Threshold temperature sensor comprising room temperature test means

[0001] The present invention relates to integrated temperature sensors, and more particularly, to a threshold temperature sensor.

[0002] Temperature sensors are frequently provided in integrated circuits to detect when the operating temperature limits provided by the manufacturer have been exceeded, for example 100 C. and 30 C. These temperature sensors deliver detection signals that change values when one of these temperatures is exceeded.

[0003] To detect these critical temperature thresholds, integrated temperature sensors use the properties of diodes or bipolar transistors to generate currents or voltages proportional to the temperature, as well as reference currents or voltages that do not vary or hardly vary with temperature. A comparison of the voltages or currents varying with temperature, and of the reference voltages or currents enables detection when critical temperature thresholds have been exceeded.

[0004] FIG. 1 represents an integrated temperature sensor 10 of the type mentioned above. The temperature sensor 10 delivers two signals S₁ and S₂ whose value depends on the temperature of the sensor, i.e., the temperature of the silicon wafer on which the sensor is integrated.

[0005] FIGS. 2A and 2B represent examples of signals S₁ and S₂. The signal S₁ is, for example, equal to 0 as long as a threshold T₁ has not been reached, for example 100 C., and goes to 1 when the threshold T₁ is exceeded. The signal S₂ is, for example, equal to 0 as long as a threshold T₂ has not been reached, for example 30 C., and goes to 1 when the threshold T₂ is exceeded. Because of errors in manufacture when integrating the sensor, the detection thresholds of the temperatures T₁ and T₂ can vary over a wide range of values. The error may be caused by dispersions of the manufacturing method when implanting the electronic components, and by variations in the supply voltage of the sensor.

[0006] Tolerance margins T₁-T₁ and T₂-T₂ are defined for which the temperature sensor is considered as valid. For a threshold T₁ of 100 C., the temperatures T₁ and T₁ are, for example, equal to 90 C. and 110 C., i.e., a tolerance margin of 10 C. For a threshold T₂ equal to 30 C., the temperatures T₂ and T₂ are, for example, equal to 40 C. and 20 C.

[0007] It is therefore advisable to check the detection thresholds of such a temperature sensor. Such a verification can, for example, take place during electrical testing of the integrated circuit where the sensor is incorporated. This enables one to make sure that the detection thresholds correspond to the temperatures T₁ and T₂, or at the very least, that they adhere to the tolerance margins T₁-T₁ and T₂-T₂.

[0008] Still, verification of the detection thresholds requires the silicon wafer to be heated to the temperatures T₁ and T₂, which are generally far apart from one another. One is very high and the other is very low in order to verify that the signals S₁ and S₂ change values. Moreover, verification that the detection thresholds adhere effectively to the tolerance margins T₁-T₁ and T₂-T₂. requires that that at least four test temperatures T₁, T₁, T₂, T₂ are provided. Thus, testing a temperature sensor in an integrated circuit is a rather complex and expensive procedure to implement.

[0009] In view of the foregoing background, an object of the present invention is to enable room temperature testing of the detection thresholds of a temperature sensor of the type mentioned above.

[0010] This and other objects, advantages and features in accordance with the present invention are met by an integrated temperature sensor delivering overshoot detection signals of temperature thresholds, and comprising means for detecting a first temperature threshold exhibiting a first detection threshold, and means for detecting a second temperature threshold exhibiting a second detection threshold.

[0011] The sensor may further comprise means for detecting a third temperature threshold between the first and second temperature thresholds. The third detection threshold may be linked with the first detection threshold so that a deviation of the first detection threshold causes a corresponding deviation of the third detection threshold. The sensor may also comprise means for detecting a fourth temperature threshold between the first and second temperature thresholds. The fourth detection threshold may be linked with the second detection threshold so that a deviation of the second detection threshold causes a corresponding deviation of the fourth detection threshold. The third and fourth temperature thresholds delineate a temperature window for testing the temperature sensor for detecting a deviation of the first and second detection thresholds.

[0012] The first temperature threshold may be greater than the second temperature threshold while the third temperature threshold is less than the fourth temperature threshold. The first detection threshold may be linked with the second detection threshold so that a deviation of the first detection threshold causes a corresponding deviation of the second detection threshold, and vice versa.

[0013] According to one embodiment, the sensor comprises means to deliver a first signal varying with temperature according to a first variation law, a second signal varying with temperature according to second variation law, a third signal varying with temperature according to a third variation law, and a fourth signal varying with temperature according to a fourth variation law so that a deviation of the first signal implies a corresponding deviation of the third signal as well as a deviation of the second signal implies a corresponding deviation of the fourth signal.

[0014] The sensor may also comprise means to deliver at least one reference signal that does not vary or hardly varies with temperature, and means to compare each of the signals varying with temperature with the reference signal. Detection signals are delivered when the first, second, third and fourth temperature thresholds have been exceeded.

[0015] According to one embodiment, the sensor comprises an input to receive a selection signal for a detection threshold. The sensor may also comprise means to select the first or the third signal varying with temperature according to the value of the selection signal, compare the signal selected with the reference signal and deliver an overshoot detection signal of the first or of the third temperature threshold. There is also means to select the second or the fourth signal varying with temperature according to the value of the selection signal, compare the signal selected with the reference signal, and deliver an overshoot detection signal of the second or of the fourth temperature threshold.

