Abstract:
A driving arrangement for light emitting diode (LED) based illumination constituted of: a comparison circuitry arranged to compare an integral of a target current over a target illumination time for at least one LED based luminaire with an integral of an illumination current over an illumination time for the at least one LED based luminaire, the comparison circuitry arranged to output a comparison signal; and a control circuitry in communication with the comparison circuitry and arranged to alternately: allow the flow of electrical current through the at least one LED based luminaire responsive to a first condition of the comparison signal; and prevent the flow of electrical current through the at least one LED based luminaire responsive to a second condition of the comparison signal, the second condition different from the first condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/466,969 filed Mar. 24, 2011, entitled “Brightness Control for LED Lighting”, the entire contents of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of solid state lighting, and in particular to an arrangement wherein the on-time of a LED string is controlled responsive to a comparison of an effective brightness signal, representing both the desired on time and a brightness level, with a signal representing both the actual current through the LED string and the actual amount of time for which the current is flowing through the LED string. 
       BACKGROUND OF THE INVENTION 
       [0003]    Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications. 
         [0004]    In a large LCD matrix display, and in large solid state lighting applications, such as street lighting and signage, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string share a common current. 
         [0005]    LEDs providing high luminance exhibit a range of forward voltage drops, denoted V f , and their luminance is primarily a function of current. For example, one manufacturer of LEDs suitable for use with a backlight application for a portable computer, such as a notebook computer, indicates that V f  for a particular high luminance white LED ranges from 2.95 volts to 3.65 volts at 20 mA and a LED junction temperature of 25° C., thus exhibiting a variance in V f  of greater than ±10%. Furthermore, the luminance of the LEDs vary as a function of junction temperature and age, typically exhibiting a reduced luminance as a function of current with increasing temperature and increasing age. In order to provide backlight illumination for a portable computer with an LCD matrix display of at least 25 cm measured diagonally, at least 20, and typically in excess of 40, LEDs are required. In order to provide street lighting, in certain applications over 100 LEDs are required. 
         [0006]    In order to provide a balanced overall luminance, it is important to control the current of the various LED strings to be approximately equal. In one embodiment a power source is supplied for each LED string, and the voltage of the power source is controlled in a closed loop to ensure that the voltage output of the power source is consonant with the voltage drop of the LED string, however the requirement for a power source for each LED string is quite costly. 
         [0007]    In another embodiment, as described in U.S. Patent Application Publication US 2007/0195025 to Korcharz et al, entitled “Voltage Controlled Backlight Driver” and published Aug. 23, 2007, the entire contents of which is incorporated herein by reference, this is accomplished by a controlled dissipative element placed in series with each of the LED strings. In another embodiment, binning is required, in which LEDs are sorted, or binned, based on their electrical and optical characteristics. Thus, in accordance with the prior art, in order to operate a plurality of LED strings from a single power source, at a common current, either binning of the LEDs to be within a predetermined range of V f  is required, or a balancing element, such as the dissipative element of the aforementioned patent application, must be supplied to drop the voltage difference between the strings caused by the differing V f  values so as to produce an equal current through each of the LED strings. Either of these solutions adds to cost and/or wasted energy. 
         [0008]    Dimming of an LED string is often performed responsive to a pulse width modulated, or otherwise modulated, dimming signal. To save energy, the power source supplying power to the various LED strings may be shut down when the incoming dimming signal is in an off, or inactive condition. Unfortunately, for low duty cycles, the amount of time required to start up the power source may be longer than the length of the active portion of the actual dimming signal, thus requiring a complex control circuitry, again adding to cost. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, it is a principal object of the present invention to overcome at least some of the disadvantages of the prior art. This is provided in certain embodiments by an arrangement wherein the on-time of a LED string is controlled responsive to a comparison of an effective brightness signal, representing both the desired on time and a brightness level, with a signal representing the actual current through the LED string and the actual amount of time for which the current is flowing through the LED string. 
         [0010]    In one exemplary embodiment, a driving arrangement for LED based illumination is provided, the driving arrangement comprising: a comparison circuitry arranged to compare an integral of a target current over a target illumination time for at least one LED based luminaire with an integral of an illumination current over an illumination time for the at least one LED based luminaire, the comparison circuitry arranged to output a comparison signal; and a control circuitry in communication with the comparison circuitry and arranged to alternately: allow the flow of electrical current through the at least one LED based luminaire responsive to a first condition of the comparison signal; and prevent the flow of electrical current through the at least one LED based luminaire responsive to a second condition of the comparison signal, the second condition different from the first condition. 
         [0011]    In one further embodiment, the driving arrangement further comprises a balancing network, wherein the at least one LED based luminaire comprises a plurality of LED based luminaries connected to receive power in parallel from a common power source via the balancing network. In one yet further embodiment the common power source is responsive to an output of the control circuitry, wherein the allowance of the flow of electrical current through the at least one LED based luminaire responsive to the first condition of the comparison signal comprises activation of the common power source responsive to the first condition of the comparison signal, and the prevention of the flow of electrical current through the at least one LED based luminaire responsive to the second condition of the comparison signal comprises deactivation of the common power source responsive to the second condition of the comparison signal. 
