Voltage-level shifter

Abstract: In a voltage-level shifter, a P-channel MOS transistor (early cut-off circuit) (Qp1603) is interposed between a voltage source (Vpp) of the voltage-level shifter and a source of a P-channel MOS transistor (Qp1602) which tends to be turned on when a voltage of a voltage source of an input signal supplies a low voltage or a large potential difference exists between the voltage source of the input signal and the voltage source of the voltage-level shifter, and is supplied on its gate with the input signal of the voltage-level shifter. Accordingly, the interposed P-channel MOS transistor (Qp1603) is turned off prior to the P-channel MOS transistor (Qp1602) having a tendency of being turned on, so that the voltage level of the output signal of the voltage-level shifter can be rapidly fixed at the "L" level.

Description: The present invention relates to a voltage level shifter as set forth in the preamble of claim 1. A voltage level shifter of this kind is known from EP-A-0 405 441. The aforementioned document discloses a voltage level shifter having a plurality of level shifter units, called buffer circuits in the aforementioned document, connected in series, in which each of the units receives a signal of a predetermined potential and an outputs signal of a potential higher than that of the input signal, wherein the voltage level shifter unit at the initial stage receives an external signal as input signal and each of the voltage level shifter units other than that in the initial stage receives the output signal of the voltage level shifter unit at the preceding stage as its input signal, and each of the voltage level shifter units has a power source forming a voltage source of its own output signal and generates a potential lower than that of the voltage source as a preceding stage. Such voltage level shifters are used in semiconductor devices using a plurality of voltage sources, where such voltage level shifters are required for shifting potentials of signals to potential levels of the respective voltage sources. An example of a voltage level shifter of the prior art will be described below. Fig. 3 shows a circuit structure of a conventional voltage-level shifter in the prior art, and Fig. 4 shows an operation timing of the conventional voltage-level shifter. In these figures, I30 indicates an input signal, O30 indicates an output signal, VCC indicates a first voltage source, VPP indicates a second voltage source, VSS indicates a ground voltage source, 3001 indicates a voltage-level shifter, Qn3001 and Qn3002 indicate N-channel MOS transistors, Qp3001 and Qp3002 indicate P-channel MOS transistors, and N3001 indicates a node name. A circuit structure in Fig. 3 will be described below. The input signal I30 is connected to a source of the N-channel MOS transistor Qn3001 and a gate of the N-channel MOS transistor Qn3002. A gate of the N-channel MOS transistor Qn3001 is connected to the first voltage source VCC, and a source of the N-channel MOS transistor Qn3002 is connected to the ground voltage source VSS. A source of the P-channel MOS transistor Qp3001 and a source of the P-channel MOS transistor Qp3002 are connected to the second voltage source VPP. A drain of the N-channel MOS transistor Qn3001, a drain of the P-channel MOS transistor Qp3001 and a gate of the P-channel MOS transistor Qp3002 are connected together. The output signal O30 is connected to a drain of the N-channel MOS transistor Qn3002, a gate of the P-channel MOS transistor Qp3001 and a drain of the P-channel MOS transistor Qp3002. Fig. 4 shows a basic structure of the voltage-level shifter in Fig. 3. The structure in Fig. 4 differs from that in Fig. 3 in that an N-channel MOS transistor Qn3001' is supplied on its gate with a signal formed by inverting the input signal by an inverter INV32, has a source connected to the voltage source VSS of the input signal, and has a drain connected to the P-channel MOS transistor Qp3202. In other words, the structure in Fig. 31 eliminates the inverter circuit INV32 by employing the appropriate connection structure of the N-channel MOS element Qn3001, but performs the same operation as the basic structure in Fig. 32. Accordingly, the operation will be described below only in connection with the structure in Fig. 3. Referring to an operation timing diagram of Fig. 5, the operation will be described below. When the input signal I30 is at the "L" (Low) level, the node N3001 is at the "L" level, the N-channel MOS transistor Qn3002 is off, P-channel MOS transistor Qp3002 is on, output signal O30 is at the "H" level set by the second voltage source VPP, and the P-channel MOS transistor Qp3001 is completely off. When the input signal I30 transits from the "L" level to the "H" level, the N-channel MOS transistor Qn3002 is completely turned off, the node N3001 attains a potential (VCC-Vtn) which is lower than the potential of the first voltage source VCC by a threshold voltage (Vtn) of the N-channel MOS transistor Qn3001, and the P-channel MOS transistor Qp3002 is substantially turned off. Then, the output signal 030 attains the "L" level, the P-channel MOS transistor Qp3001 is completely turned off, the node N3001 attains the potential of the second voltage source VPP, and the p-channel MOS transistor Qp3002 is completely turned off. Fig. 6 shows another known voltage-level shifter. This voltage-level shifter amplifies both the maximum and minimum values of an amplitude of an input signal to produce a signal having a