Level shelter, semiconductor integrated circuit and information processing system

[0001] 1. Field of the Invention

[0002] The present invention relates to a level shifter, which level-converts an input signal and then outputs the resulting signal, a semiconductor integrated circuit, and an information processing system. In particular, it is related to a level shifter, which converts an input low-level signal into a high-level signal, a semiconductor integrated circuit, and an information processing system.

[0003] 2. Description of the Prior Art

[0004] In recent years, computer motherboards are equipped with multiple devices such as ASIC, microprocessors, memories, and peripheral circuits, and an increasing number of microcomputers are designed to fulfill desired functions. Particularly, since operation with low amounts of power consumption and at high frequencies is required with the ASIC and microprocessors, amplitude of the supply voltage to be internally utilized has been designed to decreased. For example, given a 2.5 V internal supply voltage, this voltage is inclined to decrease to 1.8 V, 1.5 V and 1.2 V in the future.

[0005] In contrast, there are many devices such as peripheral circuits that operate at 3.3 V where input/output of data between each device is executed at 3.3 V based on JEDEC system interface standards or the like. Consequently, the condition where the peripheral circuits operate at 3.3 V and the ASIC and microprocessor operate at a different voltage of 2.5 V has been increasing. Accordingly, the ASIC and microprocessor are equipped with input/output buffers with a 3.3 V supply voltage in order to level shift by the voltage difference. In other words, the ASIC and microprocessor run on two power supplies, where an internal power supply is 2.5 V and an input/output buffer power supply is 3.3 V.

[0006] Such level shifting function-equipped conventional input/output buffer is illustrated in FIG. 24. The conventional input/output buffer (level shifter) is one that converts a 2.5 V amplitude input signal S_IN to the level of a 3.3 V amplitude output signal S_OUT, and is configured from an inverter INV₁, 3.3 V-tolerant P-channel MOS transistors M_P1 and M_P2, and 3.3 V-tolerant N-channel MOS transistors M_N1 and M_N2. Hereafter, a MOS transistor is simply referred to as a transistor.

[0007] The inverter INV₁ has a CMOS structure, which is configured from a P-channel transistor and an N-channel transistor, and inverts the 2.5 V amplitude input signal S_IN so as to output an inverted input signal S_INB with an amplitude of 2.5 V. The transistor M_P1 has a 3.3 V supply voltage V_DD applied to its source, its drain connected to the transistor M_P2 gate, and its gate connected to the transistor M_P2 drain. The transistor M_P2 has a 3.3 V supply voltage V_DD applied to its source, and the output signal S_OUT with amplitude of 3.3 V is output from its drain. The transistor M_P1 has the input signal S_IN applied to its gate, its drain connected to the transistor M_P1 drain, and its source connected to a ground. The transistor M_P2 has the inverted input signal S_INB applied to its gate, its drain connected to the transistor M_P2 drain, and its source connected to a ground.

[0008] Next, the operation of the above-configured level shifter is described. To begin with, when the input signal S_IN with a 2.5 V amplitude is at an “H” level, the transistor M_N1 turns on, and at the same time since the inverted input signal S_INB becomes at an “L” level, the transistor M_N2 turns off. Accordingly, the transistor M_P2 turns on whereas the transistor M_P1 turns off. Therefore, the “H” level output signal S_OUT of 3.3 V amplitude is output from this level shifter.

[0009] In contrast, when the input signal S_IN is at the “L” level, the transistor M_N1 turns off, and at the same time since the inverted input signal S_INB becomes the “H” level, the transistor M_N2 turns on. Accordingly, the transistor M_P2 turns off whereas the transistor M_P1 turns on. Therefore, the “L” level output signal S_OUT is output from this level shifter.

[0010] It should be noted that disclosed in Japanese Patent Application Laid-open No. Hei 11-239051 is a conventional negative logic level shifter where the output signal S_OUT differs from the configuration described above.

[0011] The conventional level shifter mentioned above operates normally when the “H” level of the inverted input signal S_INB is substantially higher than the threshold voltage Vt (typically 0.7 V) of the transistor M_N2 gate voltage, and the resistance between the source and the drain of the off transistor M_P2 is sufficiently smaller than that of the on transistor M_N2. However, since the transistors M_N1 and M_N2 are not fully turned on when the “H” level of the inverted input signal S_INB drops until near the threshold voltage Vt of the transistor M_N2 gate voltage, the resistance between the source and the drain of the transistor M_N2 that is insufficiently turned on abruptly increases to a value matching that of the Off transistor M_P2, whereby driving the subsequent transistors M_P1 and M_P2 is no longer possible. As a result, even when the input signal S_IN is at the “L” level, the output signal S_OUT stays at the “L” level, namely it can no longer decrease to 0 V, where the level shifter is unable to operate normally.

[0012] Input and output waveforms and voltages are described based on FIG. 25 through FIG. 30. It should be noted that in FIG. 25 through FIG. 30, the vertical axis represents input signal voltage (V) and the horizontal axis represents time (nS (nanoseconds)). Furthermore, the gate width of the transistors M_P1 and M_P2 is set as 10 μm and the gate width of the transistors M_N1 and M_N2 is set as 30 μm.

[0013] It can be understood from the waveform of FIG. 25 that with an input signal amplitude up to 2.0 V, level shifting to 3.3 V is possible without any change in the duty ratio. From the waveforms of FIG. 26 and FIG. 28, however, it can be understood that the duty ratio cannot be maintained regardless that level shifting has been performed as the input voltage drops from 1.5 V to 1.35 V.

[0014] Moreover, when the input signal voltage drops and becomes 1.2 V and 1.0 V, the output signals change at only near 3 V as illustrated in FIG. 29 and FIG. 30, respectively. In other words, the output signals no longer drop to 0 V. As a result, it is the same as if the level shifter actually outputs an “H” level signal, thereby becoming unable to transmit the input signal changes to the subsequent circuit.

[0015] Furthermore, with a first level shifter, it is evident that it takes long for the output signal to change and the level to reach a stable level since the input signal has changed, in other words, the response speed is slow, and particularly the slower as the input voltage drops more. For example, when the input signal is 2.0 V, as illustrated in FIG. 25, approximately 0.5 nanoseconds has lapsed from the time when the input signal begins rising to when the output voltage reaches a stable level; however, it is evident that when the input signal is 1.4 V, as illustrated in FIG. 27, approximately 1.5 nanoseconds has lapsed from the time when the input signal begins rising to when the output voltage reaches a stable level.

[0016] Moreover, since the transistors M_N1 and M_N2 are driven in the non-saturation region (insufficient ON state), if noise is superimposed at the input signal S_IN and/or ground lines, resistance between the sources and drains of the transistors M_N1 and M_N2 abruptly fluctuates. As a result, there are cases where the ON-current widely varies and the capability of driving the subsequent transistors M_P1 and M_P2 is hindered. In this case, the waveform of the output signal S_OUT becomes significantly irregular.

