Abstract:
An apparatus for monitoring a load current drawn by an electrical circuit in a wire includes: 1) a Lorentz force MOS transistor having a first drain current (ID 1 ) and a second drain current (ID 2 ), wherein the Lorentz force MOS transistor is disposed proximate the wire carrying the load current and wherein a magnetic force generated by the load current increases a first current difference between the first drain current and a second drain current; 2) a current difference amplification circuit for detecting the first current difference between the first drain current and the second drain current and generating an amplified output signal; and 3) a current monitoring circuit coupled to the current difference amplification circuit capable of detecting and measuring the amplified output signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is generally directed to a current monitoring circuit and, more specifically, to a circuit capable of detecting a latchup condition or other over-current condition using a magnetic field detection transistor (or magFET). 
     BACKGROUND OF THE INVENTION 
     The power and complexity of integrated circuits, such as microprocessor chips, random access memory (RAM) chips, application specific integrated circuit (ASIC) chips, and the like, has increased dramatically in the last twenty years. This complexity has increased the likelihood of a manufacturing defect occurring on the chip. It also has increased the likelihood that a poor chip design may make the integrated circuit more susceptible to error conditions, such as latch-up, during times when the integrated circuit is operating under adverse conditions, such as high noise, power supply over-voltage conditions, high temperature, and the like. A common technique for screening integrated circuits (IC) is to measure the I DDQ  of an integrated circuit under test. I DDQ  is the power supply current in a quiescent operating condition. Faulty integrated circuits have a different I DDQ  signature compared to non-faulty integrated circuits. 
     To increase the reliability of integrated circuits and to detect error conditions and defective chips more rapidly, it is common practice to incorporate built-in self test (BIST) circuitry in many types of integrated circuits. However, adding built-in self test circuitry presents additional problems. As the level of sophistication of self-testing increases, so does the size and complexity of the BIST circuitry. This results in a tradeoff between silicon area and detection sensitivity. Furthermore, the BIST circuitry itself may cause errors. This is particularly true as the complexity of the BIST circuitry increases. Finally, it is essential that the built-in self test (BIST) circuitry be able to monitor voltages and currents in an integrated circuit without interfering with the operation of the circuits that are being tested. 
     Therefore, there is a need in the art for improved circuitry for detecting error conditions in integrated circuits. In particular, there is a need in the art for built-in self test (BIST) circuitry that is simple and reliable and yet capable of performing relatively complex and sensitive testing. More particularly, there is a need in the art for BIST circuitry that is capable of accurately and non-intrusively monitoring current levels in an integrated circuit. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an apparatus for monitoring a load current drawn by an electrical circuit in a wire. In an advantageous embodiment of the present invention, the apparatus comprises: 1) a Lorentz force MOS transistor having a first drain current (ID 1 ) and a second drain current (ID 2 ), wherein the Lorentz force MOS transistor is disposed proximate the wire carrying the load current and wherein a magnetic force generated by the load current increases a first current difference between the first drain current and the second drain current; and 2) a current difference amplification circuit capable of detecting the first current difference between the first drain current and the second drain current and generating an amplified output signal. 
     According to one embodiment of the present invention, the apparatus further comprises a current monitoring circuit coupled to the current difference amplification circuit capable of detecting and measuring the amplified output signal. 
     According to another embodiment of the present invention, the current monitoring circuit compares the amplified output signal to a predetermined threshold level and generates an error signal if the amplified output signal exceeds the predetermined threshold level. 
     According to still another embodiment of the present invention, the apparatus further comprises a switch controlled by the current monitoring circuit capable of coupling the wire to a power supply. 
     According to yet another embodiment of the present invention, the current monitoring circuit opens the switch and measures the amplified output signal when the load current is zero to thereby determine an initial current difference reference. 
     According to a further embodiment of the present invention, the current monitoring circuit closes the switch and measures the amplified output signal to determine the first current difference. 
     According to a still further embodiment of the present invention, the Lorentz force MOS transistor is disposed within a loop formed by the wire. 
