Circuit for providing a constant current

Details Circuit for providing a constant current Circuit for providing a constant current

2001-07-10

2003-05-06

Le, Dinh T. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

With specific source of supply or bias voltage

C323S315000

Reexamination Certificate

active

06559711

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a circuit for providing a constant current.
The invention also relates to a method of providing a constant current.
SUMMARY OF THE INVENTION
Such circuits are known for the generation of a constant current, independently of variations of temperature, supply voltage, etc. They are mainly used in analog circuits for providing a reference signal for the measurement of analog signals, for example in analog-digital converters or digital-analog converters, or for generating a constant supply current for, for example, sensors. Nowadays constant current references are derived from voltage reference circuits, so-called bandgap reference circuits. The conversion of a voltage to a current depends on the accuracy of a resistor or of the combination of a capacitor and a timer circuit for charging the capacitor by means of the voltage reference and discharging it so as to generate the output current. The components which are generally used for converting a reference voltage into a reference current, i.e. resistors and capacitors, have values which are usually temperature-dependent. In addition, the accuracy of a bandgap reference circuit depends on the compensation of temperature-dependent parameters of the circuit by means of other temperature-dependent parameters. Normally, this compensation is accurate only in a limited temperature range.
It is an object of the invention to provide a circuit for supplying a constant current which does not suffer the disadvantages outlined above.
The circuit according to the invention is for this purpose characterized by means for generating a first and a second of two substantially identical currents, means for supplying a differential current which is the difference between said two substantially identical currents to a first capacitor, means for supplying a variable charging current to at least one second capacitor, means for periodically discharging and subsequently charging again the first and the at least one second capacitor, means for generating a clock signal between two periodic discharges, which clock signal is a measure for the difference in voltage across the first and the at least one second capacitor, means for generating a setting signal for setting both the variable charging current and at least one of the two substantially identical currents in dependence of said clock signal, and means for controlling an element connected as a constant current source with a same signal as the setting signal.
The invention is based on the following recognition. An electric current is formed by a flow of electrons (or holes, which will also be referred to as electrons hereinafter). An electron has a charge q. The charge Q
1
transported by a current I
1
during a time t is equal to
Q
1
=I
1
t=qN
1
,
in which N
1
is the number of transported electrons. If the transport mechanism determining I
1
is controlled by the mutual independent emission of electrons in a device across an energy barrier higher than a few times k
B
&THgr; (in which k
B
is the Boltzmann constant and &THgr; is the absolute temperature), N
1
will have a Poisson distribution with the standard deviation N
1
. The Poisson distribution may be approximated for high values of N
1
by a standard distribution with an expected value N
1
and a standard deviation N
1
. The standard deviation of Q
1
may be written as
&sgr;
Q1
=qN
1
=qQ
1
=qI
1
t
A current to which this type of statistic is applicable is said to have “shot noise”. Such a current is the saturated drain current of a MOS transistor which is set for the sub-threshold region, i.e. below the threshold voltage.
The difference &Dgr;I
1
=I
1,a
−I
1,b
between two currents I
1,a
and I
1,b
having equal expected values I
1
but uncorrelated shot noise values, for example such as generated by two MOS transistors set in the same manner, will lead to a fluctuation &Dgr;Q
1
=Q
1,a
−Q
1,b
. For N
1
=(I
1
t/q)>>1 this fluctuation by approximation has a standard distribution with an expected value zero and a standard deviation
&sgr;
&Dgr;Q
1
=(2)&sgr;
Q
1
=2
qI
1
t
Said &Dgr;I
1
is supplied to an originally discharged capacitor with capacitance C
1
. A fluctuating voltage U
1
then arises across the capacitor with capacitance C
1
, which voltage by approximation has a standard distribution with an expected value zero and a standard deviation
&sgr;
U
1
=(2
qI
1
t
)/
C
1
In addition to the capacitor with capacitance C
1
mentioned above, there is also an originally discharged capacitor with capacitance C
2
. The capacitor with capacitance C
2
is charged by a current I
2
. The voltage U
2
across this capacitor at moment t will be equal to
U
2
=(
I
2
t
)/
C
2
Provided the unequality I
2
t>>q is complied with, the shot noise of I
2
can be disregarded. Assuming that a standard distribution holds for U
1
, the probability that U
1
lies in the region (−U
2
, U
2
) is given by
P[−U
2
<U
1
<U
2
]=erf
((
U
2
)/((2)&sgr;
U
1
))
The function erf (error function) is defined as
erf
(
x
)=(2/(&pgr;))*
0
∫
x
e
−y2
dy
It will be assumed below for simplicity's sake that the probability P indicated above is equal to 0.5 because this value leads to a simple embodiment of the invention which is yet to be described in more detail. Alternative values of P are also possible and lead to other values of the factor erf
−1
.
The following relation can be derived for the current I
2
corresponding to P=0.5 at moment t by means of the relations given above:
I
2
=(2
erf
−1
(0.5))
2
*(
I
1
/I
2
)(
C
2
/C
1
)
2
(
q/t
)=0.91*(
I
1
/I
2
)(
C
2
/C
1
)
2
(
q/t
)
in which the function erf
−1
is the inverse of the error function erf.
For a fixed ratio I
1
/I
2
the probability P[−U
2
<U
1
<U
2
] is a rising function of I
2
. The probability P can be kept equal to 0.5 on average by sampling the time-dependent voltages U
1
and U
2
at a given moment T and subsequently increasing I
2
if U
2
is smaller than the absolute value of U
1
or decreasing I
2
if U
2
is greater than the absolute value of U
1
. After sampling, the capacitors C
1
and C
2
are discharged again, time t is reset to zero, and the capacitors C
1
and C
2
are charged again with the respective currents &Dgr;I
1
and I
2
, respectively, during a time period T. The resulting current I
2
depends exclusively on the time period T, on the ratio of the capacitances C
1
and C
2
, and on the ratio of the currents I
1
and I
2
. The latter two ratios can be kept constant in general, i.e. independent of temperature, supply voltage, etc., with a high degree of accuracy which is given by the mutually attuned properties of the components used. The time period T can be generated with high accuracy by means of a crystal oscillator or an oscillator with a ceramic resonator. The ratios I
1
/I
2
and C
2
/C
1
can be optimized for a fixed value of I
2
T so as to occupy a minimum circuit surface area of the integrated circuit in the design of an integrated circuit which uses the circuit according to the present invention.
It was assumed in the above that a comparison is made between the absolute value of the voltage U
1
across the capacitor having capacitance C
1
and the voltage U
2
across the capacitor having capacitance C
2
. The result of this comparison is a signal whereby the current I
2
is increased or decreased in steps.
An alternative algorithm consists in that the difference |U
1
|−U
2
is used as a measure for the error in a feedback loop which comprises an integrator which integrates the difference |U
1
|−U
2
continuously, while the capacitors with capacitance values C
1
and C
2
are periodically discharged in accordance with a given period T. The output of the integrator is then used for controlling the current I
2
such that I
2
is a continuous and monotonic rising function of the volta

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