Temperature sensor

Abstract

Temperature Sensor This description relates to a temperature sensor (2) comprising a first oscillator (LO1') and a second oscillator (LO2'), and a counter (COUNTER) of a number (P) of periods of the second oscillator (LO2') during a predetermined time of the first oscillator (LO1'). One of the first and second oscillators (LO1', LO2') comprises a resistive component (R) and has its frequency dependent linearly with the temperature by a coefficient that is a multiple of the inverse of a resistance value of the resistive component (R). The resistive component (R) is a switched capacitor (C) controlled at a frequency determined by the other of the first and second oscillators. Figure for the abbreviation: Fig. 2

Description

Title of the invention: Temperature sensor Technical field

[0001] The present description relates generally to electronic circuits, and, more particularly, to temperature sensors. Prior art

[0002] Many integrated circuits, for example on silicon, include a temperature sensor. This temperature sensor, or temperature measuring circuit, makes it possible to measure the temperature of the integrated circuit, for example to allow adjustment of the parameters of the integrated circuit, for example its supply voltage, as a function of the measured temperature.

[0003] A known type of temperature sensor uses a differential measurement of a time quantity, either between a time quantity constant as a function of temperature and a time quantity varying with temperature, or between two time quantities varying with temperature but with very different temperature coefficients. An example of a time quantity is the frequency of an oscillator.

[0004] However, known temperature sensors based on differential frequency measurement have drawbacks. Summary of the invention

[0005] There is a need to overcome all or part of the disadvantages of known integrated temperature sensors based on differential frequency measurement.

[0006] One embodiment overcomes all or part of the drawbacks of known integrated temperature sensors based on differential frequency measurement.

[0007] One embodiment provides a temperature sensor comprising: a first oscillator and a second oscillator; and a counter configured to count a number of periods of the second oscillator during a duration determined by a period of the first oscillator, wherein: one of the first and second oscillators is configured so that its frequency depends linearly on the temperature or on the inverse of the temperature according to a coefficient multiple of the inverse of a resistance value of a resistive component of said one of the first and second oscillators; said resistive component is implemented by a switched capacitor; and a control frequency of the switched capacitor is determined by the frequency of the other of the first and second oscillators.

[0008] According to one embodiment, said one of the first and second oscillators comprises: an oscillating circuit; and a current source configured to provide a bias current to the oscillating circuit, the current source being of the absolute temperature proportional type or of the absolute temperature complementary type.

[0009] According to one embodiment: the current source includes the resistive component; the resistive component is configured to convert a voltage into a first current; and the current source is configured so that the bias current is a copy of the first current up to a multiplicative factor.

[0010] According to one embodiment, the oscillating circuit is a ring oscillator.

[0011] According to one embodiment, the oscillating circuit is a relaxation oscillator including an RS flip-flop.

[0012] According to one embodiment, the oscillating circuit comprises the resistive component.

[0013] According to one embodiment: said oscillating circuit comprises at least a first inverter having its threshold voltage determining the frequency of said one of the first and second oscillators; and said one of the first and second oscillators further comprises: a second inverter identical to the first oscillator and having its output connected to its input, an error amplifier configured to provide a signal indicating a deviation between an output voltage of the second inverter and a reference threshold voltage, and a circuit configured to control a threshold voltage of the first and second inverters from the signal provided by the error amplifier such that the threshold voltage of the first and second inverters is equal to the reference voltage.

[0014] According to one embodiment, the sensor comprises a calibration circuit configured to: - receive from the counter the number P of periods of the second oscillator counted during the duration determined by the period of the first oscillator; - determine, when the calibration circuit receives a calibration request, and from a known calibration temperature and the number P, a constant coefficient J such that: a) P is equal to J times the calibration temperature if said one of the first and second oscillators is the first oscillator and has its frequency linearly dependent on the inverse of the temperature or if said one of the first and second oscillators is the second oscillator and has its frequency linearly dependent on the temperature; or b) P is equal to J times the inverse of the temperature if said one of the first and second oscillators is the second oscillator and has its frequency linearly dependent on the inverse of the temperature or if said one of the first and second oscillators is the first oscillator and has its frequency linearly dependent on the temperature.

[0015] According to one embodiment, the sensor comprises a calculation circuit configured to determine a temperature value from said counted number.

[0016] According to one embodiment, the sensor is configured so that the duration determined by the frequency of the first oscillator is at least 100 times greater than a period of the second oscillator.

[0017] According to one embodiment, the sensor is configured so that the control frequency of the switched capacitor is at least 10 times greater than the frequency of said one of the first and second oscillators.

[0018] According to one embodiment, each of the first and second oscillators is devoid of quartz. Brief description of the drawings

[0019] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0020] [Fig.l] represents, in the form of blocks, an example of a temperature sensor;

[0021] [Fig.2] represents, in the form of blocks, an embodiment of a temperature sensor;

[0022] [Fig.3] represents, in the form of blocks, an embodiment of an oscillator of the sensor of [Fig.2];

[0023] [Fig.4] represents an exemplary embodiment of a current source of the oscillator of [Fig.3];

[0024] [Fig.5] represents an exemplary embodiment of an oscillating circuit of the oscillator of [Fig.3]; and

[0025] [Fig.6] represents an example of a circuit of the oscillator of [Fig.3]. Description of the embodiments

[0026] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0027] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the various known integrated circuits in which a temperature sensor is integrated have not been detailed, the embodiments and variants described being compatible with these known integrated circuits.

