System and method for temperature sensing

In accordance with an embodiment, a temperature sensor includes a proportional to absolute temperature (PTAT) current generator having a first current output configured to provide a first temperature dependent current, a first curvature compensation circuit configured to provide a first compensating current to an internal node of the PTAT current generator, and a second curvature compensation circuit configured to add a second compensating current to the first current output. The first compensating current has a first non-linearity with respect to temperature, a portion of the first non-linearity is present in the first temperature dependent current, the second compensating current includes a second non-linearity with respect to temperature, such that the second non-linearity in the second compensating current substantially cancels out the first non-linearity in the first temperature dependent current.

TECHNICAL FIELD

The present disclosure relates generally to an electronic device, and more particularly to a system and method for temperature sensing.

BACKGROUND

Temperature sensors are commonly used in a variety of applications including thermostats for homes and for industrial use, safety systems, automotive systems, as well as various self-monitoring electronic systems. For example, a temperature sensor may be included on a same die as other electronic circuitry in order to detect increases in ambient temperature. When a high temperature is detected using such a temperature sensor that exceeds a particular limit, the system may take protective action such as shutting down the entire system or portions of the system. Temperature sensors may be further included in integrated circuits, such as a CPU to provide the temperature information for the whole IC for the purpose of thermal management. This information may be used by the integrated circuit to adjust parameters to improve the performance of the circuit over a certain temperature range.

Temperature sensors may be constructed in a variety of ways. For example, a temperature sensor may be constructed using a bi-metallic strip using two metals having different thermal expansion coefficients. The mechanical deflection of such a bi-metallic strip serves as an indication of the temperature of the bi-metallic stip.

Another way to implement a temperature sensor is electronically using solid state circuitry. For example, the junction voltage of a diode, which has an almost linear temperature dependency with negative slope, may be used to provide a measure of temperature.

In another example, a voltage difference between two diodes having two current densities may also be used to measure temperature. A circuit that uses such a voltage difference is commonly referred to as a proportional to absolute temperature (PTAT) generator, and produces an output signal that has linear temperature dependency with positive slope. To provide a digital output which is related to absolute temperature, a reference voltage is compared to an output signal of the PTAT generator. This reference voltage is expected to be temperature independent, and is typically derived from the PTAT generator by combining a junction voltage of diode and a scaled voltage difference between the two diodes. However, since the junction voltage of diode has some nonlinearity with respect to temperature, the resulting reference voltage has a nonlinear temperature dependency that produces a nonlinearity at temperature sensor's digital output.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a temperature sensor includes a proportional to absolute temperature (PTAT) current generator having a first current output configured to provide a first temperature dependent current, a first curvature compensation circuit configured to provide a first compensating current to an internal node of the PTAT current generator, and a second curvature compensation circuit configured to add a second compensating current to the first current output. The first compensating current has a first non-linearity with respect to temperature, a portion of the first non-linearity is present in the first temperature dependent current, the second compensating current includes a second non-linearity with respect to temperature, such that the second non-linearity in the second compensating current substantially cancels out the first non-linearity in the first temperature dependent current.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to preferred embodiments in a specific context, namely a temperature sensing circuit. Embodiments of the present invention may also be applied toward various systems that utilize temperature sensing circuits, other sensing circuits, and circuits directed toward linearizing non-linear behavior.

In an embodiment, temperature sensor utilizes a curvature corrected a proportional to absolute temperature (PTAT) current generator to provide a current that is linearly related to temperature. In an embodiment, a first set of curvature correction currents are added to a core of the PTAT current generator in order to provide a curvature corrected bandgap voltage. An additional curvature correction current is applied to an output current of the PTAT generator to further cancel out the residual non-linearity of current v. temperature caused by the first set of curvature corrections currents. In some embodiments, the output of the PTAT current generator is applied to an analog to digital converter in order to provide a digital output value indicative of temperature.

Conventional temperature sensors may sense a temperature by measuring a PTAT voltage, which is proportional to a voltage difference between two diodes or two base-emitter junctions (ΔVbe) having different current densities. This PTAT voltage may be compared to a reference voltage that is generated using a bandgap voltage reference. Due to the nonlinear temperature dependency of the diode junction and/or base-emitter voltage (Vbe) of bipolar transistor, the bandgap voltage has a curvature over temperature.

