Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-12-17
2004-01-27
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S316000
Reexamination Certificate
active
06683444
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a generator of at least one reference voltage with improved performance. Reference voltage generators may be used in a great number of applications such as converters in which it is necessary to have a voltage value that is precise and stable whatever the environmental conditions are. This is notably the case when the reference voltage is based on the energy band. These voltage generators are known by the English name of bandgap generators in the literature. In an integrated circuit the potential barrier of a PN junction corresponding to the forbidden bandwidth of the semiconductor, that is 1.205 volts in the case of silicon, is used as a reference voltage.
BACKGROUND OF THE INVENTION
It is tried for these reference voltage generators to have a temperature gradient that is well known and even often to be independent of temperature variations. These reference voltage generators are constituted by various electronic components which themselves have their own dependence on temperature and the control of the temperature gradient of the assembly is difficult.
The value of the reference voltage delivered by this reference voltage generator must not be dependent on the manufacturing process of the various electronic components of the generator. These reference voltage generators are produced in the form of monolithic integrated circuits and it is a known practice that components having the same characteristics finally have different cost.
Moreover, it is tried for the reference voltage delivered by such generators to be the least possible affected by faults of the supply source that feeds them. The signals delivered by the supply sources inevitably comprise disturbances: parasitic random noise, noise, voltage peaks. These faults need not affect the reference voltage delivered by the generator. In conclusion, it is tried for the reference voltage generator to have as large a power supply rejection ratio as possible on a large frequency band. It is the ratio between a variation of the output voltage of the reference voltage generator brought about by a variation of the supply voltage and said variation of the supply voltage, this magnitude is known by the English abbreviation PSRR for Power Supply Rejection Ratio.
Finally, it is also tried for the reference voltage generator to have a good load rejection and to have the shortest possible response time at the start.
The reference voltage generators of known type are such that their output voltage combines with appropriate weight factors a base-emitter voltage of a bipolar transistor and a voltage proportional to the absolute temperature T. The choice of the weight factors is made so that the voltage variations proportional to the absolute temperature compensate for those of the base-emitter voltage of the bipolar transistor.
An example of a reference voltage generator known from the article “A Simple Three-Terminal IC Bandgap Reference”, A. Paul BROKAW, IEEE Journal of Solid State Circuits, vol. SC-9, no. 6, December 1974, pp. 388 to 393, is illustrated in FIG.
1
. It is formed by an input stage
1
having two branches
10
,
11
connected between two supply terminals
20
,
21
, one terminal
20
connected to a high potential Vcc, the other terminal
21
connected to a low potential Vee, generally ground. In each of the branches
10
,
11
is found at least a bipolar transistor Q
1
, Q
2
and these transistors do not have the same size of emitter. This input circuit
1
combines a base-emitter voltage of one of the bipolar transistors Q
2
with a voltage proportional to the absolute temperature (known by the voltage name PTAT, PTAT being the English abbreviation for Proportional To Absolute Temperature) and it is the voltage resulting from this combination that forms the reference voltage Vref.
This input circuit
1
is associated with an operational amplifier
2
which, while attenuating the variations of the supply voltage Vcc−Vee, maintains the same current in the two branches
10
,
11
. The operational amplifier is configured to have a largest possible gain.
More precisely, the two transistors Q
1
, Q
2
have a common base, their collectors connected to the supply terminal
20
connected to the potential Vcc via a resistor R
2
, R
3
, respectively. The emitter of the first transistor Q
1
is connected to the other supply terminal
21
by a series combination
12
of two resistors R
1
, R
0
. The emitter of the second transistor Q
2
is connected to the other supply terminal
21
via one of the resistors R
0
of the series combination
12
. It is supposed that the emitter surface of the first transistor Q
1
is equal to n (n being an integer greater than one) times that of the second transistor Q
2
. For example, n may be equal to 8.
The operational amplifier
2
may adopt a conventional form with a differential amplifier stage
13
and an output stage
14
. In
FIG. 1
the differential amplifier stage
13
comprises a differential pair
15
of transistors Q
3
, Q
4
whose bases form the two differential inputs. The base of the transistor Q
3
is connected to the branch
11
at the transistor collector Q
2
, the base of the transistor Q
4
is connected to the branch
10
at the collector of the transistor Q
1
. The emitters of the transistors Q
3
and Q
4
are interconnected. They are connected to the supply terminal
21
connected to the potential Vee via a source resistor R
4
. The collectors of the two transistors Q
3
, Q
4
are each connected to the supply terminal
20
connected to the potential Vcc via a load resistor R
5
, R
6
, respectively. The output stage
14
comprises a follower circuit
22
with a transistor Q
5
whose emitter is connected to the supply terminal
21
connected to the potential Vee via a resistor R
7
whose collector is connected to the supply terminal
20
connected to potential Vcc and whose base is connected to the emitter of the transistor Q
4
of the differential amplifier
13
.
The output of the reference voltage generator is found at the bases of the transistors Q
1
, Q
2
of the input stage
1
which are connected to the emitter of the transistor Q
5
of the output stage
14
. The operational amplifier
2
compares the currents flowing in the two branches
10
,
11
and provides that they remain substantially equal whatever the variations of the supply power.
The voltage Vref delivered by this reference voltage generator has the value of: Vref=Vbe(Q
2
)+R
0
.I
0
, Vbe(Q
2
) representing the base-emitter voltage of the transistor Q
2
and I
0
being the current flowing in the resistor R
0
.
It may be stated that Vbe(Q
2
)−Vbe(Q
1
)=R
1
.I
1
.
But Vbe(Q
2
)−Vbe(Q
1
)=V
T
.Log(n) with V
T
being the thermal voltage. This thermal voltage V
T
is equal to kT/Q where k is the Boltzmann constant, T the temperature in degrees Kelvin and Q the charge of the electron.
The voltage at the terminals of the resistor R
0
is equal to: 2.V
T
.Log(n).R
1
/R
0
since the same currents are flowing in the transistors Q
1
, Q
2
.
The reference voltage Vref is such that:
Vref=Vbe(Q
2
)+2.V
T
.Log(n).R
1
/R
0
.
The ratio of the resistances R
1
/R
0
may thus be adjusted so that in the sum the variations of the term proportional to V
T
practically compensate for those of Vbe(Q
2
). But in an open loop arrangement the reference voltage Vref follows the variations of the supply voltage.
One of the drawbacks of this generator is that the precision of the voltage obtained is not very good if not a high gain operational amplifier is used. But a high gain amplifier has a high energy consumption and needs to be stabilized. Its passband is small and so is its supply voltage rejection.
Another drawback is that the reference voltage generator needs to have a start circuit (not shown). Actually, the circuit is found in a stable mode when no current is flowing in the transistors Q
1
, Q
2
and when they are in a blocked state. The start circuit has for its function to inject a current in the charging circuit of the d
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