Patent Publication Number: US-9411406-B2

Title: SRAM regulating retention scheme with discrete switch control and instant reference voltage generation

Description:
BACKGROUND 
     1. Technical Field 
     This invention is related to the field of integrated circuit implementation, and more particularly to the implementation of power supply management circuits. 
     2. Description of the Related Art 
     Computing systems may include one or more systems on a chip (SoC), which may integrate a number of different functions, such as, graphics processing, onto a single integrated circuit. With numerous functions included in a single integrated circuit, chip count may be kept low in mobile computing systems, such as tablets, for example, which may result in reduced assembly costs, and a smaller form factor for such mobile computing systems. 
     Since many functional blocks, such as memories, timers, serial ports, phase-locked loops (PLLs), analog-to-digital converters (ADCs) and more, may be included in an SoC, the probability that a given functional block is not in use at a given time may be high. When a functional block is not in use, the SoC may turn the block off by disabling power to it to conserve power, to reduce the internal chip operating temperature, and the like. However, when the functional block is needed again, power must be turned back on and the block must be initialized. Any data or operational settings stored in the functional block are lost when power is disabled. 
     In some SoC designs, functional blocks that are not used all of the time may be placed into a retention mode. In a retention mode, clock signals to the functional block may be disabled and the power supply to the block may be reduced to a level that allows the block to retain some or all of the operational settings and/or data contained within the block. This may allow some power savings or temperature reduction without a functional block requiring to be re-initialized when it is needed again. In order to implement a retention mode, a power supply with a voltage level below the main system operating voltage may be required. In addition, it is desirable to implement this power supply with minimal impact to the total chip power consumption. 
     Power regulation circuits may be designed in accordance with various designs styles including passive and active designs. Passive regulating circuits may employ a voltage drop across a passive circuit element such as, e.g., a resistor or a diode, to generate a voltage level below the main system operating voltage. 
     The flexibility to control the voltage output may be provided by using active power regulating circuits. Active power regulating circuits allow control over the voltage output by monitoring the output and comparing the output to one or more known voltage references. The output may be adjusted higher or lower based on this comparison. 
     SUMMARY OF THE EMBODIMENTS 
     Various embodiments of a power management apparatus are disclosed. Broadly speaking, an apparatus and a method are contemplated in which the apparatus includes a reference voltage circuit configured to provide one or more analog voltage levels in response to being enabled, and a voltage generation circuit coupled to the reference voltage circuit. The voltage generation circuit may be configured to enable the reference voltage circuit and receive one of the analog voltage levels from it. The voltage generation circuit may be configured to generate an output signal with a voltage level dependent upon the received analog voltage level. The voltage generation circuit may be configured to compare the output signal to the received analog voltage at a time after the reference voltage circuit was enabled, thereby establishing a current operational state of the voltage generation circuit. The reference voltage circuit may be disabled at a time after the comparison is made. The voltage generation circuit may be further configured to store this current operational state and then adjust the output voltage level based on one or more of the stored operational states. 
     In another embodiment, a sense amplifier circuit may be used within the voltage generation circuit to perform the comparison of the received analog voltage to the voltage generation circuit&#39;s output signal. The sense amplifier may be further configured to output one or more digital signals dependent upon the comparison. 
     In a further embodiment, the sense amplifier may be configured to output two complementary digital signals in response to successfully comparing the output signal to the received analog voltage. The complementary digital signals from the sense amplifier may be used to disable the reference voltage signal. 
     A periodic signal may be sent to the apparatus in another embodiment. In response to detecting a rising or a falling edge in the periodic signal, the voltage generating circuit may be configured to enable the reference voltage circuit. In response to determining the reference voltage circuit is ready, the voltage generation circuit may be configured to begin a comparison between the output signal and the received analog voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  illustrates an embodiment of a system-on-a-chip. 
         FIG. 2  illustrates an embodiment of a power supply system. 
         FIG. 3  illustrates an embodiment of a sense amplifier circuit. 
         FIG. 4  illustrates an embodiment of a staticizer circuit. 
         FIG. 5  illustrates an embodiment of a bias voltage selector. 
         FIG. 6  illustrates a state diagram of an embodiment of a power supply system. 
         FIG. 7  illustrates a flowchart of an embodiment of a method. 
         FIG. 8  illustrates an embodiment of a voltage reference circuit. 
         FIG. 9  illustrates example waveforms associated with the operation of an embodiment of a power supply system. 
         FIG. 10  illustrates an alternate embodiment of a power supply system. 
     
    
    
