Frequency sensing NMOS voltage regulator

[0002] This application is a continuation of U.S. patent application Ser. No. 09/692,472, filed Oct. 20, 2000, which is a continuation of U.S. patent application Ser. No. 09/386,312 filed Aug. 31, 1999 (issued as U.S. Pat. No. 6,175,221 on Jan. 16, 2001), the entirety of each of which is incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to voltage regulators, and more particularly to a frequency sensing voltage regulator that uses the system operating frequency to limit the amount of current delivered to a load, thereby regulating the variance of the supply voltage to the load.

[0005] 2. Description of the Related Art

[0006] Voltage regulator circuits are known in which a voltage supply to a load is regulated by controlling the current supplied to the load. Typical of such prior art structures is the use of a negative feedback circuit for sensing the output voltage and/or output current which is used for comparison with a reference voltage/reference current. The difference between the output and the reference signal is used to adjust the current supplied to a load.

[0007] There are problems, however, with such voltage regulators. A considerable amount of power is drawn, and thus heat dissipated, because of the use of the negative feedback circuit. In addition, the negative feedback circuit decreases the response time to sharp current fluctuations. Furthermore, the comparator circuits and reference level generating circuits take up considerable layout area when the voltage regulator is incorporated in an integrated circuit (IC) structure.

[0008] Additional problems also occur when a voltage regulator is used to regulate the supply voltage to a synchronous device, such as a synchronous memory device, for example an SRAM. In an SRAM, an external supply voltage, Vcc, must be maintained within a predetermined level. The external supply voltage Vcc must be regulated to produce a regulated Vcc value during periods of considerable current fluctuation. For example, an SRAM load current may quickly fluctuate between microamps and milliamps during use. Such changes in the load current can cause significant variation on the regulated Vcc value, which can result in improper operation of the SRAM or possibly even damage to the SRAM.

[0009] Thus, there exists a need for a voltage regulator that is easy to implement, does not occupy significant layout area when the voltage regulator is incorporated in an integrated circuit (IC), and provides a minimal variance of the supply voltage Vcc over a wide current range.

[0010] The present invention is designed to mitigate problems associated with the prior art by providing a frequency sensing NMOS voltage regulator that is easy to implement, does not occupy significant layout area when the voltage regulator is incorporated in an integrated circuit (IC), and provides a minimal variance of the supply voltage Vcc over a wide current range. The present invention takes advantage of the fact that current tracks frequency in a linear fashion for synchronous systems.

[0011] In accordance with the present invention, a NMOS source follower transistor has a gate connected to a fixed gate voltage, a drain coupled to an external supply voltage through a PMOS switching transistor, and a source connected to a load. The gate of the PMOS transistor is controlled by a delay circuit through which the clock pulse of the system is passed. Through the use of the delay circuit and the PMOS transistor, the amount of current provided by the NMOS transistor is made a function of the cycle rate of the clock pulse, tracking the current requirements of the load. This results in a reduced variance of the regulated supply voltage Vcc over a wide current range.

[0012] These and other advantages and features of the invention will become apparent from the following detailed description of the invention which is provided in connection with the accompanying drawings.

[0013] FIG. 1 illustrates a NMOS voltage regulator in accordance with the present invention;

[0014] FIG. 2 illustrates the delay circuit of FIG. 1;

[0015] FIG. 3 illustrates a delay chain that may be used in the delay circuit of FIG. 2;

[0016] FIGS. 4A and 4B illustrate timing diagrams of various clock signals;

[0017] FIG. 5 illustrates in block diagram form an integrated circuit that utilizes a voltage regulator in accordance with the present invention; and

[0018] FIG. 6 illustrates in block diagram form a processor system that utilizes a voltage regulator in accordance with the present invention.

[0019] The present invention will be described as set forth in the preferred embodiment illustrated in FIGS. 1-6. Other embodiments may be utilized and structural or logical changes may be made and equivalents substituted without departing from the spirit or scope of the present invention. Like items are referred to by like reference numerals throughout the drawings.

