PATENT DOCUMENT

Publication Number: US-9843330-B1
Application Number: US-201615272526-A
Country: US
Kind Code: B1

Title: Digital secondary control loop for voltage converter

Abstract:
An apparatus is disclosed, including a driver circuit, a comparator circuit, and a counter circuit. The driver circuit may be configured to source a current to a load circuit. The comparator circuit may be configured to perform a comparison of a reference voltage to a voltage across the load circuit. The counter circuit may be configured to modify a digital count value based on the comparison. The driver circuit may be further configured to adjust a value of the current using the digital count value.

Claims:
What is claimed is: 
     
       1. An apparatus, comprising:
 a driver circuit configured to source a current to a load circuit; 
 a comparator circuit configured to perform a comparison of a reference voltage to a voltage across the load circuit; and 
 a counter circuit configured to:
 modify a digital count value based on the comparison; and 
 store a specific digital count value in response to an indication from the load circuit that at least a portion of the load circuit is switching from a first operating state to a second operating state; 
 
 wherein the driver circuit is further configured to adjust an amount of the current sourced to the load circuit using the stored specific digital count value. 
 
     
     
       2. The apparatus of  claim 1 , wherein to indicate that the at least a portion of the load circuit is switching from the first operating state to the second operating state, the counter circuit is further configured to determine that the digital count value has been consecutively incremented for a particular number of clock cycles. 
     
     
       3. The apparatus of  claim 1 , wherein the counter circuit is further configured to:
 read the stored specific digital count value in response to an indication from the load circuit that the at least a portion of the load circuit is switching from the second operating state to the first operating state; and 
 replace a present digital count value with the stored specific digital count value. 
 
     
     
       4. An apparatus, comprising:
 a driver circuit configured to source a current to a load circuit; 
 a comparator circuit configured to perform a comparison of a reference voltage to a voltage across the load circuit; 
 a first counter circuit configured to, modify a first count value based on the comparison; 
 a second counter circuit configured to modify a second count value based on the comparison; and 
 a logic circuit configured to, in response to a determination that a magnitude of a difference between the reference voltage and the voltage across the load circuit is greater than a threshold difference, select the first count value as a digital count value; and to otherwise select the second count value as the digital count value; 
 wherein the driver circuit is further configured to adjust an amount of the current sourced to the load circuit using the digital count value from the first counter circuit. 
 
     
     
       5. The apparatus of  claim 4 , wherein to modify the first count value, the first counter circuit is further configured to:
 modify a portion of the first count value based on the comparison; and 
 concatenate the portion of the first count value with a fixed constant value to generate the digital count value. 
 
     
     
       6. The apparatus of  claim 4 , wherein the logic circuit is further configured to assert a stop signal in response to a determination that a value of a control signal has toggled since a most recent modification of the second count value. 
     
     
       7. The apparatus of  claim 6 , wherein the second counter circuit is further configured to maintain a constant value of the digital count value in response to the assertion of the stop signal. 
     
     
       8. A method, comprising:
 sourcing, by a main control circuit, a current to a load circuit; 
 performing, by a secondary control circuit, a comparison of a reference voltage to a voltage across the load circuit; 
 modifying a digital count value based on the comparison; 
 adjusting an amount of the current sourced to the load circuit using the digital count value; and 
 storing a specific digital count value in response to determining that at least a portion of the load circuit is switching from a first mode of operation to a second mode of operation. 
 
     
     
       9. The method of  claim 8 , wherein determining that the at least a portion of the load circuit is switching from the first mode of operation to the second mode of operation comprises determining that the digital count value has been consecutively incremented for a particular number of clock cycles. 
     
     
       10. The method of  claim 8 , further comprising:
 reading the stored specific digital count value in response to determining that the at least a portion of the load circuit is switching from the second mode of operation to the first mode of operation; and 
 replacing a present digital count value with the stored specific digital count value. 
 
     
     
       11. The method of  claim 8 , further comprising, at a first time, operating in a first mode in response to determining that a magnitude of a difference between the reference voltage and the voltage across the load circuit is greater than a threshold difference. 
     
     
       12. The method of  claim 11 , further comprising, in response to operating in the first mode:
 modifying a portion of a first count value based on the comparison; and 
 concatenating the portion of the first count value with a fixed constant value to generate the digital count value; and 
 selecting the concatenated first count value as the digital count value in response to determining that a magnitude of a difference between the reference voltage and the voltage across the load circuit is less than the threshold difference. 
 
     
     
       13. The method of  claim 11 , further comprising, at a second time:
 operating in a second mode in response to determining that a magnitude of a difference between the reference voltage and the voltage across the load circuit is less than a threshold difference; 
 modifying a second count value in a manner based on the comparison; and 
 selecting the second count value as the digital count value in response to determining that a magnitude of a difference between the reference voltage and the voltage across the load circuit is less than the threshold difference. 
 
     
     
       14. The method of  claim 8 , further comprising:
 adjusting, by the main control circuit, the amount of the current sourced to the load circuit at a first frequency; and 
 adjusting, by the secondary control circuit, the amount of the current using the digital count value at a second frequency; 
 wherein the second frequency is lower than the first frequency. 
 
     
     
       15. A system, comprising:
 a processing circuit; and 
 a voltage converter circuit including:
 a main control circuit configured to source a current to the processing circuit; and 
 a secondary control circuit configured to:
 perform a comparison of a reference voltage to a voltage across the processing circuit; 
 modify a digital count value based on the comparison; 
 adjust an amount of the current sourced to the processing circuit using the count value; and 
 store a specific digital count value in response to determining that at least a portion of the processing circuit is switching from a first mode of operation to a second mode of operation. 
 
 
 
     
     
       16. The system of  claim 15 , wherein to determine that the at least a portion of the processing circuit is switching from a first mode of operation to a second mode of operation, the secondary control circuit is further configured to determine that the count value has been consecutively incremented for a particular number of clock cycles. 
     
     
       17. The system of  claim 15 , wherein the secondary control circuit is further configured to:
 read the stored specific digital count value in response to determining that the at least a portion of the processing circuit is switching from the second mode of operation to the first mode of operation; and 
 replace a present digital count value with the stored specific digital count value. 
 
     
     
       18. The system of  claim 15 , wherein the secondary control circuit is further configured to operate in a first mode in response to a determination that a magnitude of a difference between the reference voltage and the voltage across the processing circuit is greater than a threshold difference, and otherwise operating in a second mode. 
     
     
       19. The system of  claim 18 , wherein, in response to operating in the first mode, the secondary control circuit is further configured to:
 modify a portion of a first count value in a manner based on the comparison; and 
 concatenate the portion of the first count value with a fixed constant value to generate the digital count value. 
 
     
     
       20. The system of  claim 15 , wherein the main control circuit is configured to adjust the amount of the current sourced to the processing circuit at a first frequency, wherein the secondary control circuit is further configured to adjust the amount of the current using the digital count value at a second frequency, and wherein the second frequency is less than the first frequency.