     While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the disclosure to the particular form illustrated, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to. 
     Various units, circuits, or other components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the unit/circuit/component can be configured to perform the task even when the unit/circuit/component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits. Similarly, various units/circuits/components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a unit/circuit/component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, paragraph six interpretation for that unit/circuit/component. More generally, the recitation of any element is expressly intended not to invoke 35 U.S.C. §112, paragraph six interpretation for that element unless the language “means for” or “step for” is specifically recited. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A system-on-a-chip (SoC) may include one or more functional blocks, such as, e.g., memories and power supplies, which may integrate the function of a computing system onto a single integrated circuit. Since an SoC may integrate multiple features into a single circuit, they are a popular choice for portable devices where space for components is limited. 
     To reduce power consumption in some SoC designs, multiple power supply voltages may be generated within the SoC to provide power to various functional blocks. In some embodiments, each power supply voltage may be employed for operating a functional block in a different operational mode. For example, one of the generated power supply voltages may be lower than a nominal supply voltage in order to conserve power or to prevent damage to the circuit. A suitable voltage may be higher than the nominal supply voltage to improve performance or for proper operation of the circuit. The suitable voltage for a given feature may change during operation as the features moves from one state to another, such as, for example, a random access memory (RAM) transitioning from a fully operational read and write state, which may require a voltage equal to the nominal supply voltage, to a lower power retention state in which the memory values are retained, but data cannot be read or written, which may require a voltage less than the nominal supply voltage. Another example is a flash memory which may require a voltage greater than the nominal supply voltage to write data but may only require a voltage equal to the nominal supply voltage to read data. 
     Multiple supply voltages may be obtained through the use of various circuits. In some embodiments, a voltage greater than the primary power supply voltage may be generated using a charge pump, a voltage doubler, or any other suitable power supply generation circuit. In other embodiments, a voltage less that the primary power supply voltage may be generated by employing a voltage regulator, a voltage divider, or any other suitable circuit. 
     Various embodiments of a voltage regulation system are described in this disclosure. The embodiments illustrated in the drawings and described below may provide techniques for providing controllable voltages to peripheral circuits within a computing system. 
     Circuit Design Overview 
     Many terms commonly used in reference to SoC designs are used in this disclosure. For the sake of clarity, the intended definitions of some of these terms, unless stated otherwise, are as follows. 
     A Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) describes a type of transistor that may be used in modern digital logic designs. MOSFETs are designed as one of two basic types, n-channel and p-channel. N-channel MOSFETs open a conductive path between the source and drain when a positive voltage greater than the transistor&#39;s threshold voltage is applied between the gate and the source. P-channel MOSFETs open a conductive path when a voltage greater than the transistor&#39;s threshold voltage is applied between the drain and the gate. 
     Complementary MOSFET (CMOS) describes a circuit designed with a mix of n-channel and p-channel MOSFETs. In CMOS designs, n-channel and p-channel MOSFETs may be arranged such that a high level on the gate of a MOSFET turns an re-channel transistor on, i.e., opens a conductive path, and turns a p-channel MOSFET off, i.e., closes a conductive path. Conversely, a low level on the gate of a MOSFET turns a p-channel on and an n-channel off. In addition, the term transconductance is used in parts of the disclosure. While CMOS logic is used in the examples, it is noted that any suitable digital logic process may be used for the circuits described in this disclosure. 
     It is noted that “high” or “high logic level” refers to a voltage sufficiently large to turn on a n-channel metal-oxide semiconductor field-effect transistor (MOSFET) and turn off a p-channel MOSFET while “low” or “low logic level” refers to a voltage that is sufficiently small enough to do the opposite. In other embodiments, different technology may result in different voltage levels for “low” and “high.” 
     The embodiments illustrated and described herein may employ CMOS circuits. In various other embodiments, however, other suitable technologies may be employed. 
     System-on-a-Chip Overview 
     A block diagram of an SoC is illustrated in  FIG. 1 . In the illustrated embodiment, the SoC  100  includes a processor  101  coupled to memory blocks  102   a  and  102   b , an analog/mixed-signal block  103 , an I/O block  104 , and a power management unit  107 , through a system bus  106 . Processor  101  is also coupled directly to a core memory  105 . In various embodiments, SoC  100  may be configured for use in a mobile computing application such as, e.g., a tablet computer or cellular telephone. 
     Processor  101  may, in various embodiments, be representative of a general-purpose processor that performs computational operations. For example, processor  101  may be a central processing unit (CPU) such as a microprocessor, a microcontroller, an application-specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). In some embodiments, processor  101  may include multiple CPU cores. In some embodiments, processor  101  may include one or more register files and memories. 
     In some embodiments, processor  101  may implement any suitable instruction set architecture (ISA), such as, e.g., PowerPC™, or x86 ISAs, or combination thereof. Processor  101  may include one or more bus transceiver units that allow processor  101  to communication to other functional blocks within SoC  100  such as, memory blocks  102   a  and  102   b , for example. 
     Memory  102   a  and memory  102   b  may include any suitable type of memory such as, for example, a Dynamic Random Access Memory (DRAM), a Static Random Access Memory (SRAM), a Read-only Memory (ROM), Electrically Erasable Programmable Read-only Memory (EEPROM), a FLASH memory, a Ferroelectric Random Access Memory (FeRAM), or a Magnetoresistive Random Access Memory (MRAM), for example. Some embodiments may include single memory, such as memory  102   a  and other embodiments may include more than two memory blocks (not shown). Memory  102   a  and memory  102   b  may be multiple instantiations of the same type of memory or may be a mix of different types of memory. In some embodiments, memory  102   a  and memory  102   b  may be configured to store program instructions that may be executed by processor  101 . Memory  102   a  and memory  102   b  may, in other embodiments, be configured to store data to be processed, such as graphics data, for example. 
