PATENT DOCUMENT

Publication Number: US-10512130-B1
Application Number: US-201916265401-A
Country: US
Kind Code: B1

Title: Multi-string LED drivers and current switching techniques

Abstract:
LED backlight circuits for a display and methods for operating the circuits are disclosed. The LED backlight circuit includes a set of drivers and a set of LED strings. A driver can be capable of coupling to any of the LED strings at a given time. Examples of the disclosure can include different configurations of drivers. In some instances, the driver may include an auxiliary transistor that allows the driver to settle before being switched to control a respective LED string. In some examples, the driver can include switches and an idle transistor that can be operated such that a low current path through the driver can exist when the driver is not coupled to a LED string.

Claims:
The invention claimed is: 
     
       1. A circuit comprising:
 a first source; 
 a second source; 
 a set of drivers, each of the set of drivers comprising:
 a set of first transistors, each first transistor coupled to a pulse-width modulation (PWM) signal; 
 a first resistor; 
 a second transistor, the second transistor coupled to the set of first transistors and the first resistor; 
 a third transistor, the third transistor coupled to the second transistor and the second source; and 
 an operational amplifier coupled to the second transistor; and 
 
 a set of strings of series-connected light emitting diodes (LEDs), wherein the set of strings of series-connected LEDs is coupled to the set of drivers and the first source, such that a respective one of the set of drivers is configured to connect to any one of the set of strings of series-connected LEDs. 
 
     
     
       2. The circuit of  claim 1 , wherein the third transistor is coupled to the set of first transistors. 
     
     
       3. The circuit of  claim 1 , wherein the third transistor is coupled to the operational amplifier. 
     
     
       4. The circuit of  claim 3 , wherein the at least one of the set of drivers further comprises a fourth transistor coupled to the set of first transistors. 
     
     
       5. The circuit of  claim 3 , further comprising:
 an inverting amplifier coupled to an input signal; 
 a first switch controlled by the input signal and capable of coupling the operational amplifier to the second transistor; and 
 a second switch controlled by an inverted input signal and capable of coupling the operational amplifier to the third transistor. 
 
     
     
       6. The circuit of  claim 5 , further comprising:
 a negative feedback loop, the negative feedback loop including:
 the operational amplifier; 
 the second transistor when the first switch couples the operational amplifier to the second transistor; and 
 the third transistor when the second switch couples the operational amplifier to the third transistor. 
 
 
     
     
       7. The circuit of  claim 1 , wherein the third transistor and the set of first transistors are turned on at different times. 
     
     
       8. The circuit of  claim 1 , wherein the PWM signals received by a first set of first transistors form a PWM bus. 
     
     
       9. A method for operating a display, the method comprising:
 turning on a set of LED strings in a round using a first set of one or more LED drivers, wherein the turning on of the set of LED strings in the round includes:
 at a first time interval, coupling a first LED string to a first driver, the first LED string included in the set of LED strings, and the first driver included in the first set of one or more LED drivers; 
 at a second time interval:
 in accordance with a determination that the first driver is coupled, coupling a second LED string to a second driver, the second LED string included in the set of LED strings, and the second driver included in the first set of one or more LED drivers; and 
 in accordance with a determination that the first driver is not coupled to a LED string, coupling the second LED string to the first driver. 
 
 
 
     
     
       10. The method of  claim 1 , further comprising:
 turning on the set of LED strings in another round using a second set of one or more LED drivers. 
 
     
     
       11. The method of  claim 1 , wherein a number of drivers in the first set of one or more LED drivers is one, and the set of LED strings comprises all LED strings in the display. 
     
     
       12. The method of  claim 1 , further comprising:
 decoupling the first LED string from the first driver, 
 wherein a time duration between the decoupling of the first LED string from the first driver and the coupling of the second LED string to the first driver is zero seconds. 
 
     
     
       13. The method of  claim 1 , wherein the turning on of one of the set of LED strings in the round includes:
 at a third time interval:
 in accordance with a determination that the first driver is not coupled to one of the set of LED strings, coupling a third LED string to the first driver, the third LED string included in the set of LED strings. 
 
 
     
     
       14. The method of  claim 1 , wherein the turning on of the set of LED strings in the round includes:
 at a third time interval:
 in accordance with a determination that the first driver is coupled to at least one LED string, determining whether the second driver is coupled to at least one LED string; and 
 in accordance with a determination that the second driver is coupled to at least one LED string, coupling a third LED string to a third driver, the third LED string included in the set of LED strings, and the third driver included in the first set of one or more LED drivers. 
 
 
     
     
       15. The method of  claim 1 , further comprising:
 turning on the set of LED strings in another round using the first set of one or more LED drivers, wherein a respective order of the first set of one or more LED drivers that the drivers in the first set of one or more LED drivers are coupled in the round differs from a respective order in the another round. 
 
     
     
       16. The method of  claim 1 , wherein the turning on of the set of LED strings in the round further comprises:
 prior to the coupling of the first LED string to the first driver:
 coupling an internal source to the first driver using an auxiliary transistor; and 
 charging the first driver. 
 
 
     
     
       17. The method of  claim 1 , further comprising:
 operating at least one driver of the first set of one or more LED drivers in an idle state, wherein the operation includes: 
 coupling of an amplifier of the at least one driver to an idle transistor; and 
 maintaining an output voltage of the amplifier. 
 
     
     
       18. The method of  claim 17 , wherein the amplifier is coupled to the idle transistor when the amplifier is not coupled to a second transistor. 
     
     
       19. The method of  claim 1 , further comprising:
 delaying the coupling of the first LED string to the first driver by a non-zero amount of time.

