Ieee Gunawan 1993

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    IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 28, NO . 6, JUNE 1993 667

    A Curvature-Correc edLow-Voltage Bandgap Reference

    Made Gunawan, Gerard C. M. Meijer, Jeroen Fonderie, and Johan H. Huijsing, Senior Member, IEEE

    Absfract- A new curv ature-corr ected bandgap refe rence ispresented which is able to function at supply voltages as lowas 1 V, at a supply current of only 100 PA. After trimmingthis bandgap reference has a temperature coefficient (TC) off4 pm/" C. The refere nce voltage is about 200 mV and it c ansimply be adjusted to higher reference voltages. The temperaturerange of this circuit is from 0 to 125C. This bandgap referenceis realized using a standard bipolar process with base-diffusedresistors.

    I. INTRODUCTIONOLTAGE references are used in many types of analogV ircuits for signal processing, such as A-D converters,

    smart sensors, D-A converters, etc. Of all the types of refer-ences, only bandgap references are suited to operate at verylow supply voltages.

    In a bandgap reference, the reference voltage is obtained bycompensating the base-emitter voltage of a bipolar transistor(Vbe) or its temperature dependence. The temperature depen-dence of the base-emitter voltage of a transistor, biased bya proportional-to-absolute-temperature PTAT) current, can berepresented by the following equation [11:

    where V,O s the extrapolated bandgap voltage at 0 K, Vbe(TR)is the base-emitter voltage at the reference temperature TR,and 71 is a process-dependent constant. Practical values forthese parameters are Vgo= 1170 mV, Vbe(!fR)= 0.65 V,

    The existing bipolar bandgap references operate generallyat a supply voltage larger than 1 V. The only reference knownto the authors that is able to operate at a supply voltage of1 V is the one introduced by Widlar [2].

    In this paper, a low-voltage bandgap reference with a lowertemperature coefficient (TC) than that of Widlar's circuit is

    Manuscript received September 8, 1992; revised February 17, 1993.M. Guna wan was with the Departme nt of Electrical Engineering, DelftUniversity of Technology, 2628 CD Delft, The Netherlands. He is now withthe Indonesian Government.G. C. M.Meijer is with the Laboratory of Electronics, Department ofElectrical Engineering, Delft University of Technology, 2628 C D Delft, TheNetherlands.J. Fonderie was with the Laboratory of Electronic Instrumentation, De-

    partment of Electrical Engineering, Delft University of Technology 2628 CDDelft, The Netherlands. H e is now with Philips Semiconductors, Sunnyvale,CA 94088.J. H. Huijsing is with the Laboratory of Electronic Instrumentation, De-partment of Electrical Engineering, Delft University of Technology, 2628 CDDelft, The Netherlands.

    TR = 300 K , and q = 3.6.

    IEEE Log Number 9209023.

    LFig. 1. The principle of the low-voltage bandgap reference.

    discussed. This low TC is achieved by a new method for thecompensation of the nonlinear temperature dependence (cur-vature correction) of &e . Further, within a certain range thebandgap reference can simply be adjusted for other referencesvoltages.

    11. PRINCIPLE AN D IMPLEMENTATION OFTHE LOW-VOLTAGEANDGAP EFERENCE

    The principle of the low-voltage bandgap reference is shownin Fig. 1 . A current proportional to v b e (21Vbe)and a nonlinearcorrection current ( 2 1 N L ) are generated. When the nonlinear(curvature) correction is performed correctly, 2(1LTbe+I N L )should consist of a constant component and a component thatis PTAT. This latter component can be compensated by usinga PTAT current source. The sum of the currents is convertedinto the voltage reference by using a resistor (Rref). he buffercircuit is applied to obtain a sufficiently low output impedance.For the PTAT current source, the circuit shown in Fig. 2 isused [3], where the emitter-area ratio of Q1 and Q 2 amountsto 4.

    The magnitude of the PTAT current is given by

    The capacitor C,, is required to obtain high-frequency sta-bility of the circuit. The transistor Qs is biased at a currentwhich is twice as large as those of Q1 and Q 2 in order tGobtain full compensation of the effect of the base currents.