[0016] According to another embodiment, the sensor comprises an input to receive a selection signal of a detection threshold, means to deliver the first or the third signal varying with temperature according to the value of the selection signal, and means to deliver the second or the fourth signal varying with temperature according to the value of the selection signal. The temperature sensor further comprises means to compare the signal delivered with the reference signal and deliver an overshoot detection signal of the first or of the third temperature threshold, and means to compare the signal delivered with the reference signal and deliver an overshoot detection signal of the second or of the fourth temperature threshold.

[0017] According to one embodiment, the sensor comprises means to deliver a first reference signal that does not vary or hardly varies with temperature, a second reference signal that does not vary or hardly varies with temperature, a third reference signal that does not vary or hardly varies with temperature, a fourth reference signal that does not vary or hardly varies with temperature so that a deviation of the first signal implies a corresponding deviation of the third signal and a deviation of the second signal implies a corresponding deviation of the fourth signal. The sensor may also comprise means to deliver at least one signal varying with temperature, and means to compare each of the reference signals with the signal varying with temperature, and deliver overshoot detection signals of the first, second, third and fourth temperature thresholds.

[0018] According to another embodiment, the sensor comprises an input to receive a selection signal of a detection threshold, means to select the first or the third reference signal according to the value of the selection signal, compare the signal selected with the signal varying with temperature and deliver an overshoot detection signal of the first or of the third temperature threshold. The sensor may also comprise means to select the second or the fourth reference signal according to the value of the selection signal, compare the signal selected with the signal varying with temperature and deliver an overshoot detection signal of the second or of the fourth temperature threshold.

[0019] According to yet another embodiment, the sensor comprises an input to receive a selection signal of a detection threshold, means to deliver the first or the third reference signal according to the value of the selection signal, and means to deliver the second or the fourth reference signal according to the value of the selection signal. The sensor may also comprise means to compare the signal delivered with the signal varying with temperature and deliver an overshoot detection signal of the first or of the third temperature threshold, and means to compare the signal delivered with the signal varying with temperature, and deliver an overshoot detection signal of the second or of the fourth temperature threshold.

[0020] Two detection means exhibit detection thresholds linked when they exhibit at least one common electrical element intervening in the detection of the temperature thresholds, such as a current source, an electrical resistor or a transistor. The temperature window may be centered on a temperature on the order of 20 to 30 C. The first temperature threshold may be greater than 50 C., the second temperature threshold may be less than 0 C., and the third and fourth temperature thresholds may be between 0 C. and 50 C.

[0021] The present invention also concerns a method to test detection thresholds of an integrated temperature sensor delivering overshoot detection signals of at least two temperature thresholds. The temperature sensor exhibits a first detection threshold of a first temperature threshold, and a second detection threshold of a second threshold temperature. The method preferably comprises offsetting the first detection threshold to obtain a third detection threshold of a third temperature threshold between the first and the second temperature thresholds so that the third detection threshold is linked with the first detection threshold, and a deviation of the first detection threshold causes a corresponding deviation of the third detection threshold. The second detection threshold is preferably offset to obtain a fourth detection threshold of a fourth temperature threshold between the first and second temperature thresholds so that the fourth detection threshold is linked with the second detection threshold, and a deviation of the second detection threshold causes a corresponding deviation of the fourth detection threshold. The threshold detection signals delivered by the temperature sensor are observed while adhering to a temperature window between the third and fourth temperature thresholds.

[0022] The temperature sensor is laid out so that the first temperature threshold is greater than the second temperature threshold, and that the third temperature threshold is less than the fourth temperature threshold. The first and the second detection thresholds may be linked so that a deviation of the first detection threshold causes a corresponding deviation of the second detection threshold, and vice-versa.

[0023] The temperature sensor may be tested at a single temperature. The temperature window may be centered on a temperature on the order of 20 to 30 C. The first temperature threshold is greater than 50 C., the second threshold temperature is less than 0 C., and the third and fourth temperature thresholds are between 0 C. and 50 C.

[0024] According to one embodiment, the step of offsetting the first detection threshold towards the third detection threshold comprises providing, in the temperature sensor, detection means of the first temperature threshold exhibiting a first detection threshold, and detection means of the third temperature threshold exhibiting a third detection threshold linked with the first detection threshold so that a deviation of the first detection threshold causes a corresponding deviation of the third detection threshold. The detection means of the third temperature threshold are used during the test of the temperature sensor. The method may further include offsetting the second detection threshold towards the fourth detection threshold and comprises providing, in the temperature sensor, detection means of the second temperature threshold exhibiting a second detection threshold, and detection means of the fourth temperature threshold exhibiting a fourth detection threshold linked with the second detection threshold so that a deviation of the second detection threshold causes a corresponding deviation of the fourth detection threshold. The detection means of the fourth temperature threshold are used during the test of the temperature sensor.

