Abstract:
An temperature sensor circuit is disclosed. In one embodiment, the temperature sensor comprises an input circuit with a current mirror for forcing a current down a reference stage and an output stage. The reference stage and the output stage include P-N junctions (e.g., using bipolar transistors) with differing junction potentials. By tailoring the resistances in the reference and output stages, the input circuit produces two output voltages, one of which varies predictably with temperature, and one which is stable with temperature. The input circuit is preferably used in conjunction with an amplifier stage which preferably receives both the temperature-sensitive and non-temperature-sensitive outputs. Through various resistor configurations in the amplifier stage, the output of the temperature sensor can be made to vary at a higher sensitivity than produced by the temperature-sensitive output of the input circuit. Moreover, as a result of the non-temperature-sensitive output, the output of the temperature sensor is additionally and beneficially tailored in its offset voltage in a temperature-independent manner. The result is a flexible circuit that can achieve very high sensitivities and near-ideal performance even at lower power supply voltages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional application of U.S. Ser. No. 11/328,694, filed Jan. 4, 2006, to which priority is claimed and which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of this invention relate to a temperature sensor with high output voltage sensitivity and the ability to function at low power supply voltages over wide temperature ranges. 
       BACKGROUND 
       [0003]    Temperature sensors are well known in the integrated circuit art. Typically, a temperature sensor provides an output voltage whose magnitude equates to the temperature that the circuit senses. 
         [0004]    One temperature sensor  100  is shown in  FIG. 1 , and is taken from U.S. Pat. No. 6,867,470 as a good illustration of the problems indicative of prior art temperature sensors. As shown, temperature sensor  100  includes a current mirror  130  comprised of P-channel transistors  135   a - e  in (in this case) five output stages  125   b , each of which passes the input of I. This current from each stage is met by five PNP (bipolar) transistors  137   a - e , which comprise in effect 5 P-N junctions in series. The base-to-emitter voltage of these P-N junctions, V be(a)-(e) , is a function of temperature, and essentially such voltage changes by about −2 mV per every degree Celsius. Aside from this temperature dependence, the V be  for each junction is on the order of about 0.6 Volts at room temperature (25 degrees Celsius). Accordingly, the output voltage, V out  is on the order of 3.0V (0.6V*the five stages), and its sensitivity is on the order of about −10 mV/C (−2 mV*5). 
         [0005]    More stages could be used to increase the temperature sensor  100 &#39;s sensitivity, but this comes at a price. While each junction added to the circuit adds sensitivity (i.e., another −2 mV/C worth at the output), it also adds another 0.6V drop. Accordingly, as more and more junctions are used, the power supply voltage, Vdd, must be increased accordingly. For example, for the temperature sensor  100  of  FIG. 1  to function as desired over an appropriate temperature range (e.g., −50 to 100 degrees C.), the power supply voltage must be at least 3.5V (i.e., about 3.0V for the P-N junctions and another 0.5V for proper V ds  voltage drops across the current mirror transistors  135 ). But this is an unfortunate limitation, especially when considering that many modern-day integrated circuits have power supply voltages that are lower than 3.5V. This minimum power supply limitation can be alleviated by removing some of the stages/junctions from the circuitry  100 , but this comes at the price of reduced sensitivity. In other words, temperature sensor circuits of the prior art tend to offer either high sensitivities, or flexible power supply operating values, but not both as would be desirable. 
         [0006]    It is therefore a goal of this disclosure to provide embodiments of temperature sensors that are both highly sensitive over extended temperature ranges and capable of working at wider power supply ranges and in particular at low power supply values. 
       SUMMARY 
       [0007]    An temperature sensor circuit is disclosed. In one embodiment, the temperature sensor comprises an input circuit. The input circuit comprises a current mirror for forcing a current down a reference stage and an output stage. The reference stage and the output stage include P-N junctions (e.g., using bipolar transistors) with differing junction potentials. By tailoring the resistances in the reference and output stages, the input circuit produces two output voltages, one of which varies predictably with temperature, and one which is stable with temperature. 
         [0008]    While the input circuit is useful as a temperature sensor in its own right and is particularly useful in its additional provision of a non-temperature-sensitive output, the input circuit is preferably used in conjunction with an amplifier stage. The amplifier stage can comprise a number of different amplifiers (e.g., operational amplifiers), and preferably receives both the temperature-sensitive and non-temperature-sensitive outputs. Through various resistor configurations in the amplifier stage, the output of the amplifier stage (i.e., the output of the temperature sensor) can be made to vary at a higher sensitivity than produced by the temperature-sensitive output of the input circuit. Moreover, as a result of the non-temperature-sensitive output, the output of the temperature sensor is additionally and beneficially tailored in its offset voltage in a temperature-independent manner. The result is a flexible circuit that can achieve very high sensitivities at lower power supply voltages. Indeed, the disclosed temperature sensor circuit can achieve near-ideal performance over a temperature range between first and second temperatures in which the temperature sensor output is approximately the power supply voltage at the first temperature, and is approximately ground at the second temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  illustrates a prior art temperature sensor circuit. 
