Abstract:
A level shifter circuit is provided that is capable of improving current drivability and executing stable operation with a low voltage by boosting a voltage level of an input signal. The level shifter circuit includes a level shifting unit for producing a boosted voltage by boosting an input signal and shifting a voltage level of the boosted voltage to output an output signal.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a semiconductor device and, more particularly, to a level shifter in a semiconductor device. 
     BACKGROUND 
     Generally, a word line driver of a semiconductor memory device uses a high voltage VPP which is higher than a power supply voltage VDD, and a driving signal, which drives the word line driver, swings between the power supply voltage VDD and a ground voltage VSS, while the word line driver needs a swing voltage level between the high voltage VPP and the power supply voltage VDD. In the case where these two circuits (that is, power supply and word line driver) are directly coupled to each other without a level conversion circuit, leakage current can be caused in the circuit that uses the high voltage VPP. Therefore, a level shifter must be disposed between these two circuits. 
       FIG. 1  is a circuit diagram illustrating a conventional level shifter. In  FIG. 1 , the reference numerals VDDL and VDDH denote a low voltage level and a high voltage level, respectively. In the conventional semiconductor memory device, the low voltage level VDDL may be a low voltage VDD and the high voltage level VDDH may be a power supply voltage VDD. 
     In  FIG. 1 , a signal INb swings between the low voltage VDDL and the ground voltage VSS and a level shifter circuit is added in order to change the signal INb into a signal which swings between the high voltage VDDH and the ground voltage VSS. 
     A problem of the conventional level shifter will be explained shortly. First, when the input signal INb of the level shifter transits from a low level to a high level, a NMOS transistor mn 1  is turned on and a NMOS transistor mn 2  is turned off. A voltage level on a node AA drops to the ground voltage VSS through the NMOS transistor mn 1  which is turned on. At this time, a PMOS transistor mp 2  is turned on so that a node BB transits to a high voltage level and a conflict occurs between a PMOS transistor mp 1  and the NMOS transistor mn 1  before the PMOS transistor mp 1  is turned off. Further, when the input signal INb transmits from a high voltage level to a low voltage level, a conflict occurs between the PMOS transistor mp 2  and the NMOS transistor mn 2 . 
     When the drivability of the PMOS transistor is lower than that of the NMOS transistor, a normal operation is not guaranteed. Accordingly, the current drivability of the NMOS transistors is to be reinforced by increasing the size of the NMOS transistors in order to make the level shifter stable and fast. However, there is a limitation of the reinforcement of the drivability of the NMOS transistors with such a size increment as the input signal INb becomes lower and lower up to the threshold voltage of the NMOS transistors mn 1  and mn 2 . 
     As mentioned above, the conventional level shifter circuit has a limitation of the current drivability which can be reinforced by increasing the size of the NMOS transistors when the input signal INb has a low voltage level. 
     SUMMARY 
     In an aspect of the present disclosure, a level shifter circuit is provided that is capable of improving current drivability and executing stable operation with a low voltage by boosting a voltage level of an input signal. 
     In an embodiment, a level shifter circuit includes a level shifting unit configured to generate a boosted voltage by boosting an input signal and shift a voltage level of the boosted voltage to output an output signal. 
     The level shifting unit includes a voltage boosting unit configured to boost a voltage level of the input signal and output the boosted voltage, and a voltage level shifter configured to shift the voltage level of the boosted voltage to output the output signal. 
     The voltage boosting unit includes a voltage level setup unit configured to receive the input signal and set up voltage levels on first and second nodes in response to a power supply voltage signal, a delay unit configured to delay the input signal, and a pumping unit configured to boost the voltage levels on the first and second nodes and output the boosted voltage, in response to an output signal of the delay unit. 
     The voltage level setup unit includes a first NMOS transistor configured to set up the voltage level on the first node in response to the power supply voltage signal, and a second NMOS transistor configured to set up the voltage level on the second node in response to the power supply voltage signal. 
