RELATED CASES 
     The present application claims priority to and is a continuation application of U.S. application Ser. No. 11/423,479 entitled “Two Pin-Based Sensing Of Remote DC Supply Voltage Differential Using Precision Operational Amplifier And Diffused Resistors” filed on Jun. 12, 2006 which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Power supply systems for supplying DC power to a device, such as core processors of digital processing devices, and the like, which are subject to varying load conditions, must continuously monitor the respective voltages at (remote) power supply terminals to which the powered device is coupled, in order to compensate for voltage drops associated with the resistance of the main DC output power rails and ground planes, and thereby ensure that the powered device will be continuously supplied with its intended target voltage differential. Typical monitoring and control circuits that have been employed for this purpose include three pin-based circuits. 
     SUMMARY OF THE INVENTION 
     The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. 
     In one embodiment a control circuit for a DC voltage supply is provided. The control circuit includes a circuit, an error amplifier and modulator. The circuit is operable to measure a voltage difference between a negative voltage rail and a ground reference in the DC voltage supply. The circuit is further operative to create an offset voltage proportional with the measured voltage difference. The circuit is further yet operative to add the offset voltage to a reference voltage to create a modified reference voltage. The error amplifier has a first input coupled to receive the modified reference voltage and a second input coupled to a positive voltage rail in the DC voltage supply. The error amplifier further has an output. The modulator is coupled to the output of the error amplifier. The modulator is operative to maintain the positive rail at a select value corresponding to the modified reference voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The single FIGURE is a schematic illustration of a two input pin-based remote differential voltage sensing architecture in accordance with a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Attention is now directed to the single FIGURE, wherein a preferred embodiment of a two input pin-based remote differential voltage sensing architecture in accordance with of the present invention is schematically illustrated. As shown therein, the differential voltage sensing architecture includes a first (negative voltage rail sensing) input pin, shown as a first remote voltage sensing terminal RGND, which is adapted to be coupled to a first remote power supply terminal through which a first supply rail voltage, such as ground (GND) potential, is supplied to a first power supply terminal of the powered device, such as a core processor of a personal computer. The differential voltage sensing architecture of the invention also includes a second (positive voltage rail sensing) input pin, shown as a second remote voltage sensing terminal VSENSE, which is adapted to be coupled to a second remote power supply terminal through which a second supply rail voltage, having some prescribed DC voltage value (e.g., +3.3 VDC) that is positive relative to the first voltage (e.g., ground), is supplied to a second power supply terminal of the powered device. 
     The first remote voltage sensing terminal RGND is coupled through a first input resistor R 1  to a first, non-inverting (+), input terminal  201  of a precision operational amplifier (op amp)  200  having a very low offset voltage. Input resistor R 1  serves to provide compensation for the inherent input bias current to the op amp&#39;s input terminal  201 . A second, inverting (−), input terminal  202  of the op amp is coupled through a second input resistor Rsense (which may be implemented as a diffused resistor) to a prescribed reference potential, which corresponds to the potential of the first (negative) DC power supply voltage (here ground (GND) potential). 
     The output  203  of op amp  200  is coupled to the control terminal (gate)  01  of a current flow control device, shown as an NMOS field effect transistor (FET) M 0 , of an offset current generator  100 , so that the source-drain current (Id/Ic M0 ) through NMOSFET M 0  is controlled in accordance with the output  203  of op amp  200 . The source-drain current through NMOSFET M 0  serves as the input current for a current mirror input PMOSFET M 1  of a first current mirror circuit  300 . For this purpose, current flow control NMOSFET M 0  has its drain terminal  02  coupled to the commonly connected gate and drain terminals  11  and  13 , respectively, of a current mirror input PMOSFET M 1 , the source terminal  12  of which is referenced to a prescribed positive DC voltage (e.g., +5.0 VDC). Current mirror circuit  30  has its output coupled to an output current reference node  35 , from which a voltage Vout is derived, as will be described. 