     Analog/mixed-signal block  103  may include a variety of circuits including, for example, a crystal oscillator, a phase-locked loop (PLL) or delay-locked loop (DLL), an analog-to-digital converter (ADC), and a digital-to-analog converter (DAC) (all not shown). In some embodiments, analog/mixed-signal block  103  may also include, in some embodiments, radio frequency (RF) circuits that may be configured for operation with cellular telephone networks. Analog/mixed-signal block  103  may include one or more voltage regulators to supply one or more voltages to various functional blocks and circuits within those blocks. 
     I/O block  104  may be configured to coordinate data transfer between SoC  100  and one or more peripheral devices. Such peripheral devices may include, without limitation, storage devices (e.g., magnetic or optical media-based storage devices including hard drives, tape drives, CD drives, DVD drives, etc.), audio processing subsystems, graphics processing subsystems, or any other suitable type of peripheral devices. In some embodiments, I/O block  104  may be configured to implement a version of Universal Serial Bus (USB) protocol, IEEE 1394 (Firewire®) protocol, or, and may allow for program code and/or program instructions to be transferred from a peripheral storage device for execution by processor  101 . In one embodiment, I/O block  104  may be configured to perform the data processing necessary to implement an Ethernet (IEEE 802.3) networking standard. 
     Core memory  105  may be configured to store frequently used instructions and data for the processor  101 . Core memory  105  may be comprised of SRAM, DRAM, or any other suitable type of memory. In some embodiments, core memory  105  may be a part of a processor core complex (i.e., part of a cluster of processors) as part of processor  101  or it may be a separate functional block from processor  101 . In some embodiments, core memory may be a cache memory. 
     System bus  106  may be configured as one or more buses to couple processor  101  to the other functional blocks within the SoC  100  such as, e.g., memory  102   a , and I/O block  104 . In some embodiments, system bus  106  may include interfaces coupled to one or more of the functional blocks that allow a particular functional block to communicate through the link. In some embodiments, system bus  106  may allow movement of data and transactions between functional blocks without intervention from processor  101 . For example, data received through the I/O block  104  may be stored directly to memory  102   a.    
     Power management unit  107  may be configured to manage power delivery to some or all of the functional blocks included in SoC  100 . Power management unit  107  may include sub-blocks for managing multiple power supplies for various functional blocks. Power management unit  107  may receive signals that indicate the operational state of one or more functional blocks. In response to the operational state of a functional block, power management unit may adjust the output of a power supply. 
     It is noted that the SoC illustrated in  FIG. 1  is merely an example. In other embodiments, different functional blocks and different configurations of functions blocks may be possible dependent upon the specific application for which the SoC is intended. It is further noted that the various functional blocks illustrated in SoC  100  may operate at different clock frequencies, and may require different power supply voltage levels. 
     System Overview 
     A block diagram of a power supply unit embodiment is illustrated in  FIG. 2 . In the illustrated embodiment, the system  200  may include a clock circuit (CLK  201 ), a voltage reference (REFERENCE VOLTAGE CIRCUIT  202 ), an input comparison circuit (COMPARE  203 ), a control logic block (CONTROL LOGIC  204 ), a state retention circuit (STATE RETENTION CIRCUIT  205 ), a D-type flip-flop, (FLOP  206 ) a bias voltage selection circuit (BIAS SELECT  207 ), an output driver (DRIVER  208 ). Blocks  203  through  208  may combine to form POWER SUPPLY UNIT  209 . One or more functional blocks (e.g., RAM ARRAYS  210   a , TIMER  210   b , SERIAL PORT  210   c ) may receive the output of DRIVER  208  in some embodiments. In other embodiments, other functional blocks may receive the output from DRIVER  208 . 
     In some embodiments, the output of CLK  201  may be used by POWER SUPPLY UNIT  209  to synchronize the various sub-blocks within POWER SUPPLY UNIT  209 . CLK  201  may, in some embodiments, be in another part of system  200  and provide an input signal to the POWER SUPPLY UNIT  209 . In other embodiments, CLK  201  may be included within POWER SUPPLY UNIT  209 . CLK  201  may output a signal continuously while system  200  is operating. In other embodiments, CLK  201  may enable and disable its output as needed by POWER SUPPLY UNIT  209 . In systems where CLK  201  is in another part of system  200 , the enabling and disabling of the output of CLK  201  may be used to enable and disable POWER SUPPLY UNIT  209 . 
     REFERENCE VOLTAGE CIRCUIT  202  may, in various embodiments, be configured to produce one or more consistent voltage outputs (signal  211 ) that may be used as reference voltages by other sub-blocks in POWER SUPPLY UNIT  209 . REFERENCE VOLTAGE CIRCUIT  202  may be designed according to one of various design styles, for example a resistor ladder, a bandgap reference, or any other suitable circuit may be employed. In various embodiments, a resistor ladder may be connected between a power supply node and ground. A power supply refers to the main operating voltage for digital logic in the SoC. Ground refers to the common ground voltage for the digital logic. The resistor ladder may have one or more “tap” points wherein the voltage at a given tap is equal to the value of the resistance between the tap and ground divided by the value of the total resistance between the power supply and ground. In some embodiments, the REFERENCE VOLTAGE CIRCUIT  202  may be further configured adjust the total value of the resistance ladder to compensate for fluctuations in the manufacturing process in order to maintain a consistent value from device to device. 
     When REFERENCE VOLTAGE CIRCUIT  202  is initially enabled, the output may require a brief amount of time to settle to a steady state where the output voltage(s) is ready to be used as a reference. For example, if REFERENCE VOLTAGE CIRCUIT  202  is of a resistor ladder type, then the output when REFERENCE VOLTAGE CIRCUIT  202  is disabled may be equal to the power supply voltage. When REFERENCE VOLTAGE CIRCUIT  202  is enabled in this case, the output will settle to a voltage level less than the power supply voltage. The transition from an output equal to the power supply voltage to the desired reference voltage level will take a finite amount of time. It should be noted, that power supply output levels typically have some amount of fluctuation due to a variety of reasons, such as, for example, switching noise in the system, the design of the power supply itself, and/or ambient electro-magnetic noise in the environment. Therefore, in the following descriptions, when the terms stable or stabilized are used in reference to a voltage, it refers the voltage being in a state steady enough to be used by the system. 
