# Patent

## Title
Low-voltage bandgap reference circuit with startup control

## Abstract

A bandgap reference circuit (BGRC) that is suitable for low-supply voltage applications outputs an adjustable reference voltage. In an operational mode, main currents flow through diodes and are controlled by a main current generator such that a positive temperature coefficient of a voltage across a resistor compensates for a negative temperature coefficient of a voltage across the diodes. The difference of the voltages across the diodes increases with temperature and is used to generate the main currents having positive temperature coefficients. The BGRC ensures sufficient current flow through the diodes during startup. In a startup mode, a startup current generator outputs startup currents that combine with the main currents and prevent the BGRC from operating at incorrect operating points that would otherwise be stable when insufficient current flows through the diodes. The startup currents are generated when the voltage drop across the resistor is less than a predetermined voltage offset.

## Background
TECHNICAL FIELD
The present invention relates generally to reference voltage circuits and, more specifically, to a bandgap reference circuit with a startup current generator that avoids incorrect operating points.
BACKGROUND
A conventional bandgap reference (BGR) circuit generates a reference voltage that remains constant with varying temperature. BGR circuits typically combine the negative temperature coefficient of the bandgap voltage of a transistor with the positive temperature coefficient of the voltage drop across a resistor with increasing current to achieve a zero overall temperature coefficient. The zero temperature coefficient typically occurs when the combined voltage drop across the transistor and resistor equals the silicon bandgap voltage of about 1.22 volts.
FIG. 1 (prior art) shows one conventional BGR circuit 10 that allows an output reference voltage (VREF) to be adjusted to a voltage that is below the bandgap voltage of silicon. Moreover, BGR circuit 10 can operate with a supply voltage (VCC) of less than 1 volt. BGR circuit 10 includes a single diode 11, a set of N diodes 12, a differential amplifier 13, a current mirror 14 and four resistors R1, R2, R3 and R4. Each of the diodes is a diode-connected CMOS transistor. Current mirror 14 includes three PMOS transistors P1, P2 and P3 having the same dimensions. Because the gates of P1, P2 and P3 are each connected to a common node with a voltage V1, three equal currents I1, I2 and I3 are generated.
The resistances of resistor R1 and resistor R2 are equal, and therefore a current I1B and a current I2B that flow through resistor R1 and resistor R2, respectively, are equal. Consequently, a current I1A and a current I2A are also equal. Current I1A flows through diode 11, and current I2A flows through resistor R3 and diode set 12. VF1 is the voltage drop across diode 11, VF2 is the voltage drop across diode set 12, and dVF is the voltage drop across resistor R3. Differential amplifier 13 operates to maintain two input voltages VA and VB at the same voltage. Therefore, VF1 equals the sum of VF2 plus dVF. The negative temperature coefficient of VF1 is compensated by the positive temperature coefficient of dVF with increasing current, and the voltage level of VA and VB remains stable over varying temperatures.
The output reference voltage VREF is generated using the mirrored current I3 and the voltage drop across resistor R4. The reference voltage VREF can therefore be adjusted by adjusting resistor R4. The reference voltage VREF equals R4(VF1/R2+dVF/R3) and can be adjusted without changing the temperature coefficient of the bandgap, which is dependent on R2 and R3, where R1 equals R2.
FIG. 2 (prior art) shows the reference voltage VREF generated by BGR circuit 10 as a function of the supply voltage VCC. The relationship between the supply voltage VCC and other voltages (VA, VB, V1 and VS) on nodes of BGR circuit 10 is also shown. FIG. 2 shows that a reference voltage significantly below the silicon bandgap voltage of 1.22 volts can be generated. For example, reference voltage VREF equals about 0.55 volts when the final operating point of voltages VA and VB is about 0.6 volts. For additional information on BGR circuit 10, see the journal article entitled “A CMOS Bandgap Reference Circuit with Sub-1-V Operation,” by Hironori Banba et al., published in the IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999, pages 670–674.
