Patent Publication Number: US-2019173381-A1

Title: Current sensing with rdson correction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/997,415 filed Jun. 4, 2018, now U.S. Pat. No. 10,205,391, which application is a divisional of U.S. patent application Ser. No. 14/572,361, filed Dec. 16, 2014, now U.S. Pat. No. 9,991,792. This application also claims priority to U.S. provisional patent application Ser. No. 62/042,521, entitled “METHOD OF ACCURATELY REPORTING CURRENT AS MEASURED BY A METAL-OXIDE SEMICONDUCTOR (MOS) ON-STATE RESISTANCE (RDS ON ) VOLTAGE DROP,” filed on Aug. 27, 2014, all such applications being incorporated herein by reference in their entireties. 
    
    
     DRAWINGS 
     Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an exemplary system that includes a driver with on-state resistance (RDS ON ) correction. 
       FIG. 2  is a diagram of an exemplary switching power supply that includes a driver with RDS ON  correction. 
       FIGS. 3A-3B  are diagrams of exemplary drivers with RDS ON  correction. 
       FIG. 4  is a flow diagram of an exemplary method for determining a current. 
       FIG. 5  is a flow diagram of an exemplary method for determining a temperature sensor gain and offset correction function. 
       FIG. 6  is a flow diagram of an exemplary method for determining a voltage correction function. 
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. 
     DC-to-DC voltage conversion can be performed by a switching power supply that converts a voltage (e.g. 12V) to a different voltage, as required by one or more load devices. Switching power supplies usually include a current sensing mechanism for various purposes, including, but not limited to, control loop inputs (current mode control), over current detection, multi-phase converter inductor current balance, output current reporting, and input current reporting. One technique for determining the current in a switching power supply is to measure the RDS ON  voltage drop of the switches that are turned on and off to produce the output voltage of the switching power supply. If the RDS ON  of the switch is known and the on-state voltage drop is measured, the current through the switch can be determined according to Ohm&#39;s Law, V=I*R. However, the RDS ON  of the switch is not easily known because it is not constant; the RDS ON  is a function of the device characteristics, gate drive voltage, and temperature. In many conventional implementations, the RDS ON  variability is not accounted for; and therefore, the current through the switch cannot be accurately calculated. 
     System Including a Driver with RDS ON  Correction 
       FIG. 1  is a block diagram of an exemplary system  100  that includes a driver with RDS ON  correction  104  and a pulse width modulation (PWM) controller  103 . As explained below, the PWM controller  103  sends control signals to the driver with RDS ON  correction  104 . In some embodiments, the PWM controller  103  and the driver with RDS ON  correction  104  can be included in the same chip. In other embodiments, the PWM controller  103  and the driver with RDS ON  correction  104  are on different chips. 
     The driver with RDS ON  correction  104  can be used to accurately report the current flow through a metal-oxide-semiconductor (MOS) transistor switch that is used in a switching power supply  102  to control the output voltage of the switching power supply  102 . In some embodiments, the MOS transistor switch can be an n-channel MOS (NMOS) transistor switch, a p-channel MOS (PMOS) transistor switch, a complementary MOS (CMOS) transistor switch, or double-diffusion (DMOS) transistor switch. The driver with RDS ON  correction  104  can be implemented as one or more of the RDS ON  correction circuits discussed in the embodiments below. The switching power supply  102  can be included in any suitable electronic device using regulated power including, but not limited to, a desktop computer, a laptop computer, or tablet computer, a set-top box, battery charger, or other device. 
     The system  100  also includes a power source  106  and a load  108 . The load  108  draws power from the power source  106  via the switching power supply  102 . The switching power supply  102  can receive unregulated voltage from the power source  106  (e.g., line power, battery power), regulate the voltage, and provide regulated supply voltage to the load  108 . The load  108  can include, but is not limited to, one or more processors (e.g., a central processing unit (CPU), a microcontroller, microprocessor, a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), a display device (e.g., a light-emitting diode (LED) display, a liquid crystal display (LCD), a cathode ray tube (CRT) display, etc.), a memory device (e.g., a conventional hard disk, a volatile or non-volatile media such as a solid state hard drive, random access memory (RAM) including, but not limited to, synchronous dynamic random access memory (SDRAM), double data rate (DDR) RAM, RAMBUS dynamic RAM (RDRAM), static RAM (SRAM), etc.), electrically erasable programmable ROM (EEPROM), and flash memory, etc.), other peripheral devices, an internal device component, or other components. 
