Abstract:
An embodiment of the present invention is directed to a method and circuit to control light emitting diode (LED) output. The method includes receiving a line voltage signal which powers a lighting circuit comprising an LED and determining an adjustment of a threshold based on a variation of the line voltage signal and/or a controller delay or other practical controller limitation or imperfection. The method further includes dynamically adjusting a threshold or other reference of a controller which controls a switch of said lighting circuit for compensating for line variations to maintain a substantially uniform LED current.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the present invention generally relate to light emitting diode driver circuit control. 
       BACKGROUND 
       [0002]    Light emitting diodes (LEDs) are increasingly used for lighting applications including home and office lighting fixtures. LEDs are current fed devices and as such control of the current allows modulation of the output light intensity. Further, LEDs have a relatively small time constant meaning that certain variations in current will quickly impact the output light intensity. Such low frequency variations may manifest as a flicker which often is unpleasant to the human eye. 
         [0003]    Conventional LED circuits have included a power source and a resistor in addition to the LED. Unfortunately with these supply types, a large fraction of the power is dissipated in the resistor and therefore the circuit is not efficient. As LEDs increase in power output and increase in current requirements, the power dissipated in the resistor increases, thus more power is wasted. 
       SUMMARY OF THE INVENTION 
       [0004]    An embodiment of the present invention is directed to a method for controlling light emitting diode (LED) output. The method includes monitoring a line voltage signal which powers a lighting circuit comprising one or more LEDs and determining an adjustment of a threshold within that lighting circuit based on a variation of the line voltage signal. The method further includes dynamically adjusting the threshold of a controller circuit (e.g., hysteretic controller, peak current controller, and the like) which controls a switch that comprises the switch mode power converter (e.g., hysteretic controller, peak current controller, and the like) powering the lighting circuit. The threshold functions to control the current in the LED lighting circuit. The determined adjustment of the threshold is apportioned so as to cancel out or substantially reduce variations and effects of time delays or other practical imperfections associated with the controller. The method may include scaling and filtering the line voltage signal to remove noise and isolate components of interest (e.g., frequencies below 120 Hz). The method is effective at reducing unwanted low frequency flicker for the LED output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows block diagram of an exemplary circuit system for compensating for a power line variation, in accordance with one embodiment of the present invention. 
           [0006]      FIG. 2  shows block diagram of an exemplary circuit system for compensating for a power line variation, in accordance with another embodiment of the present invention. 
           [0007]      FIG. 3   a  shows block diagram of an exemplary system for compensating for a power circuit variation, in accordance with another embodiment of the present invention. 
           [0008]      FIG. 3   b  shows block diagram of an exemplary system for compensating for a power circuit variation, in accordance with another embodiment of the present invention. 
           [0009]      FIG. 4   a  shows an exemplary low frequency power line voltage variation over time. 
           [0010]      FIG. 4   b  shows an exemplary upper threshold and current variation over time. 
           [0011]      FIG. 4   c  shows an exemplary lower threshold and compensated threshold over time in accordance with embodiments of the present invention. 
           [0012]      FIG. 4   d  shows an exemplary compensated current variation and adjusted threshold over time, in accordance with one embodiment of the present invention. 
           [0013]      FIG. 5  shows a flowchart of an exemplary method for controlling light emitting diode (LED) output, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Reference will now be made in detail to embodiments of the claimed subject matter, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with embodiments, it will be understood that they are not intended to limit the claimed subject matter to these embodiments. On the contrary, the claimed subject matter is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the claimed subject matter as defined by the claims. Furthermore, in the detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. However, it will be obvious to one of ordinary skill in the art that the claimed subject matter may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the claimed subject matter. 
       Example Systems 
       [0015]      FIGS. 1-3   b  illustrate example components used by various embodiments of the present invention. Although specific components are disclosed in circuits  100 ,  200 ,  300 , and  350  it should be appreciated that such components are examples. That is, embodiments of the present invention are well suited to having various other components or variations of the components recited in systems  100 ,  200 ,  300 , and  350 . It is appreciated that the components in systems  100 ,  200 ,  300 , and  350  may operate with other components than those presented, and that not all of the components of systems  100 ,  200 ,  300 , and  350  may be used to achieve the goals of systems  100 ,  200 ,  300 , and  350 . 
