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.

2 (b)(a) (a). PSR analysisofconventionalCMOS bandgapvoltagereferencecan beperformedbasedonits PSR analysis small signalmodelindicatedin Fig. 1. (b).TheoutputresistanceofM1,2isomittedto neglectthechannellengthmodulationofthecu rrentmirrorpairs inorderto gainmoreinsightofthe can get (b).Vg=A(132Vb-131vrej)+AddVdd(1)gml,2(V dd-vg)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.

3 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]. So, LDOs with highaccuracylowtemperature-coefficientha vebeenusedwidelyin noisesensitivewirelesstransceivers[4] and ADCs [5].

4 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.

5 And now is withRunxinIT Co., ,510663 is with the SchoolofElectronicandInformationEngineer ing,SouthChinaUniversityofTechnology, was with the SchoolofElectronicandInformationEngineer ing,SouthChinaUniversityofTechnology,Gua ngzhou510640 China. 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.}

6 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 . 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].

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)

8 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]. 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!

9 M14~t-f-~~M15--~I4Jv,IR5R3R2R7M3M4II~~I> ---~;:VbIf-~IIVaVrefM16C310---<""' ~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. As the gainofopamp decreaseswith the operating frequency, the PSRperformancedegradesin high frequency.)

10 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.