[0025] The detection thresholds of two detection means may be linked while providing in the detection means at least one common electrical element intervening in the detection of the temperature thresholds, such as a current source, an electrical resistor or a transistor.

[0026] The temperature sensor preferably contains two multiplexed outputs driven by a threshold selection signal, delivering respectively an overshoot detection signal of the first threshold or of the third threshold, and an overshoot detection signal of the second threshold or of the fourth threshold. The step of offsetting a detection threshold comprises the modification of the value of at least one electrical parameter in the temperature sensor, such as a bias current or an electrical resistor.

[0027] These as well as other objects, characteristics and advantages of the present invention will be disclosed in more detail in the following description of the method of the invention and of diverse embodiments of temperature sensors according to the invention, non-limiting in relation to the appended figures in which:

[0028] FIG. 1 represents diagrammatically a threshold temperature sensor according to the prior art;

[0029] FIGS. 2A and 2B represent two detection signals delivered by the temperature sensor of FIG. 1;

[0030] FIG. 3 represents diagrammatically a first type of threshold temperature sensor according to the present invention;

[0031] FIG. 4 represents diagrammatically a second type of temperature sensor according to the present invention;

[0032] FIGS. 5A and 5B represent offset detection signals delivered by a threshold temperature sensor and illustrate a first implementation example of the method according to the present invention;

[0033] FIGS. 6A and 6B represent offset detection signals delivered by a temperature sensor and illustrate a second implementation example of the method according to the present invention;

[0034] FIGS. 7A and 7B represent offset detection signals delivered by a temperature sensor and illustrate a third implementation example of the method according to the present invention;

[0035] FIG. 8 represents a first embodiment of a temperature sensor of the type represented in FIG. 3;

[0036] FIG. 9 represents a first embodiment of a temperature sensor of the type represented in FIG. 4;

[0037] FIG. 10 represents a measuring circuit available in a temperature sensor according to one of FIG. 8 or 9;

[0038] FIG. 11 represents one embodiment variation of a measuring stage available in the measuring circuit of FIG. 10;

[0039] FIG. 12 represents another embodiment variation of a measuring stage available in the measuring circuit of FIG. 10;

[0040] FIG. 13 represents a second embodiment of a temperature sensor of the type represented in FIG. 3;

[0041] FIG. 14 represents one embodiment of a measuring stage available in the temperature sensor of FIG. 13;

[0042] FIG. 15 represents another embodiment of a measuring stage available in the temperature sensor of FIG. 13;

[0043] FIG. 16 represents one embodiment example of a bias circuit available in a temperature sensor according to the present invention;

[0044] FIG. 17 represents a third embodiment of a temperature sensor of the type represented in FIG. 3;

[0045] FIG. 18 represents a second embodiment of a temperature sensor of the type represented in FIG. 4;

[0046] FIG. 19 represents a fourth embodiment of a temperature sensor of the type represented in FIG. 3; and

[0047] FIG. 20 illustrates an application of a temperature sensor according to the present invention.

[0048] In the following description, detection threshold of a temperature X designates a detection threshold internal to a temperature sensor. This is provided to correspond to the detection of a temperature X. This signal is likely to exhibit a deviation and will no longer correspond to the temperature. For purposes of simplifying the following description, detection threshold X designates the detection threshold of the temperature X, and threshold X designates the detection threshold of the temperature X or the temperature threshold X, according to the context.

[0049] FIG. 3 represents diagrammatically an integrated temperature sensor 20 according to the invention. The temperature sensor exhibits outputs OUT1 and OUT2 which deliver conventionally a normal operating mode of the sensor, that is, two detection signals S₁ and S₂ as already described above and as represented in FIGS. 2A and 2B. Thus, the signal S₁ changes values and goes to 1, for example, when a threshold T₁ is exceeded, for example, a threshold equal to 100 C. The signal S₂ changes values and goes to 1, for example, when a threshold T₂ is exceeded, for example, a threshold equal to 30 C.

[0050] According to the invention, the temperature sensor 20 exhibits an input receiving a signal TST (TEST) for switching the temperature sensor to a special operating or test mode wherein the outputs OUT1, OUT2 no longer deliver the signals S₁ and S₂, but deliver signals S₃ and S₄ for respectively detecting offset temperature thresholds T₃ and T₄, which are close to one another and close to a test temperature T_t. The test temperature T_t is preferably the temperature of a test room, i.e., generally 25 C., assuming the integrated circuits to be tested are situated at this temperature.

[0051] According to one essential aspect of the invention, the detection threshold of the temperature T₃ is linked with the detection threshold of the temperature T₁, and the detection threshold of the temperature T₄ is linked with the detection threshold of the temperature T₂. This is so that a deviation of the detection threshold of the temperature T₁ is expressed by a corresponding deviation of the detection threshold of the temperature T₃, and a deviation of the detection threshold of the temperature T₂ is expressed by a corresponding deviation of the detection threshold of the temperature T₄.

[0052] Preferably, the detection thresholds of the temperatures T₁ and T₂ are also linked so that a deviation in the detection of any of the temperatures T₁ or T₂ implies a corresponding deviation in the detection of the other temperatures T₂ or T₁, T₃ and T₄.

