Abstract:
This invention relates to a voltage regulator particularly suitable for powering a submicron DRAM. The regulator relies on a feed forward approach in which current to a load is controlled by a differential amplifier which provides a control signal to a current regulating transistor based on the difference in a voltage sensed at the regulator output and a reference voltage. The control signal is also suppied to a current sensing circuit which provides a signal for adaptively biasing the tail current of the differential amplifier during peak current drain periods.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of voltage regulators and more specifically to a method and apparatus for the control of a voltage utilized by a load, such as a DRAM, during periods where the load current fluctuates considerably. 
     2. Description of Related Art 
     Voltage regulator circuits are known in which a voltage supply to a load is regulated by regulating the current supplied to the load. One such voltage regulator is illustrated in U.S. Pat. No. 5,548,205. 
     Typical of such prior art structures is the use of a feedback circuit for sensing the output voltage which is used for comparison with a reference voltage with the difference between the output and reference voltages being used to control the current supplied to a load. With such circuits, when there is a considerable change in the current drawn by the load, the voltage regulator circuit also senses the large current drain and, compensates for it through the use of the negative feedback current sensing circuit to increase the current supplied to the load and thereby maintain the output voltage at a relatively constant level. Although such voltage regulators generally perform an adequate job of voltage regulation, a considerable amount of power and thus heat is drawn because of the use of the negative feedback circuit. In addition, the negative feedback circuit decreases the response time to sharp current fluctuations and also takes up considerable layout area when the voltage regulator is incorporated in an integrated circuit (IC) structure. 
     An adaptive voltage follower is also known which could be used as a voltage regulator and is shown in the text CMOS Circuit Design, Layout, and Simulation by Baker, R. J. et al at Chapter 26, FIG. 26.25, page 703. This circuit uses a differential amplifier to control an output voltage to a load based on changes to an inut voltage. The differential amplifier compares the output and input voltages and based on variations between the two generates a control signal which is used to control an output current control transistor to thereby control the output voltage. A feed forward current sensor formed by serially connected complementary transistors also receives the control signal and develops another control signal which partially controls the tail current to the differential amplifier. In this feed forward current sensing design, a current source is also required to ensure that an adequate tail current is always supplied to the differential amplifier. Although this curcuit could be adapted for use as a voltage regulator, and avoids the delay problem with a feedback current sensing approach, the differential amplifier used is unbalanced and a separate tail current source is required, making the circuit less accurate and more complex than desired. In addition, the output voltage is directly supplied to one input of the differential amplifier, so that output voltage connot be controlled to within desired limits, less than the limits of the supply voltage. 
     Additional problems also occur when a voltage regulator is used to regulate the supply voltage to a DRAM. In a DRAM an external voltage must be lowered and regulated during periods of considerable voltage and current fluctuation, for example, a DRAM load current may quickly fluctuate between microamps and milliamps during use. In order to accomodate such large current fluctuations a DRAM power suppy may use two separate power amplifiers for supplying operative power to the DRAM memory array, one of them a low power amplifier used to supply steady state current on the order of microamps, and another higher power amplifier for supplying transitory higher currents when needed on the order of milliamps. Typically the lower power amplifier supplies current during times of low current drain, while the higher power amplifier is switched on and operative only when needed during times of high current comsumption. 
     In addition, the higher power amplifier may in fact be constructed as a bank of lower power amplifiers, for example ten power amplifiers may be actually used, which are switched on in sequence as the required current to the load increases. That is, as more current is required additional amplifiers are turned on to meet the power demand. The control of multiamplifier power regulator circuits is complex requiring a control circuit for developing the necessary control signals for turning the various power amplifiers on and off on a dynamic basis in accordance with the required DRAM load current. 
     In addition, such multiamplifier voltage regulators tend to occupy considerable layout area when formed in an integrated circuit structure. 
     SUMMARY OF THE INVENTION 
     The present invention is designed to overcome problems associated with the response time of conventional current sensing negative feedback voltage regulators. The present invention also avoids problems associated with the use of complex multiamplifier regulated power supply designs with their attendant complex circuitry and large layout areas. 
