Abstract:
A bipolar/CMOS integrated circuit uses an on-chip amplifier to provide an intermediate voltage supply (18) to two groups of small geometry CMOS circuits. Bipolar devices (24) may use a full five volts from the outside supply rails (12, 14).

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to integrated circuits, more particularly to a MOS transistor integrated circuit having reduced voltages. 
     BACKGROUND OF THE INVENTION 
     Present day CMOS integrated circuits use a 5 volt power supply. As the size of the CMOS devices are reduced, the voltage that the device can tolerate is similarly reduced. Consequently, a smaller power supply voltage will be needed to accommodate the smaller CMOS devices. 
     In order to maintain compatibility with CMOS integrated circuits using a 5 volt power supply, as well as other technologies such as bipolar and ECL, it is desirable that the new CMOS integrated circuits also use a 5 volt power supply at its external pins. Internally, however, it is necessary to provide a smaller voltage, such as 2.5-3.0 volts for the CMOS devices. 
     One alternative is to use an on-chip voltage regulator which is powered by 5 volts and outputs a smaller voltage, such as 2.5 volts. Such an arrangement is inefficient since the excess voltage would be dissapated as heat, and is therefore undesirable from both a power and a reliability standpoint. 
     Therefore, a need has arisen in the industry to provide an integrated circuit having a 5 volt power supply input which efficiently supplies the CMOS devices comprising the integrated circuit with a reduced voltage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an integrated circuit and method of forming the same is provided which substantially eliminates problems associated with integrated circuits incorporating low voltage circuit blocks. 
     The integrated circuit of the present invention provides first and second voltage supply rails, typically biased at 0 and 5 volts, respectively. CMOS circuit blocks are connected in series between the first and second supply rails with an intermediate supply rail connected between the circuit blocks. Hence, each pair of circuit blocks receives the 5 volts across the first and second supply rail, one circuit block receiving a voltage equal to the potential between the first supply rail and the intermediate supply rail and the second circuit block receiving a voltage equal to the potential between the intermediate supply rail and the second supply rail. An amplifier is connected between the first and second supply rails to supply current to the intermediate supply rail in order to maintain the voltage thereon a predetermined intermediate voltage. 
     The present invention provides the technical advantage that devices requiring a reduced voltage may be used on an integrated circuit which receives conventional voltage supplies to be consistent with other integrated circuits on the circuit board. The present invention is more efficient than a on-chip voltage regulator, which would dissipate power at a 5 volt level, while performing at a 2.5 (or other intermediate) volt level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description now taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a block diagram of a circuit using the present invention; and 
     FIG. 2 illustrates a circuit diagram of two CMOS circuit blocks connected between the voltage supply rails and the intermediate supply rail. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention is best understood by referring to FIGS. 1-2 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 illustrates a block diagram of an integrated circuit using low voltage CMOS devices. The integrated circuit 10 receives its voltage from a 5 volt (relative to ground) voltage supply rail 12 and a ground voltage supply rail 14. Amplifier 16 is connected between the voltage supply rails 12 and 14 and is connected to a 2.5 volt supply rail 18. Low voltage CMOS circuits 20 are grouped in pairs 22 of serially connected circuits 20 such that each CMOS pair 22 is connected across the voltage supply rails 12 and 14. Hence, each CMOS circuit pair 22 receives 5 volts. The 2.5 volt supply rail 18 is connected between the CMOS circuits 20 comprising each circuit pair 22. Bipolar circuits 24 are connected across the voltage supply rail 12 and 14. 
     In operation, each CMOS circuit pair 22 receives a voltage of 5 volts. The 2.5 volt supply rail 18 is maintained at 2.5 volts by the amplifier 16 which supplies the unbalanced current to the 2.5 volt supply rail 18. Hence, each of the CMOS circuits 20 individually, receives 2.5 volts. 
     FIG. 2 illustrates a circuit diagram of two low voltage CMOS circuits 20a and 20b as used in the integrated circuit 10 of the present invention. The first CMOS circuit 20a, an inverter, comprises a PMOS transistor 26 and NMOS transistor 28. The PMOS transistor 26 has a first source/drain connected to the 5 volt supply rail 12 and a second source/drain connected to a source/drain of the NMOS transistor 28. The other source/drain of the NMOS transistor 28 is connected to the 2.5 volt supply rail 18. The second CMOS device 20b could also comprise a PMOS transistor 30 and an NMOS transistor 32. The PMOS transistor 30 has a first source/drain connected to the 2.5 volt supply rail 18 and a second source/drain connected to a source/drain of the NMOS transistor 32. The other source/drain of the NMOS transistor 32 is connected to the ground supply rail 14. 
     For purposes of illustration, CMOS circuits 20a-b are shown as inverters. These circuits are shown for exemplary purposes; more complicated circuits could be similarly implemented as would be understood by one skilled in the art. 
     In each CMOS circuit pair 22, one of the CMOS circuits 20 will have an NMOS transistor with a 2.5 volt backgate bias. Consequently, the N channel devices on opposite sides of the 2.5 volt supply rail 18 may have to be fabricated differently. Two alternative schemes may be used to compensate for the 2.5 volt backgate bias. The simplest compensation technique is to adjust the threshold voltage of one NMOS device differently than the other. The second compensation technique is to isolate each NMOS device in a P-well by using a buried N+diffusion such that each NMOS transistor sits in its own P-well. The P channel devices do not suffer from the same problem, since each P channel device typically sits in its own N-well. 
     It should be noted that while the preferred embodiment shows a 2.5 volt supply rail 18, this intermediate supply rail could be held at any voltage between 0 and 5 volts, as desired. For example, the supply rail 18 could be held at 3 volts, providing for 2 volt CMOS devices to be implemented between the intermediate supply rail 18 and the 5 volt voltage supply rail 12 and 3 volt CMOS devices being implemented between the ground supply rail 14 and the intermediate supply rail 18. 
     The present invention provides the technical advantage that CMOS circuits may be operated at reduced voltages, thereby avoiding hot electron effects, without the need for an on-chip regulator. On-chip regulators are undesirable, since they dissipate power at a 5 volt level, while providing the performance of a reduced supply. 
     It should be noted that while the present invention has been discussed in connection with 0 and 5 volt external voltage supplies, the concepts embodied in the invention would be useful regardless of the actual external voltages used. It should also be noted that a plurality of intermediate supply rails 18 could be used to split the voltage from the outside supply rails 12 and 14 into more than two increments. 
     Although the present invention has been described in detail, it should be understood that various changes substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.