Thermal measuring and testing – Temperature measurement – By electrical or magnetic heat sensor
Reexamination Certificate
1999-03-30
2001-02-06
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Temperature measurement
By electrical or magnetic heat sensor
C374S170000, C327S512000
Reexamination Certificate
active
06183131
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a digital temperature sensor and in particular to the linearization of the temperature measurement produced by a digital temperature sensor.
BACKGROUND
Digital temperature sensors are well known. Digital temperature sensors are typically used to generate a temperature reference for the circuits on the chip or other integrated chips.
Digital temperature sensors, such as voltage reference circuits, generate a temperature output signal by normalizing a thermometer voltage V
TEMP
, which is temperature dependent, to a reference voltage V
REF
. The analog to digital converter of the digital temperature sensor measures the temperature dependent voltage ratiometrically relative to the reference voltage. Thus, the output temperature signal of a digital temperature sensor can be expressed as follows:
F
(
T
)
=
V
TEMP
V
REF
equ
.
1
where F(T) is the thermometer transfer function, V
TEMP
is the temperature dependent thermometer voltage, and V
REF
is the reference voltage.
The reference voltage V
REF
is typically generated by a bandgap reference voltage circuit. As is well known in the art, a bandgap reference voltage V
REF
has two component voltages: a C*&Dgr;V
BE
(T) term, which is the scaled difference between two base emitter voltages and is proportional to absolute temperature (PTAT), and a V
BE
(T) term, which is one base emitter voltage. When these two component voltages, which have opposite and nearly equal temperature coefficients, are summed together correctly, as follows:
V
REF
=
V
BE
(
T
)
+
C
×
Δ
V
BE
(
T
)
,
equ. 1a
the resulting reference voltage V
REF
is nearly temperature independent. Unfortunately, there are second order errors intrinsic to the V
BE
(T) term that precludes the reference voltage from being truly temperature independent.
The reference voltage V
REF
can be expressed as a function of a physical constant, several device parameters, and, typically a single user driven parameter that determines the entire voltage-vs.-temperature characteristics of any conventional bandgap reference. The equation that expresses these relationships, which is derived by expansion of equation
1
a
, is the well-known Standard Model and is shown below:
V
REF
=
V
GO
+
γ
T
(
1
+
ln
T
0
T
)
;
where
γ
≡
k
q
[
4
-
α
-
η
]
equ
.
2
where V
GO
) is the so called “extrapolated bandgap voltage,” &ggr; is a combined process, device, and operating current temperature coefficient variable, &agr; and &eegr; are device dependent parameters, and T
0
is the user selectable parameter that defines the temperature at which V
REF
, reaches a peak value. Note that T
0
can be altered by adjusting the scaling factor C in equation
1
a
. When T
0
is chosen to occur at ambient temperature, i.e., T
0
=T
amb
, which is the convention in the prior art, the resulting reference voltage V
REF
has a parabolic curvature as shown in FIG.
1
. For more information relating to bandgap reference voltages and equation 2, see Gray & Meyer, “Analysis and Design of Analog Integrated Circuits,” 289-96 (John Wiley & Sons, 2nd ed. 1984), which is incorporated herein by reference.
FIG. 1
is a graph showing the well-known temperature dependence of a reference voltage V
REF
generated by a bandgap reference voltage circuit without additional correction circuitry. As shown in
FIG. 1
, the non-linear curvature of reference voltage V
REF
is parabolic over the operational temperature range, i.e., between temperatures 220° K and 400° K. Voltage reference circuits are conventionally trimmed to minimize reference voltage errors by adjusting T
0
so that the peak value of the reference voltage V
REF
occurs at ambient temperature T
amb
approximately 300° K, as shown in FIG.
1
. By assuring that the peak of voltage V
REF
is at ambient temperature T
amb
, the parabolic curvature of the reference voltage is approximately symmetrical about the peak value within the operational temperature range.