Description:
FIELD 
     This disclosure relates generally to systems, methods, and apparatuses for improving display devices using a backlight controller. More specifically, this disclosure relates to multi-string light emitting diode (LED) drivers and associated current switching techniques. 
     BACKGROUND 
     Display screens of various types of technologies, such as liquid crystal displays (LCDs), organic light emitting diode (OLED) displays, etc., can be used as screens or displays for a wide variety of electronic devices, including consumer electronics such as televisions, computers, and handheld devices (e.g., mobile telephones, tablet computers, audio and video players, gaming systems, etc.). LCD devices, for example, can provide a flat display in a relatively thin package that can be suitable for use in a variety of electronic goods. In addition, LED devices may use less power than comparable display technologies, making them suitable for use in battery-powered devices, or in other contexts where it is desirable to minimize power usage. 
     LCDs generally include a backlight that provides visible light to a liquid crystal layer. The liquid crystal layer can take the light from the backlight and can control the brightness and color at each individual pixel in the display in order to render a desired image. One metric that can be used to judge the performance of a display is the uniformity of color generated by the display over varying levels of brightness. In some displays, the brightness can be adjusted by increasing or decreasing the drive current using a LED driver, which can be referred to as analog dimming. For example, 50% brightness can be achieved by applying a drive current equal to 50% of the maximum current. In some instances, a change in drive current can result in a shift in the wavelength (i.e., color) of the light produced by the display. Additionally, analog dimming may require an analog control signal, which may not be readily available or may require complex circuitry. 
     Additionally or alternatively, the brightness may be adjusted by using pulse width modulation (PWM) dimming, where the duty cycle of the drive current can be increased or decreased. In some instances, the drive current applied may be equal to 100% of the maximum current. For example, 50% brightness can be achieved by applying a drive current equal to 100% of the maximum current at a 50% duty cycle. The duty cycle in PWM techniques can result in the drive current being applied during an on pulse, and not being applied during an off pulse. The PWM signal can include on pulses that alternate with off pulses. In some instances, the frequency of the PWM signal may need to above a certain threshold frequency (e.g., 100 Hz) to avoid the pulsing of the PWM signal being visible to the human eye. A backlight circuit that includes LED driver(s) that can perform at PWM frequencies much higher (e.g., 50 kHz) than this threshold frequency may be desired. 
     Additionally, certain devices, such as laptops and monitors, may have high-resolution displays, where global dimming (e.g., uniform dimming across all or the majority of the display) may be desired. Also, the quality of the display may benefit from precise matching of the drive current between LED strings and high linearity when the backlight circuit is operated at high PWM frequencies. 
     SUMMARY 
     Disclosed herein is a LED backlight circuit for a display. The LED backlight circuit includes a set of drivers and a set of LED strings. A driver can be capable of coupling to any of the LED strings at a given time. In this manner, the driver that controls a given LED string can change, and the order of which drivers are controlling the strings can be rotated during different rounds and/or clock cycles to reduce any mismatches between the drive currents of the set of LED strings. In some examples, the current mismatches can be reduced by controlling all of the LED strings within a set using the minimum number of drivers required. 
     Examples of the disclosure can include different configurations of drivers. In some instances, the driver may include an auxiliary transistor that allows the driver to settle before being switched to control a respective LED string. In some examples, the driver can include switches and an idle transistor that can be operated such that a low current path through the driver can exist when the driver is not coupled to a LED string. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  illustrate systems in which examples of the disclosure can be implemented. 
         FIG. 2  illustrates a cross-sectional view of a portion of a display according to examples of the disclosure. 
         FIG. 3  illustrates a block diagram of a portion of an exemplary backlight circuit including multi-string LED drivers according to examples of the disclosure. 
         FIGS. 4A-4F  illustrate timing diagrams of exemplary operations of multi-string LED drivers according to examples of the disclosure. 
         FIG. 4G  illustrates a process flow of an exemplary operation of multi-string LED drivers according to examples of the disclosure. 
         FIG. 5A  illustrates a schematic diagram of an exemplary driver including an auxiliary transistor according to examples of the disclosure. 
         FIG. 5B  illustrates a process flow of an exemplary operation of a driver including an auxiliary transistor according to examples of the disclosure. 
         FIG. 5C  illustrates a timing diagram of an exemplary operation of a driver including an auxiliary transistor according to examples of the disclosure. 
         FIG. 6A  illustrates a schematic diagram of an exemplary driver including components for operating the driver in an idle state according to examples of the disclosure. 
         FIG. 6B  illustrates a process flow of an exemplary operation of a driver in an idle state according to examples of the disclosure. 
         FIGS. 7A-7E  illustrate timing diagrams of exemplary operations, which can include phase shifting, of a set of strings according to examples of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples. 
     Disclosed herein is a LED backlight circuit for a display. The LED backlight circuit includes a set of drivers and a set of LED strings. A driver can be capable of coupling to any of the LED strings at a given time. In this manner, the driver that controls a given LED string can change, and the order of which drivers are controlling the strings can be rotated during different rounds and/or clock cycles to reduce any mismatches between the drive currents of the set of LED strings. In some examples, the current mismatches can be reduced by controlling all of the LED strings within a set using the minimum number of drivers required. 
     Examples of the disclosure can include different configurations of drivers. In some instances, the driver may include an auxiliary transistor that allows the driver to settle before being switched to control a respective LED string. In some examples, the driver can include switches and an idle transistor that can be operated such that a low current path through the driver can exist when the driver is not coupled to a LED string. 
     The various examples are described in the context of LEDs, LED displays, and associated backlight circuitry. It should be appreciated that these examples are merely illustrative and the disclosed backlight circuit and methods described herein may be implemented in other contexts in which the benefits of the disclosure are desired (e.g., for illumination of keyboards, flash components, etc.). These benefits may include reduced mismatches of the drive current between LED strings, increased linearity when the backlight circuit is operated at high PWM frequencies, increased performance at low duty cycles and enhanced display image, as discussed in detail below. 
     As used throughout this specification, a reference number without an alpha character following the reference number can refer to one or more of the corresponding reference, the group of all references, or some of the references. For example, “ 220 ” can refer to any one of the strings  220  (e.g., string  220 A, string  220 B, etc.), can refer to all of the strings  220 , or can refer to some of the strings (e.g., both string  220 A and string  220 B). 
     Exemplary Systems 
       FIGS. 1A-1C  illustrate systems in which examples of the disclosure can be implemented.  FIG. 1A  illustrates an exemplary mobile telephone  136  that can include a touch screen  124 .  FIG. 1B  illustrates an exemplary media player  140  that can include a touch screen  126 .  FIG. 1C  illustrates an exemplary wearable device  144  that can include a touch screen  128  and can be attached to a user using a strap  146 . 
     Exemplary systems may also include other types of electronic devices such as computers, laptops, tablets, set-top boxes, wireless access points, televisions, and other electronic equipment that may include LEDs. For example, electronic devices may include LEDs in displays that may be used to present visual information and status data and/or may be used to gather user input data (e.g., keyboards, flash LEDs, and/or other components). 
     The touch screens  124 ,  126 , and  128  can each include a display. A display may include 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 to control the orientation of the liquid crystals. Controlling the orientation of the liquid crystals can control the polarization of the backlight generated by a backlight unit of the display (which can include the backlight circuit according to examples of the disclosure). This polarization control, in combination with polarizers on opposite sides of the liquid crystal layer, can allow the display to selectively block or selectively allow light at the display pixels. 
     The backlight unit may include one or more strings of LEDs and an associated backlight circuit that can generate the backlight for the display. The strings of LED(s) 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 grid of LEDs. The backlight circuit can operate (e.g., control) the strings of LEDs. 
