Abstract:
A control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and producing a first signal, an analog signal processor coupled to the amplifier circuit for receiving the first signal and a second signal from the input of the power supply and a third signal from an output of the power supply. The analog signal processor is configured to produce a fourth signal as a function of the first, the second, and the third signals. An adder circuit is coupled to the fourth signal and a dimmer control signal, and the adder circuit is configured to output a fifth signal. A comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor that controls current flow in the power supply based on comparison of the fifth signal and a reference signal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Chinese Patent Application No. 200910260587.9, filed Dec. 21, 2009, by inventors Hongyue Du, et al., commonly assigned and incorporated in its entirety by reference herein for all purposes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the present invention are directed to power supply control circuits and power supply systems and their applications. More particularly, embodiments of the present invention provide methods for systems for controlling a switched mode power supply for providing constant output current in LED light systems. 
         [0003]    A DC-DC converter receives a rectified DC voltage and delivers a regulated DC output. DC-DC converters are widely used in white light-emitting diode (LED) drivers or flash LED drivers. Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency, and larger output power capability. On the other hand, they also have the disadvantages of greater noise, especially Electromagnetic Interference at the power transistor&#39;s switching frequency or its harmonics. 
         [0004]    Conventional power supplies of buck-boost topology use current control mode (CCM) or voltage control mode (VCM) loop control that needs internal or outside compensation, which can often cause circuit instability. Compared with the ordinary structure of CCM or VCM switch controller, the architecture described in  FIG. 1  tends to be more stable. Such a controller is extensively used in home-lighting, auto-motor, and backlight instruments. In LED lighting systems, the LEDs are often connected in series in the inductor loop. 
         [0005]      FIG. 1  is a schematic diagram of an LED lighting system  100  driven by a conventional switching mode power supply. As shown in  FIG. 1 , lighting system  100  includes serially connected multiple LEDs  104  coupled with a load capacitor  111 . The LEDs are driven by a power supply  120 , which includes a sense resistor  101 , an inductor  102 , and a Schottky diode  103 . Power supply  120  also includes a controller  130 , which includes a transconductance amplifier  105 , a Dim linear amplifier  106 , a current adder  107 , a resistor  108 , a comparator  109 , and a power MOSFET  110 . As shown, transconductance amplifier  105  receives inputs from both ends of sense resistor  101 , and power transistor  110  is connected to a node between inductor  102  and Schottky diode  103 . 
         [0006]    As shown in  FIG. 1 , power supply  120  receives a rectified DC input voltage Vin. During the charging period, the current from Vin flows through resistor  101  and inductor  102  through power transistor  110  to ground. In this period, energy is stored in inductor  102 . The voltage across resistor  101  is sensed by transconductance amplifier  105 , which produces an output current I 1 . Current I 1  is fed to resistor  108  through current adder  107 , and the resulting voltage is compared with an internal voltage reference Vref. When an internal turn-on reference voltage is reached, the output of comparator  109  drives power MOSFET  110  to switch off through a drive block (not shown). In the discharging period, the energy stored in inductor  102  discharges through diode  103  which, along with capacitor  111 , provide a current Iout to LEDs  104 . When the sense voltage becomes lower than an internal off reference voltage and detected by comparator  109 , power transistor  110  is turned on again, and the charging period is repeated. Controller  130  is capable of boosting input voltage Vin to a higher regulated output voltage Vout. 
         [0007]    Even though conventional LED lighting systems, such as system  100  of  FIG. 1 , can be found in many application, they suffer from many limitations. These limitations include, for example, instability in light output, which may result in flickers. 
         [0008]    Therefore, it is desirable to have improved methods and devices for controlling the output current in a power supply in LED lighting and other applications. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the present invention provide methods and systems for a buck-boost pulse width modulation (PWM) power supply. Merely as an example, some embodiments are described in the context of light-emitting diode (LED) driver applications. But it would be recognized that the invention has a much broader range of applicability. 
         [0010]    Conventional LED lighting systems such as system  100  shown in  FIG. 1 , LEDs  104  are driven by current Iout. According to embodiments of the invention, current Iout can vary with input voltage Vin if a conventional controller, such as  130 , is used to control the power supply. This variation may lead to changes and instabilities in LED light output, including flickers. 
         [0011]    Some embodiments of the invention provide a method and circuit for providing a constant current output in a Buck-Boost topology of power supply system. In a specific embodiment, the input current is sensed at a resistor with an OTA (operation Transconductance Amplifier) converting a voltage drop on the sense resistor to a current. A voltage signal derived from the resistor is compared with a reference voltage in a comparator with hysteresis to drive a switching power MOSFET. In this embodiment, an analog signal processor is used to convert the output of the OTA by a ratio of input voltage plus load voltage of LED over the input voltage 
         [0000]    
       