     The delay unit includes a plurality of inverters to delay and output the input signal. 
     The pumping unit includes first and second capacitors to pump up the voltage levels on the first and second nodes in response to the output signal of the delay unit. 
     The first and second nodes are respective input nodes of the voltage level shifter. 
     In another embodiment, a level shifter circuit includes a voltage level setup unit configured to receive an input signal, and set up voltage levels on first and second nodes in response to a power supply voltage signal, a delay unit configured to delay the input signal, a pumping unit configured to output the boosted voltage by boosting the voltage levels on the first and second nodes in response to an output signal of the delay unit, a pull-down driving unit configured to perform a pull-down operation in response to voltage levels on the first and second nodes, and a pull-up driving unit configured to perform a pull-up operation in response to an output voltage of pull-down driving unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the subject matter of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram illustrating a conventional level shifter; 
         FIG. 2  is a block diagram illustrating an example of a level shifter according to an exemplary embodiment of the present disclosure; 
         FIG. 3  is a circuit diagram illustrating an example of a configuration of the level shifter of  FIG. 2 ; and 
         FIG. 4  is a graph showing results of simulation of the level shifter of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described with reference to accompanying drawings. However, the embodiments are for illustrative purposes only and are not intended to limit the scope of the invention and the claims appended hereto. 
       FIG. 2  is a block diagram illustrating an example of a level shifter according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 2 , the level shifter includes a voltage boosting unit  10  to increase a voltage level of an input signal IN and output a boosted voltage, and a voltage level shifting unit  20  to shift a voltage level of the boosted voltage. 
       FIG. 3  is a circuit diagram illustrating an example of configuration of the level shifter of  FIG. 2 . 
     Referring to  FIG. 3 , the voltage boosting unit  10  includes a voltage level setup unit  11  configured to receive an external signal IN and set up voltage levels on first and second nodes A and B in response to a power supply voltage VDDL, a delay unit  13  to delay an input signal INb which is generated by the external signal IN, and a pumping unit  12  to boost the voltage levels on the first and second nodes A and B and output boosted voltages, in response to an output signal of the delay unit  13 . 
     The voltage level setup unit  11  includes a first NMOS transistor mn 3  to set up the voltage level on the first node A in response to the power supply voltage VDDL, and a second NMOS transistor mn 4  to set up the voltage level on the second node B in response to the power supply voltage VDDL. These first and second NMOS transistor mn 3  and mn 4  set up the voltage levels on the first and second nodes A and B to a voltage level of VDDL-Vt. 
     The delay unit  13  includes a plurality of inverters IV 1  and IV 2  to output a delayed signal by delaying the input signal INb. 
     The pumping unit  12  includes first and second capacitors C 1  and C 2  to pump up the voltage levels on the first and second nodes A and B in response to a delay signal INb_delay of the delay unit  13 . The first and second capacitors C 1  and C 2  increase the voltage levels on the first and second nodes A and B up to the voltage level of the power supply voltage VDDL, by means of a coupling effect. 
     The voltage level shifting unit  20  includes a pull-down driving unit  21  for performing a pull-down operation in response to the voltage levels on the first and second nodes A and B and a pull-up driving unit  22  for performing a pull-up operation in response to output voltages of the pull-down driving unit  21 . 
     The pull-down driving unit  21  includes a first pull-down transistor mn 1  for performing a pull-down operation of a third node AA in response to the voltage level on the first node A and a second pull-down transistor mn 2  for performing a pull-down operation of a fourth node BB in response to the voltage level on the second node B. 
     The pull-up driving unit  22  includes a first pull-up transistor mp 2  for performing a pull-up operation of the fourth node BB in response to the voltage level on the third node AA and a second pull-up transistor mp 1  for performing a pull-up operation of the third node AA in response to the voltage level on the fourth node BB. 