     COMPARE  203  may, in various embodiments, receive two analog input signals and generate a digital signal whose value is dependent upon the relationship between the two input signals. In the embodiment illustrated in  FIG. 2 , COMPARE  203  receives a reference voltage (signal  211 ) from REFERENCE VOLTAGE CIRCUIT  202  and the output (signal  216 ) of the POWER SUPPLY UNIT  209 . Compare  203  may be designed according to one of various design styles. For example, compare  203  may employ a sense amplifier, an analog comparator, or any other suitable circuit for comparing the voltage levels of two or more signals. COMPARE  203  may, in some embodiments, generate two or more signals (signals  212 ), such as, for example, one signal that is high when the first input is greater than the second and low when the second input is greater than the first, and a second signal that is the opposite of the first signal. In other embodiments, a signal may be generated to indicate that COMPARE  203  has completed a comparison and the output is valid. COMPARE  203  will be discussed in more detail later in the disclosure. 
     CONTROL LOGIC  204  may be configured to, in some embodiments, to distribute CLK  201  to other sub-blocks within POWER SUPPLY UNIT  209 . In other embodiments, CONTROL LOGIC  204  may enable various sub-blocks, such as, for example, REFERENCE VOLTAGE CIRCUIT  202  and COMPARE  203 . CONTROL LOGIC  204  may, in some embodiments, enable REFERENCE VOLTAGE CIRCUIT  202  first and then delay to allow REFERENCE VOLTAGE CIRCUIT  202  to stabilize before enabling COMPARE  203  and other sub-blocks. 
     STATE RETENTION CIRCUIT  205  may, in various embodiments, be configured to receive output signals  212  from COMPARE  203  and maintain the value of the output after COMPARE  203  stops driving. In some embodiments, STATE RETENTION CIRCUIT  205  may retain the output value of COMPARE  203  until the next valid output signals  212  from COMPARE  203  become available. In other embodiments, STATE RETENTION CIRCUIT  205  may output a digital value corresponding to a current state of an output voltage level of POWER SUPPLY UNIT  209  compared to the reference voltage. In some embodiment, the digital value may be a digital word including one or more data bits. The digital word may, in various embodiments, be encoded according to one of a variety of encoding schemes, such as, e.g., binary coded decimal (BCD), for example. Additional details of how STATE RETENTION CIRCUIT  205  operates will be described below in regards to  FIG. 4 . 
     FLOP  206  may sample the value of the output of STATE RETENTION CIRCUIT  205  in some embodiments. FLOP  206  may sample this value on a rising clock edge from CLK  201 . In some embodiments, FLOP  206  may sample the output of STATE RETENTION CIRCUIT  205  at the same time COMPARE  203  starts a new comparison. In such embodiments, FLOP  206  may consistently hold a previous state of STATE RETENTION CIRCUIT  205  output, while STATE RETENTION CIRCUIT  205  holds a current state. In some embodiments, FLOP  206  may include multiple storage circuits to create a longer history of previous states which may be used by system  200 . An alternate embodiment illustrating more previous states will be presented later in the disclosure. 
     In some embodiments, FLOP  206  may be implemented as a flip-flop. It is noted that flip-flops may be particular embodiments of single data bit storage circuit and may be designed in accordance with one of various design styles. For example, latches and flip-flops may be implemented using either dynamic or static circuits, or a combination thereof. In some embodiments, each storage circuit may include scan cells as part of the implementation of a boundary scan test circuit. 
     BIAS SELECT  207  may receive both the current state from STATE RETENTION CIRCUIT  205  and the previous state from FLOP  206 . In the example embodiment of  FIG. 2 , BIAS SELECT  207  selects a voltage to output based on the current and previous states. In other embodiments, BIAS SELECT  207  may receive additional previous states such that more bias voltage options may be selected as the output. BIAS SELECT  207  will be discussed in further detail later in the disclosure. 
     In some embodiments, DRIVER  208  may be implemented as one or more transconductance devices, such as, e.g., a Junction Field-Effect Transistor (JFET), MOSFET, or Bipolar Junction Transistor (BJT). The transconductance device(s) may couple driver output  216  to the voltage supply, such that the transconductance device may act as a pull-up device. The strength of the pull-up on driver output  216  is higher or lower based on the input voltage to DRIVER  208 . In various other embodiments, a transconductance amplifier may be used. In the embodiment illustrated in  FIG. 2 , output signal  215  from BIAS SELECT  207  provides the input to transconductance FET such that DRIVER  208  output is pulled harder to the voltage supply when bias select output  215  is a lower voltage. 
     In the example embodiment, RAM ARRAYS  210   a , TIMER  210   b , and SERIAL PORTS  210   c  are the recipients of output  216  from DRIVER  208 . These peripherals  210   a - 210   c  may enter a low power or background state before receiving the output of POWER SUPPLY UNIT  209 . In other embodiments, various other SoC peripherals may be used, such as, for example, SoC registers, data buffers, etc. 
     It is noted that the system illustrated in  FIG. 2  is merely an example. In other embodiments, different functional blocks and different configurations of functions blocks are possible dependent upon the specific application for which the system is intended. 
     Turning to  FIG. 3 , an embodiment of a sense amplifier is illustrated. In some embodiments, SENSE AMP  300 , may correspond to COMPARE  203  as illustrated in  FIG. 2 . SENSE AMP  300  may receive signals sa_enable  320 , Vref  321 , and Vout  322  as inputs from other parts of the system. For instance, Vref  321  may correspond to signal  211  from  FIG. 2  and Vout  322  may correspond to output  216  from  FIG. 2 . Vref  321  and Vout  322  are coupled to transistors Q 306  and Q 307 , respectively. Signal sa_enable  320  may correspond to a delayed version of the output of CLK  201  from  FIG. 2 . Sa_enable  320  is coupled to transistors Q 301 -Q 305 . SENSE AMP  300  may have two outputs, sa_out_H  323  and sa_out_L  324  which may correspond to signals  212  as shown in  FIG. 2 . Sa_out_H  323  and sa_out_L  324  are coupled to the outputs of inverters INV  312  and  313 , respectively. Although inverters are illustrated, INV  312  and INV  313  may be embodiments of various circuits that may buffer and/or invert the coupled input signals, node  314  and node  315 . 
     During operation SENSE AMP  300  is enabled and disabled by the input signal sa_enable  320  which controls transistors Q 301 -Q 305 . SENSE AMP  300  may be disabled when sa_enable  320  is low. In this state, Q 301 -Q 304  will conduct, thereby pre-charging nodes  314  and  315  to the power supply voltage. Nodes  314  and  315  are the input to inverters INV  312  and INV  313 , which drive outputs sa_out_H  323  and sa_out_L  324  both to low states, respectively. Nodes  316  and  317  are also pre-charged to the power supply voltage. Q 305  will not conduct with a low input from sa_enable  320 , so the path to ground is disabled, regardless of the Vout  322  and Vref  321  inputs on Q 306  and Q 307  respectively. 
