Low voltage temperature-to-voltage converter

A temperature-to-voltage converter includes a first circuit for developing a signal having a positive temperature coefficient and a second circuit for developing a signal having a voltage offset and a negative temperature coefficient. The converter also includes an adder circuit configured to subtract the negative-temperature-coefficient signal from the positive-temperature-coefficient signal. The resulting difference signal is a low voltage that exhibits linear temperature-to-voltage conversion, allowing the converter to be powered by a low operating voltage.

FIELD OF THE INVENTION 
The present invention relates generally to temperature sensors, and more 
particularly to temperature-to-voltage converters for integrated circuits. 
BACKGROUND 
Temperature sensing circuits are commonly used in integrated circuits to 
protect against operation at excessive temperatures. 
Temperature sensing circuits provide an output signal (typically a voltage 
level) that varies with temperature. This output signal conventionally 
provides temperature-dependent feedback to a control circuit in an 
integrated circuit. By monitoring the temperature-dependent feedback, the 
control circuit is able to respond to increased operating temperatures by, 
for example, shutting down the integrated circuit or decreasing the 
operating speed of the integrated circuit. 
Temperature sensing circuits conventionally convert temperature to voltage 
using the thermal voltage (i.e., KT/q), conventionally expressed as 
V.sub.T, multiplied by some constant n. Expressed mathematically, such 
temperature sensing circuits may output a voltage V.sub.O =nV.sub.T that 
varies with temperature according to a known relationship. At present, the 
accepted standard for temperature-to-voltage converters calls for a change 
in output voltage V.sub.O of 10 mV/.degree.C. (i.e., dV.sub.O /dT=10 
mV/.degree.C.). However, the thermal voltage V.sub.T changes at only about 
0.1 mV/.degree. C. (i.e., dV.sub.T /dT=0.1 mV/.degree.C). Thus, the 
constant n is selected to be approximately one hundred (i.e., 10 
mV/.degree.C.div.0.1 mV/.degree.C.) so that the output voltage V.sub.O 
varies with temperature at the standard 10 mV/.degree.C. 
Unfortunately, conventional temperature-to-voltage converters that provide 
an output voltage V.sub.O based upon the thermal voltage V.sub.T provide 
relatively high output voltage levels at relatively high expected 
operating temperatures. Thus, circuits using such temperature-to-voltage 
converters impose a fairly high lower limit on the usable power supply 
voltage. One solution to this problem has been to create a fixed offset 
for the output voltage V.sub.O using a band-gap generator; however, that 
solution is expensive, for it requires substantial silicon real estate. 
What is needed is a simple, low cost, temperature-to-voltage converter that 
provides a low output voltage V.sub.O so that it can be operated with a 
low supply voltage. 
SUMMARY 
A temperature-to-voltage converter in accordance with the present invention 
includes a first circuit for developing a signal having a positive 
temperature coefficient and a second circuit for developing a signal 
having a voltage offset and a negative temperature coefficient. The 
converter also includes a voltage adder circuit configured to subtract the 
negative temperature coefficient signal from the positive temperature 
coefficient signal. The negative temperature coefficient signal is used as 
a voltage offset to lower the output voltage. The resulting difference 
signal exhibits linear temperature-to-voltage conversion. In the preferred 
embodiment, the negative temperature coefficient signal is created by the 
base-emitter drop across a single transistor, and, hence, the resulting 
circuit is much smaller than prior circuits. In one embodiment, the 
operating voltage for the converter is 2 volts or less. 
In one embodiment, the adder circuit is implemented using an operational 
amplifier. A feedback circuit connected to the output terminal of the 
operational amplifier develops a feedback voltage, related to the 
base-emitter voltage of a transistor, having a negative temperature 
coefficient. This feedback voltage is applied to the inverting input of 
the operational amplifier. In the preferred embodiment, the transistors 
forming the operational amplifier are configured to generate the positive 
temperature coefficient signal to further reduce the size of the 
temperature sensor.

DETAILED DESCRIPTION 
FIG. 1 is a symbolic diagram of a temperature-to-voltage converter 100 in 
accordance with the present invention. Temperature-to-voltage converter 
100 includes an adder 110 connected to an amplifier 120 via a line 130. 
The output voltage V.sub.O of temperature-to-voltage converter 100 is 
provided from the output terminal of amplifier 120 on a line 140. 