       FIG. 2  illustrates a cross-sectional view of a portion of a display according to examples of the disclosure. The display  110  can include a backlight unit  202  and a liquid crystal display unit  204 . The backlight unit  202  can generate backlight  208  that is emitted in the direction of the liquid crystal display unit  204 . The liquid crystal display unit  204  can selectively allow some or all of the backlight  208  to pass through the display pixels therein to generate display light  209  visible to a user. The backlight unit  202  may include one or more subsections  206 . In some implementations, the subsections  206  may be elongated subsections that extend horizontally or vertically across some or all of the display  110  (e.g., in an edge-lit configuration for the backlight unit  202 ). In other implementations, subsections  206  may be square or nearly square subsections (e.g., in a two-dimensional array backlight configuration). The subsections  206  may include one or more strings of LEDs. In some examples, the subsections  206  may be controlled individually for local dimming of backlight  208 . 
     Exemplary Multi-String LED Drivers 
       FIG. 3  illustrates a block diagram of a portion of an exemplary backlight circuit including multi-string LED drivers according to examples of the disclosure. Backlight circuit  200  can be implemented in the backlight unit  202  shown in  FIG. 2 , for example. The backlight circuit  200  can include a set of strings  220  and a set of drivers  210 . As used herein, a set of elements (e.g., strings, drivers, transistors) can include one or more elements. 
     The set of strings  220  can each include one or more LEDs  221  connected in series. For example, the string  220 A can include a set of LEDs  221 A connected in series; the string  220 B can include a set of LEDs  221 B connected in series; and the string  220 N can include a set of LEDs  221 N connected in series. In some examples, the variable “N” can refer to the total number of strings  220  in the backlight circuit  200 . 
     The LEDs  221  can receive a voltage V O    223  at a first end of a string from, for example, a DC/DC converter (not shown). In some examples, boost regulators may generate the high voltage for the strings  220  and allow drivers  210  to have sufficient headroom. The LEDs  221  can also be coupled, at a second end of the string  220 , in series with a driver  210 . Exemplary drivers  210  are discussed in more detail below. 
     As previously mentioned, any driver  210  can be coupled to any string  220 . In some examples, the order of which a driver  210  is connected to a given string  220  can be changed. The change in order can reduce current mismatches among the set of strings  220 . In some examples, the current mismatch can be the difference between drive currents I LED    222  among the set of strings (e.g., the difference between I LED    222 A and I LED    222 B). 
     Although not shown in the figure, examples of the disclosure can further include circuitry that allows the strings  220  to couple to some or all (e.g., each) of the set of drivers  210 . Exemplary circuitry can include a set of switches. The switches may be included in the drivers  210 , or may be a component separate from the drivers  210 . For example, the driver  210 A can include five switches, each configured to couple a unique string for the driver  210 A to receive one or more of the drive currents (e.g., I LED    222 A, I LED    222 B, I LED    222 C, I LED    222 D, I LED    222 E). 
     Using multi-string LEDs in a display backlight may be beneficial for certain technologies based on its rated voltage, efficiency, and drive current regulation. A configuration where each string  220  to be coupled to a unique driver  210  can lead to increased power consumption. Additionally, using multiple strings  220  can lead to higher complexity or reduced image quality due to drive current mismatches between the strings  220 . The current mismatches can be due to, e.g., mismatches in the components (e.g., transistors, resistors, etc.) of the different drivers and mismatches in the offsets of the amplifiers in the drivers (not shown). 
     Exemplary Operations of Multi-String LED Drivers 
       FIGS. 4A-4F  illustrate timing diagrams of exemplary operations, and  FIG. 4G  illustrates a process flow of an exemplary operation of multi-string LED drivers, according to examples of the disclosure. The backlight circuit  200  can be operated based on a number of factors, such as the period of the PWM clock cycle, the duty cycle of the drive current, and the number of strings  220  in a set of strings. In some examples, the operation of the backlight circuit  200  may change while the display is being operated in real-time. 
     As discussed above, the brightness of a display can be adjusted by changing the duty cycle of the drive current. The duty cycle can be related to the length of time of the on pulses of the drive current relative to its period. For example, a larger duty cycle can be associated with a wider pulse. 
     With a set of drivers  210  capable of coupling to different strings  220 , the backlight circuit  200  can utilize a single driver or less than all of the set of drivers  210  to control some or all of the strings  220 . The number of drivers  210  activated (e.g., actively controlling one or more strings  220 ) may depend on the period  312  of the PWM clock, the width of the pulse of the drive current I LED    222 , and the number of strings  220 . In some examples, the period  312  of the PWM clock can be equal to the duration of a display frame. 
     Examples of the disclosure can include using all or less than all of the backlight circuit  200  in the backlight circuit  200 . The drivers used to control the strings  220  within a given clock cycle can be referred to as a set of drivers. Examples of the disclosure can also include using all or less than all of the strings  220 . The strings that are controlled by the drivers within a given clock cycle can be referred to as a set of strings. 
     In some examples, a single driver  210  can be used to control a set of strings  220 .  FIG. 4A  illustrates a timing diagram of an exemplary operation of a set of strings controlled by a single driver. The PWM clock  311  can have a period  312 . The strings  220  can be turned on (e.g., coupled to and actively controlled by one or more drivers  210 ), one at a time, and in a manner such that all of the strings  220  are turned on once before any string is turned on a second time (i.e., a round). In some examples, the strings  220  can be turned on and turned off (e.g., not coupled to and not being actively controlled by a driver  210 ) sequentially such that one string (e.g., string  220 A) is turned on and turned off before another string (e.g., string  220 B) is turned on. For example, the string  220 A can be turned on at time T 1  for a duration of time  313 A; the string  220 B can be turned on at time T 2  for a duration of time  313 A; the string  220 C can be turned on at time T 3  for a duration of time  313 A; the string  220 D can be turned on at time T 4  for a duration of time  313 A; and the string  220 E can be turned on at time T 5  for a duration of time  313 A. 
     The period  312  can be divided into a set of time intervals, which are five time intervals in the example of  FIG. 4A . A string  220  can be turned on at every time interval: T 1 , T 2 , T 3 , T 4 , and T 5 . In some examples, the time  313 A can be such that the same first string  220  is turned on before a second string  220  is turned on. That is, the time  313 A can be less than the time between adjacent time intervals (e.g., time  313 A is less than the difference between T 2  and T 1 ). In other words, the time  313 A can be such that it is less than the period  312  divided by the number of strings  220  (e.g., five strings). The time  313 A and the number of strings  220  can be such that all of the set of strings  220  can be turned on and turned off within the period  312 . As a result, a single driver (e.g., only driver  210 A illustrated in  FIG. 3 ) can be used to control a set of strings  220  of the display. 
     Using a single driver  210  to control a set of strings  220  can be beneficial for certain applications, such as low brightness applications. When multiple drivers are used in low brightness applications, a current mismatch can exist between different strings  220 . This current mismatch can lead to unwanted effects such as different rise and fall times, mismatches and corresponding linearity differences between different digital-to-analog converters (DACs), driver offsets, and other errors. As such, the use of a single driver  210  for low brightness application can help reduce these mismatches and linearity differences. 
     With the time  313 A being less than the period  312  divided by the number of strings, another time  313 B can exist between two strings  220  being operated by the same driver  210 . In some examples, during this time, the driver  210  may not be biased (i.e., turned off) until a subsequent string is being controlled (e.g., after time  313 B has lapsed). In some examples, during this time, the driver  210  may maintain its bias point by operating in an idle state, but the driver may not be coupled (e.g., controlling) to a string  220 ; more details about the idle state operation of a driver is described below. 
     In some examples, although a single driver  210  may be used throughout a given clock cycle  311 , the backlight circuit  200  may change to use different drivers  210  (e.g., a different set of drivers). For example, the driver  210 A can be used for the clock cycle  311 A; the driver  210 B can be used for the clock cycle  311 B; etc. As a result, mismatches (e.g., due to different drive currents I LED    222 ) can be reduced when their values are averaged over time. 
       FIG. 4B  illustrates a timing diagram of an exemplary operation of set of strings being controlled by a single driver. In some examples, the operation shown in  FIG. 4B  can be similar to that shown in  FIG. 4A , but the time that a string is on may be longer (e.g., time  313 C is greater than time  313 A). The PWM clock  311  can have a period  312 , which can be divided into a set of time intervals (T 1 , T 2 , T 3 , T 4 , and T 5 ). 
