Abstract:
An electrostatic discharge (ESD) protection circuit for protecting an input/output (I/O) circuit provided with different supply voltages against electrostatic discharge. The ESD protection circuit comprises a stacked NMOS transistor configuration, a triggering circuit and a disabling circuit. The ESD protection circuit is effectively disabled by the disabling circuit during normal operation. During an ESD event, a trigger current is generated by the triggering circuit to turn on the stacked NMOS transistor configuration and thus the ESD current is directed away. The ESD protection circuit also allows different voltages to be supplied during normal operation without damaging the transistors in the ESD protection circuit.

Description:
BACKGROUND  
       [0001]     The present invention relates to an electrostatic discharge (ESD) protection circuit, and, more particularly, to an electrostatic discharge (ESD) protection circuit protecting an input/output (I/O) circuit provided with different supply voltages against electrostatic discharge.  
         [0002]     Electrostatic discharge (ESD) commonly occurs in semiconductor devices. The ESD phenomenon may occur when excessive electrostatic charge is drained through an I/O pad or a power pad of an integrated circuit (IC), damaging the IC. To solve this problem, manufacturers may provide an ESD protection circuit in IC devices. The ESD protection circuit is initiated before the pulse of electrostatic discharge exerts excessive pressure on IC devices, directing the ESD current to a potential terminal (preferably ground) to bypass IC devices, thus reducing the potential for ESD-related damage.  
         [0003]     With the continuing demand for faster and smaller devices, however, there is a trend to scale down the dimensions of semiconductor integrated circuit (IC) devices. As a result, there arises a decrease in the gate length and gate oxide thickness of MOS devices, leaving IC devices more susceptible to damage from electrostatic discharge. That is, the safe operating voltage of these devices is reduced to a lower level. Consequently, an adequate and more effective ESD on-chip protection circuit must be designed to protect the IC against ESD-related damage.  
         [0004]     Furthermore, many ICs are required to receive input signals from peripheral devices having an operating voltage exceeding the core logic voltage of the IC devices. Such I/O signals can cause reliability problems if a suitable voltage protection circuit capable of withstanding higher input signal voltages is not incorporated. Accordingly, it is desirable to have an ESD circuit allowing suitable protection for a lower core voltage circuit when higher input/output voltage is present.  
         [0005]     Hence, there exists a need for a more effective ESD protection circuit for accommodating circuits with varying operating voltage range to overcome the problems of the related art.  
       SUMMARY  
       [0006]     The present invention is generally directed to an electrostatic discharge (ESD) protection circuit protecting an input/output (I/O) circuit provided with different supply voltages against electrostatic discharge. According to one aspect of the invention, the ESD protection circuit comprises a stacked NMOS transistor configuration, a triggering circuit and a disabling circuit. The stacked NMOS transistor configuration is coupled between a first power rail and a second power rail for receiving an ESD current from the first power rail and directing it to the second power rail during an ESD event. The stacked NMOS transistor configuration comprises at least a first NMOS transistor cascaded to a second NMOS transistor and has a gate coupled to a third power rail. The triggering circuit is coupled between the first power rail and the substrate of the stacked NMOS transistor configuration via a node. When an ESD event occurs, a trigger current is supplied to the stacked NMOS transistor configuration. The disabling circuit is coupled between the node and the second power rail for disabling the triggering circuit during normal operation. The third power rail has a third voltage level which is lower than the first voltage level of the first voltage rail but greater than the second voltage level of the second voltage rail during normal operation.  