large amplitude. The voltage-level shifter includes a signal inverter circuit 50 inverting the input signal, a positive level shifter 51 which receives the input signal and an output signal of the signal inverter circuit 50 to amplify the maximum value of the amplitude of the input signal, a negative level shifter 52 which receives the input signal and the output signal of the signal inverter circuit 50 to amplify the minimum value of the amplitude of the input signal, and a positive/negative level shifter 53 which receives output of the positive and negative level shifters 51 and 52 to compose them. However, in the voltage-level shifters of the conventional structures shown in Figs. 3 and 5, when the input signal I30 transits from the "L" level to the "H" level, the N-channel MOS transistor Qn3002 is completely off, and the P-channel MOS transistor Qp3002 is substantially off as already described. In particular, if the first voltage source VCC supplies a low voltage, and/or a large potential difference exists between the first and second voltage sources VCC and VPP, the P-channel MOS transistor Qp3002 is on. Therefore, a through current flows from the second voltage source VPP to the ground voltage source VSS via the N-channel MOS transistor Qn3002 and the P-channel MOS transistor Qp3002, which causes a disadvantage that the voltage level of the output signal O30 cannot fixed at the "L" level. The conventional voltage-level shifter shown in Fig. 6 requires three level shifters and hence many transistors forming them, and is constructed to compose the shifted outputs of the positive and negative level shifters, resulting in reduction of the operation speed. SUMMARY OF THE INVENTION An object of the invention is to provide a voltage-level shifter which can operate as follows if a voltage source of an input signal supplies a low voltage and/or a large potential difference exists between a voltage source of an input signal and a voltage source of a voltage-level shifter. When the input signal transits from an "L" level to the "H" level, an MOS transistor to be turned off is surely controlled to be turned off, and a voltage level of an output signal of the voltage-level shifter is surely fixed at the "L" level while surely preventing flow of a through current. A voltage-level shifter according to the invention includes a plurality of voltage level shifter units connected in series. Each voltage level shifter unit receives a signal of a predetermined potential and outputs a signal of a potential higher than that of the input signal. The voltage level shifter unit at an initial stage receives an external signal as its input signal. Each of the voltage level shifter units other than that in the initial stage receives the output signal of the voltage level shifter unit at a preceding stage as its input signal. Each voltage level shifter unit has a power source forming a voltage source of its own output signal. Each voltage source generates a potential lower than that of the voltage source at a preceding stage. Further, according to the voltage-level shifter of the invention, since multiple voltage level shifter units connected in series are used to change the potential of the input signal to the potential of the voltage source VPP in stepwise fashion, the voltage level of the output signal of the voltage-level shifter can be fixed even when a large potential difference exists between the voltage source VCC of the input signal and the voltage source VPP of the voltage-level shifter. Other objects, features and advantages of the invention will appear more fully from the following detailed description thereof taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings showing preferred embodiments of the invention: Fig. 1 shows a structure of a first embodiment of the voltage-level shifter of the invention; Fig. 2 shows a structure of a second embodiment of the voltage-level shifter of the invention; Fig. 3 shows a structure of a voltage-level shifter in the prior art; Fig. 4 shows a basic structure of the voltage-level shifter in the prior art; Fig. 5 shows an operation timing of the voltage-level shifter in the prior art; and Fig. 6 shows a structure of another conventional voltage-level shifter. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the invention will be described below with reference to the drawings. [First Embodiment] A voltage-level shifter of a first embodiment of the invention will now be described below. Fig. 1 shows a circuit structure. I20 indicates an input signal, O20 indicates an output signal, VCC indicates the first voltage source, VPP indicates the second voltage source, and VSS indicates the ground voltage source. 2011-2013 indicate voltage-level shifter units. 