[0053] It can be derived from the following that four linked detection thresholds enables testing of the detection thresholds of the temperatures T₁ and T₂ with a single test temperature. It will also appear in the following that detection thresholds which are linked in terms of deviation can be easily obtained by a temperature measuring means with common elements, notably current sources and/or voltage sources, resistors, transistors, etc.

[0054] FIGS. 5A and 5B, 6A and 6B, 7A and 7B illustrate three non-limiting implementation examples of the method according to the invention. The temperature thresholds T₁ and T₂ are respectively equal to 100 C. and 30 C., and the sensor is to be tested at a temperature T_t on the order of 25 C.

[0055] In FIGS. 5A and 5B, the signal S3 exhibits a detection threshold T₃ equal to 36 C. corresponding to the offset of the threshold T₁ towards the test temperature T_t. The threshold T₃ is selected so that a temperature range T₃-T₃ corresponding to a set tolerance margin, such as the range 26 C.-46 C. corresponding to a tolerance of 10, for example, is situated on the right-hand edge of the temperature T_t. Besides, the signal S₄ exhibits a detection threshold T₄ equal to 14 C. corresponding to an offset of the threshold T₂ towards the temperature T_t selected so that a temperature range T₄-T₄ corresponding to a set tolerance margin, for example 4 C.-24 C., is situated on the left-hand edge of the test temperature.

[0056] The advantages of the method according to the invention in this example are as follows. First, if the thresholds T₁ and T₃, respectively T₂ and T₄, are linked, but the thresholds T₁ and T₂ are not, a test sequence conducted at temperature T_t that includes observing the signals S₃ and S₄ (which must be equal to 0), enables detection of a left-hand offset of the threshold T₃ outside the tolerance margin (signal S₃ at 1) or a right-hand offset of the threshold T₄ outside the tolerance margin (signal S₄ at 1). If the thresholds T₁ and T₃, respectively T₂ and T₄, are linked, it can be determined that the threshold(s) T₁, T₂ of the sensor are not situated within the tolerance margin. Conversely, detecting a right-hand offset of the threshold T₃ or a left-hand offset of the threshold T₄ outside the tolerance margin calls for two other test temperatures, respectively T₄ (4 C.) and T₃ (46 C.), to delineate the test zone.

[0057] If, conversely, the thresholds T₁ and T₂ are linked, a single test sequence conducted at temperature T_t that includes observing the signals S₃ and S₄ (which must be equal to 0), enables one to simultaneously detect a left-hand or right-hand offset of the threshold T₃ outside the tolerance margin (signal S₃ at 1) or a right-hand offset of the threshold T₄ outside the tolerance margin (signal S₄ at 1). The test sequence at temperature T_t enables one to verify simultaneously that the thresholds T₁ and T₂ are situated within the tolerance margins T₁-T₁ and T₂-T₂. This can be understood easily in reference to FIGS. 2A, 2B, 5A and 5B while considering that the four tolerance margins T₁-T₁, T₂-T₂, T₃-T₃, T₄-T₄ offset together to the right or to the left (the detection thresholds T₁, T₂, T₃ and T₄ are linked). A collective offset, however small, will cause the value of one of the test signals S₃ or S₄ to change with respect to the expected value (here 0).

[0058] In FIGS. 6A and 6B, the detection thresholds offset T₃ and T₄ are interlaced. The signal S₃ exhibits a threshold T₃ equal to 14 C., corresponding to the offset of the left-hand threshold T₁ of the test temperature T_t. The signal S₄ exhibits a detection threshold T₄ equal to 36 C., corresponding to the offset of the right-hand threshold T₂ of the temperature T_t. The threshold T₃ is selected so that a temperature range T₃-T₃ corresponding to a set tolerance margin, such as the range 4 C.-24 C. corresponding to a tolerance of 10, for example, is situated on the left-hand edge of the temperature T_t. The threshold T₄ is selected so that the temperature range T₄-T₄ corresponding to the set tolerance margin, such as the range 26 C.-46 C., for example, is situated on the right-hand edge of the temperature T_t.

[0059] This implementation example offers the same advantages as described above. Another advantage due to the fact that the detection thresholds are interlaced is that the signals S₃ and S₄ are equal to 1 when the sensor is at the test temperature T_t, which enables one to verify that the means for switching the signals of the sensor operate correctly. Thus, if the detection thresholds T₁ and T₃, respectively T₂ and T₄ are linked, but the detection thresholds T₁ and T₂ are not, a single test sequence conducted at temperature T_t, which includes observing the values of the signals S₃ and S₄ (which must now be equal to 1), enables one to detect a right-hand offset of the threshold T₃ outside the tolerance margin (signal S₃ at 0) or a left-hand offset of the threshold T₄ outside the tolerance margin (signal S₄ to 0).