Description:
BACKGROUND 
     Technical Field 
     Embodiments described herein are related to the field of integrated circuit implementation, and more particularly to the implementation of buck converter circuits. 
     Description of the Related Art 
     Computing systems may include one or more systems-on-a-chip (SoC), each of which may integrate a number of different functions 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. 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. Various SoCs and/or various functional blocks on a given SoC may utilize power signals of different voltage levels. Since computing systems may include a single power source with a given output voltage level, one or more voltage converters or voltage regulators may be used to generate the power signals of different voltage levels. 
     Designers implementing voltage regulating circuits in a given computing system may select from among various types of voltage converting and voltage regulating circuits. Types of circuits for converting a DC power signal with a first voltage to a DC power signal with a second voltage include linear regulators and switching regulators. Buck converters, sometimes also referred to as buck regulators, are one example of a switching regulator. 
     SUMMARY OF THE EMBODIMENTS 
     Various embodiments of a voltage conversion apparatus are disclosed. Broadly speaking, an apparatus, a system, and a method are contemplated in which the apparatus includes a driver circuit, a comparator circuit, and a counter circuit. The driver circuit may be configured to source a current to a load circuit. The comparator circuit may be configured to perform a comparison of a reference voltage to a voltage across the load circuit. The counter circuit may be configured to modify a digital count value based on the comparison. The driver circuit may be further configured to adjust a value of the current using the digital count value. 
     In a further embodiment, the counter circuit may be further configured to store a specific digital count value in response to an indication from the load circuit that at least a portion of the load circuit is switching from a first operating state to a second operating state. In another embodiment, the counter circuit may be further configured to read the stored specific digital count value in response to an indication from the load circuit that the at least a portion of the load circuit is switching from the second operating state to the first operating state. The counter circuit may also be configured to replace a present digital count value with the stored specific digital count value. 
     In one embodiment, the counter circuit may be further configured to operate in a first mode in response to a determination that a magnitude of a difference between the reference voltage and the voltage across the load circuit is less than a threshold difference, and to otherwise operate in a second mode. In a further embodiment, to operate in the first mode, the counter circuit may also be configured to modify a first digital count value based on the comparison, and to maintain a constant value of a second digital count value. The counter circuit may be further configured to concatenate the first digital count value with a fixed constant value, and to send the concatenated first digital count value to the driver circuit for use in adjusting the value of the current. 
     In another embodiment, to operate in the second mode, the counter circuit may also be configured to maintain a constant value of the first digital count value. The counter circuit may also be configured to modify the second digital count value in a manner based on the comparison, and to send the second digital count value to the driver circuit for use in adjusting the value of the current. In a further embodiment, to operate in the second mode, the counter circuit may be further configured to maintain a constant value of the second digital count value in response to a determination that a value of a received control signal has toggled since a most recent modification of the second digital count value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  illustrates a block diagram of an embodiment of a voltage converting system. 
         FIG. 2  shows a block diagram of an embodiment of a servo control loop circuit for a voltage converter. 
         FIG. 3  illustrates a chart depicting waveforms for an embodiment of a voltage converter. 
         FIG. 4  shows a chart depicting waveforms for another embodiment of a voltage converter. 
         FIG. 5 . illustrates a flow diagram depicting an embodiment of a method for operating a servo control loop circuit for a voltage converter. 
         FIG. 6  shows a flow diagram illustrating an embodiment of a method for updating a count value in a servo control loop circuit. 
         FIG. 7  depicts a block diagram of another embodiment of a servo control loop circuit for a voltage converter. 
         FIG. 8  illustrates a chart depicting waveforms for another embodiment of a voltage converter. 
         FIG. 9  shows a flow diagram depicting an embodiment of a method for operating a servo control loop circuit for a voltage converter. 
         FIG. 10  depicts a flow diagram illustrating an embodiment of a method for operating a counter in a servo control loop circuit. 
     
    
    