         [0012]    In one further embodiment the at least one LED based luminaire comprises a plurality of LED based luminaries connected to receive power in parallel from a common power source, the common power source responsive to an output of the control circuitry, wherein the allowance of the flow of electrical current through the at least one LED based luminaire responsive to the first condition of the comparison signal comprises activation of the common power source responsive to the first condition of the comparison signal, and the prevention of the flow of electrical current through the at least one LED based luminaire responsive to the second condition of the comparison signal comprises deactivation of the common power source responsive to the second condition of the comparison signal. 
         [0013]    In one further embodiment the driving arrangement comprises a particular electronically controlled switch in series with each of the at least one LED based luminaries, each of the particular electronically controlled switches responsive to an output of the control circuitry, wherein the allowance of the flow of electrical current through the at least one LED based luminaire responsive to the first condition of the comparison signal comprises closure of the particular electronically controlled switch in series with the LED based luminaire responsive to the first condition of the output of the control circuitry, and the prevention of the flow of electrical current through the at least one LED based luminaire responsive to the second condition of the comparison signal comprises an open condition of the particular electronically controlled switch in series with the LED based luminaire responsive to the second condition of the comparison signal. In one yet further embodiment the control circuitry comprises a drive signal generator with a periodic output, wherein the allowance of the flow of electrical current through the at least one LED based luminaire is responsive to the first condition of the comparison signal and to the periodic output. 
         [0014]    In one further embodiment the comparison circuitry comprises: a first current source arranged to provide a current representative of the target current; a first capacitor arranged to be charged by the first current source; a second current source arranged to provide a current representative of the illumination current; a second capacitor arranged to be charged by the second current source; and a comparator arranged to compare the voltage drop across the first capacitor with the voltage drop across the second capacitor, the comparison signal a function of the output of the comparator. In another embodiment the comparison circuitry comprises: a first current source arranged to provide a current representative of the target current; a second source arranged to provide a current representative of the illumination current; a capacitor arranged to be charged by a first one of the first current source and the second current source and to be discharged by a second one of the first current source and the second current source; and a comparator arranged to compare the voltage drop across the capacitor with a reference voltage, the comparison signal a function of the output of the comparator. In one further embodiment the comparison circuitry is associated with a particular LED based luminaire. 
         [0015]    In one embodiment a method of illumination is enabled, the method comprising: comparing an integral of a target current over a target illumination time for at least one luminaire with an integral of an illumination current over an illumination time for the at least one luminaire; alternately, allowing the flow of electrical current through the at least one luminaire responsive to a first condition of the comparison; and preventing the flow of electrical current through the at least one luminaire responsive to a second condition of the comparison, the second condition different from the first condition. 
         [0016]    In one further embodiment the at least one luminaire comprises a plurality of luminaries, the method further comprising: balancing the current flow through each of the at least one luminaries. In another further embodiment the allowing the flow of electrical current through the at least one luminaire responsive to the first condition of the comparison comprises activating a common power source, and the preventing of the flow of electrical current through the at least one luminaire responsive to the second condition of the comparison comprises deactivating the common power source. In another further embodiment the allowing of the flow of electrical current through the at least one luminaire comprises closing an electronically controlled switch arranged in series with the luminaire, and the preventing the flow of electrical current through the at least one luminaire comprises opening the particular electronically controlled switch in series with the at least one luminaire. 
         [0017]    In one further embodiment the method comprises providing a drive signal generator with a periodic output, wherein the allowing the flow of electrical current through the at least one luminaire is responsive to the first condition of the comparison and to the periodic output. In another further embodiment the method comprises providing a comparison circuitry arranged to perform the comparing, the comparison circuitry arranged to output a comparison signal, the comparison circuitry comprising: a first current source arranged to provide a current representative of the target current; a first capacitor arranged to be charged by the first current source; a second current source arranged to provide a current representative of the illumination current; a second capacitor arranged to be charged by the second current source; and a comparator arranged to compare the voltage drop across the first capacitor with the voltage drop across the second capacitor, the comparison signal a function of the output of the comparator. In yet another further embodiment the method comprises: providing a comparison circuitry arranged to perform the comparing, the comparison circuitry arranged to output a comparison signal, the comparison circuitry comprising: a first current source arranged to provide a current representative of the target current; a second current source arranged to provide a current representative of the illumination current; a capacitor arranged to be charged by a first one of the first current source and the second current source and to be discharged by a second one of the first current source and the second current source; and a comparator arranged to compare the voltage drop across the capacitor with a reference voltage, the comparison signal a function of the output of the comparator. 
         [0018]    In one embodiment a driving arrangement for illumination is enabled, the driving arrangement comprising: a means for comparing arranged to compare an integral of a target current over a target illumination time for at least luminaire with an integral of an illumination current over an illumination time for the at least one luminaire, the means for comparing arranged to output a comparison signal; a means for allowing the flow of electrical current through the at least one luminaire responsive to a first condition of the comparison signal; and a means for preventing the flow of electrical current through the at least one luminaire responsive to a second condition of the comparison signal, the second condition different from the first condition. 
