Current generator for low power voltage

Details Current generator for low power voltage Current generator for low power voltage

2002-08-29

2004-08-03

Patel, Rajnikant B (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C313S315000

Reexamination Certificate

active

06771054

ABSTRACT:

CROSS REFERENCE
The present application claim priority from French Patent Application No. 01 11356, filed Sep. 3, 2001, the disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a reference current generator that is particularly useful for integrated circuits using low supply voltages. A generator according to the invention produces a current independent of the supply voltage.
2. Description of the Prior Art
To create currents independent of the power voltage, there are known ways of using bootstrap type reference current generators, a simplified example of which is shown in FIG.
1
.
The generator of
FIG. 1
comprises two P type transistors T
1
and T
2
, two N type transistors T
3
and T
4
and a resistor R
1
. The drain of the transistor T
1
and the drain of the transistor T
3
are connected together; a supply voltage VDD is applied to the source of the transistor T
1
and a reference voltage VSS is applied to the source of the transistor T
3
. The source of the transistor T
2
is connected to the source of the transistor T
1
, and the gate and the drain of T
2
are connected together to the gate of T
1
and to the drain of T
4
. Finally, a pole of the resistor R
1
is connected to the source of T
4
and the reference voltage VSS is applied to another pole of the resistor R
1
.
The generator of
FIG. 1
works as follows. Currents I
1
and I
2
, respectively, cross the transistors T
1
and T
2
, which form a current mirror. The currents I
1
and I
2
are proportional to each other or, possibly, equal: I
1
=a*I
2
.
The current I
1
crosses the transistor T
3
, imposing a voltage VTN
3
between the gate and the source of T
3
; where VTN
3
is the threshold voltage of the transistor T
3
, and is independent of the supply voltage VDD.
The current I
2
crosses the resistor R
1
and a voltage R
1
*I
2
appears across its terminals. Since the resistor R
1
is connected between the gate and the source of the transistor T
2
, at equilibrium, we have R
1
*I
2
=VTN
3
giving:
I
2
=
VTN
3
/
R
1
.
The current I
2
is thus independent of the supply voltage VDD, as it depends only on the threshold voltage of the transistor T
3
and the resistor R
1
.
The current I
2
obtained may be copied for other uses. For example, it may be copied by means of a copying transistor T
5
, whose gate and source are respectively connected to the poles of the resistor R
1
. The drain of the transistor T
5
is connected to the ancillary circuit which uses the reference current flowing in the transistor T
5
. The reference current is directly proportional to the current I
2
flowing in the resistor R
1
.
It will be noted that the current I
2
, while independent of the supply voltage VDD, is on the contrary dependent on the temperature of the circuit because the threshold voltage VTN
3
is itself linearly dependent on the temperature. We have:
I
2
=(
VTN
3
(
T
0
)−
K
(
T−T
0
))/
R
1
, with
T being the temperature;
T
0
being a reference temperature; and
VTN
3
(T
0
) being the threshold voltage of T
3
at the temperature T
0
.
The variation, as a function of the temperature, of the current produced by a generator is not necessarily a drawback. Indeed, certain circuits use reference currents whose value is variable as a function of the temperature.
If not, it is fairly easy to accept a variable current such as the one produced by a generator according to
FIG. 1
, inasmuch as the variations of the threshold voltage VTN
3
as a function of the temperature T are known and are, furthermore, simple: the threshold voltage VTN
3
, and therefore the current I
2
that crosses the resistor R
1
, varies linearly as a function of the temperature: I
2
is indeed equal to I
2
=I
0
*(I−b*T).
If a constant current is necessary, there are known ways of combining a generator that produces an I=I
0
*(1+b*T) type current with a generator producing an I=I
0
*(1−b*T) type current to obtain a current independent of the temperature.
To create currents, there are also known ways of using reference current generators that use a bipolar transistor. A simplified example of a reference generator of this kind is shown in FIG.
2
.
As compared with the generator of
FIG. 1
, the circuit of
FIG. 2
additionally comprises a bipolar transistor T
6
. An emitter of the transistor T
6
is connected to the source of T
3
and the reference voltage VSS is applied to a collector and a base of T
6
which are connected together. Finally, the gate of T
3
is no longer connected to the source of T
4
but to its gate.
The generator of
FIG. 2
works similarly to FIG.
1
. The current I
2
flowing in the resistor R
1
is simply equal in this case to:
I
2
=
VBE
6
/
R
1
,
VBE
6
being a threshold voltage between the base and the emitter of the transistor T
6
and being independent of the supply voltage VDD. On the contrary, VBE
6
depends on the temperature linearly.
Additional information on the making of generators such as those shown in a diagrammatic view in
FIG. 1
or
FIG. 2
may be found in the document: “CMOS Analog Circuit Design”, Editions Holt Rinehart and Winston 1987.
The generators according to
FIG. 1
or
FIG. 2
are used whenever it is desired to obtain a reference current independent of the supply voltage. This need arises frequently because the supply voltage of a circuit can often vary. Indeed, this voltage often depends on the power given to the circuit.
However, the generators according to
FIG. 1
or
FIG. 2
have a major drawback related to the value of the minimum supply voltage VDDMin to be used to supply such generators. Indeed, the supply voltage VDD applied must be sufficient to turn on or even saturate all the transistors of the generators, so that a current flows in these transistors.
For example, for the generator of
FIG. 1
, the minimum voltage VDDmin to be applied is equal to:
VDD
min=
VTN
3
+
VDS
4
+
VGS
2
, with:
VTN
3
, threshold voltage of T
3
, on the order of 0.60 V, and
VDS
4
, voltage between the drain and the source of the transistor T
4
, on the order of 0.15 V, and
VGS
2
, voltage between the gate (or the drain, since they are connected together) and the source of T
2
, on the order of 0.70 V.
Consequently, the voltage VDDmin for the circuit of
FIG. 1
is on the order of 1.5 V.
In the same way and for the same reasons, for the circuit of
FIG. 2
, the minimum supply voltage VDDmin to be used is equal to:
VDD
min=
VBE
5
+
VGS
3
+
VDS
1
, with:
VBE
5
, voltage between the emitter and the base of T
5
, on the order of 0.7 V,
VGS
3
, voltage between the gate and the source of the transistor T
3
, on the order of 0.65 V, and
VDS
1
, voltage between the drain and the source of T
1
, on the order of 0.15 V.
Consequently, the voltage VDDmin necessary to power the circuit of
FIG. 2
is on the order of 1.5 V.
Thus, whatever the known current generator used, the minimum supply voltage VDDmin to be used is on the order of 1.5 V.
Now, a minimum voltage of this kind may be prohibitive, especially for circuits made by means of the smallest submicron technologies, for example technologies at the 0.25 &mgr;m level or below, which can use only voltages lower than 1.5 V, or even 1.2 V for 0.13 &mgr;m technologies.
SUMMARY OF THE INVENTION
The present invention relates to a current generator for the production of a reference current.
According to an embodiment of the invention, the generator comprises a first P type transistor, a source of which is connected to a first pole of a resistor and a gate of which is connected to a second pole of the resistor, the reference current, flowing in the resistor, being variable as a function of a threshold voltage of the first transistor, and a second N type transistor, having a drain, a gate and a source connected respectively to the second pole of the resistor, the first pole of the resistor and the drain of the first resistor, the second transistor working in saturation mode.
The reference

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