[0028] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0029] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0030] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0031] [Fig.l] represents, in the form of blocks, an example of a temperature sensor 1, and more particularly of a temperature sensor 1 based on a differential frequency measurement.

[0032] The sensor 1 comprises a first oscillator LOI and a second oscillator LO2, as well as a counter COUNTER.

[0033] The counter COUNTER is configured to count, during a duration D determined by the period of the first oscillator LOI, that is to say by the period of an output signal of the oscillator LOI, a number P of periods of the second oscillator. The duration D corresponds to a multiple, preferably an integer, of the period of the oscillator LOI.

[0034] For example, the counter COUNTER comprises an input II configured to receive a signal sigl at a period determined by the period of the oscillator LOI, that is to say a signal sigl whose period is a multiple, preferably an integer, of the period of the output signal of the oscillator LOI. For example, the signal sigl is the output signal of the oscillator LOI.

[0035] For example, the counter COUNTER further comprises an input 12 configured to receive a number N, preferably an integer. The number N determines, with the period of the signal sigl, the counting duration D. Indeed, the counter COUNTER is configured so that the duration D is equal to N times Tl, with Tl the period of the signal sigl.

[0036] For example, the counter COUNTER further comprises an input CK configured to receive the output signal sig2 from the oscillator LO2.

[0037] For example, the counter COUNTER includes an output O configured to provide the number P.

[0038] At the end of a counting duration D, the number P is equal to N.F2 / F1 with Fl the frequency of the signal sigl, and F2 the frequency of the signal sig2.

[0039] At least one of the oscillators LOI and LO2 has its frequency which depends linearly on the temperature T or on the inverse of the temperature (i.e. 1 / T).

[0040] For example, when the frequency Fl of the signal sigl, therefore the frequency of the oscillator LOI, is independent of the temperature and the frequency F2 of the signal sig2 depends linearly on the temperature T according to the relation F2 = AT, with A a constant coefficient, then P = NAT / FL As the frequency Fl is constant with the temperature, that the number N is known and that the coefficient A is constant, P = BT with B = NA / FL By determining B, for example during a calibration step, the measured temperature T is directly equal to the ratio P / B.

[0041] For example, when the frequency Fl of the signal sigl, therefore the frequency of the oscillator LOI, is independent of the temperature and the frequency F2 of the signal sig2 depends linearly on the inverse of the temperature T according to the relation F2 = A / T, with A a constant coefficient, then P = NA / (F1.T). As the frequency Fl is constant with the temperature, the number N is known and the coefficient A is constant, P = B / T with B = NA / FL By determining B, for example during a calibration step, the measured temperature T is directly equal to the ratio B / P.

[0042] For example, when the frequency F2 of the signal sig2 is independent of the temperature and the frequency Fl of the signal sigl, therefore the frequency of the oscillator LOI, depends linearly on the temperature T according to the relation Fl = ET, with E a constant coefficient, then P = N.F2 / (ET). As the frequency F2 is constant with the temperature, the number N is known and the coefficient E is constant, P = B / T with B = N.F2 / E. By determining B, for example during a calibration step, the measured temperature T is directly equal to the ratio B / P.

[0043] For example, when the frequency F2 of the signal sig2 is independent of the temperature and the frequency Fl of the signal sigl, therefore the frequency of the oscillator LOI, depends linearly on the inverse of the temperature T according to the relation Fl = E / T, with E a constant coefficient, then P = N.F2.T / E. As the frequency F2 is constant with the temperature, the number N is known and the coefficient E is constant, P = BT with B = N.F2 / E. By determining B, for example during a calibration step, the measured temperature T is directly equal to the ratio P / B.

[0044] In the above examples, one of the oscillators must have its frequency independent of the temperature. The implementation of such an oscillator can be done using a quartz, but this is not desirable in order to limit the complexity, size and cost of the sensor 1. An oscillator providing a frequency independent of the temperature can also be implemented other than with a quartz, but at the cost of even greater complexity of the sensor 1.

[0045] Furthermore, even when using an oscillator which intrinsically has its frequency which does not depend on the temperature, in practice it is necessary to polarize this oscillator with a polarization circuit which generally includes one or more resistors. These resistors have resistance values ​​which depend on the temperature, which means that, in practice, the frequency of this oscillator will not be completely independent of the temperature. It is then necessary to take at least two calibration points to determine the relationship between the temperature T (or the inverse of the temperature 1 / T) and the number P counted.

[0046] In other examples, the frequencies F1 and F2 may both depend on the temperature. At least two calibration points are then necessary to be able to determine the measured temperature from the number P, which is not desirable.

[0047] [Fig.2] represents, in the form of blocks, an embodiment of a sensor of temperature 2 based on a differential measurement of the frequency of an oscillator.