[0017] Furthermore, since the transistors M_N1 and M_N2 are driven in the non-saturation region, the ON-current values thereof are extremely small. As a result, it takes time for the gates of the subsequent transistors M_P1 and M_P2 to charge and discharge, and also to reach a stable level for the output signal S_OUT potential since the input signal S_IN is asserted from “L” level to “H” level or deasserted from “H” level to “L” level. Accordingly, the maximum frequency of the signal that is capable of operating the level shifter of this example becomes limited. Furthermore, during the transition period from when the input signal S_IN has changed to when S_OUT reaches a stable level, both the transistors M_P1 and M_N1 are not off, or both the transistors M_P2 and M_N2 are not off. Therefore, a large current, namely a flow-through current flows from the supply voltage VDD to the ground GND via the transistors M_P1 and M_N1 or the transistors M_P2 and M_N2. Consequently, the fact that it takes time to reach a stable level for the output signal S_OUT potential since the input signal S_IN is asserted or deasserted namely the transition time is long means that the time of the flow-through current flowing is long, causing power consumption to increase.

[0018] In this regard, as illustrated in FIG. 37, it can be understood from the waveform of the power supply current supplied to the first level shifter that a current continually flows during the period from when the input signal has changed to when the output signal reaches a stable level, and it can also be understood from the fact that the consumed power is determined from the amount and flowing period of the current that the first level shifter requires a large amount of power. It should be noted that the power supply current waveform of FIG. 37 is used as an example of one with an input signal of 1.4 V for the first level shifter. Furthermore, the power supply current waveform graph applies input voltage V to the left side vertical axis, time nS (nanoseconds) to the horizontal axis, and current mA to the right side vertical axis, as with the graphs in FIG. 38 to FIG. 40.

Object of the Invention

[0019] The present invention accordingly aims to provide a level shifter, which is tolerant to noise and operates with low power consumption at a high operating frequency even in the case where difference in voltage amplitude between the input signal and the output signal is large or that level shifter operates near the transistor threshold voltage, and a semiconductor integrated circuit and an information processing system thereof.

Summary of the Invention

[0020] The level shifter of the present invention includes a first transistor of a first conduction type, which is connected between a first power supply line and a first node, has a control terminal connected to an input terminal to which an input signal is to be applied; a second transistor of a second conduction type, which is connected between a second power supply line and the first node, has a control terminal connected to an output terminal; an inverter, which is connected between the input terminal and a second node; a third transistor of the first conduction type, which is connected between the first power supply line and the output terminal, has a control terminal connected to the second node; a fourth transistor of the second conduction type, which is connected between the second power supply line and the output terminal, has a control terminal connected to the first node; and a fifth transistor of the second conduction type, which is connected between the second power supply line and the first node, has a control terminal connected to the input terminal.

[0021] In this manner, the level shifter of the present invention, by including the fifth transistor, is capable of quickly charging the first node when the input is at the “L” level.

[0022] Moreover, the level shifter of the present invention further includes a sixth transistor of the second conduction type, which is connected between the second power supply line and the output terminal, has a control terminal connected to the second node.

[0023] In this manner, the level shifter of the present invention, with the sixth transistor, is capable of quickly charging the output terminal when the input signal is at the “H” level.

[0024] Further, the level shifter of the present invention further includes a seventh transistor of the first conduction type, which is connected between the first power supply line and the output terminal, has a control terminal connected to the first node; and an eighth transistor of the first conduction type, which is connected between the first power supply line and the first node, has a control terminal connected to the output terminal.

[0025] In this manner, the level shifter of the present invention, with the seventh and the eighth transistors, is capable of quickly charging the first node when the input signal is at the “H” level, and quickly charging the output terminal when the input signal is at the “L” level.

[0026] The level shifter of the present invention includes a first transistor of a first conduction type, which is connected between a first power supply line and a first node, has a control terminal connected to an input terminal to which an input signal is to be applied; a second transistor of a second conduction type, which is connected between the first node and a second power supply line, has a control terminal connected to an output terminal; an inverter, which is connected between the input terminal and a second node; a third transistor of the first conduction type, which is connected between the first power supply line and the output terminal, has a control terminal connected to the second node; a fourth transistor of the second conduction type, which is connected between the second power supply line and the output terminal, has a control terminal connected to the first node; a fifth transistor of the first conduction type, which is connected between the first power supply line and the first node, has a control terminal connected to the output terminal; a sixth transistor of the second conduction type, which is connected between the second power supply line and a third node, has a control terminal connected to the input terminal; a seventh transistor of the first conduction type, which is connected between the first node and the third node, has a control terminal connected to the first node; an eighth transistor of the first conduction type, which is connected between the first power supply line and the output terminal, has a control terminal connected to the first node; a ninth transistor of the second conduction type, which is connected between the second power supply line and a fourth node, has a control terminal connected to the second node; and a tenth transistor of the first conduction type, which is connected between the fourth node and the output terminal, has a control terminal connected to the output terminal.

[0027] Moreover, the level shifter of the present invention further includes an eleventh transistor of the second conduction type, which is connected between the first transistor and the third node as a substitute for the seventh transistor, has a control terminal connected to the output terminal; and a twelfth transistor of the second conduction type, which is connected between the fourth node and the output terminal as a substitute for the tenth transistor, has a control terminal connected to the first node.

[0028] In this manner, including the ninth, the tenth, the eleventh, and the twelfth transistors, allows preventing flow-through current and reducing power consumption.

[0069] Hereafter, the preferred embodiments of the present invention are described with reference to the drawings. The description is carried out in detail through embodiments.

A. First Embodiment

[0070] To begin with, the first embodiment of the present invention is described while referencing FIG. 1.

[0071] The level shifter of the first embodiment is one that level-converts a 1.2 V amplitude input signal S_IN to the level of a 3.3 V amplitude positive logic output signal S_OUT and a 3.3 V amplitude negative logic output signal S_OUTB. The level shifter of this example is configured from a first level shifter 1 and a second level shifter 2, where node N_A and node N_B in the first level shifter 1 and second level shifter 2, respectively, which are described later, are connected together, and later described node N_C and node N_D are also connected together.

[0072] The first level shifter 1 is configured from an inverter INV₂, 3.3 V-tolerant P-channel transistors M_P3 and M_P4, and 3.3 V-tolerant N-channel transistors M_N3 and M_N4.

[0073] The inverter INV₂ has a CMOS structure, which is configured from a P-channel and an N-channel transistor that are connected between the ground and the power supply of a 1.2 V supply voltage, and inverts the 1.2 V amplitude input signal S_IN so as to output an inverted input signal S_INB with an amplitude of 1.2 V. The transistor M_P3 has a 3.3 V supply voltage V_DD applied to its source, its drain connected to the transistor M_P4 drain, and its gate connected to the transistor M_P4 drain, where a 3.3 V amplitude negative logic output signal S_OUTB is output from its drain. The transistor M_P4 has a 3.3 V supply voltage V_DD applied to its source, and a 3.3 V amplitude positive logic output signal S_OUT is output from its drain.

