Patent Publication Number: US-9433047-B2

Title: Single inductor multiple LED string driver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 from U.S. Provisional Application No. 61/402,106, entitled “Single Inductor Multiple LED String Driver,” filed on Aug. 23, 2010, the subject matter of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to Light-Emitting Diode (“LED”) supply, control, and protection circuits; and more specifically to controllers that drive multiple LED strings using a single inductor. 
     BACKGROUND INFORMATION 
     Light-Emitting Diodes or “LEDs” are increasingly being used for general lighting purposes. For example, LEDs are suitable for backlighting for LCD televisions, lightweight laptop displays, and light source for DLP projectors. Screens for televisions and computer displays can be made increasingly thin using LEDs for backlighting. In LED backlights, multiple strings of LEDs are arranged in parallel, and each string of LEDs has series-connected LEDs. To achieve good quality backlighting, various controllers are used to regulate the currents flowing across the multiple strings of LEDs. 
       FIG. 1  (Prior Art) is a diagram of a multiple LED string driver  10  comprising a boost converter  11  that drives multiple strings of LEDs via resistor ballasting. Boost converter  11  is driven by a feedback signal  14  across a resistor  16  that senses the current through one of the LED string  15 . The output voltage VOUT of boost converter  11  is regulated to provide the necessary current. For the other LED strings, each has an identical resistor so that the current flowing through all LED strings are approximately the same. The variation of the LED string current, however, depends on how the LED forward voltages and the feedback voltage are matched. For example, if the total forward voltages of two LED strings are different by 1V, and the feedback voltage is 2V, then the mismatch in LED string current is 1V/2V=50%. 
       FIG. 2  (Prior Art) is a diagram of a multiple LED string driver  20  comprising an LED bias controller  21  that drives multiple strings of LEDs, each biased separately by a current sync. The current syncs are inside controller  21  and coupled to terminals CTRL 1 -CTRL 6  ( 22 - 27 ) of controller  21 . A power converter provides a regulated output voltage VOUT to the top of the LED strings, and the LED string current is each regulated by the current syncs. For best efficiency, the power converter output voltage VOUT is adaptively regulated so that only a necessary working voltage is dropped across the current syncs. The advantage of this approach is the LED string currents have high matching to each other. The disadvantage is that the total forward voltage variation from string to string is significant. As a result, the voltages across the current syncs vary, resulting in significant power loss and heat generation. For example, if the total forward voltages for two long LED strings are 200V and 180V respectively, then there is an additional 20V voltage drop across the current sync for the 180V forward voltage LED string. At 120 mA bias current, such a voltage drop results in an additional 2.4 W higher dissipation on the second LED string than the first LED string. 
       FIG. 3  (Prior Art) is a diagram of a multiple LED string driver  30  comprising a DC-to-DC controller  31  that drives multiple strings of LEDs. A boost converter  32  converts a 24V input DC voltage to a regulated output DC voltage VOUT (e.g., ˜100-200V) to the top of the LED strings. Each LED string bottom is separately driven by an LED string switching converter. Each LED string switching converter (e.g., switching converter  35 ), comprises a MOSFFET  36 , an inductor  37 , a diode rectifier  38 , and a current sense resistor  39 . Each LED switching converter individually operates like a buck converter, reducing the main output voltage to match the LED string total forward voltage so that each LED string current is regulated to a target value. As a result, there is no power loss caused by the voltage difference between the main output voltage and the LED string total forward voltage. However, because each LED string needs a separate switching converter having a separate inductor, the overall cost is high. 
     SUMMARY 
     A single inductor multiple LED string driver comprises a switch control circuit and a current-sensing control circuit. The switch control circuit generates a plurality of digital control signals that are used to control a plurality of LED switches coupled to a plurality of strings of LEDs. Each switch is selectively turned on and off by each corresponding digital control signal. The current-sensing control circuit determines an integrated charge amount provided by each current that flows from an input voltage through each LED string, through each LED switch, through a common inductor, and through a main switch to ground. In response to the determined integrated charge amount, the current-sensing control circuit generates an on-time control signal that controls the on-time of each LED switch such that the average current flowing across each LED string is equal to each other. Furthermore, the total current flowing across each LED string is regulated to a predefined value. 