[0048] The sensor 2 comprises, like the sensor 1, an oscillator LOI', an oscillator LO2' and the counter COUNTER. The counter COUNTER is, for example, identical to that described in relation to [Fig.l]. In particular, the counter COUNTER is configured to count the number P of periods of the oscillator LO2' during a duration D determined by the period of the oscillator LOI'.

[0049] The oscillator LOI' provides a periodic signal sig3 of frequency F3. The frequency F3 determines the counting duration D. More particularly, the duration D is equal to N*T1, with T1 the period of a signal sig1 having a frequency Fl determined by the frequency F3 of the signal sig3, therefore the frequency F3 of the oscillator LOI'. In other words, the frequency Fl of the signal sig1 is equal to 1 / Q times the frequency F3 of the signal sig3, with Q a constant number greater than or equal to 1, Q preferably being an integer. Thus, the duration D is equal to NQ / F3 and is therefore determined by the period T3 of the signal sig3. The signal sig3 and the number N are provided to the counter COUNTER.

[0050] For example, when the coefficient Q is strictly greater than 1, the sensor 2 comprises a DIVQ circuit configured to receive the signal sig3 and to provide the signal sigl with a frequency Fl equal to 1 / Q times the frequency F3 of the signal sig3. In other words, the DIVQ circuit is a frequency divider by Q configured to provide the signal sigl from the signal sig3.

[0051] As an alternative example, when the coefficient Q is equal to 1, the DIVQ circuit can be omitted, the signal sigl then being directly the signal sig3.

[0052] As an example, the numbers N and Q are constant numbers fixed once for the entire duration of operation of the sensor 2. As an alternative example, at least one of the numbers N and Q is a parameter which is fixed by the user, so that the user can modify the value of this number, for example during the duration of life of sensor 2 and / or depending on the application incorporating sensor 2.

[0053] The oscillator LO2' provides a periodic signal sig2 at the frequency F2. The signal sig2 is supplied to the counter COUNTER.

[0054] As an example, as in [Fig.l], the signal sigl is supplied to the input II of the counter COUNTER, the number N is supplied to the input 12 of the counter COUNTER, the signal sig2 is supplied to the input CK of the counter COUNTER and the output O of the counter supplies the number P.

[0055] In a similar manner to what has been indicated in relation to [Fig.l], at the end of each counting duration D, the number P is equal to N.F2 / F1, therefore to N'.F2 / F3 with N' equal to NQ

[0056] One of the oscillators LOI' and LO2' is configured so that its frequency depends linearly on the temperature T, or on the inverse of the temperature (1 / T). Furthermore, this oscillator LOI' or LO2' is configured so that its frequency is partly determined by the inverse of a resistance value Rv of a resistive component R of this oscillator. In other words, this oscillator is configured so that the coefficient linearly linking the temperature T (or the inverse of the temperature T) to its frequency is a multiple of 1 / Rv, with Rv the resistance value of the resistive component R.

[0057] In the exemplary embodiment of [Fig.2], the oscillator LO2' is configured so that its frequency F2 depends linearly on the temperature T, according to a constant coefficient multiple of 1 / Rv, the component R then being part of this oscillator LOI'.

[0058] Furthermore, the resistive component R is implemented by a switched capacitor. In other words, the component R is a switched capacitor. The switched capacitor R comprises a capacitive element C, a switch IT1 connected in series with the component C between terminals 200 and 202 of the resistive component R, and a switch IT2 connected in parallel with the element C. The switches IT1 and IT2 are controlled in phase opposition. The control frequency Fc of the switched capacitor R is determined by the frequency of the oscillator LOI' or LO2' which does not include the element R, i.e. by the frequency F3 of the oscillator LOI' in the example of [Fig.2]. In other words, the frequency Fc is equal to H times the frequency of the oscillator LOI' or LO2' not including the component R, that is to say H times the frequency F3 in the example of [Fig.l], H being a constant positive number.Preferably, H is chosen so that the frequency Fc is at least ten times greater than the frequency of the oscillator LOI' or LO2' in which the component R is implemented, i.e. so that H.F3 > 10.F2 in the example of [Fig.2].. .

[0059] Thus, in the example of [Fig.2], where F2 is equal to GT / Rv with G a constant coefficient, as the resistance value Rv of the resistive component R is equal to l / (Cv.Fc), with Cv the capacitance value of the capacitive element C, then F2 = GTCv.Fc = GTCv.F3.H.

[0060] Replacing F2 with its expression above in the equation P = N'.F2 / F3, it follows that P is equal to N'.GTCv.H. Since N', G, Cv and H are constants, P is equal to JT, with J a constant coefficient and equal to N'.G.Cv.H.

[0061] Advantageously, the coefficient J does not depend on the frequency F3 of the oscillator LOI'. The frequency F3 of the oscillator LOI' can then depend on the temperature, without this modifying the value of the coefficient J, and therefore without this distorting the measurement of T. Indeed, to determine T from P, it is then sufficient to know the coefficient J.

[0062] According to one embodiment, the oscillator LOI' or LO2' which does not comprise the component R also does not comprise a quartz, and, more generally, neither of the oscillators LOI' and LO2' comprises a quartz. This makes it possible to reduce the complexity, the size and the cost of the sensor 2 compared to a sensor 1 in which either the oscillator LOI or the oscillator LO2 comprises a quartz.

