U.S. patent application number 12/327699 was filed with the patent office on 2010-06-03 for bandgap-referenced thermal sensor.
This patent application is currently assigned to Micrel, Incorporated. Invention is credited to Paul AD Smith, Paul Wilson.
Application Number | 20100134180 12/327699 |
Document ID | / |
Family ID | 42184286 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100134180 |
Kind Code |
A1 |
Smith; Paul AD ; et
al. |
June 3, 2010 |
BANDGAP-REFERENCED THERMAL SENSOR
Abstract
A thermal sensor for an integrated circuit including a bandgap
reference circuit. The thermal sensor includes a comparator that
compares a temperature dependent voltage generated by the bandgap
reference circuit to a temperature independent voltage, where both
temperatures are referenced to the bandgap reference voltage
generated by the bandgap reference circuit. The thermal sensor
generates a digital output control signal based on a predetermined
relationship between the temperature dependent voltage and the
temperature independent reference voltage. When used as a thermal
shutdown circuit, the comparator generates a thermal shut-down
signal when the dependent temperature voltage decreases (or
increases) with rising system temperature to equal to the
temperature independent reference voltage. The comparator is
implemented using an operational amplifier that is connected to
existing circuitry associated with the bandgap reference
circuit.
Inventors: |
Smith; Paul AD; (Glasgow,
GB) ; Wilson; Paul; (Linlithgow, GB) |
Correspondence
Address: |
BEVER HOFFMAN & HARMS, LLP;901 Campisi Way
Suite 370
Campbell
CA
95008
US
|
Assignee: |
Micrel, Incorporated
San Jose
CA
|
Family ID: |
42184286 |
Appl. No.: |
12/327699 |
Filed: |
December 3, 2008 |
Current U.S.
Class: |
327/539 |
Current CPC
Class: |
G05F 3/30 20130101; G01K
3/005 20130101; G01K 7/01 20130101 |
Class at
Publication: |
327/539 |
International
Class: |
G05F 1/10 20060101
G05F001/10 |
Claims
1. (canceled)
2. The integrated circuit according to claim 5, wherein said means
for generating a temperature independent voltage comprises a
voltage divider connected between said bandgap reference voltage
and ground.
3. (canceled)
4. (canceled)
5. An integrated circuit including: a bandgap reference circuit for
generating a bandgap reference voltage and a temperature dependent
voltage that varies in proportion to an ambient temperature of said
integrated circuit and is referenced to said bandgap reference
voltage; means for generating a temperature independent voltage
that is independent of said ambient temperature of said integrated
circuit and is referenced to said bandgap reference voltage, and a
thermal sensor including means for comparing said temperature
independent voltage and said temperature dependent voltage, and for
generating a control signal based on a predetermined relationship
between said temperature dependent voltage and said temperature
independent reference voltage, wherein said thermal sensor
comprises a thermal shutdown circuit including means for generating
a thermal shutdown signal when said temperature dependent voltage
is equal to said temperature independent voltage, wherein said
thermal shutdown circuit comprises a first operational amplifier
including an inverting input terminal connected to receive said
temperature dependent voltage, and a non-inverting input terminal
connected to receive said temperature independent voltage, and
wherein said bandgap reference circuit comprises: a second
operational amplifier having an output terminal, an inverting input
terminal, and a non-inverting input terminal; a first resistor and
a first diode connected in series between the output terminal of
the second operational amplifier and ground, wherein the
non-inverting input terminal of the second operational amplifier is
connected to a first node disposed between said first resistor and
said first diode; and a second resistor, a third resistor, a fourth
resistor, and a second diode connected in series between the output
terminal of the second operational amplifier and ground, wherein
the inverting input terminal of the second operational amplifier is
connected to a second node disposed between said second and third
resistors, wherein the inverting input terminal of the first
operational amplifier is connected to a third node disposed between
said third and fourth resistors.
6. The integrated circuit according to claim 5, wherein said means
for generating said temperature independent voltage comprises a
voltage divider including a fifth resistor and a sixth resistor
connected between the output terminal of the second operational
amplifier and ground, and wherein the non-inverting input terminal
of the first operational amplifier is connected to a fourth node
disposed between said fifth and sixth resistors.
