Report for MOSIS Education Program (Research) An Ultra-Low-Power Self-Biased Current Source

Transcription

1 Repor or MO Educaion rogram (Research) D: T3AG AQ An Ulra-Low-ower el-biased Curren ource repared by: Edgar Mauricio Camacho Galeano, uden Márcio Cherem chneider, roessor nsiuion: Federal Universiy o ana Caarina Elecrical Engineering Deparmen negraed Circuis Laboraory Dae o ubmission: May,

2 DEGN OF AN ULTRA-LOW-OWER CURRENT OURCE Edgar Mauricio Camacho-Galeano, Carlos Galup-Monoro, and Márcio Cherem chneider Deparameno de Engenharia Elérica, Universidade Federal de ana Caarina, Brazil. ABTRACT This paper presens he design o an ulra low-power sel-biased curren source. We have designed a pa curren reerence in boh.5µm and.35µm CMO echnologies. The associaion o a very simple opology, an eicien design mehodology, and low oupu conducance rapezoidal ransisors has resuled in area o.6mm and power consumpion around o nw. Eperimenal resuls or he.5µm CMO echnology validae he design and show ha he curren source can operae rom supply volage down o.v wih a regulaion beer han 8%/V o supply volage.. NTRODUCTON The increasing demand or inepensive very-low-power porable and implanable medical applicaions has resuled in he inegraion o low-volage CMO analog circuis compaible wih sandard VL echnologies [], [7]. This endency has moivaed he developmen o sysemaic mehodologies or analog design. Furhermore, eicien, simple and easy-o-design analog circui srucures are highly desirable [], [], [6]. CMO analog design based on he inversion level echnique has been shown o be a robus alernaive or high perormance in very-low-power [3] and low-volage circuis []. This echnique uses he curren as he main design variable. Thus, analog circuis based on such a design echnique require a selbiased curren source (BC) o operae a he nominal inversion level. Moreover, he generaion o on-chip curren reerences avoids he need or an era pad o communicae wih he eernal environmen. everal BC circuis are ound in he lieraure [] - [7], bu a design mehodology or easy reuse or design is no available ye. Our BC is based on he circuis proposed in [], [5], [6], and [7], which have he same curren dependence on emperaure. Despie he simpliciy o he circui proposed in [], i uses a resisor ha or small currens (pa-na) consumes a very large silicon area. To avoid he need or a resisor, he auhors o [6] use a MOFET working in he riode region o replace he resisor. Even hough simple, he BC o [6] is no suiable or low volage operaion, as poined ou in [7]. Anoher implemenaion o an BC is presened in [5] bu he large curren gains and operaion in srong inversion o some o is ransisors degrade is power eiciency. Reerence [7] presens anoher proposal o an BC, a less simple srucure han he previously menioned ones. The circui in [7] uses a sel-cascode MOFET (CM) in srong inversion and a TAT volage reerence generaed by means o a curren raio. Alhough appropriae or low volage operaion, he power eiciency o he curren source in [7] is no high due o he use o slighly more comple srucures and operaion in srong inversion o some ransisors. This paper presens he design o a sel-biased curren source dedicaed o echnology-independen inversion level biasing, which is suiable or low-volage and very low power applicaions. Our BC circui uses MOFET s only and can operae down o V supply volage (in sub-.8µm echnologies his circui can operae rom.6v) and ehibis low sensiiviy o supply volage. n ecion, he ACM model [8] and he concep o inversion level are summarized. We develop he basic design equaions or he BC using he ACM model in ecion 3. ecion inroduces he low-volage CMO BC. The design mehodology o obain he dimensions o he MOFET s is ormulaed in ecion 5. As a design eample, a very low power BC is implemened in boh.5µm and.35µm CMO processes and he associaed simulaion and eperimenal resuls are presened in ecion 6. Finally, concluding commens are presened in ecion 7.. THE ACM MODEL n he design mehodology o he sel-biased curren source, we have employed ACM, a curren-mode MOFET model ha uses he concep o inversion level [8]. According o he ACM model, he drain curren can be spli ino he orward ( F ) and reverse ( R ) currens where D = = i i ) () F R ( r W = Q = Q L Q ( ) (.a) φ = µ C' o n (.b) F ( R ) depends on gae and source (drain) volages. n orward sauraion, F >> R ; consequenly, D = F i. is he normalizaion (speciic) curren and Q is he shee speciic curren ( or W = L ), i (r ) is he orward (reverse) inversion level, and µ, n, C o, φ, and W/L= are he mobiliy, slope acor, gae oide capaciance/area, hermal volage, and he ransisor aspec raio, respecively. The relaionship beween curren and volage is given [8] by