[0063] According to one embodiment, the oscillators LOI' and LO2' have the same structure or topology, which makes it possible to reduce the dependence of the sensor 2 on the supply voltage and / or on common mode noise. For example, the two oscillators LOI' and LO2' each comprise a ring oscillator, or each comprise a relaxation oscillator, for example a relaxation oscillator comprising an RS flip-flop.

[0064] According to one embodiment, the value of the coefficient J is determined during a calibration phase. For example, when a calibration is required, a number P is counted during a corresponding duration D, knowing the calibration temperature T1 at which the sensor 2 is located. The value of the coefficient J is then equal to P / T1 in the example of [Fig.2]. Advantageously, only one calibration point is required.

[0065] According to one embodiment, the calibration phase is implemented by a calibration circuit of the sensor 2 (not shown in [Fig.2]). This calibration circuit is configured to receive the number P counted by the counter COUNTER for a duration D, and to determine, when the calibration circuit receives a calibration request corresponding to this counting duration D, the coefficient J linking the number P to the known calibration temperature T1. This assumes that the calibration request received by the calibration circuit is sent while the sensor 2 is at the calibration temperature T1.For example, the calibration request is provided to the sensor 2 and its calibration circuit by an operator who has previously placed the sensor 2 at the calibration temperature TL. The value of the calibration temperature Tl may be known beforehand and stored in a memory of the calibration circuit, or be provided to the sensor 2 and its calibration circuit together with the ca- request. liberation.

[0066] According to one embodiment, the sensor 2 comprises a calculation circuit (not shown in [Fig.2]). The calculation circuit is configured to determine the value of the temperature T from the number P. More precisely, the calculation circuit is configured to determine the value of the temperature from the number P and the constant coefficient J connecting P to the temperature T.

[0067] Preferably, to obtain sufficient measurement accuracy, the duration D is chosen to be at least 100 times greater than the duration of a period of the oscillator LO2'.

[0068] Preferably, the control frequency Fc of the switched capacitor R is at least 10 times greater than the frequency of the oscillator comprising this switched capacitor R, namely the frequency F2 of the oscillator LO2' in the example of [Fig.2]. Thus, in the example of [Fig.2], preferably, Fc > 10.F2.

[0069] [Fig. 3] represents, in block form, an exemplary embodiment of an oscillator of the sensor of [Fig. 2]. More particularly, [Fig. 3] represents, in block form, an exemplary embodiment of the oscillator of the sensor 2 which comprises the switched capacitor R, namely the oscillator LO2', using the example of [Fig. 2]. However, the oscillator LOI' can be implemented in an identical manner in the case where the switched capacitor R is part of the oscillator LOI' rather than the oscillator LO2'.

[0070] The oscillator LO2' comprises an oscillating circuit OC, for example a relaxation oscillator which may comprise an RS flip-flop, or for example a ring oscillator. The circuit OC provides the output signal of the oscillator, i.e. the output signal sig2 of the oscillator LO2' in this example.

[0071] The oscillator LO2' further comprises a current source 300 configured to provide a bias (or supply) current Ibias to the oscillating circuit OC.

[0072] The current source 300 is of the CTAT (Complementary To Absolute Temperature) or PTAT (Proportional To Absolute Temperature) type. Thus, the current Ibias varies linearly with the temperature.

[0073] For example, the current source 300 has one terminal connected to a supply potential VDD and another terminal connected to a supply terminal of the circuit OC.

[0074] According to one embodiment, although this is not illustrated by [Fig.3], the switched capacitor R ([Fig.2]) is part of the oscillating circuit OC.

[0075] Alternatively, although not illustrated by [Fig. 3], the switched capacitor R ([Fig. 2]) is part of the current source 300. Preferably, in such an alternative, the resistive element R is configured to convert a voltage across terminals 200, 202 ([Fig. 2]) of the resistive element R into a first current, and the current source 300 is configured so that its current Ibias varies proportionally, for example linearly, with this first current. More precisely, when the source 300 is of the PTAT, respectively CTAT type, the component R is configured to convert a PTAT, respectively CTAT, voltage into a first PTAT, respectively CTAT current, and the source 300 is configured so that the current Ibias varies proportionally, for example linearly, with the first current.

[0076] [Fig. 4] represents an exemplary embodiment of the current source 300 of the oscillator of [Fig. 3]. In this exemplary embodiment, the current source 300 is of the PTAT type.

[0077] The current source 300 comprises two MOS (Metal Oxide Silicon) transistors T1 and T2 in series between a supply potential VDD and a reference potential GND, the transistor T1 being connected to the potential VDD and the transistor T2 being connected to the potential GND. In this example where the potential VDD is positive with respect to the potential GND, the transistor T1 is a P-channel MOS transistor, and the transistor T2 is an N-channel transistor. For example, the source of the transistor T1 is connected to the potential VDD, the drain of the transistor T1 is connected to the drain of the transistor T2, and the source of the transistor T2 is connected to the potential GND. In addition, the transistor T2 has its drain connected to its gate.

