Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2002-03-04
2004-03-23
Le, N. (Department: 2836)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S523000, C324S527000
Reexamination Certificate
active
06710603
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to circuits which safeguard circuits against overload currents. In particular, the invention relates to an overload protection circuit for line drivers.
In order that digital signals are transmitted in an uncorrupted manner from one integrated circuit to a further integrated circuit, it must be ensured that the connecting lines do not impair the signals. The influence of the connecting lines cannot be disregarded if, in the case of steep signal edges, the propagation time through the connecting lines is at least of the order of magnitude of the rise time of the circuit. A simple connecting wire between the circuits is no longer sufficient in this case. In order to avoid serious signal deformations, lines with a defined characteristic impedance are used, which are terminated with a characteristic impedance. Two insulated wires twisted together are often used for this purpose. Such lines are called twisted-pair lines. A twisted-pair line with approximately 100 turns per meter has a characteristic impedance of approximately 100 &OHgr;. Owing to the low terminating resistance, the transmitter must supply a correspondingly high output current. In order to satisfy this requirement, line drivers are used as the transmitters or drivers.
For interference-immune signal transmission, line drivers are often configured in such a way that they feed the two wires of the twisted-pair lines with symmetrical, complementary signals. A comparator is used as a receiver. In this operating mode, the information is determined by the polarity of the differential voltage and not by the absolute value of the level. In this case, an interference pulse merely brings about a common-mode modulation, which remains ineffective owing to the difference formation in the comparator.
A known line driver has differential amplifiers. The inverting inputs of the differential amplifiers are fed by input currents. The input currents contain the information to be communicated. The input currents are identical in terms of their temporal characteristics and the magnitude of their amplitudes; they differ merely through different signs. The non-inverting inputs of the differential amplifiers are connected to a constant-voltage source. The differential amplifiers are connected up as current-voltage converters with feedback resisters. The outputs of the differential amplifiers feed the primary-side inputs of a transformer. The transformer communicates the signals to a twisted-pair line.
The line driver can be connected up to a load on the output side. The load stands as an equivalent circuit diagram for the transformer and the twisted-pair line. The load is characterized by a load resistance.
In the case of the line driver, a short circuit of the outputs with the supply voltage or earth can lead to static overload currents which, under certain circumstances, entail destruction of the line driver. In the case of an excessively small load resistance, with pulsed complementary input currents, an increased pulsed current flow through the line driver can occur.
In previous line drivers, protection against an overload and/or short circuit is essentially tackled with two different protection circuits. A first conventional protection circuit is based on a resistor connected in series between the source terminal of the driver transistor and the supply voltage. The voltage drop across the resistor is compared with a reference voltage by a read-out amplifier. The protection circuit has the disadvantage that the output dynamic range of the line driver is limited by the additional voltage drop.
Furthermore, an additional operational amplifier is required as the read-out amplifier, and must be tuned, since the offset voltage of the operational amplifier is present serially and hence additively with respect to the voltage dropped across the resistor. A further disadvantage of the protection circuit can occur with pulsed input currents. If the bandwidth of the read-out amplifier is not significantly larger than the frequency of the pulse train of the input currents that feed the line driver, a pulsed overload is not identified.
A second conventional protection circuit for line drivers provides for the output current flowing through an output transistor of the line driver to be limited by a clamping transistor. To that end, the output current is mirrored into the drain-source path of a further transistor by a current mirror. The clamping transistor is connected up in such a way that its gate voltage is a function of the mirrored output current. In the case of a gate voltage above the threshold voltage, the clamping transistor turns on and limits the gate voltage of the output transistor. As a result, the maximum output current flowing through the drain-source path of the output transistor is limited. The protection circuit has the disadvantage that its switching threshold is greatly dependent on the threshold voltage tolerances of the clamping transistor. Moreover, jitter about the changeover point leads to unreliable detection of an overload current. Furthermore, the protection circuit is subject to great dependencies on the temperature and on tolerances of the production process.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an overload protection circuit for line drivers that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which precisely detects the occurrence of overload currents above a predetermined detection threshold and whose detection threshold is adjustable in a simple manner. In particular, the intention is to provide a circuit for protecting line drivers against overload currents which contains the abovementioned detection circuit and which protects the line driver even against overload currents that occur in pulsed fashion and recur at short time intervals.
With the foregoing and other objects in view there is provided, in accordance with the invention, a detection circuit for ascertaining a presence of an overshooting of a current flowing through a line above a predetermined value. The detection circuit contains a first current mirror having a first input transistor, a first output transistor, an input, and an output outputting an output signal. A second current mirror is provided and has a second input transistor, a second output transistor, an input and an output outputting an output signal. The first input transistor has a first threshold voltage being greater than a saturation voltage of the second output transistor. The second input transistor has a second threshold voltage being greater than a saturation voltage of the first output transistor. The input of the first current mirror and the output of the second current mirror are connected to each other at a common first node and the common first node is present at an end of the line. The input of the second current mirror and the output of the first current mirror are connected to each other at a common second node. A first controllable current source is provided and has an output connected to the common first node and outputs a first current. A second controllable current source is provided and has an output connected to the common second node and outputs a second current. The first current and the second current have a linear relationship to one another, and the predetermined value is a function of the linear relationship of the first current and second current. An evaluation unit is coupled to the first current mirror and to the second current mirror. The evaluation unit ascertains the presence of the overshooting of the current flowing through the line above the predetermined value from an evaluation signal corresponding to the output signal of one of the first current mirror and the second current mirror.
The first and second input and output transistors may be configured either as MOS transistors or as bipolar transistors. An essential concept of the invention is that the input of the first current mirror and the out
Greenberg Laurence A.
Infineon - Technologies AG
Le N.
Mayback Gregory L.
Stemer Werner H.
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