US20080063027A1

US20080063027A1 - Precision temperature sensor

Info

Publication number: US20080063027A1
Application number: US11/724,358
Authority: US
Inventors: Giovanni Galli
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-03-15
Filing date: 2007-03-15
Publication date: 2008-03-13
Also published as: US7914205B2; US20090207883A1

Abstract

A temperature sensor circuit in accordance with an embodiment of the present invention includes a temperature sensing element operable to provide a temperature voltage that is linearly related to the absolute temperature of the circuit. The temperature sensing element includes a first bi-polar junction transistor and a second bipolar junction transistor connected between a supply voltage and a common ground, wherein the base of first bipolar junction transistor is connected to the base of the second bipolar junction transistor, a first resistor connected between an emitter of the first bipolar junction transistor and the common ground and a second resistor connected between the common ground and a first node, wherein the temperature voltage is provided to the first node across the second resistor. The temperature sensor circuit also includes a current supply element operable to supply a common current to a collector of the first bipolar junction transistor, the second bipolar junction transistor and to the second resistor, respectively, an early voltage element operable to compensate for variations in voltage, a base current element operable to provide a steady base current to the bases of the first and second bipolar junction transistors, a channel modulation element operable to compensate for channel modulation and a leakage element operable to compensate for epi-substrate leakage between the circuit and a substrate on which it is formed.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of and priority to U.S. Provisional Application No. 60/782,609, filed on Mar. 15, 2006, entitled PRECISION TEMPERATURE SENSOR, the entire contents of which are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a precision temperature sensor circuit that provides an output voltage that is linearly related to the absolute temperature of the circuit. More specifically, the present invention relates to a precision temperature sensor circuit that eliminates the need for trimming.
Monitoring the temperature of IC chips has long been a concern. Naturally, it is preferable, when possible, to incorporate the temperature sensor into the IC. It is common to use Bipolar Junction Transistors (BJT's) in temperature sensing circuits given the known temperature and current dependence of the forward-biased base-emitter junction voltage thereof. Specifically, the temperature can be measured by subtracting two voltages at two different bias currents in a known ratio.
Such temperature sensors make use of the relationship between the BJT's base emitter voltage to its collector current: $V_{BE} = \frac{kT}{q} \ln (\frac{I_{C}}{I_{S}})$
where k is Boltzmann's constant, T is the absolute temperature and q is the charge of an electron. I_Sis the transport saturation current of the transistor. This equations presumes a voltage of a few hundred millivolts on the collector and ignores Early effects.
While conventional temperature sensing circuits provide adequate results, they are typically not precise enough unless they are subjected to trimming, for example by an external trimming circuit. Naturally, the addition of such an external trimming circuit adds expense and complexity to the circuit.
Thus, it would be beneficial to provide a temperature sensor circuit that provides high precision and avoids the need for trimming.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a highly precise temperature sensor circuit in which the voltage is linearly related to the temperature wherein trimming is not necessary.
A temperature sensor circuit in accordance with an embodiment of the present invention includes a temperature sensing element operable to provide a temperature voltage that is linearly related to the absolute temperature of the circuit. The temperature sensing element includes a first bi-polar junction transistor and a second bipolar junction transistor connected between a supply voltage and a common ground, wherein the base of first bipolar junction transistor is connected to the base of the second bipolar junction transistor, a first resistor connected between an emitter of the first bipolar junction transistor and the common ground and a second resistor connected between the common ground and a first node, wherein the temperature voltage is provided to the first node across the second resistor. The temperature sensor circuit also includes a current supply element operable to supply a common current to a collector of the first bipolar junction transistor, the second bipolar junction transistor and to the second resistor, respectively, an early voltage element operable to compensate for variations in voltage, a base current element operable to provide a steady base current to the bases of the first and second bipolar junction transistors, a channel modulation element operable to compensate for channel modulation and a leakage element operable to compensate for epi-substrate leakage between the circuit and a substrate on which it is formed.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an illustration of an exemplary Bipolar Junction Transistor.
FIG. 2 is an illustration of a basic temperature sensor circuit.
FIG. 3 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention.
FIG. 4 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including a start up circuit.
FIG. 5 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including frequency compensation.
FIG. 6 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including early voltage compensation.
FIG. 7 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention base current compensation.
FIG. 8 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention connected to a voltage regulator.
FIG. 9 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including channel modulation compensation.
FIG. 10 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including epy-substrate leakage compensation.
FIG. 11 is an illustration of a temperature sensor circuit in accordance with an embodiment of the present invention including random process variation compensation.
FIG. 12 is a graph illustrating the results of a simulation of the circuit of FIG. 11.
FIG. 13 is an exemplary layout of an integrated circuit used to implement the circuit of FIG. 11.
FIG. 14 is an chart including simulated and measured data of the circuit of FIG. 11.
FIG. 15 is another chart including simulated and measured data of the circuit of FIG. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