         [0019]    In one further embodiment, the driving arrangement further comprises a means for balancing, wherein the at least one luminaire comprises a plurality of luminaries arranged to receive power in parallel from a common means for supplying power via the means for balancing. In one yet further embodiment the means for allowing and preventing comprises a control input to the common means for supplying power, wherein the allowance of the flow of electrical current through the at least one luminaire responsive to the first condition of the comparison signal comprises activation of the common means for supplying power responsive to the first condition of the comparison signal, and the prevention of the flow of electrical current through the at least luminaire responsive to the second condition of the comparison signal comprises deactivation of the common means for supplying power responsive to the second condition of the comparison signal. 
         [0020]    In one further embodiment the at least one luminaire comprises a plurality of luminaries arranged to receive power in parallel from a common means for supplying power, wherein the means for allowing and preventing comprises a control input to the common means for supplying power, wherein the allowance of the flow of electrical current through the at least one luminaire responsive to the first condition of the comparison signal comprises activation of the common means for supplying power responsive to the first condition of the comparison signal, and the prevention of the flow of electrical current through the at least luminaire responsive to the second condition of the comparison signal comprises deactivation of the common means for supplying power responsive to the second condition of the comparison signal. 
         [0021]    Additional features and advantages of the invention will become apparent from the following drawings and description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. 
           [0023]    With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: 
           [0024]      FIG. 1A  illustrates a high level schematic diagram of an embodiment of a driving arrangement for a plurality of parallel connected LED strings comprising a comparison circuitry constituted of a plurality of integrating capacitors, wherein the power source is switched alternately on and off responsive to an output signal from the comparison circuitry; 
           [0025]      FIG. 1B  illustrates a graph of a PWM control signal, the LED current and a power source control signal of the driving arrangement of  FIG. 1A , wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units; 
           [0026]      FIG. 2A  illustrates a high level schematic diagram of an embodiment of a driving arrangement for a plurality of parallel connected LED strings comprising a comparison circuitry constituted of a unitary integrating capacitor, wherein the power source is switched alternately on and off responsive to an output signal from the comparison circuitry; 
           [0027]      FIG. 2B  illustrates a graph of a PWM control signal, the LED current and a power source control signal of the driving arrangement of  FIG. 2A , wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units; 
           [0028]      FIG. 3  illustrates a high level schematic diagram of an embodiment of a driving arrangement for a plurality of parallel connected LED strings comprising a comparison circuitry constituted of a pair of low pass filters, wherein the power source is switched alternately on and off responsive to an output signal from the comparison circuitry; 
           [0029]      FIG. 4A  illustrates a high level schematic diagram of an embodiment of a driving arrangement for a plurality of parallel connected LED strings comprising, for each LED string, a comparison circuitry constituted of a unitary integrating capacitor, wherein an electronically controlled switch in series with the respective LED string is toggled responsive to an output signal from the respective comparison circuitry, each of the comparison circuitries receiving a respective PWM input control signal; 
           [0030]      FIG. 4B  illustrates a graph of the PWM input control signal, the voltage on the unitary integrating capacitor, a clock signal and the electronically controlled switch toggle signal for a single one of the LED strings of the driving arrangement of  FIG. 4A , wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units; 
           [0031]      FIG. 5A  illustrates a high level schematic diagram of an embodiment of a driving arrangement for a plurality of parallel connected LED strings comprising, for each LED string, a comparison circuitry constituted of a unitary integrating capacitor, wherein an electronically controlled switch in series with the respective LED string is toggled responsive to an output signal from the respective comparison circuitry, the comparison circuitries receiving a common PWM input control signal; and 
           [0032]      FIG. 5B  illustrates a graph of the PWM input control signal, the voltage on the unitary integrating capacitor, a clock signal and the electronically controlled switch toggle signal for a single one of the LED strings of the driving arrangement of  FIG. 5A , wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0034]      FIG. 1A  illustrates a high level schematic diagram of an embodiment of a driving arrangement  10  for a plurality of parallel connected LED strings  20 , driving arrangement  10  comprising: a comparison circuitry  30 ; a power source  40 ; a balancing network  50 ; a control circuitry  60 ; a differential amplifier  70 ; and a sense resistor  80 . Comparison circuitry  30  comprises: an electronically controlled switch  90 ; a one shot circuitry  100 ; a target current circuitry  110 ; an illumination current circuitry  120 ; a first charging current source  130 ; a second charging current source  140 ; an electronically controlled switch  150 ; an electronically controlled switch  155 ; a first capacitor  160 ; a second capacitor  170 ; and a comparator differential amplifier  180 . Each of target current circuitry  110  and illumination current circuitry  120  comprises: a current mirror reference arm  190 ; a differential amplifier  200 ; an electronically controlled switch  210 ; and a sense resistor  220 . In one non-limiting embodiment, each of differential amplifier  70  and differential amplifiers  200  comprises an operational amplifier. In one non-limiting embodiment, each of electronically controlled switches  150 ,  155  and  210  comprises an n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET). Differential amplifier  70  and differential amplifier  180  are each preferably implemented as comparators. 
