Abstract:
A regulating apparatus having an output node and being operative for regulating the voltage level at the output node in response to a reference signal provided as an input to the regulating apparatus. The regulating apparatus includes a linear amplifier stage operative for receiving the reference signal and being capable of sourcing current to the output node when the reference signal indicates that a present voltage level at the output node is less than a desired voltage level at the output node. The regulating apparatus also includes a switching regulator, which is controlled by the linear amplifier stage, and which is operative for sourcing current to the output node when the amount of current being sourced to the output node by the linear amplifier stage exceeds a predetermined threshold.

Description:
CLAIM OF PRIORITY 
   This patent application, and any patent(s) issuing therefrom, claims priority to U.S. provisional patent application No. 60/628,652, filed on Nov. 18, 2004, which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to an improved switching regulator and/or amplifier, and more specifically, to a novel, cost effective design for a switching regulator and/or amplifier that provides for both high efficiency and high slew rate. 
   BACKGROUND OF INVENTION 
   It has been known in the prior art to utilize switching regulators/amplifiers in applications such as, but not limited to: (1) voltage regulators utilized for supplying a relatively fixed DC voltage to a load whose current demands change very quickly such as CMOS logic processors whose activity can go from negligible (such as in standby) to very high or vise-versa in a few nanoseconds, for example, at the change of state of a control signal; and (2) “digital” amplifiers or programmable regulators where the load is relatively fixed but its voltage is changed very rapidly in response to an external command, such as DSL line drivers and supplies or modulators for communication transmitters where the power level or information signal level is changed often and abruptly over a wide dynamic range. 
   It is also noted that the foregoing applications are characterized by a step down operation where the supply voltage is relatively fixed or slowly varying (such as, for example, a battery), and the widely varying load current is sourced at a voltage that is either fixed or varying, but at a lower value than the supply voltage. 
   Prior art designs for switching regulators/amplifier to be utilized in the foregoing applications have generally included buck topology switching regulators having low value inductors, high switching frequencies and hysteretic control algorithms without loop filters to achieve high load current slew rates. As is known: 
             (         ⅆ   I       ⅆ   t       ∝       Vin   -   Vout     L       )     .         
However, the use of such low value inductors results in large values of ripple current and conduction losses, while high switching frequencies result in larger switching losses, both of which undesirably lower efficiency.
 
   In an effort to satisfy performance requirements, it has been known in the prior art to add a cascaded linear amplifier/low drop out regulator immediately before the load, even though the losses due to the load current at the required voltage overhead of the linear stage can be large. Such prior art systems are described, for example, in U.S. Pat. Nos. 4,378,530 and 5,905,407.  FIG. 1  illustrates an exemplary block diagram of such a device. 
   Referring to  FIG. 1 , the device includes a programmable switching regulator  12  cascaded with a linear amplifier stage  14 . In addition, the device includes overhead voltage reference supply  16 , and resistors R 1  and R 2 , which are coupled in series to one another and to the output node, V O . The overhead voltage reference supply  16  causes V R =V O +V B1 , which is necessary for the linear amplifier to operate, as V R  must be larger than V O  by an “overhead voltage”. Resistors R 1  and R 2  form a voltage divider circuit, and provide a feedback signal to the linear amplifier stage  14 . The output of the linear amplifier stage  14  operating in conjunction with the output of the programmable switching regulator  12  generate the output voltage, V O , of the device, which is coupled to the load (e.g., a power amplifier in a cell phone application). V SUPPLY  corresponds to the voltage source for the device (e.g., a battery in a cell phone application), and V REF  sets the output voltage needed to supply the power level required by the load. It is noted that in some applications, V REF  will represent the instantaneous power requirement of the load and will include content data (e.g., voice or data information to be transmitted) which is superimposed on the V REF  signal utilizing any suitable modulation technique. In operation, the linear amplifier stage  14  essentially functions as the power supply regulator operative to generate a substantially clean signal, V O , which is representative of the instantaneous power required for the task currently at hand. 
   However, if the output voltage of the switching regulator cannot change rapidly enough to follow voltage changes in V REF , then V R  must be set to the instantaneous peak value of V O  plus enough additional voltage margin B so that the linear amplifier does not “clip” on signal peaks. If the supply voltage, V SUPPLY , is significantly greater than V R , use of the switching regulator saves most of the power equal to I LOAD *(V SUPPLY −V R ), which would otherwise be dissipated in the linear amplifier. 
   While these known prior art devices provide for an improvement in efficiency, for example, by allowing for a reduction in the switching frequency of the switching regulator, due to the requirements of today&#39;s applications and the continued demand for reducing power requirements so as to extend battery life, a further increase in the overall operating efficiency of switching regulators/amplifiers is necessary. It is an object of the present invention to satisfy these needs. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is a primary objective of the present invention to provide a novel switching regulator/amplifier which exhibits improved efficiency and slew rate performance relative to known prior art devices. It is also an objective of the present invention to provide a cost effective design for the novel switching regulator/amplifier so that the device represents a practical solution to the aforementioned problems. 
   Specifically, the present invention relates to a regulating apparatus having an output node and being operative for regulating the voltage level at the output node in response to a reference signal provided as an input to the regulating apparatus. The regulating apparatus includes a linear amplifier stage operative for receiving the reference signal and being capable of sourcing current to the output node when the reference signal indicates that the present voltage level at the output node is less than a desired voltage level at the output node. The regulating apparatus further includes a switching regulator, which is controlled by the linear amplifier stage, and which is operative for sourcing current to the output node when the amount of current being sourced to the output node by the linear amplifier stage exceeds a predetermined threshold. 
   The switching regulator/amplifier of the present invention provides numerous advantages over the prior art. One advantage of the present invention is that it provides a highly efficient switching regulator/amplifier that minimizes the power requirements for operation. This is accomplished in-part by reducing the power dissipated by the linear amplifier contained in the device, by providing a separate current path that is capable of providing the steady state current requirements to the load (i.e., the linear amplifier is activated only during fast changing transient voltage swings in the load). As a result, as one example, the present invention advantageously allows for an extension of battery operation time of a cell phone between charges. 
   In addition, the switching regulator/amplifier provides for an increased slew rate capability. As the result of the design of the present invention, which incorporates the use of a “free-wheeling” switch, it is possible to rapidly reduce the load current to substantially zero (i.e., on the order of a few nanoseconds). Moreover, when the load current is reduced in the foregoing manner, the design of the present invention does not immediately dissipate the current (i.e., as explained below the current is temporarily stored), and therefore if the load must be increased shortly after the reduction, the stored current is again coupled/provided to the load. The foregoing operation allows the switching regulator/amplifier of the present invention to exhibit both a high slew rate capability and increased operating efficiency. 
   Yet another advantage of the present invention is that the design provides a “feed-forward” control system in which the switching regulator/amplifier reacts to changes in the desired voltage set point when adjusting the current delivered to the load. The control of the switching regulator/amplifier does not utilize the output voltage signal. As a result, the design of the present invention further improves both slew rate performance (as the load current is adjusted more rapidly in comparison to a device that modifies the current delivered to the load based on changes in the output voltage of the regulator) and efficiency performance (as there is no sense resistor coupled to the output of device, which would result in an increase in power dissipation). 
   Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description, or may be learned by practice of the invention. While the novel features of the invention are set forth below, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several aspects and embodiments of the present invention and, together with the general description given above and detailed description given below, serve to explain the principles of the invention. Such description makes reference to the annexed drawings. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be treated as limiting the invention. 
     In the drawings: 
       FIG. 1  illustrates a block diagram of a prior art implementation of a switching regulator/amplifier that utilizes a linear amplifier in the design. 
       FIG. 2  illustrates an exemplary block diagram of a switching regulator/amplifier in accordance with the present invention. 
       FIG. 3  illustrates a schematic diagram of an exemplary implementation of the switching regulator/amplifier of the present invention. 
       FIG. 4  illustrates a first alternative embodiment of the output stage of the linear amplifier stage. 
       FIG. 5  illustrates a second alternative embodiment of the output stage of the linear amplifier stage. 
   

   Throughout the above-mentioned drawings, identical reference numerals are used to designate the same or similar component parts. 
   DESCRIPTION OF THE INVENTION 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein: rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art, like numbers refer to like elements throughout. 
   Referring to  FIG. 2 , similar to the prior art design illustrated in  FIG. 1 , the switch regulator/amplifier of the present invention comprises a programmable switching regulator  12  cascaded with a linear amplifier stage  14 , as well an overhead voltage supply  16  and resistors R 1  and R 2 . The linear amplifier stage  14  receives V REF  as an input signal. The foregoing components are coupled together in the same manner as illustrated in  FIG. 1 . However, the design also includes a second switching regulator  18  coupled between the supply voltage V SUPPLY  and the output, V O , as shown in  FIG. 2 . V R  is the minimum supply voltage for the linear amplifier that allows it to follow the signal peaks of V REF  without clipping. The linear amplifier stage  14  provides control signals  17  to the second switching regulator  18 , which govern the operation of the second switch regulator  18 . 
   As explained in more detail below, the inclusion of the second switching regulator  18  disposed between the power supply, V SUPPLY , and the load, V O , provides for a second current path so as to allow for the steady state current (or slowly changing current) required by the load to be delivered to the load via the second switching regulator  18  without accessing/utilizing the linear amplifier stage  14 . In the present invention, the linear amplifier stage  14  is primarily utilized to deliver fast changing (i.e., transient) current requirements to the load. As a result of this design, the utilization of the linear amplifier stage  14 , which exhibits low efficiency and high power dissipation, is minimized thereby increasing the overall efficiency of the device. 
     FIG. 3  illustrates a schematic diagram of an exemplary implementation of the switching regulator/amplifier illustrated in  FIG. 2 . It is noted that the present invention is not limited to the specific embodiment disclosed in  FIG. 3 , as variations to the particular design are clearly possible. 
   Referring to  FIG. 3 , the programmable switching regulator  12  receives V SUPPLY  as an input voltage and generates an output voltage signal V R . In addition, overhead voltage reference supply  16  is coupled to the programmable switching regulator  12 . It is noted that the programmable switching regulator  12  furnishes an output voltage, V R , which is equal to the average value of the linear amplifier output voltage, V O , plus an additional voltage V B1 , where V B1  equals the peak to average value of V O  and a small additional voltage necessary to assure that linear amplifier  14  does not clip (i.e., voltage saturate) on peaks of the V REF  signal. As such, the programmable switching regulator  12  need only have a response time fast enough to follow the average value of V O  and not its instantaneous value or envelope. 
   More specifically, the output voltage, V R , of the programmable switching regulator follows the value of its input reference voltage, (V O +V B ), within the capability of its control bandwidth or response time as set forth by its internal clock or switching frequency. Thus, the voltage V R  follows the average value of V O  plus the additional voltage of V B1 , where the averaging period is set by the control bandwidth of the programmable switching regulator or may be adjusted to a specific value by adding an additional low pass filter in its control input line. The choice of averaging period and value of V R  are selected to match the characteristics of the V REF  signal and linear amplifier response such that the value of V B1  is equal to the maximum value of V O  average to positive peak value during any sliding time averaging period as a window. The objective is to minimize the value of V B1  to the smallest value of the average to peak during the response time of the switching regulator so that most of the voltage difference between V SUPPLY  and V O  can be absorbed by the switching regulator at typically 90% efficiency rather than be wasted as voltage drop across the linear amplifier. Thus, by choosing an appropriate V B1  to match the AC signal characteristics of V O  (and therefore V REF  when not distorting) and the response time of the switching regulator, essentially any programmable switching regulator response time and signal characteristic of V O  can be accommodated. However, it is noted that if the switching regulator response is too slow relative to the rate of change of the V REF  signal, efficiency improvements from use of the programmable switching regulator may be small and overall system efficiency inadequate. 
   Continuing, in the given embodiment the linear amplifier stage  14  includes an error amplifier  22 , a linear amplifier  24  comprising an NPN transistor, resistor R 11  coupled between the base and emitter terminals of the linear amplifier  24 , resistor R 12  coupled to the collector of the linear amplifier  24 , and capacitor Cc and resistor Rc connected in series and coupled to the output of the error amplifier  22 . The emitter of the linear amplifier  24  is coupled to the load, V O . As shown in  FIG. 3 , the output signal, V R , of the programmable switching regulator  12  is coupled to both resistor R 12  and the error amplifier  22  and functions as the amplifier supply voltage. In operation, the error amplifier  22  and the linear amplifier  24  form a linear amplifier/regulator that has sufficient bandwidth to allow the output, V O , to follow the reference V REF  in the presence of rapid time variations in V REF  and/or the load current. As shown, a portion of the output signal, V O , is fed-back to the input of the error amplifier  22  so as to allow the error amplifier to generate an output signal indicative of the difference between the desired output voltage level and the actual output voltage level, and cause V O  to follow (V REF *(R 1 +R 2 )/R 2 ). 
   Referring again to  FIG. 3 , in the given embodiment, the second switching regulator  18  comprises a first comparator  26  having a first input and a second input which are coupled across resistor R 12 , and a second comparator  28  having a first input and a second input which are coupled across resistor R 11 . The second switching regulator  18  further includes a first switch  27 , which is a pMOS device, a second switch  29 , which is an nMOS device, an inductor  31  and an active diode  32 . As shown, the output of the first comparator  26  is coupled to the first switch  27 , which has a source terminal coupled to the supply voltage, V SUPPLY . The output of the second comparator  28  is coupled to the gate of the second switch  29 . It is noted that the inductor  31  is coupled between the source and drain terminals of the second switch  29 , and the body of the second switch  29  is not connected to either its source or drain, but rather to ground as shown in  FIG. 3 . The inductor  31  is also coupled between the drain terminal of the first switch  27  and the load, V O . It is further noted that the drain terminal of the first switch  27  and the source terminal of second switch  29  are coupled together and are also coupled to diode  32 . In the preferred embodiment, the diode  32  is an “active” type diode comprising a comparator and NMOS transistor as described in pending application Ser. No. 11/094,369 filed Mar. 31, 2005, which is hereby incorporated by reference in its entirety. 
   Turning to the operation of the device as a system, it is noted that without the second switching regulator  18  in the device, the entire load current would have to pass through the linear amplifier stage  14 , and as a result the power dissipation due to the load current times the overhead voltage B 1  required for proper operation would greatly reduce the efficiency of the device. However, by including the second switching regulator  18 , which has minimal switching and conduction losses, most of the load current passes through the second switching regulator  18  and therefore bypasses the linear amplifier stage  14 , thereby greatly improving overall efficiency. It is noted, however, that the linear path is always present and can supply the entire incremental load current during transients. 
   At initial turn of the power supply, V SUPPLY , with V REF  already having a desired value and V B1  set appropriately as described earlier, V O  is zero and the input to the programmable switching regulator is V B1 . The programmable switching regulator output voltage V R  rises toward (V O +V B1 ) at a rate set by its inherent response time, and the linear regulator now has a non-zero supply voltage, and so long as V O &lt;(V REF *(R 1 +R 2 )/R 2 ), it continues to increase V O  toward V R , and therefore the output voltage V O  thus ramps up at a rate set by the slew rate of the programmable switching regulator. When V O =(V REF *(R 1 +R 2 )/R 2 ), V O  has reached steady state and its stays at that voltage until the programmable switching regulator output, V R , has reached [(V REF *(R 1 +R 2 )/R 2 )+V B1 ], at which point it remains static unless or until V REF  changes. Thus, power on requires no special function within the device design, and the operation of the second switching regulator will be the same as described in the following for all modes of operation including start up. 
   Continuing, during operation, if the second switching regulator  18  off, the load current flows through the linear amplifier stage  14  including the linear amplifier  24 . This results in an increase in the voltage drop across R 12 , which if greater than the upper threshold of the first comparator  26 , results in the turn on of first switch  27  and therefore the supply of current to the load, V O , through the inductor  31 . As the inductor current increases, the current in the linear amplifier  24  decreases because their sum is the present load current. This reduces the voltage drop across R 12  until such time that the reduction in voltage across R 12  causes it to become less than the lower threshold of the first comparator  26  and turns off the first switch  27 , thereby preventing further current from being supplied to the load from V SUPPLY . Thus, at steady state, the comparator  26  switches on and off at some duty cycle, and most of the load current flows through the inductor  31 , and consists of a DC component and an AC triangular component. The sum of the DC component and AC component of the inductor current and the linear amplifier current equals the load current. Thus, the linear amplifier AC current is 180 degrees out of phase with the AC component of the inductor current and there is no AC voltage ripple present at the load, V O . 
   The switching frequency of the second switching regulator  18  is set by the relationship between V SUPPLY −V O , the value of the inductor  31 , the value of hysteresis set by the first comparator  26  and the voltage drop from the current through resistor R 12 . It is noted that when the first switch  27  is off, inductor current flows through the diode  32 , which as noted above is preferably of the “active” type, and therefore has a forward voltage drop that is negligible with respect to V O . The actual values utilized for the various components are typically based on the specific application for which the device will be utilized n conjunction with well known design relationships. 
   From the foregoing discussion, it is clear that the circuit of the present invention is capable of handling steady state and increasing load current exceedingly well. However, the circuit is also capable of handling rapidly decreasing load currents, and does so in a manner which provides for both high slew rates and improved efficiency. In operation, during transients when the inductor current is larger than the load current, the linear amplifier stage  14  starts to turn off when the voltage across R 11  becomes less than V BE  of linear amplifier  24 , thereby turning off linear amplifier  24 . The value of R 11  is part of the amplifier design and the threshold of comparator  28  should be about 0.8*V BE  with a few millivolts of hysteresis to avoid noise effects. At this time, the second comparator  28  turns on the second switch  29 , which allows the inductor current to recirculate and slowly decay in value without being passed into the load, V O . Specifically, the inductor current recirculates in an autonomous loop formed by the inductor  31  and the second switch  29  (which is referred to herein as a free-wheeling switch). Thus, the foregoing configuration allows the load current to be rapidly reduced to substantially zero on the order of a few nanoseconds. In other words, the device allows the total current sourced by the overall regulator/amplifier to go to nearly zero during transients even though the linear amplifier stage  14  can only source current, and prevents voltage overshoots in most any dissipative load without degrading efficiency. Further, as V O  is not used to control the second switching regulator  18 , it has no ripple voltage and can precisely track V REF . 
   It is further noted that by utilizing the “free-wheeling” switch  29  in the device of the present invention, when the load current is reduced in the foregoing manner, the device of the present invention does not immediately dissipate the current (i.e., the current is temporarily stored in the inductor and autonomous loop). As such, if the load current must be increased shortly after the reduction, the stored current is again coupled to the load. This would occur upon deactivation of the second switch  29 , which occurs when the linear amplifier stage  14  becomes active again (i.e., V REF  indicates a desired increase in load voltage) and the voltage across R 11  is greater than the trip point of the second comparator  28 . This operation of not dissipating the inductor current and allowing for the reuse of the stored inductor current allows the switching regulator/amplifier of the present invention to exhibit high slew rates and increased efficiency. 
   It is also noted that by sensing the collector current of linear amplifier  24  instead of the output current of the linear amplifier stage  14 , the output impedance of the linear amplifier stage  14  is advantageously not increased by a sensing resistor. Furthermore, the value of R 11  can be relatively large so that a small current threshold of the second comparator  28  can be achieved with minimal error due to the voltage offset of the second comparator  28 . 
   While an exemplary embodiment of the present invention is set forth above in  FIG. 3  is it noted that the present invention is not intended to be limited to the disclosed embodiments as various implementations of the device are clearly possible. For example,  FIGS. 4 and 5  illustrate alternative embodiments of the output stage of the linear amplifier  24 . 
   More specifically, in a first variation, the linear amplifier  24  can comprise two matched parallel transistors  24 A and  24 B, as shown in  FIG. 4 , where the emitter area of  24   A , is K* (area of  24   B ) and R 2  is K*R 2 . Thus, with K large, R 2  can be sized more conveniently but still maintain the threshold of the first comparator  26  the same with respect to the total collector current of linear amplifier  24  of  FIG. 3 , and the total current gain of the linear amplifier  24  will not change appreciably even if transistor  24   B  voltage saturates. In a second variation, an additional linear amplifier stage consisting of transistor  25  and mirror  26  also could be added to linear amplifier  24 , as shown in  FIG. 5 , to further lower the output impedance of the linear amplifier and make its frequency compensation easier without changing the operating voltages from those of the configuration shown in  FIG. 3  or  4 . It is noted that if utilizing the alternative embodiments for the linear amplifier  24 , in addition to the linear amplifier, the resistor R 12  would be replaced by the circuit shown in  FIGS. 4 and 5 , and the inputs of the first comparator would be coupled across R 12 ′. In  FIG. 5 , the inputs of the second comparator would be coupled across resistor R 11 ′. 
   As noted above, the switching regulator/amplifier of the present invention provides numerous advantages over the prior art. One advantage of the present invention is that it provides a highly efficient switching regulator/amplifier that minimizes the power requirements for operation. This is accomplished in-part by reducing the power dissipated by the linear amplifier contained in the device, by providing a separate current path that is capable of providing the steady state current requirements to the load (i.e., the linear amplifier is activated only during fast changing transient voltage swings in the load). As a result, as one example, the present invention advantageously allows for an extension of battery operation time of a cell phone between charges. 
   Another advantage is that the switching regulator/amplifier of the present invention provides for an increased slew rate capability. As the result of the present invention, which incorporates the use of a “free-wheeling” switch, it is possible to rapidly reduce the load current to substantially zero (i.e., on the order of a few nanoseconds). Moreover, when the load current is reduced in the foregoing manner, the design of the present invention does not immediately dissipate the current (i.e., as explained above the current is temporarily stored), and therefore if the load must be increased shortly after the reduction, the stored current is again coupled to the load. The foregoing operation allows the switching regulator/amplifier of the present invention to exhibit high slew rate capabilities and improved efficiency. 
   Yet another advantage of the present invention is that the design provides a “feed-forward” control system in which the switching regulator/amplifier reacts to changes in the desired voltage set point when adjusting the current delivered to the load. The control of the switching regulator/amplifier does not utilize the output voltage signal. As a result, the design of the present invention further improves both slew rate performance (as the load current is adjusted more rapidly in comparison to a device that modifies the current delivered to the load based on changes in the output voltage of the regulator) and efficiency performance (as there is no sense resistor coupled to the output of device, which would result in an increase in power dissipation). 
   While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   For example, it is noted that the programmable switching regulator  12  operates to maintain V R −V O  greater than the linear stage drop out voltage even if the short term voltage slew of V O  exceeds V R , by choosing voltage offset B 1  appropriately. This is necessary to maintain efficiency if the V SUPPLY −V O  voltage differential is much larger than the dropout voltage of the linear regulator. In the event that the V SUPPLY −V O  voltage differential is not larger than the dropout voltage of the linear regulator, it is possible to omit the programmable switching regulator from the design. 
   The aforementioned variations are merely examples. Further, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.