7. (canceled)
8. (canceled)
9. An integrated circuit including: a bandgap reference circuit for
generating a bandgap reference voltage and a temperature dependent
voltage that is referenced to said bandgap reference voltage and is
proportional to an ambient temperature of said integrated circuit;
and a thermal shutdown circuit including means for comparing a
temperature independent voltage that is referenced to said bandgap
reference voltage and is independent of said ambient temperature of
said integrated circuit with said temperature dependent voltage,
and for generating a thermal shutdown signal when the temperature
dependent voltage is equal to the temperature independent voltage,
wherein said thermal shutdown circuit comprises a first operational
amplifier including an inverting input terminal connected to
receive said temperature dependent voltage, and a non-inverting
input terminal connected to receive said temperature independent
voltage, and wherein said bandgap reference circuit comprises: a
second operational amplifier having an output terminal, a
non-inverting input terminal, and a non-inverting input terminal; a
first resistor and a first diode connected in series between the
output terminal of the second operational amplifier and ground,
wherein the non-inverting input terminal of the second operational
amplifier is connected to a first node disposed between said first
resistor and said first diode; and a second resistor, a third
resistor, a fourth resistor, and a second diode connected in series
between the output terminal of the second operational amplifier and
ground, wherein the inverting input terminal of the second
operational amplifier is connected to a second node disposed
between said second and third resistors, wherein the inverting
input terminal of the first operational amplifier is connected to a
third node disposed between said third and fourth resistors.
10. The integrated circuit according to claim 9, further comprising
a voltage divider including a fifth resistor and a sixth resistor
connected between the output terminal of the second operational
amplifier and ground, and wherein the non-inverting input terminal
of the first operational amplifier is connected to a fourth node
disposed between said fifth and sixth resistors.
11. (canceled)
12. An integrated circuit comprising: a thermal sensor comprising a
first operational amplifier; a bandgap reference circuit including
a second operational amplifier having an output terminal, an
inverting input terminal, and a non-inverting input terminal, a
first resistor and a first diode connected in series between the
output terminal of the second operational amplifier and ground,
wherein the non-inverting input terminal of the second operational
amplifier is connected to a first node disposed between said first
resistor and said first diode, and a second resistor, a third
resistor, a fourth resistor, and a second diode connected in series
between the output terminal of the second operational amplifier and
ground, wherein the inverting input terminal of the second
operational amplifier is connected to a second node disposed
between said second and third resistors; and a voltage divider
including a fifth resistor and a sixth resistor connected between
the output terminal of the second operational amplifier and ground,
wherein a first input terminal of the first operational amplifier
is connected to a third node disposed between said third and fourth
resistors, and wherein a second input terminal of the first
operational amplifier is connected to a fourth node disposed
between said fifth and sixth resistors.
Description
FIELD OF THE INVENTION
[0001] This invention relates to integrated circuits, and more
particularly to integrated circuits that include both bandgap
reference circuits and thermal sensor circuits (e.g., thermal
shutdown circuits).
BACKGROUND OF THE INVENTION
[0002] FIG. 4 is a simplified circuit diagram showing an integrated
circuit 50 including a conventional bandgap reference circuit 51
for generating a bandgap reference voltage V.sub.bg, a conventional
thermal shutdown circuit 52 for generating a thermal shutdown
control signal TSD, and a generalized functional circuit (e.g., a
Power Management IC circuit) 53 that utilizes bandgap reference
voltage V.sub.bg during normal operation and is shutdown by thermal
shutdown control signal TSD when an operating temperature of IC 50
equals or exceeds a predetermined maximum operating
temperature.
[0003] Referring to the left side of FIG. 4, bandgap reference
circuit 51 includes an operational amplifier (op amp) C1 that
generates bandgap reference voltage V.sub.bg at its output
terminal. The non-inverting (+) input terminal of bandgap reference
circuit 51 is connected between a resistor R1 and a diode Q1, which
are connected in series between bandgap reference voltage V.sub.bg
and ground. The inverting (-) input terminal of bandgap reference
circuit 51 is connected between resistors R2 and R3, which are
connected in series with a diode Q2 between bandgap reference
voltage V.sub.bg and ground. Those skilled in the art recognize
that bandgap reference circuit 51 represents only one of several
possible circuit arrangements capable of generating the desired
bandgap reference voltage V.sub.bg. The particular circuit
structure of bandgap reference circuit 51 is disclosed in Chapter
4, Appendix A4.3 of "Analysis and Design of Analog Integrated
Circuits 2.sup.ND Edition", Paul R. Gray and Robert G. Meyer
(copyright 1977 by Wiley & Sons, Inc.).
