A Low Power CMOS Bandgap Voltage Reference with …

1 A LowPowerCMOSB andgapVoltageReferencewithEnhancedPowerS upplyRejectionWenguanLi, Ruohe Yao* , (a)CircuitsofconventionalCMOS bandgapvoltagereference.(b)PSR small signalanalysismodel ,ingenerallytheforwardvoltageofapn-junct ion,exhibitsnegativestemperature-coeffic ient,whichis about VlaC[6], [7]. Thedifferencebetweenthebase-emitterjunct ionvoltagesoftwobipolartransistorsoperat ingatdifferentcurrentdensityis directlyproportionalto theabsolutetemperature,whichexhibitsposi tivetemperature-coefficient[7]. Atemperaturestablebandgapvoltagereferenc ecan beachievedbysummingthis twovoltageshavingoppositetemperature-coe fficientswithproperweighting.(b)(a) (a). PSR analysisofconventionalCMOS bandgapvoltagereferencecan beperformedbasedonits PSR analysis small signalmodelindicatedin Fig. 1. (b).TheoutputresistanceofM1,2isomittedto neglectthechannellengthmodulationofthecu rrentmirrorpairs inorderto gainmoreinsightofthe can get (b).

2 Vg=A(132Vb-131vrej)+AddVdd(1)gml,2(Vdd-v g)rQI=13 IVrej(2)gml,2(Vdd-vg)(rQ2+RI)=132Vb(3)Ab stract-A lowpowerlowtemperature-coefficientbandga p Voltage referencefeatureshighpowersupplyrejectio n (PSR) for low dropout regulators (LDOs) ispresentedin this paper. AnoptimizedPSR enhance stage isinsertedin opampbasedCMOS Bandgap Voltage Reference ,which introduces supply spurious into the Bandgap loop so asto maintain a constantgate-sourcevoltage in the uppercurrent mirror; this improves the PSRperformanceabout38dB. Aprototypeofthe Bandgap Voltage Reference isimplementedin poly, double metal, cmos technology, occupying silicon are @ 50 kHz and -70 dB@100kHz, respectively; and, the supply current is9J1A,thetemperature-coefficientislSppm /i'C at2'"'-'5V supply Voltage ,the line regulation is54J1 isscalingdown with thereducingthicknessofgate oxide inmodemCMOS technologyand theincreasingdemandfor lowpowerportabledevices, thedynamicrangeofsystem thenoisilypowersupplywithoutadequatelyre jectedwillseriouslydegradesystemperforma nces,especiallyin RFapplications[1]-[3].

3 So, LDOs with highaccuracylowtemperature-coefficientha vebeenusedwidelyin noisesensitivewirelesstransceivers[4] and ADCs [5]. Thepowersupplyrejection(PSR),whichis ameritofLDOs'immunityagainstpowerspuriou s, ismostlylimitedby , the designoflow-powerlowtemperature-coeffici enthigh PSRbandgapvoltagereferenceis becomingmore and moreimportant[2].In this paper, the PSRofbandgapvoltagereferenceisanalyzedin detailbasedon thecorrespondingPSR smallsignalmodelin low Power , lowtemperature-coefficientbandgapvoltage referencefeatures given insectionNand sectionV, ;Lowpower;Highpowersupplyrejection.(5)(4 )WenguanLi was with the SchoolofElectronicandInformationEngineer ing,SouthChinaUniversityofTechnology,Gua ngzhou510640 China. And now is withRunxinIT Co., ,510663 is with the SchoolofElectronicandInformationEngineer ing,SouthChinaUniversityofTechnology, was with the SchoolofElectronicandInformationEngineer ing,SouthChinaUniversityofTechnology,Gua ngzhou510640 China.

4 And now is , {31=--rQ1+R3r0 2+R1132=-----rQ 2+R1+R2 Where,A=vg/vdifFGmdroubAdd=vg/vdd=Gmddro utarethe gain and978-1-4244-3870-9/09/$ 2009 IEEE300 Authorized licensed use limited to: MIT Libraries. Downloaded on March 12,2010 at 18:00:23 EST from IEEE Xplore. Restrictions apply. (6)(8)(7)(9)A. Temperature CompensationThebandgapcore consistofQI, Q2, MI, M2and Rr-, ,Q2 are parasitic vertical PNP, the emitter sizeofQ2 is 8 timesthatofQI. R2equals R3,MIand M2form a current ,Rc and Cc form aconventionaltwo-stage opamp,whose bias current is directlymirroredfrom Bandgap high gain opamp forces the nodes A and B to have thesame potential. Since QI and Q2, which have unequal sizes butequal emitter current, arebiasedat different current a PTAT loop is formed in QI, Q2 and PTAT current IRIis given asI=~el-~e2=VTIn(8)RI~R11 Where, VT=kT/q is the thermal Voltage .}

5 This current flowsthrough RIand R3,and, the bandgapreferencevoltage isgiven by~=~el+IRlR2=~el+R2~,ln(8)R1 This Reference Voltage equals to the silicon energy-gap [7]. In order to generate a suitable Reference output forLDO application, which is capableofdown to 1V outputvoltage, a resistance divider consistofR, andR,is added, CIis also added to reduce the noise of the Reference ,~,R7,C2are used to balance the two branches inbandgapcore. Theoutput Reference after potential divider is given byv;.e/=R4[~el+R2 VTIn(8)]~4+Rs~lTemperaturecompensationis achievedbyappropriatechoosing RI, R2,R3resistancesratio, which give:a~ef=R4[a~ell+ISIn(8)aVT]=0(10)etR4 +RsetT=TrRIetT=TrIn this design ,RI---Rsareimplementedin high poly resistor(HpolyR), theresistorratio in (10) the negativetemperature-coefficientofHpolyRh as noeffect on thetemperature-coefficientofthe outputreferencevoltage, if they areappropriatelysizingindesign andmatching in PSR Enhance MechanismThe PSRofthe Bandgap voltagereferenceis improvedwiththe PSR enhance stage [1], [8] consistofMIOand enhance stage not only increase loop gain, but alsoeffectivelyfeed the supply ripple into the PTAT loop, whichbenefits for PSRimprovementas it has been stated input two-stageopampexhibits anexcellentpositive PSR [7].

