Abstract:
A system includes a device configured to operate in a first mode and a second mode. The device includes a first circuit configured to receive a first band gap voltage potential from a first band gap circuit when the device is operating in the first mode, and a second circuit configured to receive a second band gap voltage potential from a second band gap circuit when the device is operating in the second mode. The device is configured to generate a mode select signal to selectively turn on and off the first band gap circuit and the second band gap circuit. A calibration circuit is configured to compare the second band gap voltage potential to the first band gap voltage potential, output a calibration signal to the second band gap circuit to adjust the second band gap voltage potential based on the comparison, and turn off the first band gap circuit in response to the second band gap voltage potential being within a predetermined range of the first band gap voltage potential.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/546,298, filed Aug. 24, 2009, which is a continuation of U.S. patent application Ser. No. 11/334,030 (now U.S. Pat. No. 7,579,822), filed Jan. 18, 2006, which is a continuation of U.S. patent application Ser. No. 10/926,185 (Now U.S. Pat. No. 7,023,194), filed Aug. 25, 2004, which is a continuation of U.S. patent application Ser. No. 10/413,927 (Now U.S. Pat. No. 6,844,711), filed Apr. 15, 2003. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to voltage reference circuits, and more particularly to band gap voltage reference circuits having high accuracy and low power consumption. 
     BACKGROUND OF THE INVENTION 
     Band gap (BG) voltage reference circuits provide a fixed voltage reference for integrated circuits. Referring now to  FIG. 1 , an exemplary BG circuit  10  is shown and includes transistors Q 1  and Q 2 , resistances R 1 , R 2 , and R 3 , a variable resistance R var  and an amplifier A. Collectors and bases of the transistors Q 1  and Q 2  are connected to a potential such as ground. The resistance R 3  has one end that is connected to an emitter of the transistor Q 1  and another end (at potential V 1 ) that is connected to the resistance R 1  and an inverting input of the amplifier A. The resistance R 1  is connected between one end of the resistance R var  and one end of the resistance R 2 . Another end of the resistance R 2  (at potential V 2 ) is connected to the emitter of the transistor Q 2  and a non-inverting input of the amplifier A. An output of the amplifier A is connected to another end of the resistance R var  which is at the BG voltage potential V bg . 
     Junctions between the emitters and the bases of the transistors Q 1  and Q 2  operate as diodes. The emitter area of Q 1  is typically larger than the emitter area of Q 2 , where K is a ratio of the emitter area of Q 1  divided by the emitter area of Q 2 . Amplifier A forces the voltage potentials V 1 =V 2 . Since the resistances R 1 =R 2 , the current flowing into the transistor Q 1  is equal to the current flowing into the transistor Q 2 . Therefore,
 
Δ V   be   =|V   be ( Q   2 )|−|V be ( Q   1 )= V   T  ln( K )
 
 V   bg   =V ( R   var )+ V ( R   2 )+| V   be ( Q   2 )|
 
     ΔV be  is applied across the resistance R 3  to establish a proportional to absolute temperature (PTAT) voltage. The voltages V(R var ) and V(R 2 ) have positive temperature coefficients. |V be (Q 3 )| has a negative temperature coefficient. Therefore, V bg  has a net temperature coefficient of approximately zero. The resistor R var  is adjusted to change V bg  and its temperature coefficient. 
     The accuracy of V bg  is related to the emitter area ratio K and the emitter area. Generally as the emitter area and the emitter area ratio K increases, the accuracy of the BG circuit also increases. As used herein, the term accuracy is used to reflect the variations that occur due to process. Higher accuracy refers to increasing invariance to process. Lower accuracy refers to increasing variance to process. 
     While increasing accuracy, the power dissipation of the transistor also increases with the area of the emitter. Therefore, the increased precision of the BG circuit is accompanied by an increase in power dissipation. Therefore, circuit designers must tradeoff accuracy and power dissipation. 