     The source-drain current (Id/Ic M0 ) through NMOSFET M 0  is derived from of the positive +5.0 VDC reference and through the source-drain path of current mirror input PMOSFET M 1  to which the drain terminal  03  of NMOSFET M 0  is coupled. This source-drain current is supplied to an input current reference node  25 , to which the source terminal  02  of NMOSFET M 0  is connected. Input current reference node  25  is coupled in common with the inverting (−) input terminal  202  of op amp  200  and with the input of a reference current source  27 . Reference current source  27  is operative to supply a prescribed reference current (e.g., 40 microamps) to the commonly connected gate and drain terminals  141  and  143 , respectively, of a current mirror input PMOSFET M 14  of a second current mirror circuit  400 , the output of which is coupled to the output current reference node  35 . PMOSFET M 14  has its source terminal  142  coupled to a prescribed negative DC voltage (e.g., −2.0 VDC), which serves as the sink for the reference current supplied by reference current source  27  to current mirror input PMOSFET M 14 . 
     As will be described, as long as the monitored negative voltage differential applied to the input terminals  201  and  202  of op amp  200  is balanced or zero, the output  203  of op amp  200  is zero, so that current flow control NMOSFET M 0  is slightly turned on, which allows a quiescent source-drain current, corresponding to that (e.g., 40 microamps) produced by the reference current source  27 , to flow therethrough from current mirror input PMOSFET M 1  of the first current mirror circuit  300  to the input current reference node  25 . With reference current source  27  supplying this same value of current from the input reference current node  25  for application to the input PMOSFET M 14  of current mirror circuit  400 , no additional current will flow into or out of the input reference current node  25  by way of the inverting (−) input terminal  202  of op amp  200 , to which grounded input resistor Rsense is coupled. As a consequence, the mirrored output currents supplied by current mirror circuits  300  and  400  to the output current reference node  35  will sum to zero or match. 
     As will be described, this will prevent any current from flowing into or out of output current reference node  35  through an output reference resistor Rref, which is used to provide, as necessary, an offset in the reference voltage being applied to an error amplifier  500 , the output of which is used to control the DC power supply&#39;s positive voltage output. However, if the sensed remote voltage at the first input terminal RGND, to which the non-inverting (+) input terminal  201  of op amp  200  is coupled by way of input resistor R 1 , is different from the ground reference, to which the inverting (−) input terminal  202  of op amp  200  is coupled by way of resistor Rsense, the output  203  of op amp  200  will change accordingly, so as to cause the magnitude of source-drain current flowing through NMOSFET M 0  to the input current reference node  25  to be different or offset from its (40 microamps) quiescent value. 
     The effect of this offset in the magnitude of the source-drain current flowing through NMOSFET M 0  is to cause current to flow either in a first direction—from input current reference node  25  through input resistor Rsense to ground, or in a second direction from ground—through input resistor Rsense into input current reference node  25 , depending upon the polarity of the departure of the monitored negative voltage from its intended or target value (e.g., ground). Namely, any offset in the sensed negative voltage from its target value is effectively converted into an equivalent current (the current through the input resistor Rsense) that is proportional to the offset in the sensed remote voltage at the first input terminal RGND. The direction and magnitude of this equivalent current is defined by the relatively simple relationship I=V−/Rsense, and is such as to bring the voltage V− applied to the inverting (−) input terminal  202  of op amp  200  into balance with the change in sensed remote voltage coupled to op amp input terminal  201 . 
     The change in source-drain current through NMOSFET M 0  necessary to bring the voltage V− at input terminal  202  into balance with the change in the sensed remote voltage is mirrored at the output of current mirror  300 , so as to cause a mismatch in the magnitudes of the mirrored output currents supplied by current mirror circuits  300  and  400  to the output current reference node  35 . This, in turn, causes current to flow either out of or into the output current reference node  35  through output reference resistor Rref (depending upon whether the source-drain current through MOSFET M 0  is greater or less than the reference current generated by reference current source  27 ). Reference resistor Rref (which, like input resistor Rsense, may be implemented as a diffused resistor) is coupled to a positive target voltage reference node  45 , to which a voltage Vdac, representative of the target voltage output of the power supply, is coupled. As pointed out above, any current flow through the reference resistor Rref will cause the voltage at node  35  to change relative to the voltage Vdac, so as to change the magnitude of the reference voltage applied to the error amplifier  500 , and thereby a change in the error voltage used by the power supply&#39;s modulator loop to control the power supply&#39;s positive DC output voltage. 