[0074] The transistor M_N3 has the input signal S_IN applied to its gate, its drain connected to the transistor M_P3 drain, and its source is grounded. The transistor M_N4 has the inverted input signal S_INB applied to its gate, its drain connected to the transistor M_P4 drain, and its source is grounded. In other words, since the first level shifter 1 is provided with the N-channel transistors M_N3 and M_N4 in the input stage, and respective gates of the P-channel transistors M_P3 and M_P4 are connected to the other's drain, it can also be called an N-channel transistor input-type cross-connect circuit. The connecting point of the transistor M_P3 drain and the transistor M_N3 drain is the node N_A, and the connecting point of the transistor M_P4 drain and the transistor M_N4 drain is the node N_C.

[0075] The second level shifter 2 is configured from the inverter INV₂, 3.3 V-tolerant P-channel transistors M_P5 and M_P6, and 3.3 V-tolerant N-channel transistors M_N5 and M_N6.

[0076] The transistor M_P5 has the 3.3 V supply voltage V_DD applied to its source, the input signal S_IN applied to its gate, its drain is connected to the transistor M_N6 gate, and the 3.3 V amplitude negative logic output signal S_OUTB is output from its drain. The transistor M_P6 has a 3.3 V supply voltage V_DD applied to its source, the inverted input signal S_INB applied to its gate, where its drain is connected to the transistor M_N5 gate.

[0077] The transistor M_N5 has its drain connected to the transistor M_P5 drain, its gate connected to the transistor M_P6 drain, and its source is grounded. The transistor M_N6 has its drain connected to the transistor M_P6 drain, its gate connected to the transistor M_P5 drain, and its source is grounded. In other words, since the second level shifter 2 is provided with the P-channel transistors M_P5 and M_P6 at the input stage, and respective gates of the N-channel transistors M_N5 and M_N6 are connected to the other's drain, it can also be called a P-channel transistor input-type cross-connect circuit. The connecting point of the transistor M_P5 drain and the transistor M_N5 drain is the node N_B, and the connecting point of the transistor M_P6 drain and the transistor M_N6 drain is the node N_D.

[0078] It should be noted that the threshold voltage Vt for the gate voltages of the transistors M_P3 through M_P6 and M_N3 through M_N6 is assumed to be 0.7 V.

[0079] Next, the operation of the above-configured level shifter is described. To begin with, the independent operation of the first level shifter 1 in the case where the input signal S_IN having the waveform given in FIG. 2 is input is described while referencing FIG. 3 through FIG. 5.

[0080] To begin with, when the input signal S_IN is at the ‘H’ level (which corresponds to a voltage V_H, hereafter) where the signal amplitude is 1.2 V, the amplitude of 1.2 V is only 0.5 V higher than the threshold voltage Vt (0.7 V) for the transistor M_N3. At this time, the transistor M_N3 is slightly turning on or it operates in the non-saturation region (linear region). In other words, the transistor M_N3 operates with very low drive capability. The transistor M_N3 thus cannot fully turn on, and operates as a high impedance device (or operates in a high impedance state).

[0081] Therefore, the potential at the node N_A shown in FIG. 1 does not perfectly reach 0 V (GND), ultimately becoming an intermediate potential (referred to as intermediate potential V_M1 hereafter) between 0 V and the supply voltage V_DD (3.3 V) determined based on the high impedance state resistance ratio of the resistance between the source and the drain of the transistor M_N3 and the OFF state (described later) resistance between the source and the drain of the transistor M_P3. Accordingly, the transistor M_P4 is applied the above intermediate potential V_M1 to its gate, thereby gradually changing into an ON state, and the potential at the node N_C, which is an output end, gradually changes into the supply voltage V_DD (3.3 V). Here, setting the resistance between the source and the drain of the high impedance state transistor M_N3 as resistance R_N3 and the resistance between the source and the drain of the Off transistor M_P3 as R_P3, the above intermediate potential V_M1 is represented by equation (1).
V_M1=(R_N3×V_DD)/(R_N3+R_P3)  (1)

[0082] The input signal S_IN is the potential V_H, and thus the inverted input signal S_INB, as illustrated in FIG. 3, is at the “L” level (referred to as potential V_L hereafter), in this case, is 0 V. Accordingly, the transistor M_N4 is applied the above potential V_L to its gate, thereby is fully turned off, allowing the potential at the node N_C in FIG. 1 to continuously rise until reaching the supply voltage V_DD (3.3 V) in accordance with transistor M_P4 operation. It should be noted that since the transistor M_P4, as mentioned above, operates due to the intermediate potential V_M1 being applied to its gate, it does not sufficiently turn on, and has resistance between the source and the drain (low impedance state). However, since the resistance between the source and the drain of the transistor M_P4 is substantially lower than that between the source and the drain of the fully off transistor M_N4, the potential at the node N_C continuously rises until nearly reaching the supply voltage V_DD (3.3 V). In this manner, since the node N_C potential continues to rise until nearly reaching the supply voltage V_DD (3.3 V), the transistor M_P3, which has the potential of this node N_C applied to its gate, ultimately turns off.

[0083] In contrast, when the input signal S_IN is the potential V_L, as illustrated in FIG. 3, the inverted input signal S_INB becomes the potential V_H. Accordingly, as with the transistor M_N3 in the case where the above input signal S_IN is the potential V_H, the transistor M_N4 operates in the non-saturation region (linear region) In other words, the transistor M_N4 operates with very low drive capability. The transistor M_N4 thus cannot fully turn on, and operates as a high impedance device (or operates in a high impedance state). Therefore, the potential at the node N_C, which is an output end shown in FIG. 1, does not perfectly reach 0 V, ultimately becoming an intermediate potential (referred to as intermediate potential V_M2 hereafter) between 0 V and the supply voltage V_DD (3.3 V) determined based on the high impedance state resistance ratio of the resistance between the source and the drain of the transistor M_N4 and the Off resistance between the source and the drain of the transistor M_P4. Accordingly, the transistor M_P3 is applied the above intermediate potential V_M2 to its gate, thereby gradually altering into an ON state, and the potential at the node N_A gradually changes to the supply voltage V_DD (3.3 V). Here, setting the resistance between the source and the drain of the high impedance transistor M_N4 as resistance R_N4 and the resistance between the source and the drain of the Off transistor M_P4 as R_P4, the above intermediate potential V_M2 is represented by equation (2).
V_M2=(R_N4×V_DD)/(R_N4+R_P4)  (2)

[0084] Since the input signal S_IN is the potential V_L, the transistor M_N3 fully turns off, allowing the potential at the node N_A illustrated in FIG. 1 to continuously rise until reaching the supply voltage V_DD (3.3 V). It should be noted that since the transistor M_P3, as mentioned above, operates due to the potential V_M2 being applied to its gate, it does not sufficiently turn on and has resistance between the source and the drain. However, since the resistance between the source and the drain of the low impedance state transistor M_P3 is substantially lower than that between the source and the drain of the full off transistor M_N3, the potential at the node N_A continuously rises until nearly reaching the supply voltage V_DD (3.3 V). In this manner, since the node N_A potential continues to rise until nearly reaching the supply voltage V_DD (3.3 V), the transistor M_P4, which has the potential of this node N_A applied to its gate, ultimately turns off.

