Abstract:
A high speed voltage level shifter for use in circuitry having core circuits operating at a very low supply voltage includes a boost circuit for producing a boosted signal in accordance with a non-inverted input signal, and a voltage shifting stage for producing an output signal in response to an inverted input signal and the boosted signal. The boost circuit translates the boosted signal into a middle voltage level when the non-inverted input signal is at logic ‘0’. When the inverted input signal and the boosted signal are both at the logic ‘0’, the voltage shifting stage provides the output signal with a high voltage level. Otherwise, the voltage shifting stage pulls down the output signal to ground when the boosted signal is at the middle voltage level and the inverted input signal is at a low voltage level equal to the very low supply voltage level.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuits and, in particular, to a high speed voltage level shifter. 
     BACKGROUND OF THE INVENTION 
     With the introduction of integrated circuits with geometry below 0.13 μm, lower power supply voltages have become necessary to maintain device reliability. Consequently, supply voltage has decreased from 3.3 volts to 1.2 volts or less. However, many interface signals still use the logic level from 0 to 3.3 or 5 volts. The circuitry therefore can be divided into “core” circuits and I/O circuits, where the core logic operates at the lower 1.2 volts, and the I/O circuits operate at 3.3 volts. To interface between circuits requiring different voltage levels, a voltage level shifter is employed to switch between the voltage levels of the respective circuits. 
     FIG. 1 shows a well-known circuit implementation of a voltage level shifter  100 . A NMOS transistor MN 1  has its gate connected to a core circuit supply voltage (V CCL ). Both PMOS transistors MP 1 , MP 2  have their sources connected to an I/O voltage source V CCH , which has a higher voltage potential than V CCL . The level shifter  100  translates a low voltage input signal at an input terminal  110  to a high voltage output signal at an output terminal  130 . For such a voltage level shifter to operate properly, the PMOS transistor MP 1  is “weak” compared to the transistors MN 1 , MN 2  and MP 2 . 
     With continued reference to FIG. 1, when the input signal is logic low, the NMOS transistor MN 2  is “on” via an inverter  120 . As a result, the NMOS transistor MN 2  electrically drives the output terminal  130  to ground. Further, the logic low output signal turns “on” the PMOS transistor MP 1  which provides the supply voltage V CCH  to a gate of the PMOS transistor MP 2 , thereby the PMOS transistor MP 2  is held “off”. 
     When the input signal goes high, the NMOS transistor MN 2  is turned “off”. The NMOS transistor MN 1  electrically connects the gate of the PMOS transistor MP 2  to an inverted input signal at logic low (i.e., ground). Hence, the PMOS transistor MP 2  is turned “on” and provides the supply voltage V CCH  to the output terminal  130 . 
     The voltage level shifter  100  is suitable for ordinary applications. However, when the supply voltage V CCL  approaches 1.2 volts or less, the NMOS transistors in the level shifter  100  cannot be conducted sufficiently due to a typical threshold voltage of 0.8 volts. The gate voltage that brings about conduction in a transistor is called the threshold voltage. In the case of an input signal having a voltage swing between 0 and 1.2 volts, an output signal at the output terminal  130  should be logic low when the gate of the NMOS transistor MN 2  is at 1.2 volts, e.g. logic high. Nevertheless, the NMOS transistor MN 2  cannot be turned “on” sufficiently since the voltage drop across the gate and source of the NMOS transistor MN 2  is no more than 0.4 volts, which barely allows it to overcome the pull-up capability of the PMOS transistor MP 2 . As a result, the output signal is weakly pulled to ground by NMOS transistor MN 2 . Referring to FIG. 2, the input signal IN shown is a substantially square wave having a low voltage level of 0 and a high voltage level of 1.2 volts. The output signal OUT has a voltage swing between 0 and 3.3 volts. It can be seen that the output signal OUT is poor and suffers from relatively slow falling time. Such a defect is further exacerbated when the supply voltage V CCL  for “core” circuits is even lower. Hence, owing to its relative slowness the prior art level shifter  100  cannot be applied to high speed circuitry with “core” circuits having a very low supply voltage. 
     Accordingly, what is needed is a high speed voltage level shifter that overcomes the disadvantages associated with the prior art. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high speed voltage level shifter for use in circuitry having “core” circuits which operate at a very low supply voltage. 