     For this purpose, the first current mirror circuit  300  includes a current mirror PMOSFET M 2  coupled in current mirror configuration with input PMOSFET M 1 . Current mirror PMOSFET M 2  has its gate  21  coupled in common with the gate  11  of PMOSFET M 1 , its source  22  referenced to the prescribed positive DC voltage (+5.0 VDC), and its drain  23  coupled to the source  32  of a current mirror output PMOSFET M 3 , the drain  33  of which is coupled to the output current reference node  35 . The gate  31  of PMOSFET M 3  is coupled to the drain  93  of an NMOSFET M 9  and to the drain  73  of a PMOSFET M 7  of a first balancing amplifier  350  comprised of cascoded MOSFETs M 6 -M 9  which serve to provide constant drain voltages for the first current mirror circuit  300 . NMOSFET M 9  has its source  92  coupled to a prescribed reference potential (ground), and its gate  91  coupled in common to the gate  81  and drain  83  of an NMOSFET M 8 , the source  82  of which is coupled to ground. The commonly connected gate  81  and drain  83  of NMOSFET M 8  are connected to the drain  63  of a PMOSFET M 6 , the source  62  of which is coupled in common with the source  72  of PMOSFET  70  to receive a relatively small valued (e.g., five microamps) fixed bias current supplied by a reference current source  360 . PMOSFET M 6  has its gate  61  coupled to the drain  13  of input PMOSFET M 1 , while PMOSFET M 7  has its gate  71  coupled to the drain  23  of current mirror PMOSFET M 2 . 
     In a similar, but polarity-complementary manner, the second current mirror circuit  400  includes a current mirror NMOSFET M 4  coupled in current mirror configuration with input NMOSFET M 14 . Current mirror NMOSFET M 4  has its gate  41  coupled in common with the gate  141  of NMOSFET M 14 , its source  42  referenced to the prescribed negative DC voltage (−2.0 VDC), and its drain  43  coupled to the source  52  of a current mirror output NMOSFET M 5 , the drain  53  of which is coupled to the output current reference node  35 . The gate  51  of NMOSFET M 5  is coupled to the drain  113  of a PMOSFET M 11  and to the drain  133  of an NMOSFET M 13  of a second current balancing amplifier  450  comprised of cascoded MOSFETs M 10 -M 13  which serve to provide constant drain voltages for the second current mirror circuit  400 . PMOSFET M 11  has its source  112  coupled to a predetermined reference potential (e.g., +5.0 VDC), and its gate  111  coupled in common to the gate  101  and drain  103  of a PMOSFET M 10 , the source  102  of which is coupled to +5 VDC. The commonly connected gate  101  and drain  1083  of PMOSFET M 10  are connected to the drain  123  of an NMOSFET M 12 , the source  122  of which is coupled in common with the source  132  of NMOSFET  13  to a relatively small valued (e.g., five microamps) fixed bias current source  460 . NMOSFET M 12  has its gate  121  coupled to the drain  114  of input NMOSFET M 14 , while NMOSFET M 13  has its gate  131  coupled to the drain  43  of current mirror NMOSFET M 4 . 
     The output current reference node  35 , to which the drains  33  and  53  of output MOSFETs M 3  and M 5  of current mirrors  300  and  400  are respectively coupled, is coupled to one end of reference resistor Rref, a second end of which is coupled to positive target voltage reference node  45  which, as described above, is coupled to receive a voltage Vdac, which corresponds to the output voltage of a digital-to-analog converter (DAC) that is used to set the target value of the positive voltage of the power supply. Output current reference node  35  is further coupled to a first, non-inverting (+) input  501  of error amplifier  500 . A second, inverting (−) input  502  of error amplifier  500  is coupled to a feedback node FB from the control loop of the power supply&#39;s modulator  600  and, via a resistor R 2 , to the second input pin or remote voltage sensing terminal VSENSE. As described briefly above, this second input pin (VSENSE) is used by error amplifier  500  to monitor a second remote power supply terminal through which a second supply rail voltage, having some prescribed DC voltage value (e.g., +3.3 VDC) that is positive relative to the first voltage (ground), is supplied to a second power supply terminal of the powered device. A compensation network  550  comprised of series connected capacitor C 1  and resistor R 3 , that are connected in parallel with capacitor C 2  is connected between the inverting (−) input  502  and the output  503  of error amplifier  500 . The output  503  of error amplifier  500  provides an error voltage that is used by the power supply&#39;s modulator loop to control the power supply&#39;s positive DC output voltage. 
