Abstract:
A mixed-voltage I/O buffer comprises an input circuit, an output circuit, an I/O pad, a pre-driver circuit coupled to the output circuit, two added coupled N-type transistors, and a dynamical gate-controlled circuit coupled to each gate of the two N-type transistors and the pre-driver circuit; one of the N-type transistors is coupled to the input circuit and the output circuit; the other N-type transistor and the dynamic gate-controlled circuit are together coupled to the I/O pad. Thereby, a mixed-voltage I/O buffer which receives 2×VDD-tolerant input signals and overcomes the hot-carrier degradation is realized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a mixed-voltage I/O buffer, particularly to a mixed-voltage I/O buffer that inhibits hot-carrier degradation. 
   2. Description of the Related Art 
   With the advance of technology and science, transistors become more and more miniaturized. Thus, the allowed maximum node voltage, such as the gate-source voltage (Vgs), the gate-drain voltage (Vgd), and the drain-source voltage (Vds), should be also decreased to guarantee the lifetime of the circuit. 
   At present, the chips fabricated with the advanced CMOS process still have to work together with the circuits fabricated with the earlier stage CMOS process. Thus, the transmission interface of the chips is likely to receive a voltage signal greater than the normal working voltage VDD thereof. The mixed-voltage I/O (Input/Output) buffer is commonly used in the transmission interface to receive a higher voltage signal and output a lower operating voltage to satisfy the requirements of high speed and low power consumption and guarantee circuit lifetime. Further, the mixed-voltage I/O buffer is usually realized with thinner gate-oxide transistors. 
   Refer to  FIG. 1  a diagram schematically showing the circuit of a conventional mixed-voltage I/O buffer. As shown in  FIG. 1 , the I/O buffer  10  receives a GND-to-2×VDD input signal (high-level voltage) and transmits a GND-to-VDD output signal (low-level voltage). The I/O buffer  10  comprises a pre-driver circuit  12 , an output circuit  14 , an input circuit  16  and an I/O pad  18 . According to an output-enable signal OE, the pre-driver circuit  12  sends out a pull-up signal PU or a pull-down signal PD to control the output circuit  14 . During the transition from receiving a 2×VDD input signal to transmitting a 0V (GND) output signal, the voltage of the I/O pad  18  is 2×VDD initially; as the NMOS transistor  144  is turned on during the transition, the source voltage of the NMOS transistor  142  is pulled down to about Vdsat (the saturated drain voltage). Thus, during the transition from receiving a 2×VDD input signal to transmitting a 0V (GND) output signal, the drain-source voltage Vds of the NMOS transistor  142  is larger than the maximum operating voltage, and the hot-carrier degradation occurs. Since the transistors feature extremely short channel length and high electric field in the deep sub-micron technologies, the hot-carrier-induced degradation becomes one of the most important reliability concerns. 
   SUMMARY OF THE INVENTION 
   One objective of the invention is to provide a mixed-voltage I/O buffer, which is achieved with thin-oxide devices to receive 2×VDD-tolerant input signals. The 2×VDD-tolerant I/O buffer is designed with a dynamic gate-controlled circuit and two blocking NMOS transistors to overcome the problems of gate oxide reliability, current leakage and hot-carrier degradation. 
   Another objective of the invention is to provide a mixed-voltage I/O buffer, which incorporate a voltage slew-rate control output circuit in a mixed-voltage I/O buffer to form an I/O buffer with voltage slew-rate control to reduce the ground-bounce effect. 
   To achieve the abovementioned objectives, the present invention proposes a mixed-voltage I/O buffer, which comprises a first N-type transistor coupled to a second N-type transistor, a dynamic gate-controlled circuit, an output circuit, and a pre-driver circuit. The first N-type transistor is coupled to an input circuit, and the second N-type transistor is coupled to an I/O pad. The dynamic gate-controlled circuit is coupled to the I/O pad, the gates of the two N-type transistors, and externally coupled to a high-level voltage. The output circuit is coupled to the first N-type transistor, the input circuit, and externally coupled to a low-level voltage. The pre-driver circuit is coupled to the output circuit and the gate-controlled circuit and controls the voltage of the output circuit according to an output-enable signal and input data. 
   Further, when the output circuit of the abovementioned mixed-voltage I/O buffer is a voltage slew-rate control output circuit with a plurality of parallel pull-up P-type transistors and a plurality of parallel pull-down N-type transistors, the I/O buffer has the function of voltage slew-rate control to reduce the ground-bounce effect. 
   To enable the objectives, technical contents, characteristics and accomplishments of the present invention to be easily understood, the embodiments of the present invention are to be described in detail in cooperation with the attached drawings below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram schematically showing the circuit of a conventional mixed-voltage I/O buffer; 
       FIG. 2  is a diagram schematically showing the circuit of the mixed-voltage I/O buffer according to a first embodiment of the present invention; 
