Abstract:
A cold cathode fluorescent lamp controller constituted of a pulse generator exhibiting a cycle period; a burst dimming control operative to set a burst dimming duty cycle for the pulse generator; a comparator arranged to receive a signal indicative of the value of current through a load driven by the pulse generator and compare the received signal to a reference signal; an error detection circuit operative to shut down the pulse generator responsive to an output of the comparator indicating that the received signal is less than the reference signal for a predetermined number of cycles of the pulse generator; and a disable circuit arranged to prevent, when the set burst dimming duty cycle is less than a predetermined value, the shut down of the pulse generator responsive to the output of the comparator indicating the received signal is less than the reference signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/116,320 filed Nov. 20, 2009, entitled “Method and Apparatus for Driving CCFL at Low Burst Duty Cycle Rates”, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to the field of cold cathode fluorescent lamp based lighting and more particularly to an arrangement in which low burst dimming duty cycles are supported. 
         [0003]    Fluorescent lamps are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors. One particular type of fluorescent lamps is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ionize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to herein as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on. Preferably the CCFL is alternately powered from each end so as to maintain an even brightness across the lamp. 
         [0004]    In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCD or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness, with the resultant AC signal preferably arranged to perform the aforementioned alternate powering. Even brightness across the panel and clean operation of the inverter system with low switching stresses, low EMI, and low switching losses is desirable. 
         [0005]    In CCFL backlight applications the inverter system typically comprises a DC to AC controller in cooperation with external components operative to generate an AC power of a few hundred Volts to over one thousand Volts RMS so as to drive the lamps at a frequency of about 30 to 70 KHz. The DC to AC controller alone is sometimes referred to as an inverter. This high voltage raises certain safety issues, and as a result CCFL controllers typically provide an open lamp detection functionality. The open lamp detection functionality monitors a current, and optionally a voltage, associated with each one of the load CCFLs to detect if the CCFLs present an open circuit to the controller, referred hereinto below as an open lamp condition. Such an open lamp condition may be the result of missing, defective or burnt out lamps. Detection of an open lamp condition is normally accomplished by comparing the sensed lamp current to a pre-set threshold. If the sensed lamp current is lower than the pre-set threshold an open lamp condition is considered detected. Upon detection of an open lamp condition, the open lamp detection functionality is operative to shut down the controller so as to prevent the appearance of a high voltage AC signal without a valid load. In order to avoid transient response, typically the open lamp detection functionality requires an open lamp condition to be found for a plurality of cycles of the high frequency AC and the open lamp condition is cleared upon detection of a sensed lamp current indicative of normal operation. The plurality of cycles may be counted digitally in a counter, or by charging a capacitor with a known current source whenever an open lamp condition is detected. The lamp current indication of normal operation is in one embodiment a current greater than the pre-set threshold, and in another embodiment a separate higher threshold is provided to clear the open lamp condition. 
         [0006]    In many such applications, such as backlighting for LCD based televisions, dimming is required to adjust the brightness of the backlight in order to produce satisfactory pictures in various ambient lighting conditions and various visual conditions. Dimming is typically achieved by one or both of analog dimming and burst dimming. Technically speaking, analog dimming controls the amplitude of the CCFL current, whereas burst dimming turns the CCFL on and off at a duty cycle so as to adjust the average brightness of the backlight over time. At any particular amplitude of the CCFL current, a lower duty cycle of burst dimming results in a dimmed light as compared to a larger duty cycle of burst dimming. The frequency of the burst dimming cycle is typically in the range of 150-250 Hz, and thus functions as an envelope for the higher frequency AC lamp voltage. 
         [0007]    One limitation of CCFL is that the light may extinguish completely when the lamp is operated at a low current level. Furthermore, the efficiency of a CCFL at low current levels is lower than the efficiency of the CCFL at higher current levels. As a result a minimum lamp current limit is defined, which effectively limits the range of analog dimming. As a result, burst dimming is almost universally used, with the lamp current set to an optimum value and the brightness controlled by the burst duty cycle. 
         [0008]      FIG. 1  illustrates a high level schematic diagram of a backlighting arrangement according to the prior art, illustrating an embodiment of the above open lamp condition detection. The backlighting arrangement comprises a plurality of CCFLs  10 , a plurality of sense resistors RS each associated with a particular cold cathode fluorescent lamp  10 , a resistor RD 1  and a resistor RD 2 , a diode OR circuit  15  comprising a plurality of diodes DS each associated with a particular CCFL  10 , a controller  20  and a step-up transformer  90 . Controller  20  comprises a pulse generator  30 , an open lamp detection functionality  35  and a lamp error amplifier  80 . Open lamp detection functionality  35  comprises a comparator  40 , a comparator  50 , an OR gate  60  and an error cycle count functionality  70 . 