           [0011]      FIG. 2  illustrates an input circuit used in preferred embodiments of the disclosed temperature sensors, and comprises a temperature-sensitive output voltage and a temperature-insensitive output voltage. 
           [0012]      FIG. 3  illustrates one embodiment of a temperature sensor using the input circuit of  FIG. 2 , in which the temperature sensor uses an amplifier stage to improve the temperature sensor&#39;s sensitivity. 
           [0013]      FIG. 4  illustrates another embodiment of a temperature sensor similar to that of  FIG. 3 , but which includes additional amplifier stages and uses the temperature-insensitive output voltage of the input circuit to control an offset of the temperature sensor&#39;s output voltage. 
           [0014]      FIG. 5  graphically illustrates differences in the output voltage offset for the temperature sensors of  FIGS. 3 and 4 , and also illustrates problems associated with limited power supply voltages. 
           [0015]      FIG. 6  illustrates another embodiment of a temperature sensor in which voltage output offset and sensitivity are independently controllable. 
           [0016]      FIG. 7  graphically illustrates the output voltage for the temperature sensor of  FIG. 6 , and shows near ideal performance and maximum sensitivity for a given power supply voltage. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 2  discloses an input circuit  10  as can be used in embodiments of the temperature sensor circuitry disclosed herein, and which will be discussed later. As shown, the input circuit  10  comprises a current mirror circuit  30 , comprised of P-channel transistors  20  and  22 . This arrangement forces a reference current, I, down both reference stage  25   a  and output stage  25   b . Output stage  25   b  comprises two resistors, R 2  and R 3 , with outputs V bg  and V ref1  tapping at either end of resistor R 2 . An NPN (bipolar) transistor  26  is also present in the output stage  25   b , and given its common collector-base configuration is essentially configured as a junction or diode. The bias to the base of transistor (junction)  26  is also used to bias the base of NPN transistor (junction)  24  in the reference stage. 
         [0018]    When this circuit arrangement in hand, one skilled in the art will appreciate that certain mathematical equations describe the operation of the input circuit  10 : 
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         [0000]    where k=Boltzmann constant (8.62×10 −5  eV/K), q=electronic charge (1.60×10 −19  coul), A=the ratio in base-emitter area between the NPN transistors  24  and  26 . To briefly explain these equations, Equations (1), (2), and (4) set forth the basic ohms law characteristics of the two stages  25   a  and  25   b ; Equation (3) is known (that the difference in the junction potentials V be1 −V be2 =ΔV be =kTln(A)/q is explained in Johns &amp; Martin, “Analog Integrated Circuit Design,” pp. 360-61 (John Wiley &amp; Sons, 1997), which is incorporated herein by reference); and equations (5) and (6) comprise the temperature differentials of Equations (1) and (2). 
         [0019]    In a preferred embodiment, the two outputs of input circuit  10 , V bg  and V ref1 , are preferably different in terms of their temperature dependencies. Specifically, it is preferred that output V bg  not be dependent on temperature, such as is the case with a band gap reference circuit. In other words, it is preferred that ∂V bg /∂T=0. Conversely, it is preferred that output V ref1  be dependent on temperature, as would be necessary for the input circuit  10  to have functionality as a temperature sensor. For example, while ∂V ref1 /∂T can be tailored depending on the values of the resistors used, assume for now that it is preferable for ∂V ref1 /∂T=−1.9 mV/C. 
         [0020]    With values for these two output voltage temperature differentials set, and by empirically observing that ∂V be1 /∂T=−2 mV/C as discussed above, relations between the three resistor values R, R 2 , and R 3  can be established by plugging ∂V bg /∂T=0, ∂V ref1 /∂T=−0.0019, and ∂V be1 /∂T=−0.002 into equations (5) and (6). Specifically: 
         [0000]        R 2 /R= 0.0019 q/k  ln( A )  (7) 
         [0000]        R 3 /R= 0.000 q/k  ln( A )  (8) 
         [0000]        R 2 /R 3=19  (9) 
         [0000]    In other words, choosing resistor values in these relative proportions will provide outputs from the input circuit  10  with the desired temperature characteristics, i.e., with V bg  not varying with temperature and with V ref1  varying −1.9 mV/C with temperature. 