One way to compensate the curvature of bandgap reference is to inject another signal with the same nonlinear temperature dependency as that of Vbe in order to cancel the nonlinear item of the bandgap voltage. Such compensation may be achieved by introducing one or few diodes or bipolar transistors biased with a temperature independent (TI) current. Consequently, the nonlinear item of Vbe is reproduced by combining the Vbe biased with PTAT current with the Vbe biased with the TI current, and then generating a nonlinear signal that is fed to the bandgap core. However, this additionally introduced nonlinear signal also appears in the PTAT signal (typically a ΔVbe related current) that is obtained from the bandgap circuit to indicate temperature information. As a result, this approach removes the curvature in the bandgap voltage, but introduces curvature in the temperature dependent PTAT signal.

FIG. 1illustrates temperature sensing system100according to an embodiment of the present invention in which a temperature measurement is made by measuring a ratio between a PTAT signal (proportional to ΔVbe) and a bandgap reference voltage. As shown, temperature sensing system100includes bandgap reference102having an output coupled to analog to digital converter104. In an embodiment, analog to digital converter104is implemented using a sigma-delta analog to digital converter. In alternative embodiments, other analog to digital converter architectures besides sigma-delta converter may be used.

Bandgap reference102includes bipolar transistors Q1and Q2, resistor R, operational amplifier112, and voltage controlled current mirror106. During operation, operational amplifier112forces the base-emitter voltage of bipolar transistor Q2to be about equal to the sum of the base-emitter voltage of bipolar transistor Q1and the voltage drop across resistor R. In one example, the output of operational amplifier112is coupled to the input of a source follower whose output determines the voltage across one or more resistors within a current mirror. In some embodiments, bipolar transistor Q1has a larger emitter area than bipolar transistor Q2and or current Iptthrough bipolar transistor Q1is smaller than current K1*Iptflowing through bipolar transistor Q2such that bipolar transistor Q1has a lower current density than bipolar transistor Q2. K1is a scaling factor between the current though bipolar transistor Q1and bipolar transistor Q2. The resulting current through bipolar transistor Q1is Ipt, and the resulting current through bipolar transistor Q2is K1*Ipt.

Voltage controlled current mirror106further produces temperature stable bandgap voltage Vgap. In an embodiment, the nonlinearity or curvature of the bandgap voltage Vgap with respect to temperature is compensated by summing compensation currents Inlfrom current sources116and K1Inlfrom current source114. Accordingly, voltage controlled current mirror106produces a temperature dependent current K2*Is=K2*(Ipt+Inl) that is proportional to a sum of current Iptthat flow through bipolar transistor Q1and compensation current Inl, where K2is a current mirror ratio. Because the non-linear compensation current term Inlis present in the current K2*(Ipt+Inl) produced by voltage controlled current mirror106, this current is non-linear with respect to temperature. In an embodiment, this non-linear current term Inlis compensated by subtracting current K2*Inlproduced by current source110, such that the resulting current is K2*Ipt. This subtraction operation is represented by summing junction108that maybe implemented using various methods of summing and subtracting signals known in the art.

FIG. 2illustrates a temperature sensor circuit200according to a further embodiment. As shown, temperature sensor circuit200includes a PTAT current generator that includes bipolar transistor Q1, bipolar transistor Q2, resistors R1, R2and R3, operational amplifier202and transistor M1. During operation, operational amplifier202forces the voltage at node A to be approximately equal to the voltage and node B, therefore causing a current that is proportional to temperature to flow through bipolar transistors Q1and Q2. The voltage at node B is the base-emitter voltage of bipolar transistor Q2and is complementary to absolute temperature (CTAT), whereas the voltage across resistor R3is proportional to temperature because a PTAT current flows through it. In some embodiments, the various components are sized and selected such that voltage Vgap is substantially constant over temperature. It should be noted that the various components may be sized such that the current through bipolar transistor Q1is not equal to the current through bipolar transistor Q1. This difference between currents, as expressed as factor K1above with respect toFIG. 1, may be determined by the ratio between resistors R2and R3, and the factor K2discussed above may be expressed as 1+K1in some embodiments.