     According to a yet further embodiment of the present invention, the Lorentz force MOS transistor is disposed within a plurality of concentric loops formed by the wire. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 illustrates an exemplary prior art magnetic field detection transistor (or MagFET); 
     FIG. 2 illustrates a current difference detection circuit using the exemplary MagFET according to one embodiment of the present invention; 
     FIG. 3 illustrates a monitoring circuit capable of monitoring the current drawn by an active circuit and detecting a latchup or other over-current condition, according to one embodiment of the present invention; 
     FIGS. 4A and 4B illustrate different configurations of current-carrying wires monitored by the exemplary MagFET according to alternate embodiments of the present invention; and 
     FIG. 5 is a flow diagram illustrating the operation of the exemplary monitoring circuit in FIG. 3 according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 5, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged current monitoring circuit. 
     FIG. 1 illustrates exemplary prior art magnetic field detection transistor (or magFET)  100 . MagFET  100  comprises source  110 , gate  120 , drain  130  and drain  140 . MagFET  100  is similar to a normal metal-oxide-silicon (MOS) transistor, except for the split drain. Source  110  contains contact pad  111 , drain  130  contains contact pad  131 , and drain  140  contains contact pad  141 . Source current (I s ) flows from contact pad  111  towards contact pads  131  and  141  in the split drain. If no external magnetic field is applied, the source current splits evenly between the drains. 
     However, when an external magnetic field is generated by the current in a wire line or wire loop positioned near magFET  100 , the Lorentz force created by the magnetic field causes an imbalance (or difference) in the drain currents, I d1  and I d2 . The greater the magnitude of the current in the wire line or wire loop, the greater the magnitude of the magnetic field and the Lorentz force, and the greater the magnitude of the difference in the drain currents, I d1  and I d2 . Lorentz force devices, such as magFET  100 , are well known in the art. An exemplary Lorentz force MOSFET (LMOS) is discussed in “Micro IDDQ Test Using Lorentz Force MOSFETs,” K. Nose and T. Sakurai, Proceedings of the 1999 Symposium on VLSI Circuits, IEEE, June 1999, pp. 169-170 (hereafter, the “Nose et al. reference”). The teachings of the Nose et al. reference are hereby incorporated by reference into the present application as if fully set forth herein. 
     FIG. 2 illustrates exemplary current difference detection circuit  200  using exemplary magFET  100  according to one embodiment of the present invention. Current difference detection circuit  200  is designed to detect and amplify any difference in the drain currents, I d1 , and I d2 , in magFET  100 . Current difference detection circuit  200  comprises p-type transistors  205 ,  210 ,  215 ,  220  and  225 , n-type transistor  230 , current source  240  and magFET  100 , which also is an n-type transistor. The split drains  130  and  140  of magFET  100  are labeled D 1  and D 2 , respectively. 
     Current source  240  creates a reference current, I ref , that flows through transistor  205  and transistor  230 . The gates of transistor  230  and magFET  100  are connected together, so that the same gate-to-source bias voltage, V gs , appears across transistor  230  and magFET  100 . This forces the drain-to-source current in magFET  100  to be equal to the drain-to-source current in transistor  230 , namely I ref . Thus, the sum of the split drain currents (i.e., I d1 +I d2 ) is equal to I ref . If no magnetic force is acting upon the gate of magFET  100 , the drain currents, I d1  and I d2 , are equal to each other, so that one-half of I ref  flows into each drain of magFET  100 . Thus, ½(I ref ) flows through the circuit branch comprising transistors  210  and  220  and ½(I ref ) flows through the circuit branch comprising transistors  215  and  225 . Under these conditions, the voltages at nodes V 1  and V 2  are equal and the difference voltage, ΔV=V 1 −V 2 , equals 0. 
     The difference between the drain currents, I d1  and I d2 , does not have to be zero initially. In alternate circuit topologies, the drain currents I d1 , and I d2  may initially be deliberately unbalanced, thereby creating an initial non-zero ΔV reference point. In one circuit topology, transistors  210  and  215  may be fabricated slightly differently, which creates an imbalance in the magnitudes of currents I d1  and I d2  through magFET  100 . In an alternate circuit topology, (optional) trim block  216  may be inserted between the gates of transistors  210  and  215 . Trim block  216  creates a slight voltage difference between the gates of transistors  210  and  215 , thereby creating an imbalance in the magnitudes of currents I d1  and I d2  through magFET  100 . 