     Operation 
     As pointed out above, using only the two input pins RGND and VSENSE, the remote differential voltage sensing architecture of the invention continuously monitors the voltages at the positive and negative supply terminals by way of which power is supplied from the power supply to a remote utility device and adjusts or offsets, as necessary, the value of the target reference voltage applied to the error amplifier  500 , so as to realize an associated adjustment of the error voltage used by the power supply&#39;s modulator loop to control the power supply&#39;s positive DC output voltage. There are three modes of operation of the circuit: 1—monitored negative voltage rail at target value; 2—monitored negative voltage rail above target value; and 3—monitored negative voltage rail below target value. 
     1—Monitored Negative Voltage Rail at Target Value 
     In this mode, the value of the (negative) voltage monitored at the first (negative voltage rail-sensing) input pin RGND, which is coupled via input resistor R 1  to the non-inverting (+) input terminal  201  of op amp  200 , is at its target value (here zero volts or ground potential—Corresponding to the value of the reference voltage coupled via input resistor Rsense to the inverting (−) input terminal  202  of op amp  200 ), so that the two inputs  201  and  202  of op amp  200  will be balanced (have a zero voltage differential therebetween). As a consequence, the output  203  of op amp is zero, so that current flow control NMOSFET M 0  will be slightly turned on, as described above, to provide a prescribed quiescent source-drain current therethrough, corresponding to that (e.g., 40 microamps) produced by the reference current source  27 , that flows out of the current mirror input PMOSFET M 1  of the first current mirror circuit  300  and into the input current reference node  25 . Since the reference current source  27 , whose input is coupled to the input current reference node  25 , supplies this same value of current to the input PMOSFET M 14  of current mirror circuit  400 , no additional current will flow into or out of the input reference current node  25  by way of the inverting (−) input terminal  202  of op amp  200 , to which grounded input resistor Rsense is coupled. 
     As a consequence, the mirrored output currents supplied by current mirror circuits  300  and  400  to the output current reference node  35  will sum to zero, so that no additional current will flow out of or into node  35  relative to the positive target voltage reference node  45 , by way of reference resistor Rref. With no current flowing (in either direction) through reference resistor Rref, there will be no associated voltage drop thereacross, so that the target positive voltage Vdac, which is representative of the target value of the positive voltage output of the DC supply, will be applied to the first, non-inverting (+) input  501  of error amplifier  500 . As long as the value of the positive DC supply rail as monitored by the second input pin VSENSE is equal to its intended target value, the error out voltage from error amplifier  500  will be zero, so that the modulator&#39;s control loop will cause no change in the magnitude of the positive voltage output of DC supply. However, any difference between the value of the positive DC supply rail, as monitored by the second input pin VSENSE, from its intended target value at the positive target voltage reference node  45  and supplied therefrom to the reference input to the error amplifier  500 , will cause the error amplifier  500  to generate a non-zero output or error voltage, in response to which the modulator&#39;s control loop will change the magnitude of the positive voltage output of DC supply to bring the monitored positive voltage to its intended target value. 