Adder 110 is connected to a positive temperature coefficient voltage 
nV.sub.T and to a negative temperature coefficient voltage V.sub.N, where 
V.sub.T is equal to the well known quotient KT/q, where K is Boltzmann's 
constant, T is temperature in Kelvin, and q is the charge of an electron. 
In one embodiment, negative temperature coefficient V.sub.N is based on 
the base-emitter voltage (V.sub.BE) drop across a transistor and varies at 
approximately -2 mV/.degree.C. (i.e., dV.sub.N /dT=-2 mV/.degree.C.). The 
"+" and "-" signs adjacent adder 110 indicate that negative temperature 
coefficient voltage V.sub.N is subtracted from positive temperature 
coefficient voltage nV.sub.T. (The term adder is being used in its formal 
sense to include both a summer and a subtractor.) 
Amplifier 120 provides amplification of the signal on line 130 by a factor 
m. Thus, the output voltage V.sub.O of temperature-to-voltage converter 
100 on output terminal 235 is: 
EQU V.sub.O =(nV.sub.T-V.sub.N)m EQ. 1! 
or 
EQU V.sub.O =mnV.sub.T -mV.sub.N EQ. 2! 
As compared with conventional temperature-to-voltage converters that rely 
on a positive temperature coefficient voltage nV.sub.T alone to provide a 
temperature coefficient, temperature-to-voltage converter 100 provides a 
lower output voltage V.sub.0, and consequently enables the use of a lower 
operating voltage in many applications. 
The temperature-to-voltage relationship of output voltage V.sub.O may be 
set by selecting appropriate values for multipliers n and m. In the 
following example, n and m are selected so that temperature-to-voltage 
converter 100 provides an output voltage V.sub.O that varies, at the 
industry standard rate of 10 mV/.degree.C., from 300 mV at room 
temperature (25.degree. C.) to 2 volts at approximately 200.degree. C. 
At room temperature (25.degree. C.), V.sub.T is 25 mV and V.sub.N is 600 mV 
(the V.sub.BE of a transistor). Solving equation 2! for n using the 
desired room-temperature output voltage V.sub.O of 300 mV and the 
respective room-temperature values of V.sub.T and V.sub.N of 25 mV and 600 
mV provides: 
##EQU1## 
Variables n and m are selected such that the output voltage V.sub.O varies 
with temperature at the rate of 10 mV/.degree.C. (i.e., dV.sub.O /dT 32 10 
mV/.degree.C.). Taking the derivative of equation 2! with respect to 
temperature T provides: 
##EQU2## 
Substituting 10 mV/.degree.C., 0.1 mV/.degree.C. and -2 mV/.degree.C. for 
dV.sub.O /dT, V.sub.T /dT, and V.sub.N /dT, respectively, into equation 
4! and solving for n: 
EQU n=100/m-20 EQ. 5! 
Combining equations 3! and 5! and solving gives values n=30 and m=2. 
Based on the foregoing analysis, the output voltage V.sub.O =60V.sub.T 
-2V.sub.N. Of course, other values of m and n may be designated to provide 
different temperature-to-voltage characteristics. 
FIG. 2 is a schematic diagram of a temperature-to-voltage converter 200 in 
accordance with one embodiment of the present invention. Converter 200 
includes an input circuit 210 connected to the non-inverting input (+) of 
an operational amplifier 220 via a line 230. Operational amplifier 220 
includes an output terminal 235 connected to ground potential via a pair 
of series-connected resistors R.sub.1 and R.sub.2. The common terminal 
between resistors R.sub.1 and R.sub.2 is connected, via a line 240, to an 
input terminal of a feedback circuit 250. Finally, feedback circuit 250 
includes an output terminal connected, via a line 260, to the inverting 
input (-) of operational amplifier 220. 
Input circuit 210 provides a positive temperature coefficient voltage 
nV.sub.T to the non-inverting input (+) of amplifier 220. In one 
embodiment, the positive temperature coefficient voltage V.sub.T varies 
with temperature at approximately n.times.0.1 mV/.degree.C. Feedback 
circuit 250, on the other hand, provides a negative temperature 
coefficient voltage V.sub.N to the inverting input (-) of amplifier 220. 
In one embodiment, the negative temperature coefficient voltage V.sub.N is 
related to a base-to-emitter voltage (Vbe) of a transistor, and varies 
with temperature at approximately -2 mV/.degree.C. 
Series-connected resistors R.sub.1, and R.sub.2 form a voltage divider that 
provides an offset voltage V.sub.OFF to which is added voltage V.sub.N . 