[0085] Organizing the independent operation of the first level shifter 1 described above results in the table given in FIG. 4, and the output signal S_OUT waveform is as illustrated in FIG. 5.

[0086] In this manner, when the first level shifter 1 has independently operated, there is the advantage that the output signal S_OUT potential rises until nearly reaching the supply voltage V_DD (3.3 V) when the input signal S_IN is the potential V_H. On the other hand, there is the disadvantage that the output signal S_OUT potential does not drop to 0 V when the input signal S_IN is the potential V_L.

[0087] It should be noted that the relationship between the input/output waveforms of the first level shifter 1 and the input voltage is as described with the prior art and is as illustrated in FIG. 25 through FIG. 30.

[0088] Next, the independent operation of the second level shifter 2 in the case where the input signal S_IN having the waveform given in FIG. 2 is input is described while referencing FIG. 6 and FIG. 7.

[0089] To begin with, when the input signal S_IN is the potential V_H, since the amplitude of 1.2 V is 0.5 V higher than the threshold voltage Vt (0.7 V) of the transistor M_P5 gate voltage, the absolute value of the potential difference between the drain and the gate is 3.3 V−1.2 V=2.1 V and 2.1 V>0.7 V. At this time, the transistor M_P5does not fully turn off but nearly turns on. Thus, the transistor M_P5 operates as a low impedance device (or operates in a low impedance state).

[0090] Meanwhile, since the input signal S_IN is the potential V_H, the inverted input signal S_INB becomes the potential V_L (0 V (GND), as illustrated in FIG. 3. The transistor M_P6 is applied the above potential V_L to its gate, thereby fully turns on, and operates to make the potential at the node N_D, which is an output end, suddenly increase to the supply voltage V_DD (3.3 V). Accordingly, since the transistor M_N5 has applied to its gate a sufficiently high voltage that is close to the supply voltage V_DD, it nearly turns on and operates to make the potential at the node N_B, which is an inverted output end, suddenly drop to 0 V.

[0091] However, as described above, since the transistor M_P5 is not fully off, the potential at the node N_B shown in FIG. 1 does not perfectly reach 0 V, and ultimately becomes an intermediate potential (referred to as potential V_M31 hereafter) between 0 V and the supply voltage V_DD (3.3 V) determined based on the resistance ratio of the resistance between the source and the drain of the low impedance state transistor M_P5 and resistance between the source and the drain of the ON state transistor M_N5. Here, setting the resistance between the source and the drain of the low impedance state transistor M_P5 as resistance R_P51 and the resistance between the source and the drain of the ON state transistor M_N5 as R_N51, the above intermediate potential V_M31 is represented by equation (3).
V_M31=(R_N51×V_DD)/(R_P51+R_N51)  (3)

[0092] In practice, since the resistance between the source and the drain of the nearly on transistor M_N5 is substantially lower than resistance between the source and the drain of the low impedance state transistor M_P5, the potential at the node N_B drops near 0 V. In this manner, since the node N_B potential drops near 0 V, the transistor M_N6, which has the potential of this node N_B applied to its gate, nearly turns off (or enters a high impedance state). As a result, the ON state of the transistor M_P6 is predominant over the high impedance state of the transistor M_N6, which assists the potential at the node N_D, which is an output end, to increase up to the supply voltage V_DD (3.3 V.) Here, the potential at the node N_D in the case where the resistance between the source and the drain of the on transistor M_P6 is set as resistance R_P61 and resistance between the source and the drain of the low impedance state transistor M_N6 is set as R_N61 is represented by equation (4) as intermediate potential V_M41.
V_M41=(R_N61×V_DD)/(R_P61+R_N61)  (4)

[0093] In contrast, when the input signal S_IN is the potential V_L, since 0 V (GND) is applied to the gate, the transistor M_P5 fully turns on. Accordingly, since the transistor M_N6 has applied to its gate a voltage that is near the supply voltage VDD, it nearly turns on, making the potential at the node N_D, which is an output end, suddenly drop to 0 V. Meanwhile, since the input signal S_IN is the potential V_L, the inverted input signal S_INB becomes the potential V_H (1.20 V), as illustrated in FIG. 3. Since the absolute value of the potential difference between the drain and the gate is 3.3 V−1. 2 V=2.1 V and 2.1 V>0.7 V, the transistor M_P6 does not fully turn off but nearly turns on. In other words, the transistor M_P6 is nearly off. Thus, the transistor M_P6 operates as a low impedance device (or operates in a low impedance state). Therefore, the potential at the node N_D, which is an output end, does not perfectly reach 0 V, ultimately becoming an intermediate potential (referred to as intermediate potential V_M42 hereafter) between 0 V and the supply voltage V_DD (3.3 V) determined based on the ratio of the resistance between the source and the drain of the low impedance state transistor M_P6 and resistance between the source and the drain of the nearly on transistor M_N6. Here, setting the resistance between the source and the drain of the low impedance state transistor M_P6 as resistance R_P62 and the resistance between the source and the drain of the nearly on transistor M_N6 as R_N62, the above intermediate potential V_M42 is represented by equation (5).
V_M42=(R_N62×V_DD)/(R_P62+R_N62)  (5)

[0094] In practice, since the resistance between the source and the drain of the nearly on transistor M_N6 is substantially lower than the resistance between the source and the drain of the low impedance state transistor M_P6, the potential at the node N_D drops close to 0 V. In this manner, since the potential at the node N_D drops close to 0 V, the transistor M_N5, which has the potential of this node N_D applied to its gate, it nearly turns off (or enters a high impedance state). As a result, since the ON state of the transistor M_P5 is predominant over the high impedance state of the transistor M_N5, the potential at the node N_B, which is an inverted output end, suddenly increases up to the supply voltage V_DD (3.3 V). Here, the potential at the node N_B in the case where the resistance between the source and the drain of the ON state transistor M_P5 is set as resistance R_P52 and the resistance between the source and the drain of the low impedance state transistor M_N5 is set as R_N52 is represented by equation (6) as intermediate potential V_M32.
V_M32=(R_N52×V_DD)/(R_P52+R_N52)  (6)

[0095] Organizing the independent operation of the second level shifter 2 described above results in the table given in FIG. 6, and the output signal S_OUT waveform of this case is as illustrated in FIG. 7.

[0096] In this manner, when the second level shifter 2 has independently operated, there is the advantage that input/output response is extremely fast since the node N_B, which is an output end, is driven by the transistors M_P6 or M_N6 that always has sufficient drive capability. On the other hand, there is the disadvantage that the output signal S_OUT potential does not rise to the supply voltage V_DD (3.3 V) when the input signal S_IN is the potential V_H, and the output signal S_OUT potential does not drop to 0 V when the input signal S_IN is the potential V_L.