       FIG. 3  is a diagram schematically showing the dynamic gate-controlled circuit according to the first embodiment of the present invention; and 
       FIG. 4  is a diagram schematically showing the circuit of the mixed-voltage I/O buffer according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention discloses a 2×VDD-tolerant mixed-voltage I/O buffer, wherein the circuit thereof adopts transistors with a thinner gate oxide layer. 
   Refer to  FIG. 2  a diagram schematically showing the circuit of the mixed-voltage I/O buffer according to a first embodiment of the present invention. In  FIG. 2 , the mixed-voltage I/O buffer  20  comprises two NMOS transistors  22  and  24 , a dynamic gate-controlled circuit  26 , an input circuit  30 , an output circuit  32  and a pre-driver circuit  34 . The dynamic gate-controlled circuit  26  is coupled to the gates of the two NMOS transistor  22  and  24 , and externally coupled to a high-level voltage VDDH. An I/O pad  28  is coupled to the dynamic gate-controlled circuit  26  and the drain of the NMOS transistor  24 . The output circuit  32  is coupled to the input circuit  30  and the source of the NMOS transistor  22  at Node  1 , and is externally coupled to a low-level voltage VDD. The pre-driver circuit  34  is coupled to the output circuit  32  and the gate-controlled circuit  26 . According to an output-enable signal OE, the pre-driver circuit  34  outputs a pull-up signal PU to control a pull-up PMOS transistor  322 , and outputs a pull-down signal PD to control a pull-down NMOS transistor  324  of the output circuit  32 . The NMOS transistor  22  is coupled to the NMOS transistor  24  at Node  2 . The input circuit  30  comprises a PMOS transistor  302  and two inverters  304  and  306 . The drain of the PMOS transistor  302  and the input the inverter  304  are coupled to Node  1 . The output end of the inverter  304  is coupled to the gate of the PMOS transistor  302  and the input end of the inverter  306 . In the output circuit  32 , the pull-up PMOS transistor  322  is externally coupled to a low-level voltage VDD, and the pull-down NMOS transistor  324  is grounded. 
   Refer to  FIG. 3  a diagram schematically showing the dynamic gate-controlled circuit according to the first embodiment of the present invention. In  FIG. 3 , the dynamic gate-controlled circuit  26  comprises a level shifter  36 , two coupled inverters  38  and  40 , a NMOS transistor  42 , and a gate-tracking circuit  44 . A level shifter  36  is coupled to the pre-driver circuit  34  and receives a voltage signal PUb output by the pre-driver circuit  34  and pulls up the voltage signal PUb to be a voltage signal PUH. The input end of the inverter  38  is coupled to the level shifter  36 , and the output end of the inverter  38  is coupled to a gate of the NMOS transistor  42  and the inverter  40 . The gate-tracking circuit  44  further comprises two coupled PMOS transistors  441  and  442 . The gate-tracking circuit  44  is coupled to the inverter  40 , the drain of the NMOS transistor  42 , the gate of the NMOS transistor  22 , the gate of the NMOS transistor  24 , and the I/O pad  28 . The bulks of the PMOS transistors  441  and  442  are coupled to the gate of the NMOS transistor  24  to maintain the bulks at a high-level voltage lest current leakage occur. 
   When the output-enable signal OE received by the pre-driver circuit  34  is a low-level voltage (0V), the mixed-voltage I/O buffer  20  is in a receive mode, and the I/O pad  28  receives an input signal and transmits the signal to the input end Din of the input circuit  30 . In the receive mode, the pre-driver circuit  34  turns off the pull-up PMOS transistor  322  and the pull-down NMOS transistor  324  of the output circuit  32  and outputs a 0V voltage signal PUb to the dynamic gate-controlled circuit  26 , and the level shifter  36  of the dynamic gate-controlled circuit  26  transforms the voltage signal PUb into a VDD-level voltage signal PUH. Thus, the gate of the NMOS transistor  22  is biased at a voltage of VDD via the inverter  40 , and the NMOS transistor  42  makes the gate of the NMOS transistor  22  more stably biased at a voltage of VDD. Consequently, the gate voltage of the NMOS transistor  22  is always biased at VDD in the receive mode. The gate-tracking circuit  44  is coupled to the gate of the NMOS  24  transistor, and the gate voltage of the NMOS transistor  24  thus is dependent on the voltage of the I/O pad  28  in the receive mode. 
   Refer to table.1. When the I/O buffer  20  receives a 0V input signal in the receive mode, the gate of the NMOS transistor  22  is biased at a voltage of VDD, and the gate of the NMOS transistor  24  is also biased at the voltage of VDD; Node  1  and Node  2  are discharged to 0V via the NMOS transistors  22  and  24 , and a 0V input signal is transmitted from the I/O pad  28  to the input end Din of the input circuit  30 . When the I/O buffer  20  receives a 2×VDD input signal in the receive mode, the gate of the NMOS transistor  22  is still biased at the voltage of VDD, but the gate of the NMOS transistor  24  is biased at a voltage of 2×VDD; Node  1  is biased at the voltage of VDD via the feedback operation of the PMOS transistor  302  of the input circuit  30  and Node  2  is biased at a voltage of (2×VDD−ΔV) to enable 1×VDD input signal transmitted from the I/O pad  28  to the input end Din of the input circuit  30  via the NMOS transistors  22  and  24 . 
   
     
       
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
                 
               The Gate of 
               The Gate of 
             
             
                 
                 
               NMOS 
               NMOS 
             
             
               Operating Mode 
               I/O Pad 28 
               Transistor 22 
               Transistor 24 
             
             
                 
             
           
           
             
               Receive Mode 
               Low (0 V) 
               VDD 
               VDD 
             
             
               Receive Mode 
               High (2xVDD) 
               VDD 
               High (2xVDD) 
             
             
               Transmit Mode 
               Low (0 V) 
               VDD 
               VDD 
             
             
               Transmit Mode 
               High (VDD) 
               2xVDD 
               2xVDD 
             
             
                 
             
           
        
       
     
   
   When the output-enable signal OE received by the pre-driver circuit  34  is a high-level voltage signal (VDD), the I/O buffer  20  is in a transmit mode, and an output signal is transmitted from the output end Dout of the pre-driver circuit  34  to an output signal to the I/O pad  28 . When the output end Dout of the pre-driver circuit  34  sends out a 0V output signal in the transmit mode, the pre-driver circuit  34  turns off the pull-up PMOS transistor  322  of the output circuit  32  and turns on the pull-down NMOS transistor  324  and then a 0V voltage signal PUb is transmitted to the dynamic gate-controlled circuit  26  and transformed into a VDD voltage signal PUH by the level shifter  36  of the dynamic gate-controlled circuit  26 . The gates of the NMOS transistors  22  and  24  are biased at a voltage of VDD, as shown in Table.1, to enable a 0V output signal transmitted from the output end Dout of the pre-driver circuit  34  to the I/O pad  28 . When the output end Dout of the pre-driver circuit  34  sends out a VDD output signal in the transmit mode, the pre-driver circuit  34  turns on the pull-up PMOS transistor  322  of the output circuit  32  and turns off the pull-down NMOS transistor  324  and then a VDD voltage signal PUb is transmitted to the dynamic gate-controlled circuit  26  and transformed into a 2×VDD voltage signal PUH by the level shifter  36  of the dynamic gate-controlled circuit  26 . As shown in Table.1, the gates of the NMOS transistors  22  and  24  are biased at a voltage of 2×VDD to enable a VDD output signal transmitted from the output end Dout of the pre-driver circuit  34  to the I/O pad  28 . 
   During the transition from receiving a 2×VDD input signal to transmitting a 0V output signal in the mixed-voltage I/O buffer  20 , Node  1  and Node  2  are respectively at voltages of VDD and (2×VDD−ΔV) initially. During the transition, the pull-down signal PD generated by the pre-driver circuit  34  turns on the NMOS transistor  324  of the output circuit  32  and the NMOS transistor  22  is turned on when its source voltage is pulled down by the NMOS transistor  324 . Then, the NMOS transistor  24  is turned on and the voltage at the I/O pad  28  is pulled down. The drain-source voltages Vds of the NMOS transistors  22  and  24  will not exceed the maximum voltage defined in a given fabrication process. Therefore, no matter whether it is in the receive mode, in the transmit mode or during the transition from receiving a 2×VDD input signal to transmitting a 0V output signal, the mixed-voltage I/O buffer  20  of the present invention is free from gate-oxide deterioration and hot-carrier degradation. 
   Refer to  FIG. 4  a diagram schematically showing the circuit of the mixed-voltage I/O buffer  50  according to a second embodiment of the present invention, wherein a voltage slew-rate control output circuit  52  replaces the output circuit  32  shown in  FIG. 2 . In  FIG. 4 , the mixed-voltage I/O buffer  50  of the present invention is a mixed-voltage I/O buffer with a voltage slew-rate control function and implemented with a voltage slew-rate control output circuit  52 , which has a plurality of parallel pull-up PMOS transistors and a plurality of parallel pull-down NMOS transistors. Thereby, the mixed-voltage I/O buffer  50  improves the problem of the ground bounce effect. 
   In summary, the present invention proposes a 2×VDD-tolerant mixed-voltage I/O buffer. No matter whether it is in the receive mode, in the transmit mode or during the transition from receiving a 2×VDD input signal to transmitting a 0V output signal, via two NMOS transistors and a dynamic gate-controlled circuit, the mixed-voltage I/O buffer of the present invention is free from the problems of gate-oxide reliability, current leakage and hot-carrier degradation. Further, the present invention incorporates a voltage slew-rate control output circuit in the mixed-voltage I/O buffer to realize a mixed-voltage I/O buffer with a voltage slew-rate control function. 
   Those described above are the embodiments to clarify the characteristics and technical thought of the present invention to enable the persons skilled in the art to understand, make and use the present invention. However, it is not intended to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.