     In one novel aspect, the single inductor multiple LED string driver has a time-shared Single-Inductor-Multiple-Output (SIMO) architecture. This architecture uses the common inductor to alternatively pump current into a holding capacitor of each LED string to generate equal average current for each LED string. The multiplexing of the common inductor allows current across each LED string to be individually regulated. Each multiplexing phase of the common inductor is essentially a buck conversion phase with individually adjustable on-time to drive each LED string separately. In one advantageous aspect, each LED string is biased without power loss due to the voltage difference between the main output voltage and the LED string total forward voltage. In addition, only a single inductor is used. 
     In one embodiment, the single inductor multiple LED string driver is part of an integrated circuit. The switch control circuit is a Pulse-Width Modulation (PWM) controller. The plurality of LED switches and the main switch are located inside or outside the integrated circuit. In one advantageous aspect, an AC-to-DC converter is used to output an unregulated DC voltage VHIGH. The unregulated DC voltage VHIGH is then directly used to drive the plurality of LED strings without using any DC-to-DC boost converter such that additional efficiency loss is eliminated. 
     Other structures and methods are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  (Prior Art) is a diagram of a multiple LED string driver comprising a boost converter that drives multiple strings of LEDs via resistor ballasting. 
         FIG. 2  (Prior Art) is a diagram of a multiple LED string driver comprising an LED bias controller that drives multiple strings of LEDs via separate current syncs. 
         FIG. 3  (Prior Art) is a diagram of a multiple LED string driver comprising a DC-to-DC controller that drives multiple strings of LEDs via separate LED string switching converter. 
         FIG. 4  is a diagram of a first embodiment of a single inductor multiple LED string driver in accordance with one novel aspect. 
         FIG. 5  is a more detailed circuit diagram of the single inductor multiple LED string driver of  FIG. 4 . 
         FIG. 6  illustrates different states during a PWM switching cycle of the single inductor multiple LED string driver of  FIG. 5 . 
         FIG. 7  illustrates waveforms of different switches as well as corresponding voltage and current waveforms during a PWM switching cycle. 
         FIG. 8  is a flow chart of a method of driving multiple LED strings using a single inductor in accordance with one novel aspect. 
         FIG. 9  is a diagram of a second embodiment of a single inductor multiple LED driver in accordance with one novel aspect. 
         FIG. 10  is a diagram of a third embodiment of a single inductor multiple LED driver in accordance with another novel aspect. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  is a diagram of a first embodiment of a single inductor multiple LED string driver  40  in accordance with one novel aspect. Single inductor multiple LED string driver  40  comprises a plurality of strings of LEDs  41 - 46 , an integrated circuit  47 , an AC-to-DC converter  48 , an output capacitor  51 , a diode rectifier  52 , and a common inductor  53 . Each LED string comprises a number of series-connected LEDs. The top of each LED string is connected to a DC voltage VHIGH as illustrated, while the bottom of each LED string is connected to an LED switch terminal S 1 -S 6  of integrated circuit  47  respectively. The bottom of each LED string is also connected to a holding capacitor  61 - 66  respectively. In addition to the six LED switch terminals S 1 -S 6 , integrated circuit  47  also comprises a CTRL terminal for control interface, a ISET terminal for reference current, a main switch terminal SW, an input terminal LIN, a supply voltage terminal VCC, as well as two ground terminals GND and GP. 
     In the example of  FIG. 4 , AC-to-DC converter  48  receives voltage from an AC voltage source  49  (e.g., 110V AC) and outputs a regulated 5V DC voltage. The 5V DC voltage is commonly used in many electronic devices. AC-to-DC converter  48  also outputs an unregulated secondary DC voltage VHIGH. The value of VHIGH is determined approximately based on the winding ratio of the AC-to-DC converter. In one advantageous aspect, the unregulated DC voltage VHIGH is then directly used to drive the plurality of LED strings, without using any additional DC-to-DC boost converter. For example, if each LED string has 45 series-connected LEDs, then the total forward voltage of the LED string is about 45×3.3=150V. The supply voltage VHIGH is about 190V, leaving ˜40V for normal operation. By directly using the unregulated voltage from AC-to-DC converter  48 , ˜20% of efficiency loss can be eliminated as compared to LED string driver  30  in  FIG. 3 . 