         
           
             
               ( 
               
                 ratio 
                 = 
                 
                   
                     ( 
                     
                       Vin 
                       + 
                       Vload 
                     
                     ) 
                   
                   Vin 
                 
               
               ) 
             
             . 
           
         
       
     
         [0000]    The output current is substantially insensitive to changes in the input supply voltage. In some embodiments, circuit safety features are also provided, such as over-voltage protection, over-current protection, and over-temperature protection, etc. Thus, accurate output current can be realized using embodiments of the invention. 
         [0012]    Various embodiments of the invention provide a stable buck-boost power supply structure that can be used in MR16 LEDs lighting and other applications. The stable output can prevent flicker conditions when LEDs are connected as load elements of an electrical transformer. 
         [0013]    In one or more embodiments, an analog signal processor is provided that can perform high speed multiplier/divider operations. In some embodiment, the base-emitter junctions of bipolar transistors are configured for performing the multiplication and division operations of currents and voltages. 
         [0014]    According to an embodiment of the present invention, a control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and providing a first signal. An analog signal processor is coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from the input of the power supply and a third signal from an output of the power supply. The analog signal processor is configured to produce a fourth signal as a function of the first, the second, and the third signals. An adder circuit is coupled to the fourth signal and a dimmer control signal, and the adder circuit is configured to output a fifth signal. Moreover, a comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison of the fifth signal and a reference signal. 
         [0015]    According to another embodiment of the present invention, an LED lighting system includes one or more light emitting diodes (LEDs) connected in series, and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also has a switched mode power supply having a control circuit described above. An output terminal of the power supply is coupled to the one or more LEDs for providing a drive current. 
         [0016]    According to yet another embodiment, a control circuit for a switched mode power supply includes a transconductance amplifier circuit for receiving a voltage signal related to a current from an input of the power supply and providing a first signal. An analog signal processor is coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from an output of the power supply. The analog signal processor is configured to produce a third signal as a function of the first and the second signals. An adder circuit is coupled to the third signal and a fourth signal related to an input of the power supply. The adder circuit is configured to output a fifth signal. Moreover, a comparator circuit is coupled to the adder circuit for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison of the fifth signal and a reference signal. 
         [0017]    According to still another embodiment of the invention, a control circuit for a switched mode power supply includes an analog signal processor coupled to an input terminal and an output terminal of the power supply. The analog signal processor is configured to receive a first signal related to a current at the input terminal, a second signal related to a voltage at the input terminal, and a third signal related to a voltage at the output terminal. The analog signal processor is also configured to provide a fourth signal related to the first, the second, and the third signal. A comparison circuit is configure for providing a control signal to a power transistor for controlling current flow in the power supply based on comparison between a reference signal with the fourth signal or a fifth signal related to the fourth signal. 
         [0018]    In an alternative embodiment of the present invention, an LED lighting system includes one or more light emitting diodes (LEDs) connected in series and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also includes a switched mode power supply having a control circuit as described above. An output terminal of the power supply is coupled to the one or more LEDs for providing a drive current. 
         [0019]    According to yet another embodiment of the present invention, a switched mode power supply includes an input terminal for receiving a rectified input voltage, an output terminal for providing a regulated output voltage and a regulated output current, a resistor, an inductor, and a diode coupled in series between the input terminal and the output terminal. The power supply also has a first voltage divider coupled to the input terminal and a second voltage divider coupled to the output terminal. The power supply also includes a control circuit that has a power transistor coupled to the inductor and the diode, an amplifier circuit coupled to the resistor for receiving a voltage signal related to a current from an input of the power supply and producing a first signal. The control circuit also has a first signal processing circuit coupled to the amplifier circuit for receiving the first signal and configured to receive a second signal from the input terminal of the power supply and a third signal from an output terminal of the power supply. The first signal processing circuit is configured to produce a fourth signal as a function of the first, the second, and the third signals. The control circuit also has a second signal processing circuit coupled to the first signal processing circuit and configured to output a fifth signal related to the fourth signal. The control circuit further has a comparator circuit coupled to the second signal processing circuit for providing a control signal to the power transistor for controlling current flow in the power supply based on comparison of the fifth signal with a reference signal. 
         [0020]    In a specific embodiment of the above power supply, the control signal is configured to enable the power supply to provide a constant current output. In some embodiments, an LED lighting system one or more light emitting diodes (LEDs) connected in series and a load capacitor coupled in parallel with the one or more LEDs. The LED lighting system also has a switched mode power supply as described above, and an output terminal of the power supply being coupled to the one or more LEDs for providing a drive current. 
         [0021]    According to another alternative embodiment of the present invention, a signal processing circuit has first, second, third, and fourth bipolar transistors connected in such a way that a sum of the first transistor&#39;s base-emitter voltage and the second transistor&#39;s base-emitter voltage is equal to a sum of the third transistor&#39;s base-emitter voltage and the fourth transistor&#39;s base-emitter voltage. The first, second, third, and fourth bipolar transistors are coupled to a first current I 1 , a second current I 2 , a third current I 3 , and a fourth current I 4 , respectively. The signal processing circuit also has a current mirror for providing an output current that mirrors the fourth current. In a specific embodiment, the first, second, third, and fourth current satisfy the following relationship: 
         [0000]    
       
