Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-04-29
2003-11-04
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S316000, C323S907000
Reexamination Certificate
active
06642699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to the field of bandgap voltage reference circuits. In particular, the present invention relates to circuits and methods for providing a temperature-stable bandgap voltage reference using differential pairs to provide a temperature-curvature compensating current.
2. The Prior State of the Art
The accuracy of circuits often depends on access to a stable Direct Current (DC) reference voltage. One class of circuits that generates DC reference voltages is called “bandgap voltage reference circuits,” or “bandgap references” for short. Bandgap references use the bandgap voltage of the underlying semiconductor material (often crystalline silicon) to generate an internal DC reference voltage that is based on the bandgap voltage.
Many bandgap references forward bias the base-emitter region of a bipolar transistor to form a voltage V
BE
across its base-emitter region. V
BE
is then used to generate the internal DC reference voltage. V
BE
does, however, have some first-order, second-order and higher order temperature dependencies. Many bandgap references substantially eliminate the first-order temperature dependency by adding a Proportional-To-Absolute-Temperature (PTAT) voltage to V
BE
.
One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 3,887,863 (hereinafter referred to as the '863 patent), which issued Jun. 3, 1975 to A. P. Brokaw. The bandgap voltage reference circuit disclosed in the '863 patent relies upon a bandgap cell that is commonly referred to as a “Brokaw cell”.
Referring to
FIG. 1
, a schematic representation of a standard Brokaw cell
100
is shown. The Brokaw cell
100
comprises a pair of bipolar transistors (Q
1
and Q
2
) and a pair of resistors (R
1
and R
2
). The area of the base-emitter regions in Q
1
and Q
2
are indicated by A and unity, respectively, wherein A is greater than unity.
Referring to
FIG. 2
, a schematic representation of a bandgap voltage reference circuit
200
is shown incorporating a Brokaw cell
100
. In addition to the Brokaw cell
100
, the bandgap voltage reference circuit
200
comprises an operational transresistance amplifier R, as well as a pair of resistors R
3
and R
4
that allow the reference output voltage (V
OUT
) to exceed the bandgap voltage.
During operation, a voltage of V
BE
develops across the base-emitter region of bipolar transistor Q
2
. In addition, a PTAT voltage (termed V
PTAT
) develops across resistor R
2
. The base-emitter voltage (V
BE
) of a bipolar junction transistor has a negative temperature coefficient generally between −1.7 mV/degree C. and −2 mV/degree C. In other words, if the operating temperature of a bipolar transistor was to increase by one degree Celsius, the base-emitter voltage would decrease by a voltage in the range of from 1.7 to 2 mV. In contrast, the PTAT voltage has a positive temperature coefficient. In other words, as the temperature increases, so does the PTAT voltage. By matching the temperature coefficient of V
BE
of Q
2
to the temperature coefficient of V
PTAT
of R
2
, the first order temperature coefficient of V
B
can be made zero (or at least very close to zero) thereby significantly reducing temperature dependency.
Although the bandgap voltage reference circuit substantially eliminates first-order temperature dependencies in the output voltage, second and higher order temperature dependencies remain. In particular, a plot with temperature on the x-axis and output voltage on the y-axis results in an approximately parabolic curve that reaches a maximum at about the ambient temperature of the bandgap reference.
Some conventional bandgap references even substantially reduce much of the second and higher order temperature variations in the output voltage. One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 5,767,664 (hereinafter referred to as the '664 patent), which issued Jun. 16, 1998 to B. L. Price.
FIG. 3
illustrates such a bandgap reference
300
.
The bandgap reference
300
includes the conventional bandgap reference
200
of
FIG. 2
, but also includes a V-to-I converter circuit
304
with two differential pair segments
306
made up of MOSFETs M
1
-M
4
. A current mirror
308
is formed with MOSFETs M
5
and M
6
so as to extract a correction current, I
CORR
, from the V
B
node. The correction current reduces a significant portion of the remaining temperature dependencies that were present in the bandgap reference
200
. Accordingly, the voltage at node V
B
is relatively temperature stable. As a consequence, the output voltage of the bandgap reference
300
is a DC voltage that is relatively stable with temperature changes as compared to the prior bandgap reference
200
.
In order for the correction current to reduce temperature errors, the differential pairs
306
are tuned to provide an appropriate current component at given temperatures. One current source
308
is provided for each differential pair
306
. A PTAT voltage is applied to the gate terminal of the left MOSFET in each differential pair (e.g., M
1
for differential pair
306
′, and M
3
for differential pair
306
″). A substantially constant voltage is tapped onto the gate terminal of the right MOSFET in each differential pair (e.g., M
2
for differential pair
306
′, and M
4
for differential pair
306
″). As the temperature varies the voltage applied to the gate of the left MOSFET in each differential pair will change. Note that the relatively constant voltage applied to the gate of MOSFET M
2
will be lower that the relatively constant voltage applied at the gate of MOSFET M
4
due to the voltage division provided by resistors R
4A
, R
4B
and R
4C
.
Each of the differential pairs
306
generates a component of the correction current. For example, consider the differential pair
306
′ which contributes a component of the correction current. At very low temperatures, the gate voltage of MOSFET M
1
is lower than the gate voltage at M
2
. Accordingly, most of the current I
1
is diverted through M
1
to contribute to I
CORR
via current mirror
308
. However, the MOSFET M
4
is substantially off. Accordingly, at lower temperatures, the corrective current is approximately proportional to current I
1
.
As the temperature rises, the gate voltage of M
1
becomes the same as the gate voltage of M
2
. Accordingly, only half of the current I
1
would pass through M
1
to contribute to curvature correction current I
CORR
. This temperature is often referred to as the “crossing point”. At very high temperatures, the gate voltage of M
1
is higher than the gate voltage of M
2
. Accordingly, very little of the current I
1
passes through M
1
to contribute to the error current.
Accordingly, by adjusting the crossing point of each differential pair, one may change the current contribution profile of each differential pair until the sum of the contributions results in a correction current that generally reduces the temperature error in the output voltage. In
FIG. 3
, the crossing points are set by fine tuning the size of the resistors R
4A
, R
4B
, and R
4C
.
The bandgap reference
300
provides a significant improvement in the art. However, there is still some degree of temperature dependency in the output voltage, despite the correction current. Accordingly, what are desired are bandgap circuits and methods for more precisely generating a correction current so that temperature dependencies in the generated output current may be even further reduced.
SUMMARY OF THE INVENTION
The foregoing problems in the prior state of the art have been successfully overcome by the present invention, which is directed to bandgap reference circuits and methods that generate a correction current by using differential pairs using positive as well as negative temperature drift voltage sources to perform current steering or diversion in each differential pair.
In accordance with the present invention, a bandgap voltage reference circuit includes a ba
AMI Semiconductor Inc.
Sterrett Jeffrey
Workman Nydegger
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