Patent Document

RELATED APPLICATIONS 
     This application is related to and claims priority to U.S. Provisional Application No. 62/032,758, entitled “System and Apparatus for Integrated Power Converter with High Bandwidth,” filed on Aug. 4, 2014, which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application is directed to switched inductor power conversion including systems and methods for controlling the output voltage of a switched inductor DC-DC power converter using a parallel linear voltage regulator. 
     BACKGROUND 
     Electronic switched-mode DC to DC converters convert one direct current (DC) voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) and/or electric field storage components (capacitors). This conversion method is more power efficient (often 75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). Efficiency has increased due to the use of power field effect transistors (FETs), which are able to switch at high frequency more efficiently than power bipolar transistors (BJTs). BJTs incur more switching losses and require a more complicated drive circuit. 
     A buck converter is a voltage step down and current step up converter. In its simplest form, a buck converter comprises two switches and an inductor in series with a load. It controls the current in the inductor by the two switches (usually a transistor and a diode). Heuristically, the buck converter is best understood in terms of the relation between current and voltage of the inductor. Beginning with the switch open (i.e., in the off position), the current in the circuit is 0. When the switch is first closed, the current will begin to increase, and the inductor will produce an opposing voltage across its terminals in response to the changing current. This voltage drop counteracts the voltage of the source and therefore reduces the net voltage across the load. 
     Over time, the rate of change of current decreases, and the voltage across the inductor also then decreases, increasing the voltage at the load. During this time, the inductor is storing energy in the form of a magnetic field. If the switch is opened while the current is still changing, then there will always be a voltage drop across the inductor. So, the net voltage at the load will always be less than the input voltage source. 
     When the switch is opened again, the voltage source will be removed from the circuit, and the current will decrease. The changing current will produce a change in voltage across the inductor, now aiding the source voltage. The stored energy in the inductor&#39;s magnetic field supports current flow through the load. During this time, the inductor is discharging its stored energy into the rest of the circuit. If the switch is closed again before the inductor fully discharges, the voltage at the load will always be greater than zero. 
     Switched inductor, DC-DC down-converters, and buck converters provide conversion of power from one potential to another lower voltage potential. These types of converters are used in a broad and diverse set of applications. One typical application is the conversion and regulation of power supplies for microprocessors and other sensitive or high performance integrated circuits. 
     Modern integrated circuits using advanced complementary metal oxide semiconductor (CMOS) technology run on power supplies with voltages at 1V or less, while the power levels delivered to a computer are typically at 120V or higher. The power is down-converted in the computer from 120V AC to 1V DC for the microprocessor through a series of power converters. AC-DC converters will generally provide a range of DC voltages such as 3.3V, 5V and 12V, and then a buck converter will take one of those power levels and down-convert to the 1V required by the microprocessor. 
     Some buck converters down-convert power by driving a low pass filter with a pulse width modulation (PWM) signal. The low pass filter comprises an inductor in series with a capacitor, which is in parallel with the load. PWM signals are rectangular pulse wave trains whose pulse width is modulated resulting in a variation of the average value of the waveform. The PWM signal is produced by power switches or transistors that modulate a DC signal by connecting the input inductor terminal to either the input power supply (DC) or ground. 
     In the idealized converter, all the components are considered to be perfect. Specifically, the switch and the diode have zero voltage drop when on and zero current flow when off and the inductor has zero series resistance. Further, it is assumed that the input and output voltages do not change over the course of a cycle (this would imply that the output capacitance is infinite). 
     Typical buck converters have some physical separation from the load that they are powering. The physical separation results in an unwanted parasitic inductance and resistance between the output of the buck converter and the load. In the event of a load current transient, the high frequency content of that transient will see high impedance due to the parasitic inductance, and consequently there will be a large deviation in the voltage of that power supply. 