6 So the ripple at theoutputofthe opamp can be neglected, thus the PSR at nodeVgin mainly depends on the"diode-connected"PSR enhancestage, the"diode-connected"MIlhas a low impedanceofI/gmll,wheregmllis thetransconductanceof PSR atnode vgis given byVDD~------ITIIIIIIIIM11104M2I'"M7 MII,1 IIt-II-II-IIIIVcVgLM11!M14~t-f-~~M15--~I 4Jv,IR5R3R2R7M3M4II~~I>---~;:VbIf-~IIVaV refM16C310---<""' ~v C1(2M5rr1 MII-R4Q~2(8)R6 IIM12 IIII IPSRIB ancfgapIIcoreOpamp~:~h!~:e_JStart upFig. opamp, ,2is thetransconductanceofMI, ,rQI,rQ2are the small signal on resistanceofQIandQ2, PSRofthe Bandgap Voltage can bederivedfrom (1) - (5), andit'sgiven by (6).vref()1-Add--=gml,2rQ 1+R3()Vdd1+gml,2rQ 2+R1A-gml,2rQ 1A()I-A'" '"gml,2QI31R A+gml,21In (6), we note that the PSRofthe Reference Voltage mainlydepends on the gain and PSRofthe 'sgain can improves the PSR of Reference , which mayalso causes stability issues.)

7 As the gainofopamp decreaseswith the operating frequency, the PSRperformancedegradesin high frequency. Soit'sessential to increaseopamp's gainband widthproduct(GBW) in order to get a wide band highPSR capability. We also note from (6), if the PSRofopamp is1 ( ),the second term in (6) is zero; thereferencewillhave a high PSR capability. That is to say, if the outputofopamp vgfollows the ripple in the Power supply, the gate-source voltagesofMj, M2maintain constant, since MIand M2is designedto have a long channel length in order to make thechannel lengthmodulationeffect small enough to be neglected,which is usually the case in current mirror based cmos Bandgap Voltage Reference , thus, the drain currentofMI, M2remains constant even with supply ripple, so the bandgapvoltagereferencehas a high conclusion,it'sdesirable that vgtracks the fluctuation in the supplyvoltage.

8 The Bandgap voltagereferenceshown in sectionII,which features high PSR, follows the above circuitimplementationofthe low Power ,lowtemperature-coefficientbandgapv oltagereferencewithenhancedPSR isillustratedin The Bandgap voltagereferenceconsistoffoursub-buildin gblocks, including thebandgapcore, opamp, PSR enhance stage and startup licensed use limited to: MIT Libraries. Downloaded on March 12,2010 at 18:00:23 EST from IEEE Xplore. Restrictions apply. below the supply Voltage , both MI4and MI 5are cutoff, thus,thepowerconsumptionof the startup circuits is zero afterstartup. MI 6is inserted to discharge C3when supply is turneddown, in order to make sure properly startup next prototypeofthe Bandgap voltagereferenceis fabricatedin TSMC , double poly, double metal cmos in thistechnologyis , respectively.

9 The microchip isillustratedin Fig. active silicon area is measuredwith HP4155 Asemiconductor \1'----ConventionalBG/'-------f-----PSRe hancedBG-fII',~'I'fII'/11,--~~V"/~~iD~-8 0n::~-100-20-120-40 Add=Vg=rd510~l=OdB(11)Vddl/gml l+rds l OAsindicatedin (11) that the ripple from the positive supplycan injected in node vgwithout being attenuated, so, vgfollows the supply ripple, thegate-sourcevoltageofMIandM2maintainco nstantalthough there is noisily ripple in thesupply. Asit'smentionedin sectionII,this can improve thePSRofreferencevoltage. The simulated PSRofthe bandgapvoltagereferencecomparingwithconv entionalone are shownin Fig. 3. The PSR enhance stage improve PSR about thebandgapvoltagereferenceFig. (V)--&-30deg C--e-60deg C~ ' ~ >Thetemperaturedependenceof thebandgapreferenceisshown in Fig. 6. The output Voltage variationin0----100 Ctemperaturerange at different supply is less than Thetemperature-coefficientsat 2, 3, 4, 5V supply Voltage arel Sppm/X', , ',respecti outputcharacteristicofthe Bandgap voltagereferenceat differenttemperatureis shown in Fig.

10 5. The resultindicatedthat thereferenceoutput regulates at supply voltagedown to 2V. The output Voltage is at roomtemperature; and, the operating current at 100 C is 9flA,correspondsto 18flWpowerconsumptionat 2V supplyvoltage. The variationofoutput Voltage for the supply voltagechange from 2 to 5V is ; thecorrespondinglineregulationis CompensationOpamp based current mirror which has both positive andnegative loops is used in PTA T loop, frequencycompensationisrequiredto make sure loop stability. As the outputimpendenceofthe PSR enhance stage is low, the pole at nodevglocate at high frequency which farbeyondthe unite gainfrequency (UGF)ofthe feedback ,is utilized toachieve stability. Thedominantpole locate at the outputofthefirst stage, a nulling resistorR,is utilized to move the RHPzero to LHP and locate itjustbeyondthe UGF to improvephase margin [7].