     Thus, one object of the invention is the provision of a voltage regulator for supplying a regulated voltage within desired limits which dissipates low power while providing responsive voltage regulation even under conditions of large scale voltage and current fluctuations which might occur, for example, during dynamic operation of a DRAM or other integrated circuit structures. 
     An additional object of the invention is the provision of a voltage regulator which has low static power consumption, reduces the response time of the circuit to load fluctuations, does not require a separate current source, and which has a small layout area for circuit architecture on a chip. 
     An additional object of the invention is the provision of an integrated circuit memory device, for example a DRAM or SRAM, having a built-in voltage regulator which dispenses with the multiamplifier design, but which is still able to responsively accomodate a wide range of voltage and current fluctuations of the load. 
     An additional object of the invention is the provision of a voltage regulator design which has a simple structure and which is easy to integrate. 
     An additional object of the invention is the provision of a voltage regulator which can better accomodate wide swings in output voltage fluctuation. 
     The foregoing and other objects, advantages, and features of the invention are achieved in a circuit configuration and method of operation for a voltage regulator in which a controlled element, such as a transistor, is used for regulating the current supplied to a load connectable to a load connection point of the regulator. The load connection point is also connected through a voltage divider to one input of a differential amplifier which receives at its other input a reference voltage. The output, taken from one leg of the differential amplifier, is coupled to control the transistor controlling the load current. The output of the differential amplifier is also supplied to a current sensing circuit formed by a pair of serially connected complimentary transistors which supply, as an output signal, a signal representing the current supplied to the load connection point based on the output from the differential amplifier. The output of the current sensing transistor pair is also applied as an input to control the tail current, i.e., bias, of the differential amplifier. 
     The circuitry as described provides an adaptive bias technique where a load current is sensed, not by feedback, but by a forward control current, and a signal representing the load current is then used to automatically adjust the internal bias current of the differential amplifier without needing an additional current source. A simple, low cost regulator is provided which can handle wide fluctuations in output voltage and current and which can also allow for regulation of the voltage within a desired range up to the full value of the supply voltage which is available for regulation. 
     The foregoing objects, features and advantages of the invention will become more apparent from the following detailed description of the invention which is provided in connection with the attached drawings in which like parts or elements throughout the figures are denoted by like reference numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a voltage regulator constructed and operated in accordance with the teachings of the invention; 
     FIG. 2 is a detailed schematic drawing of a preferred embodiment of the voltage regulator illustrated in block diagram form in FIG. 1; 
     FIG. 3 illustrates the use of the voltage regulator to power a memory array such as a submicron DRAM or SRAM as well as illustrating the manner in which the memory array may be connected to a processor and to a larger computer network; 
     FIG. 4 illustrates the output of the voltage regulator of FIG. 2 when the load draws a current spike of 100 milliamps for a period of 10 nanoseconds; 
     FIG. 5 illustrates the internal regulated voltage provided by the differential amplifier when the load draws a current spike of 100 milliamps for a period of 10 nanoseconds; and, 
     FIG. 6 illustrates the current which is supplied by the voltage regulator to the load when a current spike of 100 milliamps for a period of 10 nanoseconds occurs. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates in block diagram form the voltage regulator 25 of the invention. A load L is connectable across an output connection formed of a terminal 17 and ground. Current to the load is supplied through a control circuit 15 under control of an output signal 16 of differential amplifier 11. The output control signal 16 is also applied as an input to a current sensor circuit 13 which in turn supplies a sensed current signal 18 back to a second control circuit 14 which regulates the bias current, also called tail current, of the differential amplifier 11. A resistor voltage divider network 19 is interconnected between the output terminal 17 and one input of the differential amplifier 11. A reference voltage Vref is supplied to another input of the differential amplifier 11. 
     During operation, a change in the current drawn by the load L will result in a change in the voltage at terminal 17 and at the junction of the resistors 20, 22 of the voltage divider network 19. This, in turn, will be reflected at the voltage divider output Vdiv, which is input to the differential amplifier where it is compared to the voltage of the Vref input to the differential amplifier. An imbalance in the voltages input to the differential amplifier 11 will be reflected in the control signal 16 which is applied t o the control circuit 15 and current sensor circuit 13. 
