Abstract:
A variable efficiency and response buck converter is achieved. The device includes a multi-phase switch, the coupled coils, the filter capacitor, and the load. The multi-phase switch includes the phase control inputs, the circuit common reference, at least two pairs of complementary switches with each switch containing one upper switch and one lower switch, at least two phase control outputs from the complementary switches. The coupled inductive coils are coupled to the phase control outputs to enable weak couplings and strong couplings. Based on the working mode, equivalently the coupled coils can provide strong mutual inductances and weak mutual inductances. The filter capacitors connected to the output of the coupled coils provide high efficiency output to the load.

Description:
FIELD 
     The disclosure relates generally to variable buck converters, voltage regulators, and methods and, more particularly, to how to control the efficiency and the response of the buck converter and voltage regulators and a method thereof. 
     BACKGROUND 
     Buck converters are switching voltage regulators that operate in a step down method to provide a voltage output that is smaller than the input voltage. It accomplishes this by causing the circuit topology to change by virtue of turning on and off semiconductor devices. It uses signal switching to transfer energies into inductors. It uses a low pass filter scheme to eliminate high frequency harmonics to maintain a relatively constant output voltage and reduce the ripple of the output. 
     Typically buck converters use a feedback circuit to regulate the output voltage in the presence of load changes. They are more efficient at the cost of additional components and complexity. 
     Buck converters can be made very compact. Therefore they are popularly used for mobile devices, printed circuit boards, even in integrated circuit packages. 
     An example of a prior art buck converter circuit  600  is illustrated in a circuit schematic block diagram in  FIG. 6 a   . The circuit  600  includes the a P type switch SW 1   612 , an N type switch SW 2   614 , an energy storage inductor L  630 , the low pass filter capacitor C F    632 , and the output load R L    634 . The input voltage Vin  610  is the given high voltage. The output voltage Vout  636  is the converted voltage that is usually lower than Vin  610 . The Vcom  640  is the common reference ground of the buck converter circuit  600 . The control voltage V GS1    616  is coupled to the gate of the switch SW 1   612  to control its on and off status. The control voltage V GS2    618  is coupled to the gate of the switch SW 2   614  to control its on and off status. 
     The control voltage V GS1    616  and V GS2    618  are complementary to each other. It means that when V GS1    616  turns on the switch SW 1   612 , the V GS2    618  turns off the switch SW 2   614 . When V GS1    616  turns off the switch SW 1   612 , the V GS2    618  turns one the switch SW 2   614 . Hence, there are two working states for the buck converter in one working cycle. 
     One example of the working cycle is illustrated in  FIG. 6 b   . During the time period T s1    662 , the switch SW 1   612  is turned on. Then the switch SW 2   614  is turned off. The node voltage Vs  620  is equal to the input voltage Vin  610  since the SW 1   612  is on with almost zero voltage drop. Then the buck converter storing the magnetic energy into the inductive coil charges inductor L  630 . Then during the time period T s2    663 , the switch SW 1   612  is turned off Base on the Lenz&#39;s law, the inductor L  630  will instantaneously maintain the current flowing through it. Therefor the loop current will go through the switch SW 2   614  and turn it on. The node voltage Vs  620  is shorted to the common reference Vcom  640 . The total working cycle is T sw    666 . It is obvious that T sw =T s1 +T s2 . The inductor and the capacitor form the low pass filter that filters out the high frequency harmonics reaches the output Vout  636 . As a result, the output voltage Vout  636  is relatively constant with very small ripples. 