2001 indicates a voltage-level shifter formed of the voltage-level shifter units 2011-2013. Qn2001-Qn2032 indicate N-channel MOS transistors. Qp2011-Qp2032 indicate P-channel MOS transistors. N2001-N2004 indicate node names. The circuit structure in Fig. 1 will be described below. The voltage-level shifter 2001 is formed of the voltage-level shifter units 2011-2013 and the two N-channel MOS transistors Qn2001 and Qn2002. The voltage-level shifter unit 2011 shifts the potential of the first voltage source VCC of the input signal to the voltage level potential (VPP-2xVtn) of the node N2001, the voltage-level shifter unit 2012 shifts the voltage level (VPP-2xVtn) of the node N2001 to the voltage level (VPP-Vtn) of the node N2002, and the voltage-level shifter unit 2013 shifts the voltage level (VPP-Vtn) of the node N2001 to the second voltage source VPP of the output signal. Each of the voltage-level shifters 2011-2013 shown in the figure is the same as that in the prior art shown in Fig. 3. The node N2002 is a drain of the N-channel MOS transistor Qn2002 of which gate and source are connected to the second voltage source VPP. The node N2001 is a drain of the N-channel. MOS transistor Qn2001 of which gate and source are connected to the node N2002. The voltage-level shifter of this embodiment has such a feature that since the multiple voltage-level shifter units are used to shift the first voltage source VCC of the input signal to the potential of the second voltage source VPP of the output signal through multiple voltage levels, the voltage level of the output signal can be surely fixed even if there is a large potential difference between the first and second voltage sources VCC and VPP. [Second Embodiment] A second embodiment of a voltage-level shifter of the invention will be described below. This thirteenth embodiment is formed of a plurality of voltage-level shifter units. Fig. 2 shows a circuit structure. In the figure, I21 indicates an input signal, O21 indicates an output signal, VCC indicates the first voltage source, VPP indicates the second voltage source, and VSS indicates the ground voltage source. 2111-2113 indicate voltage-level shifter units, 2114 and 2115 indicate NOT circuits (logical circuits), 2101 indicates a voltage-level shifter, Qn2101-Qn2151 indicate N-channel MOS transistors, Qp2111-Qp2151 indicate P-channel MOS transistors, and N2101-N2106 indicate node names. In the voltage level converter unit 2111, the P-channel MOS transistor Qp2113 and the N-channel MOS transistor Qn2113 form a potential fixing unit 211. In the voltage level converter unit 2112, the P-channel MOS transistor Qp2123 and the N-channel MOS transistor Qn2123 form a potential fixing unit 212. In the voltage level converter unit 2113, the P-channel MOS transistor Qp2133 and the N-channel MOS transistor Qn2133 form a potential fixing unit 213. The circuit structure in Fig. 2 will be described below. The voltage-level shifter 2101 is formed of the voltage-level shifter units 2111-2113, the NOT circuits 2114 and 2115 and the MOS transistors. The voltage-level shifter unit 2111 shifts the potential of the first voltage source VCC of the input signal to the voltage level (VPP-2xVtn) of the node N2101, the voltage-level shifter unit 2112 shifts the voltage level (VPP-2xVtn) of the node N2101 to the voltage level (VPP-Vtn) of the node N2102, and the voltage-level shifter unit 2113 shifts the voltage level (VPP-Vtn) of the node N2101 to the second voltage source VPP of the output signal. The circuit structure of the voltage-level shifter units 2111-2113 shown in the figure will now be described below. Since all the voltage-level shifter units 2111-2113 have the same circuit structure, the following description will be given on only the voltage-level shifter unit 2113. The node 2104 supplying the first input signal is connected to a source of the N-channel MOS transistor Qn2131 and a gate of the N-channel MOS transistor Qn2132. A gate of the N-channel MOS transistor Qn2131 is connected to the node 2102 at the voltage level of the first input signal. The node 2106 supplying the second input signal is connected to a gate of the N-channel MOS transistor Qn2133 and a gate of the P-channel MOS transistor Qp2133. A drain of the N-channel MOS transistor Qn2131, a gate of the P-channel MOS transistor Qp2132 and a drain of the P-channel MOS transistor Qp2131 are connected together, and a drain of the P-channel MOS transistor Qp2132 and a source of the P-channel MOS transistor Qp2133 are connected together. The output signal O21 is connected to a drain of the N-channel MOS transistor Qn2133, a drain of the P-channel MOS transistor Qp2133 and a gate of the P-channel MOS transistor Qp2131. A source of the N-channel MOS transistor Qn2132 and a source of the N-channel MOS transistor Qn2133 are connected to the ground voltage source VSS. A source of the P-channel MOS transistor Qp2131 and a source of the P-channel MOS transistor Qp2132 are connected to the second voltage source VPP, i.e., the voltage level of the output signal. The NOT circuits 2114 and 2115 use the first voltage source VCC as their voltage sources. A structure of the voltage-level shifter 2101 will be described below. The node N2102 is a drain of the N-channel MOS transistor Qn2102 of which gate and source are connected to the second voltage source VPP. The node N2101 is a drain of the N-channel MOS transistor Qn2101 of which gate and source are connected to the node N2102. In the voltage-level shifter unit 2111, the input signal I21 is connected for supplying the first and second input signals. A gate of the N-channel MOS transistor Qn2111 is connected to the first voltage source VCC at the voltage level of the input signal. Sources of the P-channel MOS transistors Qp2111 and Qp2112 are connected to the node N2101 at the voltage level of the output signal, and the potential of the node N2103 is output as the output signal. In the NOT circuit 2114, the input signal I21 is connected to supply the input signal, and the potential of the node 2105 is outputted as the output signal. The voltage-level shifter unit 2112 is connected to the input signal I21 for receiving the first input signal and is also connected to receive the potential of the node N2105 as the second input