         [0016]    Further, systems  100 ,  200 ,  300 , and  350  include components or modules that, in various embodiments, are carried out by software, e.g., a processor under the control of computer-readable and computer-executable instructions. The computer-readable and computer-executable instructions reside, for example, in data storage features such as computer usable memory, removable storage, and/or non-removable storage. The computer-readable and computer-executable instructions are used to control or operate in conjunction with, for example, a processing unit. It should be appreciated that the aforementioned components of systems  100 ,  200 ,  300 , and  350  can be implemented in hardware or software or in a combination of both. 
         [0017]      FIG. 1  shows block diagram of an exemplary system for compensating for a power line variation, in accordance with one embodiment of the present invention. System  100  includes a node for receiving a power line voltage (V in )  102 , sense resistor  104 , sense amplifier  106 , N series of light emitting diodes (LEDs)  108 , inductor  110 , diode  112 , switch  114 , ground  111 , hysteretic controller  116 , threshold control  118 , controlled gain  120 , analog to digital converter (ADC)  122 , and filtering and scaling module  124 . It is appreciated that embodiments of the present invention may compensate for a variety of power line voltage variations including, but not limited to, periodic variations (e.g., sine waves), ripples, spikes, drops. It is further appreciated that the components of system  100  may operate in a closed loop to provide current control. 
         [0018]    Line voltage (V in ) node  102  provides power to allow N series of LEDs  108  to output light. Line voltage  102  may be from a power supply including an AC plug, a transformer, bridge rectifier, and a filter. Line voltage  102  may vary for a variety of reasons including, but not limited to, source power fluctuations (e.g., power spikes or power drops). Further, the filter size determines what power variations enter into system  100  and impact the current through N series of LEDs  108 . For example, there may be a low frequency ripple at 60 Hz or 120 Hz depending on the rectifier structure. Thus, a component of the 60 Hz or 120 Hz ripple may make its way into the current that is flowing through the LEDs (by affecting the trip points of switch  114 ) and affect light output. Such lower frequency variations (e.g., 20 Hz and frequencies below 120 Hz) may be visible by the eye and appear as an unpleasant flicker. Embodiments of the present invention, described herein, compensate for this unwanted flicker. 
         [0019]    Sense resistor  104  in combination with sense amplifier  106  function to measure the current flowing though N series of LEDs  108 . In one embodiment, sense amplifier  106  is a current sense amplifier which provides an amplified differential current reading to hysteretic controller  116 . 
         [0020]    Inductor  110  facilitates a linear change in current through N series of LEDs  108 . When switch  114  is turned off, inductor  110  by its nature according to L di/dt facilitates maintenance of the current in the circuit (e.g., by release of its stored magnetic energy). That is, inductor  110  generates voltage as switch  114  turns off. Diode  112  allows a path for current to flow to the LEDs  108  when switch  114  is turned off. 
         [0021]    The linear nature of inductor  110  allows the current to be ramped up and ramped down between an upper threshold (I peak ) and a lower threshold (I valley ) with the aim to maintain an average current (I average ) via rapid switching of switch  114 . Switch  114  may be implemented using a variety of switching elements including, but not limited to, a metal-oxide-semiconductor field-effect transistor (MOSFET), a bipolar junction transistor (BJT), or a thyristor. 
         [0022]    Hysteretic controller  116  is operable to control switch  114  at intended trip points for controlling the current through N series LEDs  108 . Switch  114  is modulated for instance by a temporal density function of hysteretic controller  116 . It is appreciated that hysteretic controller  116  may be any other controller operable to generate a temporal density function to drive switch  114  or modulate the current flowing through N series of LEDs  108  by any suitable modulation function. 
         [0023]    Hysteretic controller  116  controls the current in the circuit based on the current (e.g., measured via sense amplifier  106 ) in relation to an upper threshold (I peak ) and a lower threshold (I valley ). When the upper threshold (I peak ) is reached, hysteretic controller turns off switch  114 . This causes the current to ramp down as the current flows through diode  112  powered by inductor  110 . When the current reaches the lower threshold (I valley ), switch  114  is turned on and the current ramps up again. It is appreciated that the references for thresholds of hysteretic controller  116  can either be internal or external. The thresholds of hysteretic controller  116  are controlled by controller  118  and may be programmable by software based programmed digital to analog converts (DACs) or analog inputs. Further, hysteretic controller  116  may support a dimming function based on a density function (e.g., Delta-Sigma or Stochastic Signal Density Modulation (SSDM)). 
         [0024]    Hysteric controller  116  may utilize comparators for determining whether the current has reached the upper or lower threshold. The comparators have a finite (non-zero) delay which results in a delay of hysteretic controller  116  in responding to the current flowing through N series of LEDs  108 . During this delay time, the voltage and current continues to ramp until switch  114  activates. When the upper threshold is reached, the delay time of hysteretic controller  116  causes the current through the LEDs to exceed the upper threshold resulting in a current overshoot. The overshoot current may be governed by the equation: 
         [0000]    
       