[0097] The relationship between the input and output signal waveforms and voltage of the first level shifter 2 is given in FIG. 31 through FIG. 36. It should be noted that in FIG. 31 through FIG. 36, the vertical axis is set as the input signal voltage (V) and the horizontal axis as time (nS (nanoseconds)). Furthermore, the gate width of the transistors M_P5 and M_P6 is set as 20 μm and the gate width of the transistors M_N5 and M_N6 is set as 10 μm.

[0098] With the second level shifter, it can be understood from the waveform in FIG. 31 that when the input voltage is 2.0 V, the already established output signal “L” level floats up at 0.2 V instead of at GND (0 V). It can be understood from FIG. 31 through FIG. 36 that this “float” of the output signal “L” level becomes more evident as the input voltage decreases, and that the “L” level of the output signal increases to approximately 2.6 V when the input voltage is 1.0 V.

[0099] Furthermore, it can be understood from the waveforms in FIG. 34 through FIG. 36 that the “H” level of the output voltage drops from 3.3 V when the input signal is 1.3 V or lower, and drops to approximately 3.0 V when the input signal is 1.0 V.

[0100] It can further be understood from the waveform graphs in FIG. 31 through FIG. 36 that the second level shifter changes the output signal corresponding to the level change of the input signal, thereby having good tracking capability. For example, when the input signal is 2.0 V, as illustrated in FIG. 31, the time from when the input signal is changed to when the output signal reaches a stable level is approximately 0.3 nanoseconds, and when the input signal is 1.4 V, as illustrated in FIG. 33, the time from when the input signal is changed to when the output signal reaches a stable level is approximately 0.5 nanoseconds. Accordingly, it can be understood that the period until the output signal reaches a stable level in response to the input signal change is shorter for the second level shifter than for the first level shifter.

[0101] Accordingly, as shown in FIG. 38, the power supply current waveform of the second level shifter circuit is a waveform such that it rises for just a short period, however, it can be understood that since a flow-through current constantly flows through the transistors M_P5 and M_N5 or the transistors M_P6 and M_N6, a current of approximately 1.25 mA continuously flows even in locations without signal changes. It should be noted that the power supply current waveform graph of FIG. 38 is used as an example of one with an input signal of 1.4 V for the second level shifter.

[0102] In this manner, the first level shifter 1 and second level shifter 2 have the above advantages and disadvantages when each are utilized independently. Accordingly, the level shifter of this example is configured with the first level shifter 1 and second level shifter 2 where the node N_A and node N_B, and node N_C and N_D are connected to each other, respectively, so as to make the first level shifter 1 and second level shifter 2 operate to draw on each other's advantages while covering each other's shortcomings and provide the desired functions.

[0103] Hereafter, the operation of the level shifter of this example in which the first level shifter 1 and second level shifter 2 are connected is described while referencing FIG. 1 through FIG. 4, FIG. 6, FIG. 8 and FIG. 9.

(1) A case where the input signal S_IN potential changes from the potential V_L, to the potential V_H (see FIG. 2)

[0104] In this case, the potential V_H (1.2 V) of the input signal S_IN is applied to the respective gates of the transistors M_N3 and M_P5. As described above (see FIG. 4), the transistor M_N3 is not fully in an ON state but in a high impedance state. As a result, the transistor M_N3 operates so as to make the gate voltage of the transistor M_P4, potential at the node N_A and potential at the node N_B, which is connected to the node N_A, gradually decrease to 0 V. Furthermore, the transistor M_P5 is not fully in an OFF state but in a low impedance state, as described above (see FIG. 6), even if the input signal S_IN potential V_H (1.2 V) is applied to its gate.

[0105] Meanwhile, since the input signal S_IN is the potential V_H, the inverted input signal S_INB becomes the potential V_L (0 V (GND), as illustrated in FIG. 3, and the above potential V_L is then applied to the respective gates of the transistors M_N4 and M_P6. Accordingly, as described above (see FIG. 4 and FIG. 6), the transistor M_N4 is fully turned off as well as the transistor M_N4 is fully turned on. Due to the transistor MP6 being fully turned on, the potential at the node N_D, which is an output end, namely the potential of the output signal S_OUT suddenly increases up to the supply voltage V_DD (3.3 V), reaching a logic “H” level. At the same time, the transistor M_N5 fully turns on since the supply voltage (3.3 V) is applied to its gate (see FIG. 6). Accordingly, the potential at the node N_B, which is an inverted output end from where the inverted output signal S_OUTB is to be output, suddenly changes to 0 V, where as a result, the transistor M_N5 serves as a supplement to the transistor M_N3 operation of lowering the potential at the node N_B to 0 V. Furthermore, due to the transistor M_N5 being fully on, 0 V is applied to the gate of the transistor M_N6, therefore the transistor M_N6 fully turns off.

[0106] Through the operation described above, the logic level at the node N_B reaches the “L” level (0 V).

[0107] When the logic level at the node N_B reaches the “L” level (0 V), this signal is returned to the node N_A, which is the other inverted output end connected to the node N_B, therefore the transistor M_P4, which has this “L” level (0 V) applied to the gate, suddenly turns on, causing the potential at the node N_C, which is an output end, to suddenly increase to the supply voltage (3.3 V). This operation that suddenly turns on the transistor M_P4 causes the potential at the node N_D that is connected to the node N_C, namely the potential of the output signal S_OUT to increase to the “H” level (3.3 V); this operation is being carried out as tracking transistor M_P6 operation. Accordingly, the transistor M_P3 becomes fully off since the “H” level (3.3 V) is applied to the gate connected to the node N_C, and supplements the transistor M_N3 operation of lowering to 0 V the potentials at the nodes N_A and N_B, which are inverted output ends.

[0108] As described in the foregoing, the operation that fully puts the transistor M_P6 into the ON state triggers the other transistors to operate so as to establish in a chain reaction the potential logic levels at the respective drains, namely the output ends (nodes N_C and N_D) and the inverted output ends (nodes N_A and N_B). As a result, the output signal S_OUT changes extremely quickly from 0 V to the supply voltage V_DD (3.3 V).

(2) A case where the input signal S_IN potential changes from the potential V_H to the potential V_L (see FIG. 2)

[0109] Since the input signal S_IN is the potential V_L, the inverted input signal S_INB becomes the potential V_H (1.2 V) as illustrated in FIG. 3, and the above potential V_H is then applied to the respective gates of the transistors M_N4 and M_P6. As described above (see FIG. 4), the transistor M_N4 does not fully turn on but enters a high impedance state. As a result, the transistor M_N4 operates so as to make the gate voltage of the transistor M_P3, the potential at the node N_C and potential at the node N_D, which is connected to the node N_C, gradually decrease to 0 V. Furthermore, the transistor M_P6 does not fully turn off but enters a low impedance state, as described above (see FIG. 6), even if the input signal S_IN potential V_H (1.2 V) is applied to its gate.

