (PLL) to synchronize each matrix driver to the synchronization signal. The LED array may include multiple strings of LEDs mounted in series along the string. Each LED in each string may include a bypass switch operable to modify the current through that LED by pulse-width modulation.

TECHNICAL FIELD

The present description relates generally to electronic devices with displays, and more particularly, but not exclusively, to electronic devices with displays having backlights with local dimming.

BACKGROUND

Electronic devices such as computers, media players, cellular telephones, set-top boxes, and other electronic equipment are often provided with displays for displaying visual information. Displays such as organic light-emitting diode (OLED) displays and liquid crystal displays (LCDs) typically include an array of display pixels arranged in pixel rows and pixel columns. Liquid crystal displays commonly include a backlight unit and a liquid crystal display unit with individually controllable liquid crystal display pixels.

The backlight unit commonly includes one or more light-emitting diodes (LEDs) that generate light that exits the backlight toward the liquid crystal display unit. The liquid crystal display pixels are individually operable to control passage of light from the backlight unit through that pixel to display content such as text, images, video, or other content on the display.

DETAILED DESCRIPTION

The subject disclosure provides electronic devices such as cellular telephones, media players, tablet computers, laptop computers, set-top boxes, smart watches, wireless access points, and other electronic equipment that include light-emitting diode arrays such as in backlight units of displays. Displays are used to present visual information and status data and/or may be used to gather user input data. A display includes an array of display pixels. Each display pixel may include one or more colored subpixels for displaying color images.

Each display pixel may include a layer of liquid crystals disposed between a pair of electrodes operable to control the orientation of the liquid crystals. Controlling the orientation of the liquid crystals controls the polarization of backlight. This polarization control, in combination with polarizers on opposing sides of the liquid crystal layer, allows light passing into the pixel to be manipulated to selectively block the light or allow the light to pass through the pixel.

The backlight unit includes one or more light-emitting diodes (LEDs) such as one or more strings and/or arrays of light-emitting diodes that generate the backlight for the display. In various configurations, strings of light-emitting diodes may be arranged along one or more edges of a light guide plate that distributes backlight generated by the strings to the LCD unit, or may be arranged to form a two-dimensional array of LEDs.

In a display, control circuitry coupled to the array of display pixels and to the backlight unit receives data for display from system control circuitry of the electronic device and, based on the data for display, generates and provides control signals for the array of display pixels and for the LEDs of the backlight unit.

In some scenarios, the backlight unit generates a constant amount of light for the display pixels and the amount of light that passes through each pixel is solely controlled by the operation of the liquid crystal display pixels. In other scenarios, the amount of light generated by the backlight is dynamically controlled, based on the content to be displayed on the display. In some devices with dynamic backlight control, individual backlight LEDs or groups of backlight LEDs are separately controlled to allow local dimming or brightening of the display to enhance the contrast generated by the LCD pixels. Control circuitry for the LEDs (e.g., for backlight LEDs) may include multiple matrix drivers, each for control of a subarray of an array of LEDs and each synchronized to a synchronization signal from a common controller. The control circuitry for the LEDs may include individual bypass switches for each LED to allow for local dimming at the level of individual LEDs.

Providing local dimming of the backlight LEDs in these disclosed configurations (e.g., using multiple driver circuits each dedicated to a subarray of LEDs and/or using individual LED dimming using bypass switches) allows the backlight circuitry to adjust brightness on a zone-by-zone basis within an image to be displayed. For example, backlight zones may be illuminated only in bright image areas and backlight zones may be dimmed or turned off in dark or black areas of an image. Local dimming in this way helps facilitate high dynamic range (HDR) display of images and improvements in color, contrast, motion-sharpness, and grey level.

Because display backlight units can include, in some implementations, a large number of LEDs (e.g., an array of tens, hundreds, thousands, or millions of LEDs), thermal management for LED backlights and/or other LED arrays can be challenging. The LED drive architectures disclosed herein, in which groups of LEDs and/or individual LEDs are independently controlled, can help reduce the thermal stress and/or energy loss by heat dissipation. Control systems and methods are also disclosed that reduce or minimize the headroom voltage for the backlight, which can also increase system efficiency.