         
           
             
               I 
               4 
             
             = 
             
               
                 
                   I 
                   1 
                 
                 * 
                 
                   I 
                   2 
                 
               
               
                 I 
                 3 
               
             
           
         
       
     
         [0022]    These and other features and advantages of embodiments of the present invention will be more fully understood and appreciated upon consideration of the detailed description of the preferred implementations of the embodiments, in conjunction with the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram of an LED lighting system  100  driven by a conventional switching mode power supply; 
           [0024]      FIG. 2  is a simplified schematic diagram illustrating an LED lighting system driven by a switching mode power supply according to an embodiment of the present invention; 
           [0025]      FIG. 3  is a simplified schematic diagram illustrating an LED lighting system driven by a switching mode power supply according to an alternative embodiment of the present invention; 
           [0026]      FIGS. 4A-4C  are simplified schematic diagrams illustrating an embodiment of the analog signal processor in the power controller of  FIGS. 2 and 3 ; 
           [0027]      FIG. 5  is a simplified schematic diagram illustrating an alternative embodiment of the analog signal processor in the power controller of  FIGS. 2 and 3 ; and 
           [0028]      FIG. 6  is a simplified schematic diagram illustrating yet another embodiment of the analog signal processor in the power controller of  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    As described above, the output current in conventional LED lighting systems, such as system  100  in  FIG. 1  can vary with input voltage Vin, This variation may lead to variations in LED light output, including flickers. Therefore, it is desirable to have improved methods and devices for controlling drive current in a power supply in LED lighting and other applications. 
         [0030]    As described in detail below, embodiments of the present invention provide methods and devices for power supplies that can be used as constant current drivers for white light LEDs and other applications. 
         [0031]      FIG. 2  is a simplified schematic diagram illustrating an LED lighting system  200  driven by a switching mode power supply  220  according to an embodiment of the present invention. As shown in  FIG. 2 , lighting system  200  includes serially connected multiple LEDs  204  connected with a load capacitor  214 . The LEDs are driven by a power supply  220 , which includes a sense resistor  201 , an inductor  202 , and a Schottky diode  203 . Power supply  220  also includes a controller  230 , which includes a transconductance amplifier  205 , a Dim linear amplifier  206 , a current adder  207 , a resistor  208 , a comparator  209 , and a power MOSFET  210 . As shown, transconductance amplifier  205  receives input from both ends of sense resistor  201 , and power transistor  210  is connected to a node between inductor  202  and Schottky diode  203 . 
         [0032]    As described above, lighting system  200  and power supply  220  have a number of similar components as do light system  100  and power supply  120 , respectively. The functions of these common components are not repeated here. It is noted, however, that controller  230  has an analog signal processor  213 , which is coupled between transconductance amplifier  205  and current adder  207 . Analog signal processor  213  is also coupled to input voltage Vin and output voltage Vout. As described below, analog signal processor  213  is configured to enable the power supply to provide an output current that is substantially independent of Vin. 