     The present invention is directed to a voltage level shifter including a boost circuit and a voltage shifting stage. The voltage level shifter also has an input terminal and an output terminal. The input terminal is used to receive a non-inverted input signal having a first voltage level and a second voltage level, in which the first voltage level is a reference voltage level and the second voltage level is higher than the first voltage level. The boost circuit receives the non-inverted input signal. According to the non-inverted input signal, the boosted circuit produces a boosted signal, where the boosted signal is at a third voltage level when the non-inverted input signal is at the first voltage level, and at the first voltage level when the non-inverted input signal is at the second voltage level. In particular, the third voltage level is higher than the second voltage level. The voltage shifting stage is coupled to the boost circuit. In response to an inverted input signal and the boosted signal, the voltage shifting stage produces an output signal at a fourth voltage level when the inverted input signal and the boosted signal are both at the first voltage level, and at the first voltage level when the inverted input signal is at the second voltage level and the boosted signal is at the third voltage level. Therefore, the output terminal is used to provide the output signal having an output voltage swing between the first voltage level and the fourth voltage level. 
     In a preferred embodiment, the boost circuit includes a capacitor for producing an intermediate signal at the third voltage level when the inverted input signal is at the second voltage level. The boost circuit also has a first P-type transistor, a second P-type transistor, and a first N-type transistor. In response to the inverted input signal, the first P-type transistor charges the capacitor when the inverted input signal is at the first voltage level. When the non-inverted input signal is at the second voltage level, the first N-type transistor pulls down the boosted signal to the first voltage level. The second P-type transistor passes the intermediate signal to the boosted signal when the non-inverted input signal is at the first voltage level. Further, the capacitor is charged, by way of the first P-type transistor, from a first power source having the second voltage level, such that the intermediate signal substantially remains at the second voltage level when the inverted input signal is at the first voltage level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
     FIG. 1 is a schematic diagram of a prior art voltage level shifter; 
     FIG. 2 is a graph showing the input and output signals of the voltage level shifter of FIG. 1; 
     FIG. 3A illustrates a block diagram of the invention; 
     FIG. 3B is a schematic diagram of a preferred embodiment according to the invention; and 
     FIG. 4 illustrates a graph useful in understanding the operation of a voltage level shifter of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As illustrated in FIG. 3A, a high speed voltage level shifter  300  of the invention includes a boost circuit  340  and a voltage shifting stage  350 . The voltage level shifter  300  also has an input terminal  310  and an output terminal  330 . The input terminal  310  is used to receive a non-inverted input signal having a first voltage level and a second voltage level, in which the first voltage level is a reference voltage level and the second voltage level is higher than the first voltage level. An inverter  320  receives the non-inverted input signal from the input terminal  310  and produces an inverted input signal. 
     The boost circuit  340  receives the non-inverted input signal. According to the non-inverted input signal, the boosted circuit  340  produces a boosted signal varying within a range between the second voltage level and a third voltage level. When the non-inverted input signal is at the first voltage level, the boosted signal is at a third voltage level. When the non-inverted input signal is at the second voltage level, the boosted signal is at the first voltage level. It is noted that the third voltage level is higher than the second voltage level. 
     The voltage shifting stage  350  is coupled to the boost circuit  340 . In response to the inverted input signal and the boosted signal, the voltage shifting stage  350  produces an output signal at the output terminal  330 . When the inverted input signal and the boosted signal are both at the first voltage level, the voltage shifting stage  350  provides the output signal at a fourth voltage level. In addition, when the inverted input signal is at the second voltage level and the boosted signal is at the third voltage level, the voltage shifting stage  350  provides the output signal at the first voltage level. The output terminal  350  is used to provide the output signal having an output voltage swing between the first voltage level and the fourth voltage level. 
     The second and the fourth voltage levels herein are two different supply voltage levels used in the voltage level shifter  300 . For an integrated circuit fabricated with 0.13 μm process, the typical supply voltage level for “core” circuits (the second voltage level) is 1.2 volts and the supply voltage level for interface signals (the fourth voltage level) may be 3.3 volts. It should be understood that other voltage levels are contemplated without limiting the scope of the invention. Furthermore, the first voltage level is ground potential (e.g. 0 volts). 
     FIG. 3B illustrates a preferred embodiment of the invention. Each transistor described herein is either a P-type or N-type MOS transistor having a gate, a drain and a source. Since a MOS transistor is typically a symmetrical device, the true designation of “source” and “drain” is only possible once a voltage is impressed on the terminals. The designations of source and drain herein should be interpreted, therefore, in the broadest sense. In the preferred embodiment, the boost circuit  340  includes a capacitor CAP for producing an intermediate signal which substantially incorporates a voltage swing between the second voltage level and the third voltage level. The boost circuit  340  also has a first P-type transistor BP 1 , a second P-type transistor BP 2 , and a first N-type transistor BN 1 . The P-type transistor BP 1  has its source coupled to a first power source (i.e. “core” voltage source, V CCL ), and its gate connected to a first node (node A) to receive the inverted input signal. The capacitor CAP is connected between the gate and drain of the P-type transistor BP 1 , in which an intermediate signal is provided at the drain of the P-type transistor BP 1  (node B). The P-type transistor BP 2  has its source connected to the drain of the P-type transistor BP 1 , its gate connected to the input terminal  310  to receive the non-inverted input signal, and its drain connected to a second node (node C) providing the boosted signal. The N-type transistor BN 1  has its drain connected to the drain of P-type transistor BP 2  (node C), its gate also connected to the input terminal  310 , and its source coupled to ground (the first voltage level). 