     Switching Power Supply Including a Driver with RDS ON  Correction 
       FIG. 2  is a diagram of an exemplary switching power supply  200  that includes a driver with RDS ON  correction  204 . While only a single-phase switching power supply  200  is shown, in some embodiments, multi-phase switching power supplies  200  can be used as well. As stated above, the switching power supply  200  includes a pulse-width modulation (PWM) controller  202  that sends control signals to the driver with RDS ON  correction  204 . In response to the control signals sent by the PWM controller  202 , the driver with RDS ON  correction  204  drives the gates of the MOS transistor switches  206 ,  208 . The switching of the MOS transistor switches  206 ,  208  produce a square-wave. The square-wave can be smoothed using an LC circuit comprising an inductor  210  and a capacitor  212  to produce the desired voltage, V out    214 . V out    214  can also be digitized and fed in the PWM controller  202 . Depending on the desired V out    214 , the PWM controller  202  can maintain or vary the control pulses sent to the driver with RDS ON  correction  204 , which drive the MOS transistor switches  206 ,  208 . In exemplary embodiments, MOS transistor switch  206  is a p-channel MOS; however, in other embodiments, MOS transistor switch  206  can be other types of MOSs. In the simplified block diagram of the exemplary power supply shown in  FIG. 2 , the gates of MOS transistor switches  206  and  208  are connected together. In other embodiments, the driver with RDS ON  correction  204  may include two separate gate drivers driving the gates of MOS transistor switches  206  and  208  separately and individually. 
     In addition to driving the gates of the MOS transistor switches  206 ,  208 , the driver with RDS ON  correction  204  can calculate the current through each of the MOS transistor switches  206 ,  208 . As mentioned above, in order to do so accurately, the device characteristics and temperature dependency of the RDS ON  of the MOS transistor switches  206 ,  208  must be calculated. The driver with RDS ON  correction  204  accomplishes this, in one embodiment, as explained in  FIGS. 3A-3B . 
     Driver with RDS ON  Correction 
       FIGS. 3A-3B  are diagrams of exemplary drivers with RDS ON  correction  300 A,  300 B.  FIG. 3A  differs from  FIG. 3B  only insofar as  FIG. 3B  includes a single variable gain amplifier  313 , instead of two distinct circuit elements, a gain amplifier  312  and attenuating D/A  314 , as discussed below. The MOS transistor switch  301  is not included in the driver with RDS ON  correction  300 A,  300 B; however, it is shown in  FIGS. 3A and 3B  to better explain how the current of MOS transistor switch  301  is measured and how it is coupled to the driver with RDS ON  correction  300 A,  300 B. The MOS transistor switch  301  can be either of the MOS transistor switches  206 ,  208  shown in  FIG. 2 . The components of the driver with RDS ON  correction  300 A,  300 B will be discussed first, followed by how the driver with RDS ON  correction  300 A,  300 B calculates the on-state resistance (RDS ON ) of MOS transistor switch  301  from which the current through the MOS transistor switch  301  can be derived. 
     Components of the Driver with RDS ON  Correction 
     The driver with RDS ON  correction  300 A,  300 B includes an on-chip temperature sensor  302 . The on-chip temperature sensor  302  is configured so that the on-chip temperature sensor  302  outputs an analog voltage proportional to a temperature that is sufficiently similar to the temperature of the MOS transistor switch  301 . The temperature measured by the on-chip temperature sensor is referred to herein as the measured temperature, the on-chip temperature sensor reading or the measured temperature signal. In exemplary embodiments, the on-chip temperature sensor  302  can be thermally coupled to the MOS transistor switch  301  for which the current will be measured. The analog voltage is converted to a digital voltage by an analog-to-digital (A/D) converter  304 . 
     The driver with RDS ON  correction  300 A,  300 B also includes a processing device  305  and the digital representation of the analog voltage is sent to a processing device  305 . The gate drive voltage  306  that is supplied to the gate driver  308  (which drives the gate of the MOS transistor switch  301 ) is also converted to a digital signal using an A/D converter  310  and sent to the processing device  305 . The gate driver  308  that drives the MOS transistor switch  301  is controlled by the PWM controller  202  as explained above in  FIG. 2 . The processing device  305  can include functions with software programs, firmware, or other computer readable instructions for carrying out various process tasks, calculations, and control functions used in the present disclosure and may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). 
     The driver with RDS ON  correction  300 A,  300 B also includes memory  318  for storing correction functions  322 ,  324 , as discussed below. The memory used in the present disclosure can be any appropriate tangible and non-transitory processor readable medium used for storage of processor readable instructions or data structures. Suitable memory can include, but is not limited to, fuses, zener zap diodes, or erasable or non-erasable programmable read-only memory. 
     The driver with RDS ON  correction  300 A,  300 B includes a test and trim interface  320  that is used to communicate between external testing equipment (not shown) and the internal circuitry of the driver with RDS ON  correction  300 A,  300 B. The test and trim interface  320  can also be used for conventional trims such as amplifier offsets for the gain amplifier  312  and the buffer  316 . 