3 V V ( D) = φ ( + i ) + i ( r) + ln ( r) V (3) VG VT () n where V is he pinch-o volage and V TO is he zero bias hreshold volage. More deails abou ()-() can be ound in [8]. The sel-biased curren-source circui proposed here is an eracor o normalizaion (speciic) curren Q [5] opimized or low-volage and very low power applicaions. 3. DEGN EQUATON FOR CM AND B The core o he BC is he CM shown in Figure. The V- characerisic o he CM is very appropriae or building lowvolage analog blocks such as curren reerences and sub-mv TAT volage reerences [3] [7]. Figure. chemaic o he CM conneced in diode coniguraion (V B =V B =). The design equaions (7)-(9) ha describe he V- characerisic o he CM have been deduced using ()-(3) and he schemaic in Fig.. According o (), he drain currens o M and M can be epressed as uncions o he orward and reverse inversion levels = i (5) F ( + (6) D = F R = i ir) = ince V =V =V and V D =V, hen i r = i [5]; hus, rom (5) and (6) we can ind he relaionship beween i and i = + + i i (7) wih he raio X / deined by he gain o a MO curren mirror. The applicaion o (3) o M resuls in V V φ + ( + i ) + ln( + i ) = (8) are readily deermined. n he ollowing, we show he implemenaion o a reerence volage or V X. Figure. el-biased srucure (B) The sel-biased srucure in Fig. can be used o generae V X a he inermediae node o he CM []. The volage V re can be calculaed using (3) and assuming M 8 -M 9 in weak inversion sauraion. Noing ha V 8 =V 9, D8 = D9, and V re =V 8, hen Vre = V9 + φ ln( K ) where 8 K = () 9 n our circui opology, V 9 can be eiher zero or a TAT volage generaed by means o a second CM operaing in weak inversion, as shown in Fig. 3. The applicaion o epressions (7-9) o he CM composed o M 3 and M operaing in weak inversion gives = + V9 φ ln () 3 Boh TAT volage reerences epressed by () and () are immune o supply volage as well as o echnological parameers variaions as long as he ransisors operae in weak inversion. According o epression (), in weak inversion he CM can operae as a very-low-volage TAT reerence, which is independen o curren level and echnology. The previous equaions and some consideraions or design will be used in ecion 5 o develop a design mehodology appropriae or low volage and very low power.. THE ROOED LOW-VOLTAGE BC A simple version o our BC circui is shown in Fig. 3, where V 9 can be eiher zero or V X(W). We propose a simple power eicien BC circui ha replaces he resisor o he implemenaion in [] wih an CM operaing in moderae inversion o achieve he requiremens o low curren and low volage operaion. while, or M V = ( + i ) + ln( + i ) φ + Equaions (7), (8), and (9) wih ive unknowns (V, i, i, / and V X ) have been insrumenal in he developmen o he design mehodology o he BC. we assume ha a volage generaor ses V X a a given value and a (MO) curren mirror deines /, he inversion levels i and i as well as he currens (9) K : Figure 3. el-biased curren source circui

4 When he swich in Fig. 3 is conneced o ground, he TAT volage reerence is produced by an inenional ose (K in Fig. 3) beween MO devices M 8 and M 9 boh biased in weak inversion. n his case, he TAT volage is given by (), wih V 9 =. This circui simple opology is appropriae or applicaions where moderae perormance is oleraed. Also, his circui is sable or K > and is very accurae or K [7]. Anoher orm or implemening he TAT volage reerence can make use o a second CM (M 3, M ) biased in weak inversion and K=; hus, V re =V 9, wih V 9 given by (). This second implemenaion resuls in improved symmery and maching o he srucure and allows a simple implemenaion o M 8 -M 9 wih rapezoidal ransisors [], which are employed o improve regulaion o he curren reerence wihou requiring a large silicon area. 5. DEGN METHODOLOGY The design speciicaions o he curren reerence are usually he minimum supply volage (V DDmin ), power dissipaion, silicon area, and sensiiviies, in addiion o re, he value o he curren isel. The design mehodology can be based on a maimum power supply or a curren reerence ( re ) value, and he ransisor dimensions are opimized o achieve low volage operaion. This mehodology can be applied o eiher he simple opology (swich conneced o ground), or he symmeric opology (swich conneced o node V X(W) ) in Fig. 3. The minimum supply volage, which is deermined by he consrains imposed by he wo lemos branches in Fig. 3, can be wrien as V DD ma{ VDsa, + VG, M, VG, + VDsa, M 8 + V} (.a) where V Dsa,M8 mv since M 8 operaes in weak inversion. The p-channel ransisors are sized in order o operae in weak inversion, wih an inversion level close o or smaller; hereore, V Dsa, mv and V G, V T. ince V X is less han mv, and M operaes in moderae inversion wih V G,M V TN + mv we can use he irs-order approimaion VDD ma{ VT, VTN } + mv (.b) or he minimum supply volage. O course, epression (.b) is a rough approimaion or he minimum supply volage. n our design mehodology, we have chosen he CM o operae in moderae inversion wih i =3 or, equivalenly, V =V (M.) (please, see (8)). V 9 =, we can calculae i rom equaion (9) or, equivalenly, solve or ( + i ) + ln( K ) = + ln( ) (3) =. Once has been calculaed or a given K, e. g. K=, one can proceed o calculae / rom (7) ( + ) 3( + N) = and N re = () 3 Q For a curren reerence re << Q, 9 = keeps M 9 in weak inversion and he acor N deines a rade-o beween power consumpion and area. The aspec raio ( ) o he MO ransisors M 5 -M 7 (M ) is calculaed using (.a), and a proper choice o he inversion level, usually less han or low volage applicaions. For he design o he symmeric opology we have used he same previous mehodology wih K=, and 3, calculaed rom () or a given V 9. The sensiiviy o he circui o supply power is associaed wih he Early eec o M 7 and M 8. The Early eec can be reduced using long channel lenghs ha, however, demand large silicon area. One approach o obaining long channel lenghs wih moderae area is he rapezoidal ransisor proposed in []. 6. REULT To veriy he design mehodology and perormance o he proposed circui, he wo opologies in Fig. 3 have been designed or boh AM-.35µm and AM-.5µm CMO echnologies. A comparison o simulaed resuls is given in Table, Fig. and Fig. 5 or re =pa, N=, K=9 =.35, 9 =, / =. and or / 3 =. The ransisor dimensions or he symmeric opology in AM-.5µm are presened in Table. A symmeric BC was implemened in a.5µm CMO echnology and occupies an area o 3µm. imulaion and eperimenal resuls are compared in Fig. (b) and Fig. 5(b). These resuls validae he design and show ha he BC can operae rom supply volage slighly lower han V TN + V T. The measured sensiiviies o he TAT reerence volage and o he reerence curren o V DD are below +.%/V and +8%/V, respecively. The average curren reerence o a ive sample se wih wo dieren layous is pa wih maimum deviaion o ±% a.v o supply volage. Table : ummary o simulaion resuls arameer imple opology, K=9 ymmeric opology, K= Uni.35µ.5µ.35µ.5µ V DDmin V ower (V DDmin) nw V TAT RR(DC)+ a V DD V DDmin >7 >75 >9 >95 db Vre sensiiviy ov DD %/V Vre sensiiviy o T %/ o C re sensiiviy o V DD %/V re sensiiviy o T %/ o C ( = 65( na 8( na QN ).35 ) u QN ( ).5 = ) u Table : Transisor dimensions or he symmeric opology (AM-.5µm). arameer M M M 3 M M 5-7() M 8-9 W [µm] L [µm] 8 9 6