     However, if a current-carrying wire is disposed on or near magFET  100 , the current in the wire causes a magnetic field that creates (or increases) a difference in the drain currents, I d1  and I d2 . Thus, the currents in the two current branches coupled to drains D 1  and D 2  are no longer equal and a voltage difference appears between nodes V 1  and V 2 . The greater the current in the wire overlaid on or near magFET  100 , the greater the magnetic field caused by the wire and the greater the difference in the drain currents, I d1  and I d2 . As the difference between I d1 +I d2  grows, the difference is amplified to an even greater degree in ΔV=V 1 −V 2 . In cases where transistors  210  and  215  are fabricated differently or trim block  216  is used, the initial imbalance in the magnitudes of currents I d1  and I d2  through magFET  100  may be overcome by the magnetically induced current difference, thereby causing a difference in the state of the circuit and indicating an over-current condition. 
     In the exemplary embodiment shown in FIG. 2, current difference detection circuit  200  comprises only a single magFET capable of detecting current in a wire. However, in alternate embodiments of the present invention, current difference detection circuit  200  may comprise two or more magFETs similar to magFET  100  that are coupled in series or in parallel, or in a combination of series and parallel magFETs. 
     FIG. 3 illustrates exemplary integrated circuit (IC)  300 , which contains monitoring circuitry that monitors the current drawn by active circuit  340  and detects a latchup or other over-current condition, according to one embodiment of the present invention. Integrated circuit  300  comprises power supply  310 , which supplies power to active circuit  340  via wire  330  and switch  320 . Active circuit  340  is not intended to be any particular type of electronic circuit. Wire  330  is formed by the metallization layers in the semiconductor. Active circuit  340  may be the CPU logic of a microprocessor, a random access memory (RAM), a digital signal processor, a radio frequency (RF) transceiver, or the like. 
     The monitoring circuitry comprises current difference detection circuit  200 , current monitor  350 , and switch  320 . These elements monitor the level of current in wire  330  in order to detect a latch-up or other over-current condition. Current difference detection circuit  200  comprises a plurality of magFETs for detecting current in wire  330 , including exemplary magFET  100   a,  exemplary magFET  100   b,  and exemplary magFET  100   c,  which may be arranged in parallel, in series, or in a parallel and series combination. When switch  320  is closed, current flows from power supply  310  to active circuit  340  through wire  330  (i.e., from node A to node B). The current in wire  330  creates a magnetic field that is sensed by magFETs  100   a-c,  which produce a difference voltage signal, ΔV, that is read by current monitor  350 . Current monitor  350  comprises processing circuitry capable of measuring and storing the difference voltage ΔV, comparing it to one or more predetermined threshold values, and determining whether or not a latch-up or other over-current condition exists. The greater the value of ΔV, the more likely a latch-up or over-current condition exists. 
     FIGS. 4A and 4B illustrate different configurations  400  and  450  of current-carrying wire  330  monitored by exemplary magFET  100  according to alternate embodiments of the present invention. The magnetic field created in magFET  100  by the current in wire  330  may be increased by looping the wire around magFET  100  as shown in FIRE  4 A. Current flows from node A to node B and creates a larger magnetic field because of the amount of wire in close proximity to magFET  100 . Increasing the magnetic field increases the magnitude of the difference voltage signal, ΔV, that is read by current monitor  350 . FIG. 4B shows a more extreme example. Current in wire  330  flows from node A to node B and makes many loops around magFET  100 . Each loop adds to the size of the magnetic field and, therefore, increases the magnitude of the difference voltage signal, ΔV, read by current monitor  350 . 
     FIG. 5 depicts flow diagram  500 , which illustrates the operation of exemplary monitoring circuit  300  according to one embodiment of the present invention. Initially, no power is applied to integrated circuit  300  and switch  320  is open. When power is applied, power supply  310  comes on and provides power to current monitor  350  and current difference detection circuit  200 . Current monitor  350  then measures the difference voltage, V 0 , from current difference detection circuit  200  when no current is flowing in wire  330 . This permits calibration of current monitor  350  for no load conditions (process step  505 ). 
     Next, current monitor  350  closes switch  320  so that current flows through wire  330  and into active circuit  340 . The load current in wire  330  drawn by active circuit  340  creates a magnetic field that causes drain current differences in one or more of magFETs  100   a-c   100   c.  The current differences are detected by current difference detection circuit  200 , which generates a corresponding difference voltage, V 1 , that is measured by current monitor  300  (process step  510 ). Finally, current monitor  300  compares the measured difference voltage, V 1  to a predetermined threshold voltage, VT, and passes or fails integrated circuit  300  by generating an error (or fail) signal or a pass signal accordingly (process step  515 ). 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.