     While the embodiments described in this disclosure may be 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 embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of 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(f) interpretation for that unit/circuit/component. 
     This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment, although embodiments that include any combination of the features are generally contemplated, unless expressly disclaimed herein. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Voltage conversion circuits may be found in many computing systems providing a power supply signal of a particular voltage level to one or more integrated circuits (ICs) or to a subset of circuits in a given IC. Buck converters are one type of voltage conversion circuit that may be used to generate a signal with a given voltage level. Buck converters receive an input power signal (Vin) and generate an output power signal (Vout) with a reduced voltage level. To reduce the voltage level of Vin to a desired voltage level on Vout, a buck converter may couple Vin to Vout for given amounts of time via, for example, a switch, a transistor, or another type of transconductance device. 
     The buck regulator may include one or more control circuits that generate a power signal of a desired voltage level to a load coupled to the output (i.e. ICs or other circuits). Each of these control circuits may generate a signal that enables and disables a respective transconductance device for the given amounts of time. Components such as, e.g., capacitors and inductors may be used to store charge and regulate current during the time periods in which the transistor is off. The voltage level of Vout may be dependent upon an average amount of time that the transistor is on versus off, referred to respectively as the “on time,” or “Ton” and the “off time,” or “Toff” The control circuit, which is also referred to herein as the “main control loop” or simply “main loop,” may generate a pulse-width modulated (PWM) and/or pulse-frequency modulated (PFM) signal with a given frequency. Adjustments may be made to the generated signal over one or more cycles as the main control loop responds to feedback from the Vout signal. For example, a voltage level (Vout) or an amount of current (Tout) going to the load may be monitored, and the PWM/PFM signal may be adjusted based on this feedback to provide more or less power to match a demand from the load. 
     Increases or decreases in power demand from the load may cause respective decreases or increases in the voltage level of Vout that are difficult for the main control loop to fully match. The main loop may be capable of responding to most of the increased or decreased power demand, but may not be capable of maintaining the desired voltage level of Vout. In some embodiments, a secondary control loop, also referred to herein as a “servo control loop,” or simply “servo loop,” may be used to respond to sudden large power changes, while the main control loop primarily responds to more gradual changes. One consideration for a servo control loop is the frequency at which the servo loop can respond to changes in power demand. The frequency of the servo control loop may be designed to be less than the frequency of the main control loop, otherwise, if the frequencies of the two control loops are too close, they may cause oscillations on the Vout power signal as both control loops respond to the same feedback, and effectively “double” the response, thereby overcompensating and causing Vout to increase or decrease more than desired. 
     Some servo control loops utilize resistor and capacitor (RC) circuits to filter out higher frequency changes in Vout or Tout such that the servo loop receives the slower changes. To accomplish this, however, may require an RC circuit with a large time constant. On an IC, manufacturing components with large resistance and capacitance values may be problematic, as such components may consume a relatively large amount of die area compared to other components. 
     Various embodiments of voltage conversion circuits are described in this disclosure. The embodiments illustrated in the drawings and described below may provide methods and systems for creating digital servo control loops without a need for large time constant RC circuits. 
     The embodiments illustrated and described herein may employ CMOS circuits. In various other embodiments, however, other suitable technologies may be employed. 
     Some terms commonly used in reference to SoC designs and CMOS circuits are used in this disclosure. For the sake of clarity, 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.” 
     A block diagram of an embodiment of a voltage converter is shown in  FIG. 1 . In the illustrated embodiment, Voltage Converter  100  includes Power Source  101  coupled to Main Control Loop  102  and Servo Control Loop  106 . Main Control Loop  102  is, in turn, coupled to Inductor (L)  103 . L  103  is further coupled to Capacitor (C)  104  and Load  105 . Power Signal Vin  121  is generated by Power Source  101  and Power Signal Vout  122  is sourced to a first terminal of L  103 . Load  105  receives Vload  123  from a second terminal of L  103 . In various embodiments, Voltage Converter  100  may be configured for use in various computing applications such as, e.g., desktop computers, laptop computers, tablet computers, smartphones, or wearable devices. 
     Power Source  101  may correspond to any suitable power supply, such as, for example, a DC power supply, one or more batteries, a battery charger, or a voltage regulation circuit. Power Source  101 , in the illustrated embodiment, generates power supply signal Vin  121  at a first voltage level. Multiple circuits may be coupled to Power Source  101  to receive Vin  121 . Some circuits may use a power supply signal with a lower voltage level than the voltage level of Vin  121 . Such circuits may, instead receive Vout from Voltage Converter  100 . 
     Main Control Loop  102 , in the illustrated embodiment, receives Vin  121 , and generates Vout  122  with a desired voltage level. To attain the desired voltage level, Main Control Loop  102  switches a transconductance device, such as, for example, a CMOS transistor, on and off using a control signal, resulting in Vin  121  being coupled to L 103  for a portion of an operating time. Main Control Loop  102  receives indications of amounts of current flowing through L  103  and uses this feedback to adjust the control signal to maintain the voltage level of Vout  122  close to the desired level. In some embodiments, Main Control Loop  102  generates Vout  122  utilizing one of two modes, a pulse width modulation (PWM) mode and a pulse frequency modulation (PFM) mode. In PWM mode, Main Control Loop  102  generates the control signal using a fixed frequency PWM signal corresponding to the Ton and Toff times. The width of the pulse corresponds to an amount of current sourced to L  103 , the wider the pulse, the more current sourced. In PFM mode, Main Control Loop  102  generates the control signal using a fixed-width pulse, with the frequency of the pulses varied to adjust the amount of current sourced to L 103 . The higher the frequency of the pulses, the more current is sourced to L  103 . 
     In either mode of operation, Main Control Loop  102  updates the control signal after one or more cycles of the PWM or PFM signal. For example, in PWM mode, Main Control Loop  102  may adjust the pulse width of the PWM signal dependent on the voltage level of Vload  123  after each PWM cycle. In PFM mode, Main Control Loop  102  may adjust the frequency of the pulses after several cycles of the PFM signal. 
     L  103  corresponds to any suitable type of inductive device. L  103  may, in some embodiments, correspond to a discrete component, such as, for example, a wire coiled around a magnetic core, or a magnetic film wrapped around a length of wire. A discrete L  103  may be coupled to Main Control Loop  102  via, bond pads, terminals, or input/output pins. In other embodiments, L  103  may be fabricated on a same IC as Main Control Loop  102 . Capacitor C  104 , likewise, corresponds to any suitable type of capacitive device and may be a discrete component coupled to Main Control Loop  102 , or fabricated on the same IC. 
     Load  105  receives Vload  123 , in the illustrated embodiment, as a power supply signal. Load  105  may correspond to any active or passive circuit, including, but not limited to, a processor, a system-on-a-chip (SoC), an RF transceiver, or a sensor (e.g., light sensor, touch sensor, gyroscopic sensor, temperature sensor, and the like). At any given time, Load  105  may consume one of a wide variety of currents, depending on a current state of operation. If, for example, Load  105  corresponds to an SoC, then Load  105  may consume a small amount of current while in a reduced power state and a much larger amount of current when in a fully operational state. During a Ton time period, if Load  105  switches into a reduced power state, then excess current from L  103  may flow to C  104  rather than to Load  105 . In contrast, if Load  105  switches from a reduced power state to a higher power state, thereby drawing more current than is flowing through L  103 , then the additional current may be provided by C  104 . 
     In some embodiments, Main Control Loop  102  may not be capable of regulating Vout  122  exactly to the desired voltage level for all current requirements of Load  105 . Main Control Loop  102  may be designed for most efficient operation over one or two current ranges of Iload  124 . In embodiments in which it is desired to maintain Vload  123  as close to the desired voltage level as possible, a secondary control loop may be included. Servo Control Loop  106  corresponds to a secondary control loop for Voltage Converter  100 . 
     Servo Control Loop  106 , in the illustrated embodiment, provides a secondary control loop to compensate for deviations from the desired voltage level of Vout  122  due to varying current demands from Load  105 . During time of high current demand from Load  105 , Servo Control Loop  106  may generate a control signal to increase an amount of current sourced to L  103 . Like Main Control Loop  102 , Servo Control Loop  106  receives indications of the amounts of current flowing through L  103  and uses this feedback to adjust an additional amount of current sourced L  103  to maintain the voltage level of Vout  122  closer to the desired level than Main Control Loop  102  may be capable of reaching alone. 
     As stated, both Main Control Loop  102  and Servo Control Loop  106  receive feedback indicating, either the voltage level of Vload  123 , the current amount of Iload  124 , or a combination of the two. As referred to herein, a time interval between each adjustment to their respective control signals by Main Control Loop  102  and Servo Control Loop  106  is referred to as a “time constant” of the respective loop. If both Main Control Loop  102  and Servo Control Loop  106  have similar time constants, then the resulting adjustments may overcompensate and cause the voltage level of Vout  122  to overshoot or undershoot the desired voltage level. For example, if both Main Control Loop  102  and Servo Control Loop  106  receive an indication that the level of Vload  123  is dropping below the desired level, then both Main Control Loop  102  and Servo Control Loop  106  may make independent adjustments to compensate for the drop, thereby generating too much compensation and causing the level of Vout  122  to rise above the desired voltage level. A subsequent feedback may indicate the overshoot, causing both Main Control Loop  102  and Servo Control Loop  106  to make adjustments to compensate for the overshoot, resulting in overcompensation that undershoots the desired voltage level of Vout  122 . In some embodiments, this cycle of overshooting and undershooting may continue, resulting in an undesired oscillation on Vout  122  that propagates to Vload  123  and into circuits included in Load  105 . 
     To prevent these undesired effects, in the present embodiment, Servo Control Loop  106  is designed with a time constant that is longer than that of Main Control Loop  102 . By operating Servo Control Loop  106  with a longer time constant, adjustments made by Servo Control Loop  106  will not continuously overlap with adjustments by Main Control Loop  102  and may, therefore, avoid frequent occurrences of overcompensation and mitigate undesired overshoots and undershoots of the desired voltage level. 
     It is noted that the voltage converter illustrated in  FIG. 1  is merely an example. Only components necessary to demonstrate the disclosed concepts are shown in  FIG. 1 . Additional and/or different components may be included in other embodiments. Furthermore, different configurations of components may be possible dependent upon the specific application for which the voltage converter is intended. 
     Turning to  FIG. 2 , a block diagram of an embodiment of a servo control loop circuit for a voltage converter is illustrated. In some embodiments, Servo Control Loop  200  may correspond to at least a portion of Servo Control Loop  106  in  FIG. 1 . Servo Control Loop  200  includes Comparator  202  coupled to counting circuit (Counter)  203 , which, in turn, is coupled to Digital-to-Analog Converter (DAC)  204 . The DAC  204  generates offset current (Ioffset)  228  which is combined to an output of a main control loop (Vmain)  220  to generate output voltage (Vout)  222 . Several signals are received as inputs, Vload  223 , reference voltage (Vref)  224  and clock signal  226 . Comparator  202  generates up signal  227 . 
     Servo Control Loop  200 , in the illustrated embodiment, compares voltage levels of received signals Vload  223  and Vref  224  using Comparator  202 . Vref  224  may, in various embodiments, be generated as a signal with a constant voltage level, such as, for example, an output of a band-gap voltage reference, or an output of a voltage regulator. The voltage level of Vref  224  may be selected to correspond to a desired voltage level for Vload  223 . Vload  223  may correspond to the voltage level of Vload  123  in  FIG. 1 , or, in other embodiments, may be proportional to the voltage level of Vload  123 . For example, in one embodiment, Vref  224  may be generated by a band-gap voltage reference with a voltage level of 300 millivolts (mV). The desired voltage level for Vload  123  may be 1200 mV. In order to compare Vref  224  to Vload  223 , Vload  223  may correspond to one-fourth of Vload  123 , such that the level of Vload  223  is 300 mV when the level of Vload  123  is 1200 mV. Comparator  202  compares the voltage levels of Vload  223  and Vref  224 . If the voltage level of Vload  223  is less than the voltage level of Vref  224 , then Comparator  202  generates a logic high value on up signal  227 , and otherwise generates a logic low on up signal  227 . 
     It is noted that any one of various design styles may be used for Comparator  202 . For example, Comparator  202  may employ a sense amplifier, an analog comparator, or any other suitable circuit for comparing the voltage levels of two or more signals. 
     Counter  203  receives up signal  227  from Comparator  202 . A logic high value on up signal  227  causes Counter  203  to increment a count value in response to an active edge on clock signal  226 , while a logic low value causes Counter  203  to decrement the count value in response to the active edge. As used herein, an “active edge” refers to a high-to-low transition or low-to-high transition on a given signal that causes a reaction in a circuit coupled to the given signal. In some embodiments, both high-to-low and low-to-high transitions may be active edges for a particular circuit. 
     The count value of Counter  203 , in the illustrated embodiment, increments when the level of Vload  223  is less than the level of Vref  224  and decrements otherwise. As a result, a higher count value corresponds to the voltage level of Vref  224  being higher than the voltage level of Vload  223  for a longer time. Conversely, a lower count value corresponds to the level of Vref  224  being less than the level Vload  223  for a longer time. 
     The count value is received by DAC  204  from Counter  203 . DAC  204 , in one embodiment, is a current DAC that generates a particular amount of current (Ioffset current  228 ) based on the received count value. Higher count values correspond to a larger amount of Ioffset current  228  generated, and vice versa. Ioffset  228  is sourced into Vmain  220  to support generation of Vout  222 . Vmain, in the illustrated embodiment, corresponds to an output from a main control loop, such as, for example, Main Control Loop  102  in  FIG. 1 . Vout  222  corresponds to Vout  122  in  FIG. 1 . The increased/decreased current to Vout  222  may result in a corresponding increase/decrease to the voltage level of Vload  223 , thereby reducing/increasing a difference in the voltage level of Vload  223  from the voltage level of Vref  224 . The comparisons repeat based on a time constant of Servo Control Loop  200 . In the illustrated example, the desired time constant is used to determine a frequency of clock signal  226 . As disclosed above in regards to  FIG. 1 , the time constant of Servo Control Loop  106  may be selected to be greater than the time constant of Main Control Loop  102 . 
     In some embodiments, one or more count values from Counter  203  may be stored for later use. For example, a count value may be stored for a particular operational state of Load  105 , such as, for example, when Load  105  is in a reduced power state and/or a fully operational state. A stored count value may be read and preloaded into a count register in Counter  203  when Load  105  is known to be returning to the corresponding state. In other embodiments, the stored value may be preloaded when Load  105  is predicted to reenter the corresponding state. For example, if Load  105  is presently in a high current operational state and switches to the reduced power state, Iload  124  may drop sharply. After a couple of Iload  124  measurements, Servo Control Loop  200  may determine that Load  105  has reentered the reduced power state based on the Iload  124  measurements, and preload the counter register with the stored count value for the reduced power state. 
     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. 
     Moving to  FIG. 3 , a chart depicting waveforms for an embodiment of a voltage converter is shown. Chart  300  in  FIG. 3  shows several waveforms associated with operation of a voltage converter, such as, for example, Voltage Converter  100  in  FIG. 1 , including a servo loop, such as, e.g. Servo Control Loop  200  in  FIG. 2 . Referring collectively to  FIGS. 1-3 , waveform  301  depicts Iload current  124  (y-axis), over time (x-axis). Waveform  302  depicts an embodiment of voltage (y-axis) versus time (x-axis) for Vload  223  with Servo Control Loop  200  inactive. A value (y-axis) for Counter  203  versus time (x-axis) is represented by waveform  303 , while waveform  304  shows Ioffset current  228  supplied by DAC  204  over time. Waveform  305  illustrates an embodiment of voltage (y-axis) versus time (x-axis) for Vload  223  when Servo Control Loop  200  is active. Both waveforms  302  and  305  include a dashed line representing a voltage level for Vref  224 . 
     In the illustrated embodiment with Servo Control Circuit  200  inactive, at time t 0 , Load  105  may be in a reduced power state, with a low current demand, as shown by Iload  301 . Vload (no servo)  302  has a voltage level at or near Vref  224 . At time t 1 , Load  105  switches into an operational state and Iload  301  increases rapidly. Vload (no servo)  302  drops suddenly, but recovers at least some of the voltage drop once Main Control Loop  102  reacts to the voltage drop and adjusts its control signal to compensate. Main Control Loop  302 , however, is not able to compensate for the entire voltage drop, and Vload (no servo)  302  remains below Vref  224  until Load  105  returns to the reduced power state at time t 3 . At time t 3 , Vload (no servo)  302  rises rapidly in response to the reduced Iload  301 , and over shoots Vref  224  by a small margin. Vload (no servo)  302  settles back to Vref  224  at time t 5 . 
     With Servo Control Loop  200  enabled, at time t 0  with Load  105  in the reduced power state, count value  303  is at a low value. Ioffset  304 , in response to the low count value  303 , is also low, indicating Servo Control Loop  200  is providing little or no current into Load  105  since Main Control Loop  102  is able to generate Vload (servo)  302  at or near the desired voltage level. Again, at time t 1 , Load  105  enters an operational state and Iload  301  rises sharply. Vload (servo)  305  again drops and recovers just a portion of the voltage drop. Servo Control Loop  200  reacts to the difference between Vload (servo)  305  and Vref  224  by asserting up signal  227 , thereby incrementing count value  303  in Counter  203 . As count value  303  increases, DAC  204  increases Ioffset  304 . Vload (servo)  305  increases in response to the increasing Ioffset  304 , until Vload (servo)  305  is at or near the desired voltage level at time t 2 . In response, up signal  227  is de-asserted and count value  303  stops incrementing. In some embodiments, an additional stop signal (not shown) may be asserted to halt Counter  203 . DAC  204  maintains Ioffset  304  at the level corresponding to the present count value  303 . 
     At time t 3 , Load  105  returns to the reduced power state and Iload  301  drops accordingly. Due to the current being supplied by Ioffset  304 , Vload  305  increases suddenly, overshooting Vref  224 . In response, to the overshoot, up signal  227  is de-asserted, and Counter  203  begins to decrement count value  303 . As count value  303  decrements, DAC  204  responsively reduces Ioffset  304 . As Ioffset  304  is reduced, Vload (servo)  305  is also reduced, until Vload (servo)  305  is at or near Vref  224 . 
       FIG. 3  is merely an example of waveforms that may result from the example embodiments as presented in this disclosure. The waveforms are simplified to provide clear descriptions of the disclosed concepts. In other embodiments, the waveforms may appear different due various influences such as technology choices for building the circuits, actual circuit design and layout, ambient noise in the environment, choice of power supplies, etc. 
     Turning now to  FIG. 4 , a chart depicting waveforms for another embodiment of a voltage converter is shown. Similar to Chart  300  in  FIG. 3 , Chart  400  in  FIG. 4  also shows several waveforms associated with operation of a voltage converter, such as, for example, Voltage Converter  100  in  FIG. 1 , including a servo loop, such as, e.g. Servo Control Loop  200  in  FIG. 2 . The waveforms in  FIG. 4  correspond to the similarly named and numbered waveforms of  FIG. 3 . 
     In the illustrated embodiment, Vload (no servo)  402  depicts a waveform corresponding to Vload  123  if Servo Control Loop  200  is inactive. The behavior of Vload (no servo)  402  relative to changes in Iload  401  is as described above in regards to  FIG. 3 . Vload (no servo)  402  is included in chart  400  as a comparison to Vload (servo)  405 . 
     In an embodiment in which Servo Control Loop  200  is active, at time t 0 , Iload  401  is in a reduced power state and count value  403 , Ioffset  404 , and Vload (servo)  405  maintain steady values, with Vload (servo)  305  at or near Vref  224 . At time t 1 , Load  105  enters an operational state and increases Iload  401  accordingly. Vload (servo)  405  drops in response to the increased current demand, followed by a partial recovery due to adjustments made by Main Control Loop  102 . Up signal  227  is asserted in response to Vload (servo)  405  being below Vref  224 , causing Counter  203  to increment count value  403 . At time t 2 , however, instead of continuing to increment count value  403 , Counter  203  reads and preloads a stored value corresponding to a previous occurrence of Load  105  entering an operational state. The stored value reduces a time for count value  403  to reach a point at which Ioffset  404  brings Vload (servo)  405  close to Vref  224 . 
     At time t 3 , Load  105  returns to the reduced power state and Iload  401  falls sharply in response. The current of Ioffset  404  combined with the output from Main Control Loop  102  causes Vload  405  to rise quickly, overshooting Vref  224 . In response, to the overshoot, the output of Comparator  202  changes, resulting in up signal  227  being de-asserted. In response to the de-assertion of up signal  227 , Counter  203  begins decrementing count value  403 , causing respective drops in Ioffset  404 . At time t 4 , Counter  203  reads and preloads a count value that was stored at a previous time when Load  105  was in the reduced power state. The preloaded count value may again reduce a time for count value  403  to reach the point at which Ioffset  404  brings Vload (servo)  405  close to Vref  224 . 
     It is noted that,  FIG. 4  is an example of possible waveforms occurring from the operation of the embodiments presented in this disclosure. The waveforms are simplified for clarity. In other embodiments, the waveforms may appear different due various environmental conditions and/or the technology used to implement the circuits. 
     Moving now to  FIG. 5 , a flow diagram depicting an embodiment of a method for operating a servo control loop circuit for a voltage converter is illustrated. Method  500  may be applied to a servo control loop, such as, for example, Servo Control Loop  200  in  FIG. 2 . Referring collectively to voltage converter  100  in  FIG. 1 , Servo Control Loop  200  in  FIG. 2 , and method  500  in  FIG. 5 , the method begins in block  501 . 
     A current is sourced to a load (block  502 ). In one embodiment, Main Control Loop  102  sources a current to Load  105  via L 103 . Main Control Loop  102  monitors power consumed by Load  105  and makes periodic adjustments in order to maintain a voltage level of Vload  123  at or near a desired voltage level. Under some conditions, however, Main Control Loop  102  may not be able to keep the level of Vload  123  as close to the desired voltage level as desired. A secondary control loop, such as, for example, Servo Control Loop  106  may be utilized to assist Main Control Loop  102  in maintaining the voltage level of Vload  123  near the desired voltage level. 
     A voltage across a load is compared to a reference voltage (block  504 ). In one embodiment, Vload  223  is compared to Vref  224 . In various embodiments, Vload  223  corresponds to Vload  123  or has a voltage level proportionate to the level of Vload  123 . Similarly, the voltage level of Vref  224  either corresponds t 0 , or is proportionate t 0 , the desired voltage level of Vload  123 . Comparator  202  compares Vload  223  to Vref  224  and asserts or de-asserts up signal  227  dependent upon the difference between the voltage levels of the two signals. 
     Further operations of method  500  may depend on a determination if the voltage level across the load is less than a reference voltage (block  506 ). Up signal  227  is asserted or de-asserted based on the comparison of the voltage levels of Vload  223  and Vref  224 . If the level of Vload  223  is less than the level of Vref  224 , then up signal  227  is asserted and the method moves to block  508  to increment a count value. Otherwise, up signal  227  is de-asserted and the method moves to block  510  to decrement a count value. 
     In response to determining that the voltage level across the load is less than a reference voltage, a count value is incremented (block  508 ). Up signal  227  is received by Counter  203 . Clock signal  226  is also received by Counter  203  and a count value in Counter  203  is incremented in response to detecting an active edge of clock signal  226  while up signal  227  is asserted. 
     Otherwise, in response to determining that the voltage level across the load is greater than the reference voltage, the count value is decremented (block  510 ). Counter  203  decrements the count value in response to detecting an active edge of clock signal  226  while up signal  227  is de-asserted. In some embodiments, an additional stop signal (not shown) may be asserted if the voltage level of Vload  223  is within a predetermined threshold of the voltage level of Vref  224 . In such an embodiment, Counter  203  neither increments nor decrements the count value upon detecting an active edge on clock signal  226 . Counter  203  may, in some embodiments, not detect active edges of clock signal  226  if this stop signal is asserted. 
     An amount of current sourced to the load is adjusted based on the count value (block  512 ). After the count value has been incremented or decremented as described above, DAC  204  uses the new count value to adjust its output current, Ioffset  228 . In some embodiments, DAC  204  may be implemented as a voltage DAC coupled to a transconductance device, such that a high voltage output of the voltage DAC, the more current is sourced through the transconductance device. In other embodiments, DAC  204  may be implemented as multiple current sources arranged in parallel, with each bit of the received count value enabling or disabling a respective device. The current generated by each transconductance device may be scaled, such that the most significant bit (MSB) of the count value controls the most current and the least significant bit (LSB) controls the least current. The method returns to block  504  to make another comparison between Vload  223  and Vref  224 . 
     It is noted that the method illustrated in  FIG. 5  is one example for demonstration purposes. Only the operations necessary to illustrate the disclosed concepts are shown. In various other embodiments, additional operations may be included. Some operations may be performed in a different sequence or in parallel. 
     Turning to  FIG. 6 , a flow diagram for an embodiment of a method for updating a count value in a servo control loop circuit is illustrated. Method  600  may correspond to operations performed in an embodiment of block  512  in  FIG. 5 , and may also be applied to a servo control loop, such as, e.g., Servo Control Loop  200  in  FIG. 2 . Referring collectively to voltage converter  100  in  FIG. 1 , Servo Control Loop  200  in  FIG. 2 , and method  600  in  FIG. 6 , the method begins in block  601 . 
     Operations of the method may depend on a determination if the load has changed operating states (block  602 ). A control circuit in Counter  203  may detect a change in the operational state of Load  105 . In some embodiments, a processor or other logic in Load  105  may send a signal to Servo Control Loop  200  indicating a change in the operational state of Load  105 . In other embodiments, the control circuit in Counter  203  may predict the change of state by comparing recent values of up signal  227  from two or more recent active edges of clock signal  226 . If a new operational state is determined, then the method moves to block  604  to determine which operational state Load  105  has entered. Otherwise, the method moves to block  610  to adjust Ioffset  228  based on the present count value. 
     Upon detecting a change of state, the new state of the load is determined (block  604 ). In some embodiments, the signal indicating the change in the operational state of Load  105  includes a value corresponding to the new state. In other embodiments where no indication of the new operating state is received, the control circuit in Counter  203  makes a determination of the new state. In various embodiments, the control circuit may use, for example, samples of up signal  227 , measurements of Iload  124 , measurements of Vload  123 , or other indicators for determining the new operational state. For example, the control circuit may count a number successive active edges of clock signal  226  in which the state of up signal  227  does not change state to determine if Load  105  has entered a higher current or lower current state. A string of five successive active edges of clock signal  226  in which up signal  227  is asserted may indicate that Load  105  has switched from a reduced power state to a higher power active state. Likewise, a similar count of up signal  227  being de-asserted may indicate a switch from the active state to the reduced power state. In other embodiments, Iload  124  or Vload  123  may be measured or compared to a reference signal and an amount of a change in either signal may indicate a present operational state. If Load  105  has more than two operational states, then, in some embodiments, the control circuit in Counter  203  may not determine an exact operational state, but only if the switch is to a higher power or lower power state. 
     Further operations of method  600  may depend on a determination if a count value has been stored for the new state (block  606 ). The control circuit in Counter  203  determines if a count value has been stored that corresponds to the new state. A count value may be stored during a previous occurrence of the new state. If Load  105  has not previously entered the new state, then a corresponding count value for preloading may not exist. If a corresponding count value has been stored, then the method moves to block  608  to preload the stored count value. Otherwise, the method moves to block  610  to adjust Ioffset  228  based on the present count value. 
     The stored count value is preloaded into the counter (block  608 ). The stored count value is read from its storage location and loaded into a count register in Counter  203 . In various embodiments, the count values may have been stored into a Random Access Memory (RAM), or into one or more registers in Voltage Converter  100 . In some embodiments, only a single count value may be stored for a predetermined operational state or a most frequently encountered operational state. In other embodiments, respective count values for any suitable number of possible operational states may be stored. 
     An amount of current sourced to the load is adjusted based on the count value (block  610 ). DAC  204  uses the present count value to adjust Ioffset  228 . The present count value may correspond to either the preloaded value or the count value upon the start of method  600 . The method ends in block  612 . 
     It is noted that the method illustrated in  FIG. 6  is merely an example. Only operations for illustrating the disclosed concepts are included. In some embodiments, two or more operations may be performed in a different sequence or in parallel. Additional operations may be included in other embodiments. 
     Moving to  FIG. 7 , a block diagram of another embodiment of a servo control loop circuit for a voltage converter is illustrated. Servo Control Loop  700  may correspond to another embodiment of Servo Control Loop  106  in  FIG. 1 . In the illustrated embodiment, Servo Control Loop includes three comparison circuits, Comparators  701  and  702  coupled to AND logic gate (AND)  704  and NOR logic gate (NOR)  705 , as well as Comparator  703  coupled to counting circuit (Counter)  706 , to latching circuit (Latch)  711 , and to XOR logic gate (XOR)  712 . NOR  705  is coupled to a second counting circuit (Counter)  708 , to OR logic gate (OR)  707  (via stop signal  732 ), and to multiplexing circuit (MUX)  709 . MUX  709  is coupled to digital-to-analog converter (DAC)  710 . Signals Vload  723  and Vref  724  are received by Servo Control Loop  700 , while Ioffset  728  is generated as an output. 
     In the illustrated embodiment, Servo Control Loop  700  includes two modes of operation, an LSB mode and an MSB mode. The active mode is determined based on comparisons of the voltage levels of Vload  723  and Vref  724 . Comparators  701 - 703  are each clocked on a rising edge of clock signal  726 , although, in other embodiments, each may be clocked on a falling edge instead. In response to a rising edge of clock signal  726 , Comparator  701  samples voltage levels of Vload  723  and Vref  724  and asserts its output if the level of Vref  724  is more than a predetermined threshold greater than the level of Vload  723 , and otherwise, de-asserts its output. Comparator  702  performs a similar, but opposite function, asserting its output if the level of Vload  723  is more than the predetermined threshold greater than the level of Vref  724 , and otherwise, de-asserts its output. NOR  705  receives the outputs from Comparators  701  and  702  and asserts stop signal  732  if both outputs are low, and otherwise de-asserts stop signal  732 . In other words, if the voltage level of Vload  723  is more than the predetermined threshold away from the voltage level of Vref  724 , either higher or lower, then stop signal  732  is de-asserted. If, however, the voltage level of Vload  723  is within the predetermined threshold of Vref  724 , then stop signal  732  is asserted. 
     When stop signal  732  is asserted, in the illustrated embodiment, Servo Control Loop is in LSB mode, and Counter  708  is halted, i.e., both increments and decrements are stopped. Counter  706  may be allowed to run, either incrementing or decrementing count value  730  dependent on the state of up signal  727 . Comparator  703  samples the voltage levels of Vload  723  and Vref  724  in response to a rising edge on clock signal  726 . Comparator  703  asserts up signal  727  if the level of Vref  724  is greater than the level of Vload  723 , and otherwise de-asserts up signal  727 . Counter  706  increments (if up signal  727  is asserted) or decrements (if up signal  727  is de-asserted) count value  730  in response to a falling edge on clock signal  726 . In the illustrated embodiment, count value  730  is shown as an 8-bit value, but, in other embodiments, may be any suitable number of bit, matching a number of bits used by DAC  710 . The assertion of stop signal  732  also causes MUX  709  to pass count value  730  to DAC  710 . DAC  710 , in turn, adjusts Ioffset  728  based on count value  730 . DAC  710  may be implemented similar to DAC  204  in  FIG. 2 , and, therefore, the description of DAC  204  may also correspond to DAC  710 . 
     Counter  706 , however, halts if stop signal  733  is asserted. OR  707  asserts stop signal  733  based on states of stop signal  732  and an output of XOR  712 . If stop signal  732  is de-asserted or the output of XOR  712  is asserted, then stop signal  733  is asserted and Counter  706  is halted. In the illustrated embodiment, Latch  711  captures the state of up signal  727  in response to a rising edge of clock signal  726  (in other embodiments, Latch  711  may be clocked on a falling edge instead). XOR  712  receives up signal  727  from Comparator  703  and the output of Latch  711 . Since both Comparator  703  and Latch  711  sample on the rising edge of clock signal  726 , the output of Comparator  703  changes after Latch  711  captures a state of up signal  727 . Latch  711 , therefore, holds a previous state of up signal  727 . XOR  712  asserts its output if the current state of up signal  727  is different than the latched state in Latch  711 . XOR  712 , therefore, asserts its output when the state of up signal  727  changes from one clock cycle to the next. If the voltage levels of Vload  723  and Vref  724  are close, then the state of up signal  727  may toggle back and forth between asserted and de-asserted on each rising edge of clock signal  726 . The toggling of up signal  727  may then result in the output of XOR  712  and the state of stop signal  733  to remain asserted, thereby halting Counter  706 . This halting of Counter  706  in response to toggling of up signal  727  may prevent unnecessary adjustments to Ioffset  728  when the level of Vload  723  is close to Vref  724 , possibly preventing additional ripples in the voltage level of Vload  723 . 
