Abstract:
In a conventional differential output circuit, the output terminals are connected to the drains of a differential pair of transistors and the sources of the transistors are connected together at a first node. The bodies of the transistors are connected to a second node having a potential different from that of the first node. In the event of a HBM ESD event, discharge may take place through the differential transistors, leading to destruction of one of them. To reduce the likelihood of such discharge, in a preferred embodiment, switches are provided to connect the body of each of the differential transistors to the first node when an ESD event is sensed. In an alternative embodiment, a switch is provided to connect the first node to the second node when an ESD event is sensed.

Description:
BACKGROUND OF THE INVENTION 
     This relates to the protection of integrated circuits from electrostatic discharge (ESD). More particularly, it relates to the protection of differential circuits from ESD. 
     ESD protection has been a main concern in the reliability of integrated circuit products in various sub-micron technologies. ESD is the transient discharge of static charge that can arise from activities such as human handling, machine contact or field-induced charging of a packaged IC. Specific models have been developed to represent these discharges such as the Human Body Model (HBM), the Machine Model (MM), and the Charged Device Model (CDM), respectively. See, for example, A. Amerasekera and C. Duvvury,  ESD in Silicon Integrated Circuits , pp. 17-40 (2d Ed., Wiley, 2002), which is incorporated herein by reference. 
       FIG. 1  is a schematic diagram of an illustrative differential circuit  100  with conventional ESD protection circuitry. Differential circuit  100  comprises first and second transistors  110 ,  130 , a clamp transistor  150  and a diode  170 . Illustratively, first and second transistors  110 ,  130  are a low voltage differential signaling (LVDS) output pair. Each transistor  110 ,  130 ,  150  is a MOS transistor with a source and drain formed in a body of the transistor and an insulated gate over the body in the region between the source and drain. In the schematic diagram of  FIG. 1 , the bodies of transistors  110 ,  130 , and  150  are identified as elements  112 ,  132 ,  152 ; the sources are identified as elements  114 ,  134 ,  154 ; the drains are identified as elements  116 ,  136 ,  156 ; and the gates are identified as elements  118 ,  138 ,  158 , respectively. Sources  114  and  134  are connected together at a source node  190 . Resistors  120 ,  140 ,  160  are schematic representations of the circuitry between gates  118 ,  138 ,  158 , respectively, and a common node  180 ; and resistor  182  is a schematic representation of the circuitry between source node  190  and common node  180 . As is known in the art, the actual circuitry represented by these resistors may be considerably more complicated than a simple resistance. Input terminals  122 ,  142  are connected to gates  118 ,  138 , respectively; and output terminals  124 ,  144  are connected to drains  116 ,  136 , respectively. Diode  170  may be implemented as a dedicated diode or as the body diode of a MOSFET clamp transistor similar to transistor  150  or as both devices connected in parallel. 
     As is known in the art, the differential circuit typically comprises several other circuit elements not shown in  FIG. 1 . For example, transistor  110  is typically driven by circuitry connected to input terminal  122 . Additionally, other circuits are connected to output terminal  124  to pull this node up when transistor  110  is in the off state. 
     Typically, the transistors of differential circuit  100  are NMOS transistors with a P-type body and N-type source and drain regions. As a result, since the P-type body and the N-type source region of each transistor form a first P-N junction and the P-type body and the N-type drain region form a second P-N junction, a parasitic lateral bipolar transistor is present in each transistor. In the event of a positive voltage ESD event on the output terminal  124 , circuit  100  is intended to operate so that the second P-N junction of clamp transistor  150  is driven into breakdown and avalanche and the parasitic transistor is triggered into conduction to discharge the ESD pulse. 
     However, during the ESD event, the body voltage of transistors  110  and  130  can easily float above the source voltage, also making possible bipolar triggering of transistors  110  and  130 . For example, as shown in the voltage vs. time plot of  FIG. 2A , in the case of a positive ESD event on output terminal  124 , the voltages on nodes  180  and  190  will both rise until the voltage on node  180  reaches the threshold voltage of transistor  130  at time t 1 . Transistor  130  then begins to pull down the voltage at node  190  while the voltage at node  180  is basically pinned at one Vbe above ground by diode  170 . While the voltage on node  190  keeps decreasing, the avalanche current in transistor  110  and the voltage at output terminal  124  keep increasing. Eventually, destructive bipolar triggering will occur in transistor  110  when the body-source junction becomes fully forward biased at time t 2  leading to a rapid drop in the output voltage. To prevent this, the clamp transistor  150  must trigger before it happens; but it is difficult to assure consistent, timely triggering without significant additional circuitry. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is an ESD protection circuit that significantly reduces the likelihood of this failure mechanism. In a preferred embodiment of the invention, switches are provided to connect the body of each of the differential transistors to the source node when an ESD event is sensed. In an alternative embodiment, a switch is provided to connect the source node to the common node when an ESD event is sensed. 
