Abstract:
An input signal is received by a level shift circuit to generate a plurality of level-shifted output signals which have different shift amounts to each other. A switch circuit, selectively outputs the level-shifted output signals in response to a logic level of the input signal. The switch circuit selects a signal having a higher potential from the level-shifted output signals when the logic level of the input signal indicates a first level, and selects a signal having a lower potential from the level-shifted output signals when the logic level of the input signals indicates a second level.

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to a level conversion circuit for converting binary logic signals having a certain amplitude into binary logic signals having another amplitude. 
     2) Description of the Related Art 
     Generally, since digital circuit elements have a general purpose, a so-called standard logic is predetermined for the digital circuit elements. As the standard logic, there are emitter-coupled logic (ECL) having an amplitude of about 1 V, transistor-transistor logic (TTL) having an amplitude of about 1 V, CMOS logic having an amplitude of about 2 V, and the like. Therefore, a logic level conversion is required in signal transmission between different standard logic levels, and a level conversion circuit is used for that purpose. 
     In a prior art level conversion circuit, binary logic input signals having an amplitude are received by two input transistors, and the potentials of the binary logic input signals are shifted down by diodes. Then, the level-shifted binary logic signals are supplied to a differential amplifier, thereby obtaining binary logic output signals having another amplitude, which will be explained later in detail. 
     In the prior art, however, the differential amplifier cannot operate at high speed unless a large current is supplied thereto. Therefore, the prior art level conversion circuit having high speed of operation also has a large power consumption. 
     Also, in the prior art, various levels of binary logic output signals cannot be generated. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a level conversion circuit having reduced power consumption and being capable of easily generating various levels of binary logic output signals. 
     According to the present invention, a input signal is received by a level shift circuit to generate a plurality of level-shifted output signals which have different shift amounts to each other. A switch circuit, selectively outputs the level-shifted output signals in response to a logic level of the input signal. The switch circuit selects a signal having a higher potential from the level-shifted output signals when the logic level of the input signal indicates a first level, and selects a signal having a lower potential from the level-shifted output signals when the logic level of the input signal indicates a second level. 
     In the present invention, the switch is used instead of the differential amplifier, thus reducing the power consumption. Also, the number of stages of the level shift circuit can be changed to generate various levels of binary logic output signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood with reference to the accompanying drawings, wherein: 
     FIGS. 1 and 2 are block circuit diagrams illustrating memory devices including a level conversion circuit; 
     FIG. 3 is a circuit diagram illustrating a prior art level conversion circuit; 
     FIG. 4 is a circuit diagram illustrating an embodiment of the level conversion circuit according to the present invention; 
     FIGS. 5 and 6 are timing diagrams showing the operation of the circuit of FIG. 4; 
     FIG. 7 is a detailed circuit diagram of the level shift circuit of FIG. 4; and 
     FIG. 8 is a circuit diagram of a modification of the level shift circuit of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before the description of an embodiment, a prior art level conversion circuit will be explained in detail. 
     In FIG. 1, which illustrates a bipolar complementary MOS (BiCMOS) static random access memory, a TTL level signal is input and a TTL level signal is output. In FIG. 1, reference numeral 1 designates an address buffer which receives address signals Ai (i=0, 1, 2, . . . ) of a TTL level from 0.5 V to 1.5 V to generate signals Ai and their inverted signals Ai. The signals Ai and Ai are decoded by an address decoder 2 which accesses, i.e., reads one cell from a memory cell array 3. Data read from the memory cell array 3 is sensed by a sense amplifier 4. Although the sensed data is output as output data D OUT  from an output buffer 5, a level conversion circuit 6 is required therebetween. That is, the sense amplifier 4 is operated at an ECL level, while the output buffer 5 is operated at a TTL level. Therefore, the ECL level in the sense amplifier 4 is converted by the level conversion circuit 6 to the TTL level. 