     Single inductor multiple LED string driver  40  is commonly used in applications such as backlighting for LCD televisions, LCD monitors, lightweight laptop displays, and light source for DLP projectors. In order to efficiently regulate the currents that flow across each of the six LED strings, each LED string is individually biased through the use of six LED switch terminals S 1 -S 6 , common inductor  53 , and main switch terminal SW. First, each LED switch terminal is connected to an LED switch (not shown) that provides an active current sync for each LED string. In addition, the main switch terminal SW is connected to a main switch (not shown) that drives common inductor  53 . As a result, when both the main switch and one of the LED switches are turned on, an LED string current (I LED1  to I LED6 ) flows from VHIGH, through an LED string, through a corresponding LED switch, through common inductor  53 , and then through the main switch to ground. The main switch operates cooperatively with the six LED switches such that, together with a single inductor, they provide independently controllable current syncs for the six LED strings. 
     In one novel aspect, single inductor multiple LED string driver  40  has a time-shared Single-Inductor-Multiple-Output (SIMO) architecture. This architecture uses common inductor  53  to alternatively pump current into the holding capacitors ( 61 - 66 ) of each LED strings ( 41 - 46 ) to generate equal average current for each LED string. The multiplexing of common inductor  53  allows current across each LED string (I LED1 -I LED6 ) to be individually regulated. For example, during a first on-time, the first LED switch is turned on. The first LED string current I LED1  flows from VHIGH, through the first LED string  41 , through terminal S 1 , through common inductor  53 , and through terminal SW to ground (denoted by a thick dotted line  91 ). When the integrated charge from I LED1  reaches a target value, the first LED switch is then turned off. Next, the second LED switch is turned on during the second on-time so that the second LED string current I LED2  flows from VHIGH, through the second LED string  42 , through terminal S 2 , through common inductor  53 , and through terminal SW to ground (denoted by a thick dotted line  92 ). Similar to the first on-time, the second LED switch is turned off when the integrated charge from I LED2  reaches the same target value. The same process is repeated for each LED string. While each LED string current varies when the corresponding LED switch is turned on and off, each holding capacitor ( 61 - 66 ) averages the LED string current over time. Because the amount of charge pumped into each holding capacitor is equal to the same target value, the average current of each LED string is the same. Thus, each multiplexing phase of common inductor  53  is essentially a buck conversion phase with individually adjustable on-time to drive each LED string separately. In one advantageous aspect, each LED string is biased without power loss due to the voltage difference between the main output voltage and the LED string total forward voltage. In addition, only a single inductor  53  is used as compared to multiple inductors in  FIG. 3 . 
       FIG. 5  is a more detailed circuit diagram of the single inductor multiple LED string driver  40  of  FIG. 4 . In the example of  FIG. 5 , integrated circuit  47  comprises an interface module  55 , an oscillator  56 , a reference and bias module  57 , an current reference IREF module  58 , a switch control circuit  60 , a plurality of switches QS 1 -QS 6  ( 71 - 76 ), a discharge switch QD  77 , a main switch QM  78 , and a current-sensing control circuit  80 . The plurality of switches QS 1 -QS 6  are the six LED switches described above (but not shown) with respect to  FIG. 4 , and the main switch QM  78  is the main switch described above (but not shown) with respect to  FIG. 4 . Although the LED switches QSn, the main switch QM, and the discharge switch QD are all located inside integrated circuit  47  in the example of  FIG. 5 , any of the LED switches QSn, QM, and QD may be located outside integrated circuit  47  in other circuitry implementations. 
     Switch control circuit  60  in  FIG. 5  is a Pulse-Width Modulation (PWM) controller, comprising a shifter  68  and an AND gate  69 . PWM controller  60  receives a clock signal TCLK  101  from oscillator  56  that controls the period of a PWM switching cycle. PWM controller  60  also receives an on-time control signal QTON  102  from current-sensing control circuit  80 , and in response generates a plurality of switch control signals  111 - 116  to control the plurality of LED switches QS 1 -QS 6  respectively. Switch control signals  111 - 116  are supplied into AND gate  69  and buffer  82  to generate a first main switch control signal  103  that controls main switch QM  78 . Switch control signals  111 - 116  are also supplied into AND gate  69  and inverter  81  to generate a second main switch control signal  104  that controls discharge switch QD  77 . 
