# Patent

## Title
Low power and high accuracy band gap voltage reference circuit

## Abstract

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.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 10/926,185, filed Aug. 25, 2004, which is a continuation of U.S. patent application Ser. No. 10/413,927, 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 Q1 and Q2, resistances R1, R2, and R3, a variable resistance Rvar and an amplifier A. Collectors and bases of the transistors Q1 and Q2 are connected to a potential such as ground. The resistance R3 has one end that is connected to an emitter of the transistor Q1 and another end (at potential V1) that is connected to the resistance R1 and an inverting input of the amplifier A. The resistance R1 is connected between one end of the resistance Rvar and one end of the resistance R2. Another end of the resistance R2 (at potential V2) is connected to the emitter of the transistor Q2 and a non-inverting input of the amplifier A. An output of the amplifier A is connected to another end of the resistance Rvar, which is at the BG voltage potential Vbg.
Junctions between the emitters and the bases of the transistors Q1 and Q2 operate as diodes. The emitter area of Q1 is typically larger than the emitter area of Q2, where K is a ratio of the emitter area of Q1 divided by the emitter area of Q2. Amplifier A forces the voltage potentials V1=V2. Since the resistances R1=R2, the current flowing into the transistor Q1 is equal to the current flowing into the transistor Q2. Therefore,

ΔVbe=|Vbe(2)|−|Vbe(1)|=VT ln(K)

Vbg=V(Rvar)+V(R2)+|Vbe(2)|

ΔVbe is applied across the resistance R3 to establish a proportional to absolute temperature (PTAT) voltage. The voltages V(Rvar) and V(R2) have positive temperature coefficients. |Vbe(2)| has a negative temperature coefficient. Therefore, Vbg has a net temperature coefficient of approximately zero. The resistor Rvar is adjusted to change Vbg and its temperature coefficient.
The accuracy of Vbg 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 VbgH. The low power BG circuit 54 provides a BG voltage reference potential VbgL.
The BG voltage potential VbgL and the BG voltage potential VbgH are input to the calibration circuit 56. The calibration circuit 56 compares the BG voltage potential VbgL to the BG voltage potential VbgH 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 VbgL. In other words, the higher accuracy of the BG voltage potential VbgH is used to increase the accuracy of the BG voltage potential VbgL.
In one embodiment, the calibration signal is used to adjust a variable resistance 64, which alters the BG voltage potential VbgL, although other methods may be used. When the BG voltage potential VbgL and the BG voltage potential VbgH 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 a higher current levels than the low power BG circuit 54. As a result, the high power BG circuit 52 provides the BG voltage reference VbgH that is generally more accurate than the BG voltage potential VbgL 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 52 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 VbgL using the BG voltage potential VbgH 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 VbgL. 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 VbgH and the BG voltage potential VbgL, respectively, in step 74. The calibration circuit 56 compares the BG voltage potential VbgH to the BG voltage potential VbgL in step 76. In step 78, the calibration circuit 56 determines whether the BG voltage potential VbgL is within a predetermined threshold of the BG voltage potential VbgH. If step 78 is true, the high power BG circuit 52 is powered down in step 80.
If the BG voltage potential VbgL 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 VbgL based on the calibration signal. If the adjustment brings the BG voltage potential VbgL 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 VbgH from the high power BG circuit 52. The comparing circuit 92 also receives the BG voltage potential VbgL from the low power BG circuit 54. The comparing circuit 92 determines whether the BG voltage potential VbgL is within a predetermined threshold Vth of the BG voltage potential VbgH.
In other words, the comparing circuit 92 determines whether VbgH+Vth>VbgL>VbgH−Vth. For example, the threshold Vth may be 2 mV or any other threshold. If the BG voltage potential VbgL is not within the threshold Vth of the BG voltage potential VbgH, the output of the comparing circuit 92 is a first state. If the BG voltage potential VbgL is within the threshold Vth of the BG voltage potential VbgH, the output of the comparing circuit 92 is a second state. Alternatively, a simple comparison between VbgH and VbgL may be used without the threshold Vth.
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 VbgL. If the BG voltage potential VbgL is less than the BG voltage potential VbgH, the value of the variable resistance 64 is adjusted, thereby adjusting the BG voltage potential VbgL.
A default value that is stored by the counter 96 ensures that the BG voltage potential VbgL is lower than the BG voltage potential VbgH 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 VbgL until it is approximately equal to the BG voltage potential VbgH.
Calibration continues until the calibration circuit 90 determines that the BG voltage potential VbgL is equal to or approximately equal to the BG voltage potential VbgH. 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 VbgL downward from an initial value that is greater than the first BG voltage reference potential VbgH.
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 VbgL is less than BG voltage potential VbgH minus Vth. A second comparator 134 outputs a digital “1” if the BG voltage potential VbgL is greater than the BG voltage potential VbgH plus Vth. Therefore, if the BG voltage potential VbgL is too low, as determined by the threshold Vth, the counter 130 is incremented. If the BG voltage potential VbgL is too high, as determined by the threshold Vth, the counter 130 is decremented. Once the BG voltage potential VbgL 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 VbgH and the BG voltage potential VbgL 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.

## Claims

1. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that includes a variable resistance and that outputs a second BG voltage potential that is related to a value of said variable resistance; and
a calibration circuit that communicates with said first and second BG circuits, that adjusts said variable resistance based on said first BG voltage potential and said second BG voltage potential, and that selectively shuts down said first BG circuit,
wherein said first BG circuit is biased by a first current level and said second BG circuit is biased by a second current level, and wherein said first current level is greater than said second current level.

2. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that includes a variable resistance and that outputs a second BG voltage potential that is related to a value of said variable resistance; and
a calibration circuit that communicates with said first and second BG circuits, that adjusts said variable resistance based on said first BG voltage potential and said second BG voltage potential, and that selectively shuts down said first BG circuit,
wherein said calibration circuit generates a calibration signal that is used to adjust said second BG voltage potential.

3. The band gap voltage reference circuit of claim 1 wherein said calibration circuit includes a comparing circuit that compares said first BG voltage potential to said second BG voltage potential.

4. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that generates a second BG voltage potential; and
a calibration circuit that communicates with said first and second BG circuits and that adjusts said second BG voltage potential based on said first BG voltage potential,
wherein said first BG circuit is biased by a first current level and said second BG circuit is biased by a second current level, and wherein said first current level is greater than said second current level.

5. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that generates a second BG voltage potential; and
a calibration circuit that communicates with said first and second BG circuits and that adjusts said second BG voltage potential based on said first BG voltage potential,
wherein said calibration circuit sets said second BG voltage potential approximately equal to said first BG voltage potential.

6. The band gap voltage reference circuit of claim 4 wherein said calibration circuit shuts down said first BG circuit when said second BG voltage potential is approximately equal to said first BG voltage potential.

7. The band gap voltage reference circuit of claim 4 wherein said calibration circuit generates a calibration signal that is used to adjust said second BG voltage potential.

8. The band gap voltage reference circuit of claim 7 wherein said second BG circuit includes an adjustment circuit that receives said calibration signal and that adjusts said second BG voltage potential.

9. The band gap voltage reference circuit of claim 4 wherein said calibration circuit includes a comparing circuit that compares said first BG voltage potential to said second BG voltage potential.

10. The band gap voltage reference circuit of claim 8 wherein said adjustment circuit includes a variable resistance.

11. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that generates a second BG voltage potential;
a device that communicates with said first and second BG circuits, that includes a first circuit and a second circuit, that operates at least one of said first circuit and said second circuit in a first mode, that operates said second circuit in a second mode, and that generates a mode signal based on said first mode and said second mode,
wherein said first BG circuit turns off when said mode signal corresponds to said second mode, said second BG circuit includes a variable resistance, and said second BG voltage potential is adjusted by said variable resistance; and
a calibration circuit that communicates with said first and second BG circuits, that adjusts said variable resistance based on said first BG voltage potential and said second BG voltage potential, and that selectively shuts down said first BG circuit.

12. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that generates a second BG voltage potential; and
a device that communicates with said first and second BG circuits, that includes a first circuit and a second circuit, that operates at least one of said first circuit and said second circuit in a first mode, that operates said second circuit in a second mode, and that generates a mode signal based on said first mode and said second mode,
wherein said first BG circuit turns off when said mode signal corresponds to said second mode, wherein said first BG circuit is biased by a first current level and said second BG circuit is biased by a second current level, and wherein said first current level is greater than said second current level.

13. A band gap voltage reference circuit comprising:
a first band gap (BG) circuit that generates a first BG voltage potential;
a second BG circuit that generates a second BG voltage potential;
a device that communicates with said first and second BG circuits, that includes a first circuit and a second circuit, that operates at least one of said first circuit and said second circuit in a first mode, that operates said second circuit in a second mode, and that generates a mode signal based on said first mode and said second mode,
wherein said first BG circuit turns off when said mode signal corresponds to said second mode; and
a summer that communicates with said first and second BG circuits, that sums said second BG voltage potential and said first BG voltage potential, and that outputs said sum to said device.