         [0033]    As shown in  FIG. 2 , analog signal processor  213  is configured to receive three inputs: I 1 , I 2 , and I 3 , and to produce an output I 4  as a function of I 1 , I 2 , and I 3 . I 1  is the output from transconductance amplifier  205 , I 2  is related to Vin through voltage divider R 1 /R 2 , and I 3  is related to Vin and also related to Vout (also referred to as Vload) through voltage divider R 3 /R 4 . As described in more detail below, in some embodiments, I 4  can be expressed as a function of I 1 , I 2 , and I 3 : 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     4 
                   
                   = 
                   
                     
                       
                         I 
                         1 
                       
                       * 
                       
                         I 
                         2 
                       
                     
                     
                       I 
                       3 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0034]    In an embodiment, output current Iout of system  200  can be written as an equation of Vin, Vdim, Vload (or Vout), and efficiency η, as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     out 
                   
                   = 
                   
                     K 
                     * 
                     
                       
                         
                           V 
                           dim 
                         
                         * 
                         
                           V 
                           in 
                         
                       
                       
                         
                           R 
                           sense 
                         
                         * 
                         
                           ( 
                           
                             
                               V 
                               in 
                             
                             + 
                             
                               V 
                               load 
                             
                           
                           ) 
                         
                         * 
                         η 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where K is a proportionality constant, Rsense is the resistance of resistor  201  in  FIG. 2 , and Vdim is a voltage at a light adjustment pin DIM which is used to linearly adjust the output current through LEDs. Alternatively, the DIM pin can receive an external DC voltage or a Pulse Width Modulation (PWM) dimming signal for dimming control. As shown, Iout is affected by changes in Vin and Vload. In Eq. 2, efficiency η, may be related to the on-resistance of the power switch, parasitic resistance in the inductor or Schotty diode, or deterioration of various components in the power supply. 
         [0035]    In embodiments of the present invention, the DIM pin is a multi-function On/Off and brightness control pin. In some embodiments, when the Vdim is within a first voltage range, the DIM pin can be used to adjust the brightness of the lighting device. When the Vdim is within a second voltage range, Vdim is not used for the dimming function, and the DIM pin can be coupled to the input (as shown in  FIG. 3 ) and used in controlling the output current in the power supply. Additionally, the DIM pin can also be used in a soft start function. 
         [0036]    In the embodiment shown in  FIG. 2 , where I 2  is related to Vin and I 3  is related to Vin+Vload, controller  230  is configured such that Iout can be expressed as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     out 
                   
                   = 
                   
                     K 
                      
                     
                         
                     
                      
                     1 
                     * 
                     K 
                      
                     
                         
                     
                      
                     2 
                     * 
                     
                       
                         V 
                         dim 
                       
                       
                         