Under some conditions, however, BGR circuit 10 outputs a reference voltage the does not equal R4(VF1/R2+dVF/R3). As BGR circuit 10 is powered up, differential amplifier 13 can stabilize at incorrect operating points. Under these conditions, BGR circuit 10 outputs an inaccurate reference voltage that may lie significantly below the voltage defined by R4(VF1/R2+dVF/R3).
A method is sought for generating an adjustable bandgap reference voltage that is not rendered inaccurate due to stabilization at incorrect operating points.
SUMMARY
A bandgap reference (BGR) circuit outputs a reference voltage that can be adjusted below the bandgap voltage of silicon, allowing for low-supply voltage applications. The reference voltage can be adjusted without affecting the combined zero temperature coefficient of the circuit.
In an operational mode, two main currents that flow through diodes are controlled by a main control current generator such that a positive temperature coefficient of a voltage drop across a resistor compensates for a negative temperature coefficient of a voltage drop across the diodes. Although the diodes have negative temperature coefficients, the difference between the voltage drops across the diodes has a positive temperature coefficient. The difference of the voltages across the diodes increases with increasing temperature and is used to generate the two main currents having positive temperature coefficients.
As the BGR circuit biases up, the difference between the voltage drops across the diodes might not result in a positive temperature coefficient if insufficient current flows through the diodes. In a startup mode, a startup current generator therefore outputs two startup currents that combine with the two main currents and prevent the BGR circuit from operating at incorrect operating points that would otherwise be stable when insufficient current flows through the diodes. At an incorrect operating point, the BGR circuit would output an incorrect reference voltage. The startup currents are generated when the voltage drop across the resistor is less than a predetermined voltage offset. When the BGR circuit enters the operational mode, the voltage drop across the resistor exceeds the predetermined voltage offset and the startup current generator stops generating the startup currents.
Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.
FIG. 1 (prior art) is a simplified schematic diagram of a bandgap reference circuit that allows an output reference voltage to be adjusted to a voltage below the bandgap voltage of silicon.
FIG. 2 (prior art) is voltage waveform diagram illustrating the operation of the bandgap reference circuit of FIG. 1.
FIG. 3 is a schematic circuit diagram of a bandgap reference circuit with a startup control loop according to one embodiment of the invention.
FIG. 4 is a diagram of equations showing the derivation of the relationship between resistors of the bandgap reference circuit of FIG. 3 that results in a combined temperature coefficient of zero.
FIG. 5 is a waveform diagram showing the relationship between the supply voltage VDD and the reference voltage VOUT output as the bandgap reference circuit of FIG. 3 biases up.
FIG. 6 is a waveform diagram showing the response of various internal voltages of the bandgap reference circuit of FIG. 3 after power is turned on.
FIG. 7 is a waveform diagram showing the response of two internal voltages output by two operational amplifiers as the bandgap reference circuit of FIG. 3 biases up.
FIG. 8 is a more detailed waveform diagram showing the relationship between the supply voltage VDD and the internal voltages shown in FIGS. 6 and 7.
FIG. 9 is a more detailed schematic diagram of an operational amplifier in a main control current generator of the bandgap reference circuit of FIG. 3.
FIG. 10 is a more detailed schematic diagram of a bias current generator in the main control current generator of the bandgap reference circuit of FIG. 3.
FIG. 11 is a more detailed schematic diagram of an operational amplifier in a startup current generator of the bandgap reference circuit of FIG. 3.