In an embodiment, the curvature over temperature with respect to bandgap voltage Vgap may be compensated using temperature independent (TI) current source206, bipolar transistor Q3and resistors R4and R5. In some embodiment, TI current source206may be implemented by applying bandgap voltage Vgap or a replica thereof to a resistor that tracks resistors R1, R2, R3, R4and R5over process. As shown, bipolar transistor Q3is biased using TI current source206. Accordingly, the base emitter voltages of bipolar transistors Q2and Q3may be expressed as:

Vbe⁢⁢2=Vg⁢⁢0-(Vg⁢⁢0-Vbe⁢⁢2Tr)⁢TTr-(η-1)⁢VT⁢ln⁢TTrVbe⁢⁢3=Vg⁢⁢0-(Vg⁢⁢0-Vbe⁢3⁢⁢_⁢⁢Tr)⁢TTr-η⁢⁢VT⁢ln⁢TTr.
where T is the device temperature, Tris a reference temperature, Vbe2is the base emitter voltage of bipolar transistor Q2, Vbe2Tris the base emitter voltage of bipolar transistor Q2at the reference temperature Tr, Vbe3is the base emitter voltage of bipolar transistor Q3, Vbe3Tris the base emitter voltage of bipolar transistor Q3at the reference temperature Tr, VTis the thermal voltage, η is a factor determined by IC technology, and Vg0is the bandgap voltage of silicon. The voltage across resistors R4and R5may be expressed as:

Δ⁢⁢Vbe⁢⁢23=(Vbe⁢⁢2⁢_⁢⁢Tr-Vbe⁢⁢3⁢_⁢⁢Tr)⁢TTr+VT⁢ln⁢TTr.
Therefore, the nonlinearity of Vbe2is reproduced in the current through R4and R5. By appropriately choosing the value of the resistors, the curvature of Vgap originating from the curvature of Vbe2may be reduced and/or removed. In one embodiment, the following component value ratios may be used:
R4/R5=R2/R3, and
R2/R4=η−1.

Accordingly, the current though resistors R4and R5provide curvature compensation to bandgap voltage Vgap. It should be appreciated that other component value ratios may be used in alternative embodiments of the present invention. In alternative embodiments of the present invention, bipolar transistors Q1, Q2, and Q3, may be implemented using diodes and or other devices that have a p-n semiconductor junction.

In an embodiment, nonlinearities in the curvature correction current present in the temperature dependent current at the output of transistor M1may be compensated by performing a voltage to current conversion of the voltage between nodes B and C and summing the resulting current to the drain of transistor M1. As shown, a voltage-to-current (VI) converter implemented by transconductance amplifier204converts ΔVbe23into a current and sums this current with the output of the PTAT generator at the drain of transistor M1. In one embodiment, transconductance amplifier204has a transconductance of about 1/R4+1/R5. Thus, the output current is about the sum of the currents through R4and R5. As a result, the final output current K2*Iptis the sum of the current through Q1and Q2, which are purely PTAT.

In an embodiment, further temperature measurement accuracy may be achieved by using chopping in the transconductance amplifier and/or in the operational amplifier to remove its DC offset as shown inFIG. 3athat illustrates temperature sensor circuit300according to a further embodiment of the present invention. Temperature sensor circuit300is similar to temperature sensor circuit200to shown inFIG. 2with the exception that operational amplifier302is implemented as a chopper stabilized amplifier and chopping is implemented for transconductance amplifier304. Chopper stabilized operational amplifier302may be implemented using chopper stabilized amplifier circuits known in the art.

In an embodiment, sample and hold circuit308is used to reduce ripple in bandgap voltage Vgap. As shown, a similar sample and hold circuit is not shown at the output of chopper stabilized transconductance amplifier304. Sample and hold circuit308may be implemented using sample and hold circuits known in the art.

FIG. 3billustrates a schematic of an embodiment chopped transconductance amplifier304that may be used, for example, to implement chopped transconductance amplifier304depicted inFIG. 3a. As shown, chopped transconductance amplifier304includes an input stage that includes NMOS transistors M14and M15coupled to current sources322and324and degeneration resistors R6and R7. Accuracy of the transconductance of the input stage is enhanced by placing each of NMOS transistors M14and M15in feedback with operational amplifiers318and316respectively. For example, the gain of operational amplifier318forces the source of NMOS transistor M14to match the voltage at the positive input terminal of operational amplifier318. Similarly, the gain of operational amplifier316forces the source of NMOS transistor M15to match the voltage at the positive input terminal of operational amplifier316. The drains of NMOS transistors M14and M15are coupled to a cascode current mirror that includes PMOS transistors M10, M11, M12, and M13.