                           R 
                           sense 
                         
                         * 
                         η 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where K 1  and K 2  are constants. It can be seen from Eq. (3) that Iout is not a function of Vin or Vload, when the current relationship described in Eq. (1) is implemented. Thus, a constant output current Iout can be obtained. 
         [0037]    In an alternative embodiment, as described below in connection with  FIG. 3 , when I 2  is a constant internal current related to the voltage at DIM through a voltage divider as shown in  FIG. 3 , Iout can be expressed as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     out 
                   
                   = 
                   
                     K 
                      
                     
                         
                     
                      
                     1 
                     * 
                     K 
                      
                     
                         
                     
                      
                     3 
                     * 
                     
                       1 
                       
                         
                           R 
                           sense 
                         
                         * 
                         η 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0038]    In equations (3) and (4), η represents the transformation efficiency, I 1  represents the transconductance amplifier current, I 2  and I 3  are related to Vin and Vload (Vout) converter current as shown in  FIG. 3 . As shown in equations (3) and (4), embodiments of the present invention provides constant output current Iout, which is substantially independent of Vin. 
         [0039]    As shown in  FIG. 3 , another embodiment of the present invention provides an LED driver circuit. As shown, a current adjustment linear amplifier  306  is coupled between resistors R 1  and R 2  at the input, and signal processor  313  is coupled between resistors R 3  and R 4  which couple load capacitor  314  to ground. Moreover, signal processor  313  is coupled between transconductance amplifier  305  and current adder  307 . That is, current flows from load capacitor  314  and divider resistor R 3  and enters signal processor  313 , and current from transconductance amplifier  305  also enters signal processor  313 . As shown in  FIG. 3 , V dim  adjusts the voltage between two terminals of sampling resistor  301 . Therefore, V 301 =K 1 ×V dim . 
         [0040]    The operations of LED driver circuits in  FIGS. 2 and 3  can be briefly analyzed as follows. In the output of the power supply, 
         [0000]        L×I=V in× D×T  
 
         [0000]    where I is the current through inductor  202 , and D is the duty cycle for charging and discharging the inductor  202 . Moreover, 
         [0000]        L×I=V out× T× (1− D ),
 
         [0000]    where I is the inductor current and D is the duty cycle of the charging circuit. Then 
         [0000]    
       
         
           
             D 
             = 
             
               Vout 
               
                 ( 
                 
                   Vin 
                   + 
                   Vout 
                 
                 ) 
               
             
           
         
       
     
       Additionally, 
       [0041]    
       
         
           
             
               I 
               out 
             
             = 
             
               
                 η 
                 * 
                 
                   V 
                   in 
                 
                 * 
                 
                   I 
                   in 
                 
               
               
                 
                   V 
                   out 
                 
                 * 
                 D 
               
             
           
         
       
     
         [0000]    where η is the efficiency of the driver and 
         [0000]    
       
         
           
             Iin 
             = 
             
               
                 
                   V 
                    
                   
                       
                   
                    
                   201 
                 
                 
                   R 
                    
                   
                       
                   
                    
                   201 
                 
               
               . 
             
           
         
       
     
         [0000]    Substituting in the expression for D, Iout can be expressed as 
         [0000]    
       
         
           
             Iout 
             = 
             
               
                 
                   η 
                   × 
                   Vin 
                   × 
                   V 
                    
                   
                       
                   
                    
                   101 
                 
                 
                   
                     ( 
                     
                       Vin 
                       + 
                       Vout 
                     
                     ) 
                   
                   × 
                   R 
                    
                   
                       
                   
                    
                   101 
                 
               
               . 
             
           
         
       
     
         [0000]    As can be seen, Iout can be kept constant, if 
         [0000]    
       
         
           
             Vsense 
             = 
             
               
                 Vin 
                 × 
                 V 
                  
                 
                     
                 
                  
                 101 
               
               
                 ( 
                 
                   Vin 
                   + 
                   Vout 
                 
                 ) 
               
             
           
         
       
     
         [0000]    is kept constant. In  FIG. 2 , signal processor  213  is configured to provide such a function. 
         [0042]    In  FIG. 3 , the following relationship holds: 
         [0000]        V 301= K×V dim= K 1 ×K 2× V in.
 