Conventionally, a pair of bipolar transistors forced to operate at a fixed non-unity current density ratio is used to generate a &Dgr;V
BE
(T) term, which is then scaled by C to produce the V
TEMP
term. The &Dgr;V
BE
(T) term is the difference between two base-emitter voltages, e.g., V
BE1
and V
BE2
, of two diodes or transistors operating at a constant ratio between their collector-current densities, where collector-current densities are defined as the ratio between the collector current to the emitter size. Thus, &Dgr;V
BE
(T) can be expressed accordingly:
Δ
V
BE
(
T
)
=
V
BE1
-
V
BE2
equ
.
3
where:
Δ
V
BE
(
T
)
=
kT
q
ln
(
J
1
J
2
)
equ
.
4
and V
BE1
and V
BE2
are the respective base-emitter voltages of a two transistors or diodes, k is Boltzman's constant, T is the absolute temperature (°K), q is the electronic charge, J
1
and J
2
are the respective current densities in two transistors or diodes, the ratio of which is intended to be fixed with regard to temperature.
As can be seen in equation 4, under the above conditions, the &Dgr;V
BE
(T) term is precisely linear with temperature. The temperature dependent thermometer voltage V
TEMP
is generated by combining the &Dgr;V
BE
(T) term with a scaling factor C as follows.
V
TEMP
=
Δ
V
BE
(
T
)
×
C
==
C
β
T
;
where
β
≡
k
q
ln
(
J
1
J
2
)
equ
.
5
The scaling factor C is typically used to amplify and adjust the value of the temperature dependent voltage V
TEMP
.
FIG. 2
is a graph showing a temperature dependent voltage thermometer V
TEMP
generated by a circuit consisting of a pair of bipolar transistors operating at a fixed non-unity current density ratio to produce a &Dgr;V
BE
, which is then scaled by C.
It is necessary to distinguish here between the thermometer voltage V
TEMP
, which is produced in this example by the scaled &Dgr;V
BE
from two bipolor devices, and the &Dgr;V
BE
term in equation
1
a
, which is a component of V
REF
. Both are produced by the same mechanism in this example, i.e., by a pair of bipolar transistors, but they are not the same voltage. They may be from different pairs of bipolar devices in one embodiment of this invention, or may be produced from a single pair of bipolar devices, as an engineering choice, in another embodiment of this invention. In the following description, the term V
TEMP
is used to refer strictly to the thermometer voltage, which may or may not be reused to adjust the reference voltage V
REF
. Additionally, the thermometer voltage V
TEMP
may be produced by other means, and not only by the use of a pair of bipolor transistors, as discussed elsewhere in the present disclosure.
Combining equations 2 and 5 completes the right hand side of equation 1. Now, the thermometer transfer function F(T) for a conventional temperature sensor may be expressed as:
F
(
T
)
=
C
β
T
V
GO
+
γ
T
(
1
+
ln
T
0
T
)
.
equ
.
6
Because a digital temperature sensor system divides the precisely linear voltage V
TEMP
by the reference voltage V
REF
, which has a parabolic curvature error, the thermometer transfer function F(T) of the digital temperature sensor system has a corresponding but inverted parabolic curvature error.
FIG. 3
is a graph showing the temperature error in the thermometer transfer function F(T) generated by a conventional temperature sensor, which uses the ratio of V
TEMP
to V
REF
as stated in equation 1. As shown in
FIG. 3
, the resulting error of the temperature output signal is a non-linear curve that is the inverse of the parabolic reference voltage V
REF
, shown in FIG.
1
.
Because of individual part errors, each digital temperature sensor will generate a different amount of error at ambient temperature
Holloway Peter R.
Sheehan Gary S.
Subrahmayan Ravi
Bennett G. Bradley
Halbert Michael J.
Kwok Edward C.
National Semiconductor Corporation
Skjerven Morrill & MacPherson LLP
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