DETAILED DESCRIPTION
Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.
FIG. 3 is a simplified schematic diagram of a bandgap reference (BGR) circuit 20 with a startup control loop 21 in which an output reference voltage VOUT can be adjusted without changing the combined temperature coefficient of the circuit. BGR circuit 20 includes a current generator 22, two diodes D1 and D2, and four resistors R1, R2, R3 and R4. Current generator 22 includes a main control current generator 23 and a startup current generator 24. Startup current generator 24 is part of startup control loop 21. Current generator 22 generates a first current I1, a second current I2 and a third current I3.
In an operational mode, startup current generator 24 does not generate current, and currents I1 and I2 are substantially equal to a fourth current I4 and a fifth current I5, respectively. Fourth current I4 and fifth current I5 are generated by a current mirror that is part of main control current generator 23. In a startup mode, startup current generator 24 generates a sixth current I6 and a seventh current I7 that contribute to currents I1 and I2, respectively. Currents I6 and I7 prevent BGR circuit 20 from operating at incorrect operating points that would otherwise be stable when currents I4 and I5 are insufficient to flow through diodes D1 and D2.
First current I1 is split into two current portions I1A and I1B. Current portion I1A flows through first diode D1, and current portion I1B flows through resistor R1. Second current I2 is split into two current portions I2A and I2B. Current portion I2A flows through both resistor R3 and second diode D2. Current portion I2B flows through resistor R2. Each of first diode D1 and second diode D2 is a diode-connected bipolar transistor. In this embodiment, second diode D2 is eight times larger than first diode D1, and resistor R2 is six times larger than resistor R1.
Main control current generator 23 includes an operational amplifier 25, a bias generator 26, a capacitor 27, and three cascode current sources. The three cascode current sources form the current mirror that generates third current I3, fourth current I4 and fifth current I5. Six PMOS transistors 28–33 form the cascode current sources. Each cascode current source is a stack of two PMOS transistors that has a higher output impedance than would a single transistor. For example, transistors 32 and 33 generate third current I3, whose magnitude varies less with changes in supply voltage VDD than would a current generated by transistor 32 alone. In this embodiment, the sizes of transistors 28–33 is such that fourth current I4 is six times larger than fifth current I5, and fifth current I5 has the same magnitude as third current I3. Other embodiments are configured such that fourth current I4 is some other fixed multiple of fifth current I5. In another embodiment, for example, fourth current I4 is one times as large as fifth current I5. In yet another embodiment, fourth current I4 is one third as large as fifth current I5.
In the operational mode when startup current generator 24 does not generate current, fourth current I4 substantially equals first current I1, and fifth current I5 substantially equals second current I2. An inverting input lead 34 of operational amplifier 25 is coupled to the anode of first diode D1. A voltage VA is present on the anode of first diode D1. A noninverting input lead 35 of operational amplifier 25 is coupled to resistor R3. A voltage VB is present on noninverting input lead 35 of operational amplifier 25. Main control current generator 23 controls fourth current I4 and fifth current I5 in order to maintain voltage VA and voltage VB at the same level.
Diode D1 and diode D2 both have negative temperature coefficients. The voltage drop VF across the p-n junction of a diode is typically expressed by the equation VF=(kt/q)ln(IF/IS), where k is Boltzmann's constant (1.38×10−23 CV/K) and q is the electronic charge (1.6×10−19 C) and where C is coulombs and K is degrees Kelvin. For each degree of temperature increase of a bipolar diode, the voltage drop across the p-n junction decreases by about two millivolts.
BGR circuit 20 compensates for the negative temperature coefficient of diodes D1 and D2 by generating a PTAT current (proportional to absolute temperature). Although both diodes D1 and D2 have negative temperature coefficients, the difference between the voltage drop (VF1) across diode D1 and the voltage drop (VF2) across diode D2 has a positive temperature coefficient. The difference in the bandgap voltages (VF1–VF2) is used to generate currents I1 and I2 that increase in magnitude with increasing temperature. The voltage drop (dVF) across resistor R3 increases proportionately to the increase in second current I2, and consequently dVF has a positive temperature coefficient. Voltage drop dVF is the difference between voltage VB on one node of resistor R3 and a voltage VC on the other node of resistor R3. Voltage VC is also present on the anode of diode D2. The magnitudes of the resistances of resistors R1, R2 and R3 are adjusted such that the negative temperature coefficient of VF2 across diode D2 is offset by the positive temperature coefficient of dVF across resistor R3. The positive temperature coefficient of the difference between the voltage drop VF1 across diode D1 and the voltage drop VF2 across diode D2 is achieved by flowing more current per diode area through first diode D1. Approximately forty-eight times more current per diode area flows through first diode D1 than through second diode D2 because second diode D2 is eight times larger than first diode D1, first current I1 is six times larger than second current I2, and resistor R2 is six times larger than resistor R1.