     It is noted that although pre-charge devices, feedback devices, pull-up devices, and pull-down devices may be illustrated as individual transistors, in other embodiments, any of these devices may be implemented using multiple transistors or other suitable circuit elements. 
     In response to sa_enable  320  transitioning to a high state, Q 301 -Q 304  stop conducting and Q 305  starts conducting, thereby opening a path to ground. Now the comparison of Vout  322  to Vref  321  begins. If Vout  322 &lt;Vref  321 , then node  316  will be pulled to ground faster than node  317 , which results in Q 309  conducting before Q 311 . This in turn pulls node  314  to ground which causes Q 310  to conduct and Q 311  to not conduct which keeps node  315  pulled to the power supply voltage. The sense amp will stabilize with sa_out_H  323  transitioning to a high state and sa_out_L  324  remaining in a low state. If Vout  322 &gt;Vref  321 , the opposite will occur and sa_out_H  323  will remain in a low state and sa_out_L  324  will transition to a high state. 
     It is noted that static CMOS inverters, such as those shown and described herein, may be a particular embodiment of an inverting amplifier that may be employed in the circuits described herein. However, in other embodiments, any suitable configuration of inverting amplifier that is capable of inverting the logical sense of a signal may be used, including inverting amplifiers built using technology other than CMOS. 
       FIG. 3  is merely an example of a comparison circuit. In other embodiments, various other sense amps circuits may be employed. Still other embodiments may implement comparison circuits based on differential amplifier circuit techniques. 
       FIG. 4  illustrates an embodiment of a state retention circuit, STATICIZER  400 . In some embodiments, STATICIZER  400  may correspond to STATE RETENTION CIRCUIT  205 , although other possible embodiments are known. In this illustrated embodiment, sa_out_H  423  and sa_out_L  424  may be inputs coming from a comparison circuit, such as, e.g., sa_out_H  323  and sa_out_L  324  from SENSE AMP  300  as illustrated in  FIG. 3 . Sa_out_H  423  is coupled to the input of INV  407 . Sa_out_L  424  is coupled to transistors Q 402  and Q 403 . STATICIZER  400  outputs a signal Qc  425  which is coupled to the output of INV  409 . 
     During operation, note that if SENSE AMP  300  is enabled and its outputs are stable, sa_out_H  423  and sa_out_L  424  may be at complementary states, in other words, if sa_out_H  423  is high then sa_out_L  424  may be low and vice versa. When sa_out_H  423  is low and sa_out_L  424  is high, inverter INV  407  outputs a high state which results in p-channel transistor Q 401  not conducting. N-channel transistor Q 402  will conduct due to the high state from sa_out_L  424 . Q 403  will not conduct and Q 404  will conduct. This results in node  410  being pulled to a low state. Both INV  408  and INV  409  will output high states. The high state output by INV  408  causes Q 405  to not conduct and Q 406  to conduct. Since both Q 404  and Q 406  are conducting, node  410  is pulled strongly to a low state. The output Qc  425  will therefore be in a high state. 
     When the comparison circuit is disabled, sa_out_H  423  and sa_out_L  424  may both transition to low states after a short delay while the comparison circuit stabilizes. This may cause both Q 401  and Q 402  to not conduct, and place Q 403  and Q 404  in a conductive state. Since node  410  has already stabilized at a low state, Q 405  is not conducting and Q 406  is conducting. Therefore, node  410  may remain in a low state and output Qc  425  remains in a high state. 
     Also illustrated in  FIG. 4  is FLOP  430 , which, in some embodiments, may correspond to FLOP  206  in  FIG. 2 . FLOP  430  is coupled to Qc  425  such as to receive Qc  425  as an input. FLOP  430  is also coupled to sa_enable  420 , such as to receive sa_enable as a clock source. The output of FLOP  430  is Qp  426 . 
     During operation, sa_enable  420  may transition to a high state to start another comparison of Vout  322  to Vref  321  in SENSE AMP  300 , this may cause FLOP  206  to capture the current state of Qc  425 , which, in our current example, is a high state, as delays in the stabilization of outputs from SENSE AMP  300  will delay the update of Qc  425 . The output of FLOP  430 , Qp  426 , may then be in a high state. After a short delay, determined by the technology and design of the transistors used to create the circuits, Qc  425  will transition to its new value based on the latest SENSE AMP  300  output. Since FLOP  430  captured the state of Qc  425  before it updated, Qp  426  now holds the previous state of Qc  425 . 
     The embodiment illustrated in  FIG. 4  is merely an example. It is noted that various other embodiments of staticizer circuits are possible and contemplated. In other embodiments, a latch or flip-flop may be used as STATE RETENTION CIRCUIT  205 . 
       FIG. 5  illustrates an example embodiment of a digital hybrid amplifier, HYBRID AMP  500 . In some embodiments, HYBRID AMP  500  may correspond to BIAS SELECT  207  from  FIG. 2 , although other possible embodiments are known. HYBRID AMP  500  may receive signals Qc  525  and Qp  526  which may correspond to Qc  425  and Qp  426  from STATICIZER  400  from  FIG. 4 . Qc  525  is coupled to the input of inverter INV  509  and Qp  526  is coupled to the input of inverter INV  510 . HYBRID AMP  500  has an output Vbias  521 , which is coupled to transistors Q 502 , Q 504 , Q 506 , and Q 508 . 
     In this illustrated embodiment, Qc  525 , representing the current state of Vout  522  (high state if Vout  522  is greater than Vref  321 , low state if Vout  522  is less than Vref  321 ) and Qp  526 , representing a previous state of Vout  522 , are the input signals to HYBRID AMP  500 . Qc  525  and Qp  526  may be inverted by inverters INV  509  and INV  510 . Inverted Qc  525  is coupled to the gates of transistors Q 501 , Q 503 , Q 505  and Q 507 . Inverted Qp  526  is coupled to the gates of transistors Q 502 , Q 504 , Q 506  and Q 508 . 
     In the present example, transistors Q 501 , Q 502 , Q 506  and Q 507  are p-channel, so they conduct in response to a low state on their gates. Transistors Q 503 , Q 504 , Q 505  and Q 508  are n-channel, so they conduct in response to a high state on their gates. When a signal equal to the power supply voltage passes through a p-channel transistor, the signal is not attenuated. When the same signal passes through an n-channel transistor, the signal is attenuated by a voltage equal to the threshold voltage (also referred to herein as “Vt”) of the n-channel transistor. This value is determined by the technology and design of the transistor. The opposite is true for a signal equal to ground. In this case, the signal would not be attenuated passing through an n-channel transistor and would be attenuated by a voltage equal to Vt when passing through a p-channel transistor. Table 1 shows the resulting bias voltage, Vbias, as a result of the four combinations of states of signals Qc and Qp. The term Vpower_supply in Table 1 refers to the power supply voltage. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Bias Voltage Selection 
               