         [0009]    A received burst dimming pulse, denoted VBST is connected to an input of pulse generator  30 . The output of pulse generator  30  is connected to a first end of the primary winding of step-up transformer  90 , and the second end of the primary winding is connected to a first common point, illustrated as a chassis ground. A first end of the secondary winding of step-up transformer  90  is connected to a second common point, which may be different from the common point of the primary winding, and is illustrated as a local ground. A second end of the secondary winding of step-up transformer  90  is connected to a first end of resistor RD 1  and to a first end of each of the plurality of CCFLs  10 . The second end of each CCFL  10  is connected to a first end of the associated sense resistor RS and to the anode of a particular one of the diodes DS of diode OR circuit  15 . The second end of each of the associated sense resistors RS are connected to the second common point. 
         [0010]    The second end of resistor RD 1  is connected to a first end of resistor RD 2  and to the non-inverting input of comparator  40 , denoted input VSNS of controller  20 . The second end of resistor RD 2  is connected to the second common point. A maximum voltage level, denoted VTH 2 , is connected to the inverting input of comparator  40 . The output of diode OR circuit  15  is connected to the inverting input of comparator  50  and to the non-inverting input of error amplifier  80 , denoted input ISNS of controller  20 . An open lamp detection threshold current level, denoted VTH 1 , is connected to the non-inverting input of comparator  50 . 
         [0011]    The output of comparator  40  is connected to a first input of OR gate  60 . The output of comparator  50  is connected to a second input of OR gate  60  and via an inverter to the clear input of error cycle count functionality  70 . The output of OR gate  60  is connected to the input of error cycle count functionality  70 . The output of error cycle count functionality  70 , denoted FAULT, is connected to an input of pulse generator  30 . A lamp current reference level, denoted IREF, is connected to the inverting input of error amplifier  80 , and the output of error amplifier  80  is connected to an input of pulse generator  30 . Error cycle count functionality  70  is illustrated as a digital counter, however this is not meant to be limiting in any way. In another embodiment error cycle count functionality  70  is implemented in an analog fashion with a capacitor arranged to receive a fixed current, and a fault signal will be output when a certain voltage level is reached. 
         [0012]    VBST is illustrated as a received gating signal, however this is not meant to be limiting in any way. In one embodiment, VBST is derived from a received analog signal whose level is translated internally into the duty cycle for the burst dimming signal. 
         [0013]    In operation, pulse generator  30  is operative to generate a pulse width modulated high frequency square wave gated by a low frequency burst dimming pulse VBST and thereby drive the primary side of step-up transformer  90 . In one embodiment, pulse generator  30  drives an H-bridge switching arrangement connected to the primary side of step up transformer  90 , as described in U.S. Pat. No. 5,930,121 to Henry, the entire contents of which is incorporated herein by reference. Step-up transformer  90  steps up the voltage of the signal received at the primary, and in cooperation with self inductance of step-up transformer  90  and parasitic capacitance of CCFLs  10 , filters the resultant AC voltage to supply the AC voltage necessary for operation of CCFLs  10 . 
         [0014]    The voltage across the CCFLs  10  is divided by the voltage divider of resistors RD 1  and RD 2  and the divided voltage is presented via input VSNS to be compared with maximum voltage level VTH 2 . The current through CCFLs  10  are each sampled across the respective sense resistor RS, and the greater current is passed through diode OR circuit  15  and presented via input ISNS to be compared with open lamp detection threshold current level VTH 1 . The voltage representation of the current presented via input ISNS is further compared to lamp current reference level IREF, and any error is amplified and transmitted to pulse generator  30  which acts to increase or decrease the duty rate of the pulse width modulated output of pulse generator  30  so as to ensure that ISNS coincides with IREF when CCFLs  10  are being driven. 