[0060] Conversely, the detection of a left-hand offset of the threshold T₃ and a right-hand offset of the threshold T₄ outside the tolerance margin calls for two other test temperatures to be provided, respectively T₃ (4 C.) and T₄ (46 C.). If conversely, the thresholds T₁ and T₂ are linked, a single test sequence conducted at temperature T_t enables detection of a left-hand or right-hand offset of the threshold T₃ outside the tolerance margin (signal S₃ at 0) or a right-hand offset of the threshold T₄ outside the tolerance margin (signal S₄ at 0) This enables one to verify in a single phase that the thresholds T₁ and T₂ lie within the tolerance margins T₁-T₁ and T₂-T₂.

[0061] In FIGS. 7A and 7B, the detection thresholds T₃ and T₄ are still interlaced but respectively exhibit an additional deviation to the left (towards negative temperatures) and to the right (towards positive temperatures). For example, T₃ is equal to 10 C. and T₄ equal to 40 C., the tolerance margin T₃-T₃ is equal to 0 C.-10 C. and the tolerance margin T₄-T₄ equal to 30 C.-50 C. Here is an additional degree of freedom with regards to the selection of the test temperature T_t, which can lie between the temperatures T₃ and T₄, i.e., between 20 C. and 30 C. The sensor can in such a case be tested with less control of the temperature of the test room, at a test temperature between 20 C. and 30 C. enabling one to verify simultaneously that the thresholds T₁ and T₂ lie within the tolerance margins T₁-T₁ and T₂-T₂ (while neglecting the tolerance margin T₃-T₄).

[0062] Diverse embodiment examples of temperature sensors according to the invention will now be described. It should be noted first that, although it is sufficient to implement the method of the invention, a temperature sensor may deliver the signals S₃ and S₄ instead of the signals S₁ and S₂ during the test phase. A sensor according to the invention can also exhibit two additional outputs to deliver the signals S₃ and S₄. Thus, the temperature sensor 30 represented in FIG. 4 exhibits two outputs OUT1 and OUT2 delivering the signals S₁ and S₂, and two outputs OUT3 and OUT4 delivering signals S₃ and S₄ having detection thresholds linked with those of the signals S₁ and S₂. The outputs OUT3, OUT4 and the signals S₃, S₄ are used during the test phases of the sensor, instead of the outputs OUT1, OUT2 and signals S₁, S₂. Such a use of the signals S₃, S4 instead of the signals S₁, S₂ is equivalent, in terms of the method, to a transformation of the signal S₁ into a signal S₃ and to a transformation of the signal S₂ into a signal S₄. Both embodiments correspond to an offset of the detection thresholds T₁ and T₂ to form the close detection thresholds T₃ and T₄.

[0063] FIG. 8 represents one embodiment example of a temperature sensor 21 of the type represented in FIG. 3, exhibiting two outputs OUT1, OUT2 delivering the signals S₁, S₂ or S₃, S₄ according to the value of the signal TST. The sensor 21 comprises a measuring circuit MC1 delivering four voltages VT₁, VT₂, VT₃, VT₄ varying in proportion to the temperature according to different coefficients of proportionality, as well as a reference voltage V_ref independent of the temperature. The voltages VT₁ and VT₃ are applied to the inputs of an analog multiplexer MX1 driven by the signal TST, and the voltages VT₂ and VT₄ are applied to the inputs of an analog multiplexer MX2 also driven by the signal TST. The output of the multiplexer MX1 delivers the voltage VT₁ or the voltage VT₃ according to the value of the signal TST, and is applied to the positive input of a comparator CP1 whose negative input receives the voltage V_ref and whose output forms the output OUT1 of the sensor. The output of the multiplexer MX2 delivers the voltage VT₂ or the voltage VT₄ according to the value of the signal TST, and is applied to the negative input of a comparator CP2 whose positive input receives the voltage V_ref and whose output forms the output OUT2 of the sensor.

[0064] FIG. 9 represents one embodiment example of a temperature sensor 31 of the type represented in FIG. 4, exhibiting four outputs OUT1, OUT2, OUT3, OUT₄ delivering respectively the signals S₁, S₂, S₃, S₄. The sensor 21 comprises the measuring circuit MC1 mentioned above and four comparators CP1, CP2, CP3, CP4. The comparator CP1 receives on its positive input the voltage VT₁, on its negative input the voltage V_ref, and delivers the signal S₁. The comparator CP2 receives on its negative input the voltage VT₂, on its positive input the voltage V_ref, and delivers the signal S₂. The comparator CP3 receives on its positive input the voltage VT₃, on its negative input the voltage V_ref, and delivers the signal S₃. The comparator CP4 receives on its negative input the voltage VT₄, on its positive input the voltage V_ref, and delivers the signal S₄.

[0065] FIG. 10 represents one embodiment example of the measuring circuit MC1. The latter comprises a bias circuit BC1, a reference stage RS1 delivering the voltage V_ref, and a measuring stage MS1 delivering the voltages VT₁, VT₂, VT₃ and VT₄. The circuit BC1 is powered by a voltage Vcc, and delivers a voltage V(I_t) that should be applied to the gate of a transistor PMOS in order to form a source of current imposing a current I_t proportional to the temperature T, of the type I_tT, with being a constant.