[0078] The current source 300 further comprises a resistive element and two MOS transistors T3 and T4 in series between the supply potential VDD and the reference potential GND, the transistor T3 being connected to the potential VDD and the transistor T4 being coupled to the potential GND by the resistive element. The series association of the transistors T1 and T2 is connected in parallel with the series association of the transistors T3 and T4 and the resistive element. In this example where the potential VDD is positive with respect to the potential GND, the transistor T3 is a P-channel MOS transistor, and the transistor T4 is an N-channel. For example, the source of the transistor T3 is connected to the potential VDD, the drain of the transistor T3 is connected to the drain of the transistor T4, and the source of the transistor T4 is coupled to the potential GND by the resistive element. In addition, the transistor T3 has its drain connected to its gate.

[0079] In addition, transistors T1 and T3 have their gates connected to each other, and transistors T2 and T4 have their gates connected to each other.

[0080] The resistive element coupling the transistor T4 to the potential GND is configured to convert a voltage across its terminals into a first current, and the current source 300 is configured so that its current Ibias copies this first current, that is to say so that the current Ibias is equal to the first current to within a multiplicative factor, this multiplicative factor being constant.

[0081] In this exemplary embodiment, the resistive component R is part of the current source 300, and corresponds to the resistive element coupling the transistor T4 to the GND potential.

[0082] In another example not illustrated where the resistive component R is part of the oscillating circuit OC, the resistive element coupling the transistor T4 to the potential GND is implemented by a resistor.

[0083] The current source 300 further comprises a MOS transistor T5, with a channel of the same type as that of the transistor T3. The transistor T5 has its gate connected to the gate of the transistor T3, its source connected to the potential VDD and its drain configured to supply the current Ibias.

[0084] In the example of [Fig.4], the current Ibias is equal to ((kT) / (Rv.q)).ln(n), with k the Boltzmann constant, T the temperature in Kelvin, q the charge of an electron, n the ratio between the widths of transistors T2 and T4 and Rv the resistance value of component R in ohms.

[0085] Thus, the Ibias current varies linearly with temperature.

[0086] Although an example of a PTAT type current source has been described above, the person skilled in the art will be able to provide other examples of PTAT type current sources, and, more generally, other PTAT or CTAT type current sources 300. Preferably, in these other examples of implementation of the current source 300, when the current source 300 comprises the resistive component R, this component R is configured to convert a voltage at its terminals 200, 202 into a first current and the source 300 is configured so that the current Ibias is a copy of the first current, that is to say is equal to the first current to within a multiplicative factor, this multiplicative factor being constant.

[0087] [Fig.5] represents an exemplary embodiment of the OC circuit of the oscillator of [Fig.3].

[0088] In this example, the OC circuit is a relaxation oscillator, and, more precisely, a relaxation oscillator comprising an RS flip-flop (referenced 500 in [Fig.5]).

[0089] In this example, the component R ([Fig.2]) is not part of the circuit OC, and is part of the current source 300, the latter being for example implemented in the manner illustrated in [Fig.4].

[0090] The OC circuit comprises a supply terminal 502 configured to receive the Ibias current.

[0091] The OC circuit comprises two MOS transistors T6 and T7 in series between the node 502 and a node 504 configured to receive the reference potential GND, and two MOS transistors T8 and T9 connected in series between the nodes 502 and 504, the transistors T6 and T8 being connected to the node 502 and the transistors T7 and T9 being connected to the node 504.

[0092] In this example where the potential VDD to which the source 300 supplying the current Ibias is connected ([Fig.2]) is positive with respect to the potential GND, the transistors T6 and T8 are P-channel and transistors T7 and T9 are N-channel.

[0093] For example, transistors T6 and T8 have their sources connected to node 502 and their drains connected to the drains of transistors T7 and T9 respectively, the latter having their sources connected to node 504.

[0094] The gates of transistors T6 and T7 are connected to an output Qb of flip-flop 500, and the gates of transistors T8 and T9 are connected to an output Q of flip-flop 500.

[0095] A capacitive element C1 couples the node 504 to a connection node 506 of the transistors T6 to the transistor T7, and a capacitive element C2 couples the node 504 to a connection node 508 of the transistors T8 to the transistor T9. Preferably, and although this is not essential, the capacitive elements C1 and C2 are identical, that is to say they have the same capacitance value.

[0096] The OC circuit further comprises an even number of series inverters coupling the node 506 to an input R of the flip-flop 500 and the same even number of series inverters coupling the node 506 to an input S of the flip-flop RS.

[0097] In the example of [Fig.5], the circuit OC comprises two inverters INV1 and INV2 in series coupling the node 506 to the input R, and two inverters INV3 and INV4 coupling the node 508 to the input S. The inverter INV1 has its input connected to the node 506, and the inverter INV3 has its input connected to the node 508. For example, at least the inverters INV1 and INV3 are identical, and, preferably, the inverters INV1 to INV4 are all identical.

[0098] The output signal sig2 of the oscillating circuit OC, therefore of the oscillator LO2' in this example, has a frequency F2 equal to Ibias / (2.Clv.Vth), with Civ the capacitance value of each of the capacitive elements Cl and C2, and Vth the switching threshold of the inverters INV1 and INV3.