    The new circuit generating IVbe as well as the curvature-correction current I N L is shown in Fig. 3.The PTAT currentsin Fig. 3 are generated by the circuit shown in Fig. 2. Thecurrent Ivbe = Vbe3/R2 is derived from the base-emittervoltage of Q3. The intemal feedback in the circuit ensuresthat biasing at the indicated current levels is obtained. Thetransistors Q3 and Q4 , which have an emitter-area ratio 1 :p ,

    0018-9200/93$03.00 0 1993 IEEEi 1

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    66 8 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 28, NO. 6, JUNE 1993

    Parameter Simula tion Result Measurement result Units

    Output Voltage I 195 I 199.2 . 1 mV

    100ine Regulation

    Temp. Coefficient f 3over the temperatore rangefrom 0 to 125C

    PpmVLIVcc450

    Output Resistance 135 150 nFig. 2. The circuit gen erating the TAT current. Total Supply Current 1 90 1 95 I SA

    I 1 I I I I I , v -

    Fig. 3. The circuit generating the currents 11 bc and 1 . v ~ .

    are driven by two different currents, IPTAT and Iconst+I N L ,respectively. The voltage across R N L equals the differenceAvbe of the base-emitter voltages of Q3 and Q 4 , so that

    where I s 3 and Is4 represent the saturation currents of Q3 andQ 4 , respectively, and where it is supposed that P IS S= IS^.

    The resistor R N L is chosen in such a way that thetemperature-dependent part of the current 2(1Vbe +I N L )andIPTAT compensate each other and a temperature-independentcurrent Iconst (Fig. 3) is obtained for which it holds that

    Using (l), for the current through the resistor R2 it is foundthat

    Supply Voltage Range I 1 - 1 0 1 1 - 2 1 vI I II II0-125emperature Range 1 0 - 1 2 5

    Noiseover the temperature range n v l mfrom O t o 125'c 1 < l oo 1

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    GUNAWAN et al.: CURVATURE-CORRECTED LOW-VOLTAGE BANDG AP REFERENCE 669

    I I '-bufferamplifier1start-up PTAT-currentsource. circuit generating I-vconvertexIvbcd ' N LcircuitFig. 4. The complete circuit of the low-voltage bandgap reference.

    derived from the basic voltage Vbe and VPTATand convertedto the output voltage Vref. The conversion factors in thisprocess of signal conversion are obtained with resistors, anddetermined by resistor ratios.

    When well-matched resistors are used these ratios are in-dependent of temperature. However, the basic voltage VbeJ isalso affected by the resistor TC's, because this TC causes thebiasing current IPTATo be not really PTAT.

    In [4 ] Meijer shows that this effect causes a small nonlin-earity, which is compensated for when the condition (7) ismodified into

    where a1 is the first-order (linear) component of the TC. In ourdesign the empirical values 71 =3.5 and a1 = 1700 ppm/"Chave been used to calculate the resistor ratio R I / R N L . Finally,this ratio has been further optimized to compensate for theresidual term I L L of (9).

    The results of this analysis are found to be in good agree-ment with numerical ones obtained using the SPICE computerprogram. The SPICE simulation results are listed in Table I.

    111. MEASUREMENTESULTSND DISCUSSIONThe complete circuit is shown in Fig. 4.The start-up circuit

    generates a small current which is injected into the bases of thep-n-p current mirror and drives the circuit towards the desiredstate of biasing.

    This complete circuit is integrated using a standard bipolarprocess with base-diffused resistors. The circuit is calibratedfor the nominal value of Vrefby trimming the resistor R2 usinga fusible-link trimming method [ l ] . For Vref it holds that

    Provided that the resistor ratio R r e f / R 2 is a well-knownconstant which is completely predetermined by the layout, then

    v.t(mv)I 199.3199.1198.9198.7198.5 0 10 20 30 40 50 60 70 80 90 100 110 120 130

    Fig. 5. Measured value of l i e f (T) t a supply voltage of 1 V.

    this low-voltage reference has the typical attractive featureof bandgap references that the output voltage is temperature-independent when this voltage is adjusted for a predeterminedvalue, where also the (PTAT) offset voltage of output bufferamplifier and the mismatches of the current-mirror transistorsare compensated for.

    In practice, a slight deviation for the optimal value of Vref(i.e., the value with the lowest TC) is found (see Table I). Thisdeviation is mainly attributable to the effect of the (nonlinear)TC of the resistors.

    The temperature dependency of the output voltage is shownin Fig. 5, which depicts the measurement results for a supplyvoltage of 1 V. This measurement result agrees with the valuesfound by simulation. Other measurement results are listed inTable I. The measured line regulation differs considerablyfrom what was expected from the simulations. This effectis caused by the voltage dependency of the base-diffusedresistors, particularly the resistors R I , R z , and R N L .