     2—Monitored Negative Voltage Rail Above Target Value 
     In this mode, the value of the (negative) voltage monitored at the first (negative voltage rail sensing) input pin RGND is more positive than its target value, so that the voltage at op amp input terminal  201  will be positive relative to the voltage at its input terminal  202 . As a result, op amp  200  will increase the gate drive to NMOSFET M 0 , so as to increase the magnitude of its source-drain current being supplied to the input current reference node  25 . As the magnitude of current being coupled from node  25  to the second current mirror  400  is fixed (e.g., at 40 microamps) by the reference current source  27 , the increase in source-drain current into the input current reference node  25  will cause an offset current equal to that increase to flow out of node  25  and through the resistor Rsense to ground (which is at a lower potential than that of the positive voltage reference (+5 VDC) to which the input PMOSFET M 1  of current mirror  300  is referenced). 
     This outward flow of current through resistor Rsense from the inverting (−) terminal  202  of op amp  200  to ground will cause a voltage drop across the resistor Rsense, that is effective to increase the voltage V− applied to the inverting (−) input terminal  202  of op amp  200 , and thereby increase the value of the voltage V− at the inverting (−) input terminal  202  of op amp  200  toward the value of the voltage monitored at the input pin RGND and coupled to the non-inverting (+) input  201  of op amp  200 . The inherent operation of operational amplifier  200  is such that the magnitude of its output (the gate drive to NMOSFET M 0 ) will cause the resulting increase in source-drain current through NMOSFET M 0  and through input resistor Rsense to bring the voltage V− at op amp input terminal  202  into balance with the positive change in the sensed remote voltage that is coupled to op amp input terminal  201 . 
     This increase in the source-drain current through NMOSFET M 0  that is necessary to bring the voltage V− at op amp input terminal  202  into balance with the change in the sensed remote voltage at op amp input terminal  201  increases the magnitude of the input current of current mirror input PMOSFET M 1  of current mirror circuit  300 , which is mirrored at the drain  33  of its associated current mirror output PMOSFET M 3  and applied by output PMOSFET M 3  to output reference current node  35 . This results in a mismatch (corresponding the offset current through resistor Rsense) in the magnitudes of the mirrored output currents supplied by current mirror circuits  300  and  400  to output current reference node  35 . 
     Because the magnitude of the current flowing into node  35  from PMOSFET M 3  of current mirror circuit  300  is greater than the magnitude of the current flowing out of node  35  into NMOSFET M 5  of current mirror  400 , a current equal to that flowing through resistor Rsense will flow out of output current reference node  35  and through reference resistor Rref to the positive target voltage reference node  45 . With current flowing through reference resistor Rref outwardly from node  35  to node  45 , the resulting voltage drop across reference resistor Rref will be effective to increase the voltage Vout at node  35 , relative to the voltage Vdac at the node  45 . This increase in the value of the voltage Vout from its DAC-defined positive target voltage reference value supplied to node  45  will increase the value of the positive supply rail reference against which error amplifier  500  compares the positive DC supply rail as monitored by the second input pin VSENSE. 
     As a result, any adjustment of the positive output voltage by the DC power supply&#39;s correction loop will depend upon whether or not the monitored positive DC supply rail voltage (VSENSE) corresponds to an increased modification of the positive target value that takes into account the extent to which the negative DC supply rail has been detected to be above its target value, thereby ensuring that the intended differential between the positive and negative supply rails will be maintained. 
     3—Monitored Negative Voltage Rail Below Target Value 
     In this mode, the value of the (negative) voltage applied to the first (negative voltage rail sensing) input pin RGND is more negative than its target value, so that the voltage at op amp input terminal  201  will be negative relative to the voltage at its input terminal  202 . As a result, op amp  200  will decrease the gate drive to NMOSFET M 0 , so as to reduce the magnitude of its source-drain current, which is supplied therethrough from current mirror input PMOSFET M 1  to the input current reference node  25 . Since the magnitude of current being coupled from node  25  to the second current mirror  400  is fixed (e.g., at 40 microamps) by the reference current source  27 , this decrease in the amount of source-drain current through NMOSFET M 0  into the input current reference node  25  will cause an offset current, that equal to the decrease in the magnitude of source-drain current through NMOSFET M 0 , to from ground through the resistor Rsense and into node  25 . This inward flow of current through resistor Rsense from ground toward inverting (−) input terminal  202  of op amp  200  will cause a voltage drop across resistor Rsense, that is effective to decrease the voltage V− applied to the inverting (−) input terminal  202  of op amp  200 . The inherent operation of operational amplifier  200  is such that the magnitude of its output (the gate drive to NMOSFET M 0 ) will cause the resulting decrease in source-drain current through NMOSFET M 0  and through input resistor Rsense to bring the voltage V− at op amp input terminal  202  into balance with the negative change in the sensed remote voltage that is coupled to op amp input terminal  201 . 