         [0035]    A first end of electronically controlled switch  90  is connected to a reference voltage, denoted IREF, and a control input of electronically controlled switch  90  is connected to a pulse width modulated signal, denoted PWM. A second end of electronically controlled switch  90  is connected to the non-inverting input of differential amplifier  200  of target current circuitry  110  and to the non-inverting input of differential amplifier  70 . The inverting input of each differential amplifier  200  is connected to the source of the respective electronically controlled switch  210  and to a first end of the respective sense resistor  220 , and a second end of each sense resistor  220  is connected to a common point. The output of each differential amplifier  200  is connected to the gate of the respective electronically controlled switch  210  and the drain of each electronically controlled switch  210  is connected to an output of the respective current mirror reference arm  190 . An input of each current mirror reference arm  190  is connected to a supply voltage, denoted VDD. 
         [0036]    A first input of each of first charging current source  130  and second charging current source  140  is connected to supply voltage VDD and a control input of each of first charging current source  130  and second charging current source  140  is connected to a respective current mirror reference arm  190  so as to form a respective current mirror, such that the current output by each of first charging current source  130  and second charging current source  140  is substantially equal to the current flow through the respective current mirror reference arm  190 . 
         [0037]    An output of first charging current source  130  is connected to the drain of electronically controlled switch  150 , to a first end of first capacitor  160  and to the non-inverting input of comparator  180 . The source of electronically controlled switch  150  is connected to a second end of first capacitor  160  and to the common point. An output of second charging current source  130  is connected to the drain of electronically controlled switch  155 , to a first end of second capacitor  170  and to the inverting input of comparator  180 . The source of electronically controlled switch  155  is connected to a second end of second capacitor  170  and to the common point. The gates of electronically controlled switches  150 ,  155  are commonly connected to an output of one shot circuitry  100 . The output of comparator  180  constitutes control circuitry  60  and is connected to a control input of power source  40  and an input of one shot circuitry  100 , and is denoted signal ON/OFF. An output of power source  40  is connected to an input of balancing network  50  and each of a plurality of outputs of balancing network  50  is connected to the anode end of a particular LED string  20 . The cathode ends of plurality of LED strings  20  are commonly connected to a first end of sense resistor  80 , to the non-inverting input of differential amplifier  200  of illumination current circuitry  120  and to the inverting input of differential amplifier  70 . The output of differential amplifier  70  is connected to a second input of power source  40  and a second end of sense resistor  80  is connected to the common point. 
         [0038]      FIG. 1B  illustrates a graph of certain signals of driving arrangement  10 , particularly: signal PWM; the current flowing through the plurality of LEDs  20 , as represented by the voltage drop across sense resistor  80 , denoted ISNS; and signal ON/OFF, wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units. For the sake of clarity the operation of driving arrangement  10  of  FIG. 1A  will be described in relation to the graph of  FIG. 1B . For ease of understanding, signal PWM is described wherein the active state thereof is the high state and the inactive state is the low state, however this is not meant to be limiting in any way. 
         [0039]    In operation, signal PWM exhibits a duty cycle reflective of a target luminance. At a high state of signal PWM electronically controlled switch  90  is closed, thereby connecting reference voltage IREF to the non-inverting inputs of differential amplifier  200  of target current circuitry  110  and to differential amplifier  70 . Differential amplifier  200  of target current circuitry  110 , in cooperation with the respective electronically controlled switch  210 , is arranged as a current source passing a current, denoted I 1 , of a value such that the voltage drop developed across respective sense resistor  80  by current I 1  is equal to the voltage at the non-inverting input of differential amplifier  200  of target current circuitry  110 . Thus, when electronically controlled switch  90  is closed, current I 1  flowing through current mirror reference arm  190  of target current circuitry  110  is set to be proportional to value IREF. The current output from first charging current source  130  mirrors the current flow through the respective current mirror reference arm  190  and is denoted current I 2 . Current I 2  begins to charge first capacitor  160  and the voltage across first capacitor  160  is received at the non-inverting input of comparator  180 . As will be described further below, the potential at the inverting input of comparator  180  is near the common potential, and thus comparator  180  sets output signal ON/OFF of control circuitry  60  to a positive value so as to activate power source  40 . Thus signal ON/OFF is set to a positive value responsive to a positive value of signal PWM, and substantially contemporaneously therewith. After a start up time of power source  40 , current begins to flow through plurality of LED strings  20  connected to the output of power source  40  via balancing network  50 , and increases over time as illustrated by signal ISNS. Balancing network  50  is arranged to maintain a substantially equal current in each of plurality of LED strings  20 . In one embodiment, balancing network  50  is not provided and plurality of LEDs  20  are directly connected to the output of power source  40 , without exceeding the scope. In another embodiment, balancing network  50  is constituted of a plurality of balancing transformers, each of the balancing transformers exhibits a primary winding in series with a respective LED string  20 , and the secondary windings are connected in a closed serial loop. In yet another embodiment, balancing network  50  is constituted of a plurality of impedances significantly greater than the differences in impedance between the various LED strings  20 . 