     Although FIG. 2 also illustrates a BiCMOS static random access memory, an ECL level signal is input and an ECL level signal is output. In FIG. 2, reference numeral 1&#39; designates an address buffer for receiving an ECL level address signal Ai (i=0, 1, 2, ...) to generate signals Ai and their inverted signal Ai. 2&#39; designates an address decoder, 3&#39; a memory cell array, 4&#39; a sense amplifier, 5&#39; an output buffer. In this case, the signals in the address buffer 1&#39; are at an ECL level, while the signals in the address decoder 2&#39; are at a CMOS level. Therefore, a level conversion circuit 6&#39; is required between the address buffer 1&#39; and the address decoder 2&#39;. 
     In FIG. 3, which illustrates a prior art level conversion circuit for converting an ECL level having an amplitude of about 1 V to a CMOS level having an amplitude of about 2 V, the level conversion circuit is formed by a level shift circuit 31 and a CMOS current mirror differential amplifier 32. 
     The level shift circuit 31 includes input bipolar transistors Q 1  and Q 2 , level shift diodes D 1  and D 2 , and current sources CS 1  and CS 2 . Applied to the bases of the transistors Q 1  and Q 2  are input signals IN and IN having ECL levels opposite to each other and having a level difference of about 1 V (for example, IN=5 V and IN=4.2 V). The ECL input level of the input signals IN and IN is dropped by 0.7 V due to the base-emitter voltage V be  of the transistor Q 1  and 0.7 V of the diode D 1  and is supplied to the differential amplifier 32. 
     The differential amplifier 32 has P-channel MOS (broadly, MIS) transistors Q p1  and Q p2  and N-channel MOS transistors Q n1  and Q n2 . Applied to the gates of the MOS transistors Q n1  and Q n2  are output signals from the level shift circuit 31. The drains of the transistors Q n1  and Q n2  generate output signals OUT and OUT whose potentials are, for example, 2 V and 3 V, respectively. If the input level of the differential amplifier 32 is not at an intermediate level between the power supply V cc  such as 5 V and ground V ss  such as 0 V, the operation speed is low. That is, if the input level is too high, a discharging current from the next stage such as the address buffer 2&#39; of FIG. 2 is too large due to the ON-transistor Q n1  or Q n2 , while a charging current by the transistor Q p1  or Q p2  to the next stage is too small. Conversely, the level is too low, a discharging current from the next stage is too small due to the ON-transistor Q n1  or Q n2 , while a charging current from the transistor Q p1  or Q p2  to the next stage is too large. Both of the above-mentioned cases invites a low speed operation. Thus, the input level is shifted down by the diodes D 1  and D 2  to input to the differential amplifier 32. 
     Further, even if the input level of the differential amplifier 32 is appropriately at an intermediate level between V cc  and V ss , in order to further increase the speed of operation, the total current flowing through the differential amplifier 32 as well as the total current flowing through the level shift circuit 31 has to be increased, to thereby increase the charging current to the next stage. 
     Also, one of the input signals IN and IN is high, one of the transistors Q n1  and Q n2  is turned ON and therefore, a current path is always created between V cc  and V ss , thus increasing the power consumption. 
     Further, the output difference (MOS level difference in the above-mentioned example) cannot be easily obtained at an arbitrary level. That is, if ΔV IN  is an input difference to the differential amplifier 32 and ΔV OUT  is an output difference to the differential amplifier 32, 
     
         ΔV.sub.OUT =α·ΔV.sub.IN 
    
     where α is an amplification factor of the differential amplifier 32. Therefore, the output difference ΔV OUT  dependent only upon ΔV IN  cannot be obtained at an arbitrary level. 
     In FIG. 4, which illustrates an embodiment of the level conversion circuit according to the present invention, reference numeral 41 designates a level shift circuit and 42 designates a switch circuit. 