         
           
             Overshoot 
             = 
             
               
                 
                   ( 
                   
                     
                       V 
                       
                         i 
                          
                         
                             
                         
                          
                         n 
                       
                     
                     - 
                     
                       V 
                       LED 
                     
                   
                   ) 
                 
                 L 
               
               · 
               
                 T 
                 Delay 
               
             
           
         
       
     
         [0000]    The overshoot is dependent on magnitude line voltage  102  or V in , which can influence the LED current and brightness. Accordingly, the amount of the overshoot corresponds to how quickly the current ramp climbs which is based on the variation, change, or increase in line voltage  102 . 
         [0025]    Similarly, the delay of hysteretic controller  116  may result in undershoot where the current drops below the lower threshold. Undershoot may be governed by the equation: 
         [0000]    
       
         
           
             Undershoot 
             = 
             
               
                 
                   - 
                   
                     V 
                     LED 
                   
                 
                 L 
               
               · 
               
                 T 
                 Delay 
               
             
           
         
       
     
         [0000]    It is appreciated that the undershoot may be caused by the delay of hysteretic controller  116  and generally is independent of variations in line voltage  102  or V in . 
         [0026]    The overshoot and undershoot thus effect the average current through the LEDs and thereby the light output by N series of LEDs  108 . The average current may be governed by the equation: 
         [0000]    
       
         
           
             
               I 
               average 
             
             = 
             
               
                 
                   I 
                   peak 
                 
                 + 
                 
                   I 
                   valley 
                 
               
               2 
             
           
         
       
     
