CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 62/040,466, filed Aug. 22, 2014, and which is hereby incorporated by reference. 
    
    
     A portion of the invention of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent invention, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to power converters for lighting control systems. More particularly, the invention as disclosed herein relates to dimming current control for high power factor, constant current buck-boost converters. 
     Buck-boost converters are conventionally very good candidates for use with wide range input voltage (120-277V), high power factor non-isolated constant current LED drivers. Such converters are relatively low cost and compact in nature. A typical topology, as represented for example in  FIG. 1 , has a drawback in that the output does not share the same ground as the control IC. This makes the current control very complicated. 
     For a conventional LED driver circuit  10  as shown in  FIG. 1 , V 1  is the input AC source. L 1  is a common mode inductor for electromagnetic interference (EMI). Capacitor C 1  is an EMI filter capacitor. Inductor L 2  is a differential EMI inductor. Diodes D 1 -D 4  are input rectifier diodes for converting the AC input supply to a DC power supply. Capacitor C 2  is a high frequency filter capacitor for the converter. Resistors R 1  and R 2  define a voltage divider coupled across filtering capacitor C 2 . Inductor L 3  is a buck-boost inductor that stores that energy and releases it according to the control of IC. MOSFET Switch Q 1  is a switching element that is controlled by driver signals generated from the IC. Diode D 5  is a rectifier diode that bypasses the current from the primary winding L 3   p  of the buck-boost inductor to output capacitor C 4  when the switching element Q 1  is off. 
     The controller IC as shown in  FIG. 1  typically can be a power factor control (PFC) controller IC as is known in the art, such as for example the L6562 offered by STMicroelectronics. The controller IC has a MULT pin that senses the input line signal via a node between the voltage dividing resistors R 1  and R 2 . The controller IC also has a zero current detection (ZCD) pin that is coupled to a secondary winding L 3   s  of the buck-boost inductor via resistor R 3 , wherein the controller IC may ensure transition mode operation by controlling the turn on time of the switching element Q 1 . The controller IC also has an I sense  pin that senses the current going through the switching element Q 1  and resistor R 5 . The controller IC further includes an internal op amp with a V sense  input and COMP as output. C 3  is an integration capacitor for the control loop. 
     Typically, there is an internal voltage reference in the controller IC which is used as a control reference. The controller IC compares this internal reference with the external V sense  signal to tightly control the output. For constant current control, V sense  needs to be a current feedback signal that comes from the load. 
     However, the controller IC does not share the same ground as the output load, as shown in  FIG. 1 . As a result, an expensive isolated signal coupler is typically required to transfer the real current sensing signal from the output stage to the IC stage. Resistor R 6  is the load current sensing resistor. 
     This isolated signal coupler is not only expensive, but also introduces error and complicates the control scheme. Therefore, it would be desirable to eliminate this type of isolated signal coupler in a buck-boost converter topology. 
     It would further be desirable to have a dimming control circuit that has the same ground as GND main  so that only one dimming signal is required in order to control multiple channels of a buck-boost converter. 
     BRIEF SUMMARY OF THE INVENTION 
     The floating IC driven buck boost converter of the present invention will effectively solve this problem. The floating IC driven high power factor constant current buck-boost converter has a very compact size, simple control scheme, extremely low cost and high efficiency. 
     In one embodiment, a buck-boost LED driver circuit is provided with floating IC driving control. A DC power supply is provided with first and second inputs, the second input coupled to a mains ground. A PFC switching circuit is coupled to the first input and operable to drive an LED load. A current sensor is coupled to the switching circuit and configured to provide feedback signals representative of current through the LED load, and a dimming control circuit is coupled to the mains circuit ground and effectively superposes an external dimming control signal with the load feedback signal. A PFC controller is configured to provide driver signals to a switching element based on the superposed dimming and load feedback signals as compared to an internal reference. Each of the switching element, the current sensor and the controller are commonly coupled to a floating circuit ground. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram representing a high power factor constant current buck-boost converter as conventionally known in the art. 