     In the preferred embodiment, each switch is implemented with a pair of transistors. One transistor is a PMOS transistor that is connected between the body of one of the differential transistors and the source node; and the other transistor is an NMOS transistor that is connected between the body and the common node. The gate of the PMOS transistor is connected to the output terminal of the other differential transistor; and the gate of the NMOS transistor is connected to a control voltage. 
     During normal operation of the differential pair, the NMOS transistors are kept on by the control voltage, thereby connecting the bodies of the differential transistors to the common node. The PMOS transistors are kept off by the common voltage in the output signal. If there is an ESD event on the output terminal of one of the differential transistors relative to the other output terminal, the NMOS transistor connected to the body of that differential transistor is turned off and the PMOS transistor is turned on. As a result, the body of that transistor is disconnected from the common node and connected to the source node 
     In an alternative embodiment, the switches are implemented by a pair of PMOS transistors connected between the source node and the common node. The gate of each of the PMOS transistors is connected to a different one of the output terminals of the differential pair. Again, during normal operation of the circuit the PMOS transistors are kept off by the common voltage in the output signal. If there is an ESD event on the output terminal of one of the differential transistors relative to the other output terminal, one of the PMOS transistors is turned on, thereby connecting the source node and common node and connecting the body to the source node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will be apparent to those of ordinary skill in the art in view of the following detailed description in which: 
         FIG. 1  is a schematic diagram of a differential pair with a conventional ESD protection circuit; 
         FIGS. 2A-2B  are plots of voltage vs. time at various points in the circuit of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an illustrative embodiment of the present invention; 
         FIGS. 4A-4C  are plots of voltage vs. time at various points in the circuit of  FIG. 3 ; and 
         FIG. 5  is a schematic diagram of an alternative implementation of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic diagram of a differential circuit  200  with ESD protection circuitry of the present invention. Differential circuit  200  comprises first and second transistors  210 ,  230  and a clamp transistor and a diode (not shown) that are substantially the same as clamp transistor  150  and diode  170  of  FIG. 1 . Illustratively, transistors  210 ,  230  are a low voltage differential signaling (LVDS) output pair. Each transistor is a MOS transistor with a source and drain formed in a body of the transistor with an insulated gate over the body in the region between the source and drain. In the schematic diagram of  FIG. 3 , the bodies of transistors  210  and  230  are identified as elements  212 ,  232 ; the sources are identified as elements  214 ,  234 ; the drains are identified as elements  216 ,  236 ; and the gates are identified as elements  218 ,  238 , respectively. Sources  214  and  234  are connected together at a source node  290 . Again, resistors  220 ,  240  are schematic representations of the circuitry between gates  218 ,  238 , respectively, and a common node  280 ; and resistor  282  is a schematic representation of the circuitry between source node  290  and common node  280 . Again, the actual circuitry may be considerably more complicated than a simple resistance. Input terminals  222 ,  242  are connected to gates  218 ,  238 , respectively; and output terminals  224 ,  244  are connected to drains  216 ,  236 , respectively. As in the case of the circuit of  FIG. 1 , the differential circuit typically includes other circuit elements. 
     In addition, the circuit of  FIG. 3  comprises fourth and fifth transistors  310 ,  330 , which illustratively are PMOS transistors, and sixth and seventh transistors  350 ,  370 , which illustratively are NMOS transistors. In the schematic diagram of  FIG. 3 , the bodies of transistors  310 ,  330 ,  350 ,  370  are identified as elements  312 ,  332 ,  352 ,  372 ; the sources are identified as elements  314 ,  334 ,  354 ,  374 ; the drains are identified as elements  316 ,  336 ,  356 ,  376 ; and the gates are identified as elements  318 ,  338 ,  358 ,  378 , respectively. As shown in  FIG. 3 , the bodies  312 ,  332  of transistors  310 ,  330  are connected to a control voltage Vccn; the bodies  352 ,  372  of transistors  350 ,  370  are connected to sources  354 ,  374 , respectively; the drains  316 ,  356  of transistors  310  and  350  are connected to the body  212  of transistor  210 ; and the drains  336 ,  376  of transistors  330  and  370  are connected to the body  232  of transistor  230 . The gates of transistors  350  and  370  are connected to a control voltage Vcc. The gate of transistor  310  is connected to output terminal  244 ; and the gate of transistor  330  is connected to output terminal  224 . 
     During normal operation of circuit  200 , transistors  350 ,  370  are kept on by the control voltage Vcc., thereby connecting the bodies  212 ,  232  of transistors  210 ,  230  to common node  280 . Transistors  310 ,  330  are kept off by the common voltage in the output signal from the differential pair. If there is an ESD event on the output terminal of one of the differential transistors relative to the other output terminal, the NMOS transistor connected to the body of that differential terminal is turned off and the PMOS transistor is turned on. As a result, the body of that transistor is disconnected from common node  280  and connected to source node  290 . For example, if there is an ESD event on output terminal  224 , NMOS transistor  350  is turned off while PMOS transistor  310  is turned on, thereby switching the connection of body  212  from common node  280  to source node  290 . 