     The level shift circuit 41 includes input bipolar transistors Q 1  and Q 2 , level shift diodes D 11 , D 12 , . . . , D 21 , D 22 , . . . , and current sources CS 1  and CS 2 . In this level shift circuit 41, as shown in FIG. 5, the input signals In and IN having a first level amplitude are applied to the bases of the transistors Q 1  and Q 2 , the potentials of the input signals IN and IN are level-shifted by the base-emitter voltage V be  of the transistors Q 1  and Q 2 , to obtain the potentials at nodes A and A. Also, the potentials of the signals A and A are level-shifted by one stage of the diodes D 11  and D 21 , to obtain the potentials at nodes B and B. Further, the potentials of the signals B and B are level-shifted by one stage of the diodes D 12  and D 22 , to obtain the potentials at nodes C and C. 
     On the other hand, the switch circuit 42 includes a pair of transistors comprising P-channel MOS transistor Q p1  and N-channel MOS transistor Q n1 , and a pair of transistors comprising P-channel MOS transistor Q p2  and N-channel MOS transistor Q n2 . The pair of transistors comprising P-channel MOS transistor Q p1  and N-channel MOS transistor Q n1  are connected between nodes A and C, and therefore, the transistors Q p1  and Q n1  are powered by the potentials at nodes A and C. Also, the gates of the transistors Q p1  and Q n1  are connected to nodes C and A, respectively, and therefore, the transistors Q p1  and Q n1  are controlled by the potentials at nodes C and A, respectively. Similarly, the pair of transistors comprising P-channel MOS transistor Q p2  and N-channel MOS transistor Q n2  are connected between nodes A and C, and therefore, the transistors Q p2  and Q n2  are powered by the potentials at nodes A and C. Also, the gates of the transistors Q p2   and Q n2  are connected to nodes C and A, respectively, and therefore, the transistors Q p2  and Q n2  are controlled by the potentials at nodes C and A, respectively. 
     When the potentials of the input signals IN and IN are high and low, respectively, the P-channel MOS transistor Q p1  and the N-channel MOS transistor Q n2  are turned ON and the P-channel MOS transistor Q p2  and the N-channel MOS transistor Q n1  are turned OFF. As a result, the potential at the output terminal OUT is the same as that at node A, and the potential at the output terminal OUT is the same as that at node C. In this case, the amplitude of the output terminals OUT and OUT is A - C, as shown in FIG. 6. 
     Similarly, when the potentials of the input signals IN and IN are low and high, respectively, the P-channel MOS transistor Q p2  and the N-channel MOS transistor Q n1  are turned ON and the P-channel MOS transistor Q p1  and the N-channel MOS transistor Q n2  are turned OFF. As a result, the potential at the output terminal OUT is the same as that at node C, and the potential at the output terminal OUT is the same as that at node A. In this case, the amplitude of the output terminals OUT and OUT is A- C, as shown in FIG. 6. 
     In a stationary state, one of the transistors Q p1  and Q n1 , and one of the transistors Q p2  and Q n2  are turned OFF, and therefore, there is no power consumption. Therefore, since power is consumed only in the level shift circuit 41, the operation current can be reduced to suppress the power consumption. Also, a level conversion at an arbitrary level can be easily carried out only by changing the number of stages of diodes. That is, when the connections between the level shift circuit 41 and the switch circuit 42 are changed, various level binary output signals can be obtained. 
     In FIG. 7, which is a detailed circuit diagram of the level shift circuit 41 of FIG. 4, the diodes D 11 , D 12 , . . . , D 21 , D 22 , . . . are constructed by base-collector-connected bipolar transistors, and the current sources C S1  and C S2  are constructed by N-channel MOS transistors whose gates receive a definite volta V REF  such as 2V. 
     In FIG. 8, which is a modification of the circuit of FIG. 7, the level shift circuit 41 includes two input N-channel MOS transistors Q 1  &#39; and Q 2  &#39;, and diodes D 11  &#39;, D 12  &#39;, . . . , D 21  &#39;, D 22  &#39;, . . . which are formed by gate-drain connected N-channel MOS transistors. In this case, no current sources are necessary. 
     The level shift circuit 41 as illustrated in FIG. 8 can operate in the same way as the level shift circuit 41 as illustrated in FIG. 7. 
     According to the present invention, since the switch circuit has very low power consumption, the operation current can be reduced to suppress the power consumption, and level conversion at an arbitrary level can be easily carried out at high speed.