         [0021]    Note that resistor R 3  is small, and generally it can be omitted from circuit. This is shown by noting that when ∂V bg /∂T=0 and ∂V ref1 /∂T=−0.0019 are plugged into equations (5) and (6), the R 3  term falls out. However, R 3  can still be used to set a temperature sensitivity (e.g., ∂V ref1 /∂T=−0.0019) that is different from the temperature sensitivity of the P-N junction (i.e., −0.002). 
         [0022]    Input circuit  10  therefore comprises a temperature sensor in its own right, and is unique in its output of an output voltage indicative of temperature (V ref1 ) and also in its simultaneous output of a temperature stable reference voltage (V bg ). However, as used by itself, input circuit  10  has a relatively low temperature sensitivity (∂V ref1 /∂T=−1.9 mV/C). However, it beneficially operates at low power supply voltages (e.g., less than 1.5V), because only one junction  26  is present in the circuit. 
         [0023]    In any event, while input circuit  10  is novel and useful in its own right, preferred embodiments of the invention use the input circuit as an input stage to various amplifier stages to follow. As will be appreciated, when the input circuit is used in conjunction with the amplifier stages, the result is a temperature sensor circuit with high sensitivities and the capability to run at low power supply voltages and over wide temperature ranges. 
         [0024]    A first embodiment of a temperature sensor circuit  40  that uses the input circuit  10  in conjunction with an amplifier stage  45  is shown in  FIG. 3 . Temperature sensor  40  uses an operational amplifier (op amp)  42 , to which is input only the temperature-dependent output V ref1  from input circuit  10 . The voltage divider formed by R 5  and R 4 , which is fed back to the inverting input of the op amp  42 , establishes the output V ref2  of the temperature sensor  40  as follows: 
         [0000]        V   ref2 =( R 5 +R 4)/ R 4 *V   ref1   =n*V   ref1   (10) 
         [0000]        ∂V   ref2   /∂T=n*∂V   ref1   /∂T   (11) 
         [0000]    where the scalar n=(R 5 +R 4 )/R 4 . 
         [0025]    Thus by choosing R 4  and R 5  appropriately, n can be set to a value such as 1.7. With the values of the resistors so chosen, note that the sensitivity of the temperature sensor  40 , i.e., ∂V ref2 /T, equals, 1.7*−1.9 mV/C=−3.2 mV/C. Note further that this sensitivity value is possible at lower power supply voltages (e.g., Vdd=1.5V); such sensitivity at low power supply voltages were not possible using the prior art circuit of  FIG. 1 , because such sensitivity required the use of multiple serially-connected junctions, which in turn require higher power supply values to function. 
         [0026]      FIG. 4  shows another embodiment of a temperature sensor  50 . In this embodiment, the temperature sensor  40  of  FIG. 3  is used, but additionally, the amplifier stage  40  includes further op amps  44  and  46 . Op amp  44  receives as in input the temperature-independent output V bg  from the input circuit  10 . As configured, temperature sensor  50  is similar in its function to temperature sensor  40 , but the output of the sensor (V out ) includes a controllable offset (V b1 ). In other words, V out =V ref2 +V b1 , where V b1  is a controllable bias voltage. Specifically, V b1 =(R 9 /(R 6 +R 9 ))*V bg , where R 6  and R 9  comprise the resistance to either side of V b1  tap of variable resistor  49 . Variable resistor  49  may be one-time programmed to set R 6  and R 9  during manufacture, or may be controllable via control signals (not shown, but well within the understanding of one skilled in the art). The voltage at the input of the non-inverting input of op amp  46  is (V ref2 +V b1 )/2 as established by voltage divider resistors R 8 . Because op amp  46  will encourage this same voltage at the inverting terminal of op amp  46 , the output voltage of twice this amount (V out =V ref2 +V b1 ) is established by virtue of voltage divider resistors R 8 ′. 
         [0027]    Because V bg  is not dependent on temperature, neither is V b1 . Of course, V ref2  is temperature dependent, as explained with reference to temperature sensor  40  of  FIG. 3 . Because the output V out  of the temperature sensor  50  equals V ref2 +V b1 , the magnitude of the output voltage is scalable by a temperature-independent offset. In other words, the temperature sensor  50  of  FIG. 4  allows for the same temperature sensitivity in its output as is formed by the temperature sensor  40  of  FIG. 3 , but at a different magnitude, as shown in  FIG. 5 . This ability to adjust the offset of the output without the worry of adding unforeseen temperature dependence can provide improved design flexibility. The ability to adjust the offset is further useful should process variations require the output voltages to be modified from chip to chip or from wafer to wafer. 