[0110] In this case, however, the potential V_L (0 V) of the input signal S_IN is applied to the respective gates of the transistors M_N3 and M_P5. Accordingly, as described above (see FIG. 4 and FIG. 6), the transistor M_N3 fully turns off as well as the transistor M_P5 fully turns on. Due to the transistor M_P5 turning fully on, the potential at the node N_B, which is an inverted output end, namely the potential of the inverted output signal S_OUTB suddenly changes up to the supply voltage V_DD (3.3 V), reaching a logic “H” level. At the same time, the transistor M_N6 has the supply voltage (3.3 V) applied to its gate, thereby fully turns on (see FIG. 6). Accordingly, the potential at the node N_D, which is an output end from which the output signal S_OUT is to be output, suddenly changes to 0 V, and as a result, the transistor M_N6 supplements the operation of the transistor M_N4 of lowering the node N_D potential to 0 V. Furthermore, due to the transistor M_P5 turning fully on, 0 V is applied to the transistor M_N5 gate, whereby the transistor M_N5 fully turns off.

[0111] Through the operation described above, the logic level at the node N_D reaches the “L” level (0 V).

[0112] When the logic level at the node N_D reaches the “L” level (0 V), this signal is returned to the node N_A, which is the other inverted output end connected to the node N_B, therefore the transistor M_P4 that has this “L” level (0 V) applied to its gate suddenly turns on, causing the potential at the node N_C, which is an output end, to suddenly rise to the supply voltage (3.3 V). This operation that suddenly turns on the transistor M_P3 causes the potential at the node N_B that is connected to the node N_A, namely the potential of the output signal S_OUT to increase to the “H” level (3.3 V); this operation is being carried out as tracking transistor M_P5operation. Accordingly, the transistor M_P4fully turns off since the “H” level (3.3 V) is applied to the gate connected to the node N_A, and supplements the transistor M_N4 operation of lowering to 0 V the potentials at the nodes N_C and N_D, which are inverted output ends.

[0113] As described in the foregoing, the operation that fully turns on the transistor M_P5 triggers the other transistors to operate so as to establish in a chain reaction the potential logic levels of the respective drains, namely the output ends (nodes N_C and N_D) and the inverted output ends (nodes N_A and N_B). As a result, the output signal S_OUT changes extremely quickly from the supply voltage V_DD (3.3 V) to 0 V (GND).

[0114] As described above, in the level shifter of this example, the second level shifter 2 has the potential V_L (0 V) of the input signal S_IN always applied to either the gate of the transistor M_N5 or the transistor M_P6. Accordingly, the transistor M_P5 or M_P6 applied with the potential V_L (0 V) carries out a fast drive operation in the saturation region, operates to raise to the supply voltage (3.3 V) the potential at the output end (referred to as first output end for convenience), which is the drain end, and operate to lower to 0 V the potential at the output end (referred to as second output end for convenience), which is the drain end, of the other transistor M_P6 pr M_P5. Simultaneously, the first level shifter 1 operates to raise the potential at the output end to the supply voltage (3.3 V) and to lower to 0 V the potential at the output end so as to concur with the second level shifter 2 operation.

[0115] As a result, according to the level shifter of this example, stably establishing the logic levels of the output signal S_OUT and the inverted output signal S_OUTB to be at either one of the “L” level or “H” level is possible. Here, an example of the waveform of the output signal S_OUT and the inverted output signal S_OUTB to be output from the level shifter of this example is given in FIG. 8 and FIG. 9.

[0116] Furthermore, as described above, the respective output ends (node N_A and node N_B) of the first level shifter 1 and the second level shifter 2 are connected to each other, and the respective inverted output ends (node N_C and node N_D) are also connected to each other. Accordingly, the signal output from the drain of either of the transistor M_P5 or M_P6 turned on is returned to the first level shifter 1 so as to completely complement the operation of either of the transistor M_P3 or M_P4; where the transistors M_P5 and M_P6 configure the second level shifter 2, and the transistors M_P3 and M_P4 configure the first level shifter 1. At the same time, the drain of either of the transistor M_P6 or M_P5 that enters the low impedance state is always lead to establishment of its potential by the high impedance state transistor M_N4 or M_N3; where the transistors M_N4 and M_N3 configures the first level shifter 1. Therefore, the slow response speed can be complemented.

[0117] Accordingly, this exemplary level shifter is capable of normally operating even if the output signal voltage amplitude is more than double the input signal voltage amplitude.

[0118] Furthermore, this exemplary level shifter, as described above, is tolerant to input noise and power noise and is thus capable of stable operation since the operation of the high impedance state transistors M_N3 and M_N4, which configure the first level shifter 1, are always supplemented by the fully on transistors M_N5 and M_N6, which configure the second level shifter 2; in addition, the fully on transistors M_N3 and M_N4, which configure the first level shifter 1, supplement the operation of the low impedance state transistors M_N5 and M_N6, which configure the second level shifter 2.

[0119] Furthermore, this exemplary level shifter operates at a high operating frequency since the first level shifter 1 operates so as to supplement the operation of the second level shifter 2 while taking advantage of the fast response time of the second level shifter 2. Accordingly, this exemplary level shifter is capable of reliably performing level shift operations even for high frequency input signals.

[0120] Furthermore, since this exemplary level shifter has a short period from when the input signal has changed to when the output signal reaches a stable level, the time a current passes through the level shifter is also short, thereby reducing power consumption.

[0121] Furthermore, this exemplary level shifter, in the case where the difference in voltage level between the input signal and the output signal is small, may have desired characteristics without systematically unbalancing the drive capability (gate width) of the transistor M_P4 and drive capability (gate width) of the transistor M_N4, which configure the level shifter.

[0122] Next, simulation results when input voltage is varied for the level shifter of FIG. 1 are given in FIG. 10 through FIG. 15 based on the assumption that the transistor M_P3 gate width to be 2 μm, the transistor M_P4 gate width to be 2 μm, the transistor M_N3 gate width to be 60 μm, the transistor M_N3 gate width to be 60 μm, the transistor M_P5 gate width to be 8 μm, the transistor M_P6 gate width to be 8 μm, the transistor M_N5 gate width to be 2 μm, and the transistor M_N4 gate width to be 2 μm. It should be noted that in FIG. 10 through FIG. 15, the vertical axis represents input signal voltage (V) and the horizontal axis represents time (nS (nanoseconds)).

[0123] It can be understood from FIG. 10 through FIG. 15 that the output voltage amplitude remains steady at roughly 3.3 V even if the input voltage is varied as 2.0 V, 1.5 V, 1.4 V, 1.3 V, 1.2 V, and 1.0 V, and that the duty ratio barely changes. Nevertheless, there is the problem that the output voltage lower limit (“L” level) floats slightly above 0 V as the input voltage decreases. It can be seen from FIG. 15 that this “float” in voltage is approximately 0.2 V when 1.0 V is input. This arises from a flow-through current passing through the second level shifter 2, whereby the level shifter waveform itself becomes cause for power consumption increase although waveform shaping may be easily performed through an inverter receiving the output voltage. Accordingly, level shifters with reduced power consumption are described hereafter with the second and the third embodiment.