     SUMMARY OF THE INVENTION 
     A band gap voltage reference circuit comprises a first band gap (BG) circuit that generates a first BG voltage potential. A second BG circuit includes a variable resistance and outputs a second BG voltage potential that is related to a value of said variable resistance. A calibration circuit communicates with said first and second BG circuits, adjusts said variable resistance based on said first BG voltage potential and said second BG voltage potential, and selectively shuts down said first BG circuit. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  illustrates an exemplary BG circuit according to the prior art; 
         FIG. 2  is a functional block diagram of a BG circuit including low power and high power BG circuits according to the present invention; 
         FIG. 3A  illustrates power consumption of a high power BG circuit according to the prior art; 
         FIG. 3B  illustrates the power consumption of a low power BG circuit according to the prior art; 
         FIG. 3C  illustrates the power consumption of a BG circuit with power on calibration of the low power BG circuit according to the present invention; 
         FIG. 3D  illustrates the power consumption of a BG circuit with periodic calibration of the low power BG circuit according to the present invention; 
         FIG. 3E  illustrates the power consumption of a BG circuit with non-periodic calibration of the low power BG circuit according to the present invention; 
         FIG. 4  is a flow diagram illustrating steps that are performed by a calibration circuit according to the present invention; 
         FIG. 5  illustrates an exemplary calibration circuit according to the present invention; 
         FIGS. 6A and 6B  illustrate exemplary variable resistance circuits according to the present invention; 
         FIG. 7  illustrates a calibration circuit incorporating an up/down counter according to the present invention; 
         FIGS. 8A and 8B  are functional block diagrams of a device including high power and low power circuits that are selectively powered by high power and low power BG circuits; and 
         FIG. 9  is a functional block diagram of the circuits in  FIG. 8A  with a calibration circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. 
     Referring now to  FIG. 2 , a BG circuit  50  according to the present invention includes a high power BG circuit  52 , a low power BG circuit  54 , and a calibration circuit  56 . As used herein, the terms high and low power are relative terms relating to the emitter area ratio K and the current density of the devices. The high power BG circuit has a larger emitter area and emitter area ratio, higher power dissipation and greater accuracy than the low power BG circuit. The degree to which the high and low power BG circuits differ will depend upon the accuracy and power consumption that is desired for a particular application. The high power BG circuit  52  provides a BG voltage reference potential V bgH . The low power BG circuit  54  provides a BG voltage reference potential V bgL . 
     The BG voltage potential V bgL  and the BG voltage potential V bgH  are input to the calibration circuit  56 . The calibration circuit  56  compares the BG voltage potential V bgL  to the BG voltage potential V bgH  and generates a calibration signal. The calibration signal  62  is fed back to the low power BG circuit  54  to adjust the BG voltage potential V bgL . In other words, the higher accuracy of the BG voltage potential V bgH  is used to increase the accuracy of the BG voltage potential V bgL . 
     In one embodiment, the calibration signal is used to adjust a variable resistance  64 , which alters the BG voltage potential V bgL , although other methods may be used. When the BG voltage potential V bgL  and the BG voltage potential V bgH  are approximately equal, the calibration circuit  56  turns the high power BG circuit  52  off to reduce power consumption. 
     In general, the current density for bipolar transistors in the high power and low power BG circuits  52  and  54 , respectively, is approximately the same. The emitter area ratio of the bias current level for the high power and low power BG circuits  52  and  54  is approximately equal to the emitter area ratio of the emitter areas for the high power and low power BG circuits  52  and  54 . For example, the ratio can be a factor of 4 or larger. Therefore, the high power BG circuit  52  uses bipolar transistors having larger emitter areas that are biased at higher current levels than the low power BG circuit  54 . As a result, the high power BG circuit  52  provides the BG voltage reference V bgH  that is generally more accurate than the BG voltage potential V bgL  that is provided by the low power BG circuit  54 . 
     Referring now to  FIG. 3A , power consumption of a high power BG circuit according to the prior art is shown. The high power BG circuit is biased by a higher current level. For example, a bias current level of 60 μA is output to the high power BG circuit. Conversely, a low power BG circuit is biased by a lower current level and has lower power dissipation as shown in  FIG. 3B . For example, a bias current level of 10 μA may be used. 