FIG. 4 shows a derivation of the relationship between the resistances of resistors R1, R2 and R3 that results in a combined temperature coefficient of zero for BGR circuit 20. A first equation 37 represents the voltage drop VF1 across diode D1. Because the cathode of diode D1 is coupled to analog ground (GND), voltage drop VF1 equals the voltage VA present on the anode of first diode D1 and on inverting input lead 34 of operational amplifier 25. The current I1A through diode D1 is expressed in terms of voltage VA. Equation 38 indicates that diode D2 is eight times larger than diode D1. Equation 39 indicates that current I1A through diode D1 is six times larger than current I2B through diode D2 as a consequence of transistors 28 and 29 being six times larger than transistors 30 and 31. Equation 40 indicates that main control current generator 23 controls currents I4 and I5 to maintain voltages VA and VB at the same level. Equation 41 indicates that the current mirror formed by the cascode current sources generates third current I3 and fifth current I5 having equal magnitudes. Moreover, fifth current I5 equals second current I2 in the operational mode when startup current generator 24 does not generate current.
Equation 42 expresses the first derivative of the output reference voltage VOUT as a function of temperature. The change in reference voltage VOUT as a function of temperature is expressed as the sum of a positive temperature coefficient and a negative temperature coefficient. The contribution of the positive temperature coefficient depends on the magnitude of resistor R3, and the contribution of the negative temperature coefficient depends on the magnitude of resistor R2. Applying the standard accepted negative temperature coefficient of −2 mV for a bipolar bandgap results in a ratio of 6 for the relationship R2/R3 in order to achieve a zero temperature coefficient. In the embodiment of FIG. 3 based on a 0.35 micron TSMC SPICE simulation, a ratio of 5.76 for R2/R3 resulted in a zero temperature coefficient for BGR circuit 20. Thus, the output reference voltage VOUT can be set by adjusting the magnitude of resistor R4 without disturbing the balance of positive and negative temperature coefficients. The ratios of R4/R3 and R4/R2 are independent of temperature changes because the resistors are all of the same type and manufactured in the same process.
FIG. 5 is a voltage waveform diagram showing the relationship between supply voltage VDD and reference voltage VOUT output as BGR circuit 20 “biases up.” In this embodiment, resistor R4 is adjusted so that reference voltage VOUT is about 1.25 volts in normal operation. Although supply voltage VDD continues to rise from about 1.6 volts at about 0.5 milliseconds to about 3.4 volts at about 1.0 milliseconds, VOUT stabilizes at about 1.25 volts shortly after about 0.5 milliseconds. BGR circuit 20 operates in startup mode until about 1.0 milliseconds when VDD levels off, at which time BGR circuit 20 switches to the operational mode.
Returning to FIG. 3, resistor R4 is comprised of a string of resistor portions 43–49. In one embodiment, some resistor portions are themselves comprised of multiple resistor components. In one example, resistor portion 43 comprises forty-seven closed transistors coupled in series. The magnitude of resistor R4 is adjusted by bypassing certain resistor portions by closing certain of switches 50–55. In one embodiment, switches 50–55 are NMOS transistors that are controlled by digital signals B2–B7, respectively. For example, to achieve a resistance only through resistor portions 43–47 but not through resistor portions 48–49, signals B2–B3 are asserted, and signals B4–B7 are deasserted. Thus, switches 52–55 remain open, and switches 50–51 are closed, bypassing resistor portions 48–49.
In one embodiment, BGR circuit 20 is part of a microcontroller that also includes an analog-to-digital converter. Output reference voltage VOUT is used to calibrate the internal reference of the analog-to-digital converter. The level of reference voltage VOUT output by BGR circuit 20 is programmable and is adjusted by the microcontroller asserting signals B2–B7 in a predetermined manner. In another embodiment, BGR circuit 20 is part of a 1.8-volt core memory cell, and output reference voltage VOUT is programmed to be a voltage significantly below the bandgap voltage of silicon.
Bandgap reference circuits typically have stable operating points at their final operating points, as well as at voltages of VA and VB that correspond to no current flowing through their components. Typical bandgap reference circuits therefore have startup circuits that prevent only the condition where no current flows across the p-n junctions of diode-connected transistors. BGR circuit 20, however, has more than two incorrect operating points that can become stable operating points, including points at which some current flows across the p-n junctions.
FIG. 6 is a voltage waveform diagram showing how voltages VA, VB and VC respond over time as the supply voltage VDD increases and BGR circuit 20 biases up. The period in FIG. 6 corresponds to the period in FIG. 5 over which VOUT varies in relation to VDD. In the example of FIG. 6, voltages VA and VB achieve a final operating point 56 of about 7.3 volts within less than about one millisecond from power on. At the final operating point, the difference between voltage VB and voltage VC is about 100 mV. Thus, dVF is about 100 mV during normal operation in the operational mode. At around 0.5 milliseconds from power on, operational amplifier 25 detects that voltage VA does not equal voltage VB and increases currents I4 and I5 until voltage VA equals voltage VB.
In a region of potential incorrect operating points 57 before about 0.5 milliseconds from power on, however, operational amplifier 25 does not detect that voltages VA and VB have not achieved final operating point 56 if voltages VA and VB do not diverge. Without startup control loop 21, the reference voltage VOUT output by BGR circuit 20 would not correspond as expected to the equation

