Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2003-02-13
2004-08-31
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S313000
Reexamination Certificate
active
06784724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a constant voltage generating circuit for generating a reference voltage needed by a circuit such as a power supply circuit, and in particular to a constant voltage generating circuit that can detect the starting of a low voltage without fail despite operating from a supply voltage lower than is conventionally required.
2. Description of the Prior Art
Circuits that operate with high accuracy and stability from a wide range of operating voltages and in a wide range of operating temperatures, such as power supply circuits, D/A converters, and A/D converters, require as their reference voltage a constant voltage that is highly accurate and stable. For example, in a PWM-type switching regulator, the accuracy and stability of the reference voltage fed thereto greatly affects, among others, the differential voltage amplification factor and the in-phase component suppression ratio of the error amplifier and the PWM comparator provided within the IC used in the switching regulator. For this reason, where high accuracy is required, the constant voltage generating circuit incorporated in the IC generates a constant voltage by the use of a band-gap circuit that exhibits almost no dependence on temperature.
Such a constant voltage generating circuit operates from a supply voltage input thereto, and accordingly the output voltage of the constant voltage generating circuit rises as the input supply voltage rises. Thus, whether the output voltage remains stable at a predetermined voltage or not is greatly affected by how the input supply voltage rises and how it fluctuates. Therefore, it is essential to check first whether the voltage generated by the constant voltage generating circuit has certainly reached the predetermined constant voltage and whether the input supply voltage has reached a voltage that permits the generation of the predetermined constant voltage, before feeding the constant voltage to circuits that need it as a reference voltage. Failing to do this may lead to malfunctioning and, with a power supply circuit, even destruction of the circuit connected as a load thereto or of the power supply circuit itself.
FIG. 3
shows an example of a conventional constant voltage generating circuit. In
FIG. 3
, the constant voltage generating circuit
10
generates an output voltage Vref by the use of, for example, a band-gap circuit
1
. The output voltage Vref is fed to the inverting input terminal (−) of a comparator A
1
, and a division voltage Vd obtained by dividing an input supply voltage V
IN
with resistors R
4
and R
5
is fed back to the non-inverting input terminal (+) of the comparator A
1
. How the band-gap circuit
1
is configured and how it operates to generate the output voltage will be described later.
The output terminal of the comparator A
1
is connected to the base of an NPN-type, common-emitter connection transistor Tr
1
whose collector is pulled up through a resistor R
3
to the input supply voltage V
IN
. When the output voltage Vref has reached a predetermined voltage, the transistor Tr
1
outputs, at its collector, a low-level starting signal.
Next, with reference to
FIG. 4A
, how the conventional constant voltage generating circuit
10
operates will be described.
FIG. 4A
is a graph showing how the output voltage Vref and the division voltage Vd, obtained by dividing the input supply voltage V
IN
, vary according to the input supply voltage V
IN
. Along the horizontal axis is taken the input supply voltage V
IN
, and along the vertical axis are taken the division voltage Vd and the output voltage Vref.
FIG. 4B
shows the timing with which the conventional constant voltage generating circuit
10
produces a starting signal. Now, suppose that electric power starts being supplied. Then, as shown in
FIG. 4A
, in a region A, as the input supply voltage V
IN
rises, the division voltage Vd rises linearly in proportion thereto. The output voltage Vref also rises as shown in
FIG. 4A
, and reaches a stable voltage starting point T
2
, at which the output voltage Vref stabilizes, at the boundary between regions B and C.
However, the stable voltage starting point T
2
varies greatly with variations in temperature and in the constants of the circuit elements. Thus, in the conventional circuit configuration, a starting signal is output after checking whether or not the input supply voltage V
IN
is sufficiently high to permit the output voltage Vref to certainly reach the stable voltage starting point T
2
even under the influence of variations in temperature and in the constants of the circuit elements. Accordingly, in the comparator A
1
shown in
FIG. 3
, the division voltage Vd of the input supply voltage V
IN
is compared with the output voltage Vref. Then, as indicated by a comparator detection point T
3
in
FIG. 4A
, the comparator A
1
outputs a high-level signal as late as when Vd≧Vref, i.e., when the region C has been passed through. This high-level signal turns the transistor Tr
1
on, which then outputs, for example, a low-level starting signal as shown in FIG.
4
B. The relationship Vd≧Vref holds also in the region A in
FIG. 4A
, but, in this region, since the input supply voltage V
IN
is very low, the comparator A
1
does not operate and thus does not output a signal, and accordingly the transistor Tr
1
is not turned on.
Likewise, when, as a result of electric power being shut off, or a fault in the circuitry, or a variation in the load, the input supply voltage V
IN
falls below the comparator detection point T
3
, the starting signal is stopped (turned to a high level) before it becomes impossible to keep the output voltage Vref at the predetermined voltage. In this way, it is possible to stop the operation of other circuits that use the output voltage Vref as a reference voltage before they start malfunctioning.
As described above, the stable voltage starting point T
2
, at which the output voltage Vref stabilizes, varies greatly with variations in temperature and in the constants of the circuit elements, and accordingly, in the conventional circuit configuration, a starting signal is output as late as at the comparator detection point T
3
after checking whether or not the input supply voltage V
IN
is sufficiently high to permit the output voltage Vref to certainly reach the stable voltage starting point T
2
even under the influence of variations in temperature and in the constants of the circuit elements. This makes it necessary to secure a margin, i.e., the region C shown in FIG.
4
A. As a result, an IC circuit that uses as a reference voltage the output from a conventional constant voltage generating circuit cannot be designed to operate from a low input supply voltage, that is, it is impossible to lower the minimum operating voltage from which it can operate. This often makes it impossible to realize operation at a low voltage as required in information technology equipment.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a constant voltage generating circuit that does not require the checking, as required in a conventional one, of whether or not the input supply voltage is sufficiently high to permit the generation of an output voltage stable at a predetermined voltage and that requires less circuit elements, operates from a lower input supply voltage, and thus consumes less electric power than a conventional one.
To achieve the above object, according to one aspect of the present invention, a constant voltage generating circuit is provided with: a reference voltage generating circuit of which that output voltage is so controlled as to be a constant voltage when the output voltage has risen, with an increase in the input supply voltage, to reach a predetermined voltage and that outputs the constant voltage as a reference voltage; a first transistor that turns on when the output voltage reaches the predetermined voltage in order to control the constant voltage output from the reference voltage generating ci
Cunningham Terry D.
Morgan & Lewis & Bockius, LLP
Rohm & Co., Ltd.
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