[0004] In general, bandgap reference circuits function as
temperature independent voltage reference circuits to provide a
bandgap reference voltage V.sub.bg at a voltage level typically
around 1.25 V, which is close to the theoretical 1.22 eV bandgap of
silicon at 0.degree. K. In the example shown in FIG. 4, when power
is supplied to IC 50, bandgap reference circuit 51 operates as
follows. Assuming a stable operating point exists, then the
differential input voltage of op amp C1 must be zero, and resistors
R1 and R2 have equal voltages across them. Thus, the two currents
I.sub.1 and I.sub.2 must have a ratio determined by the ratio of
resistors R1 to R2. These two currents are the collector currents
of the two diodes Q1 and Q2 (e.g., diode-connected transistors),
assuming base currents are negligible. Thus, the difference between
their base-emitter voltages can be represented by Equation 1:
.DELTA. V be = V T ln I 1 I 2 I S 2 I S 1 = V T ln R 2 R 1 I S 2 I
S 1 Eq . 1 ##EQU00001##
where V.sub.T is the threshold voltage across resistor R3, and
I.sub.S1 and I.sub.S2 represent the saturation currents of diodes
Q1 and Q2, respectively. The same current that flows through
resistor R3 also flows through resistor R2, so the voltage across
resistor R2 is represented by Equation 2:
V R 2 = R 2 R 3 .DELTA. V be = R 2 R 3 V T ln R 2 R 1 I S 2 I S 1
Eq . 2 ##EQU00002##
Note that Eq. 2 implies that the currents I.sub.1 and I.sub.2 are
both proportional to temperature if the resistors have zero
temperature coefficient. The output voltage of op amp C1 (i.e.,
bandgap reference voltage V.sub.bg) is thus represented by Equation
3:
V bg = V be + R 2 R 3 V T ln R 2 R 1 I S 2 I S 1 = V be + KV T Eq .
3 ##EQU00003##
Eq. 3 implies that bandgap reference voltage V.sub.bg is
substantially independent of temperature, with the constant K set
by the ratios of R2/R1, R2/R3 and I.sub.S2/I.sub.S1.
[0005] Referring again to FIG. 4, due to process variations and
component mismatches typically encountered during the production of
integrated circuits, bandgap reference circuit 51 is typically
designed in a way that allows "trimming" of resistors R1, R2 and R3
at the end of the fabrication process in order to produce the
desired bandgap reference voltage V.sub.bg. This trimming process
typically involves providing several resistors connected in series,
and a mechanism (e.g., programmable elements) that can be used to
bypass some of the resistors in order to supply accurate voltages
to op amp C1. An exemplary trimmable resistor circuit is shown in
FIG. 5, where resistors R21 to R24 are connected in series, with
each node N21 to N23 between adjacent resistors connected to op amp
C1 by way of programmable elements (e.g., fuses, anti-fuses or pass
gates) P1 to P3. In this example, resistor R2 (see FIG. 4) is
trimmed by selectively turning on zero or more of programmable
elements P1 to P3. For example, the resistance of resistor R2 is
minimized by closing programmable element P1 such that op amp C1 is
coupled to bandgap reference voltage V.sub.bg by way of node N21,
whereby the total resistance of resistor R2 is equal to the
resistance of resistor R21. Conversely, resistor R2 is maximized by
leaving open all programmable elements P11 to P13 such that op amp
C1 is coupled to bandgap reference voltage V.sub.bg by way of node
N24, whereby the total resistance of resistor R2 is equal to the
sum of the resistances of resistors R21+R22+R23+R24.
[0006] Referring again to FIG. 4, one further common practice
associated with the production of ICs including bandgap reference
circuits is to provide a series of resistors R.sub.A to R.sub.D
that are connected to bandgap reference voltage V.sub.bg, and
provide temperature independent reference voltage V.sub.REF1 to
V.sub.REF3, which are tapped from node N.sub.AB, N.sub.BC and
N.sub.CD, respectively. Reference voltages between V.sub.bg and GND
are easily generated for various PMIC functions.