VOUT=R4([(kT/qR3)ln(48)]+[(kT/qR2)ln(I1A/IS)])

as derived in FIG. 4, even though voltages VA and VB would be substantially equal. Without startup current generator 22 to force sufficient current through diodes D1 and D2, BGR circuit 20 might stabilize at an incorrect operating point in region 57. After startup current generator 22 forces current through diodes D1 and D2, BGR circuit 20 maintains VF1 across diode D1 equal to the sum of VF2 across diode D2 plus dVF across resistor R3.
VF1, VF2 and dVF all equal about zero volts during startup and before appreciable current flows through diodes D1 and D2. During this period, substantially all of first current I1 flows through resistor R1, and substantially all of second current I2 flows through resistor R2. Before appreciable current flows through diodes D1 and D2, VA equals VB because VA=I1·R1, VB=I2·R2, I1=6I2 and 6R1=R2. In the startup mode, startup current generator 24 forces current through diodes D1 and D2 and thereby prevents BGR circuit 20 from stabilizing at an incorrect operating point within region 57 when VF1, VF2 and dVF all equal about zero volts.
As shown in FIG. 3, startup control loop 21 includes an operational amplifier 58 that has 55 mV of input offset. Voltage VB is present on a noninverting input lead 59 of operational amplifier 58. Voltage VC is present on an inverting input lead 60 of operational amplifier 58. Operational amplifier 58 generates a voltage VE that turns on two PMOS transistors 61 and 62 when dVF across resistor R3, which equals VB–VC, is less than about 55 mV. In the startup mode, when dVF is less than 55 mV, sixth current I6 and seventh current I7 flow through transistors 61 and 62, respectively. Startup control loop 21 generates current I7 with a magnitude that results in current I2A through resistor R3 having a magnitude of 55 mV/R3. The magnitude of current I2A at which second diode D2 is placed “in conduction” is about half the magnitude of current I2A under normal operating conditions, which is about 100 mV/R3.
FIG. 7 is a voltage waveform diagram showing the response over time of voltage VD output by operational amplifier 25 and of voltage VE output by operational amplifier 58. As voltage VE increases to about 3.4 volts, transistors 61 and 62 gradually turn off, and currents I6 and I7 cease flowing. Voltage VD stabilizes at about 2.5 volts in the operational mode, which allows sufficient magnitudes of currents I1 and I2 to pass through transistors 28–31 so as to maintain voltage VA substantially equal to voltage VB. FIG. 7 shows that up to and including the region of potential incorrect operating points 57 (as labeled in FIG. 6), where operational amplifier 25 does not yet detect a difference in the voltages VA and VB, voltage VD output by operational amplifier 25 continues to rise, gradually turning off transistors 28–31. Around region 57, startup control loop 21 detects that dVF is less than 55 mV and thereupon decreases voltage VE and increases currents I6 and I7. Currents I1A and I2A begin to flow through diodes D1 and D2 causing voltages VA and VB to diverge. As voltages VA and VB diverge, main current control generator 23 drops voltage VD to almost zero volts. Transistors 28–31 turn on fully, and currents I1 and I2 increase dramatically. After the elapse of about 0.5 milliseconds, voltage VD increases and currents I1 and I2 are then reduced and stabilize at a magnitude where dVF is maintained at about 100 mV.