B. Second Embodiment

[0124] Next, the second embodiment of the present invention is described.

[0125] FIG. 16 is a circuit diagram illustrating the configuration of a level shifter, which is the second embodiment of the present invention. In this diagram, the same reference numerals are applied to the same parts corresponding to each part of FIG. 1, wherewith duplicate descriptions will be omitted. Replacing the second level shifter 2 shown in FIG. 1, a second level shifter 10 is newly provided to the level shifter illustrated in FIG. 16. The first level shifter 1 and the second level shifter 10 have a node N_A and node N_B that are connected, as well as a node N_C and node N_D that are connected. This exemplary level shifter operates such that the first level shifter 1 and second level shifter 10 complement each other's shortcomings, thus can be called a complementary level shifter.

[0126] The second level shifter 10 is configured from the inverter INV₂, 3.3 V-tolerant P-channel transistors M_P5, M_P6, M_P10 and M_P11, and 3.3 V-tolerant N-channel transistors M_N5 and M_N6. The transistor M_P5 has a 3.3 V supply voltage V_DD applied to its source and an input signal S_IN applied to its gate, where its drain is connected to the transistor M_P10 source. The transistor M_P6 has a 3.3 V supply voltage V_DD applied to its source and an inverted input signal S_INB applied to its gate, where its drain is connected to the transistor M_P11 source. The transistor M_P10 has its gate connected to the transistor M_P11 drain, where that drain is connected to the transistor M_N5 drain, as well as a negative logic 3.3 V amplitude inverted output signal S_OUTB is output from that drain. The transistor M_P11 has its gate connected to the transistor M_P10 drain, where that drain is connected to the transistor M_N6 drain, as well as a positive logic 3.3 V amplitude output signal S_OUT is output from that drain. The transistor M_N5 has its gate connected to the transistor M_P11 drain, and its source is grounded. The transistor M_N6 has its gate connected to the transistor M_P10 drain, and its source is grounded.

[0127] In other words, since the second level shifter 10 is provided with the P-channel transistors M_P5 and M_P6 at the input stage, and respective gates of the N-channel transistors M_N5 and M_N6 are connected to the other's drain, it can also be called a P-channel transistor input-type cross-connect circuit. The connecting point of the transistor M_P10 drain and the transistor M_N5 drain is the node N_B, and the connecting point of the transistor M_P11 drain and the transistor M_N6 drain is the node N_D.

[0128] It should be noted that the threshold voltage Vt for the gate voltage of all the transistors M_P5, M_P6, M_P10, M_P11, M_N5 and M_N6 is assumed to be 0.7 V.

[0129] In the above-configured level shifter, the second level shifter 10 has the transistor M_P10 directly connected between the transistor M_P5 drain and the transistor M_N5 drain, and the output signal S_OUT used as feedback input for the transistor M_P10 gate. Furthermore, the transistor M_P11 is directly connected between the transistor M_P6 drain and the transistor M_N6 drain, and the inverted output signal S_OUTB is used as feedback input for the transistor M_P11 gate. This is due to the reason given below.

[0130] Namely, with the level shifter according to the first embodiment described above, the transistors M_P5 and M_P6 enters a low impedance state, as indicated in FIG. 6, when the input signal S_IN or the inverted input signal S_INB with the potential V_H (1.2 V) is applied to the respective gates. Consequently, a steady yet minute flow-through current passes through the respective source and drain of the transistors M_P5 and M_P6, even after the potentials of the output signals S_OUT and S_OUTB have reached stable levels.

[0131] In this example, due to the transistor M_P10 directly connecting between the transistor M_P5 drain and the transistor M_N5 drain, and using the output signal S_OUT as feedback input for the transistor M_P10 gate, the transistor M_P10 fully turns off by the “H” level (3.3 V) output signal S_OUT so as to completely interrupt at the transistor M_P10 the above steady flow-through current that flows through between the transistor M_P5 source and drain. Similarly, due to the transistor M_P11 directly connecting between the transistor M_P6 drain and the transistor M_N6 drain, and using the inverted output signal S_OUTB as feedback input for the transistor M_P11 gate, the transistor M_P11 fully turns off by the “H” level (3.3 V) inverted output signal S_OUTB so as to completely interrupt at the transistor M_P11 the above steady flow-through current that passes through between the source and the drain of the transistor M_P6.

[0132] In this manner, according to this exemplary configuration, the advantages that can be achieved in the first embodiment described above, as well as complete interruption of flow-through current allows further reduction in power consumption.

[0133] In this second embodiment, simulation results when input voltage is varied are given in FIG. 17 through FIG. 22 based on the assumption that the transistor M_P3 gate width to be 2 μm, the transistor M_P4 gate width to be 2 μm, the transistor M_N3 gate width to be 60 μm, the transistor M_N3 gate width to be 60 μm, the transistor M_N5 gate width to be 2 μm, the transistor M_N4 gate width to be 2 μm, the transistor M_P10 gate width to be 2 μm, and the transistor M_P11 gate width to be 2 μm. It should be noted that in FIG. 17 through FIG. 22, the vertical axis represents input signal voltage (V) and the horizontal axis represents time (nS (nanoseconds)).

[0134] It can be understood from FIG. 17 through FIG. 22 that the output voltage amplitude remains steady at roughly 3.3 V even if the input voltage is varied as 2.0 V, 1.5 V, 1.4 V, 1.3 V, 1.2 V, and 1.0 V, and that the duty ratio also barely changes. Furthermore, there was the problem of the output voltage lower limit slightly floating above 0 V in the first embodiment, however it is understood in the second embodiment that this “float” does not occur even if the input voltage is reduced.

[0135] Here, source current waveforms of the first embodiment and the second embodiment are described.

[0136] The source current waveform of the first embodiment, as shown in FIG. 39, has a large current flowing only during the period from when the input signal has changed to when the output signal reaches a stable level, therefore further reduction in power consumption than in the prior art shown in FIG. 37 and FIG. 38 can be seen. Nevertheless, in the first embodiment as described above, since the flow-through current is flowing through the second level shifter, an unintended current of approximately 0.8 mA is constantly flowing. In contrast, in the case of the source current waveform of the second embodiment, as shown in FIG. 40, a current that flows only during the period from when the input signal has changed to when the output signal reaches a stable level without the current that was constantly passing through. Accordingly, further reduction in power consumption than in the first embodiment can be seen. It should be noted that in FIG. 39 and FIG. 40, for the sake of comparing with the conventional waveforms in FIG. 37 and FIG. 38, the example used for the level shifter of the present invention is one where the input signal is 1.4 V.

[0137] Hereafter is described a third embodiment, which is a modification of the second embodiment.

C. Third Embodiment

[0138] Next, the third embodiment of the present invention is described.