[0066] The reference stage RS1 comprises a transistor PMOS TP1 receiving the voltage Vcc on its source S and the voltage V(I_t) on its gate G. The drain D of this transistor is connected to the end of a resistor R₁ whose other end is connected to the emitter E of a bipolar transistor TB1 of the type PNP having its base and its collector connected to ground (diode). The voltage V_ref is drawn from the connection node of the drain D of the transistor TP1 and of the resistor R₁. The stage RS1 conducts through it the current It, and the resistor R₁ exhibits a value selected so that the voltage V_ref is independent of the temperature, wherein:

[0067] V_be(T) is the emitter-base voltage of the transistor TB1, whose variations in relation to the temperature are compensated for by the variations of the term R₁*T.

[0068] The stage MS1 comprises a transistor PMOS TP2 receiving on its source S the voltage Vcc and on its gate G the voltage V(I_t). The transistors TP2 and TP1 are thus laid out symmetrically for conducting current. The drain D of the transistor TP2 is connected to ground by five series resistors R₁₀, R₁₁, R₁₂, R₁₃, R₁₄. The cathode of the resistor R₁₄ is connected to ground. The voltage VT₁ is drawn from the anode of the resistor R₁₄, the voltage VT₂ is drawn from the anode of the resistor R₁₁, the voltage VT₃ is drawn from the anode of the resistor R₁₂, and the voltage VT₄ is drawn from the anode of the resistor R₁₃.

[0069] The detection thresholds of the temperatures T₁ to T₄ are linked, as readily understood by those skilled in art. A deviation of an operating point of the measuring circuit MC1 causes a corresponding deviation of the voltages VT₁, VT₂, VT₃ and VT₄ and consequently a deviation of detection thresholds. A deviation of the voltage V_ref has an identical effect on the detection thresholds by acting on the triggering thresholds of the comparators. A deviation of the operating point can be due to a variation of the voltage Vcc, a variation of the voltage V(I_t), a dispersion of the ratio W/L (gate width over length) of a transistor TP1 or TP2, a dispersion of the resistivity by square SQR of the material forming the resistors R₁, R₁₀ to R₁₄.

[0070] It should be noted that a deviation of a detection threshold T₁, T₂ causes a corresponding deviation of the offset detection threshold T₃, T₄ when the deviation is expressed as a percentage. Thus, for example, the detection threshold of the temperature T₁ that will be designated as ST₁ is defined by the following relation:

[0071] Similarly, the detection threshold of the temperature T₃ that will be designated as ST₃ is defined by the following relation:

[0072] The ratio between both thresholds is therefore:

[0073] If one of the parameters V_ref, , SQR varies and causes a deviation of the thresholds ST₁ and ST₃, the ratio between the thresholds remains the same. For example, a 5% deviation of ST₁ causes a 5% deviation of ST₃.

[0074] It has been suggested in the foregoing, for simplification purposes, to provide for the offset thresholds T₃ and T₄ tolerance margins which are of the same value as the tolerance margins of the useful thresholds T₁ and T₂. In order to take into account the observation above, it is possible to provide a contraction of the tolerance margins defined for the thresholds T₁ and T₂ when the tolerance margins of the thresholds T₃ and T₄ are determined. On the other hand, the term corresponding in the expression corresponding deviation which is used in this description and in the claims, must be construed in the light of the observation above.

[0075] FIG. 11 represents a variation MS2 of the measuring stage. The stage MS2 comprises two transistors PMOS TP2-1, TP2-2 receiving the voltage Vcc on their sources S and the voltage V(I_t) on their gates G. The drain D of the transistor TP2-1 is connected to the ground by three series resistors R₂₀, R₂₁, R₂₂. The cathode of the resistor R₂₂ is connected to ground. The voltage VT₁ is drawn from the anode of the resistor R₂₂ and the voltage VT₃ is drawn from the anode of the resistor R₂₁. The drain D of the transistor TP2-2 is connected to ground by three series resistors R₂₃, R₂₄, R₂₅. The cathode of the resistor R₂₅ is connected to ground. The voltage VT₂ is drawn from the anode of the resistor R₂₄ and the voltage VT₄ is drawn from the anode of the resistor R₂₅.

[0076] The detection thresholds of the temperatures T₁ to T₄ are also linked in this embodiment in case of a variation of the parameters V_ref, and SQR, unless there is an isolated error regarding the value of one of the resistors. There is little probability of the later happening. However, in such a case, the voltages VT₁ and VT₃ remain linked, and consequently the detection thresholds of the temperatures T₁ and T₃. The voltages VT₂ and VT₄ also remain linked, and consequently the temperature thresholds T₂ and T₄.

[0077] FIG. 12 represents another variation MS3 of the measuring stage. The stage MS3 comprises four transistors PMOS TP2-1, TP2-2, TP2-3, TP2-4 each receiving the voltage Vcc on their sources S and the voltage V(I_t) on their gates G. The drain D of the transistor TP2-1 is connected to ground by two series resistors, and the voltage VT₁ is drawn from the middle point of both resistors. Similarly, the drain of the transistor TP2-2 is connected to ground by two series resistors, and the voltage VT₂ is drawn from the middle point of both resistors. The drain of the transistor TP2-3 is connected to ground by two series resistors, and the voltage VT₃ is drawn from the middle point of both resistors. The drain of the transistor TP2-4 is connected to ground by two series resistors, and the voltage VT₄ is drawn from the middle point of both resistors.

