Inductor current measurement compensation for switching voltage regulators
A current estimation method for a switching voltage regulator that delivers current to a load through an inductor includes measuring a voltage across a capacitor of an RC current sense network coupled in parallel with the inductor, generating an estimate of the current through the inductor based on the voltage measured across the capacitor of the RC current sense network and adjusting the estimate of the current through the inductor by a compensation filter that compensates for variation in the inductance of the inductor as a function of at least one of temperature and current.
1. A method of current estimation for a switching voltage regulator that delivers current to a load through an inductor, the method comprising:
measuring a voltage across a capacitor of an RC current sense network coupled in parallel with the inductor;
generating a digitized current value which estimates the current through the inductor, based on the voltage measured across the capacitor of the RC current sense network and a specified DCR (non-ideal DC resistance) value for the inductor at nominal conditions; and
adjusting the digitized current value by a compensation filter that compensates for at least one of temperature and current variation of the specified DCR value and the inductance of the inductor.
2. The method of claim 1 , further comprising measuring a temperature of the switching voltage regulator, wherein the RC current sense network accounts for the DCR of the inductor and the compensation filter compensates for temperature variation between the specified DCR value and the inductance of the inductor as a function of the measured temperature, the compensation filter comprising a plurality of filter coefficients that are a function of the measured temperature.
3. The method of claim 2 , further comprising:
calculating a first one of the plurality of filter coefficients as a function of a time constant (τ C ) for the RC current sense network, a time constant (τ L ) for the inductor and a temperature parameter (Δ T ) that is a function of the difference between the measured temperature and a temperature at which the DCR value of the inductor is specified; and
calculating a second one of the plurality of filter coefficients as a function of τ L , Δ T and a sampling frequency at which the digitized current value is estimated.
4. The method of claim 3 , further comprising:
calculating Δ T =α(T−T 0 ) where α is a temperature coefficient of resistivity for the inductor, T is the measured temperature and T 0 is the temperature at which the DCR value of the inductor is specified.
5. The method of claim 2 , wherein the measured temperature is used to compensate for the temperature variation between the specified DCR value and the inductance of the inductor during transient load conditions.
6. The method of claim 2 , further comprising providing over-temperature protection for the switching voltage regulator based on the same measured temperature used to compensate for the temperature variation between the specified DCR value and the inductance of the inductor.
7. The method of claim 1 , wherein the RC current sense network accounts for the DCR of the inductor and the compensation filter compensates for a current-dependency of the inductance of the inductor, the compensation filter comprising a plurality of filter coefficients that are a function of the current-dependency of the inductance.
8. The method of claim 7 , further comprising:
calculating a first one of the plurality of filter coefficients as a function of a time constant (τ C ) for the RC current sense network and a time constant (Σ ΔI ) for the inductance of the inductor as a function of current through the inductor; and
calculating a second one of the plurality of filter coefficients as a function of τ ΔI and a sampling frequency at which the digitized current value is estimated generated.
9. The method of claim 8 , further comprising:
calculating τ ΔI =L(I)/DCR and L(I)=m L I+L b where L(I) is an estimate of the inductance as a function of current, DCR is the specified DCR value of the inductor, m L is a slope of decrease for the inductance as a function of current, I is current, and L b is an intercept of an extrapolated slope for the inductance versus current.
10. The method of claim 9 , wherein the compensation filter assumes a linear decrease m L .
11. The method of claim 9 , further comprising disabling the compensation filter over a range of the inductor current for which m L is relatively flat.
12. A controller for controlling operation of a switching voltage regulator that delivers current to a load through an inductor, the controller comprising:
an RC current sense network coupled in parallel with the inductor;
a current estimator operable to generate a digitized current value which estimates the current through the inductor based on a voltage measured across the capacitor of the RC current sense network and a specified DCR (non-ideal DC resistance) value for the inductor at nominal conditions; and
a compensation filter operable to adjust the digitized current value by compensating for at least one of temperature and current variation of the specified DCR value and the inductance of the inductor.
13. The controller of claim 12 , further comprising a temperature sensor operable to measure a temperature of the switching voltage regulator, wherein the RC current sense network accounts for the DCR of the inductor and the compensation filter is operable to compensate for temperature variation between the specified DCR value and the inductance of the inductor as a function of the measured temperature, the compensation filter comprising a plurality of filter coefficients that are a function of the measured temperature.
14. The controller of claim 13 , wherein:
a first one of the plurality of filter coefficients is calculated as a function of a tune constant (τ C ) for the RC current sense network, a time constant (τ L ) for the inductor and a temperature parameter (Δ T ) that is a function of the difference between the measured temperature and a temperature at which the DCR value of the inductor is specified; and
a second one of the plurality of filter coefficients is calculated as a function of τ L , Δ T and a sampling frequency at which the digitized current value is estimated.
15. The controller of claim 14 , wherein Δ T =α(T−T 0 ), α is a temperature coefficient of resistivity for the inductor, and T is the measured temperature and T 0 is the temperature at which the DCR value of the inductor is specified.
16. The controller of claim 13 , further comprising an over-temperature protection circuit operable to provide over-temperature protection for the switching voltage regulator based on the same measured temperature used to compensate for the temperature variation between the specified DCR value and the inductance of the inductor.
17. The controller of claim 13 , wherein a time constant of the RC current sense network does not match the time constant of the inductor and the specified DCR of the inductor.
18. The controller of claim 12 , wherein the RC current sense network accounts for the DCR of the inductor and the compensation filter is operable to compensate for a current-dependency of the inductance of the inductor, the compensation filter comprising a plurality of filter coefficients that are a function of the current-dependency of the inductance.
19. The controller of claim 18 , wherein:
a first one of the plurality of filter coefficients is calculated as a function of a time constant (τ C ) for the RC current sense network and a time constant (τ ΔI ) for the inductance of the inductor as a function of current through the inductor; and
a second one of the plurality of filter coefficients is calculated as a function of τ ΔI and a sampling frequency at which the digitized current value is estimated.
20. The controller of claim 19 , wherein τ ΔI =L(I)/DCR and L(I)=m L I+L b , L(I) is an estimate of the inductance as a function of current, DCR is the specified DCR value for the inductor, m L is a slope of decrease for the inductance as a function of current, I is current, and L b is an intercept of an extrapolated slope for the inductance versus current.
21. The controller of claim 20 , wherein the controller is further operable to disable the compensation filter over a range of the inductor current for which m L is relatively flat.
22. A multiphase switching voltage regulator, comprising:
a plurality of phases operable to deliver current to a load, each phase being connected to the load through an inductor;
an RC current sense network coupled in parallel with each of the inductors;
a current estimator operable to generate a digitized current value which estimates the individual currents through the inductors based on a voltage measured across the capacitors of the RC current sense networks and a specified DCR (non-ideal DC resistance) value for the inductors at nominal conditions, and generate a total digitized current value based of the digitized current values for the inductors; and
a compensation filter operable to adjust the total digitized current value by compensating for variation in at least one of temperature and current of the specified DCR value for the inductors and the inductance of the inductors.