Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-02-28
2002-05-21
Sherry, Michael J. (Department: 2858)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C232S009000, C324S765010
Reexamination Certificate
active
06392392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an over-current detecting circuit (thereafter, “over-current detecting circuit” is abbreviate to “OCD Circuit”.) for detecting the over-current state of an output transistor having an open drain structure, and a semiconductor integrated circuit (SI Circuit) having the OCD Circuit built-in.
2. Description of the Related Art
In many cases, SI Circuits which drive a solenoid or the like have therein an output transistor having an open drain structure. In such SI Circuits, a large current passes through the output transistor which frequently destroys the output transistor when an external load such as either a solenoid suffers catastrophic failure or the state of an external power source varies abnormally. In order to prevent such failure, it is necessary to have a mechanism for monitoring the amount of current that passes through the output transistor at all times and immediately sever the current towards the output transistor when detected that the current is in an over-current state. A circuit including such a mechanism is called an OCD Circuit. The following will describe a conventional SI Circuit having this mechanism built-in.
FIG. 1
is a conventional view showing an SI Circuit disclosed in Japanese Patent Application Laid-Open No. 2-87817.
An SI Circuit
70
, disclosed in the above-mentioned publication, is a device for controlling whether or not a driving current is supplied to the load R
L
′ which connects an output terminal
73
and the external power source V
B
′. And the SI Circuit
70
operates in accordance with an input signal supplied to the input terminal
76
from the outside. Whether or not the driving current is supplied to the load R
L
′ is decided by whether output transistor Q
71
, which has an open drain structure, is in a state of conduction or non-conduction. The output transistor Q
71
is controlled by a logic circuit
75
, which receives the input signal. That is, if the logic circuit
75
outputs a high-level signal in response to the input signal, the output transistor Q
71
becomes conductive, and consequently passes the driving current through the load R
L
′. On the other hand, if the logic circuit
75
outputs a low-level signal in response to the input signal, the output transistor Q
71
becomes nonconductive, thus severing the supply of the driving current toward the load R
L
′.
However as previously described, in the case that either the load R
L
′ short-circuits or an abnormally high voltage is generated in the external power source V
B
′, an unexpected large current is sent in the conductive state to output transistor Q
71
and it may be destroyed. To prevent this, the SI Circuit
70
has an OCD Circuit, comprised of reference voltage generating circuit (RVG Circuit)
71
, resistors R
75
and R
76
, diodes D
71
-D
7n
, and a comparator
72
. Such an OCD Circuit is a circuit for detecting an over-current state of the transistor Q
71
and for comparing the voltage at the output terminal
73
with a reference voltage V
r
′.
The reference voltage V
r
′ for detecting the over-current, can be obtained by dividing the output voltage from the RVG Circuit
71
through a voltage divider wherein the resistors R
75
and R
76
and n diodes D
71
-D
7n
are connected in series. The comparator
72
continually compares the reference voltage V
r
′ with the output voltage from the output transistor Q
71
, so that whenever the detection of an over-current is found on the basis of an abnormal rise in the output voltage from the output transistor Q
71
. The over-current detecting terminal
74
outputs an over-current detecting signal. The over-current detecting signal is fed back to the logic circuit
75
to trigger the output transistor Q
71
into an OFF-state. As a result, the output transistor Q
71
is protected from destruction based on an over-current.
The diodes D
71
-D
7n
are set to compensate the temperature characteristic of the ON-state resistance of the output transistor Q
71
. For this reason, any over-current is detected under good dependency at any temperature.
An output voltage V
BG
′ from the RVG Circuit
71
is also used for detecting temperature by a temperature detecting circuit. Therefore, dependency, (contingent of the output voltage V
BG
′ from the RVG Circuit
71
) of the temperature must be small. It is impossible to cause the output voltage V
BG
′, itself, of the RVG Circuit
71
to have a temperature characteristic for compensating the temperature characteristic of the output transistor Q
71
. For this reason, in order to perform over-current-detection with good dependency upon temperature, the diodes in series are used to generate the reference voltage V
r
′, which has the same temperature characteristic as the output transistor Q
71
by using the output voltage V
BG
′ having no dependency upon temperature.
However, the reference voltage V
r
′ for detecting an over-current is generated from a voltage divider to which the diodes D
71
-D
7n
and R
75
and R
76
are connected in series, causing a problem that the range wherein detection values thereof are set, is limited to a low range by a voltage-drop at the diodes connected in series.
For example, in the case that
5
diodes are used, the reference voltage V
r
′ for detecting an over-current, is limited to lower than the output voltage V
BG
′ from the RVG Circuit
71
, by the forward direction voltage reduced through the 5 diodes.
For this reason, the margin for setting the detection range is low. As the case may be, in order to obtain a desired reference voltage, it is necessary to sacrifice the temperature characteristic to some degree. Alternatively, in order to obtain a desired temperature characteristic, it is necessary to shift the reference voltage from a desired value by some degree.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an OCD Circuit, making it possible to set the value for over-current detection to a desired value, while compensating the temperature characteristic of an output transistor sufficiently, with the SI Circuit having the detecting circuit.
According to one aspect of the present invention, an OCD Circuit is a circuit for comparing a voltage drop by an ON-state resistance of an output transistor with a reference voltage to detect an over-current state of the output transistor. The OCD Circuit comprises a first power source and an RVG Circuit which outputs a first reference voltage on the basis of a voltage supplied from the first power source. The OCD Circuit also comprises a constant-current source, which generates a constant current having a second temperature characteristic on the basis of the first reference voltage, and a current mirror circuit which inputs the constant current. Moreover, the OCD Circuit comprises a current-voltage converting circuit, which converts an output current from the current mirror circuit to a voltage then outputs a reference voltage having a temperature characteristic in proportion to the second temperature characteristic. A first temperature characteristic of the output transistor is compensated by the temperature characteristic of the reference voltage.
According to another aspect of the present invention, an OCD Circuit is a circuit for detecting an over-current passing through an output transistor which connects an output terminal and a power source. The OCD Circuit comprises a constant-current source which generates a first constant current having a given temperature characteristic on the basis of a reference voltage. The given temperature characteristic is substantially equal to a temperature characteristic of the output transistor. The OCD Circuit also comprises a current mirror circuit which generates a second constant current on the basis of the first constant current, and a current-voltage converting circuit which generates a reference voltage on the basis of the second constant current. Moreove
NEC Corporation
Patel Paresh
Sherry Michael J.
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