     Furthermore, the driver with RDS ON  correction  300 A includes a gain amplifier  312  to amplify a measured RDS ON  voltage drop across the MOS transistor switch  301 . The output of the gain amplifier  312  is attenuated by an attenuating digital-to-analog (D/A) converter  314 , as determined by the processing device  305  and the voltage correction function  324 , in order to obtain a corrected RDS ON  voltage drop across the MOS transistor switch  301 , as explained in more detail below. In some embodiments, the signal from the attenuating D/A converter  314  can be buffered using a buffer  316  if the corrected RDS ON  voltage drop is used outside the driver with RDS ON  correction  300 A,  300 B. Moreover, in some embodiments, a sample and hold circuit element (not shown) can be added between the gain amplifier  312  and the attenuating D/A converter  314  or between D/A  314  and the buffer  316  if it is desirable to remember the measured RDS ON  voltage drop or the corrected RDS ON  voltage drop while the MOS transistor switch  301  is off. It will be appreciated by those of ordinary skill in the art that different arrangements of the amplifier  312  and the attenuating D/A converter  314  may be used; moreover, the amplifier  312  and the attenuating D/A converter  314  can be merged into a single variable gain amplifier  313  with the gain set to the correct output, as shown in  FIG. 3B . 
     Calibration and Operation of the Driver with RDS ON  Correction 
     As explained above, the current through a MOS transistor switch  301  can be accurately determined from its RDS ON  voltage drop if the on-state resistance (RDS ON ) is known. However, the RDS ON  is a function of the device characteristics, temperature and gate drive voltage (V gs )  306 . Therefore, the RDS ON  as a function of temperature must be calibrated. To do so, the driver with RDS ON  correction  300 A,  300 B can be calibrated as follows. 
     Temperature Sensor Gain and Offset Correction Function Calibration 
     In exemplary embodiments, the driver with RDS ON  correction  300 A,  300 B is tested at two or more temperatures, T 1  and T 2 . In exemplary embodiments, T 1  and T 2  are chosen at opposite ends of the working temperature range of the switching power supply  102  or the automatic test equipment (ATE) that is aiding the calibration, whichever is more restrictive. A wide separation in T 1  and T 2  may improve the accuracy of the driver with RDS ON  correction  300 A,  300 B. At T 1 , the on-chip temperature sensor  302  is read and stored in memory  318 , which will be referred to as T original . Moreover, the actual chip temperature as measured by the external testing equipment is stored in memory  318 , via the test and trim interface  320 . The actual chip temperature as measured by external testing equipment will be referred to as T actual . At T 2 , this process is repeated. Next, by comparing T original  and T actual  at both T 1  and T 2 , either the processing device  305  or external testing equipment can calculate a temperature sensor gain and offset correction function  322  for the on-chip temperature sensor  302  by fitting a regression function to the data. In some embodiments, the regression function can be linear; or, in some embodiments, the regression function can be higher order. After the temperature sensor gain and offset correction function  322  is determined, it can be stored in memory  318 . 
     Furthermore, in some embodiments, the temperature sensor gain and offset correction function  322  can be extrapolated outside T 1  and T 2 , so that a corrected temperature can be determined outside T 1  and T 2 . Thus, at any temperature, the processing device  305  can read a measured temperature from the on-chip temperature sensor  302  and apply a temperature gain factor to the measured temperature to yield a corrected temperature. 
     Correcting a Measured Temperature 
     This temperature sensor gain and offset correction function  322  can be retrieved from memory  318  and applied by processing device  305  to the measured temperature signal produced by the on-chip temperature sensor  302 , which will yield a corrected temperature, for any operation requiring an accurate chip temperature reading from the on-chip temperature sensor  302 . In some exemplary embodiments, the processing device  305  applies the temperature sensor gain and offset correction function  322  by multiplying the measured temperature signal produced by the on-chip temperature sensor  302  by a temperature gain factor to yield a product and adding a temperature offset factor to the product to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined according to the temperature sensor gain and offset correction function  322  and the measured temperature. In other exemplary embodiments, the processing device  305  applies the temperature sensor gain and offset correction function  322  by adding a temperature offset factor to the measured temperature to yield a sum and multiplying the sum by a temperature gain factor to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined according to the temperature sensor gain and offset correction function  322  and the measured temperature. That is, the order for which the temperature offset factor and the temperature gain factor are applied is determined according the temperature gain and offset correction function  322  and can be reversed. As an example, assume the on-chip temperature sensor  302  produces a measured temperature signal of 50 degrees Celsius. The processing device  305  will retrieve the temperature sensor gain and offset correction function  322  from memory  318  and determine at 50 degrees Celsius that the measured temperature needs to be multiplied by a 0.98 temperature gain factor to yield a product and a 4 degree temperature offset factor added to the product. After the temperature gain factor and the temperature offset factor are applied to the measured temperature (50 degrees Celsius), a corrected temperature of 53 degrees Celsius is rendered. 