As an initial matter it is useful to review some of the features of the Bi-polar Junction Transistor (BJT). FIG. 1 illustrates an example of a BJT 1. BJT's operate is substantially two regions. In the Active forward region, the collector current I_Cis represented by the following equation I_C=I_s(e^Vbe/Vt) and the emitter current I_E=−I_S(e^Vbe/Vt)/α_Fwhere I_Sis the transport saturation current of the BJT and α_Fis the large signal forward current gain in common base configuration. V_tis equal to KT/q where K is Boltzmann's constant, T is the absolute temperature in degrees Kelvin and q is the charge of an electron, as is noted above. At room temperature, V_tis approximately 26 mV. In the Active reverse region, I_E=I_S(e^Vbe/Vt) and I_B=I_S(e^Vbe/Vt)/β_Rwhere β_R=α_R/(1−α_R). Thus, the emitter current may be expressed as I_E=β_RI_B. In general I_Sis the transport saturation current and J_Sis the transport saturation current density. The other equations illustrated in FIG. 1 are based on or derived from the commonly known Ebers-Moll model of a BJT and thus are not described in further detail.
FIG. 2 illustrates a basic temperature sensor circuit 20 that provides a voltage V_TSthat is linearly related to the ambient temperature. As illustrated, two BJT transistors Q₁, Q₂are provided with their base terminals connected to each other. The reference symbol A₂refers to the emitter area of the transistor Q₂and the reference symbol A₁refers to the emitter area of transistor Q₁. The collector current I_Cis expressed in the following equation:
I _C =I _S(e ^Vbe/Vt); where
V _be =V _Tln I_C /I _S= V _tln I _C /J _S A ₁
In the circuit of FIG. 2, presuming that the same collector current I_Cis provided in all three branches, it follows that:
V _tln I _C /J _S A2−V _Tln I _C /J _S A1=I _E R ₁and
I _C =V _t(ln A1/A2)/R1=(KT/q)(lmA1/A2)/R1(PTAT urrent)
and V _TS =I _C R ₂=(KT/q)(ln A ₁ /A ₂)R ₂ /R ₁ =K′T
Thus, the temperature voltage V_TSis determined by a constant value K′ multiplied by the absolute temperature T. Thus, the temperature voltage V_TSacross resistor R₂may be used to indicate the temperature of the circuit, or die, on which the circuit 20 is formed.
As noted above, the temperature voltage V_TSindicates the temperature of the circuit when the current I_Cis the same in each branch of the circuit illustrated in FIG. 2. There are several ways to ensure that this is so, for example, a simple current mirror may be provided and connected to the collector of each of the transistors Q₁, Q₂and to the node at which V_TSis provided.
In FIG. 3, a plurality of MOSFETs M₁, M₂, M₃, M₄are provided to form two current gain loops 31, 32 that ensure that the current I_Csupplied to each of the BJTs Q₁and Q₂and provided to the first node, at which the temperature voltage V_TSis provided across the resistor R₂is the same. The MOSFETs M₁, M₂, M₃, M₄preferably have substantially the same electrical and physical traits and are interconnected to form the two current gain loops 31, 32. Specifically, the MOSFETs M₁, M₂form the positive feedback loop 31 that provides the current I_Cto the transistor Q₂. The MOSFETs M₂, M₃form the negative feedback loop 32 to provide the collector current I_Cto the transistor Q₁. The MOSFET M₄is used to provide the collector current I_Cto the node at which the temperature voltage V_TSis provided. The transistors Q₂and Q₁are also selected to be matched such that the emitter area A₁of transistor Q₁is set such that the ratio of the area A₁to the emitter area A₂of the transistor Q₂is 4. Further, the resistors R₁and R₂are selected such that the ratio of R₂to R₁is 20. As illustrated, the circuit of FIG. 3 is substantially similar to that of FIG. 2 with the addition of two gain loops 31, 32. Together, these gain loops form a current supply for the circuit illustrated in FIG. 2.
FIG. 4 illustrates a temperature sensing circuit 40 similar to that of FIG. 3 that includes a start up circuit 42. The start up circuit 42 allows the circuit to reach a desired operating point and is then disengaged. The circuit includes two MOSFETs MS₁, MS₂which allow the collector current I_Cto built to a sufficient point prior to disengaging after a period of time.
FIG. 5 illustrates a temperature sensor circuit 50 similar to that of FIG. 4 that includes frequency compensation for the gain loops 30, 32. The capacitor C₁provides frequency compensation for the positive feedback loop 30. The capacitor C2 and resistor R3 provide frequency compensation for the negative feedback loop 32. Together, these components smooth the effect of frequency variations on the common collector current I_C.
In order to avoid the need for trimming, the temperature sensor circuit preferably provides the compensation that would otherwise be provided by the trimming circuit. First, early voltage compensation should be supplied for the Bipolar NPN transistor(s). In addition, base current compensation should be supplied, sometimes referred to as βeta compensation. In addition channel modulation compensation should be provided to the Pmos device(s). The problem of Epi-substrate diode leakage should also be addressed and compensated for. In addition, the circuit should compensate for power supply variations. Finally, random process variation compensation should also be provided.