         [0040]    ISNS is received at the non-inverting input of differential amplifier  200  of illumination current circuitry  120 . Differential amplifier  200  of illumination current circuitry  120 , in cooperation with the respective electronically controlled switch  210 , is arranged as a current source passing a current, denoted I 3 , of a value such that the voltage drop developed across respective sense resistor  80  by current I 3  is equal to the value of signal ISNS. The current output from second charging current source  140  mirrors the current flow through the respective current mirror reference arm  190  and is denoted current I 14 . Current I 14  begins to charge second capacitor  170  and the voltage across second capacitor  170  is received at the inverting input of comparator  180 . 
         [0041]    When signal PWM changes to a low state, electronically controlled switch  90  is opened, thereby ceasing the flow of currents I 1  and I 2 . The voltage drop across first capacitor  160  is thus reflective of an integral of a target current, as represented by signal IREF, over a target illumination time, as represented by the duty cycle of signal PWM. Power source  40  is active responsive to the output of control circuitry, and ISNS rises over time until reaching the target current, i.e. voltage representation ISNS becomes equal to reference voltage IREF, and is maintained equal by the closed loop operation of power source  40  and differential amplifier  70 . As a result of the gradual approach of ISNS towards the target current IREF, and consequently the gradual approach of current I 14  towards current I 12 , second capacitor  170  is initially charged at a slower pace than first capacitor  160 . Thus, when electronically controlled switch  90  is opened responsive to signal PWM no longer being asserted, the voltage across second capacitor  170  is smaller than the voltage across first capacitor  160  and control circuitry  60  maintains power source  40  in an activate state. 
         [0042]    Second capacitor  170  continues to charge until the voltage across second capacitor  170  becomes equal to the voltage across first capacitor  160 . When the voltage across second capacitor  170  exceeds the voltage across first capacitor  160 , comparator  180  de-asserts signal ON/OFF, and thus control circuitry  60  deactivates power source  40 . The voltage across second capacitor  170  is reflective of an integral of the illumination current over the illumination time and since the voltage across second capacitor  170  is equal to the voltage across first capacitor  160  at the time of de-assertion of signal ON/OFF, the integral of the actual illumination current ISNS over the actual illumination time is equal to the integral of the target current represented by IREF over the target illumination time represented by the active portion of signal PWM. Thus, plurality of LEDs  20  achieve the target overall luminance output reflected by the duty cycle of signal PWM. 
         [0043]    Simultaneously with the deactivation of power source  40  by the de-assertion of signal ON/OFF, control circuitry  60  is arranged to control one shot circuitry  100  to close electronically controlled switches  150  and  155  for a predetermined time period, thereby discharging first capacitor  160  and second capacitor  170 . As power source  40  is deactivated, the current through LED strings  20 , as represented by signal ISNS begins to decrease over time and second capacitor  170  again charges, after the predetermined time period of one shot circuitry  100 , until signal ISNS reaches zero. The voltage across second capacitor  170  represents a luminance surplus, i.e. effective luminance greater than the total target luminance, which will be compensated for during the next cycle of signal PWM, as it will take less time for the voltage across second capacitor  170  to reach the value of the voltage across first capacitor  160 . 
         [0044]    Control circuitry is described herein as a direct pass through connection between the output of comparator  180  and the control input of power source  40 , and thus may be implemented by a direct connection, however this is not meant to be limiting in any way. 
         [0045]      FIG. 2A  illustrates a high level schematic diagram of an embodiment of a driving arrangement  300  for a plurality of parallel connected LED strings  20 , comprising: a comparison circuitry  310 ; a power source  40 ; a balancing network  50 ; a control circuitry  60 ; a differential amplifier  70 ; and a sense resistor  80 . Comparison circuitry  310  comprises: an electronically controlled switch  90 ; a target current circuitry  110 ; an illumination current circuitry  120 ; a first charging current source  130 ; a second charging current source  140 ; a capacitor  320 ; and a comparator  180 . Each of target current circuitry  110  and illumination current circuitry  120  comprises: a current mirror reference arm  190 ; a differential amplifier  200 ; an electronically controlled switch  210 ; and a sense resistor  220 . 
         [0046]    A first end of electronically controlled switch  90  is connected to a reference voltage, denoted IREF, and a control input of electronically controlled switch  90  is connected to a pulse width modulated signal, denoted PWM. A second end of electronically controlled switch  90  is connected to the non-inverting input of differential amplifier  200  of target current circuitry  110  and to the non-inverting input of differential amplifier  70 . The inverting input of each differential amplifier  200  is connected to the source of the respective electronically controlled switch  210  and to a first end of the respective sense resistor  220  and a second end of each sense resistor  220  is connected to a common point. The output of each differential amplifier  200  is connected to the gate of the respective electronically controlled switch  210  and the drain of each electronically controlled switch  210  is connected to an output of the respective current mirror reference arm  190 . An input of each current mirror reference arm  190  is connected to a supply voltage, denoted VDD. 
         [0047]    A first input of first charging current source  130  is connected to supply voltage VDD. An output of first charging current source  130  is connected to a first input of second charging current source  140 , to a first end of capacitor  320  and to the non-inverting input of comparator  180 . An output of second charging current source  140  and a second end of capacitor  320  is connected to the common point. A control input of each of first charging current source  130  and second charging current source  140  is connected to a respective current mirror reference arm  190  to form a current mirror, such that the current output by each of first charging current source  130  and second charging current source  140  is substantially equal to the current flow of the respective current mirror reference arm  190 . The inverting input of comparator  180  is connected to an offset voltage, denoted voltage VOFF. 