signal. A gate of the N-channel MOS transistor Qn2121 is connected to the node 2101 at the potential of the voltage level of the first input signal. Sources of the P-channel MOS transistors Qp2121 and Qp2122 are connected to the node 2102 at the voltage level of the output signal, and the potential of the node 2104 is outputted as the output signal. The NOT circuit 2115 is connected to receive the potential of the node N2105 as the input signal, and is also connected to output the potential of the node N2106 as the output signal. In the voltage-level shifter unit 2113, the node N2104 is connected for receiving the first input signal, the node N2106 is connected for receiving the second input signal, and a gate of the N-channel MOS transistor Qn2131 is connected to the node 2102 at the voltage level of the first input signal. Sources of the P-channel MOS transistors Qp2131 and Qp2132 are connected to the second voltage source VPP at the voltage level of the output signal. Further, the output signal O21 is outputted. The voltage-level shifter of this embodiment has such a feature that, since the multiple voltage-level shifter units are used to shift the first voltage source VCC of the input signal to the potential of the second voltage source VPP of the output signal forming the output signal through multiple voltage levels, the voltage level of the output signal can be surely fixed even if there is a large potential difference between the first and second voltage sources VCC and VPP. For example, even if the voltage level of the voltage level shifter unit 2111, i.e., the voltage level of the output signal of the node N2101 is low, and the potential of the node N2103 forming the output signal, i.e., the voltage level at the "H" level is insufficient, the signal at the voltage level of the first voltage source VCC is inputted as the second input signal to the voltage-level shifter unit 2112, whereby the voltage-level shifter 2112 can be surely operated, and the voltage level of the output signal of the voltage level shifter 2101 can be fixed.

Claims: A voltage-level shifter which shifts a potential of an input signal which is equal to a potential of a first voltage source (Vcc) to a potential of a second voltage source (VPP), wherein the voltage-level shifter comprises a plurality of voltage level shifter units (2011, 2012, 2013) connected in series, the voltage level shifter unit (2011) at an initial stage receives an external signal as its input signal, and each of the voltage level shifter units (2012, 2013) other than that at the initial stage receives an output signal from the voltage level shifter unit at each preceding stage as its input signal, each of the voltage level shifter units (2011, 2012, 2013) has a voltage source for its own output signal, a voltage generated by the voltage source increases as the stage advances, each of the voltage level shifter units (2011, 2012, 2013) includes a circuit comprising: a first P-channel MOS transistor (Qp2012, Qp2022, Qp2032) which connects the voltage source to an output node (N2003, N2004, O20); and a first N-channel MOS transistor (Qn2012, Qn2022, Qn2032) connecting the output node to a ground voltage source (VSS), and a gate of each first N-channel MOS transistor (Qn2012, Qn2022, Qn2032) receives directly the input signal, characterized in that a gate of each first P-channel MOS transistor (Qp2012, Qp2022, Qp2032) receives the input signal via a second N-channel MOS transistor (Qn2011, Qn2021, Qn2031), and is connected via a second P-channel MOS transistor (Qp2011, Qp2021, Qp2031) to the voltage source, a gate of the second N-channel MOS transistor (Qn2011) in the voltage level shifter unit at the initial stage is connected to the first voltage source (VCC), while a gate of each second N-channel MOS transistor (Qn2021, Qn2031) in each voltage level shifter unit other than that at the initial stage is connected to the voltage source of the voltage level shifter unit at each preceding stage, and a gate of each second P-channel MOS transistor (Qp2011, Qp2021, Qp2031) is connected to the output node (N2003, N2004, O20). The voltage-level shifter according to claim 1, wherein each of the voltage level shifter units (2011, 2012, 2013) further comprises: a third P-channel MOS transistor (Qp2113, Qp2123, Qp2133) connected between the first P-channel MOS transistor and the output node; and a third N-channel MOS transistor (Qn2113, Qn2123, Qn2133) connected between the output node and the ground voltage source (VSS), each of the voltage level shifter units (2012, 2013) other than that at the initial stage is connected with a logic circuit (2114, 2115), the logic circuits (2114, 2115) using the first voltage source (VCC) in common with one another, gates of the third P-channel MOS transistor (Q12123, Qp2133) and the third N-channel MOS transistor (Qn2123, Qn2133) included in each voltage level shifter unit other than that at the initial stage are connected to an output node (N2105, N2106) of the logic circuit (2114, 2115), and an output signal of each logic circuit (2114, 2115) connected to each voltage level shifter unit is inputted as an input signal to the logic circuit connected to the voltage level shifter unit at each subsequent stage. The voltage-level shifter according to claim 2, wherein each of the logic circuits is formed of a NOT circuit inverting the input signal.

Inventors: HIRANO HIROSHIGE, HIRANO, HIROSHIGE
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H03K19
IPC codes: H03K19