     The control circuit 15 in response to control signal 16 adjusts the output current Iout supplied to the load and to the voltage divider 19. The bias or tail current of the differential amplifier is further controlled by the output signal 18 of the current sensor circuit 13. The control signal 18 operates control circuit 14 which controls the bias or tail current 12 to the differential amplifier 11. Consequently, since the current sensor circuit 13 operates in response to the forward control signal 16 supplied to the control circuit 15 which regulates load current, the amount of current which is supplied by the control circuit 15 is immediately sensed by circuit 13 and used as a control signal 18 to control the tail current, i.e., bias current, of differential amplifier 11. This improves the responsiveness of the voltage regulator circuit. 
     Moreover, unlike the circuit shown in the CMOS text noted above, this circuit does not require a separate tail current current source, thus simplifying circuit design and reducing its cost and size. In addition, as shown below in greater detail, in a preferred embodiment the two arms of the differential amplifier are balanced thereby providing a more accurate and responsive circuit. Finally, the use of the voltage divider 19 permits easy selection of the control range of the voltage regulator circuit within the limits of the supply voltage VDD. 
     During static or steady state operations the voltage regulator configuration illustrated in FIG. 1 will have a decreased power consumption compared with circuits which use a negative feedback technique. In addition, because the current control signal 16 is applied directly to current sensor circuit 13 the response time of the voltage regulator 25 is reduced. Moreover, since feedback circuits are not required, the voltage regulator 25 can be fabricated in an integrated circuit with a reduced layout area. 
     FIG. 2 illustrates in electrical schematic form a preferred voltage regulator circuit which can be used to carry out the invention. 
     The FIG. 2 circuit utilizes eight MOS FET transistors T1 . . . T8. Transistor T1 functions as the control circuit 15 while transistors T2 and T3 which are complementary form the current sensor circuit 13. The drain of transistor T2 is connected to the source of transistor T3. The source of transistor T2 is connected to the supply voltage VDD, and the drain of transistor T3 is connected to ground. The voltage divider 19 is formed by the interconnection of two resistors of 20 and 22. Resistor 22 has one of its ends connected to ground. One end of resistor 20 is connected to terminal 17. The differential amplifier is formed by transistors T4, T5, T7 and T8, with T6 forming the tail current control circuit 14. Complementary transistors T4 and T5 form one leg of the differential amplifier 11 while complementary transistors T7 and T8 form the other leg. The drains of transistors T4 and T7 are connected to the respective source of transistors T5 and T8. Each of the legs of the differential amplifier 11 is connected in common via the drains of transistors T5 and T8 to the source of transistor T6 which controls the tail current or bias current through the differential amplifier 11. Transistor T5 has its gate connected to receive the input reference voltage Vref while transistor T8 has its gate connected to receive as an input the output of the voltage divider 19. Although not necessary, a capacitor 28 is also illustrated as provided in parallel across the load L. Voltage is supplied from a supply terminal VDD to the sources of each of transistors T1, T2, T4 and T7 and the ground points are as shown in FIG. 2. As shown therein, transistors T3 and T6 have their drains connected to ground. Current to the load is controlled by transistor T1 which has its source connected to VDD and its drain connected to output terminal 17. The gates of transistors T4 and T7 are connected to their respective drains, while the gate of transistor T3 is connected to its source. 
     In operation, the output voltage Vout is provided through the voltage divider 19 to the gate of transistor T8 which controls the current flow through the differential amplifier leg formed by complementary transistors T7 and T8. Current through the leg formed by complementary transistors T4 and T5 is contacted by the voltage Vref which is connected to the gate of transistor T5. A tail or bias current through the differential amplifier is provided through transistor T6 which has its gate connected to the output of the current sensor circuit 13, being taken off the interconnection point of the transistors T2 and T3. Since each of the legs of the differential amplifier is formed of a pair of complementary transistors T4, T5 and T7, T8, the differential amplifier 11 is balanced. 