Startup control loop 21 has a gain of about 50 dB and about 69 degrees of phase margin, whereas the main feedback control loop that includes main control current generator 23 has a gain of about 85 dB and about 62.5 degrees of phase margin. Therefore, as main control current generator 23 begins to generate current after the region of potential incorrect operating points 57, startup control loop 21 is overpowered by currents I4 and I5.
FIG. 8 is a voltage waveform diagram showing the response of the voltages VA, VB, VC and VE in more detail as the supply voltage VDD of BGR circuit 20 increases over a narrower time period around region 57 of FIG. 6. FIG. 8 shows that startup control loop 21 prevents voltage VA from equaling VB in region 57. When voltage VE output by operational amplifier 58 decreases from about 400 microseconds to about 450 microseconds from power on, currents I6 and I7 increase and voltages VA and VB diverge. In this example, voltage VE tracks the increase in supply voltage VDD after voltages VA and VB converge at their final operating point at around 510 microseconds. Startup current generator 23 nevertheless continues to generate some amount of current I6 and current I7 until the startup mode ends at around one millisecond from power on and transistors 61 and 62 are completely turned off.
FIG. 9 is a circuit diagram showing operational amplifier 25 of main control current generator 23 in more detail. Operational amplifier 25 includes fourteen PMOS transistors 63–76, sixteen NMOS transistors 77–92 and two inverters 93–94. Transistor 75 is coupled to inverting input lead 34, and transistor 76 is coupled to noninverting input lead 35. Based on the voltages VA and VB present on input leads 34 and 35, respectively, operational amplifier 25 outputs voltage VD onto an output lead 95. An external bias select signal (BSEL) is present on an input lead 96. BSEL is used to select between an internal bias current IBIAS generated by bias generator 26 and an external bias current source. The external bias current EBIAS is present on an input lead 97. Internal bias current IBIAS is received from bias generator 26 on an input lead 98. When BSEL is deasserted, BGR circuit 20 uses the internal bias current IBIAS, and the external bias current EBIAS is not used. A power up signal PUP is present on an input lead 99 and is used to power up BGR circuit 20 when supply voltage VDD is always on. In another embodiment, BGR circuit 20 powers up when voltage VDD is turned on.
FIG. 10 is a circuit diagram of bias generator 26. Bias generator 26 includes nine PMOS transistors 100–108, two NMOS transistors 109–110, a bipolar transistor 111 and a resistor 112. Bias generator 26 outputs internal bias current IBIAS onto an output lead 113. Bias generator 26 is powered down by asserting a power down signal PDOWN that is received on an input lead 114.
FIG. 11 is a circuit diagram showing operational amplifier 55 of startup current generator 24 in more detail. Operational amplifier 55 includes six PMOS transistors 115–120 and twelve NMOS transistors 121–132. Transistor 119 is coupled to inverting input lead 60, and transistor 120 is coupled to noninverting input lead 59. Based on the voltages VB and VC present on input leads 59 and 60, respectively, operational amplifier 55 outputs voltage VE onto an output lead 133.
Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Although the voltage drops VF1 and VF2 are described above as the voltages across the p-n juctions of diodes, BGR circuit 20 is configured in other embodiments such that the voltages VF1 and VF2 are voltage drops across p-n junctions of bipolar, NMOS or PMOS transistors that are not diode connected. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.

## Claims

1. A device comprising:
a current generator that generates a first current with a first magnitude and a second current with a second magnitude, wherein the first magnitude is a fixed multiple of the second magnitude, wherein the current generator comprises a main control current generator and a startup current generator;
a p-n junction having a node, wherein the first current passes through the p-n junction, and wherein a first voltage is present on the node of the p-n junction; and
a resistor with a first node and a second node, wherein the second current passes through the resistor, wherein a second voltage is present on the first node and a third voltage is present on the second node, wherein the main control current generator controls the first magnitude and the second magnitude such that the first voltage equals the second voltage, and wherein the startup current generator increases the first magnitude and the second magnitude when the second voltage exceeds the third voltage by less than a predetermined voltage offset.

2. The device of claim 1, wherein the startup current generator comprises an operational amplifier that detects when the second voltage exceeds the third voltage by less than the predetermined voltage offset.

3. The device of claim 1, wherein the p-n juction has a negative temperature coefficient.

4. The device of claim 1, wherein the main control current generator comprises a current mirror.

5. The device of claim 1, further comprising:
a second p-n junction having a node, wherein the node of the second p-n junction is coupled to the second node of the resistor.

6. The device of claim 1, wherein the p-n junction has a temperature, and wherein the first voltage remains substantially constant as the temperature of the p-n junction varies.

7. A method comprising:
(a) in a startup mode, generating a startup current and a second current in a bandgap reference circuit, wherein the bandgap reference circuit comprises a resistor having a first node and a second node, wherein a first voltage is present on the first node and a second voltage is present on the second node, wherein the startup current flows through the resistor when the first voltage exceeds the second voltage by less than a predetermined voltage offset; and
(b) in an operational mode, generating the second current, wherein the second current flows through the resistor, wherein the bandgap reference circuit has a temperature, and wherein the first voltage remains substantially constant as the temperature of the bandgap reference circuit varies.

8. The method of claim 7, wherein the startup current is not generated in the operational mode.

9. The method of claim 7, wherein a diode is coupled in series to the resistor, and wherein in the startup mode the startup current passes through the resistor and the diode.

10. The method of claim 7, wherein the generating the startup current in (a) is performed using an operational amplifier.

11. The method of claim 7, further comprising:
(c) in the startup mode, generating a second startup current when the first voltage exceeds the second voltage by less than the predetermined voltage offset, wherein the startup current has a first magnitude and the second startup current has a second magnitude, and wherein the first magnitude is a fixed multiple of the second magnitude.

12. The method of claim 11, wherein the second startup current passes through a diode.

13. A method comprising:
(a) generating a first current with a first magnitude and a second current with a second magnitude, wherein the first magnitude is a fixed multiple of the second magnitude;
(b) passing the first current through a p-n junction having a node, wherein a first voltage is present on the node of the p-n junction;
(c) passing the second current through a resistor, wherein a second voltage is present on a first node of the resistor, and a third voltage is present on a second node of the resistor;
(d) detecting a difference between the second voltage and the third voltage;
(e) increasing the first magnitude and the second magnitude when the difference between the second voltage and the third voltage is less than a predetermined voltage offset; and
(f) controlling the first magnitude and the second magnitude until the first voltage substantially equals the second voltage.

14. The method of claim 13, wherein the p-n junction is part of a diode.

15. The method of claim 13, wherein the second node of the resistor is coupled to a second p-n junction.

16. The method of claim 13, wherein the increasing in (e) is performed using an operational amplifier.

17. The method of claim 13, wherein the generating in (a) is performed using a current mirror.

18. The method of claim 13, further comprising:
(g) generating an output voltage, wherein the p-n junction has a temperature, and wherein the output voltage remains substantially constant as the temperature of the p-n junction varies.

19. A device comprising:
a bandgap reference circuit that generates a reference voltage, wherein the bandgap reference circuit has a resistor with a first node and a second node, wherein a first voltage is present on the first node and a second voltage is present on the second node, wherein the bandgap reference circuit has a p-n junction, wherein a third voltage is present on a node of the p-n junction, wherein the bandgap reference circuit has an operating point when the first voltage substantially equals the third voltage; and
means for preventing the bandgap reference circuit from operating at the operating point when the first voltage exceeds the second voltage by less than a predetermined voltage offset.

20. The device of claim 19, wherein the means generates a startup current when the first voltage exceeds the second voltage by less than the predetermined voltage offset.

21. The device of claim 19, wherein the bandgap reference circuit has a temperature, and wherein the reference voltage remains substantially constant as the temperature of the bandgap reference circuit varies.