     Voltage Correction Function Calibration 
     Furthermore, at T 1 , a calibrated current can be sent through the MOS transistor switch  301  and the resulting voltage read at the buffer  316 . In some embodiments, the calibrating current can be sent by external testing equipment. After which, the voltage correction function  324  at T 1  can be calculated (by either the processing device  305  or by external testing equipment) by comparing the voltage drop that should have been read at the buffer  316  (referred to as the theoretical RDS ON  voltage drop) and the voltage drop that was actually read at the buffer  316  (referred to as the measured RDS ON  voltage drop), as a function of the known calibration current that was passed through the MOS transistor switch  301 . This calculated voltage correction function  324  can be stored in memory  318  and can be applied by the attenuating D/A converter  314  so that the correct theoretical voltage is measured at buffer  316  at temperature T 1  Similarly, this process can be repeated at T 2  and the voltage correction function  324  at T 2  can be stored in memory  324 , as well. After the temperature sensor gain and offset correction function  322  and the voltage correction function  324  are calculated at T 1  and T 2 , either the processing device  305  or external testing equipment can extrapolate the voltage correction function  324  as a function of temperature, by fitting a regression function to the data points, and this voltage correction function  324  can be stored in memory  318 . Since the difference between the theoretical RDS ON  voltage drop and the measured RDS ON  voltage drop MOS transistor switch  301  is known at different temperatures when a calibrating current is passing through the MOS transistor switch  301 , the RDS ON    326  of the MOS transistor switch  301  can be calculated using Ohm&#39;s Law, V=I*R for a given temperature. The RDS ON    326  of the MOS transistor switch  301  can be stored in memory and can be used in the future to determine the current through the MOS transistor switch  301  when RDS ON  voltage drop of the MOS transistor switch  301  is measured, according to Ohm&#39;s Law. 
     In most embodiments, the MOS transistor switch  301  RDS ON  varies with the gate drive voltage  306  as well as temperature. As a result, the above calculations for the attenuation values  324  and RDS ON  correction  326  can also be done using different gate drive voltages  306 . In these embodiments, four or more calibration points can then be used in these embodiments to determine the attenuation values  324  from which the RDS ON  correction  326  can be derived. For example, a first calibration point can be at T 1  with a low drive supply voltage, a second calibration point can be at T 1  with a high drive supply voltage, a third calibration point can be at T 2  with a low drive supply voltage and a fourth calibration point can be at T 2  with a high drive supply voltage. The processing device  305  or external testing equipment can then interpolate the relationship between the temperature sensor gain and offset correction function  322  and the attenuation values  324  for both sets of conditions. In some embodiments, if the voltage dependency is small enough, a linear adjustment to RDS ON  versus the temperature dependent gate drive voltage  306  may suffice or ignored completely. 
     Similarly to the temperature sensor gain and offset correction function  322 , in some embodiments, the voltage correction function  324  can be extrapolated outside T 1  and T 2 , so that a corrected temperature and corrected RDS ON  voltage drop can be determined outside T 1  and T 2 . Thus, at any temperature, the processing device  305  can read a measured temperature from the on-chip temperature sensor  302  and apply a temperature gain factor to the measured temperature to yield a corrected temperature, retrieve the stored relationship between the corrected temperature and a gate drive voltage, and determine the proper attenuation to be applied by the attenuating D/A converter  314  to a measured RDS ON  voltage drop. 
     In some embodiments, the MOS transistor switch  301  can have a linear, 2 nd  order or piecewise linear relationship between RDS ON  and temperature. This can be determined by characterizing samples of the MOS transistor switch  301  over the desired temperature range. If the MOS transistor switch  301  does not have a linear RDS ON  versus temperature characteristic, then a 2 nd  order equation could be processed by the processing device  305 . In some embodiments, the MOS transistor switch  301  might be found to have a variable shape to its RDS ON  versus temperature curve. In these embodiments, a 3 rd  order or more necessary RDS ON  measurement can be made and a piecewise linear or variable 2 nd  order equation can be used. 