         [0048]    The output of comparator  180  constitutes control circuitry  60  and is connected to a first input of power source  40 , and denoted signal ON/OFF. An output of power source  40  is connected to an input of balancing network  50  and each of a plurality of outputs of balancing network  50  is connected to the anode end of a particular LED string  20 . The cathode ends of plurality of LED strings  20  are commonly connected to a first end of sense resistor  80 , to the non-inverting input of differential amplifier  200  of illumination current circuitry  120  and to the inverting input of differential amplifier  70 . The output of differential amplifier  70  is connected to a second input of power source  40  and a second end of sense resistor  80  is connected to the common point. In one embodiment balancing network  50  is not provided and plurality of LED strings  20  is directly connected to power source  40 , without exceeding the scope. In another embodiment, balancing network  50  is constituted of a plurality of balancing transformers, each of the balancing transformers exhibits a primary winding in series with a respective LED string  20 , and the secondary windings are connected in a closed serial loop. In yet another embodiment, balancing network  50  is constituted of a plurality of impedances significantly greater than the differences in impedance between the various LED strings  20 . 
         [0049]      FIG. 2B  illustrates a graph of certain signals of driving arrangement  300 , particularly: signal PWM; the current flowing through the plurality of LEDs  20 , as represented by the voltage drop across sense resistor  80 , denoted ISNS; and signal ON/OFF, wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units. For the sake of clarity the operation of driving arrangement  300  of  FIG. 2A  will be described in relation to the graph of  FIG. 2B . For ease of understanding, signal PWM is described wherein the active state thereof is the high state and the inactive state is the low state, however this is not meant to be limiting in any way. 
         [0050]    The operation of driving arrangement  300  is in all respects similar to the operation of driving arrangement  10  of  FIG. 1A , with the exception that first charging current source  130  charges capacitor  320  and second charging current source  140  discharges capacitor  320 . Responsive to assertion of signal PWM, current I 12  charges capacitor  320  to a value above voltage VOFF, thus asserting signal ON/OFF responsive to the operation of comparator  180 , which enables power source  40 . ISNS rises responsive to the enabling of power source  40 , thus enabling second current source  140  with a value which mirrors signal ISNS. Second current source  140  draws current I 14  from the nexus of capacitor  130  and the output of first current source  130  thereby reducing the rate of increasing charge of capacitor  320 . When electronically controlled switch  90  opens, responsive to de-assertion of signal PWM, thereby ceasing flow of current I 12  as described above, the voltage across capacitor  320  represents the overall charge from the difference between current I 2  times the amount of time that signal PWM was asserted, i.e. the difference between the integral of the target current over the target illumination time, and the integral of current I 14  over time responsive to the output of power source  40 , i.e. the integral of the actual illumination current over the illumination time. 
         [0051]    Responsive to signal ISNS, second charging current source  140  begins to discharge capacitor  320 . When the voltage across capacitor  320  falls below voltage VOFF, responsive to comparator  180 , control circuitry  60  de-asserts signal ON/OFF thus disabling power source  40 . As described above, signal ISNS falls over time, thereby capacitor  320  continues to discharge and the voltage across capacitor  320  below voltage VOFF represents a luminance surplus which will be compensated for during the next cycle of signal PWM. Particularly, during the next cycle of signal PWM the luminance surplus causes the voltage across capacitor  320  to begin from a value below voltage VOFF, and thus the assertion of signal ON/OFF will be delayed in relation to the assertion of signal PWM so as to result in a reduced actual illumination. Advantageously, capacitor  320  does not need to be discharged every cycle. 
         [0052]      FIG. 3  illustrates a high level schematic diagram of a driving arrangement  400  for a plurality of parallel connected LED strings  20 , comprising: a comparison circuitry  410 ; a power source  40 ; a control circuitry  420 , in one non-limiting embodiment comprising an SR flip-flop; a differential amplifier  70 ; and a sense resistor  80 . Comparison circuitry  410  comprises: an electronically controlled switch  90 ; a one shot circuitry  100 ; a target low pass filter  430 ; an illumination low pass filter  440 ; a differential amplifier  180 , preferably arranged as a comparator; and an electronically controlled switch  470 . Each of target low pass filter  430  and illumination low pass filter  440  comprises: a resistor  450 ; and a capacitor  460 . In one non-limiting embodiment, resistors  450  and capacitors  460  are chosen such that the time constant of illumination low pass filter  440  is smaller than the time constant of target low pass filter  450 . In one embodiment, electronically controlled switch  470  comprises an NMOSFET. In one embodiment differential amplifier  70  is arranged as a comparator. 