         [0000]    Where I peak  is the actual peak of the current caused by overshoot and I valley  is the actual current of the LED including undershoot. Thus, as the average current fluctuates, based on the delay of hysteretic controller  116  and variations in line voltage  102 , so does the brightness of N series of LEDs  108 . It is appreciated that switch  114  is operated at such high frequencies that its normal operation does not cause any noticeable flicker from the LEDs  108 . However, low frequency power line variations may affect the overshoot current in such a way as to be visible to the eye, e.g., 60 Hz or below, for instance. Dynamic threshold control circuit  118  compensates for these low frequency power line variations. 
         [0027]    Filtering and scaling system  124  removes noise from line voltage  120 . Filtering and scaling system  124  may further separate certain components (e.g., ripples in line voltage  102 ) and isolate components of interest. For example, filtering and scaling system  124  may isolate and respond to only certain frequencies which are of interest (e.g., ripples and variations below 120 Hz). Filtering and scaling system  124  thus removes noise and other variations in line voltage  102  thereby enabling system  100  to respond appropriately to variations in line voltage  102 . It is appreciated that filtering and scaling system  124  may be optional and may facilitate increasingly precise current control. It is further appreciated that in other embodiments line voltage signal  120  may be scaled or digitally filtered. 
         [0028]    Analog to digital converter (ADC)  122  digitally samples the analog power source signal or line voltage  102 . ADC  122  outputs the digital value of the line voltage to controlled gain circuit  120 . It is appreciated that ADC is coupled to line voltage  120 . 
         [0029]    Controlled gain module  120  determines a compensation for a power variation in line voltage  120 . The compensation determined by controlled gain module is used by threshold control circuit  118  to dynamically adjust the threshold of hysteretic controller  116 . In one embodiment, controlled gain module  120  may perform an inversion of the line voltage value and determine a factor for the threshold to be multiplied by to compensate for the variation in line voltage  102 . The compensation may be determined based on a variety of techniques including, but not limited to, polynomials, lookup tables, which may be in firmware or software. For example, line voltage  120  may have a 0.1 volt (V) amplitude sine-wave-based variation on a 1 V signal and controlled gain  120  may sample line voltage  120  and remove the 1 V signal. Controlled gain  120  may invert the 0.1 V sine wave value and send the inverted value to threshold control  118 . 
         [0030]    In accordance with embodiments of the present invention, threshold control module  118  modifies the threshold of hysteretic controller  116  such that the modified threshold compensates for the sampled power line variations on line voltage  102  thereby removing the impact of low frequency variations on line voltage  102  on the LED current. By altering the threshold supplied to hysteretic controller  116 , the trip point it sets for switch  114  is dynamically altered in response to the power line variations. Threshold control module  118  may include a threshold generation function to generate a compensated threshold, which substantially cancels out the effect of power variations in line voltage  102 . The compensated threshold may then be applied to hysteretic controller  116 . Referring to the above example, threshold module  118  receives the inverted 0.1 V sine variation or compensation value and applies (e.g., adds) the compensation value to the current threshold to determine a compensated threshold. 
         [0031]    The compensated threshold may cause hysteretic controller  116  to turn switch  114  off/on earlier e.g., dynamically alter the trip point. For example, where the delay of hysteretic controller  116  and/or an increase in power from line voltage  102  would result in an overshoot of the upper current threshold, controlled gain module  120  and threshold control  118  set the upper threshold lower such that hysteretic controller  116  turns off switch  114  earlier such that current does not substantially exceed the original or intended current threshold. This is done dynamically. As another example, where the delay of hysteretic controller  116  would result in an undershoot of the lower current threshold, controlled gain module  120  and threshold control  118  may increase the lower threshold such that hysteretic controller  116  turns on switch  114  earlier such that the current does not go substantially lower than the original current threshold. The latter example is of course independent of power line variations since the inductor  110  is supplying the current. In both examples, system  100  facilitates maintaining the average current through N series of LEDs  108 . Controlled gain  120  and threshold control  118  may be software implemented or controlled. 