[0020] The present invention provides a frequency sensing NMOS voltage regulator that is easy to implement, does not occupy significant layout area when the voltage regulator is incorporated in an integrated circuit (IC), and provides a minimal variance of the supply voltage Vcc over a wide current range. FIG. 1 illustrates a voltage regulator 10 in accordance with the present invention. Voltage regulator 10 includes a NMOS source follower transistor 12 connected to a control circuit 14 via line 16. The drain of transistor 12 is coupled to an external supply voltage Vcc 20 through a PMOS transistor 22. The source of transistor 12 provides a regulated voltage Vreg to a load 18. In accordance with the present invention, the output 26 of a delay circuit 40 is connected to the gate of PMOS transistor 22. The input 25 of delay circuit 40 is connected to the clock pulse signal CLK PULSE 24 which is the output of a pulse generator 25 driven by the CLK 27 of the system in which the voltage regulator is installed.

[0021] Control circuit 14, which provides a predetermined gate voltage Vgate to transistor 12, includes a pair of PMOS transistors 30, 31, NMOS transistors 33, 34, 35, and resistors 37, 38, and 39. External supply voltage Vcc 20 and a reference voltage Vref 29 are used to supply the fixed gate voltage Vgate 16 to the gate of transistor 12 during operation of the voltage regulator 10. It should be understood that although one method of supplying a predetermined gate voltage to transistor 12, i.e., control circuit 14, has been illustrated, any method as is known in the art may be used with the present invention.

[0022] FIG. 2 illustrates the delay circuit 40 of FIG. 1. Delay circuit 40 includes a plurality of delay chains 50a-50eeach having a signal input, a signal output and a reset input, connected in series. The input 51 of the first delay chain 50ais connected to ground in this embodiment. The output 53 of delay chain 50ais connected to the input of delay chain 50b, the output of the delay chain 50bis connected to the input of delay chain 50cand so forth up to delay chain 50e. While five delay chains 50a-50eare illustrated, the invention is not so limited and any number of delay chains 50a-50emay be used depending upon the desired delay, nor are the types of delay elements used within 50a-50erequired to be identical.

[0023] The clock pulse signal CLK PULSE 24 is connected to the reset input of each delay chain 50a-50e. The output of the last delay chain 50eis connected to a plurality of inverters 52, of which three are shown in this embodiment, connected in series.

[0024] FIG. 3 illustrates a delay chain 50athat can be used in the delay circuit 40 of FIG. 2. Delay chain 50aincludes three inverters 55, 56, 57 connected in series and a NAND gate 58 having a first input 60 connected to the output of the last inverter 57 and a second input 62 connected to the clock pulse signal CLK PULSE 24 via the reset input.

[0025] The operation of the voltage regulator 10 of FIG. 1 will be described with respect to the CLK 27 and CLK PULSE 24 clock signals illustrated in FIGS. 4A and 4B. FIGS. 4A and 4B illustrate clock signals having a respective frequency which are generated by the respective system in which the voltage regulator 10 is installed. For example, the system may have a clock frequency of 100 MHz or 300 MHz. The pulse generator 25 generates a fixed-width, low going pulse for each rising edge of the system clock, CLK 27. The clock signal CLK PULSE 24 is input to delay circuit 40 and specifically to the reset input of each delay chain 50a-50eas illustrated in FIG. 2. The reset input of each delay chain 50a-50eis connected to input 62 of NAND gate 58 within each delay chain as illustrated in FIG. 3. Thus, the input 62 to NAND gate 58 will alternate between a high logic level and a low logic level corresponding to the clock pulse signal CLK PULSE 24 of the system.

[0026] As noted with respect to FIG. 2, the input 51 of the first delay chain 50ais connected to ground. Thus, the signal input to the input 60 of NAND gate 58 of delay chain 50awill be a logic high signal. The output 53 of delay chain 50awill thus go high when the CLK PULSE 24 signal goes low and go low when the CLK PULSE 24 signal returns high after some time period t_adue to the delay of NAND gate 58. The outputs from delay chains 50b-50ewill be similar to that of the output of delay chain 50a, except for an additional time delay for each successive delay chain, as shown in FIG. 4A. Thus, the low ground signal input to input 51 of delay chain 50awill ripple through each delay chain and be input to the series of inverters 52 if CLK PULSE 24 remains at a logic high level long enough. By varying the number of delay chains in delay circuit 40, the total time delay for the ground signal to reach the inverters 52 can be set to a predetermined time.