            
           
           
               
               
               
               
            
               
                   
                 Qc 
                 Qp 
                 Vbias 
               
               
                   
               
               
                   
                 low 
                 low 
                 ground 
               
               
                   
                 low 
                 high 
                 Vt 
               
               
                   
                 high 
                 low 
                 Vpower_supply-Vt 
               
               
                   
                 high 
                 high 
                 Vpower_supply 
               
               
                   
               
            
           
         
       
     
     The resulting Vbias  521  may be coupled to the gate of a transconductance FET to implement DRIVER  530 . In some embodiments, DRIVER  530  may correspond to DRIVER  208  illustrated in  FIG. 2 . The output of DRIVER  530  may be Vout  516 , which may correspond to output  216  from  FIG. 2 . 
     Transconductance refers to how well a transistor conducts current when a given voltage is applied to its gate. In CMOS logic circuits, the MOSFETs are typically designed to operate in one of two modes, “on” with maximum conductance and “off” with minimum conductance. In some embodiments, to create voltage levels between the power supply voltage and ground, a transistor may be operated such that its transconductance responds proportionately to the voltage level applied to its gate. In this example, since a p-channel FET is used, the lower the Vbias  521  voltage is, stronger Vout  516  will be pulled to the power supply voltage. In some embodiments, the relationship between the gate voltage and the transconductance may be approximately linear, while, in other embodiments, the gate voltage and the transconductance may have a different relationship. 
       FIG. 5  is merely one example of bias selection and output driver circuits. In other embodiments, various adjustable amplification circuits may be used, such as digital-to-analog converters (DACs). Still other embodiments may implement other state-controlled power supplies. 
       FIG. 6  illustrates an embodiment of a state diagram for a power supply system such as described in  FIGS. 1-4 . In this example embodiment, operation of the regulator is simplified to four states,  601 - 604 , as determined by the current state, Qc and the previous state, Qp. In state  601 , Vout is below Vref and was also below Vref during the previous comparison. This may be the state when the voltage regulator system  200  is initially powered on. In state  601 , DRIVER  208  may pull Vout hard to the power supply voltage to quickly move Vout to be greater than Vref. There may be two transitions while in this state, transitions  605  and  606 . If Vout remains below Vref during the next comparison, then transition  605  may occur, in which case, system  200  may remain in state  601 . If Vout is greater than Vref during the next comparison, then transition  606  may occur and system  200  may transition into state  602 . 
     In state  602 , Vout has increased from being less than Vref in the previous comparison to being greater than Vref in the current comparison. In state  602 , DRIVER  208  may pull Vout to the power supply voltage with a weak pull up to avoid overshooting the Vref target voltage. Two transitions may be available to leave state  602 , transitions  607  and  608 . Transition  607  may be taken if Vout is less than Vref at the next comparison, transitioning system  200  to state  604 . Otherwise, if Vout remains greater than Vref, transition  608  may transition system  200  to state  603 . 
     In state  603 , Vout is greater than Vref and was also greater than Vref in the previous comparison. In state  603 , DRIVER  208  may stop pulling Vout to the power supply voltage to allow Vout to droop down below Vref, thereby keeping the voltage centered around the Vref target level. Two transitions may be available from state  603 , transitions  609  and  610 . Transition  609  may be taken if Vout remains greater than Vref, which keeps system  200  in state  603 . If Vout falls below Vref, then transition  610  may be taken and system  200  moves to state  604 . 
     In state  604 , Vout has fallen from being higher than Vref in the previous comparison to lower than Vref in the current comparison. In state  604 , DRIVER  208  may pull Vout to the power supply voltage harder than in state  602 , but not as hard as in state  601 , with a goal of raising Vout above Vref without overshooting the Vref target level. Two transitions may be available from state  604 , transitions  607  and  611 . Transition  607  may be taken if Vout increases to greater than Vref in the next comparison, moving system  200  into state  602 . Otherwise, transition  611  may take system  200  to state  601 . 
     It is noted that  FIG. 6  is merely an example of a state diagram for a power supply system such as POWER SUPPLY UNIT  209 . The number of states and the transitions between states may differ in other embodiments based on the application for which the power supply is intended. 
     Power Management Methods 
       FIG. 7  illustrates a flowchart depicting an embodiment of a method for managing the output of a power supply circuit, such as, e.g., POWER SUPPLY UNIT  209  illustrated in  FIG. 2 . Referring collectively to POWER SUPPLY UNIT  209  and the flowchart in  FIG. 7 , the method may begin in block  701 . RAM ARRAYS  210   a  may enter a retention mode (block  702 ). In response to RAM ARRAYS  210   a  entering a retention mode, CLK  201  may begin to toggle. In other embodiments, a control signal may be asserted that signals a RAM is entering a retention mode. 
     In response to RAM ARRAYS  210   a  entering a retention mode, POWER SUPPLY UNIT  209  may be enabled. POWER SUPPLY UNIT  209  may be enabled by CLK  201  transitioning from a static state to a toggling state. In other embodiments, POWER SUPPLY UNIT  209  may be enabled by the control signal that signaled the RAM retention mode. In response to receiving a rising clock edge from CLK  201 , CONTROL LOGIC  204  may enable REFERENCE VOLTAGE CIRCUIT  202  (block  703 ). In other embodiments, a falling edge of CLK  201  may trigger CONTROL LOGIC  204  to enable REFERENCE VOLTAGE CIRCUIT  202 . 