[0007] Referring again to FIG. 4, thermal shutdown circuit 52
includes a thermal sensor 55 whose voltage is compared by an op amp
57 to a supplied reference voltage V.sub.REF, whereby thermal
shutdown control signal TSD when the sensor voltage V.sub.SENSOR is
equal to the reference voltage. Such thermal shutdown circuits are
described, for example, in "Development of a 1 MHz MOSFET
gate-driver for integrated converters", M. A. de Rooij, J. T.
Strydom and J. D. van Wyk, P. Beamer (IEEE publication
0-7803-7420-7/02 (2002)), in which the disclosed thermal sensor
uses is a thermistor whose output signal is dedicated to the
thermal shutdown of a functional circuit.
[0008] A problem associated with including thermal shutdown circuit
52 is that thermistor 55 and its associated circuitry are
additional to any other thermally sensitive circuitry present on an
IC having a bandgap reference circuit, and therefore take up a
significant amount of valuable silicon area, consume a significant
amount of power, and typically require a separate trimming
operation (i.e., in addition to the trimming operation mentioned
above with reference to bandgap reference circuit 51).
[0009] What is needed is a thermal shutdown circuit that minimizes
silicon area and power consumption, and simplifies the trimming
operations associated with its host IC.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to a thermal sensor that
compares a temperature dependent voltage to a temperature
independent voltage, both temperatures being referenced to a
bandgap reference voltage, and generates a digital output control
signal based on a predetermined relationship between the
temperature dependent voltage and the temperature independent
reference voltage. For example, the thermal sensor may serve as a
thermal shutdown circuit that generates a thermal shut-down signal
when the dependent temperature voltage is equal to the temperature
independent reference voltage.
[0011] In accordance with an aspect of the invention, the
temperature dependent and temperature independent voltages utilized
by the thermal shutdown circuit are tapped from existing bandgap
reference circuitry, thereby providing an accurate thermal sensor
output signal simply by adding a circuit that operably compares two
selected voltages. The conventional bandgap reference circuits
provided on most integrated circuits (ICs) typically include
circuit structures (e.g., series-connected resistors/diodes) that
that produce temperature dependent voltages having diverging
temperature coefficients, and these temperature dependent voltages
are combined by the bandgap reference circuit to provide the
substantially temperature independent bandgap reference voltage. In
addition, these conventional bandgap reference circuits include
mechanisms (e.g., trimmable resistors, discussed above) that allow
accurate adjustment (trimming) of the bandgap reference voltage to
the desired voltage level. Further, each such IC typically includes
one or more voltage dividers referenced to the bandgap reference
voltage that provide reliable temperature independent reference
voltages utilized by the functional circuitry of the IC. The
present invention takes advantage of this existing bandgap
reference circuitry simply by adding a selected comparator (e.g.,
an op amp) and providing metal lines that connect to selected nodes
at which selected temperature dependent and temperature independent
voltages are already generated. The present invention thereby
facilitates minimizing the amount of space and power utilized by a
thermal sensor added to an IC having a bandgap reference circuit,
particularly when compared to conventional thermal shutdown
approaches that utilize a dedicated thermistor and associated bias
circuitry.
[0012] Another advantage of the present invention is that, in
trimming the bandgap voltage, the temperature dependent and
temperature independent voltages utilized by the thermal sensor are
also trimmed "for free". That is, the present invention eliminates
the separate trim operations for the bandgap reference circuit and
the thermal sensor that are required by conventional approaches,
thereby further reducing manufacturing costs by automatically
trimming of the thermal sensor when the bandgap reference is
trimmed (i.e., the bandgap reference circuit and the thermal sensor
are trimmed simultaneously).
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other features, aspects and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings,
where:
[0014] FIG. 1 is a simplified circuit diagram showing a integrated
circuit including a thermal shutdown circuit according to a
generalized embodiment of the present invention;
[0015] FIG. 2 is simplified circuit diagram showing an integrated
circuit including a thermal shutdown circuit according to a
specific embodiment of the present invention;
[0016] FIG. 3 is a diagram depicting operating voltages associated
with the thermal shutdown circuit of FIG. 2;
[0017] FIG. 4 is a circuit diagram illustrating a conventional
bandgap reference circuit; and
[0018] FIG. 5 is a simplified diagram illustrating a trimmable
resistor circuit utilized by conventional bandgap reference
circuits.
