Bandgap type reference voltage source with low supply voltage

Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage

Reexamination Certificate

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Reexamination Certificate

active

06680643

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bandgap type reference voltage source with low supply voltage.
2. Description of the Related Art
In most electronic devices with a high integration scale, there are analogue blocks which require a reference voltage that is independent of the temperature and of the supply voltage. Examples of these electronic devices are voltage regulators for programming and erasing non volatile memories and DC/DC voltage reduction converters which generate internal supply voltages regulated at a fixed value.
The generation of reference voltages is generally obtained through a source circuit which supplies a bandgap output voltage.
Various bandgap reference sources are known. The simplest is formed by bipolar transistors, present in standard CMOS technology, of a vertical type, as shown in FIG.
1
.
The bandgap source
1
of
FIG. 1
comprises a bandgap stage
18
, an operational amplifier
15
of transconductance type, and an output stage
19
.
The bandgap stage
18
comprises a first and second branch
2
,
3
flowed by a first and a second current I
1
, I
2
. The first branch
2
is formed by a first PMOS transistor
5
and by a first diode connected bipolar transistor, shown in
FIG. 1
as a diode
6
; the second branch
3
is formed by a second PMOS transistor
7
, by a first resistor
8
and by a second diode
9
. The PMOS transistors
5
,
7
are identical, have source terminals connected to a supply line
12
, and drain terminals connected to a first and, respectively, to a second output node
10
,
11
. The output nodes
10
,
11
are set respectively at voltages V
A
, and V
B
. The first output node
10
is connected to an anode terminal of the first diode
6
; the second output node
11
is connected to an anode terminal of the second diode
9
through the first resistor
8
. The diodes
6
,
9
have an area ratio 1:n and have their cathodes connected to ground
16
. The first resistor
8
has a resistance R
1
.
The operational amplifier
15
has an inverting input connected to the first output node
10
, a non-inverting input connected to the second output node
11
of the bandgap stage
18
and an output connected to the gate terminals of the PMOS transistors
5
,
7
.
The output stage
19
comprises a PMOS output transistor
20
, an output resistor
21
and an output diode
22
. The PMOS output transistor
20
is equal to the first and second PMOS transistors
5
,
7
(and thus it is formed using the same technology and has the same dimensions as the transistors
5
,
7
) and has source terminal connected to the supply line
12
, gate terminal connected to the output of the operational amplifier
15
, and drain terminal defining an output terminal
24
on which there is a bandgap voltage V
BG
. The output terminal
24
is connected, through the output resistor
21
, to the anode of the output diode
22
, the cathode of which is connected to ground
16
. The output resistor
21
has a resistance R
2
; on the output diode
22
there is a voltage V
D
and in the output stage
19
flows a current I
3
.
Since the PMOS transistors
5
,
7
are identical and have the same gate-to-source voltage V
gs
, this gives:
I
1
=I
2
,
moreover the operational amplifier
15
maintains V
A
=V
B
.
When the equations of the dipole
13
formed by the first diode
6
and of the dipole
14
formed by the resistor
8
and by the second diode
9
are written, the conditions of equality of current and voltage indicated above occur only when:
I
1
=I
2
=(
V
T
/R
1
)ln(
n
).  (1)
Moreover, as the PMOS output transistor
20
is identical and has the same gate-to-source voltage V
gs
as the first and the second PMOS transistor
5
,
7
, it conducts a current I
3
=I
1
=I
2
.
Consequently, in the PMOS transistors
5
,
7
,
20
there flows a current proportional to V
T
/R. The bandgap voltage V
BG
present on the output terminal
24
is therefore equal to:
V
BG
=V
D
+I
3
* R
2
=V
D
+K
(
V
T
/R
1
)
R
2
  (2)
In (2), the resistance ratio R
2
/R
1
is insensitive to temperature variations, since the two resistors
8
,
21
vary in the same way; vice versa the terms V
T
and V
D
are variable with temperature. However, by acting on the coefficient K (through the mirroring ratio n) and on the number of diodes in parallel, it is possible to ensure that the temperature variations of V
T
and V
D
are compensated and that the bandgap voltage V
BG
present on the output terminal
24
is substantially insensitive to temperature.
The circuit in
FIG. 1
, however, has the problem that the inputs of the operational amplifier
15
have a temperature dynamics of 300 mV (−2 mV/° C.) and consequently, when the power supply falls below 1.5 V, the operational amplifier
15
does not work correctly. In fact, on the outputs of the operational amplifier
15
there are transistors (whether of the N-type or the P-type) which, at least in certain temperature intervals, work below threshold.
Moreover the bandgap voltage V
BG
generated by the output stage
19
is equal to about 1.25 V, so the supply voltage must be kept above 1.5 V.
Another known bandgap type reference source uses NMOS transistors operating below threshold instead of the first and the second diode
6
,
9
. This solution solves the problem of operation at a low supply voltage as regards the bandgap stage, but it suffers from other problems. In fact its PSSR value (Power Supply Rejection ratio) in DC is not very high; consequently, a supply voltage decrease leads to an unacceptable variation of the output voltage. Moreover, in a dynamic condition, the rejection of the noise coming from the power supply is not very good. Finally, also this solution uses an output stage similar to that of
FIG. 1
, so it is affected by the same problem of limitation of the minimum usable power supply voltage.
BRIEF SUMMARY OF THE INVENTION
An embodiment of the invention solves the problems affecting the known bandgap reference sources.
According to an embodiment of the present invention a bandgap type reference voltage source and a method for generating a reference voltage in a bandgap type reference voltage source are provided.
A bandgap type reference voltage source using an operational transimpedance amplifier is provided. The bandgap stage is formed by a first and a second bandgap branch, parallel-connected. The first bandgap branch comprises a first diode and a transistor, series-connected and forming a first output node; the second bandgap branch comprises a second diode and a second transistor series-connected and forming a second output node. The operational transimpedance amplifier has inputs connected to the output nodes of the bandgap stage. An amplifier current detecting stage is connected to the outputs of the operational amplifier and supplies a current related to the current drawn by the operational amplifier. A diode current detecting stage is connected to the output of the amplifier current detecting stage and to an output of the operational amplifier and supplies a current related to the current flowing in the first diode. An output stage transforms this current into a stabilized voltage.
A method of operation is also provided.


REFERENCES:
patent: 5394026 (1995-02-01), Yu et al.
patent: 5568045 (1996-10-01), Koazechi
patent: 6028482 (2000-02-01), Herrle
patent: 6147548 (2000-11-01), Doyle
patent: 0 598 578 (1994-05-01), None
Jiang et al., A 1.2 V Bandgap Reference Based on Transimpedance Amplifier, IEEE International Symposium on Circuits and Systems, May 28-31, 2000, Geneva, Switzerland.

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