Patent Publication Number: US-2013234692-A1

Title: Voltage supply and method with two references having differing accuracy and power consumption

Description:
FIELD 
     The present invention relates generally to voltage reference circuits and methods and, in particular, to voltage reference circuits and methods having references for accuracy. 
     BACKGROUND 
     Conventionally, contemporary electronic circuitry often operate on the basis of voltage references. A voltage reference may be configured to produce a voltage output of a particular magnitude relative to a source voltage, often ground. The voltage reference may produce a relatively stable output which may be used by the circuitry to provide particular voltage outputs for powering the circuitry or to operate relative to known parameters. For instance, the output of a voltage reference may be utilized by a power source to provide power at a known ratio to the reference output to power some or all of the electronic circuitry. Similarly, other circuitry may utilize the reference output as a point of reference for conducting logical operations; a comparator, for instance, may make a comparison based on comparing an input with an output of the voltage reference. 
     But a variety of types of voltage reference circuits incorporate various characteristics. Certain voltage references may consume relatively little power but may provide a relatively unstable output. Conversely, other voltage references may provide relatively stable voltage outputs but may require relatively large amounts of power to do so. 
     One such stable voltage reference which consumes relatively high power is known in the art as a bandgap reference. A bandgap reference seeks to utilize the bandgap of the substrate on which the bandgap reference is built, conventionally silicon. Because the bandgap of a substance is essentially fixed by physical laws that are constant over temperature, a bandgap reference may produce a comparatively steady and reliable output voltage. In the case of a substrate of silicon, a bandgap reference may tend to produce an output approximately equivalent to the 1.22 electron Volts bandgap of silicon which, depending on the structure of the particular bandgap reference circuit, may tend to be steady in the range of approximately 1.2 to 1.3 Volts. 
     However, as noted above, bandgap reference circuits tend to consume relatively large amounts of power in comparison to voltage reference circuits which do not depend on a bandgap. In applications with abundant power sources, such relatively large power consumption may be of little consequence. In applications with relatively little available power or in which it may be advantageous to limit power consumption bandgap references may be impractical to use in spite of their steady voltage output. 
     In particular, implantable medical devices may have internal power sources which either cannot be recharged, perhaps requiring the replacement of the implantable medical device altogether, or which can be recharged only inconveniently. Thus, in such implantable medical device applications, the conventional use of a bandgap reference may result in power depletion levels which may necessitate more frequent medical procedures, including surgical procedures, to continue treating the patient. However, the conventional use of non-bandgap reference circuits may not provide an output voltage suitably stable for reliable operation of the implantable medical device. 
     SUMMARY 
     A voltage reference circuit has been developed which combines a bandgap voltage reference with a lower-power voltage source to provide a relatively stable voltage reference with relatively low power consumption. The bandgap reference is duty cycled so that the bandgap reference is powered on, and thus consuming relatively large amounts of power, only comparatively infrequently. Rather than necessarily being used as a reference voltage for a variety of different components unrelated to the voltage reference circuit, however, the duty cycled bandgap reference output may be utilized instead to adjust the lower-power voltage source. 
     The lower-power voltage source may be one or more of different, relatively low-power consumption sources. In an embodiment, the voltage source is a low power voltage reference which is relatively unstable over time. The bandgap voltage reference may be configured to provide a reference for a trim circuit which trims the low power voltage reference to compensate for the relative instability of the low power voltage reference. Either alternatively or in supplement to the low power voltage reference, a sample and hold circuit may be charged and periodically refreshed by the bandgap reference based on the decay characteristics of the sample and hold circuit. 
     While the lower power voltage reference may be relatively unstable over lengthy periods of time, i.e., days or more, the lower power voltage reference may provide adequate stability for periods of time which are nevertheless considerable for electronics applications, i.e., minutes to hours. A voltage reference circuit which incorporates both a bandgap reference and a low power reference may thus turn on the bandgap reference for a period of time only long enough to compare the output of the bandgap reference with the output of the low power reference. 