         [0015]    Open lamp detection functionality  35  is operative to compare the divided representation of the voltage across CCFLs  10  with maximum voltage level VTH 2 , and the representation of the greater current through the CCFLs  10  with open lamp detection threshold current level VTH 1 , and output an error signal to error cycle count functionality  70  whenever the divided representation of the voltage across CCFLs  10  exceeds maximum voltage level VTH 2  or the representation of the greater current through CCFLs  10  is less than open lamp detection threshold current level VTH 1 . It is to be understood that the maximum voltage is developed across CCFLs  10  when no current is flowing, and thus voltage exceeding maximum voltage level VTH 2  is indicative of an open lamp condition. Error cycle count functionality  70  is operative to count a predetermined number of error conditions, i.e. an error condition maintained for a predetermined number of cycles of pulse generator  30 , and in the event that the error is maintained to assert the FAULT signal to pulse generator  30  thereby disabling pulse generator  30 . In the event that the representation of the greater current through CCFLs  10  is greater than or equal to the open lamp detection threshold current level VTH 1  before the FAULT signal is asserted, the output of comparator  50  clears error cycle count functionality  70 . 
         [0016]    One problem with the above described burst dimming is audible noise. When the CCFL is turned on and off electro-mechanical vibration occurs due to the sharp change of electro-magnetic force in the associated components, especially in the transformers. As indicated above, the dimming frequency is in the range of 150-250 Hz, which is well within the audible frequency range. 
         [0017]    An effective method to reduce such electro-mechanical vibration is to control the profile of the burst lamp current so as to ramp up gradually when the burst dimming control changes from one state to another. Thus, when turning the lamp on, the current resultant from the high frequency AC voltage is ramped up to the nominal value, and when turning the lamp off, the current resultant from the high frequency AC voltage is ramped down from the nominal value until the lamp is off. 
         [0018]    Controlling the profile of the burst lamp current successfully suppresses the audible noise, however because of the ramp up and the ramp down of the burst current, there will be a small period at the bottom of the ramp slope wherein the lamp current is lower than the pre-set threshold, described above in relation to VTH 1 , and open lamp detection functionality  35  will assert the FAULT signal thereby shutting down pulse generator  30 . At low duty cycles of burst dimming the whole or a significant portion of the burst on period can result in a false open lamp protection of the inverter, as will be described below in relation to  FIG. 2 . Therefore most CCFL inverters cannot work at low burst duty cycles, specifically as low as several percent. This is disadvantageous as in today&#39;s market full darkness of the screen becomes one of the important performance requirements for high quality displays. To fulfill such a requirement the burst dimming operation has to be able to work stably at very small duty cycles down to zero percent. 
         [0019]      FIG. 2  illustrates a graph of certain signals of the backlighting arrangement of  FIG. 1 , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. The burst dimming signal, denoted VBST, in addition to VTH 1  and ISNS are illustrated. An additional analog representation of error cycle count functionality  70  is shown, denoted Error_Count, and a trigger value above which assertion of the FAULT signal is asserted, denoted Fault_Level. 
         [0020]    At each burst dimming pulse of VBST, the current through CCFLs  10  is ramped up to its nominal value as shown by the ISNS representation. The burst dimming is illustrated as being at a very low duty cycle, for instance around 10%, and therefore ISNS does not reach the VTH 1  level before being ramped back down. Thus, for each burst dimming pulse, an additional error count is accumulated, as illustrated by the climbing of signal Error_Count. Error_Count is not cleared because of the failure of ISNS to be equal to, or greater than, VTH 1 . After a few cycles, Error_Count exceeds Fault Level, and the FAULT signal will thus be asserted, resulting in shut down of controller  20 . 
         [0021]    What is desired, and not provided by the prior art, is a backlighting arrangement that can provide a full range of burst dimming, while maintaining support for open lamp detection. 
       SUMMARY 
       [0022]    Accordingly, it is a principal object to overcome at least some of the disadvantages of prior art. This is provided in certain embodiments by a backlighting arrangement in which shut down of the pulse generator responsive to open lamp current detection derived from the lamp current feedback signal is prevented whenever the burst dimming duty cycle is less than a predetermined value. 
         [0023]    In certain embodiments the shut down is prevented by disabling a comparator associated with the lamp current feedback signal. In other embodiments the shut down is prevented by overriding the lamp current feedback signal with a predetermined voltage. 
         [0024]    In certain embodiments the shut down is prevented by disabling a comparator associated with the lamp current feedback signal for a predetermined number of cycle periods of the pulse generator coincident with the rising edge of the burst dimming cycle. In other embodiments the shut down is prevented by overriding the lamp current feedback signal with a predetermined voltage for a predetermined number of cycle periods of the pulse generator coincident with the rising edge of the burst dimming cycle. 