DETAILED DESCRIPTION OF THE DRAWINGS
[0019] The present invention relates to an improvement in
integrated circuits incorporating bandgap reference voltages and
thermal shutdown functionality. The following description is
presented to enable one of ordinary skill in the art to make and
use the invention as provided in the context of a particular
application and its requirements. The terms "coupled" and
"connected", which are utilized herein, are defined as follows. The
term "connected" is used to describe a direct connection between
two circuit elements, for example, by way of a metal line formed in
accordance with normal integrated circuit fabrication techniques.
In contrast, the term "coupled" is used to describe either a direct
connection or an indirect connection between two circuit elements.
For example, two coupled elements may be directly connected by way
of a metal line, or indirectly connected by way of an intervening
circuit element (e.g., a capacitor, resistor, inductor, or by way
of the source/drain terminals of a transistor). Various
modifications to the preferred embodiment will be apparent to those
with skill in the art, and the general principles defined herein
may be applied to other embodiments. Therefore, the present
invention is not intended to be limited to the particular
embodiments shown and described, but is to be accorded the widest
scope consistent with the principles and novel features herein
disclosed.
[0020] FIG. 1 is a simplified circuit diagram showing an IC 100
including a generalized bandgap reference circuit 110 and a
functional circuit 120 that are similar to those found in
conventional ICs. Bandgap reference circuit 110, which is intended
to represent any of several well-know architectures, includes
voltage sources S1 and S2 that respectively generate a bandgap
reference voltage V.sub.bg and a temperature dependent voltage
V.sub.A according to known techniques, where temperature dependent
voltage V.sub.A is referenced to bandgap reference voltage
V.sub.bg, and is proportional to absolute temperature (PTAT). As
used herein, the phrase "referenced to bandgap reference voltage
V.sub.bg" is intended to mean that bandgap reference voltage
V.sub.bg serves as a supply voltage from which the referenced
voltage (e.g., temperature dependent voltage V.sub.A) is derived.
Referring to the right side of FIG. 1, in addition to application
specific circuitry (not shown) designed to perform a predetermined
function (e.g., to perform the functions of a Power Management IC),
functional circuit 120 includes series-connected resistors R.sub.5
and R.sub.6 that are connected between bandgap reference voltage
V.sub.bg and ground. As described above with reference to FIG. 4,
such series-connected resistors (voltage dividers) are typically
included on integrated circuits having bandgap circuitry to provide
temperature independent reference voltages that are less than
bandgap reference voltage V.sub.bg.
[0021] In accordance with the present invention, IC 100 also
includes a thermal sensor 130 including a comparator 135 that is
operably connected to compare temperature dependent voltage V.sub.A
with a selected temperature independent voltage V.sub.B, and to
generates a digital output control signal CS (e.g., a thermal
shut-down signal) whose voltage/current level is determined by a
selected relationship between temperature dependent voltage V.sub.A
and temperature independent voltage V.sub.B. For example, source S1
and resistors R.sub.5 and R.sub.6 may be selected such that
temperature dependent voltage V.sub.A and temperature independent
voltage V.sub.B when both voltages are equal when die temperature
of IC 100 has reached or exceeded a safe operating point.
Alternatively, the voltage level of control signal CS may increase
or decrease in response to ambient temperature, thereby
facilitating data that may be used for thermal analysis of IC
100.
[0022] As set forth above, in accordance with an aspect of the
invention, temperature dependent voltage V.sub.A and temperature
independent voltage V.sub.B utilized by thermal sensor 130 are
tapped from existing bandgap reference circuitry. That is,
temperature dependent voltage V.sub.A is generated by bandgap
reference circuit 110 in order to generate bandgap reference
voltage V.sub.bg in a manner similar to that described above with
reference to FIG. 4. Similarly, temperature independent voltage
V.sub.B is generated by series-connected resistors R.sub.5 and
R.sub.6, which are typically provided in functional circuit 120 as
a voltage divider for providing temperature independent reference
signals to the application specific circuitry (not shown) of
functional circuit 120. For example, referring to FIG. 4, resistors
R.sub.5 and R.sub.6 may be implemented by tapping node N.sub.AB,
whereby resistor R.sub.5 has the effective resistance of resistor
R.sub.A, and resistor R.sub.6 has the effective resistance of
resistors R.sub.B+R.sub.C+R.sub.D. Alternatively, resistors R.sub.5
and R.sub.6 may be implemented by tapping node N.sub.BC, whereby
resistor R.sub.5 has the effective resistance of resistors
R.sub.A+R.sub.B, and resistor R.sub.6 has the effective resistance
of resistors R.sub.C+R.sub.D. The value of control signal CS is
dependent on how the voltages V.sub.A and V.sub.B are selected, but
crucially they must remain referenced to bandgap reference voltage
V.sub.bg so that they are trimmed correctly, as described below. As
such, thermal sensor 130 provides an accurate thermal reading
simply by adding comparator 135 to an existing bandgap design, and
connecting the input terminals of comparator 135 to carefully
selected nodes that are referenced to bandgap reference voltage
V.sub.bg by way of metal lines metal lines added to the existing
metallization process that extend between input terminals of
comparator 135 and nodes on which temperature dependent voltage
V.sub.A and temperature independent voltage V.sub.B are generated.