     Similar to the operation of  FIG. 4A , the strings  220  can be turned on, one at a time, and all of the strings  220  can be turned on a first time before any string is turned on a second time. In some examples, the strings  220  can be turned on and turned off sequentially such that one string (e.g., string  220 A) is turned on and turned off before another string (e.g., string  220 B) is turned on. Additionally, a string  220  can be turned on at every time interval. For example, the string  220 A can be turned on at time T 1  for a duration of time  313 C; the string  220 B can be turned on at time T 2  for a duration of time  313 C; the string  220 C can be turned on at time T 3  for a duration of time  313 C; the string  220 D can be turned on at time T 4  for a duration of time  313 C; and the string  220 E can be turned on at time T 5  for a duration of time  313 C. 
     The time  313 C can be such that a first string  220  can be turned off before a second string  220  is turned on. In some examples, the time  313 C can be equal to a time interval (e.g., the difference between T 2  and T 1 ). In this operation, the time  313 C can be such that it is equal to the period  312  divided by the number of strings  220  (e.g., five strings). The time  313 C and the number of strings  220  can be such that all of the set of strings  220  can be turned on and turned off within the period  312 . As a result, a single driver (e.g., only driver  210 A illustrated in  FIG. 3 ) can be used to control a set of strings  220  of the display. The time  313 C may be different (e.g., longer) from the time  313 A of  FIG. 4A , and as such, there may not be an off time (e.g., time  313 B of  FIG. 4A ) for the driver  210  as it switches between strings  220 . The “off time” for a driver  210  may refer to the driver being in an off state where it does not operating at all or is operating in an idle mode, as discussed below. In some instances, a driver  210  may remain on for the entire duration of the period  312  and may be coupled to a different string  220  for different time intervals of the period  312 . For example, the string  220 B can be turned on immediately after the string  220 A is turned off while the driver  210 A remains on. 
     In some examples, similar to as discussed above with  FIG. 4A , the backlight circuit  200  may change between multiple drivers  210  for, e.g., different clock cycles. For example, the driver  210 A can be turned on for the clock cycle  311 A; the driver  210 B can be turned on for the clock cycle  311 B; etc. Additionally, like  FIG. 4A , a single driver  210  can be used to control a set of strings  220 , and the benefits can be correspondingly similar. For example, mismatches (e.g., due to different drive currents I LED    222 ) can be reduced when their values are averaged over time. 
     In some examples, the strings  220  may be not be controlled sequentially.  FIG. 4C  illustrates a timing diagram of a set of strings  220  controlled by a set of drivers  210 . The PWM clock  311  can have a period  312 , which can be divided into a set of time intervals (T 1 , T 2 , T 3 , T 4 , and T 5 ). The strings  220  can be turned on, one at a time, where a string can be turned on at every time interval. For example, the string  220 A can be turned on at time T 1  for a duration of time  313 D; the string  220 B can be turned on at time T 2  for a duration of time  313 D; the string  220 C can be turned on at time T 3  for a duration of time  313 D; the string  220 D can be turned on at time T 4  for a duration of time  313 D; and the string  220 E can be turned on at time T 5  for a duration of time  313 D. 
     The time  313 D can be greater than a time interval (e.g., the difference between T 2  and T 1 ). In some instances, the time  313 D can be equal to the duration of two time intervals (e.g., the difference between T 3  and T 1 ). In this operation, the time  313 D can be such that it is greater than the two times the period  312  divided by the number of strings in a set of strings (e.g., five strings). In some examples, the time  313 D can also be less than three times the period  312  divided by the number of strings in a set of strings. 
     In some instances, the time  313 D may not allow all of the strings  220  to be turned on, one at a time, and then also turned off during a single clock cycle  311 . As one way to reduce the total time for controlling a set of strings  220 , in some examples, a string can be turned on, while another string is already on. For example, at time T 2 , the string  220 A may already be on (e.g., coupled to a driver), while the string  220 B is turned on. In some examples, the minimum number of drivers  210  needed to activate the strings for a given clock cycle  311  can be equal to the number of time intervals that the time  313 D is equal to. In this example, the minimum number of drivers  210  needed may be two. 
     Examples of the disclosure can include using different drivers  210  to control a set of strings  220 . As one example, a first driver (e.g., driver  210 A) can be used to control the string  220 A, the string  220 C, and the string  220 E. In this manner, there may not be an off time for the driver  210 A as it switches between strings  220 . Additionally, a second driver (e.g., driver  210 B) can be used to control the string  220 B and the string  220 D. In some instances, there may not be an off time for the driver  210 B as it switches between strings  220 . As such, two drivers  210  can be used to control the strings  220  for a given clock cycle  311 . 
     In some examples, similar to as discussed above with  FIG. 4A , the backlight circuit may change between multiple drivers  210  for, e.g., different clock cycles. In the previous example, discussed immediately above, the drivers  210 A and  210 B can be used for the clock cycle  311 A. In clock cycle  311 B, different drivers, e.g., the drivers  210 C and  210 D, can be used. 
     In other examples, more than two drivers  210  can be used to control a set of strings  220  for a given clock cycle  311 . For example, the driver  210 A can be used to control the string  220 A and the string  220 D; the driver  210 B can be used to control the string  220 B and the string  220 E; and the driver  210 C can be used to control the string  220 C. In this manner, there may be an off time for the drivers  210 . 
     Additionally, examples of the disclosure can include repeating the use of a set of drivers. For example, at a third clock cycle (not shown), the same set of drivers from the first clock cycle  311 A can be used. Similarly, at a fourth clock cycle (not shown), the same set of drivers from the second clock cycle  311 B can be used, etc. 
     As another example, only the minimum number of drivers needed may be used. In the example of  FIG. 4C , a minimum of two drivers  210  is needed, as discussed above. In such instances, the same two drivers can be used to turn on and turn off the strings, which can reduce current mismatches when their values are averaged over time. 
       FIG. 4D  illustrates a timing diagram of an exemplary operation of a set of strings controlled by a set of drivers. In some examples, the operation shown in  FIG. 4D  can be similar to that shown in  FIG. 4C , but the time that a string is on may be longer (e.g., time  313 F is greater than time  313 D). The PWM clock  311  can have a period  312 , which can be divided into a set of time intervals (T 1 , T 2 , T 3 , T 4 , and T 5 ). 
     Similar to the operation of  FIG. 4C , the strings  220  can be turned on, one at a time, where a string can be turned on at every time interval. Referring back to  FIG. 4D , the duration of which a string can be controlled by a driver  210  can be time  313 F. The time  313 F can be greater than a time interval (e.g., the difference between T 2  and T 1 ). In some instances, the time  313 F can be equal to the duration of three time intervals (e.g., the difference between T 4  and T 1 ). 
     In this operation, the time  313 F can be such that it is greater than three times the period  312  divided by the number of strings  220  in a set of strings (e.g., five strings). In some examples, the time  313 F may also be less than four times the period  312  divided by the number of strings. 
     In some instances, the time  313 F may not allow all of the strings  220  in a set of strings to be turned on, one at a time, and then also turned off during a single clock cycle  311 . As one way to reduce the total time for controlling a set of strings  220 , in some examples, a string can be turned on, while multiple strings are already on. For example, at time T 3 , the strings  220 A and  220 B may be on (e.g., coupled to a driver), while the string  220 C is turned on. In some examples, the minimum number of drivers  210  needed to activate the set of strings for a given clock cycle  311  can be equal to the number of time intervals that the time  313 F is equal to. In this example, the minimum number of drivers  210  needed may be three. 
     Examples of the disclosure can include using different drivers  210  to control a set of strings  220 . As one example, a first driver (e.g., driver  210 A) can be used to control the string  220 A and the string  220 D. In this manner, there may not be an off time for the driver  210 A as it switches between strings  220 . Additionally, a second driver (e.g., driver  210 B) can be used to control the string  220 B and the string  220 E. In some instances, there may not be an off time for the driver  210 B as it switches between strings  220 . Further, a third driver (e.g., driver  210 C) can be used to control the string  220 C. In this example, three drivers  210  are used to control the set of strings  220  for a given clock cycle  311 . 
     In some examples, similar to as discussed above with  FIG. 4A , the backlight circuit may change between multiple drivers  210  for, e.g., different clock cycles. In the previous example, discussed immediately above, the drivers  210 A,  210 B, and  210 C can be used for the clock cycle  311 A. In clock cycle  311 B, different drivers, e.g., the drivers  210 D and  210 E, can be used. 
     In other examples, more than three drivers  210  can be used to control a set of strings  220  for a given clock cycle  311 . For example, the driver  210 A can be used to control the string  220 A and the string  220 E; the driver  210 B can be used to control the string  220 B; the driver  210 C can be used to control the string  220 C; and the driver  210 D can be used to control the string  220 D. In this manner, there may be an off time for the drivers  210 . 