     On the basis of the comparison, a trim circuit may trim the low power reference so that the output of the low power reference is approximately equal to that of the bandgap reference. Once the bandgap reference has provided its output sufficient to conduct the comparison, the bandgap reference may disabled. After the trimming is complete, the low power reference provides a reference output approximately equivalent to what the bandgap reference would provide if it were still enabled. In various embodiments, the bandgap reference utilizes a duty cycle of one percent or less in order to provide a relatively stable low power reference. 
     In an embodiment, a voltage supply comprises a first reference and a second reference. The first reference has an operation mode configured to supply a first reference voltage at a first accuracy and consume an operation power and a standby mode configured to consume standby power less than the operation power. The second reference is configured to supply a second reference having a second accuracy less than the first accuracy of the first reference and which consumes a second reference power less than the operation power of the first reference, the second reference voltage being trimmable based, at least in part, on a comparison of the first reference voltage to the second reference voltage. 
     In an embodiment, the first reference is a bandgap reference. 
     In an embodiment, the second reference is a threshold reference. 
     In an embodiment, the threshold reference is configured with the second accuracy being more accurate with trimming based on the bandgap reference voltage than without trimming based on the bandgap reference voltage. 
     In an embodiment, the operation mode comprises not more than approximately a one percent duty cycle relative to the standby mode. 
     In an embodiment, the operation mode comprises a duty cycle based, at least in part, on an environmental factor. 
     In an embodiment, the environmental factor is temperature. 
     In an embodiment, the environmental factor is change in temperature. 
     In an embodiment, the voltage supply is configured to supply the second reference voltage to a load having a voltage accuracy requirement, and wherein the operation mode comprises a duty cycle based, at least in part, on the voltage accuracy requirement of the load. 
     In an embodiment, the second reference is a threshold reference. 
     In an embodiment, voltage supply has a voltage reference and a storage circuit. The voltage reference has an operation mode configured to supply a reference voltage at a first accuracy and consume an operation power and a standby mode configured to consume standby power less than the operation power. The storage circuit is configured to store the reference voltage when the voltage reference is in the operation mode and supply an output voltage approximately equivalent to the reference voltage when the voltage reference is in the standby mode, wherein the output voltage is delivered at a second accuracy less than the first accuracy. 
     In an embodiment, the voltage reference is a bandgap reference configured to supply a bandgap reference voltage. 
     In an embodiment, wherein the storage circuit supplies the output voltage approximately equivalent to the bandgap reference voltage for a supply duration, and wherein the standby mode operates for not longer than the duration. 
     In an embodiment, the storage circuit stores obtains the reference voltage during a storage duration, and wherein the operation mode operates for not longer than the storage duration. 
     In an embodiment, the operation mode comprises not more than a one percent duty cycle relative to the standby mode. 
     In an embodiment, the operation mode comprises a duty cycle based, at least in part, on an environmental factor. 
     In an embodiment, the environmental factor is temperature. 
     In an embodiment, the environmental factor is change in temperature. 
     In an embodiment, voltage supply has a first reference, a trim circuit, a second reference comparator and a controller. The first reference has an operation mode configured to supply a first reference voltage and consume an operation power and a standby mode configured to consume standby power less than the operation power. The trim circuit is a trim circuit output. The second reference is operatively coupled to the trim circuit and configured to supply a second reference voltage having an accuracy less than an accuracy of the first reference voltage and consume a reference power less than the operation power of the first reference, the second reference voltage being trimmable based, at least in part, on the trim circuit output. A comparator is operatively coupled to the first reference, the trim circuit and the second reference and configured to compare the second reference voltage with a voltage based, at least in part, on the first reference voltage and generate a comparison, wherein the trim circuit output is selectable based, at least in part, on the comparison. The controller is operatively coupled to the first reference and configured to operate the first reference in the operation mode and in the standby mode, with a duty cycle in the operation mode of not greater than approximately one percent. The first reference voltage is supplied to the comparator and the trim circuit provides the trim circuit output to trim the second reference voltage while the first reference is in the standby mode. 