[0099] Thus, in this example, because the current Ibias depends linearly on the temperature T and is partly determined by the value 1 / Rv, the frequency F2 will also depend linearly on the temperature T according to a constant coefficient multiple 1 / Rv. In other words, F2 is equal to GT / Rv with G a constant coefficient. As the value Rv is equal to l / (Cv.Fc), then F2 = GTCv.Fc, from which it follows that F2 = GTCv.F3.H from the fact that Fc = H.F3. By injecting the expression of F2 above the equation P = N'.F2 / F3, it follows that P is equal to N'.GTCv.H, therefore JT, with J a constant coefficient and equal to N'.G.Cv.H.

[0100] More particularly, in the case where the source 300 is implemented in the manner illustrated in relation to [Fig.4] and where the oscillator is implemented in the manner described in relation to [Fig.5], the current Ibias is equal to ((kT) / (Rv.q)).ln(n), therefore to ((kTCv.Fc) / q).ln(n) = ((kTCv.F3.H) / q).ln(n), and the frequency F2 is equal to Ibias / (2.Clv.Vth). It follows that F2 = (kTCv.F3.H.ln(n)) / (q.2.Clv.Vth). By replacing F2 by its expression above in the equation P = N'.F2 / F3, it follows that: P = (N'.kTCv.F3.H.ln(n)) / (q.2.Clv.Vth.F3), so P = (N'.kTCv.H.ln(n)) / (q.2.Clv.Vth), or again P= JT with J = (N'.k.Cv.H.ln(n)) / (q.2.Clv.Vth), the coefficient J being constant and independent of the temperature, at least to the first order.

[0101] An advantage of the particular example above is that the coefficient J does not depend on any resistance value which might vary with temperature.

[0102] Another advantage of this particular example is that the capacitance value ratio Cv / Clv is independent of temperature to the first order, because it is a ratio between values ​​which, if they vary with temperature T, have identical coefficients of variation with temperature.

[0103] Although the threshold Vth may depend on the temperature T, this dependence of the threshold Vth on the temperature is negligible compared to the dependence of the current Ibias on the temperature, and the threshold Vth can therefore be considered as constant.

[0104] To determine the coefficient J, a calibration phase as described previously in relation to [Fig.2] can be implemented, in particular, in the particular example above, to determine the value of the threshold Vth which can depend on the manufacturing process.

[0105] The particular example described above illustrates an embodiment in which the oscillator LO2' has its frequency F2 which depends linearly on T according to a constant coefficient multiple of 1 / Rv, and where the switched capacitor R of value Rv is implemented in the current source 300.

[0106] As an alternative example, the oscillator LO2' can be implemented in the manner described in relation to [Fig.3] and its circuit OC is a ring oscillator.

[0107] In other embodiments, the switched capacitance R is implemented in the OC circuit of the oscillator LO2', and the switched capacitance R of the current source 300 is replaced by a resistive component.

[0108] As indicated in relation to the particular example of [Fig.5], the threshold Vth of an inverter depends on the temperature T, but this dependence can be neglected.

[0109] More generally, in an oscillator of the type described in relation to [Fig.3], when the frequency of the oscillating circuit OC of this oscillator is at least partly determined by the threshold voltage Vth of one or more inverters of the oscillating circuit OC, the dependence of the threshold Vth on the temperature is considered to be negligible.

[0110] However, to improve the measurement accuracy of a sensor 2 in which the oscillator comprising the switched capacitor R is of the type described in relation to [Fig.3] and has its frequency at least partly determined by the threshold voltage Vth of one or more inverters of the oscillating circuit OC of this oscillator, a mode of rea The development plans to control the threshold voltage Vth of these inverters to make it constant and independent of temperature.

[0111] In such an embodiment, this oscillator is a circuit configured to maintain the voltage Vth constant and independent of the temperature, for example by modifying the power supply supplied to these inverters and / or by modulating the back gate voltage of the transistors implementing these inverters in SOI technology (from the English "Silicon On Insulator" - silicon on insulator).

[0112] [Fig.6] illustrates an example of such a circuit for controlling the voltage Vth of inverters of the oscillating circuit OC of an oscillator, for example inverters INV1 and INV3 of the oscillating circuit OC of [Fig.5].

[0113] The circuit, referenced 700 in [Fig.6], comprises an inverter INVref identical to the inverter or inverters whose threshold Vth is controlled by the circuit 700.

[0114] The inverter INVref has its output connected to its input, from which it follows that its output is placed at the threshold voltage Vth of the inverter INVref.

[0115] The circuit 700 further comprises an error amplifier ERR configured to provide a signal Vth_CTRL representative of the error, or difference, between the output voltage of the inverter INVref, equal to Vth, and a reference or setpoint voltage Vref.

[0116] The circuit 700 further comprises a CTRL circuit configured to provide a signal fb from the signal Vth_CTRL. The ERR circuit is configured to control the threshold voltage of the inverter INVref and of the inverters, for example INV1 and INV3, of the circuit OC, so that the output voltage of the inverter INVref, therefore the threshold voltage Vth of this inverter INVref and of the inverters of the circuit OC having their voltage Vth controlled by the circuit 700, is equal to the voltage Vref.