     Additionally, examples of the disclosure can include repeating the use of a set of drivers. For example, at a third clock cycle (not shown), the same set of drivers from the first clock cycle  311 A can be used. Similarly, at a fourth clock cycle (not shown), the same set of drivers from the second clock cycle  311 B can be used, etc. 
     As another example, only the minimum number of drivers needed may be used. In the example of  FIG. 4D , a minimum of three drivers  210  is needed, as discussed above. In such instances, the same set of three drivers can be used to turn on and turn off the strings, which can reduce current mismatches when their values are averaged over time. 
       FIG. 4E  illustrates a timing diagram of an exemplary operation of a set of strings being controlled by a set of drivers. In some examples, the operation shown in  FIG. 4E  can be similar to that shown in  FIG. 4D , but the time that a string is on may be greater (e.g., time  313 H is greater than time  313 F). The PWM clock  311  can have a period  312 , which can be divided into a set of time intervals (T 1 , T 2 , T 3 , T 4 , and T 5 ). 
     The strings  220  can be turned on, one at a time, where a string can be turned on at every time interval. For example, the string  220 A can be turned on at time T 1  for a duration of time  313 H; the string  220 B can be turned on at time T 2  for a duration of time  313 H; the string  220 C can be turned on at time T 3  for a duration of time  313 H; the string  220 D can be turned on at time T 4  for a duration of time  313 H; and the string  220 E can be turned on at time T 5  for a duration of time  313 H. 
     The time  313 H can be greater than a time interval (e.g., the difference between T 2  and T 1 ). In some instances, the time  313 H can be equal to the duration of four time intervals (e.g., the difference between T 5  and T 1 ). In this operation, the time  313 H can be such that it is greater than four times the period  312  divided by the number of strings in a set. Similar to  FIG. 2E , a string can be turned on, while multiple strings are already on. In some examples, the minimum number of drivers  210  needed to activate the set of strings for a given clock cycle  311  can be equal to the number of time intervals that the time  313 H is equal to. In this example, the minimum number of drivers  210  needed may be four. 
     Examples of the disclosure can include using different drivers  210  to control a set of strings  220 . As one example, a first driver (e.g., driver  210 A) can be used to control the string  220 A; a second driver (e.g., driver  210 B) can be used to control the string  220 B; a third driver (e.g., driver  210 C) can be used to control the string  220 C; a fourth driver (e.g., driver  210 D) can be used to control the string  220 D; and a fifth driver (e.g., driver  210 E) can be used to control the string  220 E. In some examples, similar to as discussed above, a given string may be controlled by a different set of drivers for different clock cycles, which can reduce current mismatches for that string when their values are averaged over time. For example, the string  220 A can be controlled by the driver  210 A during the clock cycle  311 A, but may be controlled by the driver  210 B during the clock cycle  311 B. 
     In some examples, only the minimum number of drivers needed may be used. In the example of  FIG. 4E , a minimum of four drivers  210  is needed, as discussed above. In such instances, the same set of four drivers can be used to turn on and turn off the strings, which can reduce current mismatches when their values are averaged over time. 
     In some instances, the time that a string  220  is turned off may be less than the duration of a time interval. As shown in  FIG. 4F , the time  313 J may represent the duration that a string  220  is on, and the time  313 K may representation the duration that a string is off. The time  313 K may be less than the duration of a time interval (e.g., difference between T 2  and T 1 ). In such instances, the minimum number of drivers needed to control the set of strings  220  can be equal to the number of strings in the set, which can be five drivers  210  and five strings  220  in the example of  FIG. 4F . Example of the disclosure can include changing the driver  210  used to control a given string  220 , changing the order of the drivers  210  activating the strings, etc., as discussed above. 
       FIG. 4G  illustrates a corresponding exemplary process. The process  450  can begin with step  452 , where a selected driver can be coupled a selected string. For example, as described in  FIGS. 4A-4F , driver  210 A is the selected driver that controls a first string  220 A, the selected string, at time T 1 . At step  454 , a determination of whether all of the strings within a set have been coupled for a given round. If so, the process  450  can proceed with changing the selected driver in step  462 . 
     If not, the process  450  can proceed controlling the remaining strings within the set with step  456 . In step  456 , at the next time interval, a determination is made as to whether the selected driver is still coupled to the selected string. The selected driver may still be coupled to the selected string if the selected driver is still controlling the selected string. For example, at time T 2  of  FIG. 4C , the driver  210 A may still be coupled to the string  220 A. If the selected driver is not coupled to the selected string, then the same selected driver can be coupled to the next string (step  458  of process  450 ). For example, at time T 2  of  FIG. 4B , the driver  210 A may not be coupled to the selected string  220 A, so the driver  210 A may be coupled to the next string, string  220 B. 
     If the selected driver is coupled to the selected string, then the next driver can be coupled to the next string (step  460  of process  450 ).  FIG. 4D  illustrates such an example. At time T 2 , the selected driver may be driver  210 A coupled to the selected string  220 A. The next driver may be the driver  210 B, which can be coupled to the next string  220 B. 
     The process  450  proceeds to step  462  when a round of controlling all strings within a set of strings has been completed. As an example, in  FIG. 4A , a first round can include controlling all of the strings  220 A- 220 E during the clock cycle  311 A using driver  210 A. The second round can occur during the clock cycle  311 B, where the driver  210 B can be used to control the set of strings. 
     In step  462 , a determination is made as to whether all of the drivers within a set have been coupled to the set of strings. If so, then the process  450  repeats with the next set of drivers switched in step  464 . Step  464  is shown with the example of  FIG. 4C , where at clock cycle  311 B, drivers  210 C and  210 D can be used, instead of drivers  210 A and  210 B (used in clock cycle  311 A). 
     If not all drivers within a set have been coupled to the set of strings, then in step  466 , the process continues with the next driver within the same set. 
     Exemplary Driver 
     A driver  210  can include a set of circuit components. One exemplary driver is shown in the schematic diagram of  FIG. 5A . The driver  210 - 1  can be coupled to a string  220  and can receive a voltage signal V REF    338 , a PWM bus  310 , and drive currents I LED    222  as inputs. In some examples, each driver  210  can receive a unique signal from the PWM bus  310  that the other drivers  210  may not receive. In some examples, the PWM bus  310  can include individual signals. The individual signals may be coupled to a unique transistor  308  to turn it on. In some examples, the PWM bus signals can be generated by inputting a PWM signal having a duty cycle to a decoder (not shown) coupled to all of the transistors  308 ; at a given time, the PWM signal can drive the decoded output. In some examples, the PWM bus  310  can include one PWM signal that drives a gate of one transistor  308 . In these examples, the strings  220  can couple to the inputs of a multiplexer (not shown), and an output of the multiplexer can couple to a source or drain of the one transistor  308 . The multiplexer can be controlled to selectively couple a selected string to the transistor  308 . The driver  210 - 1  can include a set of transistors  308  and transistor  332  (e.g., field effect transistors such as metal oxide semiconductor field effect transistors (MOSFETs)) that can control the drive current I LED    222 . Although the driver  210  is illustrated as including transistors, it is understood that switching elements other than transistors can also be used. 
     The transistor  308  can be a PWM-controlled transistor having a first source/drain terminal coupled to a string  220 , a gate terminal capable of receiving a signal from the PWM bus  310 , and a second source/source terminal. The transistor  308  can be coupled to a PWM signal source or sources (not shown) that provides the signals of the PWM bus  310 , to control the drive current I LED    222  through a string  220  by reducing the total charge allowed according to the duty cycle of a PWM bus  310 . 
     The transistor  332  can be a current regulation transistor having a first source/drain terminal coupled to the second source/drain terminal of the transistor  308 , a gate terminal coupled to an output of an amplifier  330 , and a second source/drain terminal coupled to ground through a resistor  216 . In some examples, the resistor  216  can determine the gain (e.g., transconductance) of the driver  210 . The transistor  332 , in combination with a source that supplies a reference voltage V REF    338  and the operational amplifier  330 , can form a current regulation controller for the LEDs  221  to control the drive current I LED    222  through a string  220 . 
     In this manner, the driver  210  can include a cascade of two switching devices to control the drive current I LED    222 , one configured for current regulation (for analog dimming) and the other implemented as a switch for PWM pulse-width current control (for PWM dimming). 