According to various aspects of the subject disclosure, multiple LED matrix drivers are provided for an array of backlight LEDs, each matrix driver having a phase lock loop (PLL) for synchronizing to a common synchronization signal such as a line or horizontal synchronization signal for the display. According to other aspects of the subject disclosure, a dedicated bypass switch is provided for each LED in a string. According to other aspects, headroom feedback circuitry is provided in combination with multiple matrix drivers and/or dedicated bypass switches for further increased display efficiency and reliability.

An illustrative electronic device of the type that may be provided with light-emitting diodes is shown inFIG. 1. In the example ofFIG. 1, device100has been implemented using a housing that is sufficiently small to be portable and carried by a user (e.g., device100ofFIG. 1may be a handheld electronic device such as a tablet computer or a cellular telephone). As shown inFIG. 1, device100may include a display such as display110mounted on the front of housing106. Display110may be substantially filled with active display pixels or may have an active portion and an inactive portion. Display110may have openings (e.g., openings in the inactive or active portions of display110) such as an opening to accommodate button104and/or other openings such as an opening to accommodate a speaker, a light source, or a camera.

Display110may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch-sensitive. Display110may include display pixels formed from light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), plasma cells, electrophoretic display elements, electrowetting display elements, liquid crystal display (LCD) components, or other suitable display pixel structures. Arrangements in which display110is formed using LCD pixels and LED backlights are sometimes described herein as an example. This is, however, merely illustrative. In various implementations, any suitable type of display technology may be used in forming display110if desired.

Housing106, which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials.

The configuration of electronic device100ofFIG. 1is merely illustrative. In other implementations, electronic device100may be a computer such as a computer that is integrated into a display such as a computer monitor, a laptop computer, a somewhat smaller portable device such as a wrist-watch device, a pendant device, or other wearable or miniature device, a media player, a gaming device, a navigation device, a computer monitor, a television, or other electronic equipment.

For example, in some implementations, housing106may be formed using a unibody configuration in which some or all of housing106is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). Although housing106ofFIG. 1is shown as a single structure, housing106may have multiple parts. For example, housing106may have upper portion and lower portion coupled to the upper portion using a hinge that allows the upper portion to rotate about a rotational axis relative to the lower portion. A keyboard such as a QWERTY keyboard and a touch pad may be mounted in the lower housing portion, in some implementations. An LED backlight array may also be provided for the keyboard and/or other illuminated portions of device100.

In some implementations, electronic device100may be provided in the form of a computer integrated into a computer monitor. Display110may be mounted on a front surface of housing106and a stand may be provided to support housing (e.g., on a desktop).

FIG. 2is a schematic diagram of display110in which the display is provided with a liquid crystal display unit204and a backlight unit202. As shown inFIG. 2, backlight unit202generates backlight208and emits backlight208in the direction of liquid crystal display unit204. Liquid crystal display unit204selectively allows some or all of the backlight208to pass through the liquid crystal display pixels therein to generate display light210visible to a user. Backlight unit202includes one or more subsections206.

In some implementations, subsections206may be elongated subsections that extend horizontally or vertically across some or all of display110(e.g., in an edge-lit configuration for backlight unit202). In other implementations, subsections206may be square or other rectilinear subsections (e.g., subarrays of a two-dimensional LED array backlight). Accordingly, subsections206may be defined by one or more strings and/or arrays of LEDs disposed in that subsection. Subsections206may be controlled individually for local dimming of backlight208.

Although backlight unit202is shown implemented with a liquid crystal display unit, it should be appreciated that a backlight unit such as backlight unit202may be implemented in a backlit keyboard, or to illuminate a flash device or otherwise provide illumination for an electronic device.

FIG. 3shows a schematic diagram of exemplary display circuitry including control circuitry300that may be implemented in backlight unit202or other LED lighting devices. In the example ofFIG. 3, control circuitry300includes multiple subarrays302of LEDs304that, in combination, for a two-dimensional array of LEDs. Each subarray302may include one or more strings of LEDs that each include multiple LEDs304in series. Subarrays302may each include multiple strings of LEDs that are coupled, in parallel, between a common supply voltage source and a current controller for that string.

Each subarray302includes a dedicated matrix driver circuit306(sometimes referred to simply as driver circuits for convenience) that operates the LEDs304in that array. Each matrix driver circuit306operates the LEDs304of its associated array302to provide local dimming of the entire array or local dimming of individual strings of LEDs in that array. Each matrix driver circuit306provides local dimming of LEDs304, which may enhance the relative brightness and darkness of display content controlled by LCD unit204. Accordingly, matrix driver circuitry306may operate the LEDs of their associated arrays304based, at least in part, on the content being displayed using LCD unit204.