     The power consumption of the BG circuit  50  of  FIG. 2  is shown in  FIG. 3C . Initially, the high power BG circuit  52  is biased by the higher current level. The low power BG circuit  54  is biased by the lower current level. This results in a higher initial power consumption. After the calibration is completed, however, the calibration circuit  56  shuts off the high power BG circuit  52 . This is represented by the reduction in power consumption at the end of the calibration period in  FIG. 3C . With the high power BG circuit shut off, only the low power BG circuit  54  continues to consume power. As a result, the average power consumption is reduced. 
     Referring now to  FIG. 3D , periodic calibration can also be performed. The calibration of the BG voltage potential V bgL  using the BG voltage potential V bgH  is performed after a predetermined period. Referring now to  FIG. 3E , calibration can also be performed on a non-periodic basis. For example, the calibration can be performed at power on and when a predetermined event occurs. One example event could be a detected change in the BG voltage potential V bgL . Degradation in performance of the device could also be a basis for non-periodic calibration. As another example, calibration can also occur when the operating temperature changes. Still other types of events are contemplated. 
     Referring now to  FIG. 4 , steps  70  for calibrating the low power BG circuit in  FIG. 2  are shown. In step  72 , both BG circuits  52  and  54  receive power at the beginning of calibration. Calibration may occur at an initial power up  72 , at regular intervals, after specific events, or in any other circumstances. The foregoing description will describe calibration at start-up. However, skilled artisans will appreciate that the present invention is not limited to start-up. 
     After power up in step  72 , the high power and low power BG circuits  52  and  54  generate the BG voltage potential V bgH  and the BG voltage potential V bgL , respectively, in step  74 . The calibration circuit  56  compares the BG voltage potential V bgH  to the BG voltage potential V bgL  in step  76 . In step  78 , the calibration circuit  56  determines whether the BG voltage potential V bgL  is within a predetermined threshold of the BG voltage potential V bgH . If step  78  is true, the high power BG circuit  52  is powered down in step  80 . 
     If the BG voltage potential V bgL  is not within the predetermined threshold, the calibration circuit  56  generates a calibration signal in step  82 . The low power BG circuit  54  receives the calibration signal in step  84  and adjusts the BG voltage potential V bgL  based on the calibration signal. If the adjustment brings the BG voltage potential V bgL  within the predetermined threshold, the high power BG circuit  52  powers down in step  80 . Otherwise, the calibration  70  continues with steps  82  and  84 . 
     Referring now to  FIG. 5 , an exemplary calibration circuit  90  includes a comparing circuit  92 , a D-type latch  94 , and a counter  96 . The comparing circuit  92  receives the BG voltage potential V bgH  from the high power BG circuit  52 . The comparing circuit  92  also receives the BG voltage potential V bgL  from the low power BG circuit  54 . The comparing circuit  92  determines whether the BG voltage potential V bgL  is within a predetermined threshold V th  of the BG voltage potential V bgH . 
     In other words, the comparing circuit  92  determines whether V bgH +V th &gt;V bgL &gt;V bgH −V th . For example, the threshold V th  may be 2 mV or any other threshold. If the BG voltage potential V bgL  is not within the threshold V th  of the BG voltage potential V bgH , the output of the comparing circuit is a first state. If the BG voltage potential V bgL  is within the threshold V th  of the BG voltage potential V bgH , the output of the comparing circuit  92  is a second state. Alternatively, a simple comparison between V bgH  and V bgL  may be used without the threshold V th . 
     The D latch  94  receives the output from the comparing circuit  92 . An output of the D latch  94  is determined by the output of the comparing circuit  92 . The output of the D latch  94  is generated periodically based on a clock signal  98 . If the D latch  94  receives an output of the first state from the comparing circuit  92 , the D latch outputs a digital “1” at an interval determined by the clock signal  98 . Conversely, if the D latch receives an output of the second state from the comparing circuit  92 , the D latch outputs a digital “0” at the interval determined by the clock signal  98 . 
     The counter  96  receives the digital “1” or “0” from the D latch. The counter  96  will receive the signal periodically as determined by the clock signal  98 . The value stored by the counter  96  determines the value of a variable resistance  64  in the low power BG circuit  54 . If the counter  96  receives a digital “1” from the D latch, the counter  96  increments the stored value, which increases the value of the variable resistance  64 . If the counter  96  receives a digital “0”, the stored value does not change. 
