Current sensing with rdson correction

Abstract

Current sensing with RDS ON correction is disclosed. In an embodiment, a method comprises: 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 drop and the voltage correction factor.

What is claimed is: 
     
         1 . A method for operating a MOS transistor switch comprising:
 calculating a voltage correction factor as a function of a gate drive voltage of the MOS transistor switch;   measuring a RDS ON  voltage drop across the MOS transistor switch to yield a measured RDS ON  voltage drop; and   calculating a current of the MOS transistor switch using the measured RDS ON  voltage drop and the voltage correction factor.   
     
     
         2 . The method of  claim 1 , further 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; and   measuring the gate drive voltage for the MOS transistor,   wherein the stored voltage correction function is a further function of the corrected temperature.   
     
     
         3 . The method of  claim 2 , wherein the stored temperature sensor gain and offset correction function are determined using a regression fit function during a calibration process. 
     
     
         4 . The method of  claim 3 , wherein the regression fit function is a linear regression fit function. 
     
     
         5 . The method of  claim 1 , 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's Law. 
     
     
         6 . The method of  claim 1 , wherein the stored voltage correction function is determined using a regression fit function during a calibration process. 
     
     
         7 . The method of  claim 6 , wherein the regression fit function is a linear regression fit function. 
     
     
         8 . 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;   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; 
 instruct at least one circuit element to multiply a measured RDS ON  voltage drop by a voltage correction factor to yield a corrected RDS ON  voltage drop, wherein the voltage correction factor is determined using the stored voltage correction function; and 
 calculate a current of the MOS transistor switch from the corrected RDS ON  voltage drop for use by a voltage regulator to regulate a voltage signal. 
   
     
     
         9 . The current sensor of  claim 8 , wherein the at least one circuit element is configured to measure the RDS ON  voltage drop across the MOS transistor switch to yield the measured RDS ON  voltage drop. 
     
     
         10 . The current sensor of  claim 8 , 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. 
     
     
         11 . The current sensor of  claim 8 , 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. 
     
     
         12 . The current sensor of  claim 8 , wherein the at least one circuit element comprises single variable gain amplifier. 
     
     
         13 . The current sensor of  claim 8 , 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.   
     
     
         14 . The current sensor of  claim 13 , further comprising a buffer, coupled to the attenuating digital-to-analog converter, and configured to buffer the corrected RDS ON  voltage drop. 
     
     
         15 . The current sensor of  claim 8 , wherein the current sensor is included in the voltage regulator. 
     
     
         16 . 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, 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; 
 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; 
 instruct at least one circuit element to multiply a measured RDS ON  voltage drop by a voltage correction factor to yield a corrected RDS ON  voltage drop, wherein the voltage correction factor is determined using the stored voltage correction function; and 
 calculate a current of the MOS transistor switch from the corrected RDS ON  voltage drop for use by a voltage regulator to regulate a voltage signal.