Abstract:
Various embodiments of the invention improve the dynamic response and output voltage accuracy in buck DC-DC converters. In certain embodiments, power efficiency is improved by storing and recycling energy that is otherwise lost during successive load transient events.

Description:
BACKGROUND 
     A. Technical Field 
     The present invention relates to DC-DC buck voltage circuits, and particularly, to systems, devices, and methods of suppressing transient load responses in DC-DC buck voltage converters. 
     B. Background of the Invention 
     Buck or step-down DC-DC converters efficiently convert a high input DC voltage into a specified lower output DC voltage at a higher current. Buck converters are widely used in processor and power applications for consumer electronic equipment, such as desktop and portable computers; in communications equipment, such as handheld battery powered devices; in industrial applications; and in automotive applications. Buck converters often require exceptional output voltage accuracy, a reduced transient load response, and fast output voltage programming. Transient load requirements are important in low-voltage applications which are subject to fast changing load conditions or require frequent transitions between output voltage settings during operation. High-speed, high-accuracy system loads include central processing units, baseband, application and graphics processors, as well as power amplifiers. Slew rate and efficiency are important in dynamic voltage scaling and power amplifier applications. 
     The inductance of a DC-DC buck converter is sized to store adequate magnetic energy and keep output current and voltage ripple at reasonably low values. However, the transient output response to fast changes in system voltage and load current is limited by the current slew rate of the inductor. Additionally, prior art DC-DC buck converters which allow large output slew rate, suffer from low operating efficiency. Therefore, a need exists to overcome the deficiencies and inadequacies of the prior art. 
     SUMMARY OF THE INVENTION 
     Various embodiments of the invention allow for improved transient output response in buck DC-DC converters. This is accomplished simultaneously by passing and storing energy followed by subsequent release of the stored energy during successive output transient events; output load transients or voltage programming commands. In particular, certain embodiments of the invention allow for reducing output voltage deviations for a given driving voltage when a dynamic system voltage or load changes occur. This is accomplished by operating one or more switches in various states. In one embodiment, a switch is coupled in parallel with an energy storage element and operates in a linear mode, which increases the total current slew rate to a system load, thereby eliminating inductor current slew rate limitations inherent in buck DC-DC converters. In another embodiment, the switch allows a circulating current to temporarily store energy within an inductor until, in a subsequent positive voltage or current step, the energy is released to the system load, thereby allowing for a reduced output voltage deviation, reduced output capacitance and improved power efficiency of the circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments. 
         FIG. 1A  is a schematic of a prior art buck converter with transient system load during an “ON” transient. 
         FIG. 1B  is a schematic of a prior art buck converter with transient system load during an “OFF” transient. 
         FIG. 2  shows exemplary voltage and current output waveforms of a prior art buck converter in response to load transients. 
         FIG. 3  is a schematic of an illustrative buck converter during a positive load transient according to various embodiments. 
         FIG. 4  shows a buck converter during a negative load transient according to various embodiments. 
         FIG. 5  shows exemplary voltage and current output waveforms of a buck converter in response load transients according to various embodiments. 
         FIG. 6  is a flowchart of an illustrative process for reducing transient load responses in DC-DC buck voltage converters in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for the purpose of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, described below, may be performed in a variety of ways and using a variety of means. Those skilled in the art will also recognize additional modifications, applications, and embodiments are within the scope thereof, as are additional fields in which the invention may provide utility. Accordingly, the embodiments described below are illustrative of specific embodiments of the invention and are meant to avoid obscuring the invention. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase “in one embodiment,” “in an embodiment,” or the like in various places in the specification are not necessarily all referring to the same embodiment. 
     Furthermore, connections between components or between method steps in the figures are not restricted to connections that are effected directly. Instead, connections illustrated in the figures between components or method steps may be modified or otherwise changed through the addition thereto of intermediary components or method steps, without departing from the teachings of the present invention. 
       FIG. 1A  is a schematic of a prior art buck converter with transient system load during an “ON” transient. Buck converter  100  regulates its output voltage by adjusting the operating duty cycle of switches  102  and  104  that are turned ON during their conduction state and OFF during their non-conduction state. The duty cycle of buck converter  100  is equal to the ratio of ON time of switch  102  to the total switching interval. Assuming that the step of output load current  120  has an instantaneous slew rate and, assuming further, that the response of controller  106  is instantaneous, it can be shown that the output voltage deviation, ±ΔV OUT , is proportional to the load current step size and to the slew rate, ±dI OUT /dt, of output load current  120 . 
     During a positive load transient, for example when load  110  is coupled to capacitor  114 , output load current  120  is stepped to the level of I OUT =V OUT /RL, as shown in  FIG. 2 . Controller  106  responds to the feedback signal by activating switch  102  ON and switch  104  OFF. Assuming that input voltage  116  and output voltage  118  are constant, the positive inductor slew rate +d(I L1 )/dt through inductor  112  can be calculated as: 
     
       
         
           
             
               
                 + 
                 
                   ⅆ 
                   
                     ( 
                     
                       I 
                       
                         L 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     ) 
                   
                 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 
                   
                     + 
                     Δ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     I 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                 
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     T 
                     ON 
                   
                 
               
               = 
               
                 
                   
                     V 
                     IN 
                   
                   - 
                   
                     V 
                     OUT 
                   
                 
                 
                   L 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
       FIG. 1B  is a schematic of a prior art buck converter with transient system load during an “OFF” transient. During a negative load transient, for example when load  110  is uncoupled from output capacitor  114 , excess energy stored in inductor  112  is released to output capacitor  114  resulting in an overcharging of output capacitor  114 , as shown in  FIG. 2 . Controller  106  responds to the feedback signal by activating switch  102  OFF and switch  104  ON. Assuming that input voltage  116  and output voltage  118  are constant, the negative inductor slew rate +d(I L1 )/dt through inductor  112  can be calculated as: 
     
       
         
           
             
               
                 - 
                 
                   ⅆ 
                   
                     ( 
                     
                       I 
                       
                         L 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     ) 
                   
                 
               
               
                 ⅆ 
                 t 
               
             
             = 
             
               
                 
                   
                     - 
                     Δ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     I 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                 
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     T 
                     OFF 
                   
                 
               
               = 
               
                 
                   - 
                   
                     V 
                     OUT 
                   
                 
                 
                   L 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     As can be seen from the previous two equations, one possible approach for increasing the inductor slew rate is to reduce the size of inductor  112 , which is an impractical approach due to poor DC pass filtering and high output voltage ripple seen at the load. Some prior art solutions provide for a ground switch having a lossy component for the purpose of dissipating the excess energy stored in inductor  112  to prevent an overcharging of output capacitor  114 . However, this approach is very inefficient. 
       FIG. 2  shows exemplary voltage and current output waveforms of a prior art buck converter in response to load transients and graphically highlights the voltage output overshoot and undershoot problem. The buck converter is a prior art buck converter, such as the one depicted in  FIG. 1 . In response to a positive load transient caused by connecting the buck converter to a purely resistive load, output load current  202  instantly steps from 0 A to V OUT /R L . Output voltage  218  depicts the voltage present at the output capacitor C 1 , shown in  FIG. 1 . Inductor current  204  through the inductor is rate limited by a slew rate such that, following the positive load transient, inductor current  204  reaches steady state condition  206  only after an initial time period. At steady state, the controller alternates between turning ON switches S 1  and S 2  in a standard duty cycle to maintain an average inductor current that is equal to the output current, i.e. I L1     —     AVG =I OUT . 
     In  FIG. 2 , area  210  represents the difference between output load current  202  and inductor current  204  prior to the two values crossing each other during the time depicted as ON response time  224 . Area  210  is proportional to the amount of charge removed from output capacitor C 1  shown in  FIG. 1 . It is also proportional to output voltage undershoot  212 , i.e. to the drop in output voltage  218  that exists during ON response time  224 . 
     In response to a negative load transient caused by disconnecting purely resistive load R L , output load current  202  instantly steps from V OUT /R L  to 0 A. Output voltage overshoot  222  depicts the output voltage present at the output capacitor C 1 . Inductor current  204  through the inductor drops from its steady state level V OUT /R L  to a negative current value. However, inductor current  204  is limited in its rate of change by the negative inductive slew rate. Thus, during the initial time period after the system load is removed, the buck converter is unable to instantly reduce the current flow through the inductor. The time period before inductor current  204  is reduced to an average of OA is depicted as OFF response time  228 . 
     Area  220  represents the difference between load current  202  and inductor current  204 . Area  310  is proportional to the amount of charge added to the output capacitor C 1  and also proportional to output voltage overshoot  222 . 
       FIG. 3  shows a buck converter during a positive load transient according to various embodiments. Controller  316  is coupled to control one or more switches. Switch  302  is coupled between a DC voltage source  311  and inductor  312 . Switch  304  is shunted from one leg of inductor  312  to a ground reference  306 . Switch  308  is coupled between controller  316  and system load  310 , for example, via an output switch (not shown). Switch  308  is further coupled in parallel to inductor  312 . The parallel combination of switch  308  and inductor  312  is coupled to output capacitor  312  which is shunted to ground reference  306 . Switches  302  and  304  can be any type of switch, such as bipolar devices, complementary metal-oxide-semiconductors, junction gate field-effect transistors, metal-oxide semiconductor field-effect transistors, or other switches known to one skilled in the art, whereas switch  308  is a bidirectional switch capable of passing current in two directions. In certain embodiments, switch  308  is capable of being controlled by controller  316  to operate in multiple modes, examples of which include: ON, OFF, and in linear mode. 
     In various embodiments, output capacitor  314  is shunted to ground reference  306  and couples buck converter  300  to system load  310  that may be purely resistive or contain additional reactive components. Buck converter  300  is designed to convert a relatively high input DC voltage signal  311 , for example 24V to another relatively lower output DC voltage signal level  418 . A positive load transient is generated as a result of coupling buck converter  300  to system load  310 . In one embodiment, following the positive load transient, controller  316  responds to the feedback signal and controls 1) switch  302  to turn ON to produce a low impedance condition that allows maximum current flow, 2) switch  304  to turn OFF to produce a high impedance condition that allows only a minimum current to flow, and 3) switch  308  to operate in a linear voltage regulation mode. 
     When switch  308  operates in linear mode, it functions like a linear regulator that passes an additional current  324  from input source  311  to output capacitor  314 . Switch  308  is sized to maintain load current  320  for a given differential input voltage of source  311  to output voltage  318 . In  FIG. 3 , switch  308  is depicted as a variable resistor that is controlled by controller  316  to carry current  324  that allows the circuit to maintain a predetermined output voltage  318 , which would otherwise not be possible when switch S 1  is closed. The additional current  324  in the circuit allows load current  312  in system load  310  to ramp up more quickly than it would without the existence of switch  308 . The resulting increase in current slew rate in system load  310  reduces the output voltage undershoot. The current slew rate is determined by the speed and capability of controller  316  to drive switch  308  in response to the feedback signal. 
     In one embodiment, following a negative load transient, in response to the feedback signal controller  316  controls switch  302  to turn OFF, switch  304  to turn OFF, and switch  308  to turn ON so as to allow the energy that has built up in inductor  312  to be maintained by circulating through  308  during the OFF response until, in the subsequent load step when system load  310  is recoupled to buck converter  300 , the energy can be recycled by being redirected to system load  310  to sustain load current  320 . 
     At steady state, controller  316  times switch  308  to turn OFF, such that all of load current  320  passes through inductor  312 , and the circuit operates in the same manner as a prior art buck converter. At steady state, controller  316  times the duty cycle of switches  302  and  304  to alternately turn ON such as to maintain an average current through inductor  312  that equals load current  320 . 
     It will be appreciated by one skilled in the art that only one of many possible configurations for switches  302 ,  304  and  308  are illustrated  FIGS. 3 and 4 , and that multiple switches may be coupled to multiple system loads to achieve the same improvement in load transient response. 
       FIG. 4  shows a buck converter during a negative load transient according to various embodiments. Switch  408  is a bi-directional switch coupled between controller  416  and system load  410 , for example via an output switch (not shown). Switch  408  is further coupled in parallel to inductor  412  and is shunted to ground  406  via output capacitor  414 . System load  410  is typically a resistive load. 
     A negative load transient is generated as a result of uncoupling buck converter  400  from system load  410 , for example, by removing the output load resistor RL (OFF response). During the OFF response, controller  416  may respond to the feedback signal which detects an overshoot at the output voltage and control switch  402  to turn OFF, switch  404  to turn OFF, and switch  408  to turn ON. In ON mode, negative current  424  (−IS3) is maintained through switch  408 . Negative current  424  flows in a direction opposite to the linear mode and maintains previously stored electrical energy within inductor  412 . During the subsequent load step (ON response), the output load resistor is decreased and switch  408  is turned OFF again in response to the feedback signal, which allows stored energy  412  to be redirected to system load  410 . The stored inductor energy decreases the energy required to build up in inductor  412  to sustain output current  420  through system load  410 , thereby, increasing the operating efficiency of circuit  400 . 
       FIG. 5  shows exemplary voltage and current output waveforms of a buck converter, such as the buck converter depicted in  FIG. 4 , in response to a positive and a negative load transient according to various embodiments. In response to a positive load transient caused by, for example, connecting a resistive load to the output of the buck converter, output load current  502  instantly steps from 0 A to V OUT /R L . Output voltage  518  depicts the voltage present at the output capacitor C 1 . Although inductor current  504  is rate limited by a slew rate, the bi-directional switch allows for an additional current path for current  508  that flows parallel to inductor current  504 . Current  508  that passes through the bi-directional switch increases the rate at which current  502  becomes available to the output load. As inductor current  504  increases, the amount of current  508  passing through the bi-directional switch is reduced until the inductor current  504  overlaps with 508 in a manner that the sum of both currents equals the current demanded by the system load. The increase in the positive output current slew rate results in a reduced output voltage undershoot  512  that exists during ON response time  514 . 
     In  FIG. 5 , area  510  is proportional to the charge removed from the output capacitor C 1 . This charge is proportional to output voltage undershoot  512 . Because additional current  508  is provided through the bi-directional switch during ON response time  514 , less inductor current  504  is needed to recharge the output capacitor C 1 . Therefore, output voltage undershoot  512  is significantly reduced, and inductor current  504  exceeds output load current  502  less than when compared to the typical output voltage undershoot of a prior art buck converter. As a result, output capacitor C 1  can be designed to have a smaller capacitance than in prior art buck converters. 
     At steady state  516 , switch S 3  remains turned OFF and the controller alternates between turning ON switches S 1  and S 2  in a standard duty cycle to maintain an average inductor current that is equal to the output current. In response to a negative load transient following the steady state condition, output load current  502  may instantly step from V OUT /RL to 0 A, while inductor current  504  decreases with slight negative slope  526  from its steady state level V OUT /R L  to a level determined by the voltage drop across switch S 3 . The inductor current slew rate during OFF response time  524  is determined by the voltage drop across switch S 3  and the inductor DC winding resistance (RDC) as: 
     
       
         
           
             
               
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                         RDC 
                       
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     wherein RDS S3  is the DC resistance of switch S 3  and RDC is the inductor&#39;s DC winding resistance. Output voltage overshoot  522  depicts the increased output voltage present at the output capacitor C 1  for negative load transients. Area  520  is proportional to the charge added to the output capacitor and proportional to output voltage overshoot  522 . Inductor current  504  will decay to zero when ON response time  514  is equal to or exceed a maximum OFF response time defined as: 
     
       
         
           
             
               T 
               
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                 ⁡ 
                 
                   ( 
                   MAX 
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     At T OFF(MAX)  the energy stored in the inductor is lost. 
     In one embodiment, ON response time  514  is adjusted to prevent output voltage  518  from deviating beyond a predetermined value, which further reduces the power consumption of the buck converter. 
       FIG. 6  is a flowchart of an illustrative process for reducing transient load responses in DC-DC buck voltage converters in accordance with various embodiments of the invention. 
     At step  602 , the controller receives a signal regarding a load transient condition. 
     At step  604 , the controller determines whether the output voltage of the DC-DC buck voltage converter exceeds a predetermined range. 
     If the DC-DC buck voltage converter falls outside a predetermined range, then, at step  606 , the DC-DC buck voltage converter determines whether the outside exceeds a predetermined value. 
     If the predetermined value is a negative output voltage as determined by the feedback signal, then, at step  608 , the controller activates a first switch S 1  to turn ON, a second switch S 2  to turn OFF, and a third switch S 3  to operate in a linear voltage regulation mode until, at step  610 , a load current is stabilized to operate in a steady state condition by turning switch S 3  OFF and timing the duty cycle of switches S 1  and S 2  to alternately turn ON to maintain an average output current. 
     If at step  606  the predetermined value is a positive output voltage, then, at step  808 , the controller activates the first switch S 1  to turn OFF, the second switch S 2  to turn OFF, and the third switch S 3  to turn ON until, in a subsequent step  606  the predetermined value is again a negative output voltage. 
     It will be appreciated to those skilled in the art that the preceding examples and embodiments are exemplary and are for the purposes of clarity and understanding and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, combinations, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention. It is, therefore, intended that the claims in the future non-provisional application will include all such modifications, permutation and equivalents as fall within the true spirit and scope of the present invention.