Because the present invention only requires the addition of
comparator circuit 135 (and associated connections), the present
invention is implemented using a smaller amount of costly silicon
area and exhibits lower power consumption in comparison to
conventional approaches that utilize a dedicated thermistor and
associated bias circuitry.
[0023] According to another aspect of the present invention,
because both temperature dependent voltage V.sub.A and temperature
independent voltage V.sub.B are referenced to bandgap reference
voltage V.sub.BE, the act of trimming bandgap reference voltage
V.sub.BE (e.g., as described above with reference to FIGS. 4 and
5), both temperature dependent voltage V.sub.A and temperature
independent voltage V.sub.B are also trimmed "for free". That is,
the trimming operation performed to trim bandgap reference voltage
V.sub.BE also serves to trim thermal sensor 130. As such, the
present invention eliminates the separate trim operation required
in conventional approaches, thereby reducing manufacturing
costs.
[0024] FIG. 2 is a simplified circuit diagram showing an IC 100A
according to an exemplary specific embodiment of the present
invention. IC 100A includes bandgap reference circuit 110A, which
is substantially identical to conventional bandgap reference
circuit 50 (described above with reference to FIGS. 4 and 5), a
functional circuit 120 including resistors R5 and R6 (both
described above), and a thermal shutdown circuit (sensor) 130A.
[0025] Bandgap reference circuit 110A and functional circuit 120
are arranged and operate substantially in the manner described
above with reference to FIG. 4. Bandgap reference circuit 110A
includes an op amp (comparator) C.sub.1 that generates bandgap
reference voltage V.sub.bg at its output terminal, a resistor
R.sub.2 connected in series with a diode Q1 between bandgap
reference voltage V.sub.bg and ground, and resistors R.sub.1,
R.sub.3 and R.sub.4 that are connected in series with a diode Q2
between bandgap reference voltage V.sub.bg and ground. The
non-inverting (+) terminal of op amp C.sub.1 is connected to a node
between resistor R.sub.2 and diode Q1, and the inverting (-)
terminal of op amp C.sub.1 is connected to a node between resistors
R.sub.1 and R.sub.3. Similar to the arrangement described above
with reference to FIGS. 4 and 5, bandgap reference circuit 110A is
trimmed by adjusting the resistance of one or more of resistors
R.sub.1 to R.sub.4 until bandgap reference voltage V.sub.bg
achieves a desired voltage level. Similar to the generalized
embodiment (described above with reference to FIG. 1), functional
circuit 120 includes series-connected resistors R.sub.5 and R.sub.6
that are connected between bandgap reference voltage V.sub.bg and
ground.
[0026] In accordance with the present invention, thermal shutdown
circuit (sensor) 130A includes an op amp (comparator) 135A having a
non-inverting (+) terminal connected to receive a temperature
dependent voltage V.sub.A, and a inverting (-) terminal connected
to receive a temperature independent voltage V.sub.B. In the
present embodiment, temperature dependent voltage V.sub.A is
generated by tapping node N.sub.34 between resistors R.sub.3 and
R.sub.4 of bandgap reference circuit 110A (e.g., corresponding to
node N21 between resistor R21 and R22 of resistor R2; see FIG. 5).
