Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Slope control of leading or trailing edge of rectangular or...
Reexamination Certificate
1999-09-10
2001-07-24
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Slope control of leading or trailing edge of rectangular or...
C327S134000
Reexamination Certificate
active
06265921
ABSTRACT:
TECHNICAL FIELD
This invention relates to an electric circuit configuration for shaping the slew rate of a pulsed output voltage occurring at an output terminal and for detecting a short circuit at the output terminal.
BACKGROUND OF THE INVENTION
Such a circuit configuration conventionally has a control loop with an operational amplifier whose inverting input is connected with the output terminal via a capacitor, whose noninverting input is connected with a reference current source and whose output is connected with the control input of a transistor. The output terminal is connected via a resistor with a first supply voltage terminal, which normally has a positive potential, and via the transistor with a second supply voltage terminal, which usually has ground potentials
The capacitor supplies the inverting input of the operational amplifier with a capacitor current
Ic=C•dU/dt, (1)
where C is the capacitance of the capacitor, U the voltage drop across the resistor and t the time. At constant voltage, i.e., outside the edges of a pulsed signal, the capacitor current is Ic=0, while it is nonzero during the presence of pulse edges. The operational amplifier adjusts the capacitor current to the value of the reference current, which corresponds to a very definite steepness of the voltage drop across the resistor.
One performs slew rate control for example to reduce the high-frequency electromagnetic interference connected with steep pulse edges.
The output terminal of the control circuit can be connected for example to a bus line, e.g., one of the double lines of a CAN bus system as used nowadays in motor vehicles. The abbreviation CAN stands for controlled area network.
In such bus systems, for example, it can happen that the bus line connected to the output terminal of the control circuit has at some place a short circuit to the positive supply voltage terminal. In this case the resistor between the output terminal of the control circuit and said positive supply voltage terminal is short-circuited. Since such a short circuit leads to a constant potential value at the output terminal of the control circuit, the capacitor current becomes zero and the control circuit attempts to drive the transistor to maximum current output in order to bring the capacitor current back to the current strength of the reference current source.
The problems entailed by such a short circuit, namely high current consumption and high power dissipation, can be remedied by providing a short-circuit protecting circuit that causes a switch-off of the control circuit when the transistor current has exceeded a certain protective threshold.
Although such a protecting circuit offers protection from lengthy short circuits, serious problems still remain.
The load acting at the output terminal of the control circuit is virtually always inductive, at least because a line connected to said output terminal has a line inductance. A high short-circuit current before the time of protective switch-off results in accordingly high magnetic energy collecting in the load inductance and leading at the time of protective switch-off to inductive voltage pulses that can assume relatively high voltage levels.
Such voltage pulses result in relatively high electromagnetic interference, on the one hand, and involve the danger of signal decoders in the form of comparators connected to the line misinterpreting such inductive voltage pulses as signal or data pulses, resulting in a falsification of data transferred via the line, on the other hand.
SUMMARY OF THE INVENTION
An embodiment of a circuit configuration disclosed herein includes a switchover control circuit for controlling the slew rate of the output voltage as a function of a voltage curve occurring across an internal resistor in a first switching state, and feedback for controlling it as a function of the output voltage curve in a second switching state, and which is in a substantially dead state in a third switching state. The internal resistor is a different resistor from the resistor connected to the output terminal. The internal resistor is thus not impaired by a short circuit bridging the resistor connected to the output terminal.
Furthermore the circuit configuration has a detector circuit that provides a detection signal when the output voltage differs by at least a predetermined value from the output voltage level occurring before edge onset. A timer circuit is also provided for switching the control circuit from the first to the second switching state a predetermined length of time after edge onset if the detection signal is present at this time, and from the first to the third switching state if the detection signal is not present at this time.
The disclosed circuit configuration thus begins controlling the slew rate of the output voltage occurring at the output terminal, this control being guided by an element, in one embodiment the internal resistor, which is not affected by a short circuit at the output terminal. During this phase, in which the control circuit is in the first switching state, no higher current can thus flow, even if it corresponds to the desired slew rate, if a short circuit is present at the output terminal of the circuit configuration.
The detection circuit is used to compare whether the instantaneous voltage level of the edge to be controlled has a predetermined minimum distance from that voltage level present before edge onset. If such a minimum distance from the voltage level before edge onset is given, the detection circuit provides a detection signal signaling this.
The timer circuit is used to determine whether the detection signal is present at the end of a predetermined length of time since edge onset, i.e., the instantaneous output voltage level has reached the minimum distance from the voltage level occurring before edge onset.
If this is the case it can be assumed that no short circuit is present. However, if the instantaneous voltage level has not reached the minimum distance from the voltage level occurring before edge onset at the end of this length of time, the presence of a short circuit is assumed.
The distance the edge voltage must have from the voltage occurring before edge onset in order to provide the detection signal is selected to be so great that it cannot be reached under short circuit conditions.
If the detection signal, and thus no short circuit, is present at the end of the stated length of time, the control circuit is switched to the second switching state in which there is a switchover from slew rate control as a function of the voltage drop across the internal resistor to feedback control of the slew rate as a function of the voltage occurring at the output terminal.
If the detection signal is not present at the end of the stated length of time and a short circuit is therefore to be assumed, the control circuit is switched to the third switching state in which the control circuit is made dead or substantially dead.
The disclosed circuit configuration reacts to a short circuit only when a predetermined maximum current strength possible only under short circuit conditions is exceeded, but it already ascertains the presence of a short circuit toward the onset of the particular edge while the slew rate is still being controlled as a function of the internal resistor, so that a short circuit at the output terminal cannot yet have a current-increasing effect.
Since the current flowing at the output terminal increases more and more as the edge is increasingly traversed but the circuit configuration is already made dead upon detection of a short circuit in the area of the onset of the particular edge, at which time the current flowing at the output terminal is still relatively low, little magnetic energy is stored in the load inductance at the time of switchover of the control circuit to the third switching state, i.e., at the time the circuit configuration is made dead. One consequently avoids higher inductive voltage glitches when the circuit configuration is made dead, and the accompanying
Galanthay Theodore E.
Lam Tuan T.
Seed IP Law Group PLLC
STMicroelectronics GmbH
Tarleton E. Russell
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