Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2001-04-19
2004-10-19
Sircus, Brian (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S093100
Reexamination Certificate
active
06807040
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to over-current protection circuits, which also are referred to herein as current limit circuits. More particularly, the present invention relates to an adjustable over-current protection circuit which achieves a fast response time and a high degree of accuracy despite large manufacturing process variations and despite large chip operating temperature variations, without use of a sense resistor or a separate external adjustment terminal.
Many electronic circuits include components which limit output current of the circuit to protect output transistors and/or other circuit components, such as load circuits driven by output transistors of the electronic circuits, from excessive output currents. Power amplifiers for driving low resistance loads usually include over-current protection circuitry to prevent the power amplifiers, especially output transistors therein, from being damaged by an overload current caused by a short-circuit of the amplifier output. Various techniques have been used to sense and limit output currents of various circuits and to protect output transistors from excessive output currents, i.e., from over-currents. These methods are used primarily in two classes of protection circuits: 1) those including a sense resistor to sense current in an output transistor, and 2) those which do not include such a sense resistor.
The first class of protection circuits, those with a sense resistor, conventionally place a small value resistor in the current path of the output transistor to sense the output current therein. For relatively large values of output transistor current, the voltage drop across the sense resistor reduces the available “headroom”, i.e., the available voltage range or available voltage swing of signals such as the output signal of the electronic circuit. Also, the increased temperatures caused by power dissipation in the sense resistor can become excessive and cause damage to output transistors or other circuitry that is sensitive to high temperatures.
A prior art example of the first class of protection circuits is shown in
FIG. 1
, wherein protection circuit
10
senses the output current flowing through two sense resistors Rsc
1
and Rsc
2
by measuring the voltage across each of them. When the voltage across either Rsc
1
or Rsc
2
exceeds the base-to-emitter voltage (Vbe) of transistor Q
3
or Q
4
, the current through transistor Q
1
or transistor Q
2
, respectively, is limited and the output current therefore is also limited. For example, when the current through Rsc
1
exceeds the base-to-emitter voltage of transistor Q
3
, then transistor Q
3
“robs” base current from transistor Q
1
. This limits the current through resistor Rsc
1
and hence through transistor Q
1
, and thereby protects output transistor Q
1
and also limits the output current. The current through output transistor Q
2
is limited in a similar way. The current limit value is determined by the values of Rsc
1
and Rsc
2
, and because it Rsc
1
and Rsc
2
are fixed resistors, the current limit of the protection circuit must also be fixed, or there must be external terminals through which Rsc
1
and Rsc
2
may be adjusted. The current flowing through Rsc
1
and Rsc
2
causes those resistors to dissipate power and increase temperatures of nearby components on the integrated circuit chip. The tolerance of the current limit is no more accurate than the Vbe (base-to-emitter voltage) voltage of Q
3
or the Vbe voltage of Q
4
. Because of the large tolerances, and because the Vbe of each of the transistors changes with temperature, the output current protection (or current limit value) is undesirably imprecise.
U.S. Pat. No. 5,739,712 by Fujii (April, 1998) discloses a number of other similar over-current protection schemes.
Protection circuits in the second class do not directly limit the current, but instead usually rely on a feedback circuit to control the current limit. An example of the second class of protection circuits is disclosed in U.S. Pat. No. 5,519,310 to Bartlett (May 1996), titled “Voltage-to-Current Converter Without Series Resistor.” Referring to
FIG. 2
herein, which is a reproduction of
FIG. 3
of the Bartlett patent, the output current lout of voltage-to-current converter circuit
12
is controlled by adjusting the current flowing through transistor M
1
. The differential amplifier OA
2
compares the voltage across transistor M
1
to a voltage between transistors M
2
and M
5
, and adjusts the gate voltage of transistor M
5
accordingly. The current flowing through transistors M
3
and M
5
is mirrored through transistor M
4
and resistor R
1
. This mirrored current creates a feedback voltage VF voltage across resistor R
1
. The feedback voltage VF is compared with an input voltage Vin by differential amplifier OA
1
, which adjusts the gate voltage of M
1
so as to increase Iout enough to force the voltage across resistor R
1
to be equal to Vin. This causes Iout to be proportional to the current through resistor R
1
and hence to Vin. However, the feedback delay in limiting the output current Iout may allow it to exceed the current desired to be established by Vin, and thereby cause chip overheating and damage to the output transistor M
1
and other circuit components. The feedback delay also slows the overall response of a system including the circuit of FIG.
2
.
Thus, there is an unmet need for an improved over-current protection circuit that does not dissipate excessive power and raise chip temperature, does not reduce operating voltage “head room”, and does not cause substantial signal propagation delay.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an over-current protection circuit which achieves a higher degree of accuracy than previous protection circuits.
It is another object of the present invention to provide an over-current protection technique which more reliably protects output transistors of integrated circuits than previous techniques.
It is another object of the present invention to provide an over-current protection circuit or current limit protection circuit having faster response times than the closest prior art.
It is another object of the invention to provide an over-current protection circuit or a current limit circuit for protecting output transistors of integrated circuits which is adjustable over a wide range.
It is another object of the invention to provide an over-current protection circuit or a current limit protection circuit with high accuracy over large chip temperature variations and large manufacturing process variations.
It is also an object of the invention to provide a method of current control which can have symmetrical positive and negative current limits with a high degree of accuracy.
It is another object of the present invention to provide an over-current protection circuit in amplifier circuitry or output driver circuitry which drives a load circuit so as to provide a wide range of adjustability of output limit currents.
It is another object of the present invention to provide an over-current protection circuit which drives a load circuit so as to provide “on the fly” adjustability of output limit currents supplied to the wide range of load circuits.
Briefly described, and in accordance with one embodiment, the present invention provides a method and apparatus for directly limiting the output current of an over-current protection circuit in an electronic device without being subject to the delays caused by feedback loops. By directly limiting the voltage at the gate of the output transistor of the over-current protection circuit to a maximum voltage value, the current through the output transistor is correspondingly limited. By generating the maximum voltage value in reference to a current which is representative of the maximum current desired for the over-current protection circuit, the desired over-current protection is accurately achieved.
In one embodiment of the invention, an over-current protection circuit is prov
Baum David R.
Ivanov Vadim V.
Brady W. James
Demakis James A
Sircus Brian
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
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