     A PWM switching cycle comprises a main on-time and a main off-time. The main on-time is multiplexed among the six QSn switches, while the main switch QM is also on. During the main off-time, main switch QM and all the six QSn switches are off, while the discharge switch QD is on. In other words, during a PWM main on-time, shifter  68  selectively turns on one of the LED switches QS 1 -QS 6 , while the main switch QM is also turned on and the discharging switch QD is turned off. On the other hand, during a PWM main off-time, only the discharge switch QD is turned on. The main on-time and off-time of the PWM switching cycle is either controlled by the PWM clock or by a minimum off-time mechanism. The on-time and off-time of each of the QSn switches, on the other hand, are controlled by on-time control signal QTON  102  such that the average current flowing across each QSn is equal to each other. On-time switch control signal QTON  102  is in turn controlled by current-sensing control circuit  80  by sensing the LED string current (I LED1 -I LED6 ) that flows through main switch QM during the main on-time. 
     Current-sensing control circuit  80  comprises a current mirror  83 , an error amplifier  86 , a comparator  87 , a compensation capacitor CCOMP  88 , an integrating capacitor CINT  89 , and a one-shot circuitry  93 . During a PWM main on-time, when shifter  68  selectively turns on one of the LED switches QSn (i.e., QS 1 ) via switch control signals  111 - 116  (i.e., control signal  111 ), current flows from VHIGH through one of the selected LED strings (i.e., I LED1  flows across LED string  41 ), through the selected QSn, through common inductor  53 , and through switch QM to ground (denoted by thick dotted line  91 ). That is, if QS 1  is on, then the average inductor current I LX  is equivalent to I LED1  that flows across LED string  41 . Current mirror  83  detects the inductor current I LX  through main switch QM and outputs two mirrored currents (denoted as 1X, also referred to as a current sense signal), one flows into integrating capacitor CINT  89 , and the other flows into current error amplifier  86 . The two mirrored currents are used for two different purposes. 
     First, when the current sense signal of inductor current I LX  flows into integrating capacitor CINT  89 , the voltage across CINT  89  VCINT increases from zero Volts. Voltage VCINT indicates the amount of charge accumulated through I LX  over time (i.e., I LED1  when QS 1  is on). VCINT is then compared with a voltage VCOMP by comparator  87 . When VCINT becomes higher than VCOMP, on-time switch control signal QTON  102  is generated to turn off one of the selected LED switches QSn (i.e., QS 1 ). VCINT is then reset to zero Volts for the next QSn on-time. For example, VCINT may be reset by switch  90  by a one-shot reset signal  106  generated by the on-time switch control signal QTON  102 . Because each LED string is current biased, the average LED string current can be regulated by regulating the amount of charge accumulated through the LED string current. Assume that VCOMP remains as a constant voltage value, by comparing VCINT to VCOMP to control the on-time of each LED switch, the amount of charge accumulated through each LED string during the on-time of each LED switch also remains the same. As a result, the average LED string current flowing across each LED string is regulated to be equal to each other. 
     Second, the current sense signal of inductor current I LX  is compared with a reference current IREF  105  by error amplifier  86 . An output voltage signal VCOMP is generated by error amplifier  86  for all LED strings. If the combined average inductor current I LX  is less than IREF  105 , then the voltage VCOMP outputted by error amplifier  86  increases. Otherwise, if the combined average inductor current I LX  is more than IREF  105 , then the voltage VCOMP outputted by error amplifier  86  decreases. Therefore, by regulating the combined current sense value to reference current I REF    105 , VCOMP remains the same, and the total current flows across each LED string is regulated to a predefined value. The LED string current I LEDn  is typically equal to IREF multiplied by a constant. Thus, by selecting an appropriate IREF value, the LED string current I LEDn  can be regulated to a desired value. 
       FIG. 6  illustrates different states during a PWM switching cycle of the single inductor multiple LED string driver  40  of  FIG. 5 . Single inductor multiple LED string driver  40  starts with an initial OFF state, during which it is disabled or does not have good supply voltage. Single inductor multiple LED string driver  40  enters demagnetize (or discharge) state after it is enabled and receives good supply voltage. During any PWM switching cycle, single inductor multiple LED string driver  40  goes through state ST 1 , ST 2 , ST 3 , ST 4 , ST 5 , ST 6 , and then goes back to demagnetize state before repeating a next PWM switching cycle. Four PWM switching cycles are illustrated in  FIG. 6 , and the main-on time in each PWM switching cycle is divided among the six QSn switches. State ST 1  represents the state where the first switch QS 1  is turned on during TON 1 , state ST 2  represents the state where the second switch QS 2  is turned on during TON 2 , and so on so forth. From any of the states, single inductor multiple LED string driver  40  goes back to the OFF state if it is disabled or does not have good supply voltage. 
     Because at any moment only one QSn switch is turned on by PWM controller  60  during the main on-time of a PWM switching cycle, the LED string current I LEDn  across each LED string flows through the inductor only when its corresponding QSn switch is turned on. As a result, in any steady state of ST 1 -ST 6 , the average current flowing through each LED string is equal to the average current through the inductor during the on-time of the corresponding QSn switch: 
                     Average   ⁡     (     I   LEDn     )       =       TONn   TPERIOD     ·     Average   ⁡     (     I   LXQSn     )                 (   1   )               
Where
         I LEDn  is the average current of LED string n   TONn is the on-time of the QSn switch   TPERIOD is the main switching cycle period   I LXQSn  is the average inductor current during QSn switch on-time
 
For all six LED strings, the total average current flowing through all six LED strings is thus equal to the total average current through the inductor during the main on-time. Therefore, if equation (1) is added up for all six LED strings, the result becomes:
       

                       ∑     n   =   1       n   =   6       ⁢     Average   ⁡     (     I   LEDn     )         =       ∑     n   =   1       n   =   6       ⁢       TONn   TPERIOD     *     Average   ⁡     (     I   LXQSn     )                   (   2   )               
Furthermore, because the on-time TONn for each switch QSn is controlled such that the average current flowing across each LED string is equal to each other, and because the total of TONn on-time is equal to the main on-time, the average current flowing across each LED string is thus equal to the total average current through the inductor during the main on-time divided by six. Equation (2) then becomes:
 
                     Average   ⁡     (     I   LED     )       =       1   6     *     Average   ⁡     (     I   LXQM     )                 (   3   )               
Where
         I LED  is the average current for each LED string   I LXQM  is the average inductor current during the main on-time       

       FIG. 7  illustrates waveforms of different switches as well as corresponding voltage and current waveforms during a PWM switching cycle. In the example of  FIG. 7 , TON represents the ON and OFF time of each LED switches QSn, QM ON  represents the ON and OFF time of the main switch QM, VS represents the voltages at terminals S 1 -S 6 , I LX  represents the current that flows across the common inductor  53 , I CAP1  represents the current that flows across the first holding capacitor  61  of the first LED string  41 , and I LED1  represents the current that flows across the first LED string  41 . For illustration purpose, the waveforms with regard to the first LED string  41  are denoted as thick dotted lines in  FIG. 7 . During a first QS 1  on-time, LED string current I LED1  flows from VHIGH, through LED string  41 , through switch QS 1 , through inductor  53 , and through switch QM to ground (see dotted line  91  in  FIG. 5 ). The LED string current I LED1  gradually increases as inductor current I LX  gradually charges. The voltage across holding capacitor  61  (VS 1 ) decreases when current flowing out from holding capacitor  61  (I CAP1  is negative) as it discharges. During a second QS 2  on-time (after switch QS 1  is turned off and switch QS 2  is turned on), the LED string current I LED1  decreases because current flows into its holding capacitor  61  (I CAP1  is positive), and the voltage VS 1  increases as the capacitor charges. The inductor current I LX  continues to increase because switch QS 2  is turned on. The waveforms of I LED2 , VS 2 , and I CAP2  are similar to the waveforms of I LED1 , VS 1 , and I CAP1 , respectively. The inductor current I LX  continues to increase through the entire main on-time when QS 1 , QS 2  . . . and QS 6  are turned on one by one. 