     CONTROL LOGIC  204  may wait for a period of time to allow REFERENCE VOLTAGE CIRCUIT  202  to stabilize (block  704 ). This period of time may be set by propagation delays through one or more logic gates. In other embodiments, the period of time may be determined by monitoring the REFERENCE VOLTAGE CIRCUIT  202  output to sense when it is stable. 
     Once the period of time has elapsed, CONTROL LOGIC  204  may assert a signal to COMPARE  203  to begin comparing the output of REFERENCE VOLTAGE CIRCUIT  202 , Vref, to the regulator output of system  200 , Vout (block  705 ). Upon completion of the comparison of Vref to Vout, COMPARE  203  may output two signals, one, sa_out_H and sa_out_L, as described above in respect to  FIG. 3 . In some embodiments, these signals may be complementary upon COMPARE  203  being enabled and a comparison being complete. 
     STATE RETENTION CIRCUIT  205  may capture the output from COMPARE  203  and FLOP  206  may hold the previous output from COMPARE  203  as described above in respect to  FIG. 4 . STATE RETENTION CIRCUIT  205  may receive sa_out_H to capture and use as a current state. In alternate embodiments, STATE RETENTION CIRCUIT  205  may receive sa_out_L to capture and use as a current state. BIAS SELECT  207  may receive the value of the current state, Qc, from STATE RETENTION CIRCUIT  205  and may receive the value of the previous state, Qp, from FLOP  206 . BIAS SELECT  207  may compare Qc to Qp (block  706 ). 
     Based on the values of Qc and Qp, BIAS SELECT  207  output a specific voltage, Vbias (block  707 ). DRIVER  208  may receive Vbias and DRIVER  208  may adjust its output based on the level of Vbias (block  707 ). In some embodiments, DRIVER  208  may be a MOSFET transistor, configured as described above in respect to  FIG. 5 , which may be used to pull Vout up to a power supply. In other embodiments, BIAS SELECT  207  and DRIVER  208  may be a single block which may directly output Vout. More details on BIAS SELECT  207  will be provided below. 
     The complementary state of the output of COMPARE  203  described above may be used by CONTROL LOGIC  204  to disable REFERENCE VOLTAGE CIRCUIT  202  (block  708 ). In other embodiments, COMPARE  203  may assert a signal to signify a completed comparison which CONTROL LOGIC  204  may use to disable REFERENCE VOLTAGE CIRCUIT  202 . 
     POWER SUPPLY UNIT  209  may determine if the RAM retention mode is still active (block  709 ). In some embodiments, this may be determined by receiving another rising or falling edge from CLK  201 . In other embodiments, RAM retention mode may be sensed as being active by the continued assertion of a control signal. If the RAM retention mode is determined to still be active, then the method may return to block  703 . Otherwise, the method may end in block  710 . In other embodiments, the method may wait for a start signal before starting another comparison cycle at block  703 . 
     It is noted that the method illustrated in  FIG. 7  depicts operations being performed in a sequential fashion with block  707  and block  708  being performed in parallel. In various other embodiments, other operations may be performed in parallel or in a different sequence. Block  707  and block  708  may be performed in series in other embodiments. 
     Turning to  FIG. 8 , an example embodiment of a voltage regulator circuit is illustrated. In some embodiments, voltage regulator circuit  800  might be used for REFERENCE VOLTAGE CIRCUIT  202  in System  200  as illustrated in  FIG. 2 , although various other embodiments are also possible. In some embodiments, voltage regulator circuit  800  may receive inputs sa_out_H  823  and sa_out_L  824  as outputs from a sense amplifier, such as, e.g., SENSE AMP  300  from  FIG. 3 . Sa_out_H  823  and sa_out_L  824  may correspond to sa_out_H  323  and sa_out_L  324  from  FIG. 2 . In some embodiments, voltage regulator circuit  800  may also receive input vr_enable  820 , which may come from a clock source such as, e.g., CLK  201  from  FIG. 2 . In alternate embodiments, vr_enable may come from a processor within an SoC, such as, e.g., PROCESSOR  101  in  FIG. 1 . 
     In this illustrated embodiment, a resistor ladder is formed by resistors R 801 -R 803 . Various voltage levels can be output based on where on the resistor ladder a tap point is made. In this illustrated embodiment, two reference voltages may be output, Vref_a  825  and Vref_b  826 . Vref_a  825  is taken from the node between R 801  and R 802 . Vref_b  826  is taken from the node between R 802  and R 803 . Voltage regulator circuit  800  may be enabled when the two transistors, Q 804  and Q 805  are both conducting. Q 804  may conduct when the NOR logic gate, NOR  806 , is high, which may occur when input signals sa_out_H  823  and sa_out_L  824  are both low. Q 805  will conduct when input signal vr_enable  820  is high. Therefore, the inputs must be vr_enable  820  set high, sa_out_H  823  set low and sa_out_L  824  set low. Any other combination and the voltage regulator circuit  800  is disabled. 
     It is noted that in some embodiments, resistors R 801  through R 803  may reside on an SoC circuit, such as, e.g., SOC  100  illustrated in  FIG. 1 , and may be constructed from polycrystalline silicon, n-type or p-type diffused silicon, copper or aluminum metal, or any other suitable material available on a semiconductor manufacturing process. In other embodiments, resistors R 801  through R 803  may reside outside of an SoC circuit such as, e.g., SOC  100  illustrated in  FIG. 1  and coupled to the circuit through a circuit board or equivalent medium. In various embodiments, resistors R 801  through R 803  may be active resistors, i.e., transistors biased to provide a fixed impedance or resistor. 
     When voltage regulator circuit  800  is disabled, no current may flow through R 801 -R 803 . Therefore, Vref_a  825  and Vref_b  826  will rise to a voltage level equivalent to the power supply voltage. When voltage regulator circuit  700  is enabled, current will flow from the power supply to ground through resistors R 801 -R 803  and through Q 804  and Q 805 . In this case, Vref_a  825  will be at a voltage level determined by equation 1 and Vref_b  826  will be at a voltage level determined by equation 2. 
     