     Correcting a Measured RDS ON  Voltage Drop 
     At any temperature from T 1  to T 2 , processing device  305  can receive a measured temperature signal from the on-chip temperature sensor  302 , and calculate a corrected temperature from the measured temperature signal using the temperature sensor gain and offset correction function  322 , as described above. Then, processing device  305  can use the corrected temperature, retrieve the voltage correction function  324  and receive a gate drive voltage  306  to calculate a voltage correction factor using voltage correction function  324 . The voltage correction factor is the overall correction gain that is applied to a measured RDS ON  voltage drop yield a correction RDS ON  voltage drop. In exemplary embodiments, a corrected RDS ON  voltage drop can be calculated from the measured RDS ON  voltage drop by multiplying the measured RDS ON  voltage drop by the voltage correction factor to yield a corrected RDS ON  voltage drop, wherein the voltage correction factor is determined according to the voltage correction function  324 . The attenuating D/A converter  314  can be used to multiply the RDS ON  voltage drop measured by the gain amplifier  312  by the voltage correction factor. As a result, the corrected RDS ON  voltage drop is measured at the buffer  316  for any temperature between T 1  and T 2 . For example, assume the gate drive voltage  306  is 5 V and the corrected temperature is 50 degrees Celsius. The processing device  305  will retrieve the voltage correction function  322  from memory  318  and determine that at 50 degrees Celsius with a gate drive voltage of 5V, that the measured RDS ON  voltage drop needs to be multiplied by a 0.98 voltage correction factor. After this voltage correction factor is applied to the measured RDS ON  voltage drop of 50 mV, a corrected RDS ON  voltage drop of 49 mV is rendered. Using the corrected RDS ON  voltage drop, a current  328  through the MOS transistor switch  301  can be determined, as described above in paragraphs [0011] and [0026], for use in the various applications mentioned above. 
     Method for Sensing Current with RDS ON  Correction 
       FIG. 4  is a flow diagram of an exemplary method  400  for determining a current with RDS ON  correction. Method  400  comprises measuring an approximate temperature of a MOS transistor switch by a temperature sensor to yield a measured temperature (block  402 ). After which, method  400  further comprises calculating a corrected temperature from the measured temperature using a stored temperature sensor gain and offset correction function (block  404 ). In some exemplary embodiments, calculating a corrected temperature from the measured temperature comprises multiplying the measured temperature by a temperature gain factor to yield a product and adding a temperature offset factor to the product to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined using the stored temperature sensor gain and offset correction function and the measured temperature. In other exemplary embodiments, calculating a corrected temperature from the measured temperature comprises adding a temperature gain factor to the measured temperature to yield a sum and multiplying the sum by a temperature gain factor to measured to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined using the stored temperature sensor gain and offset correction function and the measured temperature. In exemplary embodiments, the temperature sensor gain and offset correction function can have some or all of the same characteristics as the temperature sensor gain and offset correction function  322  above. For example, the temperature sensor gain and offset correction function could have been determined using a regression fit function during a calibration process, as explained in  FIG. 5  below. Moreover, in some embodiments, the regression fit function is a linear regression fit function. 
     Method  400  further comprises measuring the gate drive voltage for the MOS transistor switch (block  406 ). As stated above, the gate drive voltage is a factor in calculating the RDS ON  of the MOS; therefore, the gate drive voltage for the MOS is measured and known. 
     Method  400  further comprises calculating a voltage correction factor using a stored voltage correction function, wherein the stored voltage correction function is a function of the corrected temperature and the gate drive voltage (block  408 ). In exemplary embodiments, the voltage correction function and the voltage correction factor can have some or all of the same characteristics as the voltage correction function  324  and the voltage correction factor discussed above. For example, the voltage correction function could have been determined using a regression fit function during a calibration process, as explained in  FIG. 6  below. Moreover, in some embodiments, the regression fit function is a linear regression fit function. 
     Method  400  further comprises measuring a RDS ON  voltage drop across the MOS transistor switch to yield a measured voltage drop (block  410 ). The RDS ON  voltage drop across the MOS transistor switch can be measured using any of the embodiments described above. For example, in some embodiments, an amplifier can be coupled to the MOS transistor switch to measure the RDS ON  voltage drop across the MOS transistor switch. 
     Method  400  further comprises calculating a current using the measured RDS ON  voltage drop and the voltage correction function (block  412 ). In exemplary embodiments, the current can be calculated by multiplying the measured RDS ON  voltage drop by the voltage correction function to yield a corrected RDS ON  voltage drop. The current can then be determined from the corrected RDS ON  voltage drop according to V=I*R and as described above in paragraphs [0011] and [0026] (block  412 ). As described above, this current can be used for many purposes in DC-to-DC voltage conversion, including, but not limited to control loop inputs, over current detection, multi-phase converter inductor current balance, output current reporting, and input current reporting. 
