Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Reexamination Certificate
2001-12-31
2002-10-15
Riley, Shawn (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
C361S093900
Reexamination Certificate
active
06466422
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of power devices, and, more particularly, to a current limit protection circuit for limiting output current from a voltage regulator or other similar circuit.
BACKGROUND OF THE INVENTION
Voltage regulators are designed to provide a constant voltage over a variety of load impedances. As the impedance of the load increases, the voltage regulator requires less output current to keep the load at a constant voltage. Conversely, as the impedance of the load decreases, more current is required to maintain a same constant voltage. When the output current required to maintain a constant voltage is greater than the safe operating condition of the power transistor of the voltage regulator, a current limit protection circuit is required to limit the output current.
A voltage regulator
10
with a current limit protection circuit
12
according to the prior art is illustrated in FIG.
1
. The voltage regulator
10
includes an error amplifier
14
having a non-inverting input receiving a reference voltage Vref, which corresponds to the desired output voltage Vout of the voltage regulator
10
. An inverting input of the error amplifier
14
is connected to an output terminal
16
of the voltage regulator
10
. This connection between the error amplifier
14
and the output terminal
16
forms a negative feedback loop for stabilizing the output voltage Vout.
The error amplifier
14
drives the control terminal of the power transistor
18
proportional to the amount of current necessary to maintain the output voltage Vout at the reference voltage Vref. If the output voltage Vout begins to fall below the reference voltage Vref, the output of the error amplifier
14
increases the voltage for the control terminal of the power transistor
18
, thereby driving more current to the output terminal
16
, which in turn raises the output voltage Vout.
The current limit protection circuit
12
illustrated in
FIG. 1
includes a limit switch transistor
20
and a current sense resistance
22
. The current sense resistance
22
is typically a very low resistance resistor which can handle large currents of the power transistor
18
. As the current through the power transistor
18
and the current sense resistance
22
increases, the voltage drop across current sense resistance likewise increases. The resistance of the current sense resistance
22
may be selected so that the limit switch transistor
20
turns on when the current reaches an unsafe level.
As the load current increases, the voltage drop across the current sense resistance
22
causes the limit switch transistor
20
to conduct. Bias current Ib from a current source
24
connected to the first conduction terminal of the limit switch transistor
20
shunts away available drive current Id for the power transistor
18
. This limits the output current Iout.
As the output load increases, the drive current Id for the power transistor
18
decreases. The characteristics of the current source
24
, power transistor
18
, and limit switch transistor
20
may be selected to limit the maximum output current Iout that can be delivered by the power transistor
18
to a load. The limit switch transistor
20
and the current sense resistance
22
thus limit the output current Iout in the power transistor
18
during an over-current condition by controlling the drive current Id to the power transistor
18
.
To illustrate operation of the current limit protection circuit
12
, the safe operating current of the power transistor
18
may be limited to 1 amp and the limit switch transistor
20
may be forward biased at about 0.7 volts. A resistance of the current sense resistance is about 0.7 ohms (i.e., 0.7 volts/1 amp).
A resistance of about 0.7 ohms would be required from the current limit protection circuit
12
for limiting the output current Iout to 1 amp. At 1 amp, the voltage across the current sense resistance
22
is about 0.7 volts. The limit switch transistor
20
thus begins to shunt the current Id from the control terminal of the power transistor
18
so that it is the same as the operating output current Iout.
Even though the current limit protection circuit
12
provides a constant voltage over a variety of load impedances, the voltage regulator disclosed in
FIG. 1
has two drawbacks. First, the current sense resistance
22
dissipates a significant amount of power. If the output current Iout is 1 amp, then the resistance of the current sense resistance
22
will consume 0.7 watts, for example.
Second, the output current Iout is sensitive to temperature variations. For example, assume that the limit switch transistor
20
has a negative temperature coefficient Tcf of −2 mV/° C., and the current sense resistance
22
has a positive temperature coefficient Tcf of several thousand ppm/° C. If the temperature increases to 100° C., the voltage applied to the control terminal of the power transistor
18
decreases from 0.7 V to 0.55 V, and the resistance of the current sense resistance
22
increases. Consequently, the output current Iout drops from 1 amp to 0.8 amps.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is an object of the present invention to minimize power dissipation of a current sense resistance used to sense an output current from a voltage regulator.
Another object of the present invention is to limit output current from a voltage regulator so that the output current is not sensitive to temperature variations.
These and other objects, features and advantages in accordance with the present invention are provided by a voltage regulator comprising a power transistor receiving a drive current, and a current limit protection circuit connected to the power transistor.
The current limit protection circuit preferably comprises a first resistance, i.e., a current sense resistance, connected to the power transistor for sensing an output current, a limit switch transistor connected to the power transistor and to the first resistance, and a current generator and second resistance connected thereto. The current generator and second resistance biases the limit switch transistor to divert drive current from the power transistor based upon the output current through the first resistance exceeding a threshold. The first resistance has a value less than a value of the second resistance. Accordingly, the first resistance can advantageously be made considerably smaller than otherwise to thereby reduce power consumption.
The first resistance preferably has a temperature coefficient less than a temperature coefficient of the second resistance. More particularly, the temperature coefficient for the second resistance is based upon the temperature coefficient for the first temperature coefficient so that the output current is not sensitive to temperature variations. In other words, the second resistance is selected so that a desired temperature coefficient is balanced with respect to the temperature coefficient of the first resistance. This advantageously allows the voltage regulator to have a maximum output current that is not sensitive to temperature variations.
The current generator preferably comprises a current source, and at least one transistor connected to the current source. The at least one transistor preferably comprises first and second transistors connected together. The first transistor includes a first conduction terminal connected to a first voltage reference, and a second conduction terminal connected to the first resistance. The second transistor includes a control terminal connected to a control terminal of the first transistor, a first conduction terminal connected to the first voltage reference and to a control terminal of the limit switch transistor, and a second conduction terminal connected to the second resistance.
The first transistor, the second transistor and the power transistor each preferably comprises an NPN bipolar transistor. The second conduction terminal of the first transistor defines an emitter having a first area, and the second co
Jorgenson Lisa K.
Regan Christopher F.
Riley Shawn
Shenzhen STS Microelectronics Co. Ltd.
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