An input chopper340is coupled between the input of chopped transconductance amplifier304and the positive inputs of operational amplifiers316and318, and an output chopping stage314is coupled between the drains of NMOS transistors M14and M15and the cascode current mirror. An additional chopping stage312is coupled between PMOS cascode transistors M12and M13and PMOS current mirror transistors M10and M11to ensure that the mismatch between the current mirror transistors M10and M11has negligible influence to the overall DC offset of transconductance amplifier304. It should be appreciated that the illustrated embodiment of chopped transconductance amplifier304is just one of many circuits that could be used to implement an embodiment transconductance amplifier. Alternatively, other transconductance amplifier circuits and topologies may be used.

FIG. 4illustrates a temperature measurement system400according to an embodiment of the present invention. Temperature measurement system400includes embodiment temperature sensor circuit401interfaced to sigma-delta analog to digital converter420. Embodiment temperature sensor circuit401may be implemented according to the various embodiment temperature sensor circuits described herein. As shown, temperature sensor circuit401provides bandgap voltage Vgap to a voltage to operational amplifier402, which in conjunction with transistor M2and resistor R6, converts bandgap voltage Vgap into a substantially temperature independent current TI. Operational amplifier402forces the voltage across resistor R6to be about bandgap voltage Vgap. Accordingly, current TI that flows though transistor M2may be expressed as Vgap/R6. This current is mirrored back to temperature sensor circuit401via current mirror404and is used by temperature sensor circuit401for curvature compensation as described herein. Current mirror404also mirrors current TI to form temperature independent current Irefthat is used as a reference current for sigma-delta analog to digital converter420. In various embodiments, the mirror ratio of current mirror404may be set to define the value of Irefso that a full scale of the output code corresponds to a particular temperature. To cancel the mismatch of the current mirror404, dynamic element matching can be implemented in the current mirror404as known in the art.

In an embodiment, sigma-delta analog to digital converter420is implemented using a first order modulator implemented using switch406operational amplifier408, comparator410and register412that is coupled to decimation filter414. Switch406effectively functions as a one-bit digital to analog converter, operational amplifier408and capacitor C functions as a continuous time integrator, comparator410functions as a one-bit quantizier. The output of register412determines whether switch406is opened or closed. In alternative embodiments of the present invention, the first order modulator may be implemented using discrete-time circuit. For example, the integrator may be implemented using a switched-capacitor integrated instead of a continuous time integrator. In some embodiments, other modulator architectures known in the art may be used and/or a higher order sigma delta modulator may be used and/or a multi-stage sigma delta modulator may be used.

In an embodiment clock signal fs is use to clock register412and decimation filter414. Decimation filter414may be implemented, for example, using a simple comb filter or sinc filter to process the bitstream produced by register412. Alternatively, other decimation filter structures known in the art may also be used.

In an embodiment, the mirror ratio of current mirror404is set to one. Since PTAT current K2*Iptfrom bandgap circuit is used as the input current of the sigma-delta modulator, the percentage of logic “1” in the bitstream is given by:

μ=R6⁢K2⁢IptVgap.
Accordingly, the average value produced by decimation filter414is propositional to the measured temperature, which may be, for example, an absolute ambient temperature.

FIG. 5illustrates a flowchart of an embodiment method500of sensing a temperature. In step502, a first temperature dependent current is generated using a PTAT current generator. Next, in step504a first compensating current having a first nonlinearity with respect to temperature is provided to the PTAT current generator. This first compensating current may be used, for example, to provide curvature correction for a bandgap voltage. In step506, a second compensating current having a second non-linearity with respect to temperature is provided to an output of the PTAT current generator. In an embodiment, providing this second compensating current includes summing the second compensating current with the first temperature dependent current to provide an output current, such that the second non-linearity in the second compensating current substantially cancels out the first non-linearity.

In accordance with an embodiment, a temperature sensor includes a proportional to absolute temperature (PTAT) current generator having a first current output configured to provide a first temperature dependent current, a first curvature compensation circuit configured to provide a first compensating current to an internal node of the PTAT current generator, and a second curvature compensation circuit configured to add a second compensating current to the first current output. The first compensating current has a first non-linearity with respect to temperature, a portion of the first non-linearity is present in the first temperature dependent current, the second compensating current includes a second non-linearity with respect to temperature, such that the second non-linearity in the second compensating current substantially cancels out the first non-linearity in the first temperature dependent current. The temperature sensor may also include an analog to digital converter having an input coupled to the first current output.