     Method for Determining a Temperature Sensor Gain and Offset Correction Function 
       FIG. 5  is a flow diagram of an exemplary method  500  for determining a temperature sensor gain and offset correction function. Method  500  comprises comparing a first on-chip temperature sensor reading of a MOS transistor switch with a first off-chip temperature sensor reading of a MOS transistor switch at a first temperature (block  502 ) and comparing a second on-chip temperature sensor reading of a MOS transistor switch with a second off-chip temperature sensor reading of a MOS transistor switch at a second temperature (block  504 ). The devices and circuit elements used to measure off-chip temperature and the on-chip temperature, respectively, can have some or all of the same characteristics as described above in  FIGS. 3A-3B . 
     Method  500  further comprises fitting a regression function that corresponds to the temperature sensor gain and offset correction function using at least the comparison of the first on-chip temperature sensor reading with the first off-chip temperature sensor reading and at least the comparison of the second on-chip temperature sensor reading with the second off-chip temperature sensor reading (block  506 ). In some embodiments, the regression fit function can be a linear regression function and in other embodiments the regression function can be higher order. After which, method  500  further comprises storing the regression function in an on-chip memory of a driver chip for the MOS transistor switch (block  508 ). 
     Method for Determining a Voltage Correction Function 
       FIG. 6  is a flow diagram of an exemplary method for determining a voltage correction function. Method  600  comprises comparing a first measured RDS ON  voltage drop across a MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a first theoretical RDS ON  voltage drop across the MOS transistor switch at a first temperature (block  602 ) and comparing a second measured RDS ON  voltage drop across the MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a second theoretical RDS ON  voltage drop across the MOS transistor switch at a second temperature (block  604 ). The devices and circuit elements used to produce the calibrating current and to measure the RDS ON  voltage drop across the MOS transistor switch, respectively, can have some or all of the same characteristics as described above. 
     Method  600  further comprises fitting a regression function that corresponds to the voltage correction function using at least the comparison of the first measured RDS ON  voltage drop with the first theoretical RDS ON  voltage drop and at least the comparison of the second measured RDS ON  voltage drop with the second theoretical RDS ON  voltage drop (block  606 ). In some embodiments, the regression fit function can be a linear regression function and in other embodiments the regression function can be higher order. 
     The above measurements can be repeated at different gate drive voltages at different temperatures to determine the relationship between the gate drive voltage, temperature and a RDS ON  measured voltage drop. More specifically, method  600  can include comparing a third measured RDS ON  voltage drop across the MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a third theoretical RDS ON  voltage drop across the MOS transistor switch at the first temperature, wherein the third theoretical RDS ON  voltage drop is different than the first theoretical RDS ON  voltage drop. Thereafter, the regression function can use the comparison of the first measured RDS ON  voltage drop with the first theoretical RDS ON  voltage drop, the comparison of the second measured RDS ON  voltage drop with the second theoretical RDS ON  voltage drop, and the comparison of the third measured RDS ON  voltage drop with the third theoretical RDS ON  voltage drop. While this is done at the first temperature, the same process can be completed at the second temperature as well with a fourth measured RDS ON  voltage drop and a fourth theoretical RDS ON  voltage drop. 
     After the voltage correction function is fitted, method  600  further comprises storing the regression function in an on-chip memory of a driver chip for the MOS transistor switch (block  608 ). 
     The present methods can be implemented by computer executable instructions, such as program modules or components, which are executed by at least one processor. Generally, program modules include routines, programs, objects, data components, data structures, algorithms, and the like, which perform particular tasks or implemented particular abstract data types. 
     Instructions for carrying out the various process tasks, calculations, and generation of other data used in operation of the methods described herein can be implemented in software, firmware, or other computer- or processor-readable instructions. These instructions are typically stored on any appropriate computer program product that includes a computer readable medium used for storage of computer readable instructions or data structures. Such a computer readable medium can be any available media that can be accessed by a general purpose or special purpose computer or processor, or any programming logic device. 
     Example Embodiments 
     Example 1 includes a method for sensing a current, the method comprising: measuring an approximate temperature of a MOS transistor switch by a temperature sensor to yield a measured temperature; calculating a corrected temperature from the measured temperature using a stored temperature sensor gain and offset correction function; measuring a gate drive voltage for the MOS transistor; calculating a voltage correction factor using a stored voltage correction function, wherein the stored voltage correction function is a function of the corrected temperature and the gate drive voltage; measuring a RDS ON  voltage drop across the MOS transistor switch to yield a measured RDS ON  voltage drop; and calculating the current using the measured RDS ON  voltage drop and the voltage correction factor. 
     Example 2 includes the method of Example 1, wherein calculating a corrected temperature from the measured temperature comprises multiplying the measured temperature by a temperature gain factor to yield a product and adding a temperature offset factor to the product to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined using the stored temperature sensor gain and offset correction function and the measured temperature. 