     In an embodiment, the voltage supply further comprises a storage element, operatively coupled to the bandgap reference and the comparator, configured to store the bandgap reference voltage and supply the bandgap reference voltage to the comparator when the bandgap reference is in the standby mode. 
     In an embodiment, the storage element is a capacitor. 
     In an embodiment, the threshold reference has an operation mode and a standby mode and operates in the operation mode on a threshold reference duty cycle. 
     In an embodiment, the threshold reference duty cycle is at least ten times greater than the duty cycle of the bandgap reference. 
     In an embodiment, the voltage supply further comprises a storage element operatively coupled to the threshold reference and configured to store the threshold reference voltage and provide the threshold reference voltage when the threshold reference operates in the standby mode. 
     In an embodiment, the voltage supply further comprises a voltage scaling circuit operatively coupled to the bandgap reference and the comparator and configured to provide an adjusted bandgap reference voltage to the comparator based, at least in part, on the bandgap reference voltage. 
     In an embodiment, the voltage scaling circuit is a voltage divider. 
     In an embodiment, the voltage scaling circuit is adjustable and configured to selectively provide one of a plurality of adjusted bandgap reference voltage to the comparator. 
     In an embodiment, the adjusted bandgap reference voltage is approximately equal to the threshold reference voltage. 
     In an embodiment, the operation mode comprises a duty cycle based, at least in part, on an environmental factor. 
     In an embodiment, the environmental factor is temperature. 
     In an embodiment, the environmental factor is change in temperature. 
     In an embodiment, the second reference is a threshold reference. 
     In an embodiment, voltage supply comprises a voltage reference and a storage circuit. The voltage reference has an operation mode configured to supply a reference voltage at a first accuracy and consume an operation power and a standby mode configured to consume standby power less than the operation power. The a storage circuit has a buffer and a capacitive element operatively coupled to the voltage reference and configured to store the reference voltage when the voltage reference is in the operation mode and supply an output voltage approximately equivalent to the reference voltage when the voltage reference is in the standby mode. The output voltage is delivered at a second accuracy less than the first accuracy. 
     In an embodiment, the voltage reference is a bandgap reference configured to supply a bandgap reference voltage. 
     In an embodiment, the buffer is a first buffer and wherein the storage circuit further comprises a second buffer operatively coupled to the first buffer and the capacitive element. 
     In an embodiment, the buffer supplies the output voltage approximately equivalent to the bandgap reference voltage for a supply duration, and wherein the standby mode operates for not longer than the duration. 
     In an embodiment, the buffer stores the reference voltage during a storage duration, and wherein the operation mode operates for not longer than the storage duration. 
     In an embodiment, the operation mode comprises not more than a one percent duty cycle relative to the standby mode. 
     In an embodiment, a method of providing an operation reference voltage has the steps of operating a first reference in an operation mode configured to supply a first reference voltage at a first accuracy and consume an operation power, operating the first reference in a standby mode configured to consume standby power less than the operation power, providing a second reference configured to supply a second reference voltage having a second accuracy less than the first accuracy of the first reference voltage and which consumes a second reference power less than the operation power of the first reference, trimming the second reference voltage based, at least in part, on a comparison of the first reference voltage to the second reference voltage, and providing the second reference voltage as the operation reference voltage. 
     In an embodiment, the trimming step provides the second accuracy being more accurate with the trimming step based on the bandgap reference voltage than without the trimming step based on the bandgap reference voltage. 
     In an embodiment, the operating steps comprise a duty cycle based, at least in part, on an environmental factor. 
     In an embodiment, a method of providing an operation reference comprises the steps of operating a first reference in an operation mode configured to supply a first reference voltage and consume an operation power and in a standby mode configured to consume standby power less than the operation power, supplying a second reference voltage having less accuracy than the first reference voltage and consuming a second reference power less than the operation power, and trimming the second reference voltage based upon a comparison between the first reference voltage and the second reference voltage while operating the first reference in the operation mode. The first reference is operated in the operation mode with a duty cycle of not greater than approximately one percent. 