In an embodiment, the PTAT current generator includes a first current branch having a first diode coupled in series with a first resistor, a second current branch having a second diode, an amplifier having inputs coupled to the first and second branches and an output configured to provide current to the first and second branches. The output of the amplifier may be configured to provide a temperature stable bandgap voltage and/or the temperature sensor may further include a transistor coupled to the output of the amplifier, such that a source node of the transistor is coupled to the first current branch and the second current branch, and a drain node of the transistor is coupled to the first current output.

In an embodiment, the first curvature compensation circuit includes a current source coupled to a third diode at a compensation node, a second resistor coupled between the compensation node and the first current branch of the PTAT generator, and a third resistor coupled between the compensation node and second current branch of the PTAT generator. The second curvature compensation circuit may include a transconductance amplifier having a first input coupled to the compensation node and a second input coupled to one of the first current branch and the second current branch of the PTAT generator, and an output coupled to the first current output of the PTAT current generator.

In accordance with a further embodiment, a method of sensing a temperature includes generating a first temperature dependent current using a proportional to absolute temperature (PTAT) current generator, providing a first compensating current to an internal node of the PTAT current generator, and providing a second compensating current comprising a second non-linearity with respect to temperature. The first compensating current includes a first non-linearity with respect to temperature, a portion of the first non-linearity is present in the first temperature dependent current, and providing the second compensation current includes summing the second compensating current with the first temperature dependent current to provide an output current, such that the second non-linearity in the second compensating current substantially cancels out the first non-linearity. The method may further include performing an analog to digital conversion on the output current. Performing this analog to digital conversion may include, for example, using a sigma-delta modulator.

In an embodiment, generating the first temperature dependent current includes using a first amplifier having an output coupled to a first current branch and to a second current branch of the PTAT current generator. The first current branch includes a first diode coupled in series with a first resistor, and the second current branch comprises a second diode. Generating the first compensating current includes using a first curvature compensation circuit having a current source coupled to a third diode at a compensation node, a second resistor coupled between the compensation node and the first current branch of the PTAT generator, and a third resistor coupled between the compensation node and second current branch of the PTAT generator.

In an embodiment, generating the second compensating current includes using a second curvature compensation circuit with a transconductance amplifier having a first input coupled to the compensation node and a second input coupled to one of the first current branch and the second current branch of the PTAT generator, and an output coupled to an output of the PTAT current generator. The method may further include performing chopping on the first amplifier and the transconductance amplifier.

In accordance with a further embodiment, a temperature sensing circuit includes a first current branch having a series circuit having first diode coupled in series with a first resistor, a second current branch that has a second diode, an interface circuit configured to provide a current proportional to a sum of a first current in the first current branch and a second current in the second current branch, a third current branch comprising a third diode, a first amplifier having a first input node coupled to the series circuit, a second input node coupled to the second diode, and a first output node coupled to the first current branch and to the second current branch, and a second amplifier having a third input node coupled to the third diode, a fourth input node coupled to one of the first input node and the second input node of the first amplifier, and a second output node coupled to an output of the interface circuit. In an embodiment, the second amplifier is implemented using a transconductance amplifier. The temperature sensor may also include an analog to digital converter having an input coupled to an output of the interface circuit.

In an embodiment, the temperature sensing circuit further includes a second resistor coupled between the first input node of the first amplifier and the third diode, and a third resistor coupled between the second input node of the first amplifier and the third diode. The interface circuit may include a transistor having a first load path node coupled to the first current branch and the second current branch, and a second load path node coupled to the output of the second amplifier. In some embodiments, the interface circuit is implemented using a current mirror.

According to various embodiments, the first output node of the first amplifier is configured to provide a temperature stable bandgap voltage. The temperature sensing circuit may further include a voltage to current converter circuit having an input coupled to the temperature stable bandgap voltage and an output coupled to the third current branch. In some embodiments, a sample and hold circuit is coupled to an output of the first amplifier. Chopping may be applied to at least one of the first amplifier and the second amplifier.

In an embodiment, the first diode, the second diode and the third diode each comprise a diode connected transistor.

Advantages of some embodiments include the ability to provide an accurate temperature measurement that is linear with temperature. A further advantage of some embodiments includes the ability to generate a highly accurate and linear temperature measurement using simple hardware. In some embodiments, high accuracy may be achieved without using multiple A/D conversion and/or high order digital processing.