     Example 3 includes the method of any of Examples 1-2, wherein calculating a corrected temperature from the measured temperature comprises adding a temperature offset factor to the measured temperature to yield a sum and multiplying the sum by a temperature gain factor to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined using the stored temperature sensor gain and offset correction function and the measured temperature. 
     Example 4 includes the method of any of Examples 1-3, wherein the stored temperature sensor gain and offset correction function was determined using a regression fit function during a calibration process. 
     Example 5 includes the method of Example 4, wherein the regression fit function is a linear regression fit function. 
     Example 6 includes the method of any of Examples 1-5, wherein calculating the current comprises multiplying the measured RDS ON  voltage drop by the voltage correction factor to yield a corrected RDS ON  voltage drop and calculating the current from the corrected RDS ON  voltage drop using Ohm&#39;s Law. 
     Example 7 includes the method of any of Examples 1-6, wherein the stored voltage correction function was determined using a regression fit function during a calibration process. 
     Example 8 includes the method of Example 7, wherein the regression fit function is a linear regression fit function. 
     Example 9 includes a method for determining a temperature sensor gain and offset correction function, the method comprising: comparing a first on-chip temperature sensor reading of a MOS transistor switch with a first off-chip temperature sensor reading of the MOS transistor switch at a first temperature; comparing a second on-chip temperature sensor reading of the MOS transistor switch with a second off-chip temperature sensor reading of the MOS transistor switch at a second temperature; fitting a regression function that corresponds to the temperature sensor gain and offset correction function using at least the comparison of the first on-chip temperature sensor reading with the first off-chip temperature sensor reading and at least the comparison of the second on-chip temperature sensor reading with the second off-chip temperature sensor reading; and storing the regression function in an on-chip memory of a driver chip for the MOS transistor switch. 
     Example 10 includes the method of Example 9, wherein the regression fit function is a linear regression function. 
     Example 11 includes a method for determining a voltage correction function, the method comprising: comparing a first measured RDS ON  voltage drop across a MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a first theoretical RDS ON  voltage drop across the MOS transistor switch at a first temperature; comparing a second measured RDS ON  voltage drop across the MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a second theoretical RDS ON  voltage drop across the MOS transistor switch at a second temperature; fitting a regression function that corresponds to the voltage correction function using at least the comparison of the first measured RDS ON  voltage drop with the first theoretical RDS ON  voltage drop and at least the comparison of the second measured RDS ON  voltage drop with the second theoretical RDS ON  voltage drop; and storing the regression fit function in an on-chip memory of a driver chip for the MOS transistor switch. 
     Example 12 includes the method of Example 11, wherein the regression fit function is a linear regression function. 
     Example 13 includes the method of any of Examples 11-12, further comprising: comparing a third measured RDS ON  voltage drop across the MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a third theoretical RDS ON  voltage drop across the MOS transistor switch at the first temperature, wherein the third theoretical RDS ON  voltage drop is different than the first theoretical RDS ON  voltage drop; and wherein fitting the regression function uses at least the comparison of the first measured RDS ON  voltage drop with the first theoretical RDS ON  voltage drop, at least the comparison of the second measured RDS ON  voltage drop with the second theoretical RDS ON  voltage drop, and at least the comparison of the third measured RDS ON  voltage drop with the third theoretical RDS ON  voltage drop. 
     Example 14 includes the method of any of Examples 11-13, further comprising: comparing a fourth measured RDS ON  voltage drop across the MOS transistor switch when a calibrating current is passing through the MOS transistor switch with a fourth theoretical RDS ON  voltage drop across the MOS transistor switch at the second temperature, wherein the fourth theoretical RDS ON  voltage drop is different than the second theoretical RDS ON  voltage drop; and wherein fitting the regression function uses at least the comparison of the first measured RDS ON  voltage drop with the first theoretical RDS ON  voltage drop, at least the comparison of the second measured RDS ON  voltage drop with the second theoretical RDS ON  voltage drop, and at least the comparison of the fourth measured RDS ON  voltage drop with the fourth theoretical RDS ON  voltage drop. 
     Example 15 includes a current sensor comprising: a processing device; an on-chip temperature sensor, coupled to provide a measured temperature signal of a MOS transistor switch to the processing device; at least one circuit element coupled to the processing device and configured to: measure an RDS ON  voltage drop across the MOS transistor switch to yield a measured RDS ON  voltage drop, multiply the measured RDS ON  voltage drop by a voltage correction factor, responsive to the processing device to yield a corrected RDS ON  voltage drop and output the corrected RDS ON  voltage drop; a memory device coupled to the processing device and configured to store a temperature sensor gain and offset correction function and a voltage correction function; and wherein the processing device is configured to: receive a gate drive voltage for the MOS transistor switch; receive the measured temperature signal and calculate a corrected temperature from the measured temperature signal using the stored temperature sensor gain and offset correction function; and instruct the at least one circuit element to multiply the measured RDS ON  voltage drop by a voltage correction factor to yield the corrected RDS ON  voltage drop, wherein the voltage correction factor is determined using the stored voltage correction function; and calculate a current from the corrected RDS ON  voltage drop for use by the voltage regulator to regulate the voltage signal. 