     Because the current source  66  of the BG circuit  54  is constant, adjusting the value of the variable resistance  64  also adjusts the value of the BG voltage potential V bgL . If the BG voltage potential V bgL  is less than the BG voltage potential V bgH , the value of the variable resistance  64  is adjusted, thereby adjusting the BG voltage potential V bgL . 
     A default value that is stored by the counter  96  ensures that the BG voltage potential V bgL  is lower than the BG voltage potential V bgH  at power up. Because the counter  96  is only able to increment in a positive direction, the calibration circuit  90  increases the BG voltage potential V bgL  until it is approximately equal to the BG voltage potential V bgH . 
     Calibration continues until the calibration circuit  90  determines that the BG voltage potential V bgL  is equal to or approximately equal to the BG voltage potential V bgH . Then, the calibration circuit  90  turns the high power BG circuit  52  off. For example, a power off timer  102  may be used to determine that the D latch  94  failed to output a digital “1” for a predetermined period. Additionally, the power off timer  102  prevents the high power BG circuit  52  from being powered off for an initial period after the power up. This ensures that the BG circuits  52  and  54  have an opportunity to stabilize. 
     Referring now to  FIGS. 6A and 6B , exemplary variable resistances are shown. In  FIG. 6A , the variable resistance  100  includes multiple resistive elements  110 - 1 ,  110 - 2 , . . . , and  110 - x  in series with a base resistive element  111 . The resistive elements  110  and  111  can be resistors, variable resistances, or any other type of resistive circuit. The resistive elements  110  are added and/or removed using parallel switches  112 - 1 ,  112 - 2 , . . . , and  112 - x . In one embodiment, the switches  112  are transistor circuits. An output of the counter  96  in  FIG. 5  is used to control the switches  112 . 
       FIG. 6B  shows another exemplary embodiment of a variable resistance  120 , which includes the multiple resistive elements  110 - 1 ,  110 - 2 , . . . , and  110 - x  in series with the base resistive element  111 . The resistive elements  110  are added and/or removed using switches  122 - 1 , 122 - 2 , . . . , and  122 - x . Skilled artisans will appreciate that any other device that provides a variable resistance can be used. 
     There are numerous methods for implementing the calibration circuit  90 . For example, a down counter may be substituted for the up counter  96 . In this embodiment, the calibration circuit  90  would adjust the second BG voltage reference potential V bgL  downward from an initial value that is greater than the first BG voltage reference potential V bgH . 
     Referring now to  FIG. 7 , a calibration circuit  128  that includes an up/down counter  130  is shown. A first comparator  132  outputs a digital “1” if the BG voltage potential V bgL  is less than BG voltage potential V bgH  minus V th . A second comparator  134  outputs a digital “1” if the BG voltage potential V bgL  is greater than the BG voltage potential V bgH  plus V th . Therefore, if the BG voltage potential V bgL  is too low, as determined by the threshold V th , the counter  130  is incremented. If the BG voltage potential V bgL  is too high, as determined by the threshold V th , the counter  130  is decremented. Once the BG voltage potential V bgL  stabilizes, the value of the counter  130  will no longer increment or decrement. 
     Referring now to  FIG. 8A , a device  150  includes high power circuits  152  and low power circuits  154 . When operating in the high power mode, the device  150  requires high power to operate the high power circuits  152 . When operating in the low power mode, the device  150  requires lower power to operate the low power circuits  154 . The low power circuits  154  may also be powered in both the high power and low power modes. 
     For example, the device  150  may be a transceiver that has a powered up mode and a sleep or standby mode. The device  150  generates a mode select signal that is used to turn on/off a high power BG circuit  160  and/or a low power BG circuit  164  as needed. In  FIG. 8B , the BG voltage potential V bgH  and the BG voltage potential V bgL  are summed by a summer  170  before being input to the device  150 . The device  150 , in turn, distributes the supplied power to the high power circuits  152  and the low power circuits  154  as needed. 
     Referring now to  FIG. 9 , a calibration circuit  180  is used to calibrate the low power BG circuit  164 . The low power BG circuit  164  includes a variable resistance  184  that is adjusted by the calibration circuit  180  as was described above. As can be appreciated, the circuit in  FIG. 9  can also include a summer  170  as shown in  FIG. 8B . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.