         [0053]    A first end of electronically controlled switch  90  is connected to a reference voltage, denoted IREF, and to the non-inverting input of differential amplifier  70 . A control input of electronically controlled switch  90  is connected to a pulse width modulated signal, or other time modulated signal, denoted signal PWM. A second end of electronically controlled switch  90  is connected to a first end of resistor  450  of target low pass filter  430  and a second end of resistor  450  of target low pass filter  430  is connected to a first end of capacitor  460  of target low pass filter  430 , to the drain of electronically controlled switch  470  and to the inverting input of comparator  180 . A second end of capacitor  460  of target low pass filter  430  and the source of electronically controlled switch  470  are connected to a common point. The output of comparator  180  is connected to the reset input of control circuitry  420 . The set input of control circuitry  420  is connected to signal PWM and the non-inverted output of control circuitry  420  is connected to a control input of power source  40  and to an input of one shot circuitry  100 . The output of one shot circuitry  100  is connected to the gate of electronically controlled switch  470 . 
         [0054]    The anode ends of plurality of LED strings  20  are commonly connected to an output of power source  40 . The cathode ends of plurality of LED strings  20  are commonly connected to a first end of sense resistor  80 , to a first end of resistor  450  of illumination low pass filter  440  and to the inverting input of differential amplifier  70 . A second end of sense resistor  80  is connected to the common point. A second end of resistor  450  of illumination low pass filter  440  is connected to a first end of capacitor  460  of illumination low pass filter  440  and to the non-inverting input of comparator  180 . A second end of capacitor  460  of illumination low pass filter  440  is connected to the common point and the output of differential amplifier  70  is connected to a second input of power source  40 . In one embodiment (not shown), the anode ends of plurality of LED strings  20  are commonly connected to a balancing network, as described above in relation to  FIGS. 1A and 2A , to ensure equal current flow through each of plurality of LED strings  20 . 
         [0055]    In operation, responsive to an active state of signal PWM, control circuitry  420  is set thereby enabling power source  40 . Current thus begins to flow through plurality of LEDs  20  and capacitor  460  of illumination low pass filter  440  begins to charge responsive to the voltage drop across sense resistor  80 , denoted signal ISNS. Additionally, electronically controlled switch  90  is closed and capacitor  460  of target low pass filter  440  begins to charge responsive to reference voltage IREF. As described above in relation to first capacitor  160  of  FIG. 1A , the voltage across capacitor  460  of target low pass filter  430  represents an integral of a target current, represented by reference voltage IREF, over a target illumination time, the target illumination time represented by the duty cycle of signal PWM. Specifically, the voltage across capacitor  460  of target low pass filter  430  equals an average of reference voltage IREF over the duty cycle of signal PWM. As described above in relation to second capacitor  170  of  FIG. 1A , the voltage across capacitor  460  of illumination low pass filter  440  represents an integral of the actual current flowing through plurality of LED strings  20 , as represented by signal ISNS, over the actual illumination time. Specifically, the voltage across capacitor  460  of illumination low pass filter  440  equals the integral of signal ISNS over time. 
         [0056]    When signal PWM is de-asserted, electronically controlled switch  90  opens and capacitor  460  of target low pass filter  430  ceases to charge. Capacitor  460  of illumination low pass filter  440  continues to charge, response to signal ISNS, until the voltage across capacitor  460  of illumination low pass filter  440  becomes greater than the voltage across capacitor  460  of target low pass filter  430 , thereby causing comparator  180  to reset control circuitry  420 , which in turn disables power source  40 . Additionally, control circuitry  420  activates one shot circuitry  100  which closes electronically controlled switch  470 , thereby discharging capacitor  460  of target low pass filter  430 . As signal ISNS drops to below the value of the charge across capacitor  460  of illumination low pass filter  440 , capacitor  460  of illumination low pass filter  440  discharges through sense resistor  80  to the common point. As described above, differential amplifier  70  is arranged to control the current output by power source  40  to be at the desired level responsive to reference voltage IREF. 
         [0057]      FIG. 4A  illustrates a high level schematic diagram of an embodiment of a driving architecture  500  for a plurality of parallel connected LED strings  20  comprising: a power source  40 ; a plurality of comparison circuitries  510 ; a plurality of electronically controlled switches  520 ; a plurality of sense resistors  530 ; and a plurality of control circuitries  540 . Each comparison circuitry  510  is in all respects similar to comparison circuitry  310  of  FIG. 2A , with the exception that the first end of capacitor  320 , the output of first charging current source  130  and the input of second charging current source  140  are connected to the inverting input of comparator  180  and the non-inverting input of comparator  180  is connected to an offset voltage VOFF. Each control circuitry  540  comprises a clock generator  550  and an SR flip-flop  560 . Each of plurality of LED strings  20  has associated therewith a particular comparison circuitry  510 , a particular control circuitry  540 , a particular electronically controlled switch  520  and a particular sense resistor  530 . In one embodiment, each electronically controlled switch  520  comprises an NMOSFET. 