     If the ratio of T s1  over T sw  is defined as the duty cycle D, D=T s1 /T sw . During the period T s1    662 , the current through the inductor  630  L ramps up linearly. During the period T s2    664 , the current through the inductor  630  L ramps down linearly. To make sure the ending current of the ramping down is equal to the starting current of the ramping up so that the buck converter  600  can maintain balance, the ratio of the output Vout  636  to the input voltage Vin  610  must be equal to the duty cycle:
 
 V out/ V in= D  
 
     The output voltage Vout  636  can be further controlled through feedback schemes. One popular method is the pulse width modulation (PWM) method. The PWM mode operates switches in synchronization with a clock that has a predetermined cycle. The magnetic energy stored in the inductor is repeatedly increased and decreased periodically. Hence, the power is transferred from the input Vin  610  to the output Vout  636 . The output can be stabilized to a desired level by turning on and off the switch during synchronization with the clock. This mode is optimal for mid and high load current. However, it is not very efficient at lower load currents. Then if a buck converter  600  is to operate efficiently over a relatively wide range of load currents, including low load currents, the pulse frequency modulation (PFM) will be used. 
     The PFM is similar to PWM in the sense that the switch SW 1   612  can be used to produce a series of inductor current pulses that are applied to the filter capacitor C F    632 . However, the frequency of the pulses is not fixed. It varies in order to maintain a regulated output voltage between the upper regulated output voltage level and the lower regulated voltage level. At low load currents, PFM can provide increased efficiency as compared to PWM for the same current output. This is particularly true since the PWM operation has been optimized for efficient mid and high load current operation. 
     In view of the foregoing, buck converters have been designed to operate in a PWM mode for mid and high load currents and PFM for low load currents. 
     It is commonly known that a smaller inductor L  630  in the buck converter  600  give faster transient response, and the larger inductors give higher efficiency. High efficiency is important in all modes. But the link between the inductance of the inductor L  630  and the efficiency is particularly critical in the pulsed frequency modulation (PFM) mode. Fast load transient response on the other hand is most important in modes optimized for high load currents that can be handled by the pulse width modulation modes (PWM). 
     Inductors are just becoming available where several coils are embedded in the same package. These differ from those previously available in that their coupling ratio is much lower. Traditional multi-inductor packages were designed for use as transformers. Therefore they have a coupling ratio approaching 100%. However, with the new manufacturing techniques, multi-inductor packages are available where the coupling ratio is around 10%. 
     SUMMARY 
     If the buck converter is designed to make use of multi-inductor packages, driving the inductors with the correct architecture can bring benefits to both efficiency and load transient response. 
     It is known that small value inductors allow faster responses to the load current changes because the current variation rate dI/dt is proportional to L. Large value inductors are more efficient for a number of reasons. In PFM mode, the charge delivered to the output is fixed per cycle (for discontinuous mode). The energy wasted can also be assumed to be fixed per cycle. Anything that increases the charge delivered (without increasing the energy wasted) will increase the efficiency. A higher value inductor increases the charge delivered as it slows down the current ramp and increases the area under the current ramp curve. 
     It has been found that traditionally the load transient response (speed) and efficiency were mutually exclusive. The fast load transient response is achieved by the required small inductors. The higher efficiency is achieved by the large value inductors. The buck converter must compromise on one of the requirements. As will become clear in the following Description of the disclosure, using weakly coupled inductors, where they are in a single package, or simply deliberately coupled by their layout, the buck converter can make the most of both speed and efficiency simultaneously. 
     A principal object of the present disclosure is to provide a switch converter. 
     A further object of the present disclosure is to provide a variable efficiency and transient response voltage conversion circuit device 
     Another further object of the present disclosure is to provide a method a method to tune the inductance of the mutually coupled inductor coils to provide a lower inductance for fast converter response or a higher inductance for suppression of current ripples. 
     In accordance with the objects of this disclosure, a switch converter is achieved. The device comprises a multi-phase switch having an input voltage, mutually coupled inductive coils connected to the multi-phase switch, wherein the switching converter is capable of tuning the inductance of the mutually coupled inductor coils to provide a lower inductance for fast converter response or a higher inductance for suppression of current ripple. The multi-phase switch is capable of receiving phase control signals and transferring them into phases needed for sync modes and sleep modes. It comprises at least two pairs of complementary signals as the input to multi-phase switches, at least two pairs of complementary switches receiving complementary signals as inputs to generate sync mode or sleep mode complementary phases, and at least one pair of phase signals generated as the output from complementary switches. The two pairs of complementary signals are capable of generating the sleep mode phase when they are in phases (0°) and the sync mode phase when they are out-of-phase (180°). The pair of output phase signals are instantiated in pairs, such as 0°, 90°, 180°, 270° for 4 pairs of complementary input signals. The mutually coupled inductive coil connected to output phase signals of multi-phase switches is capable of enabling both strong and weak couplings to optimize efficiencies and speed requirements of switches, the mutually coupled inductive coil further comprises 2 inductors with mutual couplings for 0° and 180° phases, or  4  inductors with mutual couplings for 0°, 90°, 180°, 270°, or multi-inductors with more phases. The mutually coupled inductive coil connected to output phase signals of multi-phase switches has the coupling ratio less than 100%. The mutually coupled inductive coil connected to output phase signals of multi-phase switches has the coupling ratio in between 5% and 30%. The mutually coupled inductive coil connected to output phase signals of multi-phase switches has a bigger inductance when the phase signal is in the sleep mode and a smaller inductance when the phase signal is in the sync mode. The coupled inductive coil connected to output phase signals of multi-phase switches in the sleep mode has 1% higher efficiency for low output currents. The coupled inductive coil connected to output phase signals of multi-phase switches in the sync mode has faster response for high output currents. 
     Also In accordance with the objects of this disclosure, a variable efficiency and transient response voltage conversion circuit device is achieved. The device comprises a converter control unit, an array of phase control circuit units controlled by the converter control unit, a multi-phase switch controlled by the array of phase control circuit units, a mutually coupled inductive coil at the output of the multi-phase switch, a plurality of capacitors and resistors circuits at the output of the coupled inductive coil; and an output monitor circuit connected to the output of the plurality of capacitors and resistors circuits and the input of the converter control unit. The converter control unit is capable of receiving control commands and transferring them into phase control signals needed for phase control circuits. The converter control unit further comprises circuit accepting control signals for efficiency and response specifications, and a regulation circuit accepting signals from the output monitor circuit. The array of phase control units connected to the control unit is capable of converting signals from outputs of the control unit to analog complementary control signals. Each phase control circuit unit further comprises a circuit accepting control signals from the converter control unit, and a conversion circuit with certain embedded algorithms to implement complementary phase control signals. The multi-phase switch controlled by the array of phase control circuit units is capable of creating an array of complementary phase signals that enables sleep modes, sync modes, or other multi-phase modes according to the number of input complementary phase control signals. A multi-phase switch further comprises at least two pairs of complementary signals as the input to multi-phase switches, at least two pairs of complementary switches receiving complementary signals as inputs to generate sync mode or sleep mode complementary phases, and at least one pair of phase signals generated as the output from complementary switches. The mutually coupled inductive coil connected to output phase signals of multi-phase switches is capable of enabling both strong and weak couplings to optimize efficiencies and speed requirements of switches. The mutually coupled inductive coil further comprises 2 inductors with mutual couplings for 0° and 180° phases, or 4 inductors with mutual couplings for 0°, 90°, 180°, 270°, or multi-inductors with more phases. The plurality of capacitors and resistors circuits at an output of the coupled inductive coil is capable of filtering out high frequency harmonics to deliver relatively constant voltages. A plurality of capacitors and resistors circuits further comprises a plurality of capacitors shunt in between the output and the common reference, a plurality of resistors shunt in between the output and the common reference, or a plurality of capacitors and resistors shunt in between the output and the common reference. The output monitor circuit connected to the output of the plurality of capacitors and resistors circuits and the input of the converter control unit is capable of detecting the load voltage or current and feed back the control signal into the control unit for the pulse frequency modulation (PFM) or the pulse width modulation (PWM). The output monitor circuit further comprises a high impedance receiving circuit to sample the load current or voltage, a signal convertion circuit with embedded algorithms to transfer the sampled load current or voltage into control signals, and an output driving circuit to port the generated port signal into the converter control unit to facilitate the pulse frequency modulation (PFM) or the pulse width modulation (PWM). 
     Also in accordance with the objects of this disclosure, a method for controlling inductance of a switch converter is achieved. The method comprises a multi-phase switch having an input voltage, mutually coupled inductive coils connected to the multi-phase switch; and tunes the inductance of the mutually coupled inductor coils to the provide a lower inductance for fast converter response or a higher inductance for suppression of current ripple. The multi-phase switch comprises complementary switches with complementary input voltages to generated output signals with various controlled phases. The mutually coupled inductive coils comprise weakly two or more coupled coils connected to the output of the multi-phase switches. The bigger effective inductances are obtained by controlling input phases of the coupled inductance coils to be in phase and bigger effective inductances are obtained by controlling input phases of the coupled inductance coils to be out-of-phase. The buck works in the “sleep mode” when the input phases of the coupled inductive coils are in phase and in the “sync mode” when the input phases of the coupled inductive coils are out of phase. The efficiency under the “sleep mode” is highest while the load response under the “sync mode” is the fastest. The efficiency under the “sync mode” does not matter much at large load current due to the weak coupling of the inductive coils. The method can be extended to other type of bucks such as boosts and buck-boosts. 
     As such, a novel variable efficiency and response buck converter with the controlled effective inductance from the coupled coils and a method to vary the efficiency and the response needed for the voltage conversion circuit through the weakly coupled coils are herein described. The circuit provides various efficiencies and response speed to the load. The efficiency is achieved with the larger effective inductance from the weakly coupled inductive coils. The faster response is achieved with the smaller effective inductance from the weakly coupled inductive coils. The device and method are applicable to a variety of switching converters, including buck converters, boost converters and buck-boost converters. The device and method are applicable to a variety of phase control schemes, the pulse frequency modulation (PFM), and the pulse width modulation (PWM) methods. The device and method are extensible to add more pairs of weakly coupled coils. The device and method are extensible to use strongly coupled coils. Other advantages will be recognized by those of ordinary skills in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure and the corresponding advantages and features provided thereby will be best understood and appreciated upon review of the following detailed description of the disclosure, taken in conjunction with the following drawings, where like numerals represent like elements, in which: 
         FIG. 1  is a circuit schematic diagram illustrating one example of a variable efficiency and transient response buck converter in accordance with one embodiment of the disclosure; 
         FIG. 2  is a circuit schematic diagram illustrating one example of a variable efficiency and transient response buck converter with the feedback circuit and the phase control system in accordance with one embodiment of the disclosure; 
         FIG. 3  is the signal diagram illustrating one example of the inductor coil currents in a variable efficiency and transient response buck converter using the in-phase “sleep mode” in accordance with one embodiment of the disclosure; 
         FIG. 4  is the signal diagram illustrating one example of the inductor coil currents in a variable efficiency and transient response buck converter using the out-of-phase “sync mode” in accordance with one embodiment of the disclosure; 
         FIG. 5  is the efficiency diagram illustrating one example of a variable efficiency and transient response buck converter using the in-phase “sleep mode” and the out-of-phase “sync mode” in accordance with one embodiment of the disclosure; 
         FIG. 6  is a circuit schematic block diagram illustrating a prior art, buck converter circuit. 
         FIG. 7  is the flow chart illustrating the methodology of using weakly coupled inductive coils to tune both speed and efficiency simultaneously. 
     
    
    
     DESCRIPTION 
       FIG. 1  is a circuit schematic diagram illustrating one example of a variable efficiency and response buck converter  100  in accordance with one embodiment of the disclosure. The device  100  includes a multi-phase switch  110 , the coupled coils  140 , the filter capacitor  150 , and the load  164 . The multi-phase switch  110  includes input Vin  112 , the circuit common reference Vcom  114 , the upper switch SW 11   124 , the low switch SW 12   126 , the upper switch SW 21   128 , the low switch SW 22   130 , the input V GS11    116 , the input V GS12    118 , the input V GS21    120 , the input V GS22    122 , the output  132  from the upper switch SW 11   124  and the low switch SW 12   126 , and the output  134  from the upper switch SW 21   128  and the low switch SW 22   130 . The coupled coils  140  includes inductor L 1   142 , the inductor L 2   144 , the mutual inductance L 12   146 , the input  148  to the inductor L 1   142 , the input  149  to the inductance L 2   149 , the output  162 . The filter capacitor  150  includes the filter capacitor C F    152 . The load is simply represented by the load resistor R L    164 . The output of the buck converter is Vout  170 . 
     In the multi-phase switches  110 , the switch SW 11   124  and SW 12   126  form a pair and are preferably coupled at the node  125 . The switches may be implemented in any available technology, such as MOS or bipolar or mixed technology. The input V GS11    116  is preferably coupled to the switch SW 11   124 . The input V GS12    118  is preferably coupled to the switch SW 12   126 . The output  132  is preferably coupled at the node  125  to both the upper switch SW 11   124  and the low switch SW 12   126 . The switch SW 21   128  and SW 22   130  form a pair and are preferably coupled at the node  129 . The switches may be implemented in any available technology, such as MOS or bipolar or mixed technology. The input V GS21    120  is preferably coupled to the switch SW 21   128 . The input V GS22    122  is preferably coupled to the switch SW 22   130 . The output  134  is preferably coupled at the node  129  to both the upper switch SW 21   128  and the low switch SW 22   130 . 
     In the coupled coils  140 , the input  148  is preferably coupled to the output  132  from the multi-phase switches  110 , and the input  149  is preferably coupled to the output  134  from the multi-phase switches  110 . The inductor L 1   142  is preferably coupled to the inductor L 2   144  at the node  145 . The other end of inductor L 1   142  is preferably coupled to the input  148 . The other end of inductor L 2   144  is preferably coupled to the input  149 . The output  162  is preferably coupled to the coupled coils  140  at the node  145 . 
     In the filter capacitor  150 , one side of the filter capacitor C F    152  is coupled to the input  162 . The other side of the filter capacitor C F    152  is coupled to the common reference ground Vcom  114 . The output  154  of the filter capacitor  150  is coupled to one side of the load resistor R L    164  at the node  160 . The output Vout  170  is preferably coupled to the load resistor R L    164  at the node  160 . 
     While the embodiment illustrates one coupled coils with only two pairs of switches, it should be understood that multiple coupled coils with multiple pair of switches may be used in the present disclosure. 
     In the preferred embodiment, the input V GS11    116  and the input V GS12    118  are controlled by complementary pulsed signals. Thereby the switch SW 11   124  has opposite on/off status relative to the SW 12   126 . The input V GS21    120  and the input V GS22    122  are controlled by complementary pulsed signals. Thereby the switch SW 21   128  has opposite on/off status relative to the SW 22   130 . Because the symmetry of the circuit configuration in the multi-phase switches  110 , the output  132  and  134  from the multi-phase switches  110  will have identical waveforms except the phase. The output  132  and  134  could be in-phase or out-of-phase. When the output  132  and  134  are in-phase, the buck converter  100  is working under the “sleep mode”. When the output  132  and  134  are out-of-phase, the buck converter  100  is working under the “sync mode”. The sleep mode is a mode for the low output currents and high efficiency. It is preferably for the pulse frequency modulation (PFM). The sync mode is a mode for the larger load transients. It is preferably for the pulse width modulation (PWM). 
     The inductor L 1   142  and L 2   144  form a pair of inductors. They have the mutual inductance L 12   146 . If the inductor L 1   142  and L 2   144  are weakly coupled, the mutual inductance L 12   146  is small. The current in the inductor L 1   142  will affect the value of the current in the inductor L 2   144  if the mutual inductance L 12  is not equal to zero. If the input  148  of the inductor L 1   142  and the input  149  of the inductor L 2   144  are in-phase, both inductors are operated in the same polarity. Hence, the current in the inductor L 1   142  and the current in the inductor L 2   144  will both ramp with the same polarity. The coils will interfere constructively. The current in the inductor L 1   142  will act together with the current in the inductor L 2   144 , and vice versa. This constructive interference increases the effective inductance to the buck converter  100 . As one example, if the two inductors are matched and coupled with 10% coupling ratio, the increase in the effective inductance is also 10%. If the input  148  of the inductor L 1   142  and the input  149  of the inductor L 2   144  are out-of-phase, both inductors are operated in the opposite polarity. Hence, the current in the inductor L 1   142  and the current in the inductor L 2   144  will ramp with the opposite polarity. The coils will interfere destructively. The current in the inductor L 1   142  will act inversely with the current in the inductor L 2   144 , and vice versa. This destructive interference decreases the effective inductance to the buck converter  100 . 
     In the proposed embodiment, the coupled coils can be implemented in the same IC package. With the new manufacturing techniques, the weakly coupled coils are available in package where the coupling ratio is around 5% to 30%. It is also understandable that the proposed embodiment does not have a limit to the coupling ratio. Hence, coupled coils made by other manufacturing techniques with strong inductive coupling are also included in the proposed embodiment. 
     In the proposed embodiment, the buck converter  100  has at least two pairs of switches in the multi-phase switches  110 . The switches SW 11   124  and the SW 12   126  are one pair with the output  132 . The switches SW 21   128  and the SW 22   130  are another pair with the output  134 . The inductor L 1   142  is controlled by the output  132 . The inductor L 2   144  is controlled by the output  134 . 
     The buck converter  100  works in the “sleep mode” when the output  132  and  134  are in-phase. In this case, if the current in the inductor L 1   142  ramps up, the current in the inductor L 2   144  also ramps up. Hence, the coupled coils  140  appear to have larger effective buck converter inductance than the nominal inductance value of each individual inductor L 1   142  and L 2   144 . Because the larger buck inductance results in higher efficiency, the “sleep mode” will show higher efficiency, especially for the low output currents in the PFM mode. 
     The buck converter  100  works in the “sync mode” when the output  132  and  134  are out-of-phase. In this case, if the current in the inductor L 1   142  ramps up, the current in the inductor L 2   144  ramps down. Hence, the coupled coils  140  appear to have a smaller effective buck converter inductance than the nominal inductance value of each individual inductor L 1   142  and L 2   144 . Because a smaller buck inductance allows the output current to change more rapidly in the response to a load step, the buck converter  100  will have better load transient response, especially for the larger load transient in the PWM mode. The efficiency in this mode is not typically limited by the coil value. Hence, the decease in the efficiency caused by the smaller inductance value is negligible. 
     Referring now to  FIG. 3 , it is one example diagram of the currents in the inductor L 1   142  and inductor L 2   144  in the “sleep mode” where high efficiency is needed. In this mode, the current Icoil (L 1 )  312  in the inductor L 1   142  is in-phase with the current Icoil (L 2 )  322  in the inductor L 2   144 . They ramp in phase. This increases the effective buck converter inductance. Hence, the current Icoil (L 1 )  312  under the in-phase coupling ramps slower than the current Icoil (L 1 )  314  that has no coupling. It is noted that the current Icoil (L 1 )  312  has greater area underneath it than the current Icoil (L 1 )  314 . It implies that the energy delivered under the coupling case is higher than the energy delivered without the coupling. This justifies why the efficiency is higher in the “sleep mode”. 
     Referring now to  FIG. 4 , it is one example diagram of the currents in the inductor L 1   142  and inductor L 2   144  in the “sync mode” where the larger transient response is needed. In this mode, the current Icoil (L 1 )  412  in the inductor L 1   142  is out-of-phase with the current Icoil (L 2 )  422  in the inductor L 2   144 . Hence, they ramp out of phase. This decreases the effective buck converter inductance. Hence, the current Icoil (L 1 )  412  under the out-of-phase coupling ramps faster than the current Icoil (L 1 )  414  that has no coupling. It implies that the buck converter  100  under the coupling case responses faster to the load step than the case without the coupling. This justifies why better load transient response can be achieved in the “sync mode”. 
     The proposed embodiment can be implemented in exactly the opposite way. The polarity of the coupled coils  140  can be reversed. As a result, the “sleep mode” will be invoked when the output  132  and  134  are out of phase while the “sync mode” will be invoked when the output  132  and  134  are in phase. 
     Referring now to  FIG. 5 , it shows one example of the calculated efficiency of the buck converter  100  from the proposed embodiment. The efficiency  512  vs. the load current shows the efficiency of the buck converter  100  when it is working in the “sleep mode” with the mutual inductance L 12   146  equal to 1.2 uH. The efficiency  514  vs. the load current shows the efficiency of the buck converter  100  when it is working in the “sleep mode” with the mutual inductance L 12   146  equal to 0.8 uH. The efficiency  522  vs. the load current shows the efficiency of the buck converter  100  when it is working in the “sync mode” with the mutual inductance L 12   146  equal to 1.2 uH. The efficiency  524  vs. the load current shows the efficiency of the buck converter  100  when it is working in the “sync mode” with the mutual inductance L 12   146  equal to 0.8 uH. The improvement of the efficiency in the “sleep mode” has 1% for the same load transient response. The decrease in “sync mode” efficiency is negligible for significant load currents.  FIG. 5  has been verified both experimentally and in simulation. Hence, the proposed embodiment is confirmed to be effective. 
     Referring now to  FIG. 7 , it shows the flowchart of the methodology of using the weakly coupled inductive coils to achieve both speed and efficiency from the proposed embodiment. It begins with the load level judgment  702 . If the load is low, the sleep mode will be invoked through  710 . Then the weakly coupled inductive coils  140  will be fed by the in-phase control signals. Large equivalent inductance will be achieved. Then the generate bulk output at  730  will have very good efficiency. If the load is high, the sync mode will be invoked through  720 . Then the weakly coupled inductive coils  140  will be fed by the out-of-phase control signals. Less equivalent inductance will be achieved. Then the generate bulk output at  730  will have better response speed. At the output of the bulk  730 , a sample of the output will be taken through  740 . The sampled load information will be fed to the input of  702 . 
     Referring now to  FIG. 1 , it shows one example when the multi-phase switches  110  contains only two pairs of complementary switches. One pair includes the switches SW 11   124  and SW 12   126 . Another pair includes SW 21   128  and SW 22   130 . However, the proposed embodiment can be extended to more pairs in the multi-phase switches  110 . For example, there can be four pairs of complementary switches in the multi-phase switches  110 . As a result, the buck converter  100  becomes a four-phase converter. In the PFM mode, the four phases are typically in phase (in the “sleep mode”) to achieve the maximum efficiency. But in the “sync mode”, the phases would typically be equally spaced with 0, 90, 180, and 270 degree delays. The phases would therefore be paired as 0, 90, 180, and 270 degree. 
     In the proposed embodiment, the coupled coils  140  can be extended to include more than one pair of coupled inductances L 1   142  and L 2   144 . The drive scheme with different phases can be applied together with the combination of many pairs of coupled coils to adjustable efficiency and load transient response. It shall be able switch to achieve the maximum effective inductance for a high efficiency mode and or the fast response for the low output mode. 
     An alternative embodiment is to disable one phase. If a phase is disabled, then the output of the disabled phase may be set to go high-impedance. In this case the voltage on the secondary coil—the one on the disabled phase—may vary with the voltage on the primary coil. Thus the primary coil will act like a simple uncoupled inductor. For this embodiment, preferably, both phases through the one pair of coupled inductances L 1   142  and L 2   144  can be driven out-of-phase in the “sync mode” to minimize the effective capacitance. Then in the “sleep mode” the secondary phase can be disabled. Consequently the effective inductance will rise back to the nominal (uncoupled) value. This will increase the efficiency. 
     Referring now to  FIG. 2 , a circuit schematic diagram illustrate one example of the variable efficiency and response buck converter  200  using the control unit  210 , the phase control units  212  and  214 , and the output monitor  230  with one embodiment of the disclosure. The circuit  200  could be on a mobile device, such as a cellular phone, or on an integrated circuit chip, such as CPU. The system  200  includes a control unit  210 , the phase control units  212  and  214 , the output monitor  230 , the multi-phase switches  216 , the coupled coils  222 , the filter cap  224 , and the load resistor R L    226 . The control unit  210  generates phase control signals  252  and  254  based on the pulse frequency modulation (PFM) or the pulse width modulation (PWM) need. The control unit  210  is also preferably coupled to the input V MNT    234  from the output monitor circuit  230 . The output  252  and  254  from the control unit  210  are coupled to the two phase control units  212  and  214  respectively. The phase control unit I  212  generates complementary analog signals  262  and  264  coupled to the multi-phase switches  216 . The phase control unit II  214  generates complementary analog signals  266  and  268  coupled to the multi-phase switches  216 . The multi-phase switches  216  includes at least 2 pairs of complementary switches (the upper switch and the low switch). If there are only 2 pairs of complementary switches in the multi-phase switches  216 , the phase output  272  and  274  are coupled to the input of the coupled coils  222 . The coupled coils  222  includes at least a pair of weakly coupled inductive coils. The output  240  of the coupled coils  222  is preferably coupled to the filter capacitor  224 . The filter capacitor  224  and the load resistor R L    226  are in parallel. The output voltage Vout  228  is coupled to the load resistor R L    226 . The output voltage Vout  228  is preferably coupled to the input V FB    232  of the output monitor  230 . The output monitor  230  monitors the output voltage Vout  228  or the load current Iout  242  and generates the control signal V MNT    234  coupled to the control unit  210 . 
     In the embodiment, the control unit  210  contains the circuit for automatically switching between PWM and PFM modes based on the input V MNT    234  coupled from the output monitor  230 . The output monitor  230  includes a reference generator to generate the reference signal for the error amplifier. The error amplifier generates the error signal between the feedback signal V FB    232  from the circuit output and the reference signal. The error signal is coupled to the control unit  210  to decide which of the PWM and PFM modes will be used. 
     The number of the coupled coils  222  can be more than one. Correspondingly the multi-phase switch  216  coupled to the inputs of the coupled coils can be more than one. The number of the multi-phase switch  216  will be equal to the number of the coupled coils  222 . Consequently there will be more the phase control units  212  and  214 . The number of the phase control unit is equal to the 2 times of the multi-phase switch  216 . 
     As such, a novel variable efficiency and response buck converter with weakly coupled coils are herein described. The circuit uses the mutually coupled coils in a multi-phase switching converter to effectively achieve an electrically tunable inductance for energy storage, and to tune the inductance according to whether a low inductance is desired for the fast converter response, or a large inductance to suppress the current ripple. 
     The device and method are applicable to a variety of programmable buck converters and buck schemes. The device and method are extensible to add more pairs of coupled coils and more multi-phase switches. Other advantages will be recognized by those of ordinary skill in the art. 
     The above detailed description of the disclosure, and the examples described therein, has been presented for the purposes of illustration and description. While the principles of the disclosure have been described above in connection with a specific device, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the disclosure.