         FIG. 2  is a circuit block diagram representing an embodiment of a power converter current control circuit topology according to the present invention. 
         FIG. 3  is a circuit block diagram representing another embodiment of a power converter current control topology according to the present invention. 
         FIG. 4  is a circuit block diagram representing an embodiment of dimming current control system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. 
     The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. Terms such as “wire,” “wiring,” “line,” “signal,” “conductor,” and “bus” may be used to refer to any known structure, construction, arrangement, technique, method and/or process for physically transferring a signal from one point in a circuit to another. Also, unless indicated otherwise from the context of its use herein, the terms “known,” “fixed,” “given,” “certain” and “predetermined” generally refer to a value, quantity, parameter, constraint, condition, state, process, procedure, method, practice, or combination thereof that is, in theory, variable, but is typically set in advance and not varied thereafter when in use. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     The terms “controller,” “control circuit” and “control circuitry” as used herein may refer to, be embodied by or otherwise included within a machine, such as a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed and programmed to perform or cause the performance of the functions described herein. A general purpose processor can be a microprocessor, but in the alternative, the processor can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     Referring generally to  FIGS. 2-4 , various embodiments of an LED driver circuit  20  as disclosed herein include an output block  24  rearranged so that it shares the same floating ground GND floating  as a power factor correction (PFC) switching block  22 , and further includes a dimming control block  26  using a main ground GND main . Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Referring more particularly now to an embodiment as represented in  FIG. 2 , an LED driver  20  includes a PFC switching block  22  which has its own floating ground GND floating . The entire output block  24   a  is connected in series with resistor R 5  and switching element Q 1 , and has its own ground GND output . However, electrically speaking GND floating  and GND output  are the same point. 
     Because the PFC switching block  22  and output block  24   a  share the same ground, the output current sensing signal I sense  can be used to directly feedback to the controller IC for current regulation. No isolated signal coupler is needed for constant current control and the controller IC operations will be extremely simplified. 
     To ensure that the power factor correction controller IC functions correctly, the average voltage between controller IC ground (GND floating ) and mains ground (GND main ) must be zero in steady state, so that the low frequency voltage (input line frequency) at MULT pin (multiplier pin of power factor correction controller IC) is effectively proportional to the output of the input diode bridge rectifier D 1 -D 4 . The controller IC can therefore regulate the input current to follow the input voltage waveform to achieve its power factor correction goal. 
     Because the DC resistance is very small for a magnetic, the DC voltage across the primary winding L 3   p  of the buck-boost inductor is zero in steady state operation. Therefore, the requirement discussed above (i.e., zero voltage across the controller IC ground and the mains ground) is satisfied in the exemplary circuit shown in both of  FIGS. 2 and 3 . 
     However, the high frequency voltage and the output voltage of the input diode rectifier bridge D 1 -D 4  are superimposed across resistors R 1  and R 2 . To filter out the high frequency noise across resistor R 2 , a high frequency noise filter capacitor C 5  is connected in parallel with resistor R 2  to filter out the high frequency noise coming from the primary winding L 3   p  of the buck-boost inductor. 
     The LED driver  20  further includes a dimming control block  26   a  ( FIG. 2 ). V control  is a dimming control voltage that can be changed by an external dimming signal (not shown). A resistor R 8  is added to the original current sensing circuit, in the present example coupled between the current sensing resistor R 6  and the error amplifier input terminals of the controller IC. R 8  and C 3  form a low pass filter. As a result, the voltage across capacitor C 3  (V C3 ) may be provided as a relatively pure DC signal with respect to the I sense  feedback signal, which might otherwise have some small AC signal component. Resistor R 7  is provided within the dimming control block  26   a  to superpose the dimming control voltage V control  on capacitor C 3 . The voltage on capacitor C 3  follows the relation: 
     