         [0028]    As noted earlier, the temperature sensors  40 ,  50  of  FIGS. 3 and 4  allow for improved temperature sensitivity even at lower power supply voltages than were permissible in the prior art (e.g.,  FIG. 1 ). However, in either of these circuits, increased sensitivities (higher ∂V ref2 /∂T) also equate to higher output voltages (i.e., V ref2 ), as shown by arrow A in  FIG. 5 . This means at some point, i.e., at lower temperatures, the output voltage may exceed the power supply voltage, Vdd, which is improper. It would therefore be beneficial to have a temperature sensor in which both magnitude and sensitivity (i.e., slope) were independently controllable. In this way, an increased sensitivity response (arrow A) could be brought back into range of the power supply voltage via a negative offset (arrow B), such that the temperature sensor has high sensitivity, and yet works within the entire desired operating temperature range. This would allow for an optimal temperature sensor, one in which the output voltage approaches Vdd at its lowest operating temperature and approaches zero at its highest operating temperature, as shown by arrow B, in  FIG. 5 . 
         [0029]    A temperature sensor  60  that achieves such optimal performance is shown in  FIG. 6 . This temperature sensor  60 , like sensor  50  of  FIG. 4 , uses both the temperature-sensitive output from the input circuit  10  of  FIG. 2  (V ref1 ), and the non-temperature-sensitive output from the input circuit (V bg ). Central to the temperature sensor  60  of  FIG. 6  is modification of the input voltage (V 1 ) to op amp  42  of  FIG. 3 , and in this regard note that the amplifier stage  40  of  FIG. 3  is used as the last amplifier in  FIG. 6 . The modified input voltage, V 1 , is related to the outputs V ref1  and V bg  of the input circuit  10  ( FIG. 2 ) by the following equations: 
         [0000]        V   1 =( R 13*( R 10 +R 11))/( R 10*( R 13 +R 12))*V ref1 −( R 11 /R 10) V   bg   (12) 
         [0000]        ∂V   1   /∂T =( R 13*( R 10 +R 11))/( R 10*( R 13 +R 12)*V ref1   /∂T   (13) 
         [0030]    As with V out  of  FIG. 4 , V 1  of  FIG. 6  is a function of V ref1  and V bg , although in this instance the V bg  term allows a non-temperature-dependent offset ((R 11 /R 10 )*V bg ) to be subtracted from the temperature-dependent V ref1  term. Moreover, V 1  can be tailored to a specific value via adjustment of the various resistor values R 10  through R 13 . In one example, RIO is chosen to equal 2R 11  and R 13  is chosen to equal 2R 12 , in which case Equations (12) and (13) simplify to: 
         [0000]        V   1   =V   ref1 −( V   bg /2)  (14) 
         [0000]        ∂V   1   /∂T=∂V   ref1   /∂T   (15) 
         [0031]    V 1  is input to an op amp  42  similar to that of  FIG. 3 , which has a voltage divider formed by resistors R 4  and R 5  on its output, V out . This forms an amplifying scalar n as discussed earlier, such that: 
         [0000]        V   out =( R 5 +R 4)/ R 4 *V   1   =n*V   1   (16) 
         [0000]        ∂V   out   /∂T=n*∂V   1   /∂T   (17) 
         [0000]    where n=(R 5 +R 4 )/R 4 . 
         [0032]    With these equations governing the temperature sensor  60  of  FIG. 6  understood, it can be seen that V 1  and ∂V 1 /∂T can be designed separately, and hence so can V out  and ∂V out /∂T, i.e., the sensitivity of the temperature sensor  60 . Thus, if we assume the resistor values R 10  through R 13  are chosen to arrive at equations (14) and (15) above, and if R 4  and R 5  are chosen to set n=10, the sensitivity of the temperature sensor  60 , ∂V out /∂T, equals n*∂V ref1 /∂T=10*−1.9 mV/C=−19 mV/C. Moreover, we see from the simulated results of  FIG. 7  that the simulated design can be used with a power supply Vdd as low as 2.7V, and can produce essentially ideal output characteristics over a typical temperature operating range (bounded by −40 C and 100 C. in this example). Thus, as can be seen, at −40 C, the output voltage is approximately Vdd (i.e., greater than 95% of Vdd) and at 100 C, the output voltage is approximately ground (i.e., less than 5% of Vdd). 
         [0033]    Were the prior art temperature sensor  10  of  FIG. 1  used to provide the same sensitivity, its power supply voltage could not be run at such a low value, but would instead be on the order of at least 5V or more, much higher than current-day power supply voltages. The temperature sensor  60  of  FIG. 6  thus marks a significant improvement, and one subject to much greater utility in modern-day low-power-supply integrated circuits. Additionally, because the design of the temperature sensor is flexible in both its output magnitude and sensitivity (slope), even lower power supply voltages can be accommodated, although of course gain factor n would need to be reduced accordingly were the same temperature range to be sensed. 
         [0034]    It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.