     In an embodiment, the method further comprises the steps of storing the bandgap reference voltage and supplying the bandgap reference voltage to the comparator when the bandgap reference is in the standby mode. 
     In an embodiment, the supplying step has an operation mode and a standby mode and operates in the operation mode on a threshold reference duty cycle. 
     In an embodiment, the method further comprises the step of providing an adjusted bandgap reference voltage based, at least in part, on the bandgap reference voltage. 
     In an embodiment, the operating step comprises a duty cycle based, at least in part, on an environmental factor. 
    
    
     
       FIGURES 
         FIG. 1  is a block diagram of a first voltage reference; 
         FIG. 2  is an exemplary embodiment of the first voltage reference of  FIG. 1 ; 
         FIG. 3  is a voltage supply utilizing the first voltage reference of  FIG. 1  and a second voltage reference; 
         FIG. 4  is a voltage supply utilizing the first voltage reference of  FIG. 1  and a storage circuit; 
         FIG. 5  is a flowchart for utilizing the voltage supply of  FIG. 3 ; and 
         FIG. 6  is a flowchart for utilizing a voltage supply. 
     
    
    
     DESCRIPTION 
       FIG. 1  is a block diagram of a first reference  10 . In the illustrated embodiment, first reference  10  includes bandgap reference core  12 , filter block  14 , buffer  16  and voltage scaling circuit  18 . The various blocks  12 ,  14 ,  16 ,  18  of first reference  10  are selectable and configurable based on the needs and requirements of the circumstances in which first reference  10  is utilized. 
     Bandgap reference core  12  is, in various embodiments, one of various bandgap references well known in the art or a proprietary bandgap reference. In alternative embodiments of first reference  10 , voltage references known in the art other than a bandgap reference may be utilized instead of a bandgap reference core, particularly voltage references with relatively stable voltage outputs. Circuitry for bandgap reference core  12  may be configured to produce a relatively reliable output of a first reference voltage at a first accuracy of approximately 1.2 Volts to 1.3 Volts with a variation tolerance of approximately two (2) millivolts over an operational temperature range of bandgap reference core  12 . In an embodiment, the variation tolerance of bandgap reference core  12  is approximately one (1) millivolt. Bandgap reference core  12  is configured to be selectively disabled to a standby mode and enabled to an operation mode. As enabled, the first reference voltage output is produced and provided from bandgap reference core  12  to the rest of first reference  10  while consuming an operation power. In various embodiments, the operation power includes an operation current of approximately one hundred (100) nanoAmperes or more. In an embodiment, the operation current is approximately two hundred (200) nanoAmperes. As disabled, the first reference voltage output is not produced while bandgap reference core  12  consumes a standby power having a standby current. In an embodiment, the standby current is approximately six (6) nanoAmperes. 
     Various bandgap references which may be utilized for bandgap reference core  12  produce noise. In various embodiments, filter block  14  is configured to filter high frequency noise. In various embodiments, buffer  16  is configured to filter low frequency noise and provide a relatively steady output at approximately the output voltage of bandgap reference core  12 . Voltage scaling circuit  18  is configured to convert the approximately 1.2 Volt to 1.3 Volt output signal of bandgap reference core  12  to a voltage level useful for the circuitry for which first reference  10  is configured to supply a reference voltage. In certain embodiments, voltage scaling circuit  18  is a voltage divider configured to supply 850 milliVolts on output line  20 . In one such embodiment, voltage converter incorporates a low pass RC filter with a resistance of approximately three (3) megaOhms and a capacitance of approximately sixty-five (65) picoFarads. In an embodiment, voltage scaling circuit  18  is a voltage divider configured to supply a selectable output of between 850 and 925 milliVolts on output line  20  through the incorporation of a variable resistor in addition to the low pass RC filter. 
     Circuitry blocks  12 ,  14 ,  16 ,  18  may be separately selectable and configurable based on the circumstances in which they are utilized. As noted above, bandgap reference core  12  may be selectable based on the characteristics of available bandgap references and other available relatively stable voltage references and the circumstances in which first reference  10  is utilized. Certain bandgap references, including those which have been developed and which may be developed, may have characteristics such as higher or lower current consumption, particular voltage levels and relative amounts of noise which may be advantageous or disadvantageous in certain circumstances. Such bandgap references may be selectable based on such circumstances. 
     Similarly, certain of filter block  14 , buffer  16  and voltage scaling circuit  18  are configurable based on the selected bandgap reference in first reference  10  or may be dispensed with altogether based on the selected bandgap reference or in the event that a requirement of a load for output  20  of first reference  10  does not require the purpose for which each block  14 ,  16 ,  18  is directed. For instance, if the selected bandgap reference core  12  produces little high frequency noise, or if the circuitry for which the reference voltage is provided is not susceptible to high frequency noise, then filter block  14  may be excluded. Similarly, filter block  14  may incorporate different types of filters to filter different types of signals. In embodiments in which buffer  16  does not incorporate low frequency filtering, filter block  14  may incorporate low frequency filtering. In various embodiment, buffer  16  may incorporate two or more separate buffers for providing outputs with desired output impedances. 
       FIG. 2  is exemplary circuitry for a particular embodiment of bandgap reference core  12 , filter  14  and buffer  16 . In the illustrated embodiment, bandgap reference core  12  produces a reference voltage of approximately 1.2 Volts by summing a base-to-emitter voltage of P-N-P bipolar diodes  22 ,  24  with a voltage proportionate to absolute temperature, or “PTAT”, created by a difference in voltage between diodes  22 ,  24  at different current densities. In various embodiments, diode  22  is at least five (5) times larger than diode  24 . In an embodiment, diode  22  is nine (9) times larger than diode  24 . In such embodiments, the difference in size of diodes  22 ,  24  provides, at least in part, the difference in current density over diodes  22 ,  24 . In the illustrated embodiment, the voltage difference between diodes  22 ,  24  arising from the difference in current density over diodes  22 ,  24  may be calculated according to the equation V=K*(T/Q)*ln(R), where K is Boltzman&#39;s constant of 1.38*10 −23 , T is temperature in degrees Kelvin, Q is the charge on an electron of 1.602*10 −19  and ln(R) is the natural logarithm of the ratio between diodes  22  and  24 . In an embodiment, the voltage difference between diodes  22 ,  24  is approximately 56.4 milliVolts at twenty-five (25) degrees Celsius. 
     In an embodiment, diodes  22 ,  24  have a diode voltage of approximately 0.5 Volts, but a negative temperature coefficient of approximately 2.3 milliVolts per degree Celsius at thirty-seven (37) degrees Celsius. The difference in current density over diodes  22 ,  24  may produce a voltage differential proportionate to absolute temperature of approximately 0.7 Volts with a positive temperature coefficient of approximately 2.3 milliVolts per degree Celsius at thirty-seven (37) degrees Celsius. Consequently, summed together, the diode voltage and the voltage proportionate to absolute temperature produce an output voltage of approximately 1.2 Volts; as the positive 2.3 milliVolts per degree Celsius of the voltage proportionate to absolute offsets the negative 2.3 milliVolts per degree Celsius of the diode voltage, the output of bandgap reference core  12  may be substantially temperature independent. 
     Amplifier  26 , resistors  28 ,  30  and variable resistor  32  provide, at least in part, for the difference of current over diodes  22 ,  24 , the variation of current over diodes  22 ,  24 , and summing of the diode voltage and the voltage proportionate to absolute temperature. Resistor  34  provides the voltage difference between diodes  22 ,  24 . Trim input  36  coupled to variable resistor  32  provides variation of current over diodes  22 ,  24 . In certain embodiments, bandgap reference core  12  is trimmed at production and not trimmed thereafter. In various embodiments, resistors  28 ,  30  have values of approximately twenty-seven (27) megaOhms and resistor  32  has a value of approximately three (3) megaOhms, while variable resistor  32  is variable from approximately 0.8 megaOhms to approximately 7.3 megaOhms with a resolution of approximately twenty-five (25) kiloOhms producing an output voltage resolution of approximately one (1) milliVolt. Alternatively, variable resistor  32  has a resolution of approximately twenty (20) kiloOhms producing an output voltage resolution of approximately 0.8 milliVolts. 
     In various embodiments, amplifier  26  is an operational amplifier with a relatively low offset voltage. In one such embodiment, amplifier  26  is a chopper amplifier, as known in the art, to reduce low frequency noise relative to other operational amplifiers known in the art. Offset from amplifier  26  and low frequency noise may further be reduced through a dynamic element matching technique. In the above illustrative embodiment for diode  22  being nine (9) times larger than diode  24 , diodes  22  and  24  together comprise ten discrete diodes each configured to be approximately or exactly the size of diode  24 . At any give time, diode  22  comprises nine (9) of the discrete diodes switched to be electrically in parallel with respect to one another while diode  24  comprises the remaining one (1) discrete diode. The discrete diodes are periodically switched, in an embodiment with each clock cycle, so that each discrete diode comprises diode  24  in turn. In such an embodiment, over ten switches, diode  24  acts as an average of each of the ten discrete diodes while diode  22  acts as a diode of nine (9) times the average of the ten discrete diodes. In an embodiment, filter  14  may provide filtering against a voltage ripple caused by such dynamic element matching diode switching. 
     Filter block  14 , as illustrated, is configured to filter out high frequency noise. Resistor  38  is approximately twenty (20) megaOhms while capacitor  40  is approximately sixty (60) picoFarads, providing a filter with a low pass filter frequency of approximately one hundred thirty (130) Hertz. As noted above, filter block  14  is configured based on the performance of surrounding circuitry, including the particular bandgap reference core  12  utilized; in alternative embodiments, filter block  14  may be configured as appropriate to filter noise based on particular circumstances. 
     As illustrated, buffer  16  is an amplifier  42  in a unity gain configuration with six thousand Ohms output resistance. Such a configuration may provide a relatively low impedance output to output capacitor  44 , in an embodiment ten (10) nanoFarads, in comparison with other buffer amplifiers known in the art. Amplifiers with different output resistance are contemplated and selectable based on the particular circumstances in which buffer  16  is utilized. 
       FIG. 3  is an embodiment of voltage supply  46  which utilizes first reference  10  in conjunction with controller  47 , second reference  48 , comparator  50  and trim circuit  52 . In an embodiment, first reference  10  is duty cycled by controller  47  as described above so that first reference  10  consumes a standby power less than the operation power, as discussed above. Second reference  48  provides a reference voltage on output  54  that is approximately equal to the first reference voltage on output  20  of first reference  10  while consuming less power than would be produced by first reference  10  operating alone continuously. 
     Trim circuit  21  is used to scale the voltage output of first reference  10  from 1.2 Volts to 850 milliVolts and is used as the “minus” input of comparator  50 . Comparator  50  compares output  54  against the “minus” terminal and the output to trim circuit  52 . Trim circuit  52  adjusts second reference  48  to output the proper voltage on output  54 . 
     In various embodiments, second reference  48  is any trimmable voltage reference which consumes less operational power than first reference  10  and which has a second accuracy less than the first accuracy of first reference  10 . In various embodiments, second reference  48  is a CMOS threshold reference as known in the art. In certain such embodiments, second reference  48  is configured to provide a second reference voltage output approximately equal to output  20  of first reference  10 . In the above embodiment, second reference  48  is configured to generate a second reference voltage output of approximately 850 milliVolts while consuming a second reference power of approximately fifteen (15) to twenty-one (21) nanoAmperes. 
     Comparator  50  compares the voltage on output  20  and the output of second reference  48  and supply a result to trim circuit  52 . A trim circuit output of trim circuit  52  selectable based on the output of comparator  50  and is supplied to second reference  48  to adjust second reference  48  so that the output of second reference  48  is approximately equal to that of first reference  10 . In an embodiment, comparator  50  clocks at approximately four (4) kiloHertz. In an embodiment, non-overlapping phases of the output of comparator  50  may be utilized to sample output  54  of second reference  48 , in a first phase, and in a second phase compare output  54  of second reference  48  with output  20  of first reference  10  and output the comparison. In various embodiments, trim circuit  52  is implementable as a field programmable gate array, or “FPGA”, or as analog or digital circuitry. 
     In various embodiments, first reference  10  is duty cycled at a duty cycle rate. In an exemplary embodiment, first reference  10  consumes approximately two hundred (200) nanoAmperes as a relatively accurate and stable output voltage, in the above embodiment approximately 850 milliVolts, is supplied to output  20 . In various embodiments, second reference  48  is not duty cycled regularly, and instead supplies an output  54  to comparator  50  effectively continuously during normal operation. During the time first reference  10  is active, comparator  50  compares output  20  with the output  54  of second reference  48 . Trim circuit  52  trims second reference  48  until the output  54  of second reference  48  approximately matches that of output  20 . 
     In various embodiments, first reference  10  is duty cycled at least fifty (50) percent in order to provide a net reduction in total system current consumption relative to operating first reference  10  without second reference  48 . In various embodiments, however, first reference  10  is only operated approximately as long as is required to trim second reference  48  to produce an output voltage  54  approximately that of output  20  of first reference  10 . In an embodiment, first reference  10  may require approximately ten (10) milliseconds to stabilize after having been enabled, while comparator  50  and trim circuitry  52  may require approximately 0.5 milliseconds to trim second reference  48 , resulting in an active duty cycle period of approximately 10.5 milliseconds. In various embodiments, the duty cycle is not more than approximately one (1) percent. 
     In an embodiment, first reference  10  requires approximately 300 nanoAmperes to operate and has a duty cycle of approximately five percent. By comparison, second reference  48  requires only 20 nanoAmperes to operate resulting in an energy savings of approximately twelve (12) times (25 nanoAmperes versus 300 nanoAmperes). 
     In various embodiments, first reference  10  is left disabled for as long as second reference  48  may be expected to maintain a stable output voltage. Relative stability of second reference  48  may be assessed both objectively and on the basis of a voltage accuracy requirement of a load to which voltage supply  46  is supplying an output voltage. Such a period of time over which output  54  of second reference  48  maintains a stable output voltage may vary significantly based on the particular second reference  48  utilized, as well as environmental factors such as temperature. In certain embodiments, second reference  48  may maintain a stable output voltage for fifteen (15) minutes or more. However, under variable temperature conditions, the same second reference  48  may produce a stable output voltage for one (1) minute or less. In an embodiment, first reference  10  is disabled for approximately one (1) minute during every duty cycle. 
     In alternative embodiments, a temperature sensor is incorporated in controller  47  which regulates, at least in part, the duty cycle of bandgap voltage reference  10  based on measured temperature, change in measured temperature over time, and an expected impact on the stability of the output voltage of second reference  48 . In such embodiments, first reference  10  may be disabled for fifteen (15) minutes or more during duty cycles where second reference  48  is expected to be relatively stable based on measured temperature. 
       FIG. 4  is an alternative voltage supply  146  incorporating storage circuitry  148 . Storage circuitry  148  incorporates switch  150 , storage capacitor  152  and operational amplifier  154 . Switch  150  is closed during an operation time of voltage reference  10 , allowing storage capacitor  152  to charge to the voltage on output  20  of voltage reference  10 . Operational amplifier  154  is configured to supply the output voltage of capacitor  152  with a relatively low output impedance. When switch  150  opens, storage capacitor  152  is configured to deliver an output voltage for a supply duration at a second accuracy less than the first accuracy of voltage reference  10 , the output voltage being approximately equivalent to the voltage on output  20 , during which supply duration storage capacitor  152  gradually discharges. 