As such, temperature dependent voltage V.sub.A is referenced to
bandgap reference voltage V.sub.bg, and is proportional to absolute
temperature V.sub.PTAT (specifically, V.sub.PTAT equals
V.sub.bg-V.sub.A). Similarly, temperature independent voltage
V.sub.B is tapped from the node between resistors R.sub.5 and
R.sub.6 of functional circuit 120. As such, temperature independent
voltage V.sub.B is also referenced to bandgap reference voltage
V.sub.bg. As depicted in FIG. 3, with this arrangement, op amp 135A
generates a thermal shutdown signal TSD at its output terminal when
temperature dependent voltage V.sub.A decreases to the point at
which it equals temperature independent voltage V.sub.B. That is,
as when IC 100A reaches a predetermined unsafe operating
temperature, temperature dependent voltage V.sub.A equals
temperature independent voltage V.sub.B, causing op amp 135A to
transmit thermal shutdown signal TSD to functional circuit 120. As
indicated in FIG. 3, predetermined unsafe operating temperature
T.sub.TRIP (i.e., the temperature at which digital thermal shutdown
signal TSD is tripped) is determined by selecting appropriate
values for resistances R.sub.3, R.sub.4, R.sub.5 and R.sub.6,
bandgap reference voltage V.sub.bg, bias current I.sub.1 and
I.sub.2 and diode ratio Q2:Q1. That is, the temperature T.sub.TRIP
at which op amp 135A asserts thermal shut-down signal TSD is
dependent on how the voltages V.sub.A and V.sub.B are selected, but
crucially they must remain referenced to bandgap reference voltage
V.sub.bg so that they are trimmed correctly.
[0027] According to an embodiment of the present invention, thermal
trip temperature T.sub.TRIP is calculated as follows:
V PTAT = V bg - V A ; V PTAT R 4 = .DELTA. V be R 1 V PTAT =
.DELTA. V be R 4 R 1 Eq . 4 V CONST = V bg - V B = V bg - V bg R 6
R 5 + R 6 = V bg ( 1 - R 6 R 5 + R 6 ) = V bg R 5 R 5 R 6 Eq . 5 T
TRIP occurs when V A = V B V PTAT = V CONST Eq . 6 .DELTA. V be R 4
R 1 = V bg R 5 R 5 + R 6 Eq . 7 now .DELTA. V be = V be 1 - V be 2
= V T ln ( I 1 I S 1 I S 2 I 2 ) In our example I 1 = 2 I 2 and I S
2 = N I S 1 .DELTA. V be = V T ln 2 N = kT q ln 2 N Eq . 8
##EQU00004##
[0028] re-arranging for T and substituting Eq. 7 into Eq. 8
gives:
T TRIP = V bg R 5 R 5 + R 6 R 1 R 4 q k ln 2 N Eq . 9
##EQU00005##
The resistor, diode and current ratios are well matched, and q and
k are constants so the trip temperature is only dependent on
V.sub.bg variation. Trimming V.sub.bg allows this variable to be
well maintained to within an arbitrary percentage range, resulting
in a trip temperature with approximately the same percentage range,
regardless of process variation.
[0029] Example values are inserted into Equation 9 (above) as
follows:
R 1 = 96.4 k ##EQU00006## R 2 = 224.9 k ##EQU00006.2## R 3 = 32.1 k
##EQU00006.3## R 4 = 417.7 k ##EQU00006.4## V bg R 5 R 5 + R 6 =
435 mV ##EQU00006.5## N = 8 ##EQU00006.6## T TRIP = 420 K = 147
.degree. C . ##EQU00006.7##
[0030] As set forth above, thermal shutdown circuit 130A provides
an accurate thermal trip point simply by adding comparator 135A to
an existing bandgap design, and connecting the input terminals of
op amp 135A to carefully selected nodes that are referenced to
bandgap reference voltage V.sub.bg. That is, the present invention
may be incorporated into existing integrated circuits having
bandgap reference circuits by adding op amp 135A and providing
metal lines between nodes on which temperature dependent voltage
V.sub.A and temperature independent voltage V.sub.B are generated.
Because the present invention only requires the addition of an op
amp (or other comparator circuit) and associated connections, the
present invention is implemented using a smaller amount of costly
silicon area and exhibits lower power consumption in comparison to
conventional thermal shutdown approaches that utilize a dedicated
thermistor and associated bias circuitry.
[0031] Although the present invention has been described with
respect to certain specific embodiments, it will be clear to those
skilled in the art that the inventive features of the present
invention are applicable to other embodiments as well, all of which
are intended to fall within the scope of the present invention.
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