     The signals of the PWM bus  310  can include alternating on pulses and off pulses. The width (e.g., the length of time of an on pulse) of the pulses can be used to control the duty cycle of the drive current through the LEDs  221  to, for example, causing dimming of the LEDs  221  without changing the voltage V O    223  or V REF    338 . In some examples, the operational amplifier  330  may be used to help reduce or prevent voltage spikes at the gate terminal at or near the rising edge of a signal of the PWM bus  310  (e.g., when transistor  332  turns on). During the off pulses, the transistor  308  can be off, so there may be substantially no current flowing in the LEDs  221 . 
     Exemplary Driver with an Auxiliary Transistor 
     In some examples, the driver can also include an auxiliary transistor  309 . The transistor  309  can be coupled to a voltage source V INT    225 . In some examples, each driver  210  can include a unique voltage source V INT    225  that may not be connected to any other driver  210 . The transistor  309  can be enabled for a short period of time to allow the driver  210  to settle (e.g., reach and stabilize) before being switched to control the respective string  220 . In some examples, the transistor  309  turns on before a signal of the PWM bus  310  turns on (e.g., a signal controlling the transistor  309  (not shown) is an inverted version of the PWM signals). In this manner, the transistor  309  can be enabled before the respective string is coupled to the driver  210 - 1 , thereby leading to reduced rise and fall times for the drive current I LED    222  through a given string  220 . 
       FIG. 5B  illustrates a process flow, and  FIG. 5C  illustrates a timing diagram of an exemplary operation of a driver including an auxiliary transistor, according to examples of the disclosure. The driver  210 - 1  may not be coupled to the respective string  220  (e.g., the transistors  308  may be off) (step  552  of process  550 ). The transistor  309  can be turned on (step  554  of process  550 ). The transistor  309  may be turned on immediately before a time interval, such as shown in  FIG. 5C . That is, during time  313 L, the transistor  309  may be turned on. 
     The transistor  309  can be used to bias the driver  210  by creating a high current path from the source V INT    225  to the transistor  332  to charge up the gate to source capacitance of the transistor  332  (step  556  of process  550 ). In some examples, the time  313 L is determined by the time needed for the driver  210  to settle to a targeted bias point. 
     In step  558 , the corresponding driver  210  may be coupled to a string  220  (e.g., by turning one of the transistors  308 ). The time during which the driver  210  is coupled to a string  220  is indicated as the time  313 M in  FIG. 5C . In step  560 , the transistor  309  can be turned off. In some examples, step  554  and step  556  can occur simultaneously. 
     Turning on the transistor  309  before the transistor  308  can increase the speed of the driver  210  by increasing the rise and fall time of the drive current I LED    222 , which can lead to reduced occurrences of overshoots and undershoots. Additionally, the transistor  309  can be used to create a high current path to help charging parasitic capacitances, which can reduce the amount of charging and discharging current taken from a string  220  when the driver  210  is coupled to another string  220 . Increasing the transient response time of a LED driver may be suitable for high frequency operation and may help reduce harmonic noise, which may be audible. In addition, the faster transition times may result in reduce the minimum duty cycle and enhance the dimming resolution. 
     Exemplary Amplifier with an Idle State 
     Examples of the disclosure can further include other configurations for the driver, such as one that includes component(s) for maintaining the driver in an idle state.  FIG. 6A  illustrates a schematic diagram of an exemplary driver  210 - 2  including components for operating the driver in an idle state according to examples of the disclosure. The driver  210 - 2  can include one or more components that are correspondingly similar to the driver  210 - 1  of  FIG. 5A . For example, the driver  210 - 2  can include the set of transistors  308 , the transistor  332 , the resistor  216 , and the amplifier  330 . Additionally, the driver  210 - 2  can receive similar input signals: V REF    338 , the PWM bus  310 , and the drive currents I LED    222 . 
     The driver  210 - 2  may also include an idle transistor  219 , a resistor  218 , and switches  231 . The transistor  219  can have a first source/drain terminal coupled to a voltage source V INT    224 , a gate terminal coupled to an output of the amplifier  330 , and a second source/drain terminal coupled to ground through a resistor  218 . Although the figure illustrates the driver  210 - 2  as including the transistor  309  and source V INT    225 , examples of the disclosure can include a driver that does not include both the auxiliary transistor  309  and an idle transistor  219 . 
     The driver  210 - 2  can include a plurality of switches  231 A and  231 B coupled to an input signal V START    217 A and an idle signal V IDLE    217 B. The input signal V START    217 A and idle signal V IDLE    217 B can control the switches  231  to couple the operational amplifier  330  to certain components regulating the drive current I LED    222 . When the driver  210 - 2  is controlling the drive current I LED    222  of a string  220 , the switch  231 A can couple the operational amplifier  330  to the transistor  332  and the resistor  216 , thereby forming a feedback loop in the current regulation circuit. 
     When the driver  210 - 2  is not controlling the drive current I LED    222  of a string  220 , the switch  231 B can couple the operational amplifier  330  to transistor  219  and the resistor  218 . While coupling the amplifier  330  to the transistor  219  and the resistor  218 , a low current path can exist in the driver  210 - 2 , thereby maintaining the feedback and keeping the output of the amplifier  330  at a certain bias voltage. In this manner, the driver  210 - 2  can be kept in regulation (e.g., constant) regardless of the PWM state. In some examples, the idle signal V IDLE    217 B coupled to switch  231 B can be an inverted signal of input signal V START   217 . In some examples, the inverted input signal can be generated by inputting V START   217 A to an inverting amplifier (not shown). 
     In some instances, the use of both PWM dimming and analog dimming to adjust the brightness of the display may lead to increased non-linearity due to the need for transitions between PWM dimming and analog dimming methods. The use of both dimming methods may also lead to overlaps between the driver current&#39;s falling edge and subsequent rising edge at high duty cycles. 
     In other instances, the power efficiency of the driver  210 - 2  may be enhanced due to the output of amplifier  330  being capable of being kept constant. The low current path can reduce the need for the amplifier  330  to slew and respond fast, while the driver  210  is being coupled to another string  220 . In some examples, since the need for amplifier  330  to respond quickly (e.g., high bandwidth, fast response) is reduced, the required quiescent current for the drivers may be reduced. Due to the improved power efficiency, analog dimming may not be required at high brightness (e.g., PWM dimming can be used solely for all brightness levels) to improve power efficiency. As such, linearity may be increased and offset currents may be reduced. 
     As discussed above, the driver  210 - 2  can be operated such that the transistor  219  and the voltage source V INT    224  create a low current path to keep the amplifier  330  on. Keeping the amplifier  330  on can help achieve faster rise and fall times. In some examples, one or more components can be configured to achieve this low current path. For example, the transistor  219  can be configured such that it is smaller (e.g., N times smaller in size) than the transistor  332 . As another example, V INT    224  can be such that the current through the transistor  219  can be smaller than the drive current I LED    222 . Additionally or alternatively, the resistor  218  can be configured with a resistance that is larger (e.g., N times larger) than resistor  216 . 
       FIG. 6B  illustrates an exemplary process flow for operating an amplifier of a driver in an idle mode according to examples of the disclosure. The driver  210 - 2  may be coupled to the respective string  220  (step  652  of process  650 ). When the driver  210 - 2  is to be coupled to the respective string  220 , the amplifier  330  may be coupled to the transistor  332 , and the switch  231 B may cause the amplifier  330  to be decoupled from the idle mode transistor  219 . The driver  210 - 2  may then control the respective string in step  654 . 
     In step  656 , the driver  210 - 2  may be decoupled from the respective string  220 . This step may include turning off the transistor  308 . Additionally, this step may include using the signal V START  to control the switches  231 . The switch  231 A may decouple the amplifier  330  from the transistor  308 . 
     In step  658 , the switch  231 B can couple the amplifier  330  to the transistor  219 , and the transistor  219  can be turned on. The transistor  219 , along with the source V INT    222 , can create a low current path to keep the amplifier  330  on (step  660  of process  650 ). 
     Exemplary Phase Shifting Techniques 
     In some instances, the use of phase shifting in LED currents can lead to many benefits such as reduced ripple with the input and output voltage signals, reduced need for large output capacitors to dampen out the noise, reduced audible noise, reduced load transients, and reduced spreading in the light output (which can reduce the video and audio interference from the backlight unit). Conventional phase shifting techniques may consume more power and be noisy because each LED string may be separately driven by a unique driver, causing additional switching noise and offset currents when the falling edge of the drive current from one driver overlaps with the rising edge of the drive current from another driver. 
     Examples of the disclosure can include a second string that can be turned on right after a first string is turned off. In this manner, switching between drivers can be reduced. The reduced switching may lead to improved power efficiency and performance. 