[0078] The detection thresholds of the temperatures T₁ to T₄ are also linked in this embodiment, unless there is an isolated error regarding the value of one of the resistors. There is little probability of the later happening. The errors in resistivity due to a dispersion of the manufacturing method generally concern a whole set of resistors.

[0079] FIG. 13 represents one embodiment example of a temperature sensor 22 of the type represented in FIG. 3, exhibiting two outputs OUT1, OUT2 delivering the signals S₁, S₂ or S₃, S₄ according to the value of the signal TST. The sensor 22 comprises a measuring circuit MC2 receiving at input the signal TST, delivering on an output OUT1 the voltage VT₁ or the voltage VT₃ according to the value of the signal TST, delivering on an output OUT2 the voltage VT₂ or the voltage VT₄ according to the value of the signal TST, and delivering also the reference voltage V_ref independent of the temperature. The voltage VT₁/VT₃ delivered by the output OUT1 is applied to the positive input of a comparator CP1 whose negative input receives the voltage V_ref. The output of the comparator CP1 forms the output OUT1. The voltage VT₂/VT₄ delivered by the output OUT2 is applied to the negative input of a comparator CP2 whose positive input receives the voltage V_ref. The output of the comparator CP2 forms the output OUT2.

[0080] FIG. 14 represents one embodiment example of a measuring stage MS4 that may be used in the circuit MC2 to generate the voltages VT₁/VT₃ and VT₂/VT₄. As previously, the measuring stage MS4 is biased by a voltage V(I_t) delivered by a bias circuit (not represented). The measuring stage MS4 comprises a first group of two parallel transistors PMOS TP2-1, TP2-2, receiving the voltage Vcc on their sources. Their drains are interconnected and connected to ground by two series resistors RA, RB. The measuring stage MS4 comprises a second group of two parallel transistors PMOS TP2-3, TP2-4, receiving the voltage Vcc on their sources. Their drains are interconnected and connected to ground by two series resistors RA, RB. The transistors TP2-1 and TP-3 receive on their gates the signal V(I_t). The gate of the transistor TP2-2 receives the signal V(I_t) via a switch transistor SW1, and the gate of the transistor TP2-4 receives the signal V(I_t) via a switch transistor SW2. The switch SW1 is driven by the signal TST, and the switch SW2 is driven by an inverse signal /TST delivered by a inverting gate INV1.

[0081] According to the value of the signal TST, the switch SW1 is open (transistor SW1 blocked) and the switch SW2 closed (transistor SW2 conductive), or conversely. When SW1 is open and SW2 closed, a current equal to It runs through the resistors RA, RB and a current equal to ²It runs through the resistors RA, RB. The middle point of resistors RA, RB delivers the voltage VT₁ and the middle point of the resistors RA, RB delivers the voltage VT₂. When SW1 is closed and SW2 open, a current equal to 2I_t runs through the resistors RA, RB and a current equal to I_t runs through the resistors RA, RB. The middle point of the resistors RA, RB delivers the voltage VT₃ and the middle point of the resistors RA, RB delivers the voltage VT₄.

[0082] FIG. 15 represents another embodiment example of a measuring stage MS5 that may be used in the measuring circuit MC2. The generation of the voltages VT₁/VT₃ and VT₂/VT₄ are based on resistance-changing instead of current-changing. The measuring stage MS5 comprises two transistors PMOS TP2-1, TP2-2 receiving the voltage Vcc on their sources and the voltage V(I_t) on their gates G. The drain of the transistor TP2-1 is connected to ground by four series resistors R_a, R_b, R_c, R_d. The cathode of the resistor R_d is connected to ground. The drain of the transistor TP2-2 is connected to ground by four series resistors R_a, R_b, R_c, R_d. The cathode of the resistor R_d is connected to ground. A switch transistor SW1 driven by the signal TST is parallel with the resistor R_b. A switch transistor SW2 driven by the signal /TST is parallel with the resistor R_c. A switch transistor SW1 driven by the signal /TST is parallel with the resistor R_b, and a switch transistor SW2 driven by the signal TST is parallel with the resistor R_c.

[0083] When the switches SW1, SW2 are closed and the switches SW2, SW1 open, the resistors R_b, R_c are short-circuited. The anode of the resistor R_c delivers the voltage VT₁ and the anode of the resistor R_c delivers the voltage VT₂. When the switches SW1, SW2 are open and the switches SW2, SW1 closed, the resistors R_a, R_b are short-circuited. The anode of the resistor R_c delivers the voltage VT₃ and the anode of the resistor R_c delivers the voltage VT₄.