     This decrease in the source-drain current through NMOSFET M 0  that is necessary to bring the voltage V− at op amp input terminal  202  into balance with the negative change in the sensed remote voltage at op amp input terminal  201  decrease the magnitude of the input current of current mirror input PMOSFET M 1  of current mirror circuit  300 , which is mirrored at the drain  33  of its associated current mirror output PMOSFET M 3  and applied by output PMOSFET M 3  to output reference current node  35 . This results in a mismatch (corresponding the offset current through resistor Rsense) in the magnitudes of the mirrored output currents supplied by current mirror circuits  300  and  400  to output current reference node  35 . 
     Because the magnitude of the current flowing into node  35  from PMOSFET M 3  of current mirror circuit  300  is less than the magnitude of the current flowing out of node  35  into NMOSFET M 5  of current mirror  400 , a current equal to that flowing through resistor Rsense will flow inwardly from the positive target voltage reference node  45  through reference resistor Rref and into the output current reference node  35 . With current flowing through reference resistor Rref inwardly from node  45  to node  35 , the resulting voltage drop across reference resistor Rref will be effective to decrease the voltage Vout at node  35 , relative to the voltage Vdac at the node  45 . This decrease in the value of the voltage Vout from its DAC-defined positive target voltage reference value supplied to node  45  will reduce the value of the positive supply rail reference against which error amplifier  500  compares the positive DC supply rail as monitored by the second input pin VSENSE. 
     As a result, any adjustment of the positive output voltage by the DC power supply&#39;s correction loop will depend upon whether or not the monitored positive DC supply rail voltage (VSENSE) corresponds to a decreased modification of the positive target value that takes into account the extent to which the negative DC supply rail has been detected to be lower its target value, thereby ensuring that the intended differential between the positive and negative supply rails will be maintained. 
     As will be appreciated from the foregoing description, by using a relatively simple circuit implementation (operational amplifier-controlled current mirror circuit) to monitor a single input pin coupled to a first (negative) power supply rail, through which a relatively negative one (e.g., ground) of a pair of supply rail voltages (such as ground and a positive DC voltage) is supplied to a positive supply terminal for the powered device, the two input pin-based DC power supply control circuit architecture of the present invention readily derives a current representative of the voltage differential between the negative supply rail and its target voltage. This derived current is then used to modify the input current to a current mirror circuit, whose mirrored output current is coupled through an output reference resistor, to produce an offset voltage of a magnitude and polarity that is defined in accordance with the magnitude and polarity of the derived current. 
     This offset voltage is added to or subtracted from a reference voltage for an error amplifier, to which a second input pin that monitors the second, relatively positive one of the pair of supply rail voltages is applied. The output of the error amplifier is then used by the power supply&#39;s modulator loop to adjust the power supply output. Because any adjustment of the positive output voltage by the DC power supply&#39;s correction loop not only depends upon whether or not the positive DC supply rail is at its target value, but whether or not the negative DC supply rail is at its target value, the invention readily ensures that the intended differential between the positive and negative supply rails will maintained. 
     While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.
A control circuit for a DC voltage supply is provided that includes a circuit, an error amplifier and modulator. The circuit is operable to measure a voltage difference between a negative voltage rail and a ground reference in the DC voltage supply. The circuit is further operative to create an offset voltage proportional with the measured voltage difference. The circuit is further yet operative to add the offset voltage to a reference voltage to create a modified reference voltage. The error amplifier has a first input coupled to receive the modified reference voltage and a second input coupled to a positive voltage rail in the DC voltage supply. The error amplifier further has an output. The modulator is coupled to the output of the error amplifier. The modulator is operative to maintain the positive rail at a select value corresponding to the modified reference voltage.