Patent Publication Number: US-8111046-B2

Title: Technique for conveying overload conditions from an AC adapter to a load powered by the adapter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 10/034,718, filed Dec. 28, 2001, entitled “A Technique For Conveying Overload Conditions From An AC Adapter To A Load Powered By The Adapter,” incorporated by reference herein as if reproduced in full below. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to a battery-powered electronic device. More particularly, the invention relates to a technique for providing power to components in an electronic device in such a way that the devices can determine when an overload condition is occurring. 
     Most every computer system with a rechargeable battery uses an external AC to DC converter (sometimes called an “adapter”) which converts the AC line voltage to a lower DC voltage. An example is shown in  FIG. 1  in which an AC adapter  10  provides power for a computer  12  and a battery subsystem  14 . The battery subsystem includes a battery charger and a battery. The current sense circuit  16  will be discussed below. 
     Due to its size and shape, the adapter has often been referred to as the “brick.” The brick is usually external to the computer shell and is often an awkward part of the system to store and carry. While using AC power, the brick supplies power both for the normal operation of the computer and also for recharging the battery. 
     Typical AC/DC converters are provided with an input of 100 to 240 VAC and generate an output voltage of 18 VDC with a total power output capacity of 50 to 70 watts. The size (i.e., power capacity) of the AC adapter is normally established by estimating a reasonable “power budget” for the CPU. The power budget is a total of the maximum power consumption of the computer&#39;s internal devices (the CPU, core chipset, LCD panel, hard drive, etc.) plus some allocation for externally powered devices (e.g., USB, PS/2, or external storage). 
     Older notebook computers with small LCD screens and low power processors typically consumed a maximum of 10 or 15 watts while operational. Today&#39;s notebooks, however, with 15″ high resolution screen, multiple internal storage drives, and gigahertz processors can easily consume 50 to 60 watts of power. Moreover, performance requirements have demanded bigger AC/DC adapters which are designed to be sufficient for the worst case power consumption of the system. 
     While the power demands for portable computers continuously increases, the pressure to make the system “mobile” places pressure on the system designer to make the AC/DC brick as small as possible. Ergonomics discourages large AC/DC adapters which dissipate proportionately more heat. Further, cost pressures prohibit the use of more powerful or more efficient AC/DC bricks. Yet, at the same time, it is desirable for the computer to be able to charge the battery as quickly as possible. In sum, many consumers desire portables that have high performance (e.g., fast CPUs, bright displays, etc.), recharge batteries very quickly, are lightweight and small, inexpensive, and do not become hot to the touch. 
     To date, the concession to AC/DC size has been to “throttle” battery charge when the rest of the system is under full loading. In many older systems, the “power budget” and AC/DC adapter size were calculated by estimating the consumption of the computer, and then allocating an additional amount of power for recharging the battery. Today, the one common concession towards power budget allocation is that power for the recharge of the battery itself is not included in the power budget on which the adapter is designed. This means that most adapters today are rated to provide sufficient power for the computer at full load, but not for charging the battery with the computer at full load. Thus, notebooks today measure the core system power consumption and then allocate the remaining AC/DC power (if there is any remaining power) to charge the battery. 
     Such conventional systems include, as shown in  FIG. 1 , a current sense circuit  16  that receives the output voltage from the adapter and passes that voltage on to the computer  12  and battery subsystem  14 . The current sense circuit generally includes a low resistance current sense resistor (e.g., 50 milliohms) in series with the power flow to the computer and battery subsystem, as well as an amplifier that amplifies the voltage across the sense resistor. The amplifier circuit is designed so as to assert an output signal  18  when the current out of the adapter exceeds a certain threshold. Conventional AC adapters  10  are constant voltage (“CV”) adapters which means their output voltage is regulated to a predetermined value (e.g., 18 VDC) as illustrated graphically in  FIG. 2 . Because the output voltage is constant, the output current can be used to determine output power. Thus, the output signal  18  from the current sense circuit  16  is asserted, in effect, when the power draw on the adapter by loads  12  (the computer) and  14  (the battery subsystem) nears or exceeds the output power rating of the adapter. In  FIG. 2 , the over power condition occurs when the current output of the adapter is above Imax. 
     The current sense circuit output signal  18  typically is provided to the battery subsystem  14  to alert the battery subsystem that the adapter  10  cannot keep up with the power demands of the computer  12  and battery subsystem  14  combined. The battery subsystem  14  uses signal  18  to “throttle” back on battery charge current. Throttling back charge current means to reduce the charge current into the battery. Throttling back charge current results in a lower power draw on the AC adapter thereby alleviating the over power condition. The battery subsystem  14  may even cease battery charging altogether if necessary to protect the adapter  10 . By throttling back battery charging, the adapter&#39;s output current will not exceed Imax. 
     Although a generally satisfactory implementation, the current sense circuit  16 , which is part of the computer, is not a trivially simple circuit to design. For instance, the amplifier in the circuit may need to be operated rail-to-rail which complicates the amplifier design. Further, voltage level shifting may be required also complicating the implementation. These contribute to error in the resulting current sense output. Accordingly, an alternative system is needed which avoids the problems noted above with the current sense circuit  16 . 
     BRIEF SUMMARY OF THE INVENTION 
     The problems noted above are solved in large part by sensing output voltage of the AC adapter instead of output current, and using a constant voltage/constant power AC adapter for use in converting AC voltage to DC voltage in providing power to a plurality of loads. The adapter, for example, may be used in a computer system and the loads may comprise the computer and a battery subsystem having a charger and a rechargeable battery. The adapter provides an output characteristic which is approximately a constant voltage as long as the output current draw by the loads is less than a threshold (e.g., the Underwriters Laboratory power rating for the adapter). If, however, the load on the adapter is such that the output current exceeds the threshold, the adapter then regulates its output power to an approximately constant level. Regulating power to a constant level in the face of increasing current includes reducing the output voltage of the adapter. 
     A preferred embodiment of the adapter includes a primary circuit, which includes a transformer and voltage rectifier, and a secondary circuit. The secondary circuit includes a power regulator and a voltage feedback circuit. The voltage feedback circuit continuously compares the adapter&#39;s output voltage to a reference and provides a feedback signal to the primary circuit which responds by adjusting the output voltage so that the output voltage remains at approximately a constant level. The power regulator continuously monitors output current. If output current exceeds a threshold, however, the power regulator provides a signal to the voltage feedback current which, in turn, causes the primary circuit to reduce the output voltage. Thus, as output current increases in excess of the threshold, output voltage is decreased thereby maintaining output power at a constant, yet safe, level. 
     One or more loads that draw power from the adapter may be adapted to detect a drop in the AC adapter&#39;s output voltage. When such a voltage drop is detected, that information tells the load that too much current is being drawn from the adapter and that the load should throttle back to decrease the power draw on the AC adapter. If the load is the battery charger and battery, the charger can throttle back charging by reducing or even ceasing the charge current to the battery. Throttling back charge current results in a lower power draw on the adapter thereby alleviating the excessive power draw condition experienced by the adapter. The load could also be the computer for which the processor could be throttled back by reducing its clock frequency. Alternatively, power could be saved by dimming the display or altering the operation of any other function of the computer. 
     These and other advantages will become apparent upon reviewing the following disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
         FIG. 1  shows a conventional computer system including a constant voltage AC adapter; 
         FIG. 2  graphically depicts the constant voltage nature of the AC adapter of  FIG. 1 ; 
         FIG. 3  shows a computer system having a constant voltage/constant power AC adapter constructed in accordance with the preferred embodiment of the invention; 
         FIG. 4  graphically depicts the constant voltage/constant power approximation nature of the preferred AC adapter of  FIG. 3 ; 
         FIG. 5  shows an exemplary circuit in the adapter that provides the constant voltage/constant power output characteristic; 
         FIG. 6  shows one embodiment for how to use the adapter&#39;s output voltage to throttle back battery charge current; 
         FIG. 7  shows a second embodiment for how to use the adapter&#39;s output voltage to throttle back battery charge current; 
         FIG. 8  shows a waveform created by the second embodiment of  FIG. 7 ; and 
         FIG. 9  shows a third embodiment for how to use the adapter&#39;s output voltage to throttle back battery charge current. 
     
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, computer companies may refer to a component and sub-components by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either a direct or indirect electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. In addition, the term “throttling” or “throttling back” a device or system means to change the operating state of the device or system so that the device/system draws less power. For example, throttling back a CPU may include reducing the clock frequency of the CPU. Throttling back a LCD display can be accomplished by dimming the display. To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 3 , computer system  100 , constructed in accordance with the preferred embodiment of the invention, includes an AC adapter  102 , a computer  110 , a battery charger  120 , and a battery  124 . As shown, the AC adapter  102  converts the incoming AC line voltage to a lower DC output voltage (e.g., 18 VDC) and provides the DC power to the computer  110  and the battery charger  120  to operate those subsystems. Other components may be included as desired. 
     The computer  110  generally includes one or more CPUs  112 , a display (e.g., an LCD display)  114 , one or more peripheral or external loads (e.g., PCMCIA cards, modems, CD ROM drive, etc.), and other components known to those of ordinary skill in the art that have been omitted for sake of clarity. For purposes of the preferred embodiment, computer  110  simply represents a load on the AC adapter  102 . 
     The battery charger  120  represents another load on the adapter  102 . In general, the battery charger comprises a DC/DC converter which accepts the DC output voltage from the adapter  102  and converts and conditions that voltage to a suitable level for charging battery  124 . Although not specifically shown in  FIG. 3 , battery  120  may include one or more battery cells and a fuel gauge circuit (e.g., the BQ2058 by Texas Instruments) which monitors the current into the battery and reports battery charge status to the charger  120  and/or computer  110 . 
     The salient feature of the architecture depicted in  FIG. 3  is that the adapter  102  provides power for driving one or more loads. The loads may include a computer  110  and/or a battery subsystem comprising a charger  120  and a battery  124 . However, this disclosure and the claims which follow should not be limited to this particular architecture. The adapter may power other and/or different loads and even be used in an electronic system other than a computer. It should also be understood that the battery, although shown separate from the computer may actually be mounted in the computer  110 . It is only being shown separately in  FIG. 3  for functional purposes. 
     The AC adapter  102  of the preferred embodiment has a constant voltage/constant (“CV/CP”) power output characteristic which is depicted graphically in  FIG. 4 . In accordance with the preferred embodiment, the AC adapter&#39;s output voltage is regulated to a predetermined constant level in a constant voltage region  130  of its output voltage-current graph. However, once the power draw on the adapter reaches its maximum rating, any further increase in output current caused by loads  110  and  120  will result in a drop in input adapter output voltage. As such, the adapter&#39;s output voltage-current characteristic preferably includes a region  132  in which voltage and current vary approximately linearly in an inverse manner (i.e., as current increases, voltage decreases). Because power is the product of voltage and current, region  132  is representative of an approximation of a constant power condition. Thus, the adapter  102  of the preferred embodiment regulates its output voltage until the power draw becomes too high and then reduces its output voltage to maintain a constant (albeit preferably maximum) output power. As described below, the advantage of the constant voltage/constant power adapter  102  described above is that an external current sense circuit is unnecessary. 
     The AC adapter  102  in  FIG. 3  generally includes a primary circuit  104 , which includes a step down transformer and a voltage rectifier as would be commonly understood, and a secondary circuit  106 . The constant voltage/constant power output characteristic is implemented via circuitry in the secondary circuit  106 , an example of which is shown in  FIG. 5 . 
     Referring now to  FIG. 5 , secondary circuit  106  preferably includes a power control circuit  140  coupled to a voltage feedback circuit  142 . The circuits  140  and  142  monitor output voltage (Vout) and current (lout) and provide a feedback signal (FBS) to the primary circuit  104  ( FIG. 3 ) which causes the primary circuit  104  to adjust the output voltage to implement the constant voltage/constant power output characteristic shown in  FIG. 4 . The feedback signal may be an analog current, or voltage or a digital signal. 
     Referring still to  FIG. 5 , the voltage feedback circuit  142  includes an operational amplifier (“op amp”) U 1 , a voltage reference VR 1 , resistors R 4  and R 5 , capacitor C 1  and an optocoupler U 2 . One of ordinary skill in the art will appreciate that other components may be added the circuit  142  as desired. The output voltage, Vout, is divided down by resistors R 1 , R 2  and R 3  and provided to the inverting input of op amp U 1 . The reference voltage from VR 1  is a positive voltage relative to the output ground. The reference voltage from VR 1  is provided to the non-inverting input of op amp U 1 . The op amp essentially amplifies the difference between the reference voltage on the non-inverting input and a divided down version of the adapter&#39;s output voltage. In general, when the divided down output voltage exceeds the reference voltage, current is driven through the optocoupler U 2  which provides the feedback signal back to the primary circuit  104  which, in turn, reacts causing the output voltage to be lowered. Essentially, the op amp U 1  drives the optocoupler U 2  to interact with the primary circuit  104  to regulate the adapter&#39;s output voltage, Vout. The response of the voltage feedback circuit  142  is partly determined by various compensation components, such as the series combination of resistor R 4  and capacitor C 1  which are coupled between the inverting input of op amp U 1  and the op amp&#39;s output pin. 
     The power control circuit  140  in  FIG. 5  comes into play if the adapter is asked to provide too much power by loads  110  and  120  ( FIG. 1 ). As shown, the power control circuit  140  includes an op amp U 3 , resistors R 6 , R 7 , R 8 , R 9  and R 10 , capacitor C 2  and diode D 1 . Resistor R 6  comprises a current sense resistor which is placed in the return (ground) path, so that one end  143  of resistor R 6  is at the output ground and the other end  144  is at a lower potential when output current is delivered. A current signal  146  is formed by a resistor divider comprising resistors R 9  and R 10  between the lower potential end  144  of current sense resistor R 6  and the reference voltage produced by VR 1 . The current signal  146  is the voltage drop across the current sense resistor R 6  plus a small positive bias to ensure that the signal is above ground. The current sense signal  146  couples to the inverting input of op amp U 3  so that when the adapter&#39;s output current increases, the positive voltage at the inverting input of the op amp decreases toward ground. 
     The adapter&#39;s output voltage is divided down by resistors R 7  and R 8  and provided to the non-inverting input of the op amp U 3 . Then, when the output voltage (Vout) increases, the voltage at the non-inverting input of op amp U 3  increases. In this manner, an increase in the output voltage or output current causes the op amp&#39;s output voltage  148  to increase. The output of the op amp U 3  will react to the weighted sum of output voltage and output current, which is an approximation of output power. 
     The response of the power control circuit  140  preferably is made relatively slow by use of a long time constant in compensation capacitor C 2  which couples between the op amp U 3 &#39;s inverting input and the op amp&#39;s output pin. Having a slowly responding power control circuit helps prevent control loop interactions between the adapter  102  and downstream loads, such as computer  110  and battery charger  120 . 
     Referring still to  FIG. 5 , the output of op amp U 3  preferably is used to raise the feedback voltage of the voltage feedback circuit  142 , preferably by driving a current source that feeds current into the inverting input of voltage control amplifier U 1  described above. When power regulation amplifier U 3  generates an output voltage  148  in response to an excessive current condition, current is driven by the amplifier through diode D 1 , resistor R 2  and into voltage regulation amplifier U 1 . In this manner, power control circuit  140  can cause the voltage feedback circuit  142  to provide an appropriate feedback signal to the primary circuit  104  to cause the output voltage to be lowered during an over current condition. If the output current is low, the output of the current amplifier U 3  will be zero volts, the diode D 1  will not conduct, and there will be no effect on the output voltage. If R 3  is much smaller than R 2 , the voltage across R 3  will be small. If, however, the output current is high enough, the sum of output voltage and current will cause the current amplifier output to slowly rise enough to drive current, the voltage across R 3  will rise, and the output voltage will fall in response. The current loop will respond relatively slowly to changes in current, but the voltage amplifier U 1  will still react to changes in voltage, so there will be little effect on ripple voltage rejection. If R 3  is much smaller than R 2 , the mid frequency and high frequency gain of the voltage feedback  142  is not affected by the action of the power feedback  140 . 
     In summary, the AC adapter  102  of the preferred embodiment provides an output voltage-current characteristic that is a constant voltage until the power draw becomes too great. At that point, the adapter causes its output voltage to drop to maintain its output power at a constant level. The drop in output voltage can be used by other system components as a mechanism to initiate throttling back on some aspect of the system&#39;s operation. For instance, and without limitation, the drop in adapter voltage can be used to indicate when battery charging should be throttled back.  FIGS. 6 ,  7  and  9  include three embodiments of how to throttle back battery charging based on a drop in output voltage from a constant voltage/constant power AC adapter. 
     Referring to  FIG. 6 , the battery charger  120  is shown functionally to include an adapter voltage detection circuit  170  coupled to a battery charge circuit  150 . The battery charger circuit  150  receives output voltage from the adapter  102  and modifies that voltage to produce an appropriate charge current, Icharge, into battery  124 . The component architecture shown in  FIG. 6  comprising the battery charge circuit  150  is exemplary of a commonly known switching battery charge circuit. The circuit includes a transistor T 1 , inductor L 1 , diodes D 2  and D 3 , resistors R 11 -R 15 , capacitors C 4  and C 5  and op amp U 4 , and comparator U 5 . 
     In general, transistor T 1  is turned on and off at a rate set by comparator U 5 . Comparator U 5  receives a periodic waveform (e.g., a sawtooth wave) on its non-inverting input and, via resistor R 13 , the output signal from current amplifier U 6  on its inverting input. Resistor R 15  is a low resistance current sense resistor (e.g., 40 milliohms) that produces a voltage (Vcs) that is proportional to the charge current (Icharge). The Vcs voltage is provided through resistor R 14  to the inverting input of current amplifier U 6 . A reference voltage (e.g.,0.1V) is provided to the non-inverting input of current amplifier U 6 . If the charge current is precisely at its predetermined preferred level, then the voltage on the inverting input will be equal to the reference voltage and the output of the current amplifier U 6  will be 0 V. If, however, the charge current rises for some reason, then the voltage on U 6 &#39;s inverting input will increase and the output of U 6  will be driven lower which, via the action of U 5 , causes the duty cycle of transistor T 1  to decrease. When the output of U 5  is low, transistor T 1  is on; when U 6  output is high, T 1  is off. A decreased T 1  duty cycle causes the charge current to decrease. If, the charge current falls below its nominal level, the opposite result occurs with T 1 &#39;s duty cycle increasing and causing the charge current to increase. In this manner the charge current is regulated to a predetermined value. 
     Voltage detection circuit  170  includes an op amp U 7 , resistors R 16 -R 19  and a diode D 4 . The adapter output voltage is divided down by resistors R 16  and R 17  and coupled to the inverting input of U 7 . A suitable reference voltage couples to the non-inverting input of U 7 . Resistor R 18  comprises a feedback resistor coupled between U 7 &#39;s output and its inverting input. As shown, circuit  170  is configured as an inverting amplifier which amplifies the adapter voltage relative to the reference voltage. If the scaled adapter voltage is greater than the reference voltage on the non-inverting op amp input, the output voltage from the op amp will be 0 V and will essentially do nothing. If, however, the scaled adapter voltage falls below the reference (due to the adapter voltage falling from being overloaded), the output voltage from op amp U 7  will be driven positive and drive current through diode D 4  and resistor R 19  to the inverting input of current amplifier U 6 . As explained above, as the inverting input of U 6  increases, the duty cycle of T 1  decreases thereby causing a reduction in charge current. Further, the lower the adapter voltage falls, the greater will be associated reduction in charge current. Thus, the voltage detection circuit  170  is a means to detect a drop in adapter voltage which indicates an excessive load condition and, in response, cause charge current to be throttled back. 
