Abstract:
Various embodiments of the invention provide for an adaptive headroom controller circuit that increases the efficiency of switch mode pre-regulator circuits that supply power to multiple linear sub-regulators. Certain embodiments increase efficiency by dynamically modulating the output voltage of the pre-regulator in response to varying headroom voltages requirements, which allows sub-regulators to operate at their individually optimized headroom voltage, thereby, extending battery life and, at the same time, avoiding the triggering of a drop-out condition. 
     In certain embodiments of the invention, further efficiency improvements are provided by selectively operating low drop-out regulators in regulator and load-switch mode. The innovation is applicable to modern mobile PMIC switching pre-regulator architectures (e.g., buck, buck/boost, or boost type) powered by a single high-voltage Li-ion battery and followed by a group of low drop-out type sub-regulators that share a common, pre-regulated low voltage input that drives multiple low-voltage outputs.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims priority to U.S. Provisional Application Ser. No. 61/841,093, titled “Adaptive Headroom Control to Maximize PMIC Operating Efficiency,” filed on Jun. 28, 2013 by Stephen W. Hawley, KyungTak Lee, ChiYoung Kim, and Rui Liu which application is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A. Technical Field 
     The present invention relates to linear regulators and, more particularly, to systems, devices, and methods of improving operating efficiency of linear regulators through adaptive headroom control. 
     B. Background of the Invention 
     In order to maximize efficiency of linear regulators, it is desirable to operate at the lowest possible input voltage since, for a given linear regulator output current, the efficiency of a linear regulator is a function of its output voltage divided by the input voltage. However, reducing the input voltage of a linear regulator below its drop-out voltage results in poor output regulation and/or noise performance. 
     Some existing designs utilize a pre-regulator that is coupled between a relatively high voltage power source and a relatively low linear regulator output voltage in order to reduce the input voltage of the low voltage regulator relative to its output voltage in order to avoid the degradation of efficiency associated with high regulator input voltages. 
     In particular, PMIC in architectures with multiple low-dropout linear regulators (LDO) that operate loads requiring differing power levels LDOs are oftentimes grouped together and coupled to the common output of a single pre-regulator. For example, a number of regulators that are designed to provide an output voltage of around 1.5 V are grouped together and driven from the same voltage rail and by the same output voltage of a pre-regulator. The pre-regulator output voltage is typically preset to a fixed, maximum value required by any linear regulator within the group in order to avoid drop-out conditions. 
     In addition, the minimum required headroom for each LDO regulator is taken into account to formulate a worst-case system voltage requirement, which is typically equal to the sum of the programmed pre-regulator output voltage and the highest headroom voltage requirement within the group of LDO regulators. The headroom serves as a safety margin that accounts for expected variations encountered during regular operation. 
     However, even when PMICs drive each group of LDO regulators with a dedicated pre-regulator, such open-loop topologies use static headroom settings that are characterized by larger than necessary headroom margins and input voltages for the majority of the LDO regulators within the group and, therefore, negatively impacts system efficiency. What is needed are tools for system designers to overcome the above-mentioned limitations. 
     SUMMARY OF THE INVENTION 
     In various embodiments of the invention, an adaptive headroom control circuit initializes and controls a pre-regulator in a manner so as to maintain a minimum output voltage that the pre-regulator supplies to a group of linear regulators. In particular, the pre-regulator output voltage is determined by the linear regulator with the highest input voltage requirement within a group of regulators. In this way, the pre-regulator satisfies a worst-case operating headroom voltage requirement for each regulator under varying operating conditions. 
     The adaptive headroom control circuit achieves this in certain implementations by monitoring and adaptively minimizing the input voltages of the linear regulators utilizing a combination of analog, digital circuitry, and/or software methods. Minimizing comprises identification of headroom voltages for linear regulators and selection of the highest of the input voltages while maintaining a minimum dropout voltage common to all linear regulators within the group. The linear regulator which determines the pre-regulator output voltage is referred to as the “master” sub-regulator. The control circuit dynamically adjusts the pre-regulator output voltage as determined by the master sub-regulator operating conditions (output voltage, output load, temperature, etc.). In addition, the selection of a master sub-regulator is determined dynamically by the operating conditions of all identified sub-regulators (output voltage, output load, temperature, etc.). In this way, the control system implements an entirely adaptive headroom control mechanism. 
     Headroom voltages are adjusted in response to varying operating conditions, including on/off states of the linear regulator, device-to-device headroom voltage variations, die temperature, output noise, and output load variations. This causes the minimum dropout voltage and the pre-regulator output voltage to dynamically adjust to the operating conditions. 
     Various embodiments allow the linear regulators to transition between a regulator mode and a load-switch mode. This further reduces the dropout voltage to about zero Volts when the output voltage noise of the master linear regulator is met and all non-master linear regulators headroom voltages are met. The linear regulators transition back to regulator mode is made when these conditions are no longer satisfied. 
     Certain features and advantages of the present invention have been generally described here; however, additional features, advantages, and embodiments presented herein will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Accordingly, it should be understood that the scope of the invention is not limited by the particular embodiments disclosed in this summary section. 
    
    
     
       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. 1  illustrates a prior art illustrates a prior art regulator system utilizing a pre-regulator that supplies power to multiple sub-regulators. 
         FIG. 2  is a general illustration of an adaptive headroom control system utilizing a pre-regulator and a combination of analog, digital circuitry, and/or software control, according to various environment of the invention. 
         FIG. 3  illustrates an adaptive headroom control system utilizing a combination of analog, digital circuitry, and/or software control, according to various environment of the invention. 
         FIG. 4  is a flowchart of an illustrative process for adaptive headroom control 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 that 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 referring to the same embodiment. 
     Furthermore, connections between components or between method steps in the figures are not restricted to connections that are affected 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. 1  illustrates a prior art regulator system utilizing a pre-regulator that supplies multiple low drop-out sub-regulators. System  100  comprises battery  120 , pre-regulator  102 , Sub-regulator  1   104 , Sub-regulator X  114 , voltage output node  106 ,  116 , and load  108 ,  118 . Typically, pre-regulator  102  is a switching regulator with an external inductor rather than a linear regulator. In this example, pre-regulator  102  is a buck switching regulator that receives a DC voltage from battery  120  and converts it into a lower output voltage  103 . LDO 1   104  is a low drop-out regulator that receives output voltage  103  and converts it to an even lower output voltage  106  that drives load  108 . Similarly, LDO 2   114  converts the same output voltage  103  into another low output voltage  116  that drives load  118 , which is a time-varying system load. 
     Sub-regulator  1   104  and Sub-regulator X  114  form a group that is connected to the common output voltage  103  of pre-regulator  102 . Sub-regulator  1   104  and Sub-regulator X  114  may be used to operate different voltage rails, e.g., a 2.5 V-3.5 V battery voltage rail and a 1 V-2 V low voltage rail. Output voltage  103  of pre-regulator  102  is set to the higher voltage requirement of Sub-regulator  1   104  and Sub-regulator X  114  to protect against drop-out of the Sub-regulators, where the output voltage would fall below the specified minimum voltage. Output voltage  103  of pre-regulator  102  is then set to that higher value plus a headroom value of, e.g., 300 mV to provide each Sub-regulator with an output voltage of 1.5V with an input voltage of 1.8 V. This worst-case voltage is intended to account, such as device-to-device variations in the manufacturing process of the devices, maximum operating temperatures, output noise, and the expected maximum load condition, which all may disadvantageously contribute to the 300 mV variation in headroom voltage. 
     Since, as mentioned in the Background section, the efficiency of a linear regulator is a function of its output voltage divided by its input voltage, at constant input and output currents, it is desirable to operate Sub-regulator  1   104  through Sub-regulator X  114  at their lowest possible input voltage, i.e., the lowest possible common pre-regulator output voltage  103 , so as to maximize the efficiency of system  100 . However, since the actual lowest possible input voltage is not constant, but rather a function of the abovementioned factors, some portion of the efficiency is always sacrificed in pre-regulator  102  that operate at a constant programmed to output voltages. Therefore, it would be desirable to automatically adjust the headroom to the actual, varying headroom requirements in order to increase circuit efficiency and extend battery life of battery  120 . 
       FIG. 2  is a general illustration of an adaptive headroom control system utilizing a pre-regulator and a combination of analog, digital circuitry, and/or software control, according to various environment of the invention. System  200  comprises power supply  202 , pre-regulator  204 , sub-regulator  1   206 , sub-regulator X  276 , load  290 , and select module  270 . Pre-regulator  204  is any switching regulator, e.g., a buck converter, a boost converter, or a buck-boost converter. Sub-regulator  206 ,  276  is coupled to pre-regulator  204  and may comprise any linear regulator known in the art, such as a low drop-out regulator. Sub-regulator  206 ,  207  is coupled to load  230 ,  280  and to select module  270 . Load  230 ,  280  may be any time-varying system load, such as microcontroller chip. 
     In operation, pre-regulator  204  receives a DC voltage from power supply  202  and converts it to a different output voltage  203 , e.g., based on initial settings or in response to control signal V SET    210 . Sub-regulator  206  converts output voltage  203  into a relatively lower output voltage  222  that drives load  230 . Similarly, sub-regulator  276  converts output voltage  203  to low output voltage  272  to drive load  280 . Sub-regulator  276  generates voltage signal V MAX     —1      220 , which is representative of the maximum input voltage requirement of sub-regulator  206 . As shown in  FIG. 2 , select module  270  receives voltage signal V MAX   _   i    220 ,  214  from sub-regulator  206 ,  276 . From this, select module  270  determines which sub-regulator  206 ,  276  has the highest voltage requirement, for example, by selecting the highest value of voltage signals V MAX   _   i    220 ,  214 , at any moment in time. If the output of sub-regulator  1   206  is set, e.g., at 1.6 V with 100 mV of headroom and the output of sub-regulator X  276  is set at 1.5 V with 300 mV of headroom, select module  270  would select the latter value and output that value as control signal  210 , which pre-regulator  204  then uses to adjust output voltage V OUT    203 . In one embodiment, system  200  comprises analog and digital circuit components coupled in a closed loop circuit configuration to continuously monitor the actual output voltage  222 ,  272  of sub-regulator  206 ,  207  in order to effectively control output voltage  203  of pre-regulator  204 . 
     In practice, the actual output voltage V SR   _   OUT   _   N  of each sub-regulator  206 ,  276  is affected by on/off state of the sub-regulator, output loads, die temperature of the group of sub-regulators, source voltage  202 , output noise requirements (e.g., derived from input and output specifications of system  200 ), etc. Therefore, a headroom voltage should be built into voltage signal V MAX   _   N    220 ,  240  in order to account for these variations. As a result, select module  270  should take into account an overall headroom margin that is a function of the actual output voltages V SR   _   OUT   _   N , such that output voltage  203  of pre-regulator  204 , i.e., the input voltage to each sub-regulator  206 ,  276  changes dynamically and relatively rapidly (e.g., within microseconds) as dictated by the headroom requirement. In this way, pre-regulator  204  ensures that a sufficiently high output voltage  203  is provided to all sub-regulators  206 ,  276 . Ideally, pre-regulator  204  maintains a minimum dropout voltage for sub-regulators  206 ,  276  under all dynamic operating conditions. 
     In one embodiment, the maximum input voltage requirement for each sub-regulator  206 ,  276  is regularly updated at predetermined intervals or conditions in order to further increase efficiency. Updates may depend on predetermined events, such as a start-up event, or how often output load  230 ,  280  changes over time. 
     In one embodiment, when the actual output voltage V SR   _   OUT   _   N  of each sub-regulator  206 ,  276  is unknown, select module  270  may receive a set output voltage value V SR   _   OUT   _   N    222 ,  272  for each sub-regulator  206  and  276  within a sub-group of sub-regulators. Select module  270  then compares values V SR   _   OUT   _   N  and determines the sub-regulator with the highest set voltage value therefrom and outputs that value as control signal V SET    210 . 
     In one embodiment, output voltage values V SR   _   OUT   _   N    222 ,  272  may be determined using software to read out registers read one or more settings of programmable registers (not shown) and employing digital comparators to compare the register settings for each sub regulator  206 ,  276  to determine the sub-regulator with the highest output voltage value setting, such that pre-regulator  204  can be set appropriately to meet the headroom and voltage requirement for sub-regulator  206 ,  276 . 
     In one embodiment, sub-regulator  206 ,  276  transitions between “linear regulator mode” and “load switch mode.” This further reduces the dropout voltage to about zero Volts when the output voltage noise and other factors of the master linear regulator is met and all non-master linear regulators&#39; maximum input voltages are met. Transition back to regulator mode is made when these conditions are no longer met. If any sub-regulators defined as master are able to utilize the direct pre-regulator output voltage, while all other (non-master) sub-regulators&#39; maximum input voltage is met, then the master output voltage is set equal to the master input voltage. In this case, master sub-regulator  206 ,  276  can be replaced with a load switch that in its on-state is ideally a zero Ohm switch. This load switch can transition, as needed, into a sub-regulator and back to a load switch, as required by the controller. 
       FIG. 3  illustrates an adaptive headroom control system utilizing a combination of analog and digital circuitry, and/or software control, according to various environment of the invention. System  300  comprises battery  302 , pre-regulator  304 , linear sub-regulator  306 , duplicate linear switch  308 , which may be any saturating device, load  330 , counter  360 , scaling circuit  336 , adjustable margin voltage source  340  comparator circuit  350 , counter  360 , and voltage detector  370 . In this example, pre-regulator  304  is a Buck converter that receives a DC voltage from battery  302  and converts it into output voltage  320  that is different from the battery voltage. Linear sub-regulator  306  is a low voltage regulator, such as a low drop-out regulator (LDO). LDO  306  receives output voltage  320  and converts it to a lower output voltage  322 . LDO  306  comprises power FET  312  and control circuitry  316 . Circuit  316  controls power FET  312  by modulating its gate voltage so as to maintain a given output voltage V LDO   _   1    322  that drives time-varying load  330 . 
     The output of linear switch  308  is coupled to ground via scaling circuit, represented by current sink  336 . The output of LDO  306  is increased, via variable margin voltage source  340 , and forwarded to comparator circuit  350 . Margin voltage source  340  may be a programmable voltage source configured to generate a headroom voltage. Counter  360 , which in this example is a digital N-bit up/down counter, is coupled between comparator circuit  350  and voltage detector  370 . 
     In operation, pre-regulator  304  converts battery voltage  302  into a lower pre-regulator output voltage  320  and outputs it to LDO  306 . LDO  306 , in turn, converts pre-regulator voltage  320  into an even lower output voltage  322  that drives load  330 . Duplicate device  308 , converts pre-regulator output voltage  320  into LDO output voltage  334  which replicates pre-regulator output voltage  320  minus a minimum headroom voltage. The current through duplicate device  308  is a scaled down version of the current through LDO  306  that drives a duplicate, but scaled-down output load version of the output load current I LDO   _   1  through load  330 . The scaled current, I LDO   _   1 /G, through the scaled switch resistance, RDS LDO   _   1 *G, determines the required minimum headroom of LDO  306 . Close physical proximity of LDO  306  and duplicate device  308  aids in ensuring that both circuit components share the same operating temperature. Programmable margin voltage source  340  selectively adds headroom voltage, i.e., operating margin, to the minimum headroom voltage. The programmable margin aids in maintaining the regulation of LDO  306  under all operating conditions. Under steady state conditions, the input ports of comparator  350  receive equal signals. Therefore, the minimum headroom voltage for LDO  306  is defined by equation:
 
 V   HEADROOM   _   1   =V   OUT −(RDS LDO   _   1   *G )*( I   LDO   _   1   /G )= V   OUT −RDS LDO   _   1   *I   LDO   _   1   =V   LDO   _   1   +V   MARGIN   _   1  
 
     The minimum pre-regulator output voltage is defined by the equation:
 
 V   OUT   =V   LDO   _   1   +V   MARGIN   _   1 +RDS LDO   _   1   *I   LDO   _   1  
 
     Comparator circuit  350  monitors output voltage  334  of duplicate device  308  and converts the difference into a digital value that can be counted by digital N-bit up/down counter  360 . In one embodiment, counter  360  comprises circuitry that allows comparator  350  to set a headroom voltage for LDO  306  that accounts for variations in output voltage  322 . Comparator detects the output voltage of LDO  306  and, based on programmable margining voltage source  340 , causes N-bit up/down counter  360  to count up or down to dynamically determine the headroom voltage. Counter  360 ,  368  outputs signal  362 ,  364  representative of the maximum voltage requirement of LDO  306  and  376 , respectively. 
     Voltage detector  370  monitors the outputs of N-bit up/down counters  360 ,  368  and determines which LDO  306  through  376  has the highest voltage requirement at any moment in time. For example, assuming LDO  306  has the highest headroom requirement in system  300 . Once LDO  306  is turned off, this is detected by the control circuit via voltage detector  370 , which adjusts the headroom margin by lowering the output voltage of pre-regulator  320  accordingly. In one embodiment, voltage detector  370  comprises a comparator block (not shown) comprising a plurality of comparators to regularly monitor and determine from digital counters  360 ,  368 , and headroom information of each LDO  306 ,  376 , the highest value of voltage signals V MAX   _   i    362 ,  364  at predetermined time intervals. Various physical properties that affect the required headroom margin may be detected and used to adjust the headroom voltage accordingly. Headroom information may be stored and retrieved, e.g., through software, from a storage device (not shown) that may be implemented internal or external to voltage detector  370 . Based on output signals of counters  360 ,  368 , voltage detector  370  generates input signal  380  for pre-regulator  304 , which adjusts its output voltage  320  accordingly to save energy. 
       FIG. 4  is a flowchart of an illustrative process for adaptive headroom control in accordance with various embodiments of the invention. The process for adaptive headroom control  400  starts at step  402  when a pre-regulated input voltage is received, for example, directly from the output of a pre-regulator. 
     At step  403 , a plurality of sub-regulators that are coupled to the pre-regulator are identified. 
     At step  404 , one or more sub-regulator output voltages is detected. 
     At step  406 , based on the sub-regulator output voltages, one or more headroom voltage requirements are determined. 
     At step  408 , minimum required sub-regulator output voltages are determined, in part, based on the headroom voltage requirements. 
     At step  410 , a highest required pre-regulator output voltage value is determined based on the minimum required sub-regulator output voltages. 
     At step  412 , the pre-regulated input voltage is adjusted according to the highest required pre-regulator output voltage value, at which that the process may return to step  404  in order to continue to detect sub-regulator output voltages. 
     It will be appreciated by those skilled in the art that fewer or additional steps may be incorporated with the steps illustrated herein without departing from the scope of the invention. No particular order is implied by the arrangement of blocks within the flowchart or the description herein. 
     It will be further appreciated 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 scope of the present invention. It is therefore intended that the claims include all such modifications, permutations, and equivalents as fall within the true spirit and scope of the present invention.