Abstract:
The present invention provides a controller for controlling strings of LEDs in a liquid crystal display. The hybrid controller uses both analog and digital circuit components. Error amplifiers are used to compare analog feedback signals received from the LED strings with reference signals. The results of those comparisons are converted to digital data and processed by a digital signal processor (DSP). The DSP calculates the drive voltages for the LED strings based on the deviation between the actual current flows (represented by feedback signals) and the desired current flows (represented by reference signals) through the LED strings. Analog drivers provide the drive voltages to the LED strings.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to electronic display technology, and particularly to a hybrid architecture of analog and digital circuitry for controlling the light emitting diode (LED) strings of the backlights of electronic displays. 
       BACKGROUND OF THE INVENTION 
       [0002]    Backlights are used to illuminate liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDA), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs). 
         [0003]    The display industry is enthusiastically perusing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display. 
         [0004]    Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings.  FIG. 1  shows an exemplary flat panel display  10  with a backlighting system having three independent strings of LEDs  1 ,  2  and  3 . The first string of LEDs  1  includes  7  LEDs  4 ,  5 ,  6 ,  7 ,  8 ,  9  and  11  discretely scattered across the display  10  and connected in series. The first string  1  is controlled by the drive circuit  12 . The second string  2  is controlled by the drive circuit  13  and the third string  3  is controlled by the drive circuit  14 . The LEDs of the LED strings  1 ,  2  and  3  can be connected in series by wires, traces or other connecting elements. 
         [0005]    The strings  1 ,  2  and  3  are controlled by a controller by way of drivers  12 ,  13  and  14  respectively.  FIG. 2  shows a prior art controller  20 .  FIG. 2  specifically shows the controller  20  for controlling string  1 , by way of example. However, the controller  20  can also be used to control strings  2  and  3 . The controller  20  includes an error amplifier  22 , a continuous time loop compensation circuit  24 , summation node, a local feedback loop  27  and a system feedback loop  28 . The controller  20  provides a real time analog control of the string  1 . The error amplifier  22  receives a reference voltage V REF a s an input. The error amplifier  22  also receives a feedback signal V FB  from the LED string  1  as an input by way of the system feedback loop  28 . One of ordinary skill in the art will appreciate that the system feedback loop  28  includes the capability to scale the feedback signal such that the error amplifier  22  can properly compare the feedback signal with V REF . 
         [0006]    Typically, V REF  is indicative of the desired drive voltage that should be provided to string  1  to cause a desired current to flow through string  1 . The error amplifier  22  compares the V REF  with the feedback voltage V FB , which can be the sensed voltage indicative of the actual current flowing through string  1 , and provides a result of the comparison to the loop compensation block  24 . The output of the error amplifier  22  represents the correction that must be made to the drive voltage of string  1  to cause the desired current to flow through string  1 . The error amplifier  22  continuously receives the feedback signal in real time from string  1  and provides the correction signal to the loop compensation block  24 . 
         [0007]    The loop compensation block  24  provides the proper drive voltage to string  1  by way of the driver  12 , in response to receiving the correction signal from the error amplifier  22 . The loop compensation block  24  thus continuously adjusts the drive voltage for string  1  in real time.  FIG. 2  shows that the loop compensation block  24  is coupled to the driver  12  by way of the summation node (Σ). The summation node receives the output of the loop compensation block  24  as an input. The summation node also receives a feedback signal from string  1  by way of the local feedback loop  27 . The feedback signal received by way of the local feedback loop  27  can be representative of, for example, the noise in string  1 . The feedback signal received by way of the local feedback loop  27  can also be representative of, for example, an open circuit condition or a short circuit condition caused by string  1  or some other part of the display circuit. The summation node can provide for a quick adjustment to the driver  12 , including shutting down the driver  12  output during abnormal conditions, depending on the circuit design and goals. 
         [0008]      FIG. 3  shows another prior art controller  30  for controlling string  1 . The controller  30  includes an analog to digital (A/D) converter  31 , an analog to digital (A/D) converter  33 , a digital signal processor (DSP)  32 , a digital to analog (D/A) converter  34 , and a buffer  35 . The controller  30  provides for digital control of string  1  by way of the driver  12 . The A/D converter  33  receives a reference signal V REF  as an input. Typically, V REF  is indicative of the desired voltage that should be used to drive string  1  in order to cause a desired current to flow through string  1 . The A/D converter  33  converts the analog V REF  signal into digital data and provides the digital data to the digital signal processor (DSP)  32 . 
         [0009]    The A/D converter  31  receives a feedback signal V FB  by way of the system feedback loop  38 . V FB  can be the sensed voltage representative of the current flowing through string  1 . The A/D converter  31  converts the analog V FB  signal into digital data and provides the digital data to the DSP  32 . The DSP  32  can be programmed to use the digital data received from the A/D converter  31  to determine the drive voltage for string  1 . The DSP  32  can make intelligent decisions about controlling string one because it has access to various programs, comparison algorithms, look up tables and the like, that provide for consideration of various real-time system variables (e.g. ambient temperature) and non-real time system variables in the decision making. The DSP  32  provides the digital data related to the selected drive voltage to the digital to analog (D/A) converter  34 . The D/A converter  34  converts the digital data into an analog drive signal, and provides the analog drive signal to the driver  12 . 
         [0010]      FIG. 3  shows that the DSP  32  is coupled to the driver  12  by way of the buffer  35 . The buffer  35  can be used to store and hold the analog signals received from the D/A converter  34 . The buffer  35  can include, for example, banks of storage capacitors for storing analog signals. The buffer  35  can be used to convert the outputs of the D/A converter  34  into smooth signals, for example, square waves, for driving string  1 .  FIG. 3  also shows that DSP  32  receives a feedback signal from string  1  by way of the local feedback loop  37 . An analog to digital (A/D) converter  36  converts the analog feedback signal into digital data. The feedback signal received by way of the local feedback loop  37  can be representative of, for example, the noise in string  1 . The feedback signal received by way of the local feedback loop  37  can also be representative of, for example, an open circuit condition or a short circuit condition caused by string  1  or some other part of the display circuit. The DSP  32  can provide for a quick adjustment to the driver  12 , including shutting down the driver  12  output during abnormal conditions, depending on the algorithms and programs included in the DSP  32 . 
         [0011]    The controllers  20  and  30  shown in  FIGS. 2 and 3  have many drawbacks. Controller  20  operates singularly according to the natural properties and characteristics of the analog circuit components, such as resistors, capacitors and inductors, and cannot be programmed to perform intelligent operations. Controller  20  is also subject to noise and delays that are inherent in analog circuit components. Controller  30  is subject to a relatively slow start up and boot up periods, inherent in digital systems. Also, the analog to digital to analog conversions and the digital signal processing result in time delays, and, as a result, real time control may not be available for many applications of controller  30 . Furthermore, to program, debug or repair the DSP  32  during operation of the controller  30 , the DSP  32  freezes the digital data provided to the D/A converter  34 . That results in D/A converter  34  continuously providing the same output signal to the driver  12  during the freeze period. The feedback signal received by way of the system feedback loop  38  is ignored during the freeze period. That is undesirable. 
         [0012]    The present invention provides a low power, high speed controller with a quick start-up period that can be programmed for intelligent decision making and can also perform real time operations. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides a controller for controlling strings of LEDs in electronic displays including liquid crystal display. The hybrid controller uses both analog and digital circuit components. Error amplifiers are used to compare analog feedback signals received from the LED strings with reference signals. The results of those comparisons are converted to digital data and processed by a digital signal processor (DSP). The DSP calculates the drive voltages for the LED strings based on the deviation between the actual current flows (represented by feedback signals) and the desired current flows (represented by reference signals) through the LED strings. Analog drivers provide the drive voltages to the LED strings. The DSP outputs can be latched, so that during the initialization of the DSP or when DSP is non-operational for various reasons, the analog drivers can provide drive voltages to the LED strings. A multiplexor is used for the sequential processing of the LED strings by the DSP. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0015]      FIG. 1  illustrates an exemplary display implementing LED strings; 
           [0016]      FIG. 2  illustrates the prior art analog control architecture for controlling a LED string; 
           [0017]      FIG. 3  illustrates the prior art digital control architecture for controlling a LED string; 
           [0018]      FIG. 4  illustrates an exemplary architecture of the present invention; and 
           [0019]      FIG. 5  illustrates another exemplary architecture of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 4  illustrates an exemplary controller  40  of the present invention for controlling string  1  shown in  FIG. 1 . The controller  40  includes a combination of analog and digital circuit components. In  FIG. 4 , an error amplifier  41  is shown coupled to an analog to digital (A/D) converter  42 . The A/D converter  42  is coupled to a digital signal processor (DSP)  43 . The DSP  43  is coupled to a digital to analog (D/A) converter  44 . The D/A converter  44  is coupled to the driver  12  by way of buffer  45  and a summation node (Σ). The driver  12  is coupled to string  1 . In one embodiment, the controller  40  is implemented in an integrated circuit (IC) chip. 
         [0021]    The error amplifier  41  is an analog circuit component. The error amplifier  41  receives a reference signal V REF  as an input. The reference signal V REF  can be indicative of the desired drive voltage for string  1 . The error amplifier also receives a feedback signal V FB  by way of the system feedback loop  49  as another input. The feedback signal V FB  received by way of the system feedback loop  49  can be indicative of the current flowing through string  1 . The error amplifier  41  compares the V REF  signal with the V FB  signal, and provides a result of the comparison to the A/D converter  42 . The system feedback loop  49  can include circuitry to scale the values of the feedback signal V FB  such that the error amplifier  41  can properly compare the V REF  and the V FB  signals on the same scale. One of ordinary skill in the art will appreciate that various comparator circuits known in the art can be substituted in place of the error amplifier  41 , for comparing the V REF  signal with the V FB  signal. 
         [0022]    In one embodiment, the output of the error amplifier  41  represents the correction that must be made to the drive voltage for string  1  to cause string  1  to output the desired current. The desired current flow for string  1  depends on the images being displayed on display  10  shown in  FIG. 1 , and can be determined by DSP  43  or another component of display  10  (not shown). In one embodiment, the level of the output of the error amplifier  41  indicates the amount of deviation between the actual current flowing through string  1  and the desired current flow for string  1 . In one embodiment, a higher error amplifier  41  output represents a larger difference between the actual and desired currents of string  1  than a lower error amplifier  41  output. In one embodiment, the output of the error amplifier  41  progressively increases as the differences between the actual and desired current flows of string  1  increase. 
         [0023]    The A/D converter  42  receives the output of the error amplifier  41  and converts it into digital data. The A/D converter  42  transmits the digital data to the DSP  43 . In one embodiment, the DSP  43  includes a state machine. In one embodiment, the DSP  43  includes a programmable microprocessor. In one embodiment, the DSP  43  includes a sequencer for processing digital data by sequencing it through various processing units. The DSP  43  can process the digital data received from the A/D converter  42  using various algorithms, look up tables, subroutines, and the like, to determine the required drive voltage for string  1 . The DSP  43  provides the digital to analog (D/A) converter  44  with digital data representative of the determined drive voltage. The D/A converter  44  converts the digital data into an analog signal and transmits the analog signal to the buffer  45 . The A/D converter  42  and the D/A  44  can be components of the DSP  43 . The buffer  45  can be a bank of capacitors, for example. The buffer  45  converts the analog signal received from the D/A converter  44  into a smooth signal, for example a square wave, and transmits it to the driver  12  by way of the summation node. 
         [0024]    As shown in  FIG. 4 , the DSP  43  can communicate with the error amplifier  41  by way of the connection  48 . The connection  48  can be wired or wireless connection. The DSP  43  can shut off the error amplifier  41 . The DSP  43  can also adjust the value of the reference voltage V REF . The DSP  43  can communicate with the local feedback control circuit  46  by way of the connection  47 . The connection  47  can be wired or wireless connection. The local feedback control loop circuit  46  receives a feedback signal from string  1  and provides it to the summation node. The summation node adjusts the signal level of the output of the D/A converter  44  based on the feedback signal received from the local feedback control circuit  46 , and provides the adjusted signal to the driver  12 . The feedback signal received from the local feedback loop can include noise, for example. 
         [0025]    Also, the local feedback control circuit  46  can shut off the summation node upon the occurrence of abnormal conditions, such as open circuit or short circuit conditions. The local feedback circuitry  46  can shut off the summation node circuit by triggering a protection circuitry (not shown) in case of an abnormal condition. In one embodiment, the local feedback control circuit  46  can cut off any drive voltage to string  1  by shutting off the summation node circuit. The DSP  47  can activate or deactivate the local feedback control circuit  46 . 
         [0026]    One of ordinary skill in the art will appreciate that during start-up, the analog component of the controller  40 , namely the error amplifier  41 , and the driver  12 , require much smaller initialization periods than the digital components of the controller  40 . In one embodiment, the DSP  43  is programmed to latch the memory locations or registers of the digital data values that are provided to the D/A converter  44  during shutdown. Latching prevents the data values present in memory registers and locations from destruction, such that they are frozen. In another embodiment, the DSP  43  is programmed to latch the memory locations or registers of the digital data values that are provided to the D/A converter  44  until they are changed by the DSP. In yet another embodiment, the memory locations or registers of the DSP  43  can be adjusted prior to the initiation of the start up sequence for the controller  40 . 
         [0027]    In the above embodiments, according to one aspect of the present invention, the driver  12  can start providing drive voltages to string  1  even while the DSP  43  is executing its initialization sequence. According to another aspect of the present invention, the driver  12  can continue to provide drive voltages to string  1  even when the DSP  43  is shut off, debugged or being programmed. In that aspect of the present invention, the D/A converter  44  converts the same latched digital data into analog signals while the DSP  43  is shut off, being debugged or being programmed. According to yet another aspect of the present invention, the driver  12  can be shut down based upon the occurrence of abnormal conditions such as open circuit or short circuit at string  1  even when the DSP is in the initialization mode, shut off, being debugged or being programmed. In that aspect of the present invention, the local feedback control circuit  46  monitors the feedback signals received from string  1  for any abnormalities in the operation of string  1 , and shuts off the summation node circuit upon the occurrence of an abnormal event. 
         [0028]      FIG. 5  illustrates an exemplary implementation of the architecture of the present invention for controlling multiple LED strings of a display. The controller  50  controls six strings STR 1 , STR 2 , STR 3 , STR 4 , STR 5  and STR 6  by way of drivers  12 ,  13 ,  14 ,  15 ,  16  and  17  respectively. The controller  50  includes many of the same components included in controller  40  including the error amplifier  41 , the AID converter  42 , the DSP  43 , the D/A converter  44 , the buffer  45 , the summation node circuit and the local feedback control circuit  46 . The controller  50  also includes a driver selector circuit  51  for selecting a particular driver  12 ,  13 ,  14 ,  15 ,  16  or  17  and a string selector  52  for selecting V FB  signal for a particular string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  and STR 6 . 
         [0029]    In one embodiment, the DSP  43  selects a string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  or STR 6  for processing. In one embodiment, the DSP  43  sequentially and periodically processes the strings STR 1 , STR 2 , STR 3 , STR 4 , STR 5  and STR 6 . In another embodiment, the DSP  43  can randomly select a string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  or STR 6  for processing. In yet another embodiment, the DSP  43  can be programmed to intelligently select a string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  or STR 6  for processing, based on various factors and circuit conditions. 
         [0030]    The DSP  43  can select the driver  12 ,  13 ,  14 ,  15 ,  16  and  17  by communicating with the driver selector  51  by way of the connection  53 , and select the V FB  signal of a particular string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  or STR 6  by way of the connection  54 . In one embodiment, the driver selector circuit  51  and the string selector circuit  52  include multiplexors. In one embodiment, the driver selector circuit  51  and the string selector circuit  52  can be implemented inside the DSP  43 . The driver selector circuit  51  and the string selector circuit  52  can be implemented in hardware, software or firmware. In one embodiment, the V REF  voltage levels for the various strings STR 1 -STR 6  differ. In that embodiment, the DSP  43  provides the V REF  voltage level to the error amplifier  41 , by way of the connection  48 , for the selected string STR 1 , STR 2 , STR 3 , STR 4 , STR 5  or STR 6 . 
         [0031]    The present invention combines digital and analog control methods. The digital and analog fields are significantly different and those of ordinary skill in the art are normally skilled only in digital or analog systems. One of ordinary skill in the art will appreciate that the techniques, structures and methods of the present invention above are exemplary. The present inventions can be implemented in various embodiments without deviating from the scope of the invention.