       FIG. 1  illustrates a schematic view of an exemplary power conversion system  10  according to the prior art. The system  10  includes a buck converter chip  110  with voltage sensing feedback loop  130 . Buck converter chip  110  comprises feedback controller  120 , n-type metal oxide semiconductor field effect transistor (NMOS transistor)  180 , p-type metal oxide semiconductor field effect transistor (PMOS transistor)  170 , series inductor  140 , and shunt capacitor  150 . 
     In concert, NMOS  180  and PMOS  170  transistors are comprised by a complementary metal oxide semiconductor (CMOS) device. As is customary in the art, NMOS transistor  180  source  182  is tied to ground  185 , and PMOS transistor  170  source  172  is tied to input power (V S ). NMOS transistor  180  drain  184  is in electrical communication with PMOS transistor  170  drain  174  and CMOS output  165  (bridge voltage Vb), as illustrated in  FIG. 1 . PMOS  170  and NMOS  180  transistor gates are electrically controlled by feedback controller  120 . In one or more embodiments, PMOS  170  and NMOS  180  transistor gates are tied together in a traditional CMOS device configuration. 
     By opening and closing PMOS  170  and NMOS  180  transistor gates in a periodic, binary clock cycle, feedback controller  120  generates a pulse width modulation (PWM) signal at the CMOS device output  165 . When PMOS transistor  170  is open, NMOS transistor  180  is closed and vice-versa, thereby engendering square wave form at the bridge voltage Vb. Feedback controller  120  modulates the width (on time) of the signal train giving rise to the PWM signal. Width determination (i.e., duty cycle) by feedback controller  120  is discussed in greater detail later in the disclosure. 
     PWM signal drives current though series inductor  140  at the bridge voltage Vb. The second terminal  145  of series inductor  140  is wired to buck converter chip output voltage and shunt capacitor  150  in parallel. As can be appreciated by one skilled in the art, the present configuration creates a low pass filter at the buck converter chip output voltage assuming a resistive load thereto, at least in part. The fundamental frequency of the PWM signal generated by the PMOS  170  and NMOS  180  transistor gates is configured to be much higher than the LC resonance of the output low pass filter formed by series inductor  140  and shunt capacitor  150 . Consequently, the output of the filter is the average value of the switching signal, which is equal to the voltage of the input power supply Vs multiplied by the duty cycle of the PWM signal. 
     Power conversion system  100  incorporates feedback control through voltage sensing loop  130  of the buck converter chip  110 . Measured at load  190 , feedback control keeps a constant output voltage Vo (or approximately constant output voltage Vo such as within 5% or 10% or 15%) at load  190  with changing operating conditions such as input voltage (V s ) or load  190  current. At frequencies above the LC resonance, shunt capacitor  150  provides a low output impedance (Z out ) which maintains the output voltage Vo during load  190  current transients. At frequencies below the LC resonance, the feedback controller  120  will modulate the buck converter&#39;s duty cycle to keep the output voltage Vo static during load  190  current transients. 
     Ideally, these components are lossless, which would result in near 100% conversion efficiency for the buck converter chip  110 . In reality, series resistance in the series inductor  140 , shunt capacitor  150  and switches (NMOS  170 , PMOS  180  transistors) all result in loss. Similarly, parasitic capacitance in the series inductor  140  and capacitive switching losses from the switches (NMOS  170 , PMOS  180  transistor gates) also result in inefficiency. Moreover, parasitic inductance  160  occurs along the wiring or power delivery channel (e.g., circuit trace elements) from the buck converter chip  110  output  115  to the load  190  which also detrimentally affects the desired functionality of the power conversion system  10 . Accordingly, there is need in the art to provide for an efficient, regulated power supply which ameliorates the effects of parasitic inductance  160  et al. 
       FIG. 2  is a graphical plot representing a frequency response  20  of an exemplary buck converter without feedback control according to the prior art. Open loop output impedance (Z out-open )  21  is graphed as a function of frequency ω  24 . The frequency response  20  of the buck converter indicates how the output voltage will change in response to changes in load current at a particular load frequency. (Those in the art may recognize the similarity to the transfer function plot, |H(jω)|.) 
     Ideally, the output impedance Z out-open  is flat and very low, thereby maximizing the power transfer to the load in linear manner with respect to frequency. Frequency response  20  is an approximate, graphical representation of a buck converter without parasitic inductance between the buck converter and load. The output impedance Z out-open  here is set by the buck converter inductor and capacitor and inductor series resistance. With reference to  FIG. 2 , peak  23  is located at resonant frequency ω r =1/√(LC). Low pass response  25  rises to meet peak  23 . It can be observed that peak  23  drops off dramatically at the slope of the higher frequency response  22 . 
       FIG. 3  is a graphical plot representing a frequency response  30  of an exemplary buck converter with some feedback control according to the prior art. Closed loop output impedance (Z out-closed )  31  is graphed as a function of frequency ω  34 . With the feedback controller for the buck converter in operation, higher impedance at peak  33  (contrast to peak  23  of  FIG. 2 ) is counteracted by feedback controller to provide a more desirable output impedance Z out-closed . Low pass response  35  remains flat as a function of frequency ω  34 ; however, the area integrated under peak  33  is smaller. 
       FIG. 4  is a graphical plot representing a frequency response  40  of an exemplary buck converter with some feedback control and parasitic inductance according to the prior art. Load impedance (Z load )  41  is graphed as a function of frequency ω  44 . Up until peak  43 , low pass response  45  is similar to that shown in  FIG. 3 , with the feedback controller for the buck converter in operation by counteracting the higher impedance. Peak  43  corresponds to the LC resonance frequency discussed above. 
     Turning to  FIG. 4 , the impedance at the load Z load  is shown with the parasitic inductance between the buck converter chip output and the load. As a result, parasitic inductance causes the output impedance Z load  to increase at higher frequencies (see high frequency response  42 ), despite the low output impedance provided by the output capacitor in the buck converter. 
     With the development of highly integrated electronic systems that consume large amounts of electricity in very small areas, the need arises for new technologies that enable improved energy efficiency and power management for future integrated systems. Integrated power conversion is a promising potential solution as power can be delivered to integrated circuits at higher voltage levels and lower current levels. That is, integrated power conversion allows for step down voltage transformers to be disposed in close proximity to transistor elements. 
     Accordingly, there is a need for high quality inductors to be used in large scale CMOS integration. This provides a platform for the advancement of systems comprising highly granular dynamic voltage and frequency scaling as well as augmented energy efficiency. The present disclosure contemplates the novel fabrication of efficient and compact on-chip, high bandwidth power converters and practical methods for manufacturing operating thereof for remedying these and/or other associated problems. 
     SUMMARY 
     The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrative examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description of the disclosure when considered in conjunction with the drawings. 
     As mentioned above, the present invention relates to new and improved systems and apparatus for providing integrated, high bandwidth power converters. In particular, the present application relates to systems for controlling the output voltage of a switched inductor DC-DC power converter using a parallel linear voltage regulator, where the power converter and linear regulator are primarily residing on a single integrated circuit substrate. 
     This invention serves to regulate the power supply by reducing the effect of parasitic inductance between a power converter and a load by using a parallel linear regulator and a power converter feedback loop. The linear regulator may be on the same integrated circuit (IC) as the power converter (e.g., buck converter), but the output of the linear regulator is preferably not connected to the output of the power converter on that chip. Rather the linear regulator should have its own, independent electrical coupling to the load, so that the parasitic inductance in that interconnect provides some electrical isolation between the linear regulator output and the output capacitor of the buck converter. 
     This allows the linear regulator to rapidly swing the potential of its output, and consequently generate a large change in current through the parasitic inductance between the linear regulator and the load. The large change in current will reduce the voltage drop in the power supply at the load that would otherwise be caused by a large change in the load current. This circuit technique provides a smaller output impedance at significantly higher frequencies than what is achieved with the buck converter alone. 
     The control circuit of the linear regulator comprises common/open drain field effect transistors biased in the subthreshold region. The transistors conduct only a negligible amount of current unless the voltage sense terminal moves. If the voltage sense terminal changes, some portion of that change in voltage will be coupled onto the gate nodes of the transistors. 
     This will subsequently conduct current in a manner to counteract the change in voltage at the load. The circuit provides high frequency regulation to augment that of the buck converter without conducting quiescent current through the transistors. Thus, the present invention provides a more efficient power converter than a linear controller (non-LDO) alone. 
     In an aspect, the invention includes an apparatus comprising a switched inductor power converter, a low-pass filter, a voltage sense path, and a parallel linear regulator. The low-pass filter has an input in electrical communication with an output of the power converter and an output in electrical communication with a power delivery channel, the power delivery channel for providing an output power to a load. The voltage sense path is in electrical communication with the power delivery channel and the power converter. The parallel linear regulator is in parallel electrically with the voltage sense path. 
     In an aspect, the invention includes a method of controlling power for a load. The method includes, in a switched inductor power converter, reducing a voltage of an input power supply to a reduced voltage suitable for the load. The method also includes delivering an output power to the load through a power delivery channel. The method also includes, in a voltage sense path in electrical communication with the power delivery channel and the power converter, sensing an output voltage of the output power at a node proximal to the load. The method also includes, in a parallel linear regulator in parallel electrically with the voltage sense path, providing a regulator current to the load during an output voltage error when the output voltage to the load is different than a target voltage for the load. 
    
    
     
       IN THE DRAWINGS 
       For a fuller understanding of the nature and advantages of the present invention, reference is be made to the following detailed description of preferred embodiments and in connection with the accompanying drawings, in which: 
         FIG. 1  illustrates a schematic view of an exemplary power converter with voltage sensing feedback loop according to the prior art; 
         FIG. 2  is a graphical plot representing an open loop frequency response of the exemplary power converter according to the prior art; 
         FIG. 3  is a graphical plot representing a closed loop frequency response of the exemplary power converter according to the prior art; 
         FIG. 4  graphically plots the device output impedance with parasitic inductance at load according to the prior art; 
         FIG. 5  illustrates a schematic view of an exemplary power converter with voltage sensing feedback loop and linear control element; 
         FIG. 6  illustrates an schematic view of an exemplary power converter with voltage sensing feedback loop and linear control element according to an embodiment; 
         FIG. 7  is a graphical juxtaposition of the impedance contributions of an exemplary power converter and linear control element in frequency space; 
         FIG. 8  is a graphical superposition of an exemplary power converter and linear control element in frequency space; 
         FIG. 9  illustrates an schematic view of an exemplary linear control circuit according to an embodiment; 
         FIG. 10  is a graphical depiction of the exemplary linear control circuit&#39;s output current as a function of delta voltage sensed; and 
         FIG. 11  is a graphical plot representing the frequency response of the exemplary linear control circuit. 
     
    
    
     DETAILED DESCRIPTION 
     As mentioned above, the present invention relates to switched inductive converters that control the output voltage using a parallel linear voltage regulator and a feedback loop. One or more embodiments or implementations are hereinafter described in conjunction with the drawings, where like reference numerals are used to refer to like elements throughout, and where the various features are not necessarily drawn to scale. 
     The present application discloses a novel DC-DC power converter that can be integrated onto the same chip as a linear voltage regulator, according to some embodiments. The system regulates the power supply, despite the parasitic inductance formed between the power converter and the load, using a parallel linear regulator and power delivery channel. As stated, the linear regulator may be on the same integrated circuit (IC) as the buck converter, but the output of the linear regulator is preferably not connected to the output of the buck converter on the buck converter chip. 
       FIG. 5  illustrates a schematic view of an exemplary power conversion system  50  with voltage sensing feedback loop  530  and linear control element  585 , according to an embodiment. Buck converter chip  510  comprises feedback controller  520 , CMOS PWM signal generator  575  (comprised of a CMOS device), series inductor  540 , shunt capacitor  550 , feedback loop  530  and linear control element  585 . As discussed above, series inductor  540  and shunt capacitor  550  form a low pass filter  555  assuming a resistive load thereto. A switched inductor power converter  505  is formed by the feedback controller  520 , CMOS PWM signal generator  575 , and the low pass filter (series inductor  540  and shunt capacitor  550 ). 
     CMOS PWM signal generator  575  comprises PMOS  570  and NMOS  580  transistors to produce a periodic rectangular wave with a predetermined frequency as previously described. Feedback controller  520  compensates for high current loads  590  and variations in input power (Vs) by monitoring output voltage (Vo) proximal to the load  590  via voltage sensing loop  530 . The feedback controller  520  calculates a voltage error, which is the difference between the actual output voltage Vo and a target output voltage. The target output voltage can be set manually or pre-programmed based on the specifications of the load  590 . If there is a positive voltage error (i.e., the actual output voltage Vo is greater than the target output voltage), the feedback controller  520  responds by decreasing the duty cycle of the PWM signal generated by CMOS PWM signal generator  575 . If there is a negative voltage error (i.e., the actual output voltage Vo is less than the target output voltage), the feedback controller  520  responds by increasing the duty cycle of the PWM signal generated by CMOS PWM signal generator  575 . The switched inductor power converter  505  is configured to respond to low frequency variations in voltage error (e.g., less than the LC resonance frequency of the output low pass filter). Frequencies higher than the LC resonance frequency cannot pass through the series inductor  540 . 
     Thus, the feedback controller  520  modulates the duty cycle of the PWM signal to create a constant (or substantially constant) actual output voltage Vo. In some embodiments, the feedback controller  520  modulates the duty cycle of the PWM signal using a PID (proportional-integral-differential), PI, or PD controller. The output of the low pass filter also remains relatively constant which is the average value of the switching signal which is equal to the voltage of the input power supply (Vs) multiplied by the duty cycle of the PWM signal. 
     PWM signal drives current though series inductor  540  at the bridge voltage (Vb). The second terminal  545  of series inductor  540  is wired to output power to delivery line  555  and shunt capacitor  550  in parallel. The fundamental frequency of the PWM signal is configured to be higher than the LC resonance of the output low pass filter, which is determined by series inductor  540  and shunt capacitor  550 . 
     Series resistance in the series inductor  540 , shunt capacitor  550  and switches  570 ,  580  of the CMOS PWM signal generator  575  all result in loss. Similarly, a parasitic inductance  560  occurs along the delivery line  555  (e.g., circuit trace elements) from the buck converter chip  510  output  515  to the load  590  which also detrimentally affects the functionality of the power conversion system  50 . 
     The efficacy of load regulation by the feedback controller  520  is diminished due to the parasitic capacitances, parasitic inductance  560  and inherent resistivity in the circuit elements. In one configuration, a separate linear control element  585  is added to the buck converter chip  510 . The input  582  of the linear control element  585  monitors the output voltage Vo from the feedback loop/voltage sensing path  530 . The output  584  of the linear control element  585  contributes to the regulation of the output voltage Vo by responding to high frequency variations (e.g., greater than the LC resonance frequency of the output low pass filter) in the output voltage Vo as discussed below. 
     In some embodiments, linear control element  585  is a low-dropout (LDO) regulator. Yet, any suitable DC voltage regulator is not beyond the scope of the present invention. A low-dropout or LDO regulator is a DC linear voltage regulator that can operate with a very small input-output differential voltage. In the present configuration illustrated in  FIG. 5 , the effectiveness of the linear control element  585  is somewhat limited because the linear controller element  585  is still filtered by the shunt capacitor  550  and parasitic inductance  560 . 
       FIG. 6  illustrates a schematic view of an exemplary power converter  60  with voltage sensing feedback loop  630  and linear control element  685  according to an embodiment. As with the previous configuration, buck converter chip  610  comprises feedback controller  620 , CMOS PWM signal generator  675 , series inductor  640 , shunt capacitor  650 , feedback loop  630  and linear control element  685 . As discussed above, CMOS PWM signal generator  675  comprises PMOS  670  and NMOS  680  transistors to produce a periodic rectangular wave with a predetermined frequency. The duty cycle of the rectangular wave can be modulated to provide a constant output voltage Vo at the load  690 . 
     The efficacy of output voltage Vo regulation is augmented by the configuration of the linear control element  685  relative to the load  690 . Feedback power of the linear control element  685  is delivered by its own parallel channel  688  to the load  690 . There is an additional parasitic inductance  660 ′ (in addition to parasitic inductance  660 ) associated with the new electrical connection. However, the present embodiment provides isolation between the linear control element  685  and the shunt capacitor  650  (via parallel channel  688 ) which enables the linear control element  685  to be much more effective at reducing the effect of the parasitic inductance  660  on the output impedance at the load  690 . The isolation is enhanced by disposing the connection  665  of the parallel channel  688  and the power delivery channel  655  to be off the buck converter chip  610 . 
     In one or more embodiments, linear control element  685  employs PID (proportional-integral-differential), PI, or PD compensation with a cut-off frequency (e.g., unity gain bandwidth) slightly above the LC resonance frequency. The advantages of a LDO regulator include a lower minimum operating voltage, higher efficiency operation and lower heat dissipation. The combination of the feedback controller and output capacitor provide a broadband low output impedance of the buck converter. 
     The linear control element  685  can be configured to regulate the output voltage Vo when the output voltage error is greater than a minimum value (e.g., 5 mV). For example, when the output voltage error is less than 5% or less than 1% of the output voltage Vo, the linear control element  685  will not be activated and, thus, the linear control element  685  will not regulate the output voltage Vo. However, when the output voltage error is greater than 5% or greater than 10% of the output voltage Vo, the linear control element  685  will be activated and, thus, the linear control element  685  will regulate the output voltage Vo as described above. 
       FIG. 7  is a graphical juxtaposition of the impedance contributions  70  of an exemplary power converter and linear control element in frequency space. Load impedance (Z closed )  71  is graphed as a function of frequency ω  74 . Buck converter contribution  73  is similar to the low pass response of  FIG. 3 . Linear control element contribution  72  mitigates parasitic inductances between the buck converter chip and the load. 
       FIG. 8  is a graphical superposition  80  of an exemplary power converter and linear control element in frequency space. Load impedance (Z load )  81  is graphed as a function of frequency ω  84 . Buck converter and linear control element contributions are superposed to produce an ideal or substantially ideal (e.g., within 5%, 10%, or 15%) frequency response curve  82 . With the combination of the buck converter and linear regulator, the desirable flat output impedance can be achieved at the load, despite the parasitic inductance between the buck converter chip and load. 
       FIG. 9  illustrates a schematic view of an exemplary linear control circuit  90  according to an embodiment. Linear control circuit  90  comprises voltage sense node  91 , buffer capacitors  92 , bias resistors  93 , CMOS amplifier  96  and output node  97 . The linear control circuit  90  can comprise the linear control element  585  and/or  685  described above. In one or more embodiments, linear control circuit  90  turns on in the event of load current transients (e.g., deviations from a desired current) or output voltage errors (e.g., deviations from a desired output voltage). Linear control circuit  90  can be designed to have some dead band, or range of voltage deviations that do not result in any change in the output current. Linear control circuit  90  can rapidly respond to load current transients by having a very short and fast signal path. Thus, linear control circuit  90  can rapidly respond to high-frequency variations in the load current or output voltage. 
     This is achieved with parallel PMOS and NMOS common source amplifiers (comprised by CMOS amplifier  96 ) whose sources are connected to the input power supply (Vs) and the output  97  respectively. Their drains are electrically coupled to the output  97  and ground respectively. Their gates are biased by appropriate PMOS, NMOS bias voltages  94 ,  95  (via large resistors  93 ) in the subthreshold region of operation which can provide a dead band response behavior in some embodiments. In some embodiments, linear control circuit  90  comprises BJT emitter followers or other suitable amplification. The resistors  93  can be selected so that the RC time constant of high pass filter  98 , formed by buffer capacitors  92  and resistors  93 , is approximately equal to the LC resonant frequency of the low pass filter in the power conversion system (e.g., power conversion system  50 ). 
     CMOS amplifier  96  gates are also electrically coupled to voltage sense node  91  through buffer capacitors  92 . The voltage sense node  91  is electrically coupled to the voltage sense feedback loop (e.g., feedback loop  630 ). Thus, the voltage sense node  91  receives the output voltage Vo, sensed by the voltage sense feedback loop, as an input. The output voltage  97  is electrically coupled to the power delivery channel as discussed above. 
     By biasing the CMOS amplifier  96  in the subthreshold region, the transistors (PMOS and NMOS) conduct only a negligible amount of current unless the voltage applied to the voltage sense node  91  changes. If the output voltage Vo changes (and there is a voltage error), some portion of that change in voltage will be coupled onto CMOS amplifier  96  gates (via voltage sense node  91  and buffer capacitors  93 ), which will subsequently conduct current in a manner to counteract the change in output voltage Vo. The high pass filter  98  can decouple the voltage level on the voltage sense node  91  from the amplifier  96  gates. The supplemental current is conducted through output  97  to the load. If the output voltage Vo changes at a low frequency (e.g.,  10 × lower than the inverse of the RC time constant for high-pass filter  98 ), the change will not be communicated through buffer capacitors  92 . Thus, the linear control circuit  90  provides high frequency regulation to augment that of the buck converter, without conducting any (or only a negligible) steady state current, which would be more efficient for the power converter than using the linear controller alone. 
     In the event of a load current transient, the output power supply voltage Vo will change, resulting in a voltage error. This change will be communicated to the linear control circuit  90  by the voltage sense path (e.g., voltage sensing feedback loop  630 ). The change in output voltage will be directly coupled (through buffer capacitors  92 ) onto the gates of the common source amplifiers  96  (e.g., CMOS PWM signal generator  675 ) and cause one of the two devices (e.g., PMOS  670  and NMOS  680  transistors) to conduct current to the load (e.g., load  690 ). Since the bias voltage generated by resistors  93  is slightly less than the threshold voltage for the amplifier  96 , the change in output voltage can increase the bias voltage to be greater than or equal to the threshold voltage. The gain of these amplifiers  96  can be designed so that enough additional current is sourced from linear control circuit  90  to reduce the total variation in the output voltage Vo, consequently lowering and/or maintaining the effective impedance of the power supply at the load. The gain of the amplifiers  96  can be set, at least in part, by the bias voltage generated by resistors  93 . 
       FIG. 10  is a graphical depiction of a dead band response  1000  of an exemplary low-dropout (LDO) regulator. As demonstrated, LDO regulator output current  1010  is plotted as a function of delta voltage sensed  1020  between input voltage sense node and gate bias voltages.  FIG. 11  is a graphical plot  1100  representing the frequency response of the exemplary LDO regulator. Load impedance (Z closed )  1110  is graphed as a function of frequency ω  1120 . 
     This present invention is designed to be easily integrated with complementary metal oxide semiconductor (CMOS) and integrated circuit chip fabrication. In some embodiments, the linear control circuit with a parallel load channel can be integrated with a buck converter on a single chip or die. However, other scale and methods of manufacture are not beyond the scope of the present invention. 
     The embodiments described and illustrated herein are not meant by way of limitation, and are rather exemplary of the kinds of features and techniques that those skilled in the art might benefit from in implementing a wide variety of useful products and processes. For example, in addition to the applications described in the embodiments below, those skilled in the art would appreciate that the present disclosure can be applied to other applications. 
     The present invention should not be considered limited to the particular embodiments described above, but rather should be understood to cover all aspects of the invention as fairly set out herein. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable, will be readily apparent to those skilled in the art to which the present invention is directed upon review of the present disclosure.

Technology Category: 5