         [0032]    In one exemplary embodiment, I peak  is 1.15 A, I average  is at 1 A, I valley  is 0.85 A. The incoming voltage may cause the current surge up to 1.2 A due to a power variation and controller delay. The threshold of a controller may then be dynamically adjusted so that the set point of a comparator (or reference the comparator) is set to 1.1 A. This results in the current overshooting to 1.15 A or the intended I peak . The adjusted threshold thus compensates to keep the overshoot within I peak  thereby maintaining the average current at I average  even though line voltage  102  changes. It is appreciated that the overshoot may be substantially reduced based on the delay of controller  116 . It is further appreciated that embodiments of the present invention may perform multiple threshold adjustments. 
         [0033]      FIG. 2  shows block diagram of another exemplary system for compensating for power line variation, in accordance with another embodiment of the present invention. System  200  includes a node for receiving a line voltage (V in )  102 , sense resistor  104 , sense amplifier  106 , N series of light emitting diodes (LEDs)  108 , inductor  110 , diode  112 , switch  114 , ground  111 , hysteretic controller  116 , threshold control  218 , and analog filtering and scaling module  224 . 
         [0034]    System  200  operates in a substantially similar manner to system  100  except circuit  218  receives analog control signals. Analog filtering and scaling system  224  receives line voltage  102  and provides an analog signal to threshold control  218  which has noise removed and components of interest isolated. It is further appreciated that in other embodiments line voltage signal  120  may be scaled or analog filtered. 
         [0035]    Threshold control  218  receives the analog filtered and scaled signal, and based on that signal, generates modified/compensated controller thresholds (e.g., inversed value, threshold scaling factor, or counteracting function). Threshold control  218  then applies compensated thresholds to hysteretic controller  116  to substantially maintain the average current (I average ) flowing through N series of LEDs  108  as described above. 
         [0036]      FIG. 3   a  shows block diagram of another exemplary system for compensating for a power variation, in accordance with another embodiment of the present invention. System  300  includes a node for receiving a line voltage (V in )  302 , sense resistor  304 , N series of light emitting diodes (LEDs)  308 , inductor  310 , diode  312 , switch  414 , ground  311 , peak current controller  316 , threshold control  318 , and analog filtering and scaling module  326 . 
         [0037]    Peak current controller  316  generates a pulse width modulated signal to control switch  314 . Peak current controller  316  turns on switch  314  until the current flowing through N series of LEDs  308 , as measured via sense resistor  304 , reaches an upper threshold. Upon reaching the upper threshold, peak current controller  316  will turn off switch  314  and the current will ramp down. Peak current controller  316  after a predetermined time (e.g., at a fixed frequency) turns switch  314  on which causes the current to then ramp up again. The current will be allowed to ramp up until the upper threshold is arrived at and then switch  314  is turned off. In this fashion, only I peak  is measured thereby causing a trip point while I valley  is dependent on the predetermined delay built into controller  316 . Peak current controller  316  has a delay associated with responding to current changes and thus the current going through the N series of LEDs  308  will exceed or overshoot the upper current threshold due to variations in line voltage  302 . As described herein, controller  318  compensates for the power line variations. 
         [0038]    Analog filtering and scaling system  326  receives line voltage  302  and provides an analog signal to threshold control  318  which has noise removed and components of interest isolated. It is further appreciated that in other embodiments line voltage signal  120  may be scaled or analog filtered. 
         [0039]    Threshold control  318  receives the analog filtered and scaled signal and based on that signal generates modified/compensated controller thresholds (e.g., inversed value, threshold scaling factor, or a counteracting function) for peak current controller  316 . Threshold control  318  then applies compensated thresholds to peak current controller  316  to substantially maintain the average current flowing through N series of LEDs  308  by dynamically altering the trip point at the overshoot side, e.g., peak. 
         [0040]      FIG. 3   b  shows block diagram of another exemplary system for compensating for power line variation, in accordance with another embodiment of the present invention. System  350  includes a node for receiving line voltage (V in )  302 , sense resistor  304 , N series of light emitting diodes (LEDs)  308 , inductor  310 , diode  312 , switch  314 , ground  311 , peak current controller  316 , threshold control  318 , and controlled gain  320 , analog to digital converter (ADC)  322 , and filtering and scaling module  324 . 