     Example 16 includes the current sensor of Example 15, wherein the processing device is configured to calculate the corrected temperature from the measured temperature signal by multiplying the measured temperature signal by a temperature gain factor to yield a product and adding a temperature offset factor to the product to yield the corrected temperature, wherein the temperature gain factor is determined using the stored temperature sensor gain and offset correction function and the measured temperature. 
     Example 17 includes the current sensor of any of Examples 15-16, wherein the processing device is configured to calculate the corrected temperature from the measured temperature signal by adding a temperature offset factor to the measured temperature to yield a sum and multiplying the sum by a temperature gain factor to yield the corrected temperature, wherein the temperature gain factor and the temperature offset factor are determined using the stored temperature sensor gain and offset correction function and the measured temperature. 
     Example 18 includes the current sensor of any of Examples 15-17, wherein the at least one circuit element comprises single variable gain amplifier. 
     Example 19 includes the current sensor of any of Examples 15-18, wherein the at least one circuit element comprises: a gain amplifier coupled to the MOS transistor switch and configured to measure the RDS ON  voltage drop across the MOS transistor switch; and an attenuating digital-to-analog converter coupled to the gain amplifier and the processing device, wherein the attenuating digital-to analog converter is configured to attenuate the output of the gain amplifier to yield the corrected RDS ON  voltage drop, as instructed by the processing device. 
     Example 20 includes the current sensor of Example 19, further comprising a buffer, coupled to the attenuating digital-to-analog converter, and configured to buffer the corrected RDS ON  voltage drop. 
     Example 21 includes the current sensor of any of Examples 15-20, wherein the current sensor is included in a voltage regulator. 
     Example 22 includes a system comprising: a power source configured to provide a voltage signal; a voltage regulator configured to regulate the voltage signal from the power source and produce an output voltage signal, wherein the voltage regulator includes a MOS transistor switch used in producing the output voltage signal, and wherein the voltage regulator includes a calibrated current sensor used to sense current though the MOS transistor switch; and a load coupled to the voltage regulator to receive the output voltage signal; wherein the calibrated current sensor comprises: a processing device; an on-chip temperature sensor, coupled to provide a measured temperature signal of a MOS transistor switch to the processing device; at least one circuit element coupled to the processing device and configured to: measure an RDS ON  voltage drop across the MOS transistor switch to yield a measured RDS ON  voltage drop, multiply the measured RDS ON  voltage drop by a voltage correction factor according to the processing device&#39;s instructions to yield a corrected RDS ON  voltage drop and output the corrected RDS ON  voltage drop; a memory device coupled to the processing device and configured to store a temperature sensor gain and offset correction function and a voltage correction function; and wherein the processing device is configure to: receive a gate drive voltage for the MOS transistor switch; receive the measured temperature signal and calculate a corrected temperature from the measured temperature signal using the stored temperature sensor gain and offset correction function; instruct the at least one circuit element to multiply the measured RDS ON  voltage drop by a voltage correction factor to yield the corrected RDS ON  voltage drop, wherein the voltage correction factor is determined using the stored voltage correction function; and calculate the current from the corrected RDS ON  voltage drop for use by the voltage regulator to regulate the voltage signal. 
     Example 23 includes the system of Example 22, wherein the processing device is configured to calculate the corrected temperature from the measured temperature signal by multiplying the measured temperature signal by a temperature gain factor and adding a temperature offset factor to yield a corrected temperature, wherein the temperature gain factor is determined using the stored temperature sensor gain and offset correction function and the measured temperature. 
     Example 24 includes the system of any of Examples 22-23, wherein the at least one circuit element comprises single variable gain amplifier. 
     Example 25 includes the system of any of Examples 22-24, wherein the at least one circuit element comprises: a gain amplifier coupled to the MOS transistor switch and configured to measure the RDS ON  voltage drop across the MOS transistor switch; and an attenuating digital-to-analog converter coupled to the gain amplifier and the processing device, wherein the attenuating digital-to analog converter is configured to attenuate the output of the gain amplifier to yield the corrected RDS ON  voltage drop, as instructed by the processing device. 
     Example 26 includes the system of Example 25, further comprising a buffer coupled to the attenuating digital-to-analog converter, and configured to buffer the corrected RDS ON  voltage drop. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.