5 6 - [ A ] [ A ] imulaion Eperimen (a) ymmeric opology AM-.35µm (imulaion) Figure. Curren reerence agains supply volage.6 [ V ] (b) ymmeric opology AM-.5µm (a) ymmeric opology AM-.35µm (imulaion) (b) ymmeric opology AM-.5µm Figure 5.. TAT volage reerence ( V(w.i)) agains supply volage [ V ] imulaion Eperimen 7. CONCLUON A low-volage low-power sel-biased curren-source has been proposed. Design equaions based on he ACM model in any inversion level have been provided. The proposed circuis are process-independen and reproducible in any sandard CMO echnology. imulaion and eperimenal resuls have shown ha he sel-biased curren sources provide low-volage, ulralow-power operaion and low sensiiviy o changes in he supply volage. The BC and design mehodology proposed here are especially suied or very-low-power applicaions. 8. ACKNOWLEDGEMENT We hank CNq, he Brazilian Agency o cience and Technology, or he parial inancial suppor o his work. MO is graeully acknowledged or providing access o inegraion. 9. REFERENCE [] J.F. Duque-Carrillo e al. VERD: An acousically programmable and adjusable CMO mied-mode signal processor or hearing aid applicaions. EEE J. olid-ae Circuis, vol. 3, no. 5, pp , May 996. []. Yan and E. ánchez-inencio, Low volage analog circui design echniques: A uorial ECE Trans. Fundamenals, vol. E- A, No., pp. -7, February. [3] B.Linares-Barranco and T. errano-goarredona, On he design and characerizaion o emoampere curren-mode circuis EEE J. olid-ae Circuis, vol. 38, pp , Augus 3. [] E. Vioz and J. Fellrah, CMO analog circuis based on weak inversion operaion, EEE J. olid-ae Circuis, vol. C-, pp. -3, June 977. [5] E.A Vioz and C.C Enz, CMO low-power analog circui design, roceedings o he nernaional ymposium o Circuis and ysems (CA 96), chaper. o Tuorials. [6] H. J. Oguey and D. Aebischer, CMO curren reerence wihou resisance, EEE J. olid-sae Circuis, vol. C-3, pp. 3-35, July 997. [7] F. erra-graells and J. L. Hueras, "ub -V CMO proporionalo-absolue emperaure reerences", EEE J. olid-ae Circuis, vol. 38, no., pp. 8-88, Januery 3. [8] A.. A. Cunha, M. C. chneider, and C. Galup-Monoro, An MO ransisor model or analog circui design, EEE J. olid-ae Circuis, vol. 33, pp. 5 59, Oc [9] M.J.M.elgrom, A.C.J.Duinmaijer and A..G.Welbers, Maching properies o MO ransisors EEE J. olid-ae Circuis, vol., no. 5, pp. 33-, Ocober 989. [] C. Galup-Monoro, M. C. chneider and. J. B. Loss, "eriesparallel associaion o FET's or high gain and high requency applicaions", EEE J. olid-ae Circuis, vol. 9, no. 9, pp. 9-, epember 99.