Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With amplifier or space discharge device
Reexamination Certificate
2001-05-30
2003-09-16
Nguyen, Vinh P. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With amplifier or space discharge device
C327S053000, C327S054000, C330S257000
Reexamination Certificate
active
06621259
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to circuits for measuring bi-directional currents across a current sense element, and more particularly to a simplified circuit for measuring bi-directional currents across a current sense element.
Current sense amplifiers, sometimes referred to as current shunt amplifiers, are typically used to measure the amount of current supplied by a power supply or battery to various types of electronic equipment, and also to measure the amount of current supplied by the electronic equipment back to the power supply. Several conventional approaches to the implementation of current sense amplifiers are known, including single polarity, low-side current sense amplifiers, low-side current sense amplifiers with bi-polar sensing, high-side switching current sense amplifiers, and bi-polar, high-side current sense amplifiers which detect the magnitude and polarity of current flowing from one device to another. These are disclosed in U.S. Pat. No. 5,498,984 entitled “High Side, Current Sense Amplifier Using a Symmetric Amplifier” issued Mar. 12, 1996 to Schaffer, which is believed to be the closest prior art.
FIG. 3 of the Schaffer patent shows a high-side current sense amplifier circuit in which a reference voltage V
REF
is connected to produce an offset voltage shift on the (−) input of the operational amplifier. This allows the amplitude and direction or polarity of the voltage drop across R
SENSE
for current flow through R
SENSE
in either direction to be indicated by means of a single voltage V
OUT
on a single output terminal. However, the current sense amplifier in FIG. 3 of the Schaffer patent requires that the operational amplifier be powered by the same supply voltage applied by the battery to the load. For example, if the battery output voltage is +12 volts, the +V
DD
supply voltage applied to the operational amplifier could not be +5 volts, because for most operational amplifiers it would not be permissible to apply a voltage greater than the +V
DD
supply voltage to the (−) input of the operational amplifier.
In FIG. 5 of the Schaffer patent, the disclosed bi-polar, high-side current sense amplifier has a symmetric architecture, and includes two sense inputs and two outputs. One output is active for positive input signals corresponding to current flowing from a battery through the sense resistor to a load. The other output is active for negative input signals corresponding to current flow in an opposite direction through the sense resistor. The two outputs are logically ORed to provide only one of the two outputs at a time. The operational amplifiers are powered by the same V
CC
voltage applied on conductor
54
to the load
46
. The V
CC
voltage does not have to be equal to the battery voltage, because the input stages of the operational amplifiers
48
and
49
are constructed so that the common mode input voltage can exceed the V
CC
voltage. The circuit described in the Schaffer patent requires two output terminals, one for indicating the magnitude of the current through the current sense resistor and the other for indicating the direction of current in the current sense resistor.
An important shortcoming of the circuit disclosed in FIG. 5 of the Schaffer patent is that it is very inaccurate for very low currents through sense resistor
42
. This is because for such very low currents, the voltage differential across sense resistor
42
is so small that the current flowing through either resistor RS
1
and transistor Q
1
or resistor RS
2
and transistor Q
2
is also very small, and that causes the feedback from the output
58
to the (+) input of the associated operational amplifier
48
or
49
to be very low. The low or reduced feedback results in low loop gain, and prevents the operational amplifier
48
or
49
from accurately producing the signal I
OUT
in conductor
58
if the current through the sense resistor
42
is very small. For example, if transistor Q
2
in FIG. 5 of the Schaffer is on, but the shunt current through sense resistor
42
is nearly zero, then feedback causes the output of operational amplifier
49
to attempt to go all the way to ground in order to turn off transistor Q
2
. However, as a practical matter, operational amplifier
49
is incapable of driving its output all the way to ground. By turning transistor Q
2
nearly off, the normal low-impedance feedback loop from the output
58
to the (+) input of operational amplifier
49
becomes a slow, high-impedance feedback loop. That is what results in a dramatic increase of the amount of error in the value of I
OUT
representing the magnitude of the very low (nearly zero) shunt current through sense resistor
42
.
Furthermore, if the current flowing between battery
44
and load
46
is very small, the voltage across sense resistor
42
may be significantly lower than the algebraic sum of the offset voltages of operational amplifiers
48
and
49
. For that reason, and also for the reason that the amplifier
18
is very inaccurate for low sense resistor currents, the determination of the direction of the sense resistor current by operational amplifier
56
is very uncertain over a considerable range of low currents through sense resistor
42
.
Thus, for low sense currents, the circuit disclosed in the Schaffer patent is incapable of accurately determining either the magnitude or the direction of the current flowing through the sense resistor.
Furthermore, the circuit described in the Schaffer patent requires use of two operational amplifiers and a comparator, and therefore is more complex and costly and dissipates more power than desirable.
Thus, there has been a long-standing unmet need for an improved, less costly, more accurate current sense amplifier which (1) provides a high degree of accuracy in measurement of the magnitude of the current flowing through a current shunt element and also provides a high degree of certainty of the direction of the current, and (2) also provides a single signal which accurately represents both the amplitude and polarity or direction of a current flowing through the current shunt element.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a current sense amplifier which is more accurate and less expensive than the closest prior art, and provides a single output signal indicative of both amplitude and direction of current through the sense resistor or the like.
It is another object of the invention to provide a high-side current sense amplifier which is more accurate and less expensive than the closest prior art, and provides a single output signal indicative of both amplitude and direction of current through the sense resistor or the like.
It is another object of the invention to provide a technique for using a current sense amplifier and an analog-to-digital converter in such a way as to avoid the effect of drift of a reference voltage on a reference-dependent offset voltage component of an output of the current sense amplifier.
It is another object of the invention to provide a current sense amplifier which is especially useful in providing a single analog output signal to an analog-to-digital converter to enable it to produce a digital output signal accurately representing both magnitude and direction of current flowing through a current shunt element.
It is another object of the invention to provide a current sense amplifier which is especially useful in measuring the amount of current in a feedback loop and providing a digital signal useful for controlling the feedback loop.
Briefly described, and in accordance with one embodiment, the invention provides a current sense amplifier (
10
) for measuring current flowing through a sense resistor (
12
) coupled between first (
11
) and second (
13
) terminals, respectively, of the current sense amplifier. The current sense amplifier includes a first amplifier (
18
) having a first input (
17
) coupled by a first resistor (
16
) to the first terminal (
11
) and a second input (
20
) coupled b
Jones David M.
Metzger Heinz-Juergen
Brady W. James
Nguyen Vinh P.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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