Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-01-24
2002-12-10
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C361S091100, C361S111000
Reexamination Certificate
active
06492859
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to protection circuitry for protecting circuits against transients such as electrostatic discharge (ESD). In particular, the invention relates to circuitry for protecting analog bipolar circuits, including BJT and BICMOS based circuits against voltage transients.
BACKGROUND OF THE INVENTION
Analog circuits typically display sensitivity to excessive voltage levels. Transients, such as ESD can cause the voltage handling capabilities of the analog circuit to be exceeded, resulting in damage to the analog circuit. Clamps have been devised to shunt current to ground during excessive voltage peaks.
Typical protection clamps employ avalanche diodes such as zener diodes
50
to provide the bias voltage for the base of a subsequent power bipolar junction transistor (BJT)
52
as shown in FIG.
5
. Since separate, stable avalanche diodes in a given technology are difficult to realize, zener diodes are typically implemented as punch-through, or reverse coupled BJT's, as shown in FIG.
6
.
FIG. 6
shows a prior art clamp comprising a reverse coupled BJT
600
that acts as an avalanche diode. The BJT
600
has a defined reverse breakdown. A voltage pulse supplied to the input
602
that exceeds the breakdown voltage of the BJT
600
, causes an avalanche effect in BJT
600
and results in current flow from the emitter
604
to the collector
606
when the reverse breakdown voltage of the BJT
600
is exceeded. The current drives the base
608
of the power BJT
610
and switches BJT
610
on by raising the voltage across the resistor
612
, thus biasing the base
608
to switch on BJT
610
. Once the BJT
610
switches on, collector/emitter current is shunted to ground. To reach a higher protection voltage, several zener diodes or reverse coupled BJT's may be connected in series. For example, in a five volt power supply circuit where the BJT
610
is a 10 volt BJT, two 3.5 volt zener diodes could be used to replace BJT
600
. Until the BJT
610
switches on, the voltage
602
across the clamp, which is also the voltage applied to the protected circuit, continues to increase during a voltage peak such as a human body discharge. Such a discharge may typically be 120-125% percent of the final holding voltage.
One of the difficulties encountered in designing such protection circuitry is that the specifications for these clamps have to fit within a relatively small design window that takes into account the breakdown voltage of the circuit being protected. Thus, the clamp must be designed so as to be activated below the breakdown voltage of the circuit that is to be protected. At the same time, the design window is limited by the latchup phenomenon. To ensure that the clamp is not conducting under normal operating conditions, the latchup voltage must exceed the normal operating voltage of the protected circuit.
Since the breakdown voltage of a particular punch-through structure in the BJT clamp is determined by the particular technology process, prior art devices have difficulty complying with the small ESD protection window especially for high-voltage circuits, where maximum breakdown voltage can be only 10% higher than the operating voltage.
SUMMARY OF THE INVENTION
The present invention provides a protection clamp against transient conditions, that provides an adjustable ESD protection window. The invention uses the ability of a reverse coupled BJT to provide different breakdown voltage characteristics at different base current multiplication conditions. Base current multiplication is controlled by providing a resistor between the collector and the base of the reverse-coupled BJT.
According to the invention, there is provided an overvoltage protection circuit for protecting an input of an analog bipolar circuit comprising a first bipolar junction transistor having a base, a collector, and at least one emitter, connected between an input of the analog bipolar circuit and ground, a reverse-coupled bipolar junction transistor having a base, a collector, and at least one emitter, wherein the emitter is connected to the input, and the collector is connected to the base of the first bipolar junction transistor, and a resistor connected between the base and the collector of the reverse coupled bipolar junction transistor. The resistor is connected to the reverse-coupled transistor so as to increase base current multiplication in the reverse-coupled transistor.
Further, according to the invention, there is provided a method of improving an overvoltage protection circuit for an input to a protected circuit, wherein the overvoltage protection circuit includes a first transistor for shunting current to ground, and a reverse-coupled transistor connected to the first transistor to switch on the first transistor, the method comprising the step of lowering the reverse-breakdown voltage of the reverse-coupled transistor when a voltage transient occurs by increasing base current multiplication.
REFERENCES:
patent: 5978192 (1999-11-01), Young et al.
patent: 6028758 (2000-02-01), Sharpe-Geisler
patent: 6268990 (2001-07-01), Ogura et al.
Hopper Peter J.
Vashchenko Vladislav
Cunningham Terry D.
National Semiconductor Corporation
Tra Quan
Vollrath Jurgen
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