Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-10-04
2004-09-14
Berhane, Adolf (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06791307
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to temperature-compensated electronic reference circuits and components therefor, and is particularly directed to a new and improved voltage-controlled current generator, which is operative to generate an output current that exhibits a prescribed non-linear to linear characteristic with temperature when its control voltage range is restricted. Injecting this output current into a voltage reference circuit, such as a ‘Brokaw’ bandgap voltage reference, provides improved high-order curvature correction, yielding an output voltage whose variation over a temperature range (e.g., −20° C. to +125° C.) is extremely flat (e.g., within several hundreds of microvolts).
BACKGROUND OF THE INVENTION
FIG. 1
is a reduced complexity diagram of a conventional first-order, current-based bandgap voltage reference, which generates an output voltage that is substantially independent of temperature, by summing a plurality of components whose temperature coefficients vary in a mutually complementary manner. For this purpose, a current I
1
proportional to absolute temperature (PTAT) is supplied to a series circuit of a diode D
1
and a resistor R
1
(referenced to ground (GND)). The voltage V1 across diode D
1
has an inverse or complementary to absolute temperature (CTAT) characteristic. As a result, if the (PTAT) current I
1
is inversely proportional to the value of a resistor having the same temperature coefficient as that of the resistor R
1
, the temperature behaviors of the respective voltage drops across diode D
1
and resistor R
1
will be mutually complementary, making the output voltage V
2
at a terminal OUT substantially (to a first order) independent of temperature.
A non-limiting example of what is commonly referred to as a ‘Brokaw cell’ current mirror implementation of the temperature-compensated bandgap voltage reference of
FIG. 1
is schematically shown in
FIG. 2. A
current mirror circuit is formed of a first pair of MOSFETs including a MOSFET M
1
and a diode-connected MOSFET M
2
in a current mirror first leg containing a diode-connected MOSFET M
5
, and a second pair of MOSFETs comprised of diode-connected MOSFET M
3
and MOSFET M
4
in a second current mirror leg containing a MOSFET M
6
. MOSFETs M
1
and M
4
have their source-drain paths coupled in series with those MOSFETs M
5
and M
6
between voltage rail VDD and GND.
Diode-connected MOSFET M
2
has its gate connected in common with the gate of MOSFET M
1
, while MOSFET M
4
has its gate connected in common with the gate of diode-connected MOSFET M
3
. MOSFET M
2
has its source-drain path coupled in series with the collector-emitter path of a bipolar NPN transistor Q
1
and resistors R
2
and R
3
to GND. In a complementary manner, MOSFET M
3
has its source-drain path coupled in series with the collector-emitter path of a bipolar NPN transistor Q
2
and resistor R
3
to GND. The bases of transistors Q
1
and Q
2
are coupled to a voltage output terminal OUT. A MOSFET M
7
has its source-drain path coupled between voltage rail VDD and output node OUT, to which an output resistor RL referenced to GND is coupled. MOSFET M
7
has its gate coupled to the drain of MOSFET M
4
.
In the current mirror-based implementation of
FIG. 2
, the current flowing through MOSFETs M
2
and M
3
corresponds to the base-emitter difference voltage &Dgr;VBE divided by the value of resistor R
2
, and is PTAT. Thus, the current I
1
supplied through resistor R
3
produces a PTAT voltage thereacross which is combined with the CTAT VBE voltage V
1
across transistor Q
2
to derive an output voltage reference V
2
having a first-order compensated temperature coefficient. As shown in
FIG. 3
, the voltage V
2
of the bandgap reference circuit of
FIG. 2
varies with temperature in a substantially parabolic manner, and has a total variation on the order of 6.7 mV. A first order voltage references of the type shown in
FIG. 2
is capable of producing a reference voltage whose temperature coefficient typically falls between 20 to 100 ppm/° c.
FIG. 4
illustrates a high-order compensating modification of the current-based voltage reference of
FIG. 1
, which employs an additional current component I
2
having a non-linear temperature coefficient. This additional non-linear current is intended to compensate for high-order, temperature dependent terms from the contribution of voltage V
1
. In the voltage reference of
FIG. 4
, the resistor R
1
of reference circuit of
FIG. 1
is shown as series-connected resistors R
4
and R
5
, with an additional, high-order compensation non-linear current I
2
being supplied to the common connection of these two resistors.
One example of this type of bandgap voltage reference circuit that injects an additional non-linear current is disclosed in the U.S. Pat. No. 6,157,245 to Rincòn-Mora. (For a non-limiting example of additional prior art documentation showing another type of current-based bandgap reference circuit, attention may be directed to the U.S. Pat. No. 5,952,873 to Rincòn-Mora.) The above-referenced '245 patent describes that second-order compensation may be achieved by injecting an additional non-linear current whose temperature coefficient is proportional to the ‘square’ of PTAT (or I
2
PTAT), so that its characteristic is parabolic.
While the squared temperature coefficient of the additional current tends to provide a second-order improvement at relatively low temperatures, where the variation in slope of the parabolic (squared) temperature coefficient characteristic is relatively gradual, the performance of an I
2
PTAT current-based voltage reference undesirably degrades at higher temperatures, due to the increasingly steep slope of the injected current's parabolic characteristic at such temperatures.
SUMMARY OF THE INVENTION
In accordance with the present invention, shortcomings of conventional first- and second-order compensated voltage references, including those described above, are substantially reduced by injecting into a voltage reference circuit, such as a ‘Brokaw’ voltage reference, a high-order, compensation current derived from a voltage controlled, non-linear current generator. This non-linear current generator is configured to generate an output current whose temperature coefficient exhibits a prescribed non-linear-to-quasi-linear curvature when the input or control voltage range is restricted. As will be described, this particular current characteristic enables a voltage reference that incorporates such a non-linear current generator for high-order curvature correction to produce an output voltage whose variation over an operational temperature range (e.g., −20° C. to +125° C.) is extremely flat (e.g., within several hundreds of microvolts). The inclusion of the non-linear current generator described herein in a bandgap reference allows a simple, power and area efficient method to achieve a curvature corrected output voltage.
To this end, the non-linear current generator according to the invention comprises an input transistor, referenced to a first power supply rail and having its collector-emitter path coupled in series with a PN junction device, such as a diode-connected transistor, to series-connected resistors that are coupled to a second power supply rail. The control electrode or base of the input transistor is coupled to receive an input or ‘reference’ (control) voltage, whose value is restricted or maintained within an ‘optimum’ range, in accordance with the desired operational parameters of the diode-connected transistor. In particular, this control voltage is set to a value, such that, in the low temperature region of operational temperature range, the diode-connected bipolar transistor operates just below the non-linear transition or ‘knee’ of its non-linear transfer characteristic. An output transistor has its emitter coupled to the common connection of the series resistors and its base coupled in common with the base of the diode-connected transistor. The collector of the output
Berhane Adolf
Intersil America's Inc.
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