       
         
           
             
               
                 
                   
                     Vref 
                     a 
                   
                   = 
                   
                     VDD 
                     * 
                     
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           802 
                         
                         + 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           803 
                         
                       
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           801 
                         
                         + 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           802 
                         
                         + 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           803 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     Vref 
                     b 
                   
                   = 
                   
                     VDD 
                     * 
                     
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         803 
                       
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           801 
                         
                         + 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           802 
                         
                         + 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           803 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     It is noted that the embodiment illustrated in  FIG. 8  is merely an example. In other embodiments, different numbers of resistors and different types of resistors are possible and contemplated. It is also noted that transistors Q 804  and Q 805  could be p-channel and connected between resistors R 801 -R 803  and the power supply. NOR  806  may implemented by various other means that provide similar functionality. 
       FIG. 9  illustrates a chart of possible waveforms associated with a power supply management unit such as POWER SUPPLY UNIT  209 . Referring collectively to POWER SUPPLY UNIT  209  and the chart in  FIG. 7 , the first waveform on the bottom of the chart,  901 , shows a clock signal such as may be provided by CLK  201 , called vr_enable. Waveform  902  displays a delayed version of waveform  901 , referred to as sa_enable. Waveform  903 , Vref, shows an example of a possible output of REFERENCE VOLTAGE CIRCUIT  202 . Waveform  904 , sa_out_L, and waveform  905 , sa_out_H, show example outputs of COMPARE  203 . Waveform  906 , Qc, is an example output of STATE RETENTION CIRCUIT  205  and waveform  907 , Qp, is an example of the output of FLOP  206 . Waveform  908 , Vbias, shows an example output of BIAS SELECT  207 . Waveform  909 , Vout, is an example of the output of DRIVER  208 . 
     Turning back to illustrated waveform  901 , this signal may be an input into POWER SUPPLY UNIT  209 . In the example embodiment of system  200 , as described above, vr_enable may be an input into CONTROL LOGIC  204 . If vr_enable is high and sa_out_L and sa_out_H are low, then REFERENCE VOLTAGE CIRCUIT  202  may be enabled and the output, Vref, may drop down to its enabled reference voltage level. 
     CONTROL LOGIC  204  may also delay vr_enable to create sa_enable. The sa_enable signal may go to COMPARE  203  and FLOP  206 , which may cause COMPARE  203  to begin a comparison. FLOP  206  may also store the current state of Qc as Qp. In time period t0 in  FIG. 9 , both Qc and Qp enter the period low, therefore Qp remains low at the rising edge of sa_enable. 
     In response to the rising edge on sa_enable, COMPARE  203  may generate a comparison of Vref to Vout. In period t0, Vout is lower than Vref, so sa_out_H is set high and sa_out_L remains low. Note that when sa_out_H goes high, Vref rises back to its disabled value. As described above in reference to the example circuit for REFERENCE VOLTAGE CIRCUIT  202  in  FIG. 8 , both sa_out_L and sa_out_H must be low for REFERENCE VOLTAGE CIRCUIT  202  to be enabled. Therefore, when COMPARE  203  generates an output in which either sa_out_L or sa_out_H goes high, REFERENCE VOLTAGE CIRCUIT  202  may be disabled automatically. 
     BIAS SELECT  207  may compare the values of Qc and Qp. In period t0, both Qc and Qp are low, so, based on the example circuit of  FIG. 5 , Vbias will be ground. As previously described in reference to  FIG. 5 , the lower the value of Vbias, the stronger Vout is pulled to the power supply voltage. Therefore, with Qc and Qp both low, Vbias is set to its lowest level, i.e., GROUND, and Vout is pulled strong to the power supply voltage and will therefore begin rising. 
     It is noted that on the falling edge of sa_enable in t0, sa_out_H goes back low, making both sa_out_L and saout_H low, which may allow REFERENCE VOLTAGE CIRCUIT  202  to be enabled again. However, vr_enable has also transitioned low, so REFERENCE VOLTAGE CIRCUIT  202  will not be enabled again until the next rising edge of vr_enable. 
     In the next time period, t 1 , another comparison may be triggered by the rising edge of sa_enable. In t 1 , Vout has risen above Vref. In response, COMPARE  203  may set sa_out_L high. STATE RETENTION CIRCUIT  205  may set the value of Qc high in response to sa_out_L transitioning high. FLOP  206  may store the value of Qc just before Qc transitions high, such that Qp remains low in t 1 . BIAS SELECT  207  may change the Vbias output in response to Qc being high and Qp remaining low. The new value for Vbias, referring to Table 1, is Vpower_supply-Vt. Now DRIVER  208  may have a weak pull on Vout to the power supply voltage, which may cause the voltage level of Vout to not rise as steeply, but it may still rise. 
     The cycle may repeat in time period t 2 . In t 2 , Vout is still higher than Vref, so sa_out_L is again set high. STATE RETENTION CIRCUIT  205  may keep Qc high. However, now FLOP  206  may set Qp as high instead of low. As a result, BIAS SELECT  207  may choose a new Vbias in response to Qc and Qp both being high. From Table 1, the new Vbias level may be the power supply voltage. Now DRIVER  208  may have no pull on Vout to the power supply voltage, which may result in the voltage level of Vout to begin falling. 
     The next cycle, t 3 , may see Vout fall below Vref. So in t 3 , COMPARE  203  may set sa_out_H to high. STATE RETENTION CIRCUIT  205  may set Qc to low. FLOP  206  may store the value of Qc before it transitions low, such that Qp may remain high. In response to Qc set low and Qp set high, BIAS SELECT  207  may set Vbias to Vt, according to Table 1. Applying Vt to DRIVER  208  may result in Vout being pulled to the power supply voltage with a mid-range strength, i.e., a stronger pull than when Vbias is Vpower_supply-Vt, but not as strong as when Vbias is GROUND. 
     In the next cycle, t 4 , we may see Vout again rise above Vref. As a result, we may see COMPARE  203  set sa_out_L high, which may result in STATE RETENTION CIRCUIT  205  setting Qc to high. FLOP  206  may store the value of Qc before it transitions high, such that Qp is set low. With Qc set high and Qp set low, BIAS SELECT  207  may set Vbias to Vpower_supply-Vt, which may result in DRIVER  208  having a weak pull on Vout to the power supply voltage. 
     In the last example cycle, t 5 , Vout may remain above Vref, which may result in COMPARE  203  setting sa_out_L to high. STATE RETENTION CIRCUIT  205  may set Qc to high. FLOP  206  may store the previous value of Qc, which was also high, setting Qp to high. In response to QC and Qp both being high, BIAS SELECT  207  may set Vbias to the power supply voltage. The power supply voltage applied to DRIVER  208  may result in no pull of Vout to the power supply voltage, which may result in Vout to begin falling. 
     From the description above and waveform  909  in  FIG. 9 , it should be easy to see that the output of POWER SUPPLY UNIT  209  may not be a single consistent voltage level. Instead, Vout may undulate above and below Vref, with the average voltage level being close to Vref. This undulation is commonly referred to as voltage ripple. To reduce the size of the ripple, i.e., reduce the voltage delta between the minimum and maximum voltage levels of the ripple, more than one previous state may be saved and used by BIAS SELECT to set Vbias. 
       FIG. 9  is merely an example of waveforms that may result from the example embodiments as presented in this disclosure. Use of alternate embodiments may result in variations to the waveforms presented in  FIG. 9 . 
       FIG. 10  illustrates an example embodiment of a portion of POWER SUPPLY UNIT  1000 . In some embodiments, Staticizer  1005  may correspond to STATE RETENTION CIRCUIT  205  as illustrated in  FIG. 2 , and may provide a similar output, Qc, corresponding to the current state of POWER SUPPLY UNIT  1000 . FLOP  1006   a  may correspond to FLOP  206  from  FIG. 2 , providing an output, Qp 1 , corresponding to the most recent previous state of POWER SUPPLY UNIT  1000 . In various embodiments, POWER SUPPLY UNIT  1000  may include flip flop, FLOP  1006   b , which may store the output of FLOP  1006   a . BUFFER  1008  may be used to add a slight delay to sa_enable, so that FLOP  1006   b  will store the value of Qp 1  before it is updated to its new value. As a result, once STATICIZER  1005 , FLOP  1006   a , and FLOP  1006   b  have updated their respective outputs, then BIAS SELECT  1007  may have three inputs to use to select a Vbias output level, Qc being the current state, Qp 1  being the state from the previous comparison cycle, and Qp 2  being the state from two comparison cycles previous. With these three inputs, BIAS SELECT  1007  now may select from eight possible Vbias outputs, as shown in Table 2. The term Vpower_supply in Table 2 refers to the power supply voltage. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Bias Voltage Selection with 2 Previous States 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Qc 
                 Qp1 
                 Qp2 
                 Vbias 
               
               
                   
               
               
                   
                 low 
                 low 
                 low 
                 GROUND 
               
               
                   
                 low 
                 low 
                 high 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.15 
               
               
                   
                 low 
                 high 
                 low 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.30 
               
               
                   
                 low 
                 high 
                 high 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.45 
               
               
                   
                 high 
                 low 
                 low 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.60 
               
               
                   
                 high 
                 low 
                 high 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.75 
               
               
                   
                 high 
                 high 
                 low 
                 Vpower_supply * 
               
               
                   
                   
                   
                   
                 0.90 
               
               
                   
                 high 
                 high 
                 high 
                 Vpower_supply 
               
               
                   
               
            
           
         
       
     
     By having more possible Vbias levels with smaller voltage deltas between each level, the changes to output of the POWER SUPPLY UNIT  1000  may be more gradual, which may result in less ripple than is present when only four Vbias levels are used. The Vbias outputs shown in Table 2 are only examples. In a given application, the Vbias voltage levels may be selected to provide the best performance for the functional blocks that may be coupled to POWER SUPPLY UNIT  1000 . Additionally, more flip-flops may be used to save even more previous sates, further increasing the number of potential Vbias voltage levels that may be selected. FLOP  1006   b  may be designed according to one of various design styles. For example, FLOP  1006   b  may employ one or more flip-flops, a shift register, a small RAM or any other suitable circuit for storing a sequence of one or more bits of data. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.