    A base-diffused resistor is implemented in an epitaxialisland. This island has to be biased by a certain voltage, inthis case the positive supply voltage. The magnitude of theresistor is affected by the voltage between the epitaxial islandand the resistor (VepipSp) .The voltage dependency affectsthe performance of the circuit because the resistors R I and

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    670 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 28, NO. 6, JUNE 1993

    R2 have a Vepi-sp that is one diode voltage higher than theVepi--spof resistor RNL. t was found that when the supplyvoltage increases, the magnitude of RNL ncreases more thanthe magnitude of R1 and R2. This problem can easily besolved by placing a forward-biased diode between the epicontact and the bias voltage of R1 and R2 so that T/ep i - spof R I ,R2, and RNL ll have the same value.In view of this effect during the test, the voltage range hasbeen limited to the 1-2-V range.

    IV . CONCLUSIONThe realized circuit mainly shows the performance as ex-

    pected, based on simulations. The minimum supply voltage is1 V for an operating temperature range from 0 to 125C. Thecircuit also operates at temperatures lower than OC, but thena slightly higher supply voltage has to be tolerated. Also thecurvature-correction circuit functions as expected. The onlydifference in the performance of the circuit emanates fromthe voltage dependency of the base-diffused resistors of thestandard bipolar process used. This effect can, however, beeliminated, as was explained.

    ACKNOWLEDGMENTThe authors wish to acknowledge the Delft Institute of

    Microelectronics and Submicron Technology (DIMES) forprocessing the chips.

    REFERENCESG. C. M. Meijer, Thermal sensors based on transistors, Sensors andActuators, vol. 10, pp. 103-125, 1986.R. J. Widlar, A new breed of lineair ICs runs at 1-volt levels,Electronics, pp. 115-199, Mar. 29, 1979.H. C. Nauta and E. H. Nordholt, A new class of high performanceFTAT current sources, Elecrron. Left .,vol. 21, pp. 38&386, 1985.G . C. M. Meijer, A low power easy-to-calibrate temperature trans-ducer, IEEE J . Solid-Stare C ircuits, vol. SC-17, pp. 609-613, 1982.

    Made G unawan wa s born in Denpasar, Indonesia,on November 18, 1963. He received the M.Sc.degree in electrical engineering from the Delft U ni-versity of Technology, Th e Netherlands, in 1992.Since 1991 he has been employed by the Indone-sian Government.

    Gerard C. M. Meijer wa s bom in Wateringen,The Netherlands, on June 28, 1945. He received theingenieurs (M.Sc.) and Ph.D. degrees in electricalengineering from the Delft University of Technol-ogy, Delft, The Netherlands, in 1 972 and 1982,respectively.Since 1 972 he has been part of the Laboratory ofElectronics, Delft University of Technology, wherehe is an Associate Professor, engaged in researchand teaching on analo g ICs. In 1984 and part-timeduring 1985-1987 he was seconded to the DelftInstruments Company in Delft where he w as involved in the developmentof industrial-level gauges and temperature transducers.

    Je r oe n F onde r i e was born in Amsterdam, TheNetherlands, on July 27, 1960. He received theMS c. deg ree in electrical engineering from the DelftUniversity of Technology, Delft, The Netherlands,in 1987 and the Ph.D. degree from the same univer-sity in 1991.From 1987 he has been a Research Assistant atthe Electronic Instrumentation Laboratory, Depart-ment of Electrical Engineering, Delft University ofTechnology, where he has been working on thesubiect of low-voltage operational amplifiers. Since1992 he has been a Senior Designer at Philips Semiconductors, Sunnyvale,CA .

    Johan H. Huijsing (SM81) was born in Ban-dung, Indonesia, on May 21, 1938. He received theingenieurs (M.Sc.) degree in electrical engineeringfrom the Delft University of Technology, Delft, TheNetherlands, in 1969, and the Ph.D. degree fromthe same university in 1981 for work on operationalamplifiers.Since 1969 he has been a member of the Researchand Teaching Staff of the Electronic InstrumentationLaboratory, Department of Electrical Engineering,Delft University of T echnology, as a full Professor._He is engaged in research on analog integrated circuits and integrated sensorsfor instrumentation systems.