     In various embodiments, storage capacitor  152  is of selectable size based on size constraints and an amount of time it is desired for storage capacitor  152  to deliver the output voltage. In an embodiment, storage capacitor  152  is a seventeen (17) picoFarad capacitor. In such embodiments, storage capacitor  152  supplies a relatively stable output voltage for a supply duration of approximately 1.5 seconds. In such embodiments, voltage reference  10  is disabled for approximately 1.5 seconds and enabled for approximately 10.5 milliseconds, as described above with respect to voltage supply  46  ( FIG. 3 ), providing a duty cycle of not more than approximately one (1) percent. As described in detail with respect to voltage supply  46 , environmental factors such as temperature may impact the supply duration of voltage supply  146  as well. 
     In various embodiments, storage circuitry  148  of voltage supply  146  may be incorporated into voltage supply  46 . In various embodiments, output  54  of second reference  48  may be supplemented by storage circuitry  148  adding additional stability and longer operational duration. In other embodiments, including those which incorporate storage circuitry  148  in voltage supply  46 , second reference  48  may be duty cycled between an operational mode and a standby mode using a duty cycle based on the duty cycle of a threshold reference. In such embodiments, the threshold reference duty cycle is at least ten (10) times greater than the duty cycle of first reference  10 . 
       FIG. 5  is a flowchart for providing an operation reference voltage utilizing voltage supply  46 . First reference  10  is operated ( 500 ) in its operation mode to supply a first reference voltage while consuming an operation power, in the above exemplary embodiment approximately 1.2 Volts and approximately two hundred (200) nanoAmperes, respectively. First reference  10  is operated ( 502 ) in a standby mode configured to consume standby power less than the operation power, in the exemplary embodiment approximately six (6) nanoAmperes. Second reference  48  is provided ( 504 ) to supply the second reference voltage having a second accuracy less than the first accuracy of first reference  10  and which consumes a second reference power less than the operation power of first reference  10 . In the exemplary embodiment, the second reference power is between approximately fifteen (15) and twenty-one (21) nanoAmperes. 
     Second reference  48  is trimmed ( 506 ) by trim circuit  52  based, at least in part, on a comparison of the first reference voltage and the second reference voltage provided by comparator  50 . In various embodiments, the second reference voltage is more accurate based on being trimmed ( 506 ) that would be the case without being trimmed ( 506 ). The second reference voltage is provided ( 508 ) as the operation reference voltage to a load, as discussed above. 
       FIG. 6  is a flowchart for an alternative method of providing an operation reference using at least one of voltage supply  46  and voltage supply  146 . First reference  10  is variably operated ( 600 ) in an operation mode configured to supply the first reference voltage and consume an operation power and a standby mode configured to consume standby power less than the operation power. A second reference voltage is supplied ( 602 ) using at least one of second reference  48  and storage circuitry  148 , the second reference voltage having a less accuracy than the first reference voltage and consuming a second reference power less than the operation power of first reference  10 . 
     In an embodiment, the second reference voltage is trimmed ( 604 ) using trim circuit  52  based upon a comparison by comparator  50  between the first reference voltage and the second reference voltage while operating ( 600 ) first reference  10  in the operation mode. In an embodiment, the output of first reference  10 , such as a bandgap reference voltage, is stored ( 606 ) in storage circuitry  148  and supplied ( 608 ) to comparator  50  when first reference  10  is in standby mode. In an embodiment, voltage scaling circuit  18  provides ( 610 ) an adjusted bandgap reference voltage from first reference  10  to comparator  50 , in an embodiment, the bandgap reference voltage being approximately equal to the threshold reference voltage of second reference  48 . 
     Thus, embodiments of the invention are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.