         [0000]    Signal processor  313  is configured such that its output current can be expressed as 
         [0000]    
       
         
           
             Isensenew 
             = 
             
               
                 
                   I 
                    
                   
                       
                   
                    
                   301 
                    
                   sense 
                   × 
                   Iconst 
                 
                 
                   I 
                   
                     ( 
                     
                       Vin 
                       + 
                       Vout 
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
         [0000]    As described above, 
         [0000]    
       
         
           
             
               I 
               out 
             
             ∝ 
             
               
                 Vin 
                 
                   Vin 
                   + 
                   Vout 
                 
               
               . 
             
           
         
       
     
         [0000]    Here, the input to signal processor  313  can be expressed as 
         [0000]    
       
         
           
             
               
                 Vin 
                 + 
                 Vout 
               
               Vin 
             
             . 
           
         
       
     
         [0000]    Signal processor  313  is configured to receive V 301  and produce an output that is proportional to 
         [0000]    
       
         
           
             
               Vin 
               
                 Vin 
                 + 
                 Vout 
               
             
             , 
           
         
       
     
         [0000]    then the input to comparator  309  Vsample is also proportional to 
         [0000]    
       
         
           
             
               
                 Vin 
                 + 
                 Vout 
               
               Vin 
             
             . 
           
         
       
     
         [0000]    Thus, by maintaining Vsample at a reference voltage using the comparator circuit, a constant output current can be achieved. 
         [0043]    In another embodiment, a diode function block  212  is coupled in parallel with power MOS transistor  210  to provide over voltage protection. Although shown as a diode in  FIG. 2 , diode function block  212  can include a rectifying device and other support circuitry. A detection circuit  213  is coupled to diode block  212 . When detection circuit  213  detects an over voltage condition at transistor  210 , diode block can shut down transistor  210 . Similar features are also included in  FIG. 3 . 
         [0044]      FIGS. 4A-4C  are simplified schematic diagrams illustrating an embodiment of analog signal processor  213  in the power controller of  FIG. 2 . In some embodiments, signal processor  213  includes first, second, third, and fourth bipolar transistors connected in such a way that a sum of the first transistor&#39;s base-emitter voltage and the second transistor&#39;s base-emitter voltage is equal to a sum of the third transistor&#39;s base-emitter voltage and the fourth transistor&#39;s base-emitter voltage. The first, second, third, and fourth bipolar transistors are coupled to a first current I 1 , a second current I 2 , a third current I 3 , and a fourth current I 4 , respectively. A current mirror for providing an output current that mirrors the fourth current. In one or more embodiments, the currents satisfy the following relationship: 
         [0000]    
       
         
           
             
               I 
               4 
             
             = 
             
               
                 
                   
                     I 
                     1 
                   
                   * 
                   
                     I 
                     2 
                   
                 
                 
                   I 
                   3 
                 
               
               . 
             
           
         
       
     
         [0000]    Several specific embodiments are described below. 
         [0045]      FIG. 4A  is a simplified circuit diagram of an embodiment of the analog signal processor shown in  FIG. 2 . In this embodiment, NPN transistors  402 ,  403 ,  404 , and  405  are interconnected as shown in  FIG. 4A . With reference to  FIG. 2 , I 1  is the sense current on sense resistor  201  in  FIG. 2 , and I 4  through transistor  408  is the output current to resistor  208  in  FIG. 2 . I 4  also designates the current flowing through transistor  405  by way of a current mirror. 
         [0046]    As configured in  FIG. 4A , the base terminals of transistors  402  and  404  are connected, and the emitters of transistors  403  and  405  are connected. It follows that: 
         [0000]        VBE   405   =VBE   402   +VBE   403   −VBE   404   (5)
 