     After all the QSn switches are selectively turned on one by one during the main on-time of a PWM switching cycle, all the QSn switches are then turned off together during the main off-time. The main switch QM is also turned off while the discharging switch QD is turned on during the main off-time. Consequently, terminal LIIN is couple to ground through switch QD and the polarity of inductor  53  is reversed. Inductor  53  maintains its current I LX  by pulling the current from ground through diode rectifier  52  and then all the way to VHIGH (see a thick dot-dashed line  97  in  FIG. 5 ). Because inductor  53  is now negatively biased, it starts to discharge and its current I LX  starts to gradually go down until the next PWM switching cycle starts. As illustrated in  FIG. 7 , I LX  continues to increase through the main on-time and quickly decreases through the main off-time. Moreover, although I LX  decreases through the main off-time, it never drops to zero. Thus, inductor  53  operates in a continuous conduction mode. This can be achieved by controlling the duration of the main off-time to be short enough such that I LX  never drops to zero. 
     It can be seen from  FIG. 7 , that while the inductor current I LX  continues to increase during the main on-time for each switch QSn on-time, the amount of on-time for each switch QSn continues to decrease. This is because the total amount of charge over the time for each LED string current is regulated to be the same to ensure the average current is also the same. Thus, when the current increases, the on-time needs to decrease such that the integrated current remains the same for each LED string. In the example of  FIG. 7 , the QSn switches are turned on in the order of QS 1 , QS 2  . . . QS 6 . Ideally, the order of turning on the QSn switches does not matter because the average LED string current is regulated to be equal to each other based on the integrated current. However, if the order remains unchanged, then the on-time for QS 1  is always the longest (I LED1  is the smallest during TON 1 ), and the on-time for QS 6  is always the shortest (I LED6  is the largest during TON 6 ). It is thus preferred that each LED string operates in exactly the same manner over the time to achieve perfect matching, considering any second-order effect. In one embodiment, shifter  68  generates an alternating order sequence to turn on the QSn switches such that each QSn has on average approximately the same chance to be turned on at a given time. 
       FIG. 8  is a flow chart of a method of driving multiple LED strings using a single inductor in accordance with one novel aspect. A single inductor multiple LED string driver comprises a switch control circuit and a current-sensing control circuit. In step  801 , the switch control circuit generates a plurality of digital control signals that are used to control a plurality of switches coupled to a plurality of strings of LEDs. Each switch is selectively turned on and off by each corresponding digital control signal. In step  802 , the current-sensing control circuit determines an integrated charge amount provided by each current that flows from an input voltage through each string of LEDs, through each switch, through a common inductor, and through a main switch to ground. In step  803 , in response to the determined integrated charge amount, the current-sensing control circuit generates an on-time control signal that controls the on-time of each switch such that the average current flowing across each string of LEDs is equal to each other. In addition, the total current flowing across each LED string is regulated to a predefined value. 
       FIG. 9  is a diagram of a second embodiment of a single inductor multiple LED driver  900  in accordance with one novel aspect. Single inductor multiple LED driver  900  is very similar to the single inductor multiple LED driver  40  illustrated in  FIG. 4 . In the embodiment of  FIG. 9 , however, the DC voltage VHIGH is provided by a DC-to-DC converter  901 . The DC-to-DC converter  901 , for example, receives a DC voltage VLOW (e.g., 24V) and outputs DC voltage VHIGH (e.g., 190V) for the multiple LED strings. The use of DC-to-DC converter  901  introduces ˜20% undesirable efficiency loss. 
       FIG. 10  is a diagram of a third embodiment of a single inductor multiple LED driver  910  in accordance with one novel aspect. Single inductor multiple LED driver  910  is very similar to the single inductor multiple LED driver  40  illustrated in  FIG. 4 . In the embodiment of  FIG. 10 , however, common inductor  53  is coupled to a main switch QM that is external to an integrated circuit  911 . In addition, the main switch QM is coupled to a current-sensing resistor Rcs that is also external to the integrated circuit  911 . Thus, when main switch QM is turned on, the inductor current I LX  flows through common inductor  53 , through main switch QM, and through resistor Rcs to ground. Integrated circuit  911  controls main switch QM via terminal GATEM and receives a current-sensing signal  107  via terminal CS. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.