Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-04-13
2002-02-26
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S907000, C327S538000
Reexamination Certificate
active
06351111
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is related to commonly-owned co-pending application serial number entitled “CIRCUITS AND METHODS FOR PROVIDING A BANDGAP VOLTAGE REFERENCE USING COMPOSITE RESISTORS” filed on the same date herewith, which application is incorporated herein by reference in its entirety.
THE FIELD OF THE INVENTION
The present invention relates to the field of current reference circuits. In particular, the present invention relates to circuits and methods for providing a current reference with a controlled temperature coefficient using a series composite resistor.
BACKGROUND AND RELATED ART
Current reference circuits are used in many CMOS integrated circuits. The purpose of a current reference circuit is to provide a reference current that may be mirrored into other parts of the circuit. The reference current does fluctuate to a certain extent. Such fluctuations are acceptable in some applications, but not in others. Potential causes of reference current fluctuations are operating temperature changes and supply voltage fluctuations.
FIG. 4
illustrates a conventional current reference circuit
400
that provides a relatively stable reference current despite fluctuations in operating temperature and supply voltage. The circuit
400
includes a positive rail V
DD
and a negative rail V
SS
. A start up circuit (represented by all the circuitry above brackets
401
) includes PMOS transistors
4
M
1
,
4
M
2
, and
4
M
3
as well as capacitor
402
configured as shown in FIG.
4
. The circuitry other than the startup circuit
401
has two stable states upon startup, one that provides a reference current, and one that does not. The startup circuit
401
ensures that the remaining current reference circuitry assumes the stable state that results in a reference current.
Excluding the startup circuit
401
, the circuit
400
includes two potential current paths between the positive rail V
DD
and the negative rail V
SS
. One current path called a “reference leg” is identified as the circuitry above brackets
403
. The current path through the reference leg
403
is through the channel regions of diode-connected PMOS transistors
423
and
427
, the channel regions of NMOS transistors
431
and
435
, the resistor
439
and the bipolar transistor
441
. The other current path called the “mirror leg” is identified as the circuitry above brackets
404
. The current path through the mirror leg
404
is through the channel regions of PMOS transistors
421
and
425
, the channel regions of diode-connected NMOS transistors
429
and
433
, and the bipolar transistor
437
. The total emitter area of the bipolar transistor
441
is greater than the total emitter area of the bipolar transistor
437
.
Accordingly, the circuit
400
provides a current along the current path of the reference leg
403
that is relatively insensitive to supply voltage fluctuations. However, due to the temperature sensitivity of the bipolar transistors
437
and
441
, the voltage applied across the resistor
439
is largely proportional to absolute temperature. The current in the reference leg
403
may be mirrored into other parts of the circuit (not shown) as needed.
The circuit
400
is similar to early conventional CMOS current reference circuits, except that some conventional current reference circuits use only one PMOS transistor and/or only one NMOS transistor in each leg. However, devices that use more than one PMOS transistor and/or more than one NMOS transistor (called “cascoded” devices) reduce the dependency between supply voltage and reference current. Accordingly, cascoded devices provide more stable reference currents.
Another key difference is that in conventional current reference currents, the resistor
439
generally does not have a resistance that compensates for the voltage applied across the resistor being roughly proportional to absolute temperature. Thus, the current reference provided by conventional current reference circuits varies significantly with temperature. In contrast, the circuit
400
reduces the temperature dependency of conventional current reference circuits by providing resistor
439
as a polysilicon resistor which is custom doped so that the resistor
439
(and the corresponding reference leg) has current flowing therethrough during operation that is less dependent upon temperature within a given operating temperature range.
Therefore, the circuit
400
improves upon the prior state of the art by providing a reference current that is less dependent on supply voltage and temperature fluctuations. However, the circuit
400
also requires custom doping which generally introduces manufacturing inefficiencies.
FIG. 5
illustrates another conventional temperature compensated current reference circuit
500
. The circuit
500
provides current references that are also stable with temperature and supply voltage fluctuations. However, the circuit
500
does not require the custom doping that is required by the circuit
400
.
The circuit
500
is similar to the circuit
400
except that the circuit
500
is not cascoded. In addition, the circuit
500
includes a parallel composite resistor that includes resistor
556
and resistor
558
situated in parallel within the current flow of the reference leg. Standard processing steps may be used to form each of the parallel resistors. By having one resistor be manufactured by process steps that result in a temperature coefficient below a target temperature coefficient, and the other resistor be manufactured by process steps that results in a temperature coefficient above the target temperature coefficient, the sizes of each parallel resistor can be designed so that the parallel composite resistor generally achieves the target temperature coefficient. Thus, the circuit
500
allows for accurate temperature compensation in which the resistors may be formed by standard processes.
Parallel resistors tend to be quite large in order to provide a composite resistor having a given resistance. Accordingly, the parallel composite resistor occupies significant valuable chip space. Therefore, what is desired are circuits and methods for providing a reference current that has a controlled temperature coefficient, which is relatively stable with supply voltage fluctuations, which does not require custom doping, and which efficiently uses chip space.
SUMMARY OF THE INVENTION
In accordance with the present invention, circuits and methods for providing a reference current are described. A current reference circuit has a reference leg and a mirror leg configured such that the current in the reference leg is mirrored in the mirror leg and such that a reference current is generated in the reference leg, the reference current being relatively stable with supply voltage and temperature fluctuations. A series composite resistor is disposed in the reference leg. The series composite resistor minimizes space as compared to using a parallel composite resistor. In addition, custom doping is not required to fabricate the series composite resistor.
The current reference circuit is coupled to a high voltage source that is configured to supply a relatively high voltage during operation. In addition, the current reference circuit is coupled to a low voltage source that is configured to supply a relatively low voltage during operation. The current reference circuit defines two current paths between the high voltage source and the low voltage source, one current path being through a reference leg, and the other current path being through a mirror leg.
The reference leg includes a series of MOS transistors including at least one PMOS transistor that is electrically closer in the series to the high voltage source. In addition, the MOS transistors also include at least one NMOS transistor that is electrically closer in the series to the low voltage source.
The reference leg also includes a series composite resistor comprising at least two resistors coupled in series within the current path. The series composite resistor is disposed o
AMI Semiconductor Inc.
Berhane Adolf Deneke
Workman & Nydegger & Seeley
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