FIG. 6 illustrates a temperature sensing circuit 60 in accordance with the present invention similar to that illustrated in FIG. 5, in which the additional BJT transistor Q₃is highlighted. The transistor Q₃is configured in a diode configuration and allows the transistors Q₁and Q₂to work at the same V_CE. MOSFET M₅is positioned between the supply voltage and the transistor Q₃with its gate connected to the collector of transistor Q₂. MOSFET M₆is connected between the supply voltage and the gate of transistor Q₁with its gate connected to the collector of Q₂. The MOSFETS M₅and M₆are provided such that they are properly sized to carry the same V_DSvoltage. Together these elements provide early voltage compensation.
FIG. 7 illustrates a temperature sensing circuit 70 in accordance with an embodiment of the present invention that is similar to that of FIG. 6 except that it provides for base current compensation. As illustrated, the circuit 70 of FIG. 7 includes a resistor R₄positioned between the base terminals of the transistors Q₁and Q₂. As a result, the following equations may be used to describe the current I_C:
V _tln I _C /J _S A ₂ +I _B R ₄ =V _tln I _C /J _S A ₁ +I _E R ₁; or
V _tln A ₁ /A ₂ =I _E R ₁ −I _B R ₄; where if R ₁ =R ₄
V _tln A ₁ /A ₂=(I _E −I _B)R ₁; or
I _C =V _t(ln A ₁ /A ₂)R _1.
Thus, the collector current I_Cis directly related to the thermal voltage V_trather than the emitter current I_E. This provides for a steadying of the base current.
FIG. 8 illustrates a temperature sensing circuit 80 in accordance with an embodiment of the present invention that is similar to the circuit of FIG. 7 except that it also provides channel modulation compensation. Two additional transistors Q₄and Q₅are added in a diode configuration between the MOSFET M₄and the node at which the voltage V_TSis provided across resistor R₂. These two “diodes” ensure that the MOSFETs M₁, M₂, M₃, M₄work at the same V_DSvoltage.
FIG. 9 illustrates a temperature sensing circuit 90 in accordance with an embodiment of the present invention that is substantially similar to that illustrated in FIG. 8 except that it includes components that provide Epi-substrate leakage compensation. In particular, the circuit of FIG. 9 includes two epitaxial (epi) diodes D₁, D₂that add additional Epi-substrate junctions to address Epi-substrate leakage. The diode D₂is positioned across the resistor R2 at the node at which the temperature voltage V_TSis obtained. The diode D₂is positioned in parallel with the transistor Q₂. In addition, the transistors Q₄and Q₅described with reference to FIG. 8 and acting as diodes may also be configured as epi diodes as well. The additional of these addition epi-substrate junctions provides compensation for epi-substrate leakage.
FIG. 10 illustrates a temperature sensing circuit 100 in accordance with an embodiment of the present invention connected to a voltage regulator 102. The temperature sensor circuit 100 may be consistent with any of the embodiments described herein with reference to FIGS. 2-9 and 11. The regulator 102 preferably provides a steady 5V supply voltage to improve V_CCnoise rejection at the output.
FIG. 11 illustrates a temperature sensing circuit 110 similar to that of FIG. 9, except that it provides compensation for random process variation. In particular, as illustrated in FIG. 10, the MOSFETs M₁, M₂, M₃, M₄are matched in a centroid Structure splitting each MOSFET into four mos⁺ dummies. Similarly, the MOSFETs M₅, M₆are matched in a centroid Structure splitting each MOSFET into 2 elementary mos⁺ dummies. The transistors Q₁, Q₂are also matched in a centroid Structure splitting each NPN into 2 elementary BJTs. The resistors R₁, R₂and R₃are also matched in a centroid Structure building each resistor using the same base modules (for example, 1 kΩ) and the same number of modules. Thus, random variation is minimized in the circuit.
FIG. 12 illustrates graphs that show the results of a simulation of a temperature sensing circuit in accordance with FIG. 11, for example, of the present application. As can be seen in FIG. 12, the voltage V_TSrises is a substantially straight line as the temperature rises.
FIG. 13, illustrates one example of a layout of an Integrated Circuit that may be used to implement the temperature sensing circuit of the present invention. The exemplary layout of FIG. 13 is merely one non-limiting example of an appropriate layout.
In FIG. 14, the chart illustrates actual measure voltages at the node where the temperature voltage V_TSis provided in the column “Measur.” and the simulated values (“Sim_min”) provided in the simulation mentioned with reference to FIG. 12. The error percentage is also listed. Further, the graph illustrates that the line formed by the actual results is very similar to the line formed by the simulated results of FIG. 12.
FIG. 15 similarly includes a chart including the actual and simulated temperature voltages V_TSand a graph of the error curve showing the difference between them. It is clear based on FIGS. 14 and 15 that the simulated circuit provides results that are substantially the same as the actual results. Further, it is clear based on FIGS. 12 and 14-15 that the temperature sensor of the present invention provides precise temperature measurement based on a temperature voltage V_TSthat is clearly linearly related to the absolute temperature. Further, the compensation provided in the temperature sensor circuit of the present invention eliminates the need for trimming, such as by an external trimming circuit, thus freeing space and reducing cost.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims

1. A temperature sensor circuit comprising:

a temperature sensing element operable to provide a temperature voltage that is linearly related to the absolute temperature of the circuit, the temperature sensing element comprising:

a first bi-polar junction transistor and a second bipolar junction transistor connected between a supply voltage and a common ground, wherein the base of first bipolar junction transistor is connected to the base of the second bipolar junction transistor each bipolar junction transistor;

a first resistor connected between an emitter of the first bipolar junction transistor and the common ground; and

a second resistor connected between the common ground and a first node, wherein the temperature voltage is provided to the first node across the second resistor;

a current supply element operable to supply a common current to a collector of the first bipolar junction transistor, the second bipolar junction transistor and the second resistor, respectively;

an early voltage element operable to compensate for variations in voltage;

a base current element operable to provide a steady base current to the bases of the first and second bipolar junction transistors;

a channel modulation element operable to compensate for channel modulation; and

a leakage element operable to compensate for epi-substrate leakage between the circuit and a substrate on which it is formed.

2. The temperature sensor circuit of claim 1, further comprising a start up circuit operable to ensure that the common current is stable at start up and that is disengaged after a predetermine period of time.

3. The temperature sensor circuit of claim 2, wherein the current supply element further comprises a frequency compensation element operable to compensate for frequency changes in the current supply element such that the common current remains stable.

4. The temperature sensor circuit of claim 3, further comprising:

a voltage regulator, connected to the temperature sensor circuit and operable to provide a steady supply voltage to the temperature sensor circuit.

5. The temperature sensor circuit of claim 4, wherein the current supply element further comprises:

a positive feedback gain loop operable to provide the common current to the first bipolar junction transistor; and

a negative feedback gain loop operable to provide the common current to the second bipolar junction transistor

6. The temperature sensor circuit of claim 5, wherein the leakage element further comprises:

a first epi diode connected across the second bipolar junction transistor; and

a second epi diode connected across the second resistor, such that the additional epi-substrate junctions provided in the first and second epi diodes reduces the affect of epi-substrate leakage.

7. The temperature sensor circuit of claim 6, wherein the temperature sensor circuit is implemented in an integrated circuit.