     The timing of these steps and their effect on the voltages at various points in the circuit is depicted in the plots of  FIGS. 4A-4C  which depict a simulation of an HBM ESD event on the circuit.  FIG. 4B  depicts the voltage at output terminal  224 .  FIG. 4A  depicts the voltage vs. time at body  212 , source node  290  and common node  280 .  FIG. 4C  depicts the difference between the voltage at body  212  and source node  290 . The vertical line indicates the point in time where the body voltage and the source voltage start to diverge in the circuit of  FIG. 1 . As can be seen in  FIGS. 4A and 4C , the circuit of  FIG. 3  holds the body voltage close to the source voltage. And, as can be seen in  FIG. 4B  the circuit prevents snapback for at least another 150 psec compared to the circuit of  FIG. 1 , thereby allowing the pad voltage to increase by about 2 more volts compared to that circuit. This, in turn, gives clamp  350  more time to turn on and absorb the energy of the ESD event, thereby diminishing the likelihood of bipolar triggering in the differential transistor. 
       FIG. 5  is a schematic diagram of a differential circuit  400  with ESD protection circuitry of the present invention. Again, differential circuit  400  comprises first and second transistors  410 ,  430  and a clamp transistor and a diode (not shown) that are substantially the same as clamp transistor  150  and diode  170  of  FIG. 1 . Illustratively, first and second transistors  410 ,  430  are a low voltage differential signaling (LVDS) output pair. Each transistor is a MOS transistor with a source and drain formed in a body of the transistor with an insulated gate over the body in the region between the source and drain. In the schematic diagram of  FIG. 5 , the bodies of transistors  410  and  430  are identified as elements  412 ,  432 ; the sources are identified as elements  414 ,  434 ; the drains are identified as  416 ,  436 ; and the gates are identified as  418 ,  438 , respectively. Sources  414  and  434  are connected together at a source node  490 . Again, resistors  420 ,  440  are schematic representations of the circuitry between gates  418 ,  438  and a common node  480 ; and resistor  482  is a schematic representation of the circuitry between source node  490  and common node  480 . The actual circuitry may be considerably more complicated than a simple resistance. Input terminals  422 ,  442  are connected to gates  418 ,  438 , respectively; and output terminals  424 ,  444  are connected to drains  416 ,  436 , respectively. As in the case of the circuits of  FIGS. 1 and 3 , differential circuit  400  typically includes other circuit elements as well. 
     In addition, the circuit of  FIG. 5  comprises fourth and fifth transistors  510 ,  530 , which illustratively are PMOS transistors. In the schematic diagram of  FIG. 5 , the bodies of transistors  510 ,  530  are identified as elements  512 ,  532 ; the sources are identified as elements  514 ,  534 ; the drains are identified as elements  516 ,  536 ; and the gates are identified as elements  518 ,  538 , respectively. As shown in  FIG. 5 , the bodies  512 ,  532  of transistors  510 ,  530  are connected to a control voltage Vccn; the sources  514 ,  534  of transistors  510 ,  530  are connected to node  480 ; and the drains  516 ,  536  of transistors  510  and  530  are connected to source node  490 . The gate of transistor  510  is connected to output terminal  444 ; and the gate of transistor  530  is connected to output terminal  424 . 
     During normal operation of circuit  400 , transistors  510 ,  530  are kept off by the common voltage in the output signal from the differential pair. If there is an ESD event on the output terminal of one of the differential transistors relative to the other output terminal, one of transistors  510 ,  530  is turned on thereby connecting common node  480  to source node  490  and connecting together the bodies  412 ,  432  and sources  414 ,  434  of transistors  410 ,  430 . As a result, the voltage at source node  490  will again remain close to the voltage at common node  480  during an ESD event and the risk of bipolar triggering the one of the differential transistors will be diminished. 
     While circuit  400  of  FIG. 5  has the advantage that it can be implemented with only two additional transistors more than in a conventional circuit instead of the four additional transistors of circuit  200  of  FIG. 3 , it is not preferred because the two additional transistors must be significantly larger because they operate at a reduced Vgs during an ESD event. Illustratively, PMOS transistors  310 ,  330  have a width/length (W/L) ratio of 100 μm/0.27 μm and NMOS transistors  350 ,  370  have a W/L ration of 30 μm/0.04 μm. While these ratios were used in simulating the circuit performance depicted in  FIGS. 3A-3C , it must be emphasized that they are only examples. It is likely that any final design would employ minimum length and that the final widths would be chosen to optimize circuit performance. 
     As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention.