Abstract:
Electrostatic discharge (ESD) protection clamp circuitry including current tunneling circuitry to provide control current for controlling current shunting circuitry for shunting ESD current from the protected signal terminal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electrostatic discharge (ESD) protection circuitry, and in particular to ESD protection clamp circuitry for circuits operating at low power supply voltages. 
   2. Description of the Related Art 
   Electrostatic discharge (ESD) in semiconductor integrated circuits is a well known problem. The presence of a sudden and undesired voltage spike within or at the interface pin of an integrated circuit can cause physical destruction of one or more portions of the circuitry. This is particularly true for field effect transistors, such as metal oxide semiconductor (MOS) devices, where the thin gate oxide is easily destroyed by large voltage spikes. Similarly, the PN junctions of bipolar devices can also be degraded if not destroyed. 
   This problem becomes increasingly acute, particularly for circuits employing both complementary MOS and bipolar processes, as the power supply voltages and signal levels decrease, e.g., 3.3 volts, 2.5 volts, 1.8 volts, and so on. As these types of circuits have developed, it has become increasingly common to provide ESD protection through the use of active clamp circuitry, many types of which are well known in the art. 
   Referring to  FIG. 1 , one conventional form of ESD clamp circuitry  10   a  has the clamp circuitry coupled between the signal node, e.g., defined as the interconnection between an input/output pad P 1  and further internal circuitry  12 , and the circuit reference terminal, e.g., ground. A serially coupled diode D 1  and resistor R 1  serve as control circuitry for a bipolar transistor Q 1  which serves as the ESD current shunting device. As the ESD voltage increases and becomes sufficiently large to cause breakdown within the diode D 1 , a control current Icon flows through the diode D 1  and resistor R 1 . This produces a voltage VI across the resistor R 1  and a base current I 1  for the transistor Q 1 . This causes the transistor Q 1  to turn on and conduct current Iesd associated with the ESD pulse to the circuit reference node, or ground. 
   Referring to  FIG. 2 , a similar circuit  10   b  uses a capacitor C 1  instead of a diode D 1 . The rapid rise of the ESD voltage causes a current to flow briefly through the capacitor C 1 . This, in turn, causes the voltage VI to appear across the resistor and turn on the current shunting transistor, in this example an N-channel MOS transistor N 1 , thereby causing the ESD current Iesd to be shunted to the circuit reference node. 
   While these circuits  10   a ,  10   b , and others similar to these (e.g., where the control devices in the form of diodes and capacitors may be mixed and matched with different types of current shunting devices in the form of bipolar transistors or MOS transistors) have performed reasonably well up to now, such circuits do not perform as well with the lower power supply voltages and signal levels which have become more prevalent. At reduced power supply voltages and signal levels, leakage currents become more of a problem as the leakage currents become proportionately higher with respect to the power supply voltages and signal levels, and such leakage currents become more likely to be amplified by the clamp, thereby causing false triggering. 
   SUMMARY OF THE INVENTION 
   In accordance with the presently claimed invention, electrostatic discharge (ESD) protection clamp circuitry includes current tunneling circuitry to provide control current for controlling current shunting circuitry for shunting ESD current from the protected signal terminal. 
   In accordance with one embodiment of the presently claimed invention, electrostatic discharge (ESD) protection clamp circuitry includes first and second terminals, current shunting circuitry and current tunneling circuitry. The first terminal is to convey desired and undesired signals. The current shunting circuitry is coupled between the first and second terminals, and responsive to a control current having first and second states, wherein during the first and second control current states the current shunting circuitry provides nonconductive and conductive current paths, respectively, between the first and second terminals. The current tunneling circuitry is coupled to the first terminal and the current shunting circuitry, and responsive to the desired and undesired signals by providing the control current in the first and second states, respectively, wherein during the first and second control current states the control current has substantially zero and nonzero magnitudes, respectively. 
   In accordance with another embodiment of the presently claimed invention, electrostatic discharge (ESD) protection clamp circuitry includes current tunneling means and current shunting means. The current tunneling means is for responding to desired and undesired signals by generating a control current in first and second states, respectively, wherein during the first and second control current states the control current has substantially zero and nonzero magnitudes, respectively. The current shunting means is for responding to the control current by shunting substantially zero current corresponding to the desired signal during the first control current state and by shunting a nonzero current corresponding to the undesired signal during the second control current state. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional ESD clamp circuit. 
       FIG. 2  is a schematic diagram of another conventional ESD clamp circuit. 
       FIGS. 3–6  are schematic diagrams of ESD clamp circuits in accordance with various embodiments of the presently claimed invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
   Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. 
   Referring to  FIG. 3 , an ESD clamp circuit  100   a  in accordance with one embodiment of the presently claimed invention has a circuit topology which is similar to other ESD clamp circuits. However, rather than using a simple diode or capacitor (e.g., such as those used in the conventional circuits of  FIGS. 1 and 2 ), a current tunneling device, such as a MOS diode D 11 , is used as the primary control element for the ESD current shunting device or circuit. For example, in this embodiment  100   a , upon reception of an ESD pulse of sufficient magnitude, the diode D 11  breaks down and a control current Icon flows, thereby producing a voltage V 11  across the resistor R 11  and the base current I 11  for the bipolar transistor Q 11  used to shunt the ESD current Iesd. 
   Using a current tunneling device such as this is advantageous as compared to the use of a diode or capacitor since such a current tunneling device with proper dielectric thickness will behave as a current source so long as the voltage across it exceeds the critical level needed for current tunneling. Such devices can be easily designed for sufficiently low breakdown voltages and low on currents, while at the same time having significantly less leakage currents than diodes which are not of the current tunneling type. Further, such tunneling devices typically have a high dynamic current range, thereby minimizing the risk of such device experiencing irreversible breakdown, particularly since most of the ESD current will be shunted by the current shunting transistor. 
   Referring to  FIG. 4 , an ESD protection circuit in accordance with another embodiment  100   b  of the presently claimed invention uses the current tunneling diode D 11  to provide a control current Icon for use as a drive current for a driver transistor Q 12  which, in turn, provides the driving current I 11  and voltage V 11  needed for triggering the ESD current shunting transistor Q 11 . 
   Referring to  FIG. 5 , an ESD protection circuit in accordance with another embodiment  200   a  of the presently claimed invention has a topology similar to the circuit of  FIG. 3 , with the exception that an N-channel MOS transistor N 11  is used as the ESD current shunting device. 
   Referring to  FIG. 6 , an ESD protection circuit in accordance with another embodiment  200   b  of the presently claimed invention uses a current tunneling diode D 11  and resistor R 11  as in the circuits of  FIGS. 3 ,  4  and  5 . As before, once current tunneling occurs, the control current Icon produces a voltage V 11  across the resistor R 11 . This voltage V 11  drives a series of logic inverter circuits L 1 , L 2  to produce a logic voltage at the gate terminal of the N-channel MOS transistor N 11  used as the ESD current shunting device. It will be understood by one of ordinary skill in the art that the number of logic inverters can vary according to the amount of signal power amplification, delay or inversion is needed for driving the ESD current shunting device. 
   Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.