[0117] Thus, the circuit 700 makes it possible to control the threshold voltage of the inverters that it controls to the reference voltage Vref.

[0118] For example, when the inverters are implemented by MOS transistors on SOI, the signal fb is a control signal of the back gate of these transistors.

[0119] Exemplary embodiments have been described above in which the oscillator LO2' has its frequency F2 which is linearly dependent on the temperature and comprises the switched capacitor R controlled from the frequency F3 of the oscillator LOI'.

[0120] The person skilled in the art is able to foresee, from the description given above, embodiments in which the oscillator LO2' has its frequency F2 which depends linearly on the inverse of the temperature, i.e. on 1 / T, and comprises the switched capacitor R controlled from the frequency F3 of the oscillator LOI'. In this case, the frequency F2 is equal to G / (Rv.T) with G a constant coefficient and the value Rv of the resistive component R is equal to l / (Cv.Fc), therefore to l / (Cv.H.F3) with H a constant coefficient, from which it follows that F2 = (G.Cv.F3.H) / T. By replacing F2 by its expression above in the equation P = N'.F2 / F3, it follows that P is equal to (N'.G.Cv.H) / T, therefore to J / T, with J a constant coefficient and equal to N'.G.Cv.H. Advantageously, the coefficient J then does not depend on the frequency F3 of the oscillator LO2'. To determine T from P, it is then sufficient to know the coefficient J, for example by determining it during a calibration phase as described in relation to [Fig.2], this calibration phase being for example implemented by the calibration circuit described in relation to [Fig.2].

[0121] In these embodiments, when the frequency of the oscillator LO2', therefore the coefficient J, depends on the threshold voltage Vth of one or more inverters of the oscillator LO2', the person skilled in the art will know how to implement a control of the threshold voltage Vth of this or these inverters, for example by means of the circuit 700 described in relation to [Fig.6], so that this voltage Vth is constant and independent of the temperature.

[0122] Exemplary embodiments have been described above in which it is the oscillator LO2' which has its frequency F2 which depends linearly on the temperature or on the inverse of the temperature, and comprises the switched capacitor R controlled from the frequency F3 of the oscillator LOI'.

[0123] The person skilled in the art is able to foresee, from the description given above, embodiments in which the oscillator LOI' has its frequency F3 which depends linearly on the temperature or the inverse of the temperature, and comprises the switched capacitor R, this switched capacitor then being controlled from the frequency F2 of the oscillator LO2'. In this case, the oscillator LOI' is for example implemented in the manner described in relation to [Fig.3].

[0124] For example, the person skilled in the art is able to foresee, from the description given above, embodiments in which the oscillator LOI' has its frequency F3 which depends linearly on the temperature T and comprises the switched capacitor R controlled from the frequency F2 of the oscillator LO2'. In this case, the frequency F3 is equal to GT / Rv, with G a constant coefficient, and the value Rv of the switched capacitor R is equal to 1 / Cv.Fc, with Fc = H.F2 and H a ​​constant coefficient. It follows that F3 = G.Cv.H.F2.T. By replacing F3 by its expression above in the equation P = N'.F2 / F3, it follows that P is equal to (N'.F2) / (G.Cv.H.F2.T), therefore to J / T with J a constant coefficient and equal to N7(G.Cv.H). Advantageously, the coefficient J then does not depend on the frequency F2 of the oscillator LO2', which can vary with the temperature.To determine T from P, it is then sufficient to know the coefficient J, for example by determining it during a calibration phase as described in relation to [Fig.2], this calibration phase being for example implemented by the calibration circuit described in relation to [Fig.2].

[0125] As an alternative example, the person skilled in the art is able to predict, from the description given above, embodiments in which the oscillator L02' has its frequency F3 which depends linearly on the inverse of the temperature, 1 / T, and includes the switched capacitor R controlled from the frequency F2 of the oscillator LO2'. In this case, the frequency F3 is equal to G / (Rv.T), with G a constant coefficient, and the value Rv of the switched capacitor R is equal to 1 / Cv.Fc, with Fc = H.F2 and H a ​​constant coefficient. It follows that F3 = (G.Cv.H.F2) / T. Replacing F3 with its expression above in the equation P = N'.F2 / F3, it follows that P is equal to (N'.F2.T) / (G.Cv.H.F2), therefore to JT with J a constant coefficient and equal to N7(G.Cv.H). Advantageously, the coefficient J then does not depend on the frequency F2 of the oscillator LO2', which can vary with temperature. To determine T from P, it is then sufficient to know the coefficient J, for example by determining it during a calibration phase as described in relation to [Fig.2], this calibration phase being for example implemented by the calibration circuit described in relation to [Fig.2].

[0126] In these embodiments, when the frequency of the oscillator LO2', therefore the coefficient J, depends on the threshold voltage Vth of one or more inverters of the oscillator LO2', the person skilled in the art will know how to implement a control of the threshold voltage Vth of this or these inverters, for example by means of the circuit 700 described in relation to [Fig.6], so that this voltage Vth is constant and independent of the temperature.