     The differential amplifier 11 provides an output control signal 16 from the interconnection of transistors T4 and T5 which is applied to the gate of transistor T2 as well as to the gate of transistor T1. The control signal 16 supplied to transistor T1 controls the current supplied to the load L, while the control signal 16 supplied to transistor T2 represents the current which is to be supplied to the load by the transistor T1. During normal or quiescent operations, normal voltage variations in the output at Vout will be handled by the differential amplifier 11 adjusting the currents in legs formed by transistors T4 and T5 and T7 and T8 in accordance with a total tail current being controlled by the transistor T6 and based on the imbalance in the voltages Vout (as reduced by the voltage divider) and Vref. Thus, any variation between the respective voltages applied to the gates of T8 and T5 will result in an imbalance of currents in the legs of the differential amplifier 11 and a corresponding raising or lowering of the output control signal 16 taken from the interconnection point of T4 and T5. 
     The tail current biassing of the differential amplifier is controlled by the transistor T6 which receives the control signal 18 from the interconnection point of T2 and T3. 
     As illustrated in FIG. 2, transistors T1, T2, T4 and T7 are P-channel type MOS transistors, whereas the transistors T5, T8, T3 and T6 are N-channel type MOS transistors. 
     It should be noted that although the circuit configuration of FIG. 1 is illustrated in FIG. 2 as being formed with MOS transistors, it can be constructed and work equivalently with other types of transistors, e.g. bipolar, as well. The voltage regulator circuit shown in FIG. 2 may be implemented as a stand alone integrated circuit, or as part of a larger integrated circuit which contains other circuit package powered by the voltage regulator, such as a DRAM or SRAM memory device. 
     FIG. 3 illustrates in block diagram form the use of the voltage regulator circuit of FIGS. 1 and 2 to supply power to a memory device 27 such as a DRAM or SRAM as the load L. FIG. 3 also illustrates that the memory is connected to a CPU (central processing unit) 29 such as a microprocessor which in turn may also be part of a computer 33a connected to other computers 33b, 33c, etc. through network server 31. Processors such as a CPU 29 may store instructions and/or data in the memory device 27 to which voltage regulator 25 is connected. The CPU 29 may be part of any electronic system such as, but not limited to, the illustrated computer 33a, or a radio, pager, television, telephone, GPS receiver, other communications system, or a control system, or the like. 
     As shown in FIG. 3, the CPU 29, may also be connected into a computer or other communications network through a network server 31. FIG. 3 also illustrates one such CPU 29 system 33a including the voltage regulator 25 memory device 27 and CPU 29. Additional similar CPU based systems are shown as elements 33b and 33c. 
     The manner in which the circuit illustrated in FIGS. 1 and 2 operates in the presence of a large change in load current is best illustrated by FIGS. 4, 5 and 6. 
     FIG. 4 illustrates the output of the FIG. 2 voltage regulator when the load consumes a current spike of about 100 milliamps starting at a location of 15 nanoseconds and lasts for a period of 10 nanoseconds. 
     FIG. 5 shows the internal regulated voltage of the voltage regulator when such a 100 milliamp spike occurs. As shown, both the supply voltage VDD and the reference voltage Vref remains relatively steady, whereas the internal regulated voltage varies in response to the 100 milliamp spike. 
     FIG. 6 shows the current supplied by the voltage regulator in response to the load variations which cause the current spike illustrated in FIG. 4. As shown, at the position where the current spikes occur, additional current is supplied by the regulator to the load to thereby maintain the output voltage and current at a substantially constant value, as depicted in FIG. 5. 
     It should be noted that the FIG. 2 preferred circuit of the invention is illustrated with reference to specific MOS transistors. However, it should also be understood that wherever a P-channel MOS is shown an N-channel MOS can be substituted and vice versa with the appropriate adjustments in VDD voltage level, as well known in the art. 
     As is apparent from the foregoing, a new and improved method and circuit have been provided for the control of the output voltage of a voltage regulator which has particular utility when supplying voltage to a circuit such as a DRAM or SRAM memory device. 
     While certain preferred embodiments of the invention have been described and illustrated, it should be apparent to those skilled in the art that certain changes and/or modifications can be made without departing from the spirit and scope of the invention, which is defined solely by the scope of the following claims.