         [0007]     According to another aspect of the invention, an ESD protection circuit is disclosed. The ESD protection circuit comprises a stacked NMOS transistor configuration, a triggering circuit and a disabling circuit. The stacked NMOS transistor configuration is coupled between a first power rail and a second power rail for receiving an ESD current from the first power rail and directing it to the second power rail during an ESD event. The stacked NMOS transistor configuration comprises at least a first NMOS transistor and a second NMOS transistor. The first NMOS transistor has a drain connected to the first power rail, a gate connected to a third power rail via a first resistor, a source coupled to the drain of the second NMOS transistor and the gate of the second NMOS transistor is connected to the source of the second NMOS transistor and the second power rail. The triggering circuit is coupled between the first power rail and the substrate of the stacked NMOS transistor configuration and comprises diodes coupled in series including at least an anode coupled to the first power rail and a cathode coupled to the disabling circuit at a node wherein the diodes are conductive for supplying a trigger current to the stacked NMOS transistor configuration during an ESD event. The disabling circuit is coupled between the node and the second power rail for disabling the triggering circuit during normal operation. The third power rail has a third voltage level which is lower than the first voltage level of the first voltage rail but greater than the second voltage level of the second voltage rail during normal operation.  
         [0008]     In one embodiment of the present invention, an ESD protection circuit comprises a stacked NMOS transistor configuration, a triggering circuit and a disabling circuit. The stacked NMOS transistor configuration is coupled between a first power rail and a second power rail for receiving an ESD current from the first power rail and directing the ESD current to the second power rail during an ESD event. The stacked NMOS transistor configuration comprises at least a first NMOS transistor and a second NMOS transistor wherein the first NMOS transistor has a drain connected to the first power rail, a gate connected to a third power rail via a first resistor, a source coupled to the drain of the second NMOS transistor and the gate of the second NMOS transistor connected to the source of the second NMOS transistor and the second power rail. Moreover, the transistors in the stacked NMOS transistor configuration are thin-gate devices. The triggering circuit is coupled between the first power rail and the substrate of the stacked NMOS transistor configuration via a node. The triggering circuit comprises a PMOS transistor including a source coupled to the first power rail, a drain coupled to the node and a gate coupled to the first power rail via a second resistor wherein the NMOS transistor is a thick-gate device and is turned on to generate the trigger current to the stacked NMOS configuration during an ESD event. The disabling circuit is coupled between the node and the second power rail for disabling the triggering circuit during normal operation. The third power rail has a third voltage level which is lower than the first voltage level of the first voltage rail but greater than the second voltage level of the second voltage rail during normal operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0010]      FIG. 1  is a schematic diagram of an ESD protection circuit according to an embodiment of the invention; and  
         [0011]      FIG. 2  is a schematic diagram of a charge pump according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]     With reference to  FIG. 1 , a circuit diagram illustrates an ESD protection circuit  100  according to a first embodiment of the invention. The ESD protection circuit  100  of the invention is arranged between power rails V I/O  and V ss , and designed to protect a circuit  102  such as an I/O buffer, which is implemented by thin gate MOS transistors. Power rails V I/O  and V ss  are connected respectively to an I/O voltage level V I/O  and V ss  (preferably I/O ground). The ESD protection circuit  100  comprises a stacked NMOS configuration  106 , a triggering circuit  108  and a disabling circuit  110 .  
         [0013]     The stacked NMOS transistor configuration  106  is arranged between power rails V I/O  and V ss  and comprises at least a NMOS transistor N 1  cascaded to a NMOS transistor N 2 . More particularly, the transistor N 1  has a drain connected to power rail V I/O , a gate connected to a power rail V core  with a core voltage level V core  via a resistor R 1  and a source coupled to the drain of the transistor N 2  wherein the core voltage level V core  is lower than I/O voltage level V I/O  and I/O ground voltage V ss  is lower than V core . The gate and source of transistor N 2  are coupled together to power rail V ss . During normal operation, the gate to source voltage or the gate to drain voltage of the stacked NMOS transistor configuration  106  is within the supply voltage of core circuit, V core . Thus, the stacked NMOS transistor configuration  106  can be implemented by thin gate NMOSs used in a core circuit while maintaining the reliability of the ESD protection circuit  100 .  
         [0014]     The disabling circuit  110  comprises a NMOS transistor N 3  and a capacitor  118 . The transistor N 3  has a gate coupled to power rail V core  through a resistor R 1 , a source coupled to power rail V ss , and a drain coupled to the triggering circuit  108  and the substrate of the stacked NMOS transistor configuration  106  via a node M.  
