# CURVATURE-COMPENSATED BAND-GAP VOLTAGE REFERENCE CIRCUIT

A band-gap reference voltage is developed by a phase-clocked band-gap circuit including a single PN junction through which first and second constant currents are alternately directed. A current proportional to absolute temperature is selectively added to one of the first and second constant currents to curvature-compensate the developed band-gap reference voltage. The band-gap circuit is calibrated at any desired temperature by interrupting the curvature compensation current and trimming the one constant current to bring the un-compensated band-gap reference voltage into correspondence with a nominal band-gap voltage functionally related to the calibration temperature and circuit component values.

## Latest DELPHI TECHNOLOGIES, INC. Patents:

**Description**

**TECHNICAL FIELD**

The present invention relates to voltage reference circuits, and more particularly to a low-cost high-precision curvature-compensated band-gap voltage reference circuit.

**BACKGROUND OF THE INVENTION**

Band-gap voltage reference circuits are routinely used in integrated circuit applications that require a reference voltage that is reasonably precise and stable over a range of temperatures. Fundamentally, a band-gap voltage reference circuit includes two bipolar base-emitter junctions operating at different current densities. The reference voltage is obtained by summing one of the base-emitter voltages with a scaled difference of the two base-emitter voltages. Since the base-emitter voltage is inversely proportional to temperature and the scaled difference voltage is directly proportional to temperature, their sum is temperature independent to the first order. However, the reference voltage still exhibits some second and higher order nonlinearity (curvature) with temperature, and various techniques have been developed to compensate for these nonlinearities. For example, the circuitry of the reference generator can be trimmed or calibrated at a number of different temperature settings, and/or various non-linear waveforms can be used to cancel out the non-linearity of the reference. Also, special steps must be taken to minimize stresses caused by the circuit packaging because package stress non-uniformity can introduce additional voltage instability over time.

Conventional approaches for addressing the curvature compensation and package stress issues typically result in a component cost that is too high for many applications. Accordingly, what is needed is a high-precision curvature-compensated band-gap voltage reference circuit that is low in cost, easily calibrated, and substantially insensitive to packaging stresses.

**SUMMARY OF THE INVENTION**

The present invention is directed to an improved high-precision voltage reference circuit featuring a novel way of generating a curvature-compensated band-gap reference voltage that is substantially insensitive to package stress, and a novel calibration technique that can be carried out at any desired temperature. A band-gap reference voltage is developed by a phase-clocked band-gap circuit including a single PN junction through which first and second constant currents are alternately directed. A current proportional to absolute temperature is selectively added to one of the first and second constant currents to curvature-compensate the developed band-gap reference voltage. The band-gap circuit is calibrated at any desired temperature by interrupting the curvature compensation current and trimming the one constant current to bring the un-compensated band-gap reference voltage into correspondence with a nominal band-gap voltage functionally related to the calibration temperature and circuit component values.

**BRIEF DESCRIPTION OF THE DRAWINGS**

**DESCRIPTION OF THE PREFERRED EMBODIMENT**

Referring to the drawings, and particularly to **10** generally designates a high-precision curvature-compensated band-gap voltage reference circuit according to this invention, including a two-phase clock generator **12**, an auxiliary band-gap circuit **14**, a trimable scaling parameter **16**, a main band-gap circuit **20**, and a voltage-boost circuit **22**. The clock generator develops a pair of fixed-frequency non-overlapping opposite-phase square-wave signals φ**1** and φ**2** for the main band-gap and voltage-boost circuits **20** and **22**. The auxiliary band-gap circuit **14**, described in detail below in reference to **20**, which signals may be developed by some other means if desired. The signals developed by auxiliary band-gap circuit **14** are current signals, and include: a substantially constant current I_{C}, and a scaled current β·I_{T }that is proportional-to-absolute-temperature (PTAT). The block **16** generates a trim-able scaling parameter α, which is used to form a scaled version (α·I_{C}) of the constant current I_{C}.

The main band-gap circuit **20**, described in detail below in reference to **1** and φ**2** and develops a high-precision curvature-compensated band-gap voltage V_{BG}. In general, the two constant input currents I_{C }and α·I_{C }are used to generate a band-gap reference voltage using a single PN junction, and the scaled PTAT input current β·I_{T }is used to curvature-compensate the generated band-gap reference voltage V_{BG}, using the same PN junction. Using a single PN junction (a diode in the illustrated embodiment) to create the curvature-compensated band-gap reference voltage V_{BG }minimizes sensitivity to package stresses that diversely affect different devices on an integrated circuit because in this circuit the package stress error is subject to common mode cancellation. In the illustrated embodiment, the uncompensated band-gap reference voltage V_{UBG }has a nominal value of 1.116 VDC at a nominal room temperature of 40° C., and the curvature-compensated band-gap reference voltage V_{BG }has a nominal value of 1.18VDC at that same temperature.

The voltage boost circuit **22**, described in detail below in reference to **1** and φ**2**. The voltage boost circuit **22** boosts the curvature-compensated band-gap voltage V_{BG }by a calibrated factor to form an elevated high-precision reference voltage V_{3P3}. In the illustrated embodiment the reference voltage V_{3P3 }has a nominal value of 3.3VDC.

Referring to **14** may be a conventional band-gap circuit of average precision, including first and second bipolar transistors **24** and **26** having different active areas. The current I_{T }in resistor **28** is a PTAT current, as can be demonstrated by summing the voltages in a circuit loop including resistor **28** and the base-emitter circuits of transistors **24** and **26**. The collector current of transistor **24** (which is substantially equal to the resistor current I_{T}) is mirrored in the transistor **30** by the current mirror circuit **32**, and the scaling factor β is determined according to the active area ratio of transistor **30** and transistor **32***a *of the current mirror circuit **32**. Thus, the current in conductor **34** is the scaled PTAT current β·I_{T }supplied to main band-gap circuit **20**. The current mirror circuit **32** also forces the collector current of transistor **26** to equal that of transistor **24**, so that the voltage Vref at junction **36** is a band-gap voltage—that is, the sum of V_{BE26 }and a scaled difference of V_{BE26 }and V_{BE24}, where V_{BE24 }is the base-emitter voltage of transistor **24**, and V_{BE26 }is the base-emitter voltage of transistor **26**. The transistors **24** and **26** have different current densities due to their different active areas, and the ratio of their active areas is chosen to optimize the temperature stability of Vref. The voltage Vref is impressed across the resistor **38** to establish the constant current I_{C }in current source circuit **40**, and the current I_{C }is mirrored in the transistor **42**. Thus, the current in conductor **44** is the constant current I_{C }supplied to the main band-gap circuit **20** and to block **16** of

Referring to **20** receives as inputs the three current signals I_{C}, α·I_{C }and β·I_{T }on lines **50**, **52** and **54**, and produces the high-precision curvature-compensated band-gap reference voltage V_{BG }at output terminal **56**. The main band-gap circuit **20** includes an input current summing junction **58**, a diode **60** (the aforementioned single PN junction) connected between the summing junction **58** and ground, an operational amplifier **62** responsive to the voltage V_{BE }across diode **60**, two divider capacitors **64** (capacitance C_{1}) and **66** (capacitance C_{2}), an output filter capacitor **68** connected between the output terminal **56** and ground, three clock-controlled switches **70**, **72** and **74**, and a normally-closed calibration (CALIB) switch **76**. As indicated, the states of switches **70** and **72** are dictated by the clock signal φ**1**, and the state of switch **74** is dictated by the clock signal φ**2**—that is, switches **70** and **72** are closed when φ**1** is high, and switch **74** is closed when φ**2** is high.

For the sake of discussion, it is initially assumed that the CALIB switch **76** is open. Under this condition, the current I_{C }flows through diode **60** when clock signal φ**2** is high, and the current I_{C}·(1+α) flows through diode **60** when clock signal φ**1** is high. When φ**2** is high, the amplifier **62** is un-coupled from the output terminal **56**, and the output voltage is held by filter capacitor **68**. Also, capacitor **64** is discharged by switch **74**. The voltage across diode **60** under this condition is designated herein as V_{BE1}, and amplifier **62** operates as a voltage-follower, developing a corresponding voltage at its output (and across capacitor **66**). When φ**1** is high, the output of amplifier **62** is coupled to output terminal **56**, and capacitors **64** and **66** operate as a voltage divider. The voltage across diode **60** under this condition is designated herein as V_{BE2}, and amplifier **62** develops voltage at its output based on V_{BE2 }and the capacitance ratio C_{2}/C_{1}. Due to the alternately differential current flowing in diode **60**, and the operation of phase-clocked amplifier **62**, an uncompensated band-gap reference voltage V_{UBG }is developed at output terminal **56**, according to equation (1) below:

where V_{G0 }is the silicon band-gap voltage of diode **60**, γ is a process constant, T is an actual temperature (in ° K), T_{0 }is a reference temperature, E is a constant, V_{T}=k·T/q, q is the electron charge, and k is the Boltzmann constant.

When the CALIB switch **76** is in its normally closed state, the scaled PTAT current β·I_{T }flows through diode **60** along with the scaled and un-scaled constant currents I_{C }and α·I_{C }when the clock signal φ**1** is high. The scaled PTAT current β·I_{T }produces a V_{BE2 }component having a quadratic (i.e., second-order) relationship with temperature that curvature-compensates the uncompensated band-gap reference voltage V_{UBG }for second-order non-linearity with temperature. While third and higher order non-linearities remain, they are relatively small in magnitude for temperatures between −50° C. and +150° C. In fact, testing has shown that the curvature-compensated band-gap reference voltage V_{BG }exhibits a voltage variation as low as 0.1 percent over this 200° C. range of temperatures. The curvature-compensated band-gap reference voltage V_{BG }is given by the following equation:

*V*_{BG}*=V*_{G0}+(*C*_{2}*/C*_{1})(*k·T/q*)·ln[(1+α)+(*f·T*)/*I*_{C}]−[(*k·T*·γ)/*q*]·ln(*T/T*_{0})+(*k·T/q*)·ln{*E*·[(1+α)·*I*_{C}*I*_{CN}+(*f·T*)/*I*_{CN}]} (2)

where, f=β·k**1**, k**1**=k·ln(m)/(q·R**1**), m is the current density ratio of bipolar transistors **24** and **26** in auxiliary band-gap circuit **14**, and R**1** is the resistance of resistor **28** in auxiliary band-gap circuit **14**.

The voltage boost circuit **22** of _{BG }of main band-gap circuit **20** by a scalable factor to form an elevated high-precision reference voltage V_{3P3 }at output terminal **80**. Referring to **22** includes five clock-controlled switches **82**-**90**. As indicated, the states of switches **82**, **84** and **86** are dictated by φ**1**, and the states of switches **88** and **90** are dictated by φ**2**—that is, switches **82**, **84** and **86** are closed when φ**1** is high, and switches **88** and **90** are closed when φ**2** is high.

When φ**2** is high, the voltage boost circuit **22** is isolated from the main band-gap circuit **20**, and the switches **88** and **90** discharge the capacitors **92** and **94**. When φ**1** is high, switch **82** couples the voltage boost circuit **22** to the main band-gap circuit **20**, and capacitor **96** samples the band-gap voltage V_{BG}. The sampled voltage is applied to the inverting input of operational amplifier **98**, which controls the conduction of a FET **100** connected between source voltage V_{DD }and output terminal **80**. The current sink **102** establishes a small static current flow through FET **100**, but the amplifier **98** drives FET **100** to charge the capacitors **92**, **94** and **104** until the voltage across capacitor **94** is equal to the sampled band-gap voltage. At this point, the voltage V_{3P3 }at output terminal **80** is equal to V_{BG}·[1+(C_{4}/C_{3})], where C_{4 }and C_{3 }are the capacitance values of capacitors **94** and **92**, respectively. When φ**2** becomes high, capacitor **96** holds the previously sampled voltage, switches **88** and **90** discharge the capacitors **92** and **94**, and the capacitor **104** holds the previously settled voltage value of amplifier **98** until φ**1** becomes high. The capacitor **104** thus filters the dynamic response of the circuit, and the average value of V_{3P3 }is given by the above expression: V_{BG}·[1+(C_{4}/C_{3})]. The capacitors **92** and **94** are preferably fabricated as surface poly-to-poly integrated circuit devices, which characteristically exhibit very low sensitivity to packaging stress, voltage, and temperature; and the capacitance of capacitor **94** is trimable as described below in reference to the calibration flow diagram of

An important aspect of the present invention is the provision of a calibration procedure whereby errors due to manufacturing process variations are corrected at any one conveniently selected temperature. Prior to calibration, the scaling parameters α and β and the capacitance ratios (C_{2}/C_{1}) and (C_{4}/C_{3}) are set based on theoretical or nominal design considerations. The scaling parameter cc is set to a value that theoretically optimizes the temperature stability of the un-compensated band-gap voltage V_{UBG}, based on equations (1-2), for example. The scaling parameter β and the capacitance ratio (C_{2}/C_{1}) are set to values that theoretically provide optimal curvature compensation of V_{UBG}, based on equation (2), for example. And finally, the capacitance ratio (C_{4}/C_{3}) is set to a value that theoretically provides the desired value of output voltage V_{3P3}. In the calibration procedure, the scaling parameter cc and the capacitance C_{4 }are trimmed to account for any deviation between theoretical and actual considerations.

The flow diagram of **13**, and the capacitance C_{4}. The scaling parameter cc is trimmed by adjusting the multiplier value provided by block **16** of _{4 }is trimmed by a bank of trim capacitors (not shown). The scaling parameter β may be trimmed in various ways, such as by an adjustable multiplier. Referring to **110** and **112** to measure the calibration temperature T and to determine by calculation or table look-up the theoretical (i.e., nominal design) values for V_{UBG }and V_{BG}. As indicated in _{UBG }and V_{BG }are functionally related to the measured temperature T. In a mechanization of the illustrated embodiment, V_{UBG }has a nominal value of 1.116 VDC at 40° C., and V_{BG }has a nominal value of 1.19VDC at 40° C.

The calibration of scaling parameter α is illustrated by blocks **114**, **116**, **118**, **120** and **122**. The CALIB switch **76** of main band-gap circuit **20** is opened to suppress the curvature correction, and the actual voltage at output terminal **56** (designated as VBG_ACT) is measured. If VBG_ACT is not equal to the nominal un-compensated band-gap voltage V_{UBG}, the scaling parameter α is trimmed until equality is achieved. When VBG_ACT is equal to V_{UBG}, the CALIB switch **76** of main band-gap circuit **20** is closed to reapply the curvature correction.

An optional calibration of the scaling parameter **13** is illustrated by blocks **124**, **126** and **128**. The actual voltage VBG_ACT is re-measured and compared to the nominal curvature-compensated band-gap voltage V_{BG }determined at block **112**. If VBG_ACT is not equal to V_{BG}, the scaling parameter β is trimmed until equality is achieved.

The calibration of capacitance C_{4 }is illustrated by blocks **130**, **132** and **134**. The actual voltage at output terminal **80** of voltage boost circuit **22** (V**3**P**3**_ACT) is measured, and compared to its nominal or target value V**3**P**3**_TARGET. If V**3**P**3**_ACT is not equal to V**3**P**3**_TARGET, the capacitance C_{4 }is trimmed until equality is achieved. The calibration of capacitance C_{4 }is completed when V**3**P**3**_ACT is equal to V**3**P**3**_TARGET.

In summary, the present invention provides a high-precision curvature-compensated band-gap voltage reference circuit that is low in cost compared to other known circuits, easily calibrated at a single user-selected temperature, and substantially insensitive to packaging stresses. While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the illustrated circuit elements may be implemented in various ways (e.g., as discrete devices and/or up-integrated devices), and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.

## Claims

1. A band-gap voltage reference circuit, comprising:

- a PN junction;

- a phase-clocked input circuit receiving first and second input currents that are substantially constant and a third input current that is proportional to absolute temperature, and alternately supplying first and second output currents to said PN junction during successive clock phases to alternately establish first and second VBE voltages across said PN junction; and

- a phase-clocked amplifier circuit responsive to the first and second VBE voltages established during said successive clock phases for producing a curvature-compensated band-gap reference voltage.

2. The band-gap voltage reference circuit of claim 1, further comprising:

- an auxiliary band-gap reference generator circuit for producing said first, second and third input currents.

3. The band-gap voltage reference circuit of claim 1, where:

- said first output current is said first input current; and

- said second output current is a sum of said first, second and third input currents.

4. The band-gap voltage reference circuit of claim 1, where:

- said first and second input currents are related by a scaling parameter; and

- said phase-clocked input circuit includes a calibration switch that suppresses said third input current in a calibration mode, during which said scaling parameter is trimmed to bring an output of said phase-clocked amplifier circuit into correspondence with a first nominal design value.

5. The band-gap voltage reference circuit of claim 4, where:

- said first nominal design value is a function of temperature during said calibration mode.

6. The band-gap voltage reference circuit of claim 4, where:

- said calibration switch is closed following the trimming of said scaling parameter, and said third input current is then trimmed to bring said curvature-compensated band-gap reference voltage into correspondence with a second nominal design value.

7. The band-gap voltage reference circuit of claim 6, where:

- said second nominal design value is a function of temperature during said calibration mode.

8. The band-gap voltage reference circuit of claim 4, where:

- the trimming of said scaling parameter at any one temperature calibrates said curvature-compensated band-gap reference voltage.

9. The band-gap voltage reference circuit of claim 1, further comprising:

- a voltage boost circuit for boosting said curvature-compensated band-gap reference voltage to form an output voltage, including a trimable device that is adjusted in value to bring said output voltage into correspondence with a target value.

**Patent History**

**Publication number**: 20110102071

**Type:**Application

**Filed**: Nov 2, 2009

**Publication Date**: May 5, 2011

**Applicant**: DELPHI TECHNOLOGIES, INC. (TROY, MI)

**Inventor**: SEYED R. ZARBADI (KOKOMO, IN)

**Application Number**: 12/610,659

**Classifications**

**Current U.S. Class**:

**Using Bandgap (327/539);**To Derive A Voltage Reference (e.g., Band Gap Regulator) (323/313)

**International Classification**: G05F 1/10 (20060101);