Abstract:
In an ESD protection circuit for an analog bipolar circuit, the avalanche breakdown voltage of a reverse-coupled NPN BJT acting as an avalanche diode is adjusted to comply with breakdown voltage and latchup requirements by including a resistor between the base and collector of the BJT.

Description:
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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of one embodiment of a clamp of the present invention; 
     FIG. 2 is a schematic circuit diagram of a general application of the clamp of FIG. 1; 
     FIG. 3 is a graphical representation showing the operating window of the clamp of the invention; 
     FIG. 4 shows sets of waveforms for the voltage, current, and lattice temperature for various base-collector resistances for the reverse-coupled BJT used in the invention; 
     FIG. 5 is a schematic circuit diagram of a prior art clamp, and 
     FIG. 6 is a schematic circuit diagram of another prior art clamp. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of a protection clamp of the invention is shown in FIG.  1 . The clamp  100  comprises an avalanche diode in the form of a reverse-coupled NPN BJT  102 , and a power BJT  104 . It will be appreciated that the reverse-coupled and power transistors could, instead, be PNP transistors. A resistor  106  is connected between the base  108  and collector  110  of the BJT  102 . 
     A general implementation of the clamp  100  of the invention is shown in FIG. 2. A general bipolar analog circuit to be protected is represented by amplifier  200  which is attached to a contact pad  202 . The clamp  100  is connected in parallel between the input of the circuit  200  and ground. 
     It will be appreciated, that for the clamp  100  to work effectively it has to turn on before the breakdown voltage of the circuit  200  is exceeded. Furthermore, a typical circuit such as that represented by circuit  200  will have some typical operating voltage, such as 5 V, 12 V, 20 V, etc. To avoid the clamp  100  switching on under stable operating conditions, it will be appreciated that the clamp  100  must be designed to switch on only once the stable operating voltage is exceeded by some defined amount. Under stable operating conditions, the voltage to which the clamp is exposed comprises a DC bias and a signal applied to the protected circuit. The avalanche diode, in this case BJT  102 , can handle only brief overvoltage pulses and would typically suffer damage if it remained in conduction under normal operating conditions. Thus the clamp  100  must be designed to switch off at a voltage above the stable operating voltage. This switching voltage, known as latchup is best illustrated by the line  300  in FIG. 3, where the latchup voltage is shown to be somewhat greater than the stable operating voltage V dd  of the analog circuit. On the other hand, in order to protect the attached circuit against voltage transients, the clamp must turn on before the breakdown voltage of the attached circuit. The clamp  100  thus has a voltage window  302 , between the latchup voltage  300  and the analog circuit breakdown voltage  304  or overvoltage limit, which defines the operating range of the clamp  100 . A transient input voltage peak  306 , would typically exceed the overvoltage limit  304 . However, the clamp  100  serves to protect the analog circuit against this overvoltage peak by switching on and remaining in conduction until the input voltage returns to a level below the latchup voltage  300 . It will be appreciated by those skilled in the art that the power BJT  104  could instead be a field effect transistor. 
     FIG. 4 shows five sets of waveforms for the collector-emitter voltage of the reverse-coupled BJT, indicated generally by reference numeral  400 . Waveforms for the corresponding collector currents of the reverse-coupled BJT  102  are indicated generally by reference numeral  402 . Waveforms for the corresponding lattice temperatures of the reverse-coupled BJT  102  are indicated generally by reference numeral  404 . The waveforms  400 ,  402 ,  404  show the effects of different resistor values for the resistor  106 . When a zero resistance is chosen for resistor  106  (short circuit), a collector-emitter voltage  410  is produced having a peak breakdown voltage of about 9V. The corresponding current waveform  412  shows the collector current flow for BJT  104  increasing until the voltage drops to just below 8V thereby defining a very narrow window for protecting circuits with a breakdown voltage of more than 9V and having a latchup voltage of about 8V. The corresponding lattice temperature is shown by waveform  416 . 
     As the resistance of resistor  106  is increased, the breakdown voltage gradually decreases, as does the latchup voltage. This is shown for a resistance of 1 Ω by a voltage waveform  420  and corresponding current waveform  422 . The lattice temperature is indicated by waveform  424 . Waveform  430  shows the voltage characteristics for a resistance of 10 Ω for the resistor  106 . Waveforms  432  and  434  show the corresponding collector-current waveform and lattice-temperature waveform for the 10 Ω resistor embodiment. When the resistance of resistor  106  is increased to 20 Ω, the voltage drops even further as shown by waveform  440 . The corresponding current waveform is indicated by reference numeral  442 , while the lattice temperature is indicated by waveform  444 . Voltage waveform  450  shows the voltage characteristics at a resistance of 100 Ω for resistor  106 . The corresponding current waveform is indicated by reference numeral  452 , and the corresponding lattice temperature is given by the waveform  454 . 
     The invention has been described with reference to a specific embodiment of a clamp. It will be appreciated that the manipulation of the design window through the inclusion of a resistor to inject current into the base of a reverse-coupled BJT can equally well be applied to variations of the clamp circuit.