       FIG. 7A  illustrates a timing diagram of an exemplary operation, which includes phase shifting, of a set of strings controlled by a single driver according to examples of the disclosure. The strings  220  can be controlled by the driver  210 A. The operation of the strings can be similar to that discussed above with  FIG. 4A  and  FIG. 4B , where one string (e.g., string  220 A) can turned on and turned off before another string (e.g., string  220 B) is turned on. Specifically, in the example illustrated in  FIG. 7A , the strings turn on and off right after another. As a result, the driver  210 A can stay on and can be rotated (e.g., switched to being coupled to a different string) as the strings  220  are driven sequentially. Having the driver  210 A stay on while the strings  220  are being controlled can result in reduced overlaps, offset currents, noise, power consumption, or a combination thereof. 
     In some examples, for a second clock cycle (e.g., clock cycle  311 B; not shown), a different driver (e.g., driver  210 B) can be used to control the set of strings  220 . In a third clock cycle, yet another driver can be used, etc. In some examples, all of the drivers  210  can be used in all of the clock cycles, and the drive current I LED    222  through each string  220  can be averaged over the N clock cycles to reduce the effect of mismatches between the drive currents I LED    222 . 
     As mentioned above, a driver  210  may be activated at different times of a clock cycle. As shown in  FIG. 7A , control of the string  220 A (e.g., the pulse signal) by driver  210 A can be centered at time T 1′ ; control of the string  220 A (e.g., the pulse signal) by driver  210 A can centered at time T 2′ , etc. In some instances, T 1′  may occur after the rising edge of the PWM clock  311 . 
     Examples of the disclosure can further include applying the same technique to  FIG. 4B , as shown with the timing diagram in  FIG. 7B . In this example, each pulse associated with each of the strings  220  may be equal to the period  312  divided by N. 
     In some examples, more than one driver may be needed, such as in the operation shown in  FIG. 4C . As discussed above, the strings  220  can be turned on for the time  313 D, and then turned off sequentially such that a first string is turned off while a second string is turned on. In this example, since the pulses associated with the strings may be greater than the period  312  divided by N, more than one driver  210  may be needed to turn all the strings on and off. As illustrated, the number of drivers  210  that may be needed to drive all the set of strings during a single clock cycle  311  equals two. Strings  220 A to  220 (N−1) can turned on and turned off before another string (e.g., string  220 B) is turned on while driver  210 A stays on. Since there may be insufficient remaining time to drive string  220 N, the driver  210 B can be used to drive string  220 N. 
     In some examples, a single driver can be used to control as many strings as it can. In some examples, the maximum number of strings that a driver can control can be equal to the period  312  divided by the pulse width (e.g., time  313 D). The remainder of the strings can be controlled with another driver. As shown in the example of  FIG. 7C , the driver  210 A can control most the strings  220 . Having the driver  210 A stay on while the string  220 A to string  220 (N−1) are being controlled can result in reduced overlaps, offset currents, noise, and power consumption. Another driver, such as the driver  210 B, can be used to control the remainder string: string  220 N. 
     In some examples, for a second clock cycle (e.g., clock cycle  311 B; not shown), different drivers (e.g., drivers  210 C and  210 D) can be used to control the set of strings  220 . In a third clock cycle, yet another driver can be used, etc. In some examples, all of the drivers  210  can be used in clock cycles, and the drive current I LED    222  through each string  220  can be averaged over the N clock cycles to reduce the effect of mismatches between the drive currents I LED    222 . 
     In some examples, multiple drivers can operate simultaneously. For example, as shown in the  FIG. 7D , control of some (e.g., half) of the strings (e.g., pulse signals associated with the strings) by driver  210 A to driver  210 (N/2) can be centered at time T 1′ . For example, every other (e.g., “odd”) string of the set of strings  220 A- 220 N can be controlled at this time. The drivers may also switch to controlling different strings, for example, “even” strings of  220 A- 220 N, centered at the time T 2′ . In some examples, control of the second portion of the strings can be delayed, relative to the first half, so that the drive currents can be distributed in time. In some instances, the delay for a second driver (e.g., driver  210 B) may be equal to the period  312  minus the total pulse widths of strings being driven by the second driver (e.g., as illustrated, period  312  minus two times the time  313 F). 
     In some examples, for a second clock cycle (e.g., clock cycle  311 B; not shown), different drivers can be used to control the set of strings  220  (e.g., using driver  210 A to control strings  220 C and  220 D, using driver  210 B to control strings  220 A and  220 B). In a third clock cycle, different drivers can be used for each string, etc. In some examples, all of the drivers  210  can be used in the clock cycles, and the drive current I LED    222  through each string  220  can be averaged over the N clock cycles to reduce the effect of mismatches between the drive currents I LED    222 . 
     In some examples, the duty cycle of the LED currents may be over 50%. In other words, for a given period, a driver may not be able to drive at least two strings immediate after one another. In these examples, to ensure uniformity of the display brightness, at least two (e.g., each) of the drivers can turn on at a different time of the period. For example, the drivers can turn on at every increment of the period divided by N. 
     In some examples, multiple drivers can be operated simultaneously, such as the driver  210 A to control the string  220 A; the driver  210 C to control the string  220 C; the driver  210 (N−1) to control the string  220 (N−1); and so on, as shown in  FIG. 7E . At a second time, driver  210 B can control the string  220 B; the driver  210 D can control the string  220 D; the driver  210 N can control the string  220 N; and so on. There may also be a delay between drivers, such as a delay T 2  for the drivers  210 B,  210 D,  210 N, etc. In some instances, the delay for a second driver (e.g., driver  210 B) may be equal to the period  312  minus the time  313 H (e.g., LED current pulse width). In some examples, a driver  210  may control only one string for each clock cycle  311 . 
     In some instances, when duty cycle of the driver current is over 50%, for a second clock cycle (e.g., clock cycle  311 B; not shown), different drivers can be used to control the set of strings  220  (e.g., using driver  210 A to control string  220 B, using driver  210 B to control string  220 C, etc.). In a third clock cycle, different drivers can be used for each string, etc. In some examples, all of the drivers  210  can be used in the clock cycles, and the drive current I LED    222  through each string  220  can be averaged over the N clock cycles to reduce the effect of mismatches between the drive currents I LED    222 . 
     In some examples, as discussed above, the backlight circuit may change which driver  210  controls a given string for, e.g., different clock cycles. For example, the string  220 A can be controlled by the driver  210 A during the clock cycle  311 A, but may be controlled by the driver  210 B during the clock cycle  311 B, etc. 
     Various functions described above can be implemented in digital electronic circuit, in computer software, firmware, or hardware. The techniques can be implemented using one or more computer program products. Programmable processors and computers can be included in or packaged as mobile devices. The processes and logic flows can be performed by one or more programmable processors and by one or more programmable logic circuit. General and special purpose computer devices and storage devices can be interconnected through communication networks. 
     Some implementations include electronic components, such as microprocessors, storage, and memory that can store computer program instructions in a machine-readable or computer-readable medium (alternatively referred to as computer-readable storage media, machine-readable media, or machine-readable storage media). Some examples of such computer-readable media include RAM, ROM, read-only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, ultra-density optical discs, any other optical or magnetic media, and floppy disks. The computer-readable media can store a computer program that is executable by at least one processing unit and includes sets of instructions for performing various operations. Examples of computer programs or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     While the above discussion primarily refers to microprocessor or multi-core processors that execute software, some implementations are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some implementations, such integrated circuits execute instructions that are stored on the circuit itself. 
     As used in this specification and any claims of this application, the terms “computer,” “processor,” and “memory” all refer to electronic or other technological devices. These terms exclude people or groups of people. For the purposes of the specification, the terms “display” or “displaying” means displaying on an electronic device. As used in this specification and any claims of this application, the terms “computer readable medium” and “computer readable media” are entirely restricted to tangible, physical objects that store information in a form that is readable by a computer. These terms exclude any wireless signals, wired download signals, and any other ephemeral signals. 
     To provide for interaction with a user, implementations of the subject matter described in this specification can be implemented on a computer having a display device described herein for displaying information to the user and a keyboard and a pointing device, such as a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, such as visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speed, or tactile input. 