     In the illustrated embodiment, when stop signal  732  is de-asserted, Servo Control Loop is in MSB mode, and Counter  706  is halted. Counter  708  is allowed to increment or decrement count value  731  dependent on the state of up signal  729 . The state of up signal  729  is determined by the output of AND  704 . AND  704  receives the output of Comparator  701  and the inverse of the output Comparator  702 . Up signal  729 , therefore, is asserted when the voltage level of Vload  723  is more than the predetermined threshold below the level of Vref  724 , and is de-asserted otherwise. On falling edges of clock signal  726  (or rising edges in other embodiments), Counter  708  increments or decrements count value  731  based on the state of up signal  729 . Count value  731 , in the illustrated embodiment, is comprised of four bits from Counter  708  and an additional four grounded bits  734  (i.e., data values of ‘0’). The two sets of four bits are concatenated to create an 8-bit count value  731 , with the four grounded bits  734  used as LSBs and the four bits from Counter  708  used as MSBs. By using this arrangement, the value of count value  731  increments or decrements by 16 rather than by one as count value  730  does. 
     When stop signal  732  is de-asserted, MUX  709  passes count value  731 , rather than count value  730 , to DAC  710 . When operating in MSB mode, the accelerated changes of count value  731  may cause DAC  710  to increase or decrease Ioffset  728  faster, which in turn, may compensate for a voltage drop or voltage spike on Vload  723  more rapidly than when Servo Control Loop is in LSB mode. 
     It is noted that the count values used in the illustrated embodiment are each 8-bits. In other embodiments, any suitable size of counters and DAC may be utilized. In addition, in MSB mode, the number of bits set to ‘0’ versus the number of bits provided by Counter  708  may vary according to the needs of a particular application. 
     It is also noted that the system illustrated in  FIG. 7  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. In various embodiments, other combinations of logic gates may be used in place of AND  704 , NOR  705 , OR  707 , or XOR  712  to produce similar functionality. 
     Turning now to  FIG. 8 , a chart illustrating waveforms for another embodiment of a voltage converter is shown. Similar to Chart  300  in  FIG. 3  and Chart  400  in  FIG. 4 , Chart  800  in  FIG. 8  shows several waveforms associated with operation of a voltage converter, such as, for example, Voltage Converter  100  in  FIG. 1 . as well as a servo loop, such as, e.g. Servo Control Loop  700  in  FIG. 7 . Referring collectively to  FIGS. 1, 7, and 8 , Chart  800  includes waveform  801  depicting Iload current  124  (y-axis), over time (x-axis). A value (y-axis) for LSB Counter  706 , versus time (x-axis) is represented by waveform  802 , while waveform  803  shows a value (y-axis) for MSB Counter  708 , versus time (x-axis). Waveform  804  illustrates a value sent to DAC  710  via MUX  709 . Waveform  805  depicts an embodiment of voltage (y-axis) versus time (x-axis) for Vload  723  if only LSB Counter  706  is used to provide values to DAC  710 . Waveform  806 , in contrast, shows an embodiment of voltage (y-axis) versus time (x-axis) for Vload  223  when Servo Control Loop  700  is fully active. Both waveforms  805  and  806  include a dashed line representing a voltage level for Vref  724 . 
     At time t 0 , Load  105  may be in a reduced power state and Iload  801  is at a lower state. It is noted that Iload  801  and other waveforms in Chart  800  depict series of ripples in the waveforms. In various embodiments, Load  105  may not consume an absolutely steady amount of current. As circuits in Load  105  are activated and deactivated, current consumption may vary, even in some reduced power states. In addition, Main Control Loop  102  may not produce a perfect DC voltage level, and may instead produce a periodic ripple into Vout  122  which propagates into Iload  801  as shown. This periodic ripple may be detected by Servo Control Loop  700  and cause Counter  706  or MSB Counter  708  to increment and decrement in response to these ripples, as shown in LSB count value  802  and MSB count value  803 . 
     It is noted that the count values illustrated in waveforms  802  and  803  assume that Counters  706  and  708  are not halted, i.e., stop signals  732  and  733  are not asserted. The oscillations that occur in LSB count value  802 , if propagated to DAC  710 , can cause more pronounced ripples on Vload (LSB)  805 . This sort of negative feedback could cause even more ripples in Iload  801  which could then be detected by Servo Control Loop  700  causing even more exaggerated feedback and introducing more undesired oscillations on Vload (LSB)  805 . The circuit elements Latch  711  and XOR  712  may mitigate this ripple feedback by asserting stop signal  733  if up signal  727  oscillates between asserted and de-asserted states. 
     DAC value  804  shows how Latch  711  and XOR  712  may help to mitigate the ripple feedback depicted in waveforms  802  and  803 . DAC value  804  is illustrated with the assumption that stop signals  732  and  733  are asserted as described in regards to  FIG. 7 . The relatively stable current of Iload  801  results in Servo Control Loop  700  operating in LSB mode, with stop signal  732  asserted and MSB Counter  708  halted. In addition, Latch  711  and XOR  712  may work to reduce the oscillations in LSB count value  802  such that DAC  710  receives a relatively stable count value  730  from MUX  709 , creating a stable output of Ioffset  728 , resulting in Vload (DAC)  806  being more stable than Vload (LSB)  805 . 
     At time t 1 , Load  105  may switch into a higher power state, causing Iload  801  to rise sharply. The sharp rise in Iload  801  causes a corresponding fall in Vload (DAC)  806 . The difference between Vload (DAC)  806  and Vref, in the illustrated embodiment, is large enough to cause comparator  701  to assert its output, thereby resulting in the de-assertion of stop signal  732  and the assertion of stop signal  733 , putting Servo Control Loop into MSB mode. MUX  709  switches to count value  731  (i.e., MSB count value  803 ) from Counter  708 . Since MSB count value  803  increments by 16 rather than by 1, the value rises faster, as shown by the difference between waveforms  802  and  803 . In response to the increasing values of DAC value  804 , Ioffset  728  increases at the same rate, causing Vload (DAC)  806  to rise. At time t 2 , Vload (DAC)  806  reaches a voltage level high enough to cause Comparator  701  to de-assert, thereby asserting stop signal  732  and switching Servo Control Loop  700  back into LSB mode. MUX  709  switches back to passing count value  730  (LSB count value  802 ) to DAC  710 . The smaller increments of LSB count value may produce a smoother rise in the level of Vload (DAC) and may prevent overshooting the level of Vref. 
     Between times t 2  and t 3 , Vload (DAC) may reach a stable voltage level, until time t 3 , when Load  105  re-enters the reduced power state, causing a sharp drop in Iload  801 . The combination of the present level of Ioffset  728  and the sudden decrease in Iload  801 , Vload (DAC)  806  rises sharply, well above Vref. This difference between Vload (DAC)  806  and Vref causes Comparator  702  to assert its output, which in turn, de-asserts stop signal  732  and puts Servo Control Loop  700  back into MSB mode. Since Vload (DAC)  806  is greater than Vref, up signal  729  is de-asserted and Counter  708  decrements its count value. MSB count value  803  is passed by MUX  709  to DAC  710 . Since MSB count value  803  decrements by 16, Ioffset  728  is reduced quickly, causing Vload (DAC)  806  to fall in response. At time t 4 , however, Servo Control Loop  700  may detect the state change of Load  105 . If a count value was stored before time t 1 , then Servo Control Loop  700  may preload this stored count value, resulting in DAC value  804  falling sharply and bringing Vload (DAC)  806  back down to Vref quickly. 
     In various embodiments, control circuits in both Counter  706  and Counter  708  may store count values, while in other embodiments, a count value may be stored for one counter or the other. Servo Control Loop may, in some embodiments, switch into LSB mode upon preloading a count value, while in other embodiments, the switch to LSB mode may occur normally, after stop signal  732  asserts. 
     It is noted that,  FIG. 8  is merely an example of possible waveforms. The waveforms may be simplified and/or exaggerated to clearly illustrate the disclosed concepts. In other embodiments, the waveforms may appear different due various environmental conditions and/or the technology used to implement the circuits. 
     Moving now to  FIG. 9 , a flow diagram for an embodiment of a method for operating a servo control loop circuit for a voltage converter is illustrated. Method  900  may be applied to a servo control loop, such as, for example, Servo Control Loop  700  in  FIG. 7 . Referring collectively to voltage converter  100  in  FIG. 1 , Servo Control Loop  700  in  FIG. 7 , and method  900  in  FIG. 9 , the method begins in block  901 . 
     A current is sourced to a load (block  902 ). In one embodiment, Main Control Loop  102  sources a current to Load  105  via L 103 . Main Control Loop  102  monitors power consumed by Load  105  and makes periodic adjustments in order to maintain a voltage level of Vload  123  at or near a desired voltage level. The periodic adjustments may cause, in some embodiments, ripples in Vload  123 . Under some conditions, however, Main Control Loop  102  may not be able to keep the level of Vload  123  as close to the desired voltage level as desired. A secondary control loop, such as, for example, Servo Control Loop  106  may be included to provide additional current to Iload  124  to help maintain the voltage level of Vload  123  near the desired voltage level. 
     A difference between a load voltage and a reference voltage is compared to a threshold value (block  904 ). In various embodiments, Vload  723  may correspond to Vload  123  or have a voltage level proportionate to the level of Vload  123 . Similarly, the voltage level of Vref  724  either corresponds t 0 , or is proportionate t 0 , the desired voltage level of Vload  123 . In one embodiment, Comparator  701  subtracts Vref  724  from Vload  723 , while Comparator  702  subtracts Vload  723  from Vref  724 . Each comparator compares its respective difference to a predetermined threshold and asserts its corresponding output if the difference is greater than the threshold. 
     Further operations of method  900  may depend on a determination if the difference between voltages is greater than the threshold value (block  906 ). Each of Comparators  701  and  702  determines a difference in voltage levels of Vload  723  and Vref  724 . If either determined difference is greater than the threshold, then stop signal  732  is de-asserted and the method moves to block  908  to select a first mode. Otherwise, stop signal  732  is asserted and the method moves to block  918  to select a second mode. 
     If either determined difference is greater than the threshold, then the first mode is selected (block  908 ). If either Comparator  701  or Comparator  702  asserts its respective output, then stop signal  732  is de-asserted and Servo Control Loop  700  is in MSB mode. The de-assertion of stop signal  732  causes an assertion of stop signal  733 . 
     A count value of a first counter is modified based on a comparison of the load voltage and the reference voltage (block  910 ). In the illustrated embodiment, the output of Comparator  701  and the inverse of the output of Comparator  702  are received by AND  704 . If the output of Comparator  701  is asserted and the output of Comparator  702  de-asserted, then AND  704  asserts up signal  729 , causing Counter  708  to increment count value  731  in response to a falling transition on clock signal  726 . Otherwise, up signal  729  is de-asserted and Counter  708  decrements count value  731  in response to a falling transition on clock signal  726 . 
     A second counter circuit is prevented from incrementing or decrementing (block  912 ). Stop signal  733  is asserted which halts Counter  706 , i.e., count value  730  is prevented from incrementing or decrementing. In some embodiments, the present count value  730  may be stored for later use. 
     The first count value is generated (block  914 ). To generate count value  731 , an output of Counter  708  is concatenated with additional data bits with a fixed value. In the illustrated embodiment, Counter  708  generates a 4-bit count value that is concatenated with four grounded bits  734 . The grounded bits  734  are placed in the four LSB positions and the output of Counter  708  is placed into the four MSB positions, creating the 8-bit count value  731 . Since the 4-bit value in Counter  708  increments or decrements by one at each falling transition of clock signal  726 , the 8-bit count value  731  effectively increments by 16 at each falling edge of clock signal  726 . 
     An amount of current sourced to the load is adjusted based on the first count value (block  916 ). The de-assertion of stop signal  732  causes MUX  709  to pass count value  731  to DAC  710 . DAC  710  uses the new count value to adjust its output current, Ioffset  728 . Ioffset  728  is sourced to Load  105  to help support the Iload  124  demand from Load  105 . The method returns to block  904  to make another comparison and repeat the process. 
     If, in block  906 , neither determined difference is greater than the threshold, then the second mode is selected (block  918 ). If neither Comparator  701  nor Comparator  702  asserts its respective output, then stop signal  732  is asserted and Servo Control Loop  700  is in LSB mode. The assertion of stop signal  732  causes a de-assertion of stop signal  733 . 
     A count value of the second counter is modified based on a comparison of the load voltage and the reference voltage (block  920 ). Comparator  703  receives and compares the voltage levels of Vload  723  and Vref  724 , and asserts up signal  727  if the level of Vref  724  is higher. An asserted value of up signal  727  causes Counter  706  to increment count value  730  in response to a falling transition on clock signal  726 . Otherwise, up signal  727  is de-asserted and Counter  706  decrements count value  730  in response to a falling transition on clock signal  726 . 
     The first counter circuit is prevented from incrementing or decrementing (block  922 ). Stop signal  732  is asserted which halts Counter  708  similar to how Counter  706  is halted in block  912 . Likewise, the present count value  731  may be stored for later use in some embodiments. 
     An amount of current sourced to the load is adjusted based on the second count value (block  924 ). The assertion of stop signal  732  causes MUX  709  to pass count value  730 , rather than count value  731 , to DAC  710 . DAC  710  adjusts Ioffset  728  based on the new count value. Again, Ioffset  728  is sourced to Load  105  to help support the Iload  124  demand from Load  105 . The method returns to block  904  to make another comparison and repeat the process. 
     It is noted that the method illustrated in  FIG. 9  is an example for demonstrating disclosed concepts. Only operations for illustrating these concepts are included. In some embodiments, additional operations may be included. In other embodiments, two or more operations may be performed in parallel or in a different sequence. 
     Turning to  FIG. 10 , a flow diagram for an embodiment of a method for operating a counter in a servo control loop circuit is shown. In some embodiments, Method  1000  may correspond to operation included in block  920  in the method of  FIG. 9 . Like Method  900 , Method  1000  may be applied to a servo control loop, such as, for example, Servo Control Loop  700  in  FIG. 7 . Referring collectively to voltage converter  100  in  FIG. 1 , Servo Control Loop  700  in  FIG. 7 , and method  1000  in  FIG. 10 , the method begins in block  1001 , with Servo Control Loop operating in LSB mode. 
     The load voltage and the reference voltage are compared to set a control signal (block  1002 ). Comparator  703  receives Vload  723  and Vref  724 . The voltage levels of Vload  723  and Vref  724  are then compared at a rising transition of clock signal  726  to generate up signal  727 . If the level of Vref  724  is higher than the level of Vload  723 , then up signal  727  is asserted, and is de-asserted otherwise. 
     In parallel, a current state of a control signal is latched (block  1004 ). Latch  711  receives up signal  727  and latches a present state in response to a rising transition of clock signal  726 . Since both Latch  711  and Comparator  703  act on the rising transition of clock signal  727 , Latch  711  captures the state of up signal  727  before Comparator  703  sets a new state. The latched state becomes a stored previous state of up signal  727  once Comparator  703  sets the new state. 
     Further operations of the method may depend on the values of the latched state and present state of the control signal (block  1006 ). In the illustrated embodiment, XOR  712  receives the values for the latched state and present state of up signal  727 . If the two values are the same, i.e., the state of up signal  727  did not change on the last rising transition of clock signal  726 , then the output of XOR  712  is de-asserted. If the state of up signal  727  did change, then the output of XOR  712  is asserted. Since stop signal  732  is asserted in LSB mode, the state of stop signal  733  is dependent on the state of stop signal  732 , assuming the same state as stop signal  732 . If stop signal  733  is de-asserted, then the method moves to block  1008  to modify the second count value. Otherwise, if stop signal  733  is asserted, the method moves to block  1010  to halt the second counter. 
     If stop signal  733  is de-asserted, then the second counter value is modified based on the state of the control signal (block  1008 ). Counter  706  increments (if up signal  727  is asserted) or decrements (if up signal  727  is de-asserted) count value  730  in response to a falling transition of clock signal  726 . Since Counter  706  modifies count value  730  on a falling transition of clock signal  726 , up signal  727  and stop signal  733  have time to settle due to being updated on the rising transition. 
     If stop signal  733  is asserted, then the second counter is halted (block  1010 ). In the illustrated embodiment, stop signal  733 , when asserted, causes Counter  706  to cease changes to count value  730 . In some embodiments, stop signal  733  blocks transitions of clock signal  726  inside of counter  706 . Count value  730  maintains its current value. 
     An amount of current sourced to the load is adjusted based on the second count value (block  1012 ). In LSB mode, MUX  709  passes count value  730  to DAC  710  to adjust Ioffset  728  based on the current count value  730 . If stop signal  733  is de-asserted, then DAC  710  adjusts Ioffset  728 . If, however, Counter  706  is halted, then DAC  710  maintains a present level of Ioffset  728 . As previously disclosed, Ioffset  728  is sourced to Load  105  to help support the Iload  124  demand from Load  105 . The method ends in block  1014 . 
     It is noted that the method illustrated in  FIG. 10  is one example. Only operations for illustrating the disclosed concepts are included. In various embodiments, additional operations may be included, and/or some operations may be performed in parallel or in a different sequence. 
     Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. 
     The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.

Metadata:
Filing Date: 20160922
Publication Date: 20171212
Grant Date: 20171212
Priority Date: 20160922
Inventors: ATTAH HUBERT
FLETCHER JAY B.
Torres Erick O.
GOZZINI FABIO
Assignee: APPLE INC
CPC Classifications: [{"code": "H03K21/026", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03K19/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/157", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03K19/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M3/157", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03K21/026", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03K19/21", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03K21/026", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 59829463