In order to operate the LEDs of an associated array304based, at least in part, on the content being displayed using LCD unit204, each matrix driver circuitry306receives one or more control signals from a common controller301. As shown in the example ofFIG. 3, each matrix driver306receives the same vertical synchronization (VSYNC), line synchronization (LSYNC), serial clock (SCLK) and slave select (-SS) signal from controller301. The VSYNC, LSYNC, SCLK and/or -SS signals may be signals used to operate the LCD pixels of LCD unit204as would be understood by one skilled in the art. For example, the VSYNC signal may be provided by controller301to indicate each display refresh or each display frame to be displayed using LCD pixels of the LCD unit. The LSYNC signal may be provided by controller301to signal the start of operation of each pixel row.

Controller301may be used to provide control signals such as the VSYNC and LSYNC signals, and/or other control signals, to both backlight unit202and LCD unit204or controller301may be a dedicated backlight control unit that receives the VSYNC, LSYNC, and/or other control signals from another display controller associated with LCD unit204.

Each matrix driver306may update the brightness of its associated array302(e.g., the entire array or a subset of the array) based on the commonly received VSYNC signal (e.g., the brightness may be updated upon receipt of the rising edge of the VSYNC signal). In some implementations, each matrix driver306may include a programmable delay to set the relative timings of the various LED array updates based on the rising edge of the common VSYNC signal.

A first one of matrix drivers306(labeled LED Matrix Driver #L1R1 inFIG. 3) also receives and enable signal (EN) and a multiple-output-single-input signal (MOSI) from common controller301. LED Matrix Driver #L1R1 provides a multiple-input-single-output signal (MISO) to a next one of matrix drivers306(labeled LED Matrix Driver #L1R2 inFIG. 3), and so forth until a last one of matrix drivers306(labeled LED Matrix Driver #LMRN inFIG. 3). LED Matrix Driver #LMRN provides a MISO signal back to controller301.

In some implementations, each matrix driver306may be an integrated circuit having an internal clock. However, due to process variations in manufacturing integrated circuits, an array of matrix drivers306each having its own clock can be problematic in that the operation of the various LED arrays302can be out of sync by as much as, for example, 10 percent. In order to ensure that the local dimming of LEDs304of various arrays302are synchronized to the associated content to be displayed, matrix drivers306are operated using a common (e.g., master) clock signal SCLK with synchronization of the various matrix drivers using the common LSYNC signal.

FIG. 4shows a schematic representation of exemplary circuitry of matrix drivers306. In the example ofFIG. 4, each matrix driver306includes a programmable phase lock loop (PLL)400. Each PLL400receives the common LSYNC signal along a path404from common controller301ofFIG. 3and generates a synchronization output signal which is provided to a multiplexer402. Each multiplexer402also receives the clock signal (labeled Pixel Clock inFIG. 4and SCLK inFIG. 3) along a path406from common controller301.

Based on a selection signal “Select”, each multiplexer402generates a driver clock signal for its associated matrix driver306, the driver clock signal geared from the LSYNC synchronized PLL signal and/or the clock signal. The selected driver clock signal is provided to a pulse-width modulation (PWM) generator408that generates a PWM signal, based on the provided driver clock signal, for use in controlling the brightness of the LEDs (e.g., in one or more strings) in the array302associated with that matrix driver306.

The PWM signal from the PWM generator408of each matrix driver306is provided to LED control circuitry410of that matrix driver306for controlling the brightness of LEDs304of that array302associated with that matrix driver306. LED control circuitry410of each matrix driver306may include, for example, a DC/DC converter or switching converter (e.g., implemented as a buck converter, a boost converter, or an inverter) for providing a supply voltage to a first end of each LED string in the associated array302. The supply voltage generated by LED control circuitry410is based on the PWM signal provided by the associated LED PWM generator408.

LED control circuitry410of each matrix driver306may also include additional circuitry such as a current driver circuitry or controlling current at a second end of each string of LEDs, may include headroom voltage control circuitry, and/or may include individual LED switching circuitry (e.g., in implementations in which each LED in a string is provided with a bypass switch as described in further detail hereinafter).