[0084] FIG. 16 represents one embodiment example of the bias circuit BC1 delivering the voltage V(I_t). The circuit BC1 comprises two transistors PMOS TP10 and TP11 in parallel, receiving the voltage Vcc on their sources. The gates of transistors TP10, TP11 and the drain of the transistor TP11 are interconnected. The drain of the transistor TP10 is connected to the drain of a transistor NMOS TN10 whose source is connected to the emitter of a bipolar transistor TB10 having its base and its collector connected to ground. The drain of the transistor TP11 is connected to the drain of a transistor NMOS TN11 whose source is connected to the end of a resistor R30. The gates of the transistors TN10, TN11 and the drain of the transistor TN10 are interconnected. The other end of the resistor R30 is connected to the emitter of a bipolar transistor TB11 having its base and its collector connected to ground. The transistor TB11 is formed by 8 identical transistors TB11-1, TB11-2, . . . TB11-8. The voltage V(I_t) is drawn from the gates of the transistors TP10, TP11.

[0085] In such a circuit, the transistors TP10 and TP11 are laid out symmetrically for conducting current and conduct the same current I_t. The gate-source voltages of cascade transistors TN10, TN11 are identical, the base-emitter voltages V_be of the transistors TB10 and TB11 are identical, and the currents running through the transistors TB11-i forming the transistor TB11 are 8 times smaller than the current I_t. It can be derived that:

[0086] with:

[0087] k is Boltzmann's constant, q is the load of the electron, ln is a logarithmic function, and r is the value of the resistor R30.

[0088] As described above, the gate voltage V(I_t) enables one to impose the current I_t in other parts of the sensor to generate the reference voltage V_ref (a band gap voltage) and the voltages VT₁, VT₂, VT₃, VT₄ proportional to the temperature.

[0089] Such a temperature sensor according to the invention is susceptible to diverse other embodiments, as readily appreciated by those skilled in the art. Notably, although a detection of the temperature thresholds T₁, T₂, T₃, T₄ by voltages VT₁, VT₂, VT₃, VT₄ proportional to the temperature and a reference voltage V_ref has been described in the foregoing, a temperature sensor according to the invention can also use several reference voltages and one or several voltages proportional to the temperature.

[0090] Thus, FIG. 17 represents a sensor 23 which exhibits the same structure as the sensor 21 described above in relation to FIG. 8, but wherein the measuring circuit MC1 is replaced with a measuring circuit MC3 which delivers four reference voltages V_ref1, V_ref2, V_ref3, V_ref4 and a voltage VT proportional to the temperature. The voltages V_ref1 and V_ref3 are linked in terms of deviation, the voltages V_ref2 and V_ref4 are linked, and the voltages V_ref1 and V_ref2 are preferably linked. The voltages V_ref1, V_ref3 are applied to the inputs of the multiplexer MX1 and the voltages V_ref2, V_ref4 are applied to the inputs of the multiplexer MX2. According to the value of the signal TST, the comparator CP1 compares the voltage V_ref1 or the voltage V_ref3 with the voltage VT, and delivers the signal S₁ or the signal S₃. Similarly, the comparator CP2 compares the voltage V_ref2 or the voltage V_ref4 with the voltage VT, and delivers the signal S₂ or the signal S₄.

[0091] In the same perspective, FIG. 18 represents a sensor 32 which exhibits the same structure as the sensor 31 described above in relation to FIG. 9, but wherein the measuring circuit MC1 is replaced with the measuring circuit MC3. The voltages V_ref1, V_ref2, V_ref3, V_ref4 are applied respectively to the comparators CP1, CP2, CP3, CP4, wherein each receive the voltage VT on their second input.

[0092] Also, FIG. 19 represents a sensor 24 which exhibits the same structure as the sensor 22 described above in relation to FIG. 13, wherein a measuring circuit MC4 replaces the measuring circuit MC2. The circuit MC4 delivers on an output OUT1 the voltage V_ref1 or the voltage V_ref3 according to the value of the signal TST, and on an output OUT2 the voltage V_ref2 or the voltage V_ref4 according to the value of the signal TST, and delivers also the voltage VT. Thus, the comparator CP1 receives on an input the voltage V_ref1 or the voltage V_ref3, on another input the voltage VT, and delivers the signal S₁ or the signal S₃. The comparator CP2 receives on an input the voltage V_ref2 or the voltage V_ref4, on another input the voltage VT, and delivers the signal S₂ or the signal S₄. As previously, the voltages V_ref1 and V_ref3 are linked, the voltages V_ref2 and V_ref4 are linked, and the voltages V_ref1 and V_ref2 are preferably linked.

[0093] Moreover, although it has been suggested in the foregoing to offset both thresholds T₁ and T₂ to form the both offset thresholds T₃ and T₄, it falls within the framework of the invention to provide several offset thresholds for each threshold to be tested. For example, two detection thresholds T₃ and T₅ linked to the detection threshold T₁ can be generated, as well as two detection thresholds T₄ and T₆ linked to the detection threshold T₂.

[0094] A temperature sensor according to the invention is also susceptible to diverse applications. By way of example, FIG. 20 represents diagrammatically an integrated circuit MC for a smart card, comprising a microprocessor based or wired-logic central unit UC, a memory MEM, and a sensor 20 or 30 according to the invention. The central unit CU applies the signal TST to the sensor 20, 30 during test periods. The signals S₁ to S₄ delivered by the sensor 20, 30 are accessible via a data bus DB.