Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-09-14
2002-01-08
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
C327S103000, C323S315000, C323S316000
Reexamination Certificate
active
06337596
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-260269, filed Sep. 14, 1999; and No. 2000-098024, filed Mar.31, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a constant current circuit using a current mirror circuit. More specifically, the invention relates to a source-type constant current driver circuit and a sink-type constant current driver circuit which are used to drive an LED (Light Emitting Diode) etc.
Conventionally, as for the constant current circuit using the current mirror circuit, the source-type constant current driver circuit for driving the LED, for example, as shown in
FIG. 1
, is known. Here, description regarding circuits will be made assuming that the number of output bits is ‘8.’ In
FIG. 1
, for example, a reference voltage source
101
for supplying a reference voltage is connected to one of input ports of an amplifier (Amp.)
102
. An output port of the amplifier
102
is connected to a base of an NPN transistor (multiplying factor: ×10) Q
101
for generating a reference current. An emitter of the transistor Q
101
is connected to the other input port of the amplifier
102
and also connected to a terminal (REXT)
103
. The terminal
103
is connected to one end of an external resistance R used for control of the output currents. Other end of the resistance R is connected to a terminal (GND)
104
.
A collector of the transistor Q
101
is connected to a base of a PNP transistor (×10) Q
102
. Moreover, the collector of the transistor Q
101
is also connected to a collector of one transistor Q
103
a
of a PNP transistor (×50, ×50) pair Q
103
a,
Q
103
b.
By the way, the transistor pair Q
103
a,
Q
103
b
is configured to have a current ratio of 1:1 and constitutes a current mirror circuit
105
. Moreover, a collector of the transistor Q
102
is grounded and an emitter thereof is connected to a common node of bases of the transistor pair Q
103
a,
Q
103
b.
Further, a collector of the transistor Q
103
b
is connected to a collector of an NPN transistor (×10) Q
104
and a base of an NPN transistor (×5) Q
105
, respectively.
On the other hand, a terminal
106
for supplying a power-supply voltage VDD (for example, 5V) is connected to each emitter of the transistor pair Q
103
a,
Q
103
b
and also connected to a collector of the transistor Q
105
.
An emitter of the transistor Q
104
is connected to the terminal
104
and each emitter of NPN transistors (×10, . . . ) Q
106
a
to Q
106
h,
respectively. A base of the transistor Q
104
is connected to an emitter of the transistor Q
105
. Moreover, a base of the transistor Q
104
is connected to each base of the transistors Q
106
a
to Q
106
h
through switches
107
a
to
107
h,
respectively.
Here, the transistors Q
106
a
to Q
106
h
are provided according to the number of the output bits (in this case, 1 to 8 bits). In addition, each of the transistors Q
106
a
to Q
106
h
together with the transistor Q
104
constitute the current mirror circuit whose current ratio is set to 1:1, respectively.
Each collector of the transistors Q
106
a
to Q
106
h
is connected to a base of a PNP transistor (×10) Q
107
, respectively. Moreover, each collector of the transistors Q
106
a
to Q
106
h
is connected to a collector of one transistor Q
108
a
of a PNP transistor (×50, ×50) pair Q
108
a,
Q
108
b.
By the way, the transistor pair Q
108
a,
Q
108
b
constitutes a current mirror circuit
108
whose current ratio is set to 1:1. Here, for convenience' sake, here only the circuit for a first bit of the output is shown in the figure.
A collector of the transistor Q
107
is grounded, and an emitter thereof is connected to a common node of bases of the transistor pair Q
108
a,
Q
108
b.
Further, the collector of the transistor Q
108
b
is connected to a common node of bases of an NPN transistor (×10, ×150) pair Q
109
a,
Q
109
b
and also connected to a collector of one transistor Q
109
a
of the transistor pair Q
109
a,
Q
109
b.
Moreover, the transistor pair Q
109
a,
Q
109
b
is configured to have a current ratio of 1:15 and constitutes a current mirror circuit
109
.
Furthermore, each emitter of the transistor pair Q
108
a,
Q
108
b
and a collector of the transistor Q
109
b
are connected to a terminal
110
for supplying a power-supply voltage VCC (for example, 17V), respectively. In addition, each emitter of the transistor pair Q
109
a,
Q
109
b
are both connected to a terminal (Out)
111
.
According to the source-type constant current driver circuit of such a configuration as this can yield heavy-current outputs (in this case, 160 mA) each of which is formed by multiplying a reference current (for example 10 mA) by a factor of n according to an amplifying factor (current ratio) of the current mirror circuit
109
. However, in the conventional source-type constant current driver circuit mentioned above, a useless circuit current (consumption current) i that reaches as high as 1/12 to 1/20 or so of the output current flows. Especially when the number of the output bits is large, the power consumption of the circuit increases because of the circuit current i according to the following formula: power consumption of the circuit=VCC voltage×output current/ratio of the circuit current i×the number of bits. Therefore, the circuit has a demerit that circuits having a large number of bits of the output are not suitable for a small size package.
For example, now, assume that a heavy-current of 160 mA is outputted and the circuit current i of 1/16 times the output current flows uselessly. Then, a power consumption of the circuit is obtained, according to the formula, as power consumption of the circuit =17V×160 mA/16×8=1.36 W. Further, a fact that the useless circuit current i is large means that desired output characteristic cannot be achieved unless the transistors are designed to be in large sizes. Therefore, the circuit of this type tends to bring about a larger chip size and an largely increased cost.
As described above, although the conventional base voltage control type of source-type constant current driver circuit makes possible a stable output of constant current, the useless circuit current at the time of outputting a heavy-current is large. Therefore, the circuit has problems that its power consumption tends to become large and it is liable to have a larger chip size and a largely increased cost.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a constant current circuit capable of decreasing its power consumption and also capable of being manufactured in a smaller size at the same time with a reduced cost.
A constant current circuit according to one aspect of the present invention, comprises a first transistor for generating a reference current in conformity to a reference voltage, a generator circuit for generating a current of 1/&bgr; times the reference current that is supplied for a base current of the first transistor, a transistor pair for amplifying by a factor of n the current that is generated by the generator circuit so as to be 1/&bgr; times the reference current, and a second transistor to which a current that is an n times amplified current by the transistor pair is supplied for a base current thereof.
According to the constant current circuit of the present invention, even when a heavy-current is required, the useless circuit current consumed by the circuit can be reduced to a small amount. As a result, a constant current circuit can be constructed as a low-consumption constant current circuit comprising small-size transistors.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages o
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