         [0025]    In certain embodiments the shut down is prevented by disabling an error counter for a predetermined number of cycle periods of the pulse generator coincident with the rising edge of the burst dimming cycle. In other embodiments the shut down is prevented by ramping a reference current signal for a predetermined number of cycle periods of the pulse generator coincident with the rising edge of the burst dimming. 
         [0026]    Additional features and advantages of the invention will become apparent from the following drawings and description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    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. 
           [0028]    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: 
           [0029]      FIG. 1  illustrates a high level schematic diagram of a backlighting arrangement including open lamp detection in accordance with the prior art; 
           [0030]      FIG. 2  illustrates a graph of certain signals of the backlighting arrangement of  FIG. 1 ; 
           [0031]      FIG. 3  illustrates a high level block diagram of a system operative to disable the open lamp current detection from lamp current feedback signal at low duty cycles according to an exemplary embodiment; 
           [0032]      FIG. 4A  illustrates a high level block diagram of a system operative to pull up the lamp feedback signal to a level above the open lamp detection threshold and below the lamp current reference level for the entire burst on period according to an exemplary embodiment; 
           [0033]      FIG. 4B  illustrates a graph of certain signals of the system of  FIG. 4A ; 
           [0034]      FIG. 5A  illustrates a high level block diagram of a system operative to disable the open lamp current detection from lamp current feedback signal for a short period at the burst dimming rising edge according to an exemplary embodiment; 
           [0035]      FIG. 5B  illustrates a graph of certain signals of the system of  FIG. 5A ; 
           [0036]      FIG. 6A  illustrates a high level block diagram of a system operative to pull up the lamp feedback signal to a value above the open lamp threshold and below the lamp current reference signal for a short period at the burst dimming rising edge, according to an exemplary embodiment; 
           [0037]      FIG. 6B  illustrates a graph of certain signals of the system of  FIG. 6A ; 
           [0038]      FIG. 7A  illustrates a high level block diagram of a system operative to provide a pre-offset to the open lamp timing circuit at the burst dimming rising edge, according to an exemplary embodiment; 
           [0039]      FIG. 7B  illustrates a graph of certain signals of the system of  FIG. 7A ; 
           [0040]      FIG. 8A  illustrates a high level block diagram of a system operative to adjust the open lamp current threshold level at the burst dimming rising edge, according to an exemplary embodiment comprising an electronically controlled switch; 
           [0041]      FIG. 8B  illustrates a high level schematic diagram of a disable circuit providing a sloped rise for the effective open lamp detection threshold current level being compared with the sensed lamp current; 
           [0042]      FIG. 8C  illustrates a graph of certain signals of the system of  FIG. 8A ; 
           [0043]      FIG. 8D  illustrates a graph of certain signals of the system of  FIG. 8B ; and 
           [0044]      FIG. 9  is a high level flow chart of the method of operation of each of  FIGS. 3-8B . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0045]    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. The term connected as used herein is not meant to be limited to a direct connection, and the use of appropriate resistors, capacitors and inductors does not exceed the scope thereof. 
         [0046]      FIG. 3  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a low burst dimming level detection functionality  140 , a lamp voltage feedback signal VSNS and a lamp current feedback signal ISNS. Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to the input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120  and low burst dimming level detection functionality  140  is connected to open lamp detection from lamp current functionality  110 . Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 , with the exception that comparator  50  is replaced with a comparator having an disable input. In the event that the disable input is asserted, the output of the comparator is set to low. 
         [0047]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. At low burst duty cycles, such as around 10% and below, low burst dimming level detection functionality  140  is operative to detect the low burst dimming level and disable open lamp detection from lamp current functionality  110 . While open lamp detection from lamp current functionality  110  is disabled, if an open lamp exists it will be detected by open lamp detection from lamp voltage functionality  100 . 
         [0048]      FIG. 4A  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a low burst dimming level detection functionality  150 , a lamp voltage feedback signal VSNS, a lamp current feedback signal ISNS, and a unidirectional circuit shown as diode D. Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to the input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120 . The output of low burst dimming level detection functionality  150 , denoted VPULL, is connected via unidirectional circuit D to lamp current feedback signal ISNS. Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 . 
         [0049]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. At low burst duty cycles, such as around 10% and below, low burst dimming level detection functionality  150  is operative to detect the low burst dimming level and output a voltage clamping level VPULL, selected to be above open lamp detection threshold current level VTH 1  of  FIG. 1  and  FIG. 2  and below lamp current reference level IREF of  FIG. 1 . The output of low burst dimming level detection functionality  150  is channeled by unidirectional circuit D to pull up lamp current feedback signal ISNS. When the voltage representation of the lamp current rises above clamping voltage VPULL the clamp circuit is cut off, i.e. unidirectional circuit D becomes reverse biased and becomes open, and error amplifier  80  of  FIG. 1  resumes to control the lamp current feedback signal ISNS at lamp current reference level IREF. 