     An alternative battery charge throttling scheme based on a drop in adapter voltage is shown in  FIG. 7 . More specifically,  FIG. 7  shows an alternative voltage detection and charge current modification circuit  180  which includes a comparator U 8 , AND gate  182 , resistors R 20 -R 24 , and capacitor C 6 . The incoming adapter output voltage is divided down by resistors R 20  and R 21  and provided to the non-inverting input of comparator U 8 . Resistor R 22  is a positive feedback resistor coupled between the non-inverting input and output of U 8 . The output of the comparator, which is an open-collector device, is pulled up to a reference voltage (e.g., 2.5V). Capacitor C 8  is a timing capacitor that couples between the op amp&#39;s inverting input and ground and timing resistor R 23  couples between the inverting input and op amp output as shown. This configuration forms an oscillator, with variable frequency that is dependent on adapter voltage. 
     When the adapter voltage is above the charge threshold, the comparator output will be high (pulled up to the reference voltage). When the adapter voltage is in the constant power region in  FIG. 4 , and falls below the charge threshold, the comparator output will go low for a period of time determined primarily by the values of the timing resistor R 23  and capacitor C 6 . In one preferred embodiment, R 23  and C 6  may be selected so as to cause the comparator&#39;s output to go low for approximately 250 microseconds. The comparator U 8  output voltage waveform is depicted in  FIG. 8 . The duration of the output high state depends on how far the adapter voltage is below the charge threshold. When the adapter is at or above the charge threshold, the comparator output remains high (i.e., high time is infinite). As the adapter voltage decreases, the duration of the high time shortens. The comparator output signal is gated by AND gate  182  and enable signal if desired. 
     The comparator output signal is used to drive the Master Battery (MBAT) signal which is known in the art to be used in an Intelligent Battery Architecture (“IBA”). When MBAT pulses low, the battery in an IBA system temporarily decreases the charge current, which regulates the power drawn from the adapter. The charge current very slowly rises until another low-going MBAT pulse is generated and again the charge current decreases temporarily. The lower the adapter voltage, the closer together will be the MBAT pulses and the lower will be the average charge current. 
     Another alternative use of adapter voltage to throttle battery is shown in  FIG. 9 .  FIG. 9  shows a Smart Battery System (“SBS”) standard implementation of the charger. The embodiment  200  shown uses a MAX1772 battery charger device  190 . Connections to the battery are not shown in  FIG. 9 . The circuit  200  includes an op amp U 9 , NPN transistor T 2  and resistors R 25 -R 29 . A reference voltage (e.g., 4.20 V) is provided by the battery charger device  190  to the non-inverting input of op amp U 9 . The inverting input of U 9  is the adapter voltage divided down by resistors R 25  and R 26 . The op amp output drives the base (B) of transistor T 2 . Resistor R 29  couples the emitter of the transistor to ground. Resistor R 27 , which couples between the inverting input of U 9  and the emitter of transistor T 2 , provides gain. If the adapter voltage is above the charging threshold defined previously, the op amp output is 0 V, and transistor T 2  is off. When the AC adapter  102  falls below the charging threshold, U 9 &#39;s output rises thereby driving an emitter current. The emitter current also flows through the collector resistor, R 28 , causing a differential voltage across it. The voltage across the collector resistor is coupled to the current sense input pins of the battery charger device  190  as shown. When the adapter voltage falls to the threshold set by the reference voltage on U 9 &#39;s non-inverting input, the differential voltage across R 28  exceeds the design threshold and the battery charger  190  responds by reducing charge current. 
     In summary, the embodiments described herein are directed to an AC adapter that regulates its output voltage until current reaches a maximum level and then regulates its output power for current in excess of the maximum level. Power is regulated at a constant level by reducing voltage with increases in current. Other components in the system can be designed to throttle back on their current demand on the AC adapter in response to detecting a drop in adapter voltage. Several embodiments of throttling back battery charging are shown above. If desired, the computer  110  can throttle itself back as well. Computer throttling can include reducing CPU clock frequency, dimming the display  116  and/or modifying another aspect of computer operation that results in a lower power draw on the adapter  102 . 
     The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the particular circuit implementations shown in the figures may be modified in a number of different ways without departing from the principles and scope of this disclosure. Components can be added or removed from the circuits and different circuits altogether that provide the same benefits and functionality can be used. It is intended that the following claims be interpreted to embrace all such variations and modifications.