[0027] When the input to inverters 52 is a logic high, the output 26 from delay circuit 40 will be low, keeping transistor 22 in an on state. When the input to inverters 52 is a logic low, the output 26 from the delay circuit 40 will be high, turning transistor 22 off. Each time the CLK PULSE 24 signal goes low, each of the delay chains of delay 40 will be reset, i.e., output a logic high regardless of the logic state being input to the delay chain from a previous delay chain, turning transistor 22 on. Thus, if the logic high time of the CLK PULSE 24 signal is longer than the delay time of delay circuit 40, the low ground signal will ripple through delay circuit 40 and shut off transistor 22. If the logic high time of the CLK PULSE 24 signal is less than the delay time of delay circuit 40, the logic low time of the CLK PULSE signal will reset each delay chain before the low ground signal can ripple out, pulling the output from delay circuit 40 high, thus keeping transistor 22 on. In this manner, the delay circuit 40 regulates the amount of current delivered to the load as a function of the frequency of the clock.

[0028] FIG. 4B illustrates a timing diagram for three clock pulse signals F1, F2, and F3, each having a different frequency. Suppose the delay time of delay circuit 40 is set to some time t_delay. As shown in FIG. 4B, clock pulse signals F1 and F2 have a high time longer than the delay time t_delay, thus allowing the ground signal input to the first delay chain of delay circuit 40 to ripple through delay circuit 40 and turn transistor 22 off for remainder of the time. When the clock pulse signals F1 and F2 go to a logic low, the delay circuit 40 is reset, outputting a logic low and turning transistor 22 on again. By “pulsing” the current provided to the load in this fashion, the voltage variance of Vreg is reduced.

[0029] Clock pulse signal F3 has a shorter pulse period and thus a “high” time which is shorter than the delay time t_delay, thus not allowing the ground signal input to the first delay chain of delay circuit 40 to ripple through delay circuit 40, as each delay chain is reset each time the clock pulse signal goes low. Thus, transistor 22 remains on for the entire duration of clock pulse signal F3. Accordingly, the frequency of the clock pulse signal is used to adjust the current to the load 18 by controlling the gate voltage of transistor 22 (FIG. 1). In addition, the value of t_delayis set to correspond to the period, and thus frequency, at which the regulator begins to pulse off.

[0030] In accordance with the present invention, a frequency sensing NMOS voltage regulator is provided that is easy to implement since it only requires a simple delay circuit 40 which sets the cycle time, or frequency, at which the regulator starts pulsing off the supplied current to the load, does not occupy significant layout area when the voltage regulator is incorporated in an integrated circuit (IC), and provides a minimal variance of the regulated supply voltage Vreg over a wide current range.

[0031] FIG. 5 illustrates in block diagram form an integrated circuit 400 that uses the voltage regulator 10 according to the present invention. Integrated circuit 400 includes a memory circuit 410, such as for example a RAM. A plurality of input/output connectors 412 are provided to connect the integrated circuit to an end-product system. Connectors 412 may include connectors for the supply voltage Vcc, ground (GND), clock signal CLK PULSE 24, and input/output terminals (I/O) for data from memory 410. Memory 410 is powered by a regulated voltage Vreg from voltage regulator 10.

[0032] It should be noted that while the invention has been described and illustrated in the environment of a memory circuit, the invention is not limited to his environment. Instead, the invention can be used in any synchronous system in which current varies linearly with clock frequency.

[0033] A typical processor system which includes a memory circuit which in turn has a voltage regulator according to the present invention is illustrated generally at 500 in FIG. 6. A computer system is exemplary of a processor system having digital circuits which include memory devices. Other types of dedicated processing systems, e.g. radio systems, television systems, GPS receiver systems, telephones and telephone systems also contain memory devices which can utilize the present invention.

[0034] A processor system, such as a computer system, generally comprises a central processing unit (CPU) 502 that communicates with an input/output (I/O) device 504 over a bus 506. A second I/O device 508 is illustrated, but may not be necessary depending upon the system requirements. The computer system 500 also includes random access memory (RAM) 510. Power to the RAM 510 is provided by voltage regulator 10 in accordance with the present invention. Computer system 500 may also include peripheral devices such as a floppy disk drive 514 and a compact disk (CD) ROM drive 516 which also communicate with CPU 502 over the bus 506. Indeed, as shown in FIG. 6, in addition to RAM 510, any and all elements of the illustrated processor system may employ the invention. It should be understood that the exact architecture of the computer system 500 is not important and that any combination of computer compatible devices may be incorporated into the system.

[0035] In accordance with the present invention, voltage regulator 10 provides a minimal variance of the regulated supply voltage Vreg over a wide current range to a regulated device, e.g. a SRAM, or other synchronous device where load current varies linearly with clock frequency.

[0036] While a preferred embodiment of the invention has been described and illustrated above, it should be understood that this is exemplary of the invention and is not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.