         [0050]      FIG. 4B  illustrates a graph of certain signals of the embodiment of  FIG. 4A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST and ISNS are illustrated as well as reference levels IREF and VTH 1 . As described above in  FIG. 4A , during the burst on period, lamp current feedback signal ISNS is pulled up to a clamping level, VPULL, above open lamp detection threshold VTH 1  and below lamp current reference level IREF. As current through the CCFLs  10  begins to rise the increased value of ISNS may pass level VPULL, thereby shutting off unidirectional circuit D. 
         [0051]      FIG. 5A  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a lamp voltage feedback signal VSNS, a lamp current feedback signal ISNS, a timing circuit  160 , and a burst dimming pulse denoted VBST. Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to the input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120 . Timing circuit  160  is connected to receive burst dimming pulse VBST and to output a signal denoted VINH which is received by open lamp detection from lamp current functionality  110 . 
         [0052]    Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 , with the exception that comparator  50  is replaced with a comparator having an disable input. In the event that the disable input is asserted, the output of the comparator is set to low. Timing circuit  160  may be implemented as one of an analog and digital timing circuit. 
         [0053]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. At the rise of burst dimming pulse VBST, timing circuit  160  enables signal VINH to disable open lamp detection from lamp current feedback signal for a number of cycles. Since there is only a short period, normally less than 10 lamp frequency cycles, at the rising edge of burst dimming pulse VBST that a false open lamp would be seen from the lamp current feedback, open lamp detection from lamp current feedback can be disabled by timing circuit  160  for a short period and resume thereafter. Open lamp detection is normally disabled during the entire burst off period including the falling edge of the burst on period, therefore disabling of the open lamp detection is only necessary during the rise of burst dimming pulse VBST. 
         [0054]      FIG. 5B  illustrates a graph of certain signals of the embodiment of  FIG. 5A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST, VINH and ISNS are illustrated as well as reference level VTH 1 . As described above in relation to  FIG. 5A , at the rising edge of burst dimming pulse VBST, signal VINH disables the open lamp detection from lamp current feedback for a portion of burst dimming pulse VBST, thereby preventing false detection of an open lamp while lamp current feedback signal ISNS is below open lamp detection threshold VTH 1 . In the event of a short burst dimming cycle, VINH will effectively prevent detection of an open lamp due to failure of ISNS to achieve the level VTH 1 . 
         [0055]      FIG. 6A  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a lamp voltage feedback signal VSNS, a lamp current feedback signal ISNS, a timing circuit  170 , a burst dimming pulse VBST and a unidirectional circuit D, illustrated as a diode. Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to the input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120 . Timing circuit  170  is connected to receive burst dimming pulse VBST and to output a signal denoted VPULL which is connected via unidirectional circuit D to lamp current feedback signal ISNS, and is operative to clamp ISNS to a minimum value of VPULL when active. 
         [0056]    Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 . Timing circuit  170  may be implemented as one of an analog and digital timing circuit. 
         [0057]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. At the rise of burst dimming pulse VBST, timing circuit  170  outputs signal VPULL which pulls up ISNS, for a predetermined number of cycles, to a minimum level above open lamp detection threshold VTH 1  of  FIGS. 1 ,  2  and below lamp current reference level IREF. 
         [0058]    Since there is only a short period, normally less than 10 lamp frequency cycles, at the rising edge of burst dimming pulse VBST that a false open lamp would be seen from the lamp current feedback, open lamp detection from lamp current feedback can be effectively disabled by timing circuit  170  for a short period and resume thereafter. Open lamp detection is normally disabled during the entire burst off period including the falling edge of the burst on period, therefore disabling of the open lamp detection is only necessary during the rise of burst dimming pulse VBST. 
         [0059]      FIG. 6B  illustrates a graph of certain signals of the embodiment of  FIG. 6A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST, VPULL and ISNS are illustrated as well as reference level VTH 1  and IREF. As described above in  FIG. 6A  at the rise of burst dimming pulse VBST signal VPULL is asserted for a predetermined short period thereby clamping lamp current feedback signal ISNS to a minimum level above open lamp detection threshold VTH 1  and below current reference level IREF. In the event of a short burst dimming cycle, VPULL will effectively prevent detection of an open lamp due to failure of ISNS to achieve the level VTH 1 . 
         [0060]      FIG. 7A  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a lamp voltage feedback signal VSNS, a lamp current feedback signal ISNS, a timing circuit  180 , and a burst dimming pulse VBST. Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to the input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120 . Timing circuit  180  is connected to receive burst dimming pulse VBST and to output a pre-offset signal denoted VOFFSET which is received by fault timing circuit  120 . Fault timing circuit  120  outputs a fault signal denoted VFAULT which is received by protection trigger circuit  130 . 
         [0061]    Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 . Timing circuit  180  may be implemented as one of an analog and digital timing circuit. 
         [0062]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. As described above, in order to avoid transient response, typically fault timing circuit  120  requires the error signal to be valid for a plurality of burst cycles and only then outputs fault signal VFAULT to protection trigger circuit  130  thereby shutting down the operation of the lamp, in one embodiment by outputting the FAULT signal of  FIG. 1 . At the rising edge of burst dimming pulse VBST, timing circuit  180  sends pre-offset signal VOFFSET to fault timing circuit  120 . Pre-offset signal VOFFSET provides a pre-determined discharge for the timing capacitor if fault timing circuit  120  is an analog capacitor charging circuit, or a pre-determined decrement of the fault timing counter if fault timing circuit  120  is a digital counting circuit. The amount of pre-offset provided, i.e. the discharge for the timing capacitor or the decrement count of the fault timing counter, is set to be equal or slightly greater than the incremental timing effect caused by the open lamp detection at the rising edge of burst dimming pulse VBST. Thus the false open lamp detection as a result of ramping of the burst signal is inhibited and true open lamp conditions are still detected. Since the open lamp timing detection functionality typically requires the error signal to be valid for a plurality of burst cycles, in another embodiment pre-offset signal VOFFSET is provided at the falling edge of burst dimming pulse VBST. 
         [0063]      FIG. 7B  illustrates a graph of certain signals of the embodiment of  FIG. 7A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST, VOFFSET, ISNS and VFAULT are illustrated as well as reference level VTH 1  and FAULT_LEVEL. As described above, in order to avoid transient response, protection trigger circuit  130  requires the error signal to be valid for a plurality of burst cycles, until fault signal VFAULT rises to fault level FAULT_LEVEL, and only then protection trigger circuit  130  is operative to shut down the operation of the inverter. Before the enablement of burst dimming pulse VBST, fault signal VFAULT is below fault level FAULT_LEVEL. At the rising edge of burst dimming pulse VBST, when lamp current feedback signal ISNS is below open lamp detection threshold VTH 1  due to the ramping burst signal, pre-offset signal VOFFSET is activated thereby lowering fault signal VFAULT by a predetermined offset. Fault signal VFAULT begins to rise for each cycle where lamp current feedback signal ISNS is below open lamp detection threshold VTH 1 . In the event of a long burst dimming cycle (not shown) ISNS would rise above VTH 1 , thereby clearing VFAULT, as described above in relation to  FIG. 1 . For short burst cycles, the offset provided by VOFFSET prevents false triggering of protection trigger circuit  130 . 
         [0064]      FIG. 8A  illustrates a high level block diagram of a system comprising an open lamp detection from lamp voltage functionality  100 , an open lamp detection from lamp current functionality  110 , a fault timing circuit  120 , a protection trigger circuit  130 , a lamp voltage feedback signal VSNS, a lamp current feedback signal ISNS, an open lamp detection threshold current level VTH 1 , a burst dimming pulse VBST, and a disable circuit  200  comprising a timing circuit  210 , an electronically controlled switch  220 , and a resistor  230 . Fault timing circuit  120  is comprised of an analog capacitor charging circuit, or a digital counting circuit as described above in relation to error cycle count functionality  70  of  FIG. 1 . Lamp voltage feedback signal VSNS is connected to the input of open lamp detection from lamp voltage functionality  100 , and the output of open lamp detection from lamp voltage functionality  100  is connected to a first input of fault timing circuit  120 . Lamp current feedback signal ISNS is connected to a first input of open lamp detection from lamp current functionality  110  and the output of open lamp detection from lamp current functionality  110  is connected to a second input of fault timing circuit  120 . Protection trigger circuit  130  is connected to the output of fault timing circuit  120 . Value detection circuit  210  is connected to receive burst dimming pulse VBST and to output a corresponding signal when VBST is less than a predetermined value, with the output of timing circuit  210  connected to the gate of electronically controlled switch  220 . Open lamp detection threshold current level VTH 1  is connected to a first end of resistor  230 , and a second end of resistor  230  carrying a signal denoted VTH 1 ′ is connected to a second input of open lamp detection from lamp current functionality  110 , and to the drain of electronically controlled switch  220 . The source of electronically controlled switch  220  is connected to a common point, shown as ground. 
         [0065]    Open lamp detection from lamp voltage functionality  100  is in one embodiment implemented as described above in relation to comparator  40  of  FIG. 1 . Open lamp detection from lamp current functionality  110  is in one embodiment implemented as described above in relation to comparator  50  of  FIG. 1 . Value detection circuitry  200  may be implemented as an analog or a digital circuit. 
         [0066]    In operation, if one of open lamp detection from lamp voltage functionality  100  and open lamp detection from lamp current functionality  110  detects an open lamp, an error signal is sent to fault timing circuit  120 . If the error signal is sustained for a specific amount of time an open lamp is detected, as described above in relation to  FIG. 1  and  FIG. 2 , and protection trigger circuit  130  is operative to shut down the inverter. As described above, in order to avoid transient response, typically fault timing circuit  120  requires the error signal to be valid for a plurality of burst cycles and only then outputs fault signal VFAULT to protection trigger circuit  130  thereby shutting down the operation of the lamp, in one embodiment by outputting the FAULT signal of  FIG. 1 . 
         [0067]    Burst dimming pulse VBST is received at timing circuit  210 . At the rise of burst dimming pulse VBST, timing circuit  210  closes electronically controlled switch  220  for a number of cycles which acts to pull down VTH 1 ′ to a low level, such as ground, and preferably maintain VTH 1 ′. Since there is only a short period, normally less than 10 lamp frequency cycles, at the rising edge of burst dimming pulse VBST that a false open lamp would be seen from the lamp current feedback, open lamp detection from lamp current feedback can be disabled by timing circuit  210  by pulling down signal VTH 1 ′ received by open lamp detection from lamp current functionality  110  for a short period and allowing open lamp detection from lamp current functionality  110  to receive VTH thereafter. Open lamp detection is normally disabled during the entire burst off period including the falling edge of the burst on period, therefore disabling of the open lamp detection is only necessary during the rise of burst dimming pulse VBST. 
         [0068]    In another embodiment, as illustrated in  FIG. 8B , disable circuit  200  comprises resistor  230 , a capacitor  240  and a unidirectional circuit  250 . Open lamp detection threshold current level VTH 1  is connected to a first end of resistor  230 , and a second end of resistor  230  carrying a signal denoted VTH 1 ′ is connected to a second input of open lamp detection from lamp current functionality  110 , to a first end of capacitor  240  and to the anode of unidirectional circuit  250 . The second end of capacitor  240  is connected to a common point of low value, illustrated as ground. The cathode of unidirectional circuit  250  is connected to burst dimming pulse VBST. Preferably, values for resistor  230  and capacitor  240  are selected so that at the rising edge of VBST, VTH 1 ′ rises slower than ISNS, and remains lower than ISNS until ISNS reaches its steady state value. Thus, when VBST goes low, VTH 1 ′ is pulled down, and upon the rising edge of all burst dimming cycles, VTH 1 ′ is sloped by the charging of capacitor  240  through resistor  230 . As described above in relation to  FIG. 1  an open lamp is detected when lamp current feedback signal ISNS is below the open lamp detection threshold current level, in the present embodiment VTH 1 ′. As VTH 1 ′ was pulled down, ISNS will be greater than VTH 1 ′ thereby avoiding a false open lamp detection. Open lamp detection is normally disabled during the entire burst off period including the falling edge of the burst on period, therefore disabling of the open lamp detection is only necessary during the rise of burst dimming pulse VBST. 
         [0069]      FIG. 8C  illustrates a graph of certain signals of the embodiment of  FIG. 8A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST, VTH 1 ′ and ISNS are illustrated as well as reference level VTH 1 . At each rising edge of burst dimming pulse VBST, optionally only when the burst of VBST is less than a predetermined value, the value of VTH 1 ′ is pulled down to a low level, optionally to ground, thereby being less than the expected value of ISNS. The low level is maintained for a sufficient amount of time for ISNS to rise to its steady state value. As a result no error is detected for low burst dimming cycles. 
         [0070]      FIG. 8D  illustrates a graph of certain signals of the embodiment of  FIG. 8A , in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. Signals VBST, VTH 1 ′ and ISNS are illustrated as well as reference level VTH 1 . At each burst of VBST, the value of VTH 1 ′ is pulled down to a low level, optionally to ground, and VTH 1 ′ rises slowly after being pulled down, consistently being less than the expected value of ISNS caused by the sloped burst dimming cycle rising edge. As a result no error is detected for low burst dimming cycles. 
         [0071]      FIG. 9  is a high level flow chart of the method of operation of each of the embodiments of  FIG. 3-8B . In stage  1000 , a representation of a current through a load, such as ISNS, supplied responsive to a power source, is compared with a reference value, such as VTH 1 . In stage  1010 , in the event that the compared representation of the current of stage  1000  is not less than the reference for a predetermined period, described above as a predetermined number of cycles of the power source, stage  1000  is repeated. 
         [0072]    In the event that in stage  1010  the compared representation of the current of stage  1000  is less than the reference for the predetermined period, in stage  1020  the burst dimming value is compared with a predetermined minimum burst value. The predetermined minimum burst value is selected in cooperation with the burst dimming slope so that a false open error is not triggered. In the event that the burst dimming value is not less than the predetermined minimum burst value, i.e. an open lamp condition has been detected, in stage  1030  the power source is at least partially shut down and stage  1000 , as described above, is performed. 
         [0073]    In the event that in stage  1020  the burst dimming value is less than the predetermined minimum burst value, in stage  1040  the shut down of stage  1030  is disabled. In optional stage  1050 , the disabling of stage  1040  is accomplished by preventing the comparing of stage  1000 , optionally for a particular period coincident with the rising edge of the burst dimming cycle, as described above in relation to  FIGS. 3 ,  5 A and  5 B. 
         [0074]    In optional stage  1060 , the disabling of stage  1040  is accomplished by providing a predetermined voltage level overriding the representation of the current, optionally for a particular period coincident with the rising edge of the burst dimming cycle, as described above in relation to  FIGS. 4A ,  4 B,  6 A,  6 B. 
         [0075]    In optional stage  1070 , the disabling of stage  1040  is accomplished by preventing the marking of the period of stage  1010  being exceeded, optionally for a particular period coincident with the rising edge of the burst dimming cycle, as described above in relation to  FIGS. 7A ,  7 B. 
         [0076]    In optional stage  1080 , the disabling of stage  1040  is accomplished by adjusting the reference signal of stage  1000  coincident with the rising edge of the burst dimming cycle, as described above in relation to  FIGS. 8A-8D . Optionally, as described above in relation to  FIGS. 8A-8D , adjusting the reference is signal is accomplished by one of pulling the reference signal to a predetermined value and ramping the reference signal from a predetermined value to its nominal value. 
         [0077]    Thus the present embodiments enable burst dimming at low duty cycles without causing false open lamp detection. This is accomplished in one embodiment by disabling the open lamp current detection at low duty cycles, usually below 10%. In another embodiment the open lamp current detection is disabled completely during operation. In another embodiment the lamp current feedback signal is pulled up to a level above the open lamp detection threshold during the burst on period. In yet another embodiment lamp current detection is disabled for a number of cycles at the rise of burst dimming. Open lamp detection is normally disabled during the entire burst off period including the falling edge and therefore disabling of lamp current detection is needed only at the rising edge. In another embodiment the lamp current feedback signal is pulled up to a level above the open lamp detection threshold for a short period, the short period being equal or longer than the time to rise to the threshold, during the burst on edge. 
         [0078]    In one more embodiment a pre-offset is provided to the open lamp timing circuit at the burst dimming rising edge to prevent false open lamp detection. As described above, in order to avoid transient response, typically the open lamp detection functionality requires the error signal to be valid for a plurality of burst cycles. The pre-offset provides a pre-determined discharge for the timing capacitor, or a pre-determined decrement of the timing counter, at the rising edge of burst dimming. The amount of pre-offset, i.e. the discharge timing capacitor or the decrement count of the fault timing counter, is set to be equal or slightly greater than the incremental timing effect caused by the open lamp detection at the rising edge of the burst on duty. In yet one more embodiment the open lamp detection threshold signal is ramped up to the nominal value over a predetermined number of cycle periods of the pulse generator coincident with the rising edge of the burst dimming duty cycle. 
         [0079]    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. 
         [0080]    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. 
         [0081]    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. 
         [0082]    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 hereinabove. 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.