     As depicted, the voltage shifting stage  350  is comprised of a second N-type transistor SN 1 , a third N-type transistor SN 2 , a third P-type transistor SP 1 , and a fourth P-type transistor SP 2 . It is noted that the P-type transistor SP 1  is “weak” compared to the N-type transistor SN 1 . The N-type transistor SN 1  has its gate coupled to the first power source V CCL , and its source connected to the node A to receive the inverted input signal. The N-type transistor SN 2  has its source coupled to ground, its gate connected to the node C to receive the boosted signal, and its drain connected to the output terminal  330  to provide the output signal. Both P-type transistors SP 1  and SP 2  have their sources coupled to a second power source V CCH  having the fourth voltage level in which the second power source V CCH  is the I/O voltage source. The P-type transistor SP 1  has its drain connected to the drain of the N-type transistor SN 1  and its gate connected to the drain of the N-type transistor SN 2 . The P-type transistor SP 2  has its drain connected to the drain of the N-type transistor SN 2 , while the gate is connected to the drain of the N-type transistor SN 1 . 
     With reference to FIG.  3 B and FIG. 4, the operation of the voltage level shifter  300  of the invention will be described. If the non-inverted input signal IN is logic high (the second voltage level), the inverted input signal (V A ), through the inverter  320 , at the node A is logic low (the first voltage level). This state turns “on” the P-type transistor BP 1  but turns “off” the P-type transistor BP 2 , so the capacitor is charged through the P-type transistor BP 1  to remain at the second voltage level (V CCL ). In the preferred embodiment, the intermediate signal at the node B (V B ) is substantially equal to the second voltage level when the node A is at the first voltage level. The boosted signal at the node C (V C ) is pulled to ground through the N-type transistor BN 1  when the non-inverted input signal is at logic high, which allows the N-type transistor SN 2  to be turned “off”. As the node A transits to logic low, the N-type transistor SN 1  is turned “on” and pulls the gate of the P-type transistor SP 2  to the first voltage level (i.e., ground). The P-type transistor SP 2  is turned “on” and provides the supply voltage V CCH  to the output terminal  330 . Thus, the output signal OUT is at the fourth voltage level, and the P-type transistor SP 1  is turned “off”. 
     The output signal OUT remains at V CCH  until the non-inverted input signal at the input terminal  310  changes state. When the state of the non-inverted input signal goes from a high to a low state, the output of the inverter  320 , or the node A (V A ), goes high (the second voltage level). The P-type transistor BP 2  is turned “on” but the P-type transistor BP 1  and the N-type transistor BN 1  are turned “off”. The N-type transistor SN 1  is also turned “off” via the inverted signal at the node A. As the P-type transistor BP 1  is “off”, the intermediate signal at the node B (V B ) increases to the third voltage level due to the electrical charges stored in the capacitor CAP. In other words, since the voltage drop across a capacitor may not change abruptly, the capacitor CAP boosts the intermediate signal to the third voltage level when the inverted input signal is at the second voltage level. The P-type transistor BP 2  passes the intermediate signal to the boosted signal at the node C (V C ) as shown in FIG. 4, when the non-inverted input signal is at the first voltage level. According to the invention, the N-type transistor SN 2  is turned “on” and is conducted sufficiently by increasing the node C to a voltage of approximately 1.7˜2.0 volts, i.e., the third voltage level. The use of the boost circuit  340  ensures that the applied gate voltage of the N-type transistor SN 2  is high enough to bring about strong conduction. As such, the N-type transistor SN 2  electrically connects the output terminal  330  to ground which strongly pulls down the output signal OUT to the first voltage level. Further, the output signal OUT turns “on” the P-type transistor SP 1  which provides the supply voltage V CCH  to the gate of the P-type transistor SP 2 , thereby the P-type transistor SP 2  is held “off”. 
     The output signal OUT, as depicted in FIG. 4, is substantially improved as compared to the prior art. Accordingly, the boosted circuit  340  enhances the response time of the voltage level shifter  300  such that the falling time of the output signal is substantially improved over the prior art. In addition, the improved falling time of the voltage level shifter  300  produces a substantially improved switching speed. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. 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.