[0139] FIG. 11 is a circuit diagram illustrating the configuration of a level shifter, which is the third embodiment of the present invention. In this diagram, the same reference numerals are applied to the same parts corresponding to each part of FIG. 1, wherewith duplicate descriptions will be omitted. Replacing the second level shifter 2 shown in FIG. 1, a second level shifter 11 is newly provided to the level shifter illustrated in FIG. 11. The first level shifter 1 and the second level shifter 11 have a node N_A and node N_B that are connected to each other, as well as a node N_C and node N_D that are connected to each other. This exemplary level shifter operates such that the first level shifter 1 and second level shifter 11 complement each other's shortcomings, thus can be called a complementary level shifter.

[0140] The second level shifter 11 is configured from the inverter INV₂, 3.3 V-tolerant P-channel transistors M_P5 and M_P6, and 3.3 V-tolerant N-channel transistors M_N5 M_N6, M_N10 and M_N11.

[0141] The transistor M_P5 has a 3.3 V supply voltage V_DD applied to its source and an input signal S_IN applied to its gate, where its drain is connected to the transistor M_P10 drain. The transistor M_P6has a 3.3 V supply voltage V_DD applied to its source and an inverted input signal S_INB applied to its gate, where its drain is connected to the transistor M_P11 drain. The transistor M_N10 has its gate and source connected to the transistor M_N5 drain, as well as a negative logic 3.3 V amplitude inverted output signal S_OUTB is output from that source. The transistor M_N11 has its gate and source connected to the transistor M_N6 drain, as well as a positive logic 3.3 V amplitude output signal S_OUT is output from that source. The transistor M_N5 has its gate connected to the transistor M_N6 drain, and its source is grounded. The transistor M_N6 has its gate connected to the transistor M_N5 drain, and its source is grounded.

[0142] In other words, since the second level shifter 11 is provided with the P-channel transistors M_P5 and M_P6 at the input stage, and respective gates of the N-channel transistors M_N5 and M_N6 are connected to the other's drain, it can also be called a P-channel transistor input-type cross-connect circuit. The connecting point of the transistor M_N10 source and the transistor M_N5 drain is the node N_B, and the connecting point of the transistor M_N11 source and the transistor M_N6 drain is the node N_D.

[0143] It should be noted that the threshold voltage Vt for the gate voltage of all the transistors M_P5, M_P6, M_N5, M_N8, M_N10 and M_N11 is assumed to be 0.7 V.

[0144] In the above-configured level shifter, the second level shifter 11 has the transistor M_N10 directly connected between the transistor M_P5 drain and the transistor M_N5 drain, and the inverted output signal S_OUTB used as feedback input for the transistor M_N10 gate. Furthermore, the transistor M_N11 is directly connected between the transistor M_P6 drain and the transistor M_N6 drain, and the output signal S_OUT is used as feedback input for the transistor M_P11 gate. This is due to the reason given below.

[0145] Namely, with the level shifter according to the first embodiment described above, the transistors M_P5 and M_P6 enters a low impedance state, as indicated in FIG. 6, when the input signal S_IN or the inverted input signal S_INB with the potential V_H (1.2 V) is applied to the respective gates. Consequently, a steady yet minute flow-through current passes through the respective source and drain of the transistors M_P5 and M_P6, even after the potentials of the output signals S_OUT and S_OUTB are established.

[0146] In this example, due to the transistor M_N10 directly connecting between the transistor M_P5 drain and the transistor M_N5 drain, and using the inverted output signal S_OUTB as feedback input for the transistor M_N10 gate, the transistor M_N10 fully turns off by the “L” level (0 V) inverted output signal S_OUTB so as to completely interrupt at the transistor M_N10 the above steady flow-through current that flows between the source and the drain of the transistor M_P5. Similarly, due to the transistor M_N11 directly connecting between the transistor M_P6 drain and the transistor M_N6 drain, and using the output signal S_OUT as feedback input for the transistor M_N11 gate, the transistor M_N11 fully turns off by the “L” level (0 V) output signal S_OUT so as to completely interrupt at the transistor M_N11 the above unintended constant flow-through current that flows between the source and the drain of the transistor M_P6.

[0147] In this manner, according to this exemplary configuration, the advantages that can be achieved in the first embodiment described above, as well as complete interruption of flow-through current as in the second embodiment allows further reduction in power consumption.

[0148] To this point, the embodiments of the present invention have been described with reference to the drawings, however, the specific configuration is not limited to these embodiments, wherewith modifications and such can be included in this invention without deviating from the technical spirit and scope of the present invention.

[0149] For example, although the example where the amplitude of the input signal S_IN is 1.2 V and the output signal S_OUT is 3.3 V is given for each of the embodiments described above, the present invention is not limited thereto, but also applicable in cases where the output signal voltage amplitude is more than double the input signal voltage amplitude.

[0150] Furthermore, the example where the inverter INV₂ has a CMOS structure, which is configured from a P-channel transistor and an N-channel transistor that have 1.2 V-tolerant characteristics with a supply voltage of 1.2 V, and inverts the 1.2 V amplitude input signal S_IN so as to output an inverted input signal S_INB with an amplitude of 1.2 V is given for each of the embodiments described above, yet is not limited thereto. Namely, the transistors configuring the inverter INV₂ may be configured with a transistor having a high voltage tolerant characteristic (for example, 3.3 V) that is higher than the supply voltage (1.2 V). It should be noted that in order to improve operation speed of the level shifter, it is preferable to quickly operate the inverter INV₂, whereby configuring with a transistor with small gate capacitance and low voltage tolerance is desirable.

[0151] Furthermore, for each of the embodiments described above, an inverter is described as an example of a circuit that generates inverted signals from input signals, however, appropriate substitution is possible as long as it is a NAND, NOR, or a circuit that generates inverted signals. It should be noted that with each of the embodiments, an inverter circuit with little delay time and simple circuitry is employed.

[0152] Furthermore, it is preferable that the gate width of the transistors M_N3 and M_N4 is larger than that of other transistors in order to quickly discharge the node N_A and node N_C.

[0153] Furthermore, N-channel and P-channel MOS transistors are used for describing each of the embodiments described above, yet is not limited to the MOS transistor, where substitution of other transistors such as a bipolar transistor is also possible.

[0154] It should be noted that by mounting the level shifter of the present invention upon a chip as a semiconductor integrated circuit, the difference in voltage inside and outside the chip may be easily decreased, thereby chip versatility may be enhanced.

[0155] Furthermore, even with an information processing system that has this chip mounted on a motherboard or the like, the voltage difference from other chips and the like may easily be decreased, whereby simplicity of design of the information processing system may be enhanced.

[0156] As described above, even if the input signal voltage amplitude drops near the transistor threshold, a low power consumption level shifter that operates steadily and at high speed and is configured with the first level shifter and second level shifter is provided.

[0157] Furthermore, by adding a transistor for blocking the flow-through current to the second level shifter, further reduction in power consumption is possible.