[0127] Thus, in all the examples of embodiments and variants described, the oscillator LOI' or LO2' which does not include the switched capacitor R and which has its frequency which depends linearly on T / Rv or on l / (T.Rv) can have its frequency which depends on the temperature without this distorting the temperature measurement, due to the fact that its frequency determines the control frequency Fc of the switched capacitor R which makes it possible to eliminate the influence of the frequency of this oscillator frequency in the linear relationship connecting P and the temperature T or the inverse of the temperature 1 / T.

[0128] Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will occur to those skilled in the art. In particular, the implementation of the oscillator LOI' or LO2' comprising the switched capacitor R is not limited to the structure described in relation to [Fig. 3], and even less to the examples of current sources 300 and oscillating circuits OC described in relation to FIGS. 3 to 5. Furthermore, when the oscillator LOI' or LO2' comprising the switched capacitor R has a structure of the type of that of [Fig. 3], those skilled in the art will be able to provide a source 300 of the CTAT type rather than PT AT as described in relation to [Fig. 4].In particular, the person skilled in the art will be able, in embodiments, to implement the source 300 such that it comprises the switched capacitor R to convert a voltage into a . current determining the Ibias current.

[0129] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. A temperature sensor (2) comprising: a first oscillator (LOI1) and a second oscillator (LO21); and a counter (COUNTER) configured to count a number (P) of periods of the second oscillator (LO21) during a duration determined by a period of the first oscillator (LOI1), wherein: one (LOI1; LO2') of the first and second oscillators (LOI1, LO2') is configured so that its frequency depends linearly on the temperature or the inverse of the temperature according to a coefficient multiple of the inverse of a resistance value of a resistive component (R) of said one (LOI1; LO2') of the first and second oscillators; said resistive component (R) is implemented by a switched capacitor; and a control frequency of the switched capacitor (R) is determined by the frequency of the other (LO21; LOI') of the first and second oscillators.

2. Sensor according to claim 1, wherein said one (LOI1; LO2') of the first and second oscillators (LOI1, LO2') comprises: an oscillating circuit (OC); and a current source (300) configured to provide a bias current (Ibias) to the oscillating circuit (C), the current source (300) being of the type proportional to the absolute temperature or of the type complementary to the absolute temperature.

3. The sensor of claim 2, wherein: the current source (300) comprises the resistive component (R); the resistive component (R) is configured to convert a voltage into a first current; and the current source (300) is configured so that the bias current (Ibias) is a copy of the first current up to a multiplicative factor.

4. A sensor according to claim 3, wherein the oscillating circuit (OC) is a ring oscillator.

5. Sensor according to claim 3, wherein the oscillating circuit (OC) is a relaxation oscillator comprising an RS flip-flop (500).

6. A sensor according to claim 2, wherein the oscillating circuit (OC) comprises the resistive component (R).

7. Sensor according to any one of claims 2 to 6, wherein: said oscillating circuit (OC) comprises at least one first inverter (INV1, INV3) having its threshold voltage determining the frequency of said one (LOI1; LO2') of the first and second oscillators (LOI1, LO2'); and said one (LOI1; LO2') of the first and second oscillators further comprises: a second inverter (INVref) identical to the first oscillator and having its output connected to its input, an error amplifier (ERR) configured to provide a signal (Vth_CTRL) indicating a deviation between an output voltage of the second inverter (INVref) and a reference threshold voltage (Vref), and a circuit (CTRL) configured to control a threshold voltage of the first and second inverters (INVref; INV1, INV3) from the signal (Vth_CTRL) provided by the error amplifier (ERR) so that the threshold voltage of the first and second inverters is equal to the reference voltage (Vref).

8. Sensor according to any one of claims 1 to 7, wherein the sensor comprises a calibration circuit configured to: - receive from the counter (COUNTER) the number P of periods of the second oscillator (LO2') counted during the duration determined by the period of the first oscillator (LOI1); - determine, when the calibration circuit receives a calibration request, and from a known calibration temperature and the number P, a constant coefficient J such that: a) P is equal to J times the calibration temperature if said one of the first and second oscillators (LOI1, LO2') is the first oscillator (LOI1) and has its frequency linearly dependent on the inverse of the temperature or if said one of the first and second oscillators is the second oscillator (LO21) and has its frequency linearly dependent on the temperature;or b) P is equal to J times the inverse of the temperature if said one of the first and second oscillators is the second oscillator (LO2') and has its frequency linearly dependent on the inverse of the temperature or if said one of the first and second oscillators is the first oscillator (LOI1) and has its frequency linearly dependent on the temperature.;

9. A sensor according to any one of claims 1 to 8, wherein the sensor comprises a calculation circuit configured to determine a temperature value from said counted number (P).

10. A sensor according to any one of claims 1 to 9, wherein the sensor is configured so that the duration determined by the frequency of the first oscillator is at least 100 times greater than a period of the second oscillator (LO21).

11. A sensor according to any one of claims 1 to 10, wherein the sensor is configured so that the control frequency of the switched capacitor (R) is at least 10 times greater than the frequency of said one of the first and second oscillators.

12. A sensor according to any one of claims 1 to 11, wherein each of the first and second oscillators is crystal-free.

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