The sum of the voltages V.sub.OFF and V.sub.N is applied to the inverting 
input (-) of amplifier 220. Thus configured, the output voltage V.sub.O on 
output terminal 235 is: 
##EQU3## 
where 
##EQU4## 
As discussed above in connection with FIG. 1, m is assumed to be set equal 
to 2; therefore, resistors R.sub.1 and R.sub.2 are equal. In one 
embodiment, resistors R.sub.1 and R.sub.2 are each 100K.OMEGA.. 
FIG. 3 is a schematic diagram of an embodiment of converter 200 in which 
feedback circuit 250 includes a PNP transistor 252 and a current source 
254 configured as a common-collector amplifier. In this configuration, the 
negative temperature coefficient voltage V.sub.N is the base-emitter 
voltage Vbe of transistor 252 (i.e., V.sub.N =Vbe) . In one embodiment, 
current source 254 provides a constant current of 2 uA. The Vbe voltage 
provides an offset voltage which lowers the output voltage to enable the 
temperature sensor to operate with voltages as low as 2 volts. 
FIG. 4 is a schematic diagram an embodiment of input circuit 210 of FIG. 2. 
Circuit 210 includes a pair of NPN transistors 410 and 415 each having an 
emitter connected to one terminal of a conventional current source 420 
providing a current equal to 2I, where I is, for example, 2uA. The base 
and collector of transistor 410 are connected to a current source 425, 
which provides a current of 2I, and to the emitter of a PNP transistor 
430. The collector of transistor 415 is connected to the base of an NPN 
output transistor 435 and to a current source 440, while the base of 
transistor 415 is connected to the emitter of a PNP transistor 445 and to 
a current source 450. Each of current sources 440 and 450 provides a 
current of I. Finally, the base of transistor 445 is connected to the 
emitter of transistor 435, to the base of transistor 430 via a resistor 
R.sub.T, and to ground potential via resistor R.sub.T and a bias resistor 
R.sub.B. 
Because of current source 420, the collective current through transistors 
410 and 415 is equal to 2I. Further, neglecting the base currents of 
transistors 415 and 435, the current through transistor 415 is equal to I. 
Subtracting the current (I) through transistor 415 from the total current 
(2I) through transistors 410 and 415 indicates that the current through 
transistor 410 must also be equal to I. Therefore, the currents through 
transistors 410 and 415 are equal. 
It is commonly known in the art that the base-emitter voltage V.sub.BE of a 
transistor in the forward active region is dependent upon the current 
through that transistor and upon the saturation current I.sub.S for that 
transistor. This relationship may be expressed as follows: 
##EQU5## 
where I is the current through the transistor and V.sub.T is the thermal 
voltage (i.e., KT/q). 
It is also commonly known in the art that the saturation current I.sub.S 
for a given transistor is dependent upon. the area of the base-emitter 
region of that transistor. The relationship between saturation current 
I.sub.S and base-emitter area A is conventionally expressed as follows: 
##EQU6## 
where D.sub.n is the average effective value of the electron diffusion 
constant in the base, n.sub.i is the intrinsic carrier concentration in 
silicon, q is the charge of an electron, and Q.sub.B is the total base 
doping per unit area. 
For purposes of the present application, the important aspect of the 
relationship expressed in equation 9 is that the saturation current 
I.sub.S varies linearly with base-emitter area A. Furthermore, referring 
back to equation 8, because saturation current I.sub.S varies with 
base-emitter area A, so too does the base-emitter voltage V.sub.BE vary 
with base-emitter area A. 
The respective xA and A designations on transistors 410 and 415 indicate 
that the base-emitter area of transistor 410 is x-times greater than that 
of transistor 415. Furthermore, due to the relationship of equation 3, the 
saturation current I.sub.S410 of transistor 410 is x-times greater than 
the saturation current I.sub.S415 of transistor 415. Stated 
mathematically, 
EQU I.sub.S410 =xI.sub.S415 EQ. 10! 
Returning to equation 8, the base-emitter voltage V.sub.BE410 of transistor 
410 may be expressed as: 
##EQU7## 
Substituting for IS410 using the relationship of equation 1091, equation 
11 becomes: 
##EQU8## 
Again using equation 8, the base-emitter voltage V.sub.BE415 of transistor 
415 may be expressed as: 
##EQU9## 
Because the emitters of transistors 410 and 415 are connected together, the 
voltages on the respective bases of transistors 410 and 415 differ by the 
magnitude of the difference between respective base-emitter voltages 
V.sub.BE410 and V.sub.BE415. Further, transistors 430 and 445, being of 
similar size and having equivalent collector currents I, exhibit 
equivalent base-emitter voltages. Consequently, the voltage difference 
between the bases of transistors 430 and 445 is equal to the magnitude of 
the difference between base-emitter voltages V.sub.BE410 and V.sub.BE415. 
The voltage difference between the bases of transistors 430 and 445 is 
expressed across resistor R.sub.T, and may be represented as V.sub.BE415 
minus V.sub.BE410. Using equations 12 and 13, 
##EQU10## 
which reduces to: 
EQU V.sub.BE415 -V.sub.BE410 =V.sub.T ln(x) EQ. 15! 
Once again neglecting base current, the reference voltage V.sub.R on 
terminal 455 may be expressed as: 
##EQU11## 
For a given circuit, the quantity 
##EQU12## 
is a constant. A designator n may be used to represent that constant, so 
that equation 16 reduces to V.sub.R =nV.sub.T, as indicated in FIG. 4. In 
an embodiment in which n equals 30, the value of x is 16 and the values of 
R.sub.T and R.sub.B are 30K.OMEGA. and 294.6K.OMEGA., respectively. 
FIG. 5 is a schematic diagram of a temperature-to-voltage converter 500 
that combines the functions of input circuit 210, operational amplifier 
220, and feedback circuit 250 of FIG. 2 into a low cost and highly 
efficient circuit. The temperature-to-voltage converter 500 may be 
operated with very low supply voltages. 
Circuit 500 includes a pair of PNP transistors 510 and 515 each having an 
emitter connected to a collector of a transistor 517 configured as a 
conventional current source providing a current equal to 2I, where I is 
e.g., 2uA. 
The base of transistor 517 is connected to the bases of transistors 550 and 
555, which are in turn connected to a bias-voltage terminal V.sub.B. 
The collectors of transistors 510 and 515 are connected to the respective 
collectors of transistors 520 and 525, which are configured as a 
conventional current mirror 530. The base of transistor 510 is connected 
to ground potential via a bias resistor R.sub.B, and to the base of 
transistor 515 via a resistor R.sub.T. 
Current mirror 530 forces the current through transistors 510 and 515 to be 
equal. As described above in connection with FIG. 4, the base-emitter 
voltage VBE of a given transistor depends upon the base-emitter area A of 
that transistor. The respective A and xA designations on transistors 510 
and 515 indicate that the base-emitter area of transistor 515 is x-times 
greater than that of transistor 510. 
Using the same analysis provided above in connection with FIG. 4, the 
reference voltage nV.sub.T on the base of transistor 515 may be expressed 
as: 
##EQU13## 
As for the circuit of FIG. 4, the quantity 
##EQU14## 
may be represented as the constant n. 
The voltage nV.sub.T is applied to the emitter of a transistor 535. The 
base-emitter voltage Vbe of transistor 535 shifts the voltage nV.sub.T 
down by the voltage Vbe so that the voltage (nV.sub.T -Vbe) is available 
at the common connection of series-connected resistors R.sub.1 and 
R.sub.2. Resistors R.sub.1 and R.sub.2 form a voltage divider that 
determines the amplification factor m, as discussed above in connection 
with FIG. 2. 
The output signal from differential pair 510, 515 is taken from the 
collector of transistor 510 to the base of a transistor 540, which 
amplifies the output signal and provides the amplified signal to the base 
of an output transistor 545. A compensation capacitor C.sub.c, typically 
20pF, is connected between the base and collector of transistor 540 to 
improve the AC response of temperature-to-voltage converter 500. 
The various embodiments of temperature sensors in accordance with the 
present invention may be formed in a silicon substrate or located 
proximate to a silicon substrate to sense the temperature of the 
substrate. Thus configured, the output terminal of the temperature sensor 
may be connected to a control terminal to shut down a circuit's operation 
or to otherwise adjust the performance of an external circuit. 
While particular embodiments of the present invention have been shown and 
described, it will be obvious to those skilled in the art that changes and 
modifications may be made without departing from this invention in its 
broader aspects. For example, a voltage with a positive temperature 
coefficient can be subtracted from or added to nV.sub.T. The appended 
claims encompass within their scope all such changes and modifications.