     Many of the above-described features and applications are implemented as software processes that are specified as a set of instructions recorded on a computer readable storage medium (also referred to as a computer readable medium). When these instructions are executed by one or more processing unit(s) (e.g., one or more processors, cores of processors, or other processing units), they cause the processing unit(s) to perform the actions indicated in the instructions. Examples of computer readable media include, but are not limited to, CD-ROMs, flash drives, RAM chips, hard drives, EPROMs, etc. The computer readable media does not include carrier waves and electronic signals passing wirelessly or over wired connections. 
     In this specification, the term “software” is meant to include firmware residing in read-only memory or applications stored in magnetic storage, which can be read into memory for processing by a processor. Also, in some implementations, multiple software aspects of the subject disclosure can be implemented as sub-parts of a larger program while remaining distinct software aspects of the subject disclosure. In some implementations, multiple software aspects can also be implemented as separate programs. Any combination of separate programs that together implement a software aspect described here is within the scope of the subject disclosure. In some implementations, the software programs, when installed to operate on one or more electronic systems, define one or more specific machine implementations that execute and perform the operations of the software programs. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a standalone program or as a module, component, or subroutine, object, or other component suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, subprograms or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     It is understood that any specific order or hierarchy of blocks in the processes disclosed is an illustration of example approaches. Based upon design preferences, it is understood that the specific order or hierarchy of blocks in the processes may be rearranged, or that all illustrated blocks be performed. Some of the blocks may be performed simultaneously. For example, in some instances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but are to be accorded the full scope consistent with the language claims, wherein the reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. Headings and subheadings, if any, are used for convenience only and do not limit the subject disclosure. 
     The predicate words “configured to,” “operable to,” “capable of,” and “programmed to” do not imply any particular tangible or intangible modification of a subject, but, rather, are intended to be used interchangeably. For example, a processor configured to monitor and control an operation or a component may also mean the processor being programmed to monitor and control the operation, or the processor being operable to monitor and control the operation. Likewise, a processor configured to execute code can be construed as a processor programmed to execute code or operable to execute code. 
     The word “example” is used herein to mean “serving as an example or illustration.” Any aspect or design described herein as “example” is not necessarily to be construed as preferred or advantageous over other aspects or designs. 
     A circuit is disclosed. The circuit can comprise: a first source; a set of drivers, each of the set of drivers comprising: a set of first transistors, each first transistor coupled to a pulse-width modulation (PWM) signal; a first resistor; a second transistor, the second transistor coupled to the set of first transistors and the first resistor; and an operational amplifier coupled to the second transistor; and a set of strings of series-connected light emitting diodes (LEDs), wherein the set of strings of series-connected LEDs is coupled to the set of drivers and the first source, such that a respective one of the set of drivers is configured to connect to any one of the set of strings of series-connected LEDs. Additionally or alternatively, in some examples, the circuit further comprises a second source, wherein at least one of the set of drivers further comprises a third transistor, the third transistor coupled to the second transistor and the second source. Additionally or alternatively, in some examples, the third transistor is coupled to the set of first transistors. Additionally or alternatively, in some examples, the third transistor is coupled to the operational amplifier. Additionally or alternatively, in some examples, the at least one of the set of drivers further comprises a fourth transistor coupled to the set of first transistors. Additionally or alternatively, in some examples, the circuit further comprises: an inverting amplifier coupled to an input signal; a first switch controlled by the input signal and capable of coupling the operational amplifier to the second transistor; and a second switch controlled by an inverted input signal and capable of coupling the operational amplifier to the third transistor. Additionally or alternatively, in some examples, the circuit further comprises: a negative feedback loop, the negative feedback loop including: the operational amplifier; the second transistor when the first switch couples the operational amplifier to the second transistor; and the third transistor when the second switch couples the operational amplifier to the third transistor. Additionally or alternatively, in some examples, the third transistor and the set of first transistors are turned on at different times. Additionally or alternatively, in some examples, the PWM signals received by the first set of first transistors form a PWM bus. 
     A method for operating a display is disclosed. The method can comprise: turning on a set of LED strings in a round using a first set of one or more LED drivers, wherein the turning on of the set of LED strings in the round includes: at a first time interval, coupling a first LED string to a first driver, the first LED string included in the set of LED strings, and the first driver included in the first set of one or more LED drivers; at a second time interval: in accordance with a determination that the first driver is coupled, coupling a second LED string to a second driver, the second LED string included in the set of LED strings, and the second driver included in the first set of one or more LED drivers; and in accordance with a determination that the first driver is not coupled to a LED string, coupling the second LED string to the first driver. Additionally or alternatively, in some examples, the method further comprises: turning on the set of LED strings in another round using a second set of one or more LED drivers. Additionally or alternatively, in some examples, a number of drivers in the first set of one or more LED drivers is one, and the set of LED strings comprises all of the LED strings in the display. Additionally or alternatively, in some examples, the method further comprises: decoupling the first LED string from the first driver, wherein a time duration between the decoupling of the first LED string from the first driver and the coupling of the second LED string to the first driver is zero seconds. Additionally or alternatively, in some examples, the turning on of the set of LED strings in the round includes: at a third time interval: in accordance with a determination that the first driver is not coupled to a LED string, coupling a third LED string to the first driver, the third LED string included in the set of LED strings. Additionally or alternatively, in some examples, the turning on of the set of LED strings in the round includes: at a third time interval: in accordance with a determination that the first driver is coupled to at least one LED string, determining whether the second driver is coupled to at least one LED string; and in accordance with a determination that the second driver is coupled to at least one LED string, coupling a third LED string to a third driver, the third LED string included in the set of LED strings, and the third driver included in the first set of one or more LED drivers. Additionally or alternatively, in some examples, the method further comprises: turning on the set of LED strings in another round using the first set of one or more LED drivers, wherein a respective order that the drivers in the first set of one or more LED drivers are coupled in the round differs from a respective order in the another round. Additionally or alternatively, in some examples, the turning on of the set of LED strings in the round further comprises: prior to the coupling of the first LED string to the first driver: coupling an internal source to the first driver using an auxiliary transistor; and charging the first driver. Additionally or alternatively, in some examples, the method further comprises: operating at least one driver of the first set of one or more LED drivers in an idle state, wherein the operation includes: coupling of an amplifier of the at least one driver to an idle transistor; and maintaining an output voltage of the amplifier. Additionally or alternatively, in some examples, the amplifier is coupled to the idle transistor when the amplifier is not coupled to the second transistor. Additionally or alternatively, in some examples, the method further comprises: delaying the coupling of the first LED string to the first driver by a non-zero amount of time. 
     Although examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the various examples as defined by the appended claims.

Metadata:
Filing Date: 20190201
Publication Date: 20191217
Grant Date: 20191217
Priority Date: 20190201
Inventors: OZALEVLI, ERHAN
MOHTASHEMI, BEHZAD
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2310/08", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/46", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B45/395", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/46", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B33/0812", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B33/0827", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05B45/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y02B20/30", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B45/395", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 68841373