         [0015]     In this embodiment, the triggering circuit  108  comprises a diode string including at least an anode coupled to power rail V I/O  and a cathode coupled to the disabling circuit  110  at the node M where during normal operation, the number of diodes in the triggering circuit  108  is adjusted according to the desired leakage current at the work temperature and the desired threshold voltage for turning on the diode string during an ESD event.  
         [0016]     During normal operation, the transistor N 2  is turned off, hence, the ESD protection circuit  100  is high impedance and non-conductive during normal operation. Additionally, transistor N 3  is turned on to draw away a leakage current, if any, from the triggering circuit  108 , to avoid turning on the stacked NMOS configuration  106  during normal operation. Furthermore, the diodes in the triggering circuit  108  are turned off because the threshold voltage of diodes in the diode string is adjusted to be greater than voltage level V I/O . Therefore, no trigger current is generated to trigger the stacked NMOS configuration  106 .  
         [0017]     During an ESD event, for example, where there is a positive voltage impulse occurring in power rail V I/O  and power rail V ss  is grounded, the diodes in the triggering circuit  108  are turned on to conduct a trigger current while the ESD stress from power rail V I/O  is higher than the threshold voltage of the diode string. The capacitor  118  in the disabling circuit  110  is unable to react in time during an ESD impulse. Therefore, the gate of transistor N 3  is grounded and transistor N 3  is turned off during an ESD event. Consequently, the trigger current generated in the triggering circuit  108  is directed to the substrate of the stacked NMOS configuration  106  at node M and turns on the stacked NMOS configuration  106  to direct the ESD current to power rail V ss .  
         [0018]     The stacked NMOS configuration  106  in the ESD protection circuit  100  further comprises a parasitic bipolar  126  and a parasitic resistor R sub  wherein the parasitic bipolar  126  has a collector connected to the drain of transistor N 1 , an emitter connected to the source of the transistor N 2  and a base coupled to the node M, and R sub  is coupled between the node M and power rail V ss . During normal operation, the bipolar  126  is turned off. When an ESD event occurs, the trigger current from the triggering circuit  108  flows to the base of bipolar  126  and resistor R sub . When the bias voltage in the base of bipolar  126  is greater than the threshold voltage of bipolar  126 , the bipolar is turned on, directing the ESD current to power rail V ss . With a higher resistance of resistor R sub , the bipolar  126  is turned on earlier. As a result, the ESD protection circuit  100  is able to draw the ESD current away from power rail V I/O  earlier. The voltage on the power rail V I/O  is thus clamped to a low voltage level so as to protect the circuit  102  from ESD damage. Moreover, according to designed, during normal operation, the stress level on the ESD protection circuit MOS transistor gates are all less than or equal to V core . It would thus be desirable to utilize thin gate devices with the same gate thickness in the ESD protection circuit  100  and to reduce IC process costs.  
         [0019]      FIG. 2  illustrates another embodiment of the invention. The ESD protection circuit  200  is similar to that shown in  FIG. 1  except that the triggering circuit  108  is replaced by a thick gate NMOS transistor P 1  and transistor N 3  is also a thick gate device having a gate coupled to power rail V I/O  via a resistor R 2 . In this embodiment, a thin gate device is fabricated to operate safely when its terminals are supplied with V core  or V ss  voltage. Additionally, a thick gate device is provided for safe operation when terminals thereof are supplied with V I/O  or V ss  voltage. Transistor P 1  has a source coupled to power rail V I/O , a gate coupled to power rail V I/O  through resistor R 2  and a drain coupled to the disabling circuit  210  at node M. During normal operation, transistor P 1  is turned off. When turned on by an ESD stress from power rail V I/O , transistor P 1  generates a trigger current during an ESD event. Similarly, the trigger current is directed to the bipolar  226  and the resistor R sub . With a bias current generated in the bipolar  226 , the stacked NMOS configuration  206  is turned on to discharge ESD current.  
         [0020]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.