Each matrix driver306may also include headroom voltage control circuitry that provides feedback control of LED arrays302to help reduce energy loss by reducing or minimizing residual voltages at the end of each LED string.FIG. 5shows an example of headroom voltage control circuitry that may be included in each matrix driver306, in an implementation in which each matrix driver controls an array of LEDs that is arranged in rows500of parallel-coupled LEDs502.

In the example ofFIG. 5, each row500of LEDs502includes multiple single LED columns each having a first end that receives a common supply voltage VOUT and at a second end coupled to a common current driver504that controls the current through all of the LEDs in that row. In the example ofFIG. 5, each row500includes residual voltage sampling circuitry506(e.g., an analog-to-digital converter) coupled between a second end of the LEDs in that row and the current driver for that row. The residual voltage sampling circuitry506for each row samples the residual voltage of that row.

In the example ofFIG. 5, minimum headroom selection circuity510determines the minimum of the sampled residual voltages of the rows500. The minimum residual voltage is compared by comparator512with a target headroom voltage “Headroom Target”. The target headroom voltage is predetermined to provide sufficient voltage for operation of all LEDs while reducing dissipation of energy due to excess residual voltage. The result of the comparison is provided to supply voltage adjustment circuitry514.

Supply voltage adjustment circuitry514determines a correction to the supply voltage VOUT that will increase or decrease the headroom voltage, based on the result of the comparison. The determined correction is provided to voltage supply circuit501to generate a new common supply voltage (e.g., by modifying the duty cycle of a PWM signal generated by PWM generator408for that matrix driver).

AlthoughFIG. 5shows a single residual voltage sampling circuit506for each row (and a single current driver504), it should be appreciated that each LED502(or each string of serial coupled LEDs) in a row can be individually controlled (e.g., using a dedicated current driver for that row) and the residual voltage of each LED (or string) can be sampled. The minimum of the various sampled residual voltages of the LEDs in each row can be determined by a minimum voltage selection circuit508. That row-minimum voltage can then be provided to minimum headroom selection circuity510so that minimum headroom selection circuity510can determine the array-minimum residual voltage from among the row-minimum voltages.

In accordance with some aspects of the subject disclosure, a string of LEDs (e.g., a string of LEDs in a matrix or array302controlled by a local matrix driver306or a string of LEDs in an array of LEDs controlled by a common control circuit) may be provided with individual bypass switches for one or more LEDs in the string. In this way, the brightness of each LED can be individually controlled even with a common supply voltage provided at a first end of each string and a common current sink provided at a second end of each string.

FIG. 6shows an example of an LED string600having multiple LEDs602, coupled in series between a voltage source Vsource and a current sink604, through which a sink current i_sink can be controlled to control the current through the LEDs. Current sink604may be a constant or an adjustable current sink. As shown, each LED602(labeled LED_x1, LED_x2, LED_y1, LED_y2, and LED_z inFIG. 6) has an associated bypass switch (labeled S_x1, S_x2, S_y1, S_y2, and S_z inFIG. 6). Each bypass switch can be operated (e.g., with a desired PWM duty cycle) to individually control the current through its associated LED. In this way, single-LED local dimming can be provided (e.g., for a display backlight). Moreover, providing individually bypassable LEDs can provide an improvement in display control efficiency. In some implementations (see, e.g., the example ofFIG. 13), headroom voltage control circuitry is provided to help maintain a desired headroom voltage at a location606at the end of string600.

The LED drive efficiency, Eff, for an LED string can be calculated as Eff=Vstring/(Vstring+Vhr), wherein Vstring is the voltage applied to the string of LEDs and Vhr is the residual voltage at the second end of the string. Accordingly, the higher Vstring, the higher the efficiency given a fixed headroom voltage Vhr. Table 1 below shows example efficiencies for various headroom voltages Vhr with a Vstring of 6 Volts (V). Considering DC/DC components also can have energy loss, a backlight system operating at the example voltages in Table 1 can have an efficiency below, for example, 70%.

However, in the example configuration ofFIG. 6, the source or string voltage that can be applied is larger than in an implementation without individual bypass switches, since the current through each LED is individually controllable by a bypass switch S. In one example, a string600having 12 LEDs602in series, each with an individually controllable bypass switch S is provided, which can be operated using a string voltage of, for example, 36 volts. In this case, a relatively higher efficiency can be achieved at various headroom voltages, as shown in Table 2 below.

TABLE 2LED drive efficiency with high string voltageVhr0.81.01.21.41.6Vstring3636363636Efficiency0.9780.9730.9680.9630.957

Switches S_* ofFIG. 6can be controlled using, for example, a pulse-width-modulation signal with a duty cycle that determines the current through the corresponding LED602.FIG. 7shows a PWM signal having on pulses700in a scenario in which all LEDs have a current that is lower than a predetermined minimum current Imin. In the example ofFIG. 7, all LEDs are operated with the same peak current Imin (e.g., the peak current achieved during the on pulse700) but with varied duty cycles to achieve desired averaged current for that LED.

In operational scenarios in which at least one LED has a current that is larger than Imin, as shown inFIG. 8, the on-pulses800can be controlled such that all LEDs have a common peak current that is higher than Imin (e.g., a peak current of Iled,max) and a different duty cycle to achieve desired averaged current for that LED.

The duty cycle of an nth LED can be calculated as:

where ILED_nis the desired current through the nth LED and max(ILED_1, ILED_2, . . . , ILED_n) is the maximum desired current through any LED. The maximum desired current may be a maximum direct current to be provided through any of the LEDs such that the current through the other LEDs is reduced by PWM operation of switches S_*.

Examples of switch patterns, and corresponding LED waveforms, for the switches and LEDs shown inFIG. 6are shown inFIGS. 9A and 9B.FIG. 9Ashows switch patterns and LED waveforms when all LEDs have a relatively low current and are operated in a PWM mode. In the example ofFIG. 9A, half of LEDs602draw current during a first half of each PWM period (TPWM) and another half of LEDs602draw current during a second half of each PWM period.

FIG. 9Bshows switch patterns and LED waveforms when at least one LED (e.g., LED_z) has a current that is higher than Imin. In the example ofFIG. 9B, the other LEDs have a current that is lower that Imin and will be in PWM mode. In this case, the high current channel LED_z is operated in a direct current control mode. For the other LEDs, half of LEDs602will draw current during a first half of each PWM period and another half of LEDs602will draw current during a second half of each PWM period.

Operating LEDs602using the example switching patterns ofFIGS. 9A and 9Bmay also facilitate a reduction in the variations in the supplied source voltage. For example, the source voltage variation can be maintained between the maximum source voltage and 50% of the maximum source voltage, where the maximum source voltage is based on the voltage supplied when all LEDs are turned on.

For example,FIG. 10shows a source voltage waveform along with LED current waveforms, in an operational scenario in which all LEDs602have a relatively high current. In the example ofFIG. 10, LED_z (ch_z) has a continuous current. All remaining LEDs have a PWM current with a relatively large duty cycle corresponding to the relatively large current. As shown inFIG. 10, Vsource has ripple with the same frequency as the PWM signal of the LED and the ripple amplitude for Vsource is less than 50% of the maximum of Vsource.

FIG. 11shows a source voltage waveform along with LED current waveforms, in an operational scenario in which all LEDs have a relatively low current (e.g., in comparison with the current in the example ofFIG. 10). In the example ofFIG. 11, all LEDs have a PWM current with a relatively small duty cycle. As shown inFIG. 11, Vsource has a ripple with the same frequency as the PWM signal and the ripple amplitude is about 50% of the maximum of Vsource.

FIG. 12shows a source voltage waveform along with LED current waveforms, in an operational scenario in which all but one LED has a relatively high current. In the example ofFIG. 12, the low current LED (e.g., ch_x1) is operated with a PWM current with a small duty cycle. The highest current LED (e.g., ch_z) is operated with a continuous current. All remaining LEDs are operated with a PWM current with a large duty cycle. As shown inFIG. 12, Vsource has ripple with the same frequency as the PWM signal and the ripple amplitude is less than 50% of the maximum of Vsource.

In accordance with various aspects of the subject disclosure backlight control circuitry may include a string of series coupled LEDs, each having a bypass switch as shown inFIG. 6, and having headroom voltage control circuitry.FIG. 13shows an exemplary implementation of backlight control circuitry having a string600of series coupled LEDs602, each having a bypass switch S and having headroom voltage control circuitry.

As shown inFIG. 13, headroom control circuitry for one or more LED strings may include a headroom voltage feedback circuit such as headroom feedback loop1300that samples a headroom voltage Vhr at a location1301between a last LED in string600(e.g., LED_z) and current sink604. Although only a single string600of LEDs602is shown inFIG. 6, it should be appreciated that headroom feedback loop1300may be configured to sample residual voltages of multiple strings600that each receive a common source voltage (Vsource) at a first end and are coupled to a common or dedicated current sink604at a second end. In implementations in which multiple parallel strings600of this type are provided, headroom feedback loop1300determines a minimum of the sampled residual voltages for comparison of the minimum voltage to a target headroom voltage.

Headroom feedback loop1300provides an output voltage (e.g., the minimum residual sampled voltage) that may be combined (e.g., differenced), at1304, with a feedforward reference voltage. The feedforward reference voltage may be based on the known switching pattern and LED current of each LED, from circuitry1310. The combination of the headroom feedback loop voltage with the feedforward reference voltage can then be combined, at1306, with a predetermined reference voltage Vref to provide a control input to a DC/DC voltage regulation loop and DC power train1308that generates the desired source voltage for string(s)600. A DC feedback voltage can be provided, at1302, to help ensure that Vsource matches the desired voltage provided to DC/DC voltage regulation loop and DC power train1308.

FIG. 14depicts a flow diagram of an example process for headroom voltage reduction for a string of LEDs, each having a bypass switch in accordance with various aspects of the subject technology. For explanatory purposes, the example process ofFIG. 14is described herein with reference to the components ofFIGS. 6 and 13. Further for explanatory purposes, the blocks of the example process ofFIG. 14are described herein as occurring in series, or linearly. However, multiple blocks of the example process ofFIG. 14may occur in parallel. In addition, the blocks of the example process ofFIG. 14need not be performed in the order shown and/or one or more of the blocks of the example process ofFIG. 14need not be performed.

In the depicted example flow diagram, at block1400, a plurality of strings of light-emitting diodes (see, e.g., string600of LEDs602ofFIGS. 6 and/or 11), the LEDs coupled in series between a common voltage source (e.g., at a first end) and an associated current driver (e.g., at a second end) are operated, at least in part, by providing a common supply voltage from the common voltage source to each of the strings of LEDs.

At block1402, a plurality of bypass switches (see, e.g., switches S_* ofFIGS. 6 and/or 11), each associated with one of the light-emitting diodes, may be operated to reduce a brightness of that light-emitting diode by individually controlling the current through that LED. Operating the plurality of bypass switches may include operating the plurality of bypass switches based on a pulse-width-modulation signal for each of the plurality of bypass switches. The pulse-width-modulation signal for at least one of the bypass switches may have a duty cycle that is different from a duty cycle of the pulse-width-modulation signal for at least another one of the bypass switches.

At block1404, a residual voltage (sometimes referred to as a headroom voltage) is sampled from each LED string (e.g., between a last LED in the string and the current driver for that string).

At block1406, a minimum of the sampled residual voltages is determined (e.g., by headroom feedback loop1300).

At block1408, the supply voltage is adjusted or modified based on the determined minimum (e.g., to correct the determined minimum to match a target headroom voltage).

In accordance with various aspects of the subject disclosure, an electronic device with a display is provided. The display includes a backlight unit having an array of light-emitting diodes, the array including a plurality of subarrays of the light-emitting diodes. The backlight unit also includes a controller to provide at least one synchronization signal for the array of the light-emitting diodes. The backlight unit also includes a plurality of driver circuits, each configured to control the light-emitting diodes of an associated one of the subarrays, and each including a phase lock loop for synchronizing the control of the light-emitting diodes of the associated one of the subarrays to the synchronization signal from the controller.

In accordance with other aspects of the subject disclosure, an electronic device having a display is provided. The display includes a backlight unit having a voltage source. The backlight unit also includes a string of light-emitting diodes configured to receive, at a first end, a supply voltage from the voltage source. The backlight unit also includes a bypass switch for each light-emitting diode in the string, the bypass switch for each light-emitting diode controllable to pulse-width-modulate that light-emitting diode. The backlight unit also includes a headroom voltage feedback circuit coupled to a second end of the string.

In accordance with other aspects of the subject disclosure, a method is provided that includes providing a supply voltage from a voltage source to a plurality of strings of light-emitting diodes, each string coupled between the voltage source and at least one current driver. The method also includes operating a plurality of bypass switches, each associated with one of the light-emitting diodes, to reduce a brightness of that light-emitting diode. The method also includes sampling a residual voltage from each of the strings. The method also includes determining a minimum of the sampled residual voltages. The method also includes modifying the supply voltage based on the determined minimum.