         [0000]    Based on the current-voltage relationship of the base-emitter junction: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     C 
                   
                   = 
                   
                     
                       I 
                       s 
                     
                     * 
                     e 
                      
                     
                       
                         
                             
                           q 
                         
                          
                         
                             
                         
                          
                         
                           V 
                           BE 
                         
                       
                       
                         KV 
                         T 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    the following current relationship is derived: I402*I403=I404*I405. With reference numerals in  FIG. 4A , the relationship expressed in Eq. (1) can be obtained. 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     4 
                   
                   = 
                   
                     
                       
                         I 
                         1 
                       
                       * 
                       
                         I 
                         2 
                       
                     
                     
                       I 
                       3 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0047]      FIG. 4B  is a simplified circuit diagram illustrating an embodiment of circuit block  406  in  FIG. 4A  for voltage to current conversion. As shown, circuit block  406  converts the voltage divider  1  input to current I 2  in  FIG. 4A , and it also converts voltage divider  2  input to current I 3  in  FIG. 4A . In  FIG. 4B , resistors  805  and  806  are matched to ensure the relationships described in the above equations holds true. 
         [0048]      FIG. 4C  is a simplified circuit diagram illustrating the connection of part of circuit block  406  in  FIG. 4A  to a voltage divider. Voltage divider resistors  702  and  703  are used to scale the input voltage at  701  to meet internal voltage requirement. In  FIG. 4C , operational amplifier  704 , PMOS transistor  707 , and resistor  706  form a voltage regulator that maintains the voltage across resistor  706  to be equal to the input voltage to operational amplifier  704 . Matching PMOS transistors  708  and  707  provide an output current of the regulator. In the embodiment in  FIG. 4B , which includes two voltage-to-current converters described in  FIG. 4C , resistors  805  and  806  are matched to ensure proper current relationship. Mismatch of these resistors can cause errors in converting the voltage signals. Additionally, mismatch of transistors  707  and  708 , as well as offset in operational amplifier  704 , can also lead to signal errors. According to embodiments of the invention, these potential errors can be corrected by using cascode MOS transistors and careful design. Further, another transfer of current can be applied, when a sink current needed. It is also noted that the circuits in  FIGS. 4B and 4C  can be implemented using MOS transistors provided in a CMOS process. 
         [0049]      FIG. 5  is a simplified schematic diagram illustrating an alternative embodiment of the analog signal processor in the power controller of  FIGS. 2 and 3 . In this embodiment, substrate PNP transistors are used, which is compatible with standard CMOS processes. Here, substrate PNP transistor  502 ,  503 ,  504 ,  505  forms a signal processing circuit, substantially similar to the signal processing circuit in  FIG. 4A . Operational amplifier  507  and PMOSFET  508  form a current regulator that maintains equal voltages at the two input terminals of  507 . As a result, the following relationship is established: 
         [0000]        VBE   504   =VBE   502   +VBE   503   −VBE   505   (7)
 
         [0000]    An output current is provided by PMOSFET  509  by matching PMOS  509  with PMOS  508 . Alternatively, the output circuit can also be configured using PNP transistors. 
         [0050]      FIG. 6  is a simplified schematic diagram illustrating yet another embodiment of the analog signal processor in the power controller of  FIGS. 2 and 3 . As shown, signal processor  600  includes both NPN and PNP transistors, including NPN transistors  602  and  604  and PNP transistors  603  and  605 . The operation of signal processor  600  is substantially similar to that of signal processor  400  in  FIG. 4A  and signal processor  500  in  FIG. 5 . The following relationship is established. 
         [0000]        VBE   604   =VBE   602   +VBE   605   −VBE   603   (8)
 
         [0000]    In  FIG. 6 , MOSFET or bipolar transistor  607  is used as a current source and serves to raise the source or emitter voltage and improve the headroom of block  606 . Block  606  is receives the divider currents of Vin and Vin plus Vload, similar to the signal processors described above in connection with  FIGS. 4A and 4B . 
         [0051]    While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention.