Abstract:
ESD preventing-able level shifter, for receiving a first signal and outputting a second signal is provided. The level shifter comprises an inverter, a voltage converter, a first ESD clamp circuit and a second ESD clamp circuit. The inverter receives the first signal and outputs a first reverse signal. The voltage converter having a first input terminal for receiving the first reverse signal, a second input terminal for receiving the first signal and an output terminal for outputting the second signal. A first and second terminal of the first ESD clamp circuit is coupled to the first input terminal of the voltage converter and a second ground voltage, respectively. A first and a second terminal of the second ESD clamp circuit is coupled to the second input terminal of the voltage converter and the second ground voltage, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit of Taiwan application serial no. 93118236, filed Jun. 24, 2004.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an electrostatic discharge (ESD) protection circuit, and more particularly to ESD protection circuit of level shifters.  
         [0004]     2. Description of the Related Art  
         [0005]     Mixed-voltage integrated circuits apply system voltages with different voltage levels to internal circuits.  FIG. 1A  is a partial circuit block diagram of a prior art mixed-voltage integrated circuit. The operating voltages of the internal circuit  110  comprises the system voltage VDD 1 , e.g. 3.3 V, and the ground voltage VSS 1 , e.g. 0 V. The operating voltages of the internal circuit  130  comprises the system voltage VDD 2 , e.g. 12 V, and the ground voltage VSS 2 , e.g. 0 V. The logic level of the internal circuit  110  does not match that of the internal circuit  130 . A level shifter  110  is required and serves as an interface of these circuits. For example, the level shifter  120  receives the signal  111  output from the internal circuit  110 , transforms the signal  111 , e.g. 3.3 V. into a corresponding signal  131  and outputs the signal  131  to the internal circuit  130 , e.g. 12 V.  
         [0006]     When ESD occurs at a terminal of the mixed-voltage integrated circuit, the ESD current flows along a low impedance path. Due to the ESD current, the devices on such a path will be damaged.  FIG. 1B  is a drawing showing the ESD paths of the level shifter  120  shown in  FIG. 1A . Referring to  FIG. 1B , when ESD occurs at the ground voltage VSS 2  and the system voltage VDD 1  is grounded, the ESD current flows from the ground voltage VSS 2  to the system voltage VDD 1  through the gate capacitor of the transistor  121 , i.e. the dot line ESD 1 . When the ground voltage VSS 1  is grounded, the ESD current flows from the ground voltage VSS 2  to the ground voltage VSS 1  through the gate capacitor of the transistor  121 , i.e. the dot line ESD 2 . Accordingly, the transistors  121  and  122  may be damaged.  
         [0007]     The damage on the devices is caused due to the fact that the ground voltage VSS 1  and the ground voltage VSS 2  are not coupled to each other. The ESD current cannot reach the ground voltage VSS 2  through the ground voltage VSS 1 , but through the silicon bulk. Due to the low impedance of the silicon bulk, the ESD current damages the transistor  121 . Because of the short period of time of the ESD pulse, the impedance of the gate capacitor under ESD operation is lower than the impedance under normal operation.  
         [0008]      FIG. 1C  is a drawing showing another ESD path of the level shifter  120  shown in  FIG. 1A . Referring to  FIG. 1C , the ESD damage becomes more serious when ESD occurs at the system voltage VDD 2 , rather than on the ground voltage VSS 2 . This phenomenon is observed due to no discharge path existing in the N-well when ESD occurs at the system voltage VDD 2 . To the contrary, a discharge path can be implemented by connecting the ground voltage VSS 1  and the ground voltage VSS 2  through the silicon bulk. When ESD occurs at the system voltage VDD 2 , and because the system voltage VDD 1  is grounded, the ESD current flows from the system voltage VDD 2  to the system voltage VDD 1  through the gate capacitor of the transistor  123 , i.e. the path of ESD 1 . When the ground voltage VSS 1  is grounded, the ESD current flows from the system voltage VDD 2  to the ground voltage VSS 1  through the gate capacitor of the transistor  123 , i.e. the path of ESD 2 . Accordingly, the transistors  123  and  124  may be damaged.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention is directed to a electrostatic discharge (ESD) preventing-able level shifter capable of preventing an ESD current flowing from a set of power terminals to another set of power terminals and thereby reducing damage to the level shifter.  
         [0010]     The present invention is directed to another ESD preventing-able level shifter capable of providing other ESD route for discharging charges so as to protect the level shifter from damage.  
         [0011]     The present invention is directed to the a ESD preventing-able level shifter capable of providing another ESD route between sets of power terminals so as to protect the level shifter from damage.  
         [0012]     The present invention discloses a ESD preventing-able level shifter for receiving a first signal and outputting a second signal with a level corresponding to a level of the first signal. The first signal is transmitted between a first system voltage and a first ground voltage, and the second signal is transmitted between a second system voltage and a second ground voltage. The level shifter comprises an inverter, a voltage converter, a first ESD clamp circuit and a second ESD clamp circuit. The inverter receives the first signal and outputs a first reverse signal, wherein the first reverse signal is reverse with respect to the first signal and is transmitted between the first system voltage and the first ground voltage. A first input terminal of the voltage converter receives the first reverse signal. A second input terminal of the voltage converter receives the first signal. An output terminal of the voltage converter outputs the second signal. A first terminal of the first ESD clamp circuit is coupled to the first input terminal of the voltage converter. A second terminal of the first ESD clamp circuit is coupled to the second ground voltage. A first terminal of the second ESD clamp circuit is coupled to the second input terminal of the voltage converter. A second terminal of the second ESD clamp circuit is coupled to the second ground voltage.  
         [0013]     The present invention discloses another ESD preventing-able level shifter for receiving a first signal and outputting a second signal with a level corresponding to a level of the first signal. The first signal is transmitted between a first system voltage and a first ground voltage, and the second signal is transmitted between a second system voltage and a second ground voltage. The level shifter comprises an inverter, a voltage converter, a first ESD clamp circuit and a second ESD clamp circuit. The inverter receives the first signal and outputs a first reverse signal, wherein the first reverse signal is reverse with respect to the first signal and is transmitted between the first system voltage and the first ground voltage. A first input terminal of the voltage converter receives the first reverse signal. A second input terminal of the voltage converter receives the first signal. An output terminal of the voltage converter outputs the second signal. A first terminal of the first ESD clamp circuit is coupled to the second system voltage. A second terminal of the first ESD clamp circuit is coupled to the first input terminal of the voltage converter. A first terminal of the second ESD clamp circuit is coupled to the second system voltage. A second terminal of the second ESD clamp circuit is coupled to the second input terminal of the voltage converter.  
         [0014]     According to another embodiment of the present invention, a ESD preventing-able level shifter for receiving a first signal and outputting a second signal with a level corresponding to a level of the first signal is provided. The first signal is transmitted between a first system voltage and a first ground voltage, and the second signal is transmitted between a second system voltage and a second ground voltage. The level shifter comprises an inverter, a voltage converter and ESD clamp circuit. The inverter receives the first signal and outputs a first reverse signal, wherein the first reverse signal is reverse to the first signal and transmitted between the first system voltage and the first ground voltage. A first input terminal of the voltage converter receives the first reverse signal. A second input terminal of the voltage converter receives the first signal. An output terminal of the voltage converter outputs the second signal. A first terminal of the ESD clamp circuit is coupled to the second system voltage. A second terminal of the ESD clamp circuit is coupled to the first ground voltage.  
         [0015]     According to the exemplary ESD preventing-able level shifters of the present invention, the ESD clamp circuit comprises, for example, an N-type transistor. A drain of the N-type transistor is coupled to a first input terminal of the voltage converter. The gate, the source and the bulk of the N-type transistor are coupled to the second ground voltage. The ESD clamp circuit may comprise, for example, a diode. A cathode of the diode is coupled to the first input terminal of the voltage converter, and an anode of the diode is coupled to the second ground voltage.  
         [0016]     By using the ESD clamp circuit, the present invention provides a current route for releasing ESD currents flowing between sets of the power terminals so as to reduce the damage to the internal circuits, such as level shifter, of the integrated circuit.  
         [0017]     The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in communication with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1A  is a partial circuit block diagram of a prior art mixed-voltage integrated circuit.  
         [0019]      FIG. 1B  is a drawing showing the ESD paths of the level shifter  120  shown in  FIG. 1A .  
         [0020]      FIG. 1C  is a drawing showing another ESD path of the level shifter  120  shown in  FIG. 1A .  
         [0021]      FIG. 2A  is a schematic drawing showing a level shifter according to an embodiment of the present invention.  
         [0022]      FIG. 2B  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0023]      FIG. 3A  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0024]      FIG. 3B  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0025]      FIG. 4A  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0026]      FIG. 4B  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0027]      FIG. 5A  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0028]      FIG. 5B  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0029]      FIG. 6A  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0030]      FIG. 6B  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0031]      FIG. 7A  is a schematic drawing showing a level shifter according to another embodiment of the present invention.  
         [0032]      FIG. 7B  is a schematic drawing showing a level shifter according to another embodiment of the present invention. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0033]      FIG. 2A  is a schematic drawing showing a level shifter according to an embodiment of the present invention. Referring to  FIG. 2A , the level shifter  220  receives the first signal  211  output from the internal circuit  210  of the integrated circuit. The level shifter  220  outputs a second signal  231  with a level corresponding to the level of the first signal  211 , which is received by the internal circuit  230  of the integrated circuit. The first signal  211  is transmitted between the first system voltage VDD 1 , e.g. 3.3 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  231  is transmitted between the second system voltage VDD 2 , e.g. 12 V, and the second ground voltage VSS 2 , e.g. 0 V.  
         [0034]     In this embodiment, the level shifter  220  comprises an inverter  240 , a voltage converter  250 , a first electrostatic discharge (ESD) clamp circuit  260  and a second ESD clamp circuit  270 . The inverter  240  receives the first signal  211  and outputs a first reverse signal  241 . The first reverse signal  241  is reverse to the first signal  211 . The first reverse signal  241  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 .  
         [0035]     The inverter  240  comprises, for example, a P-type transistor  242  and an N-type transistor  244 . The source of the transistor  242  is coupled to the first system voltage VDD 1 . The gate of the transistor  242  receives the first signal  211 . The drain of the transistor  242  outputs the first reverse signal  241 . The gate of the transistor  244  receives the first signal  211 . The drain of the transistor  244  is coupled to the drain of the transistor  242 . The source of the transistor  244  is coupled to the first ground voltage VSS 1 .  
         [0036]     The first input terminal of the voltage converter  250  receives the first reverse signal  241 . The second input terminal of the voltage converter  250  receives the first signal  211 . The output terminal of the voltage converter  250  outputs the second signal  231 . The voltage converter  250  comprises, for example, the P-type transistors T 1  and T 3 , and the N-type transistors T 2  and T 4 .  
         [0037]     The first source/drain, for example, a first source hereafter, of the first transistor T 1  is coupled to the second system voltage VDD 2 . The gate of the second transistor T 2  receives the reverse signal  241 . The first source/drain, for example, a drain hereafter, of the second transistor T 2  is coupled to the second source/drain, for example, a drain hereafter, of the first transistor T 1 . The second source/drain, for example, a source hereafter, of the second transistor T 2  is coupled to the second ground voltage VSS 2 . The first source/drain, for example, a source hereafter, of the third transistor T 3  is coupled to the second system voltage VDD 2 . The second source/drain, for example, a drain hereafter, of the third transistor T 3  is coupled to the gate of the first transistor T 1 . The gate of the third transistor T 3  is coupled to the drain of the first transistor T 1 . The gate of the fourth transistor T 4  receives the first signal  211 . The first source/drain, for example, a drain hereafter, of the fourth transistor T 4  is coupled to the drain of the third transistor T 3 . The second source/drain, for example, a source hereafter, of the fourth transistor T 4  is coupled to the second ground voltage VSS 2 . The signal on the drain of the fourth transistor T 4  is the second signal  231 .  
         [0038]     The first terminal of the first ESD clamp circuit  260  is coupled to the first input terminal of the voltage converter  250 . The second terminal of the first ESD clamp circuit  260  is coupled to the second ground voltage VSS 2 . The first terminal of the second ESD clamp circuit  270  is coupled to the second input terminal of the voltage converter  250 . The second terminal of the second ESD clamp circuit  270  is coupled to the second ground voltage VSS 2 .  
         [0039]     In this embodiment, the first ESD clamp circuit  260  comprises, for example, an N-type transistor. The drain of the N-type transistor is coupled to the first input terminal of the voltage converter  250 . The gate, the source and the bulk of the N-type transistor are coupled to the second ground voltage VSS 2 . One of ordinary skill in the art will understand that the first ESD clamp circuit  260  may comprise a diode.  FIG. 2B  is a schematic drawing showing a level shifter according to an another embodiment of the present invention. Referring to  FIG. 2B , a diode is used in the first ESD clamp circuit  260 . The cathode of the diode is coupled to the first input terminal of the voltage converter  250 . The anode of the diode is coupled to the second ground voltage VSS 2 . In this embodiment, the second clamp circuit  270  is similar to the first clamp circuit  260 . Detailed descriptions are not repeated.  
         [0040]     When ESD occurs at the terminal of the second ground voltage VSS 2 , and because the first system voltage VDD 1  is grounded, the ESD current will flow from the second ground voltage VSS 2  to the first system voltage VDD 1  through the first ESD clamp circuit  260  and the transistor  242 . If the terminal of the first ground voltage VSS 1  is grounded, the ESD current flows from the second ground voltage VSS 2  to the first ground circuit VSS 1  through the first ESD clamp circuit  260  and the transistor  244 . Accordingly, the damage to the level shifter  220  can be reduced.  
         [0041]     Following are the descriptions of another embodiment of the present invention.  FIG. 3A  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 3A , the level shifter  320  receives the first signal  311  outputted from the internal circuit  310  of the integrated circuit. The level shifter  320  outputs a second signal  331  with a level corresponding to the level of the first signal  311 , which is received by the internal circuit  330  of the integrated circuit. The first signal  311  is transmitted between the first system voltage VDD 1 , e.g. 3.3 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  331  is transmitted between the second system voltage VDD 2 , e.g. 12 V, and the second ground voltage VSS 2 , e.g. 0 V. The level shifter  320  comprises an inverter  340 , a voltage converter  350 , a first electrostatic discharge (ESD) clamp circuit  360  and a second ESD clamp circuit  370 .  
         [0042]     The inverter  340  receives a first signal  311  and outputs a first reverse signal  341 . The first reverse signal  341  is reverse with respect to the first signal  311 . The first reverse signal  341  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 . In this embodiment, the inverter  340  comprises, for example, a P-type transistor  342  and an N-type transistor  344 . The source of the transistor  342  is coupled to the first system voltage VDD 1 . The gate of the transistor  342  receives the first signal  311 . The drain of the transistor  342  outputs the first reverse signal  341 . The gate of the transistor  344  receives the first signal  311 . The drain of the transistor  344  is coupled to the drain of the transistor  342 . The source of the transistor  344  is coupled to the first ground voltage VSS 1 .  
         [0043]     The first input terminal of the voltage converter  350  receives the first reverse signal  341 . The second input terminal of the voltage converter  350  receives the first signal  311 . The output terminal of the voltage converter  350  outputs the second signal  331 . The first terminal of the first ESD clamp circuit  360  is coupled to the second system voltage VDD 2 . The second terminal of the first ESD clamp circuit  360  is coupled to the first input terminal of the voltage converter  350 . The first terminal of the second ESD clamp circuit  370  is coupled to the second system voltage VDD 2 . The second terminal of the second ESD clamp circuit  370  is coupled to the second input terminal of the voltage converter  350 .  
         [0044]     The voltage converter  350  comprises, for example, the P-type transistors T 1 , T 2 , T 4  and T 5 , and the N-type transistors T 3  and T 6 . The first source/drain, for example, a source hereafter, of the first transistor T 1  is coupled to the second system voltage VDD 2 . The gate of the second transistor T 2  receives the reverse signal  341 . The first source/drain, for example, a source hereafter, of the second transistor T 2  is coupled to the second source/drain, for example, a drain thereafter, of the first transistor T 1 . The gate of the third transistor T 3  receives the first reverse signal  341 . The first source/drain, for example, a drain thereafter, of the third transistor T 3  is coupled to the second source/drain, for example, a drain hereafter, of the second transistor T 2 . The second source/drain, for example, a source hereafter, of the third transistor T 3  is coupled to the second ground voltage VSS 2 . The first source/drain, for example, a source hereafter, of the fourth transistor T 4  is coupled to the second system voltage VDD 2 . The gate of the fourth transistor T 4  is coupled to the drain of the second transistor T 2 . The gate of the fifth transistor T 5  receives the first signal  311 . The first source/drain, for example, a source, of the fifth transistor T 5  is coupled to the second source/drain, for example, a drain, of the fourth transistor T 4 . The second source/drain, for example, a drain, of the fifth transistor T 5  is coupled to the gate of the transistor T 1 . The gate of the sixth transistor T 6  receives the first signal  311 . The first source/drain, for example, a drain, of the sixth transistor T 6  is coupled to the drain of the fifth transistor T 5 . The second source/drain, for example, a source, of the sixth transistor T 6  is coupled to the second ground voltage VSS 2 . The signal on the drain of the sixth transistor T 6  is the second signal  331 .  
         [0045]     In this embodiment, the first ESD clamp circuit  360  comprises, for example, a P-type transistor. The drain of the P-type transistor is coupled to the first input terminal of the voltage converter  350 . The gate, the source and the bulk of the P-type transistor are coupled to the second system voltage VDD 2 . One of ordinary skill in the art will understand that the first ESD clamp circuit  360  may comprise a diode.  FIG. 3B  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 3B , a diode is used in the first ESD clamp circuit  360 . The anode of the diode is coupled to the first input terminal of the voltage converter  350 . The cathode of the diode is coupled to the second system voltage VDD 2 . In this embodiment, the second ESD clamp circuit  370  is similar to the first ESD clamp circuit  360 . Detailed descriptions are not repeated.  
         [0046]     When ESD occurs at the terminal of the second system voltage VDD 2  and the first system voltage VDD 1  is grounded, the ESD current will flow from the second system voltage VDD 2  to the first system voltage VDD 1  through the first ESD clamp circuit  360  and the transistor  342 . If the terminal of the first ground voltage VSS 1  is grounded, the ESD current will flow from the second system voltage VDD 2  to the first ground circuit VSS 1  through the first ESD clamp circuit  360  and the transistor  344 . Accordingly, damage to the level shifter  320  can be reduced.  
         [0047]     Following are the descriptions of another embodiment of the present invention.  FIG. 4A  is a schematic drawing showing another level shifter according to an embodiment of the present invention. Referring to  FIG. 4A , the level shifter  420  receives the first signal  411  outputted from the internal circuit  410  of the integrated circuit. The level shifter  420  outputs a second signal  431  with a level corresponding to the level of the first signal  411 , which is received by the internal circuit  430  of the integrated circuit. The first signal  411  is transmitted between the first system voltage VDD 1 , e.g. 3.3 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  431  is transmitted between the second system voltage VDD 2 , e.g. 12 V, and the second ground voltage VSS 2 , e.g. 0 V.  
         [0048]     In this embodiment, the level shifter  420  comprises an inverter  440 , a voltage converter  450  and an electrostatic discharge (ESD) clamp circuit  460 . The inverter  440  receives the first signal  411  and outputs a first reverse signal  441 . The first reverse signal  441  is reverse with respect to the first signal  411 . The first reverse signal  441  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 .  
         [0049]     The voltage converter  450  and the inverter  440  are similar to the voltage converter  350  and the inverter  340  shown in  FIG. 3A , respectively above. Detailed descriptions are not repeated.  
         [0050]     The first terminal of the ESD clamp circuit  460  is coupled to the second system voltage VDD 2 , and the second terminal of the ESD clamp circuit  460  is coupled to the first ground voltage VSS 1 . In this embodiment, the ESD clamp circuit  460  comprises, for example, a transistor. The collector of the transistor is coupled to the second system voltage VDD 2 . The emitter and base of the transistor is coupled to the first ground voltage VSS 1 . One of ordinary skill in the art will understand that the ESD clamp circuit  460  may comprise a diode.  FIG. 4B  is a schematic drawing showing another level shifter according to an embodiment of the present invention. Referring to  FIG. 4B , a diode is used in the ESD clamp circuit  460 . The anode of the diode is coupled to the first ground voltage VSS 1 . The cathode of the diode is coupled to the second system voltage VDD 2 .  
         [0051]     When ESD occurs at the terminal of the second system voltage VDD 2 , and because the first ground voltage VSS 1  is grounded, the ESD current will flow from the second system voltage VDD 2  to the first ground voltage VSS 1  through the ESD clamp circuit  460 . Accordingly, damage to the level shifter  420  can be reduced.  
         [0052]     Following are the descriptions of another embodiment present invention.  FIG. 5A  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 5A , the level shifter  520  receives the first signal  511  outputted from the internal circuit  510  of the integrated circuit. The level shifter  520  outputs a second signal  531  with a level corresponding to the level of the first signal  511 , which is received by the internal circuit  530  of the integrated circuit. The first signal  511  is transmitted between the first system voltage VDD 1 , e.g. 12 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  531  is transmitted between the second system voltage VDD 2 , e.g. 3.3 V, and the second ground voltage VSS 2 , e.g. 0 V.  
         [0053]     In this embodiment, the level shifter  520  comprises an inverter  540 , a voltage converter  550  and electrostatic discharge (ESD) clamp circuits  560  and  570 . The inverter  540  receives the first signal  511  and outputs a first reverse signal  541 . The first reverse signal  541  is reverse with respect to the first signal  511 . The first reverse signal  541  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 .  
         [0054]     In this embodiment, the inverter  540  is similar to those described above. Detailed descriptions are not repeated.  
         [0055]     In this embodiment, the voltage converter  550  comprises, for example, the P-type transistors T 1  and T 3 , and the N-type transistors T 2  and T 4 . The first source/drain, named as a source thereafter, of the transistor T 1  is coupled to the second system voltage VDD 2 . The gate of the transistor T 1  receives a reverse signal  541 . The first source/drain, for example, a drain hereafter, of the transistor T 2  is coupled to the second source/drain, for example, a drain hereafter, of the transistor T 1 . The second source/drain, for example, a source hereafter, of the transistor T 2  is coupled to the second ground voltage VSS 2 . The first source/drain, for example, a source hereafter, of the transistor T 3  is coupled to the second system voltage VDD 2 . The second source/drain, for example, a drain hereafter, of the transistor T 3  is coupled to the gate of the transistor T 2 . The gate of the transistor T 3  receives the signal  511 . The gate of the transistor T 4  is coupled to the drain of the transistor T 1 . The first source/drain, for example, a drain hereafter, of the transistor T 4  is coupled to the drain of the transistor T 3 . The second source/drain, for example, a source hereafter, of the transistor T 4  is coupled to the second ground voltage VSS 2 . The signal on the drain of the transistor T 4  is the second signal  531 .  
         [0056]     The first terminal of the first ESD clamp circuit  560  is coupled to the second system voltage VDD 2 . The second terminal of the first ESD clamp circuit  560  is coupled to gate of the first transistor T 1 . In this embodiment, the first ESD clamp circuit  560  comprises, for example, a P-type transistor. The drain of the P-type transistor is coupled to the first input terminal of the voltage converter  550 , i.e. the gate of the first transistor T 1 . The gate, the source and the bulk of the P-type transistor are coupled to the second system voltage VDD 2 . One of ordinary skill in the art will understand that the first ESD clamp circuit  560  may comprise a diode.  FIG. 5B  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 5B , a diode is used in the first ESD clamp circuit  560 . The cathode of the diode is coupled to the second system voltage VDD 2 . The anode of the diode is coupled to the first input terminal of the voltage converter  550 , i.e. the gate of the transistor T 1 .  
         [0057]     In this embodiment, the ESD clamp circuit  570  is similar to the first ESD clamp circuit. Detailed descriptions are not repeated.  
         [0058]      FIG. 6A  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 6A , the level shifter  620  receives the first signal  611  outputted from the internal circuit  610  of the integrated circuit. The level shifter  620  outputs a second signal  631  with a level corresponding to the level of the first signal  611 , which is received by the internal circuit  630  of the integrated circuit. The first signal  611  is transmitted between the first system voltage VDD 1 , e.g. 12 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  631  is transmitted between the second system voltage VDD 2 , e.g. 3.3 V, and the second ground voltage VSS 2 , e.g. 0 V.  
         [0059]     In this embodiment, the level shifter  620  comprises an inverter  640 , a voltage converter  650  and electrostatic discharge (ESD) clamp circuits  660  and  670 . The inverter  640  receives the first signal  611  and outputs a first reverse signal  641 . The first reverse signal  641  is reverse with respect to the first signal  611 . The first reverse signal  641  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 .  
         [0060]     In this embodiment, the inverter  640  is similar to those described above. Detailed descriptions are not repeated.  
         [0061]     The voltage converter  650  comprises, for example, the P-type transistors T 1  and T 4 , and the N-type transistors T 2 , T 3 , T 5  and T 6 . The gate of the first transistor T 1  receives the reverse signal  641 . The first source/drain, for example, a source hereafter, of the transistor T 1  is coupled to the second system voltage VDD 2 . The gate of the transistor T 2  is coupled to the gate of the transistor T 1 . The first source/drain, for example, a drain hereafter, of the transistor T 2  is coupled to the second source/drain, for example, a drain hereafter, of the transistor T 1 . The first source/drain, for example, a drain hereafter, of the transistor T 3  is coupled to the second source/drain, for example, a source thereafter, of the transistor T 2 . The second source/drain, for example, a source thereafter, of the transistor T 3  is coupled to the second ground voltage VSS 2 . The first source/drain, for example, a source thereafter, of the transistor T 4  is coupled to the second system voltage VDD 2 . The second source/drain, for example, a drain hereafter, of the transistor T 4  is coupled to the gate of the transistor T 3 . The gate of the transistor T 4  receives the first signal  611 . The gate of the transistor T 5  is coupled to the gate of the transistor T 4 . The first source/drain, for example, a drain, of the transistor T 5  is coupled to the drain of the transistor T 4 . The gate of the transistor T 6  is coupled to the drain of the transistor T 1 . The first source/drain, for example, a drain, of the transistor T 6  is coupled to the source of the transistor T 5 . The second source/drain, for example, a source, of the transistor T 6  is coupled to the second ground voltage VSS 2 . The signal on the drain of the transistor T 6  is the second signal  631 .  
         [0062]     The first terminal of the ESD clamp circuit  660  is coupled to the second system voltage VDD 2 , and the second terminal of the ESD clamp circuit  660  is coupled to the gates of the first and the second transistors T 1  and T 2 , respectively. In this embodiment, the first ESD clamp circuit  660  comprises, for example, a P-type transistor. The drain of the P-type transistor is coupled to the first input terminal of the voltage converter  650 , i.e. the gates of the first and the second transistors T 1  and T 2 . The gate, the source and the bulk of the P-type transistor are coupled to the second system voltage VDD 2 . One of ordinary skill in the art will understand that the first ESD clamp circuit  660  may comprise a diode.  FIG. 6B  is a schematic drawing showing another level shifter according to an embodiment of the present invention. Referring to  FIG. 6B , a diode is used in the ESD clamp circuit  660 . The anode of the diode is coupled to the first input terminal of the voltage converter  650 . The cathode of the diode is coupled to the second system voltage VDD 2 .  
         [0063]     In this embodiment, the second ESD clamp circuit  670  is similar to the ESD clamp circuit  660 . Detailed descriptions are not repeated.  
         [0064]      FIG. 7A  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 7A , the level shifter  720  receives the first signal  711  outputted from the internal circuit  710  of the integrated circuit. The level shifter  720  outputs a second signal  731  with a level corresponding to the level of the first signal  711 , which is received by the internal circuit  730  of the integrated circuit. The first signal  711  is transmitted between the first system voltage VDD 1 , e.g. 12 V, and the first ground voltage VSS 1 , e.g. 0 V. The second signal  731  is transmitted between the second system voltage VDD 2 , e.g. 3.3 V, and the second ground voltage VSS 2 , e.g. 0 V.  
         [0065]     In this embodiment, the level shifter  720  comprises an inverter  740 , a voltage converter  750  and electrostatic discharge (ESD) clamp circuits  760  and  770 . The inverter  740  receives the first signal  711  and outputs a first reverse signal  741 . The first reverse signal  741  is reverse to the first signal  711 . The first reverse signal  741  is transmitted between the first system voltage VDD 1  and the first ground voltage VSS 1 .  
         [0066]     The voltage converter  750  and the inverter  740  are similar to the voltage converter  650  and the inverter  640  shown in  FIG. 6A , respectively. Detailed descriptions are not repeated.  
         [0067]     The first terminal of the ESD clamp circuit  760  is coupled to gates of the first and the second transistors T 1  and T 2 , respectively, and the second terminal of the ESD clamp circuit  760  is coupled to the second ground voltage VSS 2 . In this embodiment, the first ESD clamp circuit  760  comprises, for example, an N-type transistor. The drain of the N-type transistor is coupled to the first input terminal of the voltage converter  750 , i.e. the gates of the first and the second transistors T 1  and T 2 . The gate, the source and the bulk of the N-type transistor are coupled to the second ground voltage VSS 2 . One of ordinary skill in the art will understand that the first ESD clamp circuit  760  may comprise a diode.  FIG. 7B  is a schematic drawing showing a level shifter according to another embodiment of the present invention. Referring to  FIG. 7B , a diode is used in the ESD clamp circuit  760 . The cathode of the diode is coupled to the first input terminal of the voltage converter  750 . The anode of the diode is coupled to the second ground voltage VSS 2 .  
         [0068]     In this embodiment, the second ESD clamp circuit  770  is similar to the ESD clamp circuit  760 . Detailed descriptions are not repeated.  
         [0069]     It can be noted that the voltage converter  450  shown in  FIGS. 4A and 4B  can be replaced by any other voltage converter, such as the voltage converters  250 ,  550  and  650  shown in  FIGS. 2A, 5A  and  6 A, respectively.  
         [0070]     Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be constructed broadly to include other variants and embodiments of the invention which may be made by those skilled in the field of this art without departing from the scope and range of equivalents of the invention.