       
         
           
             
               V 
               
                 c 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 3 
               
             
             = 
             
               
                 
                   I 
                   sense 
                 
                 · 
                 
                   
                     R 
                     7 
                   
                   
                     
                       R 
                       6 
                     
                     + 
                     
                       R 
                       7 
                     
                   
                 
               
               + 
               
                 
                   V 
                   control 
                 
                 · 
                 
                   
                     R 
                     6 
                   
                   
                     
                       R 
                       6 
                     
                     + 
                     
                       R 
                       7 
                     
                   
                 
               
             
           
         
       
     
     Current control is achieved in the controller IC by comparing the internal reference voltage to the total current sensing signal I sense     —     total : 
     
       
         
           
             
               V 
               
                 I_ref 
                 ⁢ 
                 _IC 
                 ⁢ 
                 _internal 
               
             
             = 
             
               
                 
                   I 
                   sense 
                 
                 · 
                 
                   
                     R 
                     7 
                   
                   
                     
                       R 
                       6 
                     
                     + 
                     
                       R 
                       7 
                     
                   
                 
               
               + 
               
                 
                   V 
                   control 
                 
                 · 
                 
                   
                     R 
                     6 
                   
                   
                     
                       R 
                       6 
                     
                     + 
                     
                       R 
                       7 
                     
                   
                 
               
             
           
         
       
     
     The voltage across capacitor C 3  is the total current sensing signal I sense     —     total . When the dimming control voltage V control  changes, it follows that the total feedback signal I sense     —     total  changes as well. When the dimming control voltage V control  is zero, the total feedback signal I sense     —     total  is at its relative minimum value so that the output current will be at a relative maximum. When the dimming control voltage V control  is at its maximum value, the total feedback signal I sense     —     total  is also at a relative maximum so that the output current will be at its relative minimum. 
     C byoass  is a capacitor that is capable of filtering out the high frequency voltage across the dimming control block  26   a . In the example shown, the high frequency voltage across the dimming control block  26   a  is the voltage across the primary winding L 3   p  of the buck-boost inductor. The filter capacitor C bypass  as shown may therefore effectively ensure that all the high frequency voltage will be provided across the resistor R 7 . 
     Referring next to an alternative topology for an LED driver  20  as represented in  FIG. 3 , the primary difference is that the current sensing position in the exemplary output block  24   b  shown is different. The current sensing signal in  FIG. 2  is the real current signal, but the output is floating. The current sensing signal in  FIG. 3  is the total current passing through the diode D 5 , but the AC current component is filtered out by capacitor C 2  which is coupled in parallel with the sensing resistor R 6 , so that the DC component will be the same as the current going through the LED load R 4 . 
     One advantage for the topology represented in  FIG. 3  is that the output has a reference point, which is the input diode bridge ground, GND main . This topology could offer a better output current waveform and EMI result. 
     Otherwise, implementation of a dimming control block  26   b  in the embodiment represented in  FIG. 3  is substantially identical to that as previously described. 
     Referring now to  FIG. 4 , a lighting control system  40  may implement the concepts as described herein with respect to multi-channel floating IC driven buck boost converters. As shown in  FIG. 4 , only one dimming control block  26   c  and corresponding dimming control voltage V control  is need for two or more channels with floating IC driven buck-boost converters  24 , since the current control circuit shares the same ground GND main . It may be understood that a first buck-boost converter having PFC switching block and output block may be provided as shown with resistor R 8 , while one or more additional buck-boost converters including a second buck-boost converter having respective and otherwise equivalent PFC switching block and output block may be provided with additional respective resistors, e.g., R 10 , to provide the equivalent functionality as described above with respect to either of the previously disclosed embodiments in  FIGS. 2 and 3 . 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of an invention, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.
A buck-boost LED driver circuit is provided with floating IC driving control. A DC power supply is provided with first and second inputs, the second input coupled to a mains ground. A PFC switching circuit is coupled to the first input and operable to drive an LED load. A current sensor is coupled to the switching circuit and configured to provide feedback signals representative of current through the LED load, and a dimming control circuit is coupled to the mains circuit ground and effectively superposes an external dimming control signal with the load feedback signal. A PFC controller is configured to provide driver signals to a switching element based on the superposed dimming and load feedback signals as compared to an internal reference. Each of the switching element, the current sensor and the controller are commonly coupled to a floating circuit ground.