       FIG. 4  is a graph showing results of simulation of the level shifter of  FIG. 2 . The boosting operation, in which the voltage level on the first node A is higher than the voltage level of the input signal INb, is shown in  FIG. 4  (this boosting operation on the first node A is the same as that on the second node B). The voltage levels of the input signal INb, the first node A, and the delay signal INb_delay of the input signal INb are shown in the simulation of  FIG. 4 . 
     For example, when the input signal INb transits from a low level to a high level, the voltage level of the first node A is set up to the voltage level of VDDL-Vt through the first NMOS transistor mn 3  of the voltage level setup unit  11 . 
     Subsequently, when the delay signal INb_delay from the delay unit  13  transits from a low level to a high level, the voltage level of the first node A is increased due to the coupling effect of the first capacitor C 1 . 
     Finally, the voltage level of the first node A is increased up to the voltage level of 2*VDDL-Vt. Actually, as shown in  FIG. 4 , the voltage level of the first node A is slightly lower than the voltage level of 2*VDDL-Vt because the capacitance of the transistor and the parasite capacitance. If the voltage level of the first node A is increased up to the voltage level of 2*VDDL-Vt, the voltage level of the first node A is saturated at the voltage level of 2*VDDL-Vt because the first NMOS transistor mn 3  is turned off. 
     The voltage level of the first node A which is boosted higher than the input signal INb turns on the first pull-down transistor mn 1  of voltage level shifting unit  20  while the second pull-down transistor mn 2  is turned off by an inverted signal which is produced by inverting the delay signal INb_delay of the delay unit  13  through an inverter IV 3 . 
     The first pull-down transistor mn 1 , which is turned on, drops the voltage level of the third node AA to a ground voltage level VSS. The first pull-up transistor mp 2  is turned on so that the voltage level of the fourth node BB is increased up to the voltage level of VDDH. 
     Next, the second pull-up transistor mp 1  is turned off in response to the voltage level of VDDH of the fourth node BB. 
     Accordingly, the boosted voltage of the first node A increases the drivability of the first pull-down transistor mn 1  so that the conflict which is caused by the second pull-up transistor mp 1  is prevented. 
     On the other hand, when the input signal INb transits from a high level to a low level, the voltage level of the second node B is set up to the voltage level of VDDL-Vt through the second NMOS transistor mn 4  of the voltage level setup unit  11 . 
     Subsequently, when the delay signal INb_delay from the delay unit  13  transits from a high level to a low level, the voltage level of the second node B is increased by means of the coupling effect of the second capacitor C 1 . 
     Finally, the voltage level of the second node B is increased up to the voltage level of 2*VDDL-Vt. If the voltage level of the second node B is increased up to the voltage level of 2*VDDL-Vt, the voltage level of the second node B is saturated at the voltage level of 2*VDDL-Vt because the second NMOS transistor mn 4  is turned off. 
     The voltage level of the second node B which is boosted higher than the input signal INb turns on the second pull-down transistor mn 2  of the voltage level shifting unit  20  while the first pull-down transistor mn 1  is turned off by the low level signal on the first node A. 
     The second pull-down transistor mn 2 , which is turned on, drops the voltage level of the fourth node BB to the ground voltage level VSS. The second pull-up transistor mp 1  is turned on so that the voltage level of the third node AA is increased up to the voltage level of VDDH. 
     Next, the first pull-up transistor mp 2  is turned off in response to the voltage level of VDDH of the third node AA. 
     Accordingly, the boosted voltage of the second node B increases the drivability of the second pull-down transistor mn 2  so that the conflict which is caused by the first pull-up transistor mp 2  is prevented. 
     As apparent from the above, the level shifter stably operates with the improvement of the current drivability, by using the boosted voltage of the input signal. 
     Although examples and exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the disclosure and the accompanying claims. 
     The present disclosure claims priority to Korean application 10-2008-0052692, filed on Jun. 4, 2008, the entire contents of which are incorporated herein by reference.