         [0041]    System  350  operates in a substantially similar matter to system  300  except circuit  318  receives digital control signals. Filtering and scaling system  324  removes noise from line voltage  320 . Filtering and scaling system  324  may further separate certain components (e.g., ripples in line voltage  302 ) and isolate components of interest. For example, filtering and scaling system  324  may isolate and respond to only certain frequencies which are of interest (e.g., ripples and variations below 120 Hz). Filtering and scaling system  324  thus removes noise and other variations in line voltage  302  thereby enabling system  350  to respond appropriately to variations in line voltage  302 . It is appreciated that filtering and scaling system  324  may be optional and may facilitate increasingly precise control. It is further appreciated that in other embodiments line voltage signal  120  may be scaled or digitally filtered. 
         [0042]    Analog to digital converter (ADC)  322  digitally samples a power source signal or line voltage  302 . ADC  322  outputs the digital value to controlled gain  320 . It is appreciated that ADC may be coupled to line voltage  320 . 
         [0043]    Controlled gain module  320  determines a compensation for a power variation in line voltage  302  and provides the compensation to threshold control  118 . The compensation determined by controlled gain module  320  is used to adjust the threshold. In one embodiment, controlled gain module  320  may take a line voltage value perform an inversion and determine a factor for the threshold to be multiplied by to compensate for the variation in line voltage  302 . The compensation may be determined based on a variety of techniques including, but not limited to, polynomials, lookup tables, which be in firmware or software. The threshold adjustment dynamically alters the trip point at the overshoot, e.g., I peak . 
         [0044]      FIG. 4   a  shows an exemplary low frequency line voltage (V in )  402  (e.g., line voltage  102 ) variation over time. Graph  400  includes a vertical axis corresponding to the line voltage value and horizontal axis corresponding to the time. Line voltage  402  illustrates a low frequency sine wave like variation in a line voltage or power supply. It is appreciated that embodiments of the present invention may compensate for a variety of variations in line voltage  402  including, but not limited to, periodic variations (e.g., sine waves), ripples, spikes, as well as non-periodic variations. 
         [0045]      FIG. 4   b  shows an exemplary upper threshold, corresponding to I peak , and current variation over time caused by line voltage variation. Graph  425  includes a vertical axis corresponding to the current (e.g., through N series of LEDs  108 ) and horizontal axis corresponding to the time. Line  404  corresponds to an ideal current threshold. Line  406  corresponds to the actual current in response to variations line voltage  402  as a controller (e.g., current controller  116  or peak current controller  316 ) responds to the power variations of line voltage  402 . 
         [0046]      FIG. 4   c  shows an exemplary lower threshold and a current variation over time. Graph  450  includes a vertical axis corresponding to the current (e.g., through N series of LEDs  308 ) and horizontal axis corresponding to the time. Line  410  corresponds to an ideal threshold (e.g., for a controller without a delay). Line  408  corresponds to a compensated threshold for a controller (e.g., current controller  116  or peak current controller  316 ). It is appreciated that compensated threshold  408  may be higher than the ideal threshold so that during undershoot the current reaches ideal threshold  410 . Since the inductor is supplying the voltage when the switch is off, power line variations are not present in  FIG. 4   c.    
         [0047]      FIG. 4   d  shows an exemplary modified upper threshold and compensated current over time, in accordance with one embodiment of the present invention. Graph  475  includes a vertical axis corresponding to the current and horizontal axis corresponding to the time. Line  412  corresponds the ideal average current flowing through an LED (e.g., N series of LEDs  108 ). Line  416  is an inversion of the power line ripple and therefore corresponds to the compensated thresholds of the controller (e.g., current controller  116  or peak current controller  316 ). It is noted that line  416  corresponds to modified thresholds that substantially cancel out the effects of variations in line voltage (e.g., line  402 ) in accordance with embodiments of the present invention. 
         [0048]    Lines  414   a  and  414   b  correspond to the envelope of the possible values of actual current based on modified thresholds received by the controller in accordance with embodiments of present invention. It is appreciated that substantial portions of variations in the line voltage (e.g., line  402 ) have been cancelled out. In one embodiment, variations may be caused by spread of the delay of controller. For example, if the delay is a priori determined to be 100 nanoseconds (ns) but the delay of the controller varies from 110 ns to 90 ns, the thresholds of the threshold can be varied to compensate but there may be some residual variations. 
       Example Operations  
       [0049]    With reference to  FIG. 5 , exemplary flowchart  500  illustrates example blocks used by various embodiments of the present invention. Although specific blocks are disclosed in flowchart  500 , such blocks are examples. That is, embodiments are well suited to performing various other blocks or variations of the blocks recited in flowchart  500 . It is appreciated that the blocks in flowchart  500  may be performed in an order different than presented, and that not all of the blocks in flowchart  500  may be performed. Flowchart  500  includes processes that, in various embodiments, are carried out by a processor under the control of computer-readable and computer-executable instructions. Embodiments of the present invention may thus be stored as computer readable media or computer-executable instructions including, but not limited to, a firmware update, software update package, or hardware (e.g., ROM). 
         [0050]    In particular,  FIG. 5  shows a flowchart of an exemplary process for controlling light emitting diode (LED) output, in accordance with an embodiment of the present invention. Blocks of flow chart  500  may be carried out by modules of system (e.g., systems  100 ,  200 ,  300 , and  350 ) for controlling an LED circuit. 
         [0051]    At block  502 , a line voltage signal (e.g., line voltage  102  or  302 ) is sampled. As described herein, the line voltage may be from a rectifier and power a lighting circuit comprising an LED (e.g., N series of LEDs  108 ). 
         [0052]    At block  504 , the line voltage signal may be digitally filtered and scaled. As described herein, the line voltage signal may be filtered to have noise remove and components of interest isolated (e.g., frequencies less than 120 Hz). It is appreciated that in other embodiments the line voltage signal may be scaled or digitally filtered. 
         [0053]    At block  506 , the line voltage signal is digitally sampled. As described herein, the line voltage may be digitally sampled by an analog to digital converter (ADC) (e.g., ADC  122 ). 
         [0054]    At block  508 , the line voltage signal is processed via analog filtering and scaling. As described herein, the line voltage signal may be processed to remove noise and isolate components of interest in an analog manner. It is appreciated that in other embodiments the line voltage signal may be scaled or analog filtered. 
         [0055]    At block  510 , an adjustment of a threshold is determined based on a variation of the line voltage signal. As described herein, an adjustment of a threshold may be computed via an inverse or a counteracting function of the measured variation and a scaling factor (e.g., via controlled gain  120 ). 
         [0056]    At block  512 , a threshold of a controller that controls the switch is dynamically adjusted. The controller may control a switch of a lighting circuit. As described herein, the threshold may be determined based on the adjustment and the threshold of the controller (e.g., a switch mode controller with a threshold inherent to its operation including, but not limited to, hysteric controller  116  or peak current controller  316 ) to cancel out (e.g., remove impact of an overshoot of a threshold) the effects of variations of the line voltage signal. The power supply ripple or variation rejection is thereby improved and the effect of the line ripple is removed from the current to the LED. It is appreciated that Block  502  may then be performed if multiple compensations are to be made. For example, if 70% of a line voltage signal can be compensated out then block  502  may be performed as part of a second order compensation. 
         [0057]    Thus, embodiments of the present invention may provide a compensation system to reduce or eliminate the impact of line voltages variations and controller delays on current supplied to an LED light source in an LED circuit. The compensation system derives a threshold based on the line voltage variation such that effect of the line voltage variation and controller delay is substantially cancelled out in an embodiment. Embodiments of the present invention further allow use of simplified front end DC-DC converters to supply LED driver stages because embodiments can tolerate increased line voltage variations. The simplification of front end DC-DC converters combined with an LED driver circuit in accordance with embodiments of the present invention may thus reduce cost. Moreover, embodiments of the present invention may be implemented in a power programmable system on a chip (SoC). 
         [0058]    Embodiments of the present invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.