         [0058]    A control input of each electronically controlled switch  90  is connected to a respective PWM signal, denoted PWM 1  . . . PWMK, and the first end of each electronically controlled switch  90  is connected to a respective reference voltage, denoted IREF 1  . . . IREFK. The output of each clock generator  550  is connected to the set input of the respective SR flip flop  560  and is denoted CLK 1  . . . CLKK, of which for clarity only CLK 1  is shown. The reset input of each SR flip flop  560  is connected to the output of the respective comparator  180 . The non-inverted output of each SR flip-flop  560  is connected to the gate of the respective electronically controlled switch  520  and is denoted DRIVE 1  . . . DRIVEK. The drain of each electronically controlled switch  520  is connected to the cathode end of the respective associated LED string  20 . The source of each electronically controlled switch  520  is connected to a first end of the respective sense resistor  530  and to the non-inverting input of differential amplifier  200  of illumination current circuitry  120  of the respective comparison circuitry  510 , and respectively denoted signal ISNS 1  . . . ISNSK. A second end of each sense resistor  530  is connected to the common point and the anode ends of plurality of LED strings  20  are commonly connected to an output of power source  40 . 
         [0059]      FIG. 4B  illustrates a graph of various signals of driving architecture  500  associated with a first of plurality of LED strings  20 , specifically: the voltage across capacitor  320 , denoted VC 1 ; signal DRIVEL; clock signal CLK 1 ; and signal PWM 1 , wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units. For the sake of clarity the operation of driving arrangement  500  of  FIG. 4A  will be described in relation to the graph of  FIG. 4B  and will further be described in relation to a first of plurality of LED strings  20 . For ease of understanding, clock signal CLK 1  and signal PWM 1  are described wherein the active state is of each signal is the high state and the inactive state is the low state, however this is not meant to be limiting in any way. 
         [0060]    In operation, power source  40  provides current to each of plurality of LED strings  20 . As described above in relation to  FIGS. 2A-2B , capacitor  320  is charged responsive to the high state of signal PWM. During charging of capacitor  320  the voltage there across increases to greater than offset voltage VOFF, and responsive thereto the output of comparator  180  is set to low and SR flip flop  560  is reset. The set input of SR flip flop  560  is controlled by clock generator  550 . The frequency of clock signal CLK 1  is arranged to be greater than the frequency of signal PWM 1 , preferably at least 10 times the frequency of signal PWM 1 . At an active state of clock signal CLK 1 , SR flip flop  560  is set, thus closing electronically controlled switch  520  via signal DRIVE 1  and allowing current to flow through electronically controlled switch  520 . As described above, current is drawn by second charging current source  140  responsive to signal ISNS 1  representing current flowing through sense resistor  530  and voltage VC 1  represents the difference between the overall target luminance of the particular LED string  20 , particular the integral of the current flow from first current source  130  representing value IREF over time, and the overall actual luminance of the particular LED string  20 , representing the integral of signal ISNS over time. Once voltage VC 1  drops below voltage VOFF, i.e. the overall target luminance is achieved, SR flip flop  560  is reset and electronically controlled switch  520  is opened by signal DRIVE 1 , thereby ceasing current flow there through, and setting ISNS to zero. 
         [0061]    Each LED string  20  has a different voltage drop there across, and power source  40  is arranged to supply power with a sufficient voltage for the LED string  20  exhibiting the greatest voltage drop. As a result, a LED string  20  with a lower voltage drop, such as LED string  20  associated with signal DRIVE 1 , may receive a higher current than represented by the respective target value IREF 1 , and voltage VC 1  will then drop below voltage VOFF before signal PWM 1  changes to the de-asserted state and opens electronically controlled switch  90 . The increased current is however associated with increased illumination, and thus voltage VC 1  represent the total illumination. In particular, in such an event current flow through the LED string  20  associated with PWM 1  will cease before signal PWM 1  is de-asserted, and the continued assertion of signal PWM 1  results in an increase in voltage VC 1 . At the next high state of clock signal CLK 1 , particularly responsive to the rising edge of CLK 1 , electronically controlled switch  520  closes thereby renewing current flow there through until voltage VC 1  falls to voltage VOFF responsive to signal ISNS. Thus, each electronically controlled switch  520  is turned on periodically by the respective clock generator  550  enabling current flow until the respective voltage VC 1  . . . VCK is equal to voltage VOFF. Any residual difference on capacitor  320  at the end of an active portion of signal PWM is maintained until the next PWM active portion, and compensated for at the beginning of the next PWM active portion. 
         [0062]    Advantageously, the various LED strings  20  are thus controlled by the operation of electronically controlled switches  520  in a non-dissipative manner. 
         [0063]      FIG. 5A  illustrates a high level schematic diagram of an embodiment of a driving architecture  600  for a plurality of parallel connected LED strings  20 .  FIG. 5B  illustrates a graph of various signals associated with a first one of plurality of LED strings  20  of driving architecture  600 , specifically: the voltage across capacitor  320 , denoted VC 1 ; signal DRIVE 1 ; clock signal CLK 1 ; and signal PWM, wherein the x-axis represents time and the y-axis represents amplitude in arbitrary units. The arrangement and operation of driving architecture  600 , and the description of the signals of driving architecture  600 , is in all respects similar to the arrangement and operation of driving architecture  500  of  FIG. 4A  with the exception that a common PWM signal, denoted PWM, and a common reference voltage, denoted IREF, is provided